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Energy Center Thesis and Dissertations

2019-07-28

Design, development, experimental

investigation and system improvement

of mirt-injera baking stove

kebeta, Jagama

http://hdl.handle.net/123456789/10864

Downloaded from DSpace Repository, DSpace Institution's institutional repository

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BAHIR DAR UNIVERSITY

BAHIR DAR INSTITUTE OF TECHNOLOGY

SCHOOL OF RESEARCH AND POST GRADUATE STUDIES

ENERGY CENTER

DESIGN, DEVELOPMENT, EXPERIMENTAL INVESTIGATION AND

SYSTEM IMPROVEMENT OF MIRT-INJERA BAKING STOVE

BY

JAGAMA KEBETA

BAHIR DAR, ETHIOPIA

JULY 28, 2019


SYSTEM IMPROVEMENT OF MIRT-INJERA BAKING STOVE.

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DESIGN, DEVELOPMENT, EXPERIMENTAL INVESTIGATION AND SYSTEM

IMPROVEMENT OF MIRT-INJERA BAKING STOVE

BY

JAGAMA KEBETA

ADVISOR: DR- ABDULKADIR AMAN

A thesis submitted to the School of Research and Graduate Studies of Bahir Dar Institution

of Technology BDU in Partial fulfillment of the requirements for the Degree of Masters of

Science in Sustainable Energy Engineering.

ADVISOR: DR- ABDULKADIR AMAN

BAHIR DAR, ETHIOPIA

JULY 28, 2019



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© 2019

JAGAMA KEBETA

ALL RIGHTS RESERVED



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This Thesis is dedicated to JESUS CHRIST.

For Everlasting Love, Support and Mercy

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ACKNOWLEDGEMENT

Primarily I thank the almighty God who gave the strength and the calm through my life to

accomplish this work.

I would like to express my deepest gratitude to Dr. Abdulkadir Aman, for his invaluable

comments and continuous supervision, encouraging starting from the development of

proposal to the completion of the research work. I also thanks him for the cordial

relations showed towards me, which was very helpful and very much appreciated.

I would like to send my great honor to Ato Niguse Fayissa for allowing me to produce

Prototype in his work shop , Ato Negese Yayu for advising and supporting me starting

from the development of the thesis work and Ato Yisak Soboka head of laboratory at

Ministry of Ethiopian Water , irrigation and electricity who assign other experts ,allow

me and take the responsibility to access to the laboratory instruments to conduct the test,

shared me his experience to finish my work in a better way.

Lastly but not least, I would like to send my special thanks to my wife Mekdas Brihanu

and my son Daniel Jagama for their encouragement, patience and support towards my

academic career.



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ABSTRACT

Human development, along with other factors, is very much dependent on energy

resources. To satisfy the ever-increasing need for energy, man pushed away beyond what

can be carried by the environment and this is the cause of many of the problems the

world is facing now. There are a number of efforts undertaken to improve the efficiency,

lessen the indoor air pollution and reduce emission associated with the use of open fire

stoves by introducing improved cook stoves. Mirt stove is one of the efficient stoves in

Ethiopia which has got significant effort to be disseminated. But due to its high initial

startup time, heavy weight, less thermal efficiency its acceptance is low.Mirt Stove has

thermal and fuel efficiency of 18% -23%[15] and 33%[24] respectively. In this study an

attempt has been made to increase the thermal and fuel efficiencies of Mirt stove, by re-

designing more efficient combustion chamber to increase heat transfer efficiencies.This is

done by constructing the wall of combustion chamber from locally available shiny sheet

metal and fiber glass inserted in between the walls as insulating material. The Grate is

designed and inserted in combustion chamber for smooth air flow; reduce delay initial

startup of fire. In this work the wasted heat retained in the wall of stove body, disputed to

surrounding by heat transfer, heat wasted through flue gas are recovered and used for

baking other food staff on second designed stove. The second stove is designed in such a

way that the flue gases from first combustion chamber with steam carrying copper tube

passing and circulating through it.Heat transferred to the cooked food by mode of

different heat transfer. We can re-use the heat to be wasted from circulation of fluid to

prepare coffee, tea and wot as well to condense fluid for re- use. The performance of the

newly improved stove is manufactured as prototype and tested using Control Cooking

Test (CCT). The result shows the Thermal Efficiency is increased by 6.08%, fuel saving

increased by 18% and cooking time decrease by 11% compared to Mirt Injera baking

stove .This makes the study fruitful, can create big jobs for stove supplying,

manufacturing, dissemination as well it enables the interested users to prepare Injera,

Wot, Tea and Coffee just on streets corners in small hut as it is possible to make it

movable. The customer can have fresh Injera with Wot or take home with in big towns.

The project has power to change our mode of life in addition to the impact it has on

Environmental.

Keywords: Mirt Injera Baking Stove, Fuel efficiency, Heat Recovery, Steam



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TABLE OF CONTENTS

ACKNOWLEDGEMENT ....................................................................................................... I

ABSTRACT ............................................................................................................................. II

TABLE OF CONTENTS ......................................................................................................... III

ACRONYMS AND ABBREVIATION ................................................................................... VII

LIST OF ABBREVIATIONS AND SYMBOLS ..................................................................... VIII

CHAPTER 1............................................................................................................................. 1

1.INTRODUCTION ................................................................................................................ 1

1.1. BACKGROUND AND JUSTIFICATION .................................................................................. 1

1.2. INJERA ............................................................................................................................. 3

1.3. STATEMENT OF THE PROBLEM ......................................................................................... 3

1.4. OBJECTIVES ..................................................................................................................... 4

1.4.1.GENERAL OBJECTIVE ....................................................................................................... 4

1.4.2.SPECIFIC OBJECTIVES ...................................................................................................... 4

1.5. SCOPE OF STUDY AND LIMITATION .................................................................................. 5

1.6. SIGNIFICANCE OF THE STUDY .......................................................................................... 6

CHAPTER 2............................................................................................................................. 7

2. LITERATURE REVIEW ....................................................................................................... 7

2.1. THEORETICAL BACKGROUND FOR DESIGNING WOOD BURNING COOK STOVES .............. 7

2.2. WOOD COMBUSTION ...................................................................................................... 8

2.3. STAGE 1 COMBUSTION ..................................................................................................... 9



2.6. COMBUSTION IN SMALL ENCLOSURES ............................................................................. 11



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2.7. IMPROVE COMBUSTIONIMPROVING FUEL EFFICIENCY (GET MORE .................................. 15

HEAT INTO THE COOKING FOOD) ..................................................................................... 15

2.9. THERMAL INSULATION SYSTEM ....................................................................................... 23

2.10. ADVANTAGES OF INSULATION SYSTEMS .......................................................................... 23

2.11. THERMAL CONDUCTIVITY AND THERMAL RESISTANCE OF INSULATOR ........................... 24

2.12. INSULATING MATERIAL ................................................................................................... 24

2.13. WALL LOSS CALCULATIONS ............................................................................................ 28

2.14. HEAT RECUPERATION ...................................................................................................... 28

2.15. PHU /PERCENT HEAT UTILIZATION/ OR THE THERMAL EFFICIENCY/ TESTS METHOD

STOVE TEST TYPES ................................................................................................................... 31

2.16. WATER BOILING TEST ..................................................................................................... 31

2.17. KITCHEN PERFORMANCE TEST ........................................................................................ 32

2.18. CONTROLLED COOKING TEST AND PROCEDURE .............................................................. 32

2.19. FLUID FLOW..................................................................................................................... 34

2.20. INDOORS AIR POLLUTION ................................................................................................ 35

2.21. BIOMASS USAGE IN ETHIOPIA .......................................................................................... 37

2.22. STOVE IMPROVEMENTS IN ETHIOPIA ................................................................................ 38

2.23. DEVELOPMENT AND DISSEMINATION OF MIRT STOVE ..................................................... 41

2.24. DESCRIPTION OF MIRT STOVE .......................................................................................... 42

2.25. FUEL CONSUMPTION OF MIRT STOVE .............................................................................. 44

CHAPTER THREE .................................................................................................................. 45

3. MATERIALS AND METHOD ...................................................................................... 45

3.1. INTRODUCTION ................................................................................................................ 45

3.2. MATERIAL SELECTED FOR CONSTRUCTION OF MODIFIED MIRT ...................................... 45



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STOVE ................................................................................................................................ 45

3.4. DESIGN OF COMBUSTION CHAMBER AND MATERIAL SELECTION .................................... 48

3.5. DESIGN OF COMBUSTION CHAMBER AND STOVE WALL .................................................. 48

3.6. HEAT REQUIRED FOR INJERA BAKING.............................................................................. 49

3.7. STANDARD SIZES OF INJERA AND CLAY MITAD ............................................................... 50

3.8. THE HOUSE HOLD FAMILY SIZE AND NUMBER OF INJERA BAKED WITH FREQUENCY

OF BAKING PER MONTH ............................................................................................................ 50

3.9. MATERIAL SELECTION FOR STOVE BODY CONSTRUCTION .............................................. 50

3.10. SIZE DETERMINATION OF STOVE WALL, PRIMARY AIR INLET, WOOD INSERT AND

FLUE GAS EXHAUST HOLES ...................................................................................................... 52

3.11. SELECTION OF THE BEST INSULATION MATERIAL ............................................................ 52

3.12. CRITICAL INSULATION (OPTIMIZATION OF CRITICAL INSULATION) ................................. 54

3.13. RECOVERING THE HEAT LOSS OF THE FLUE GAS ............................................................. 60

3.14. DESIGN OF SECOND STOVE .............................................................................................. 62

3.15. SIZING AND ARRANGEMENTS OF STEAM CIRCULATING TUBES ........................................ 62

3.16. USING METAL CHIPS IN THE GAP OF SECOND STOVE ...................................................... 64

3.17. STOVE MANUFACTURING................................................................................................. 64

3.18. PERFORMANCE AND THERMAL EFFICIENCY TESTING OF IMPROVED STOVES ................... 66

3.19. EQUIPMENT ...................................................................................................................... 67

3.20. DATA ANALYSIS .............................................................................................................. 69

3.21. MEASUREMENTS AND CALCULATIONS ............................................................................. 69

CHAPTER FOUR ......................................................................................................................... 71

4. RESULT AND DISCUSSION ................................................................................................ 71

4.1. EQUIVALENT DRY WOOD CONSUMED ............................................................................. 71



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4.2. SPECIFIC FUEL CONSUMPTION ......................................................................................... 71

4.3. COOKING TIME ................................................................................................................ 73

4.4. INITIAL START UP ............................................................................................................ 74

4.5. TEMPERATURE GRADIENT WITH 2ND STOVE.................................................................... 75

4.6. THERMAL EFFICIENCY IMPROVEMENT OF IMPROVED MIRT STOVE ................................. 75

4.7. TEMPERATURE DISTRIBUTION ON SECOND MITAD .......................................................... 76

CHAPTER 5............................................................................................................................. 80

5. CONCLUSION AND RECOMMENDATION ............................................................................ 80

6. REFERENCES .................................................................................................................... 82

7. ANNEX .............................................................................................................................. 86



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ACRONYMS AND ABBREVIATION

CCT Controlled Cooking Test

WBT Water Boiling Test

CSA Central Statistical Agency

EEA Ethiopian Energy Authority

ESD Energy for Sustainable Development

EREDPC Ethiopian Rural Energy Development and promotion Center

ETB Ethiopian Birr

GIZ German International Cooperation

IEA International Energy Agency

MoWIE Ministry of Water Irrigation and Electric

NGO Non-Governmental Organizations

WBISPP Woody Biomass Inventory and Strategic Planning Project

ESD Energy for Sustainable Development

EEPCo Ethiopian Electric Power Cooperation

PHU Ppercentage heat utilization

VITA Volunteers in technical Assistance



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LIST OF ABBREVIATIONS AND SYMBOLS

A air openings =Area of air openings in m2

A chimney in let = Area of chimney in let in m2

Cc = Weight of char with container in gram

∆Cc= Weight of char remaining from combustion in gram

dc = Diameter of chimney in meter

Pd = Natural draught pressure in / 2

fd = Equivalent dry wood consumed in grams

ff= Final weight of fuel wood in grams

fi = Initial weight of fuel wood in grams

g = Gravitational acceleration in / 2

h = Convective Heat Transfer, Wm-2

K

k = Thermal conductivity, W m-1

K-1

w = Weight of container for char in grams

L = Stove height in meter

m = Moisture Content of Fuel Wood (%-wet basis)

P=Stove Power, Watt

S=Conduction Shape Factors

N = Number of Possible Alternative Combinations

Pj = Empty weight of pot in grams, j ranging 1-4

Pjf = Final weight of each pot with cooked food in grams, j ranging 1-4

r1 = Insulations inside radius in meter

r2 = Insulation outside radius in meter

SC = Specific fuel consumption in /

ta = Ambient air temperature in o

C

t c= Combustion air temperature in o

C

Ti = Inside temperature of insulation in K

To = Outside temperature of insulation in K

tf = Final time of cooking in min

ti= Initial time of cooking in min

t = Total cooking time in min

Wf = Total weight of cooked food in grams



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i = Combustion air density in / 3

o = Ambient air density in / 3



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LIST OF FIGURES

Table 1.1 Share of Ethiopian Energy Supply 2009 (%) ........................................................................ 1

Figure 1.1 Ethiopian Injera ................................................................................................................... 3

Figure 2.1 Fire Triangle ....................................................................................................................... 8

Figure 2.2 Processes and temperatures in a burning of wood ............................................................... 9

Figure 2.3 Wood Combustion ............................................................................................................. 12

TABLE 2.1 Different Factors that influences Stove Efficiency ......................................................... 14

Figure 2.4. Pictorial representation of heat transfer phenomenon in stove design [44]. ..................... 16

TABLE 2.2. TYPICAL PROPERTY VALUE OF SOME MATERIALS AT ROOM

TEMPERATURE [50] ........................................................................................................................ 17

Figure 2.5 Parameter for Conductive Heat Transfer [50] ................................................................... 17

Figure 2.6 Additive of clay with ash ................................................................................................... 26

Figure 2.7 Additive of clay with chip Wood Heat Conduction Property .......................................... 26

Table 2.3 Health affecting limits according to WHO’s Exposure Guidelines .................................... 36

Table 2.4 Improved Household Injera Baking Stoves trend in Ethiopia [781] ................................... 38

Figure 2.8 Pictorial Description of Mirt-Injera Stove [15] ............................................................... 43

Figure 2.9 Assembled Whole Picture of Mirt Stove [15] ............................................................... 43

Table 2.5 Specific fuel consumption of Mirt Stove ............................................................................ 44

Table 3.1 Selection of Body Materials for Improved Mirt Stove ..................................................... 46

Table 3.2 Percent Satisfaction in achieving the criteria ...................................................................... 46

Table 3.5 Detail Specification and BOQ of Body Construction Material .......................................... 47

Table 3.7 Percent Satisfaction in achieving the criteria ...................................................................... 53

Table 3.8 Best Insulation Material Selected ....................................................................................... 54

Figure 3.1 Critical Insulation [38]..................................................................................................... 55

Figure 3.2 Air Entrance Hole .............................................................................................................. 56

Figure. 3.3 Wood Insertion Hole ........................................................................................................ 57



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Figure 3.4 Flue Gas Exhaust Hole ...................................................................................................... 57

Figure 3.5 Drawings of first Stove Scale 1:2 ...................................................................................... 58

Figure 3.6 Drawing of Second Small Stove Scale 1:2 ........................................................................ 59

Figure 3.5 Assembled Stove Systems during Injera Baking ............................................................... 59

Figure 3.7. The Photo of Newly Designed Mirt Stove ....................................................................... 60

Figure 3.8 Picture of Circulating Copper Coil .................................................................................... 62

Figure 3.9 Shape Factors [38] ............................................................................................................. 63

Figure 3.10 View of Newly Mitad after Assembly of Coil. ............................................................... 64

Figure 3.11 AutoCAD Three Dimensional View of Improved Mirt Stove ........................................ 65

Figure 3.12 on construction with steam and flue gas system for second stove................................... 66

Figure 3.13 Fuel .................................................................................................................................. 67

Figure 3.14 Dough for Cooking .......................................................................................................... 68

Figure 3.15 weighting scale ................................................................................................................ 68

Figure 4.2 Specific fuel consumption of improved Mirt stove and open fire ..................................... 72

Figure 4.3 Specific fuel consumption of Mirt stove and Improved Mirt Stove .................................. 72

Figure 4.4 Specific Fuel Consumption of Mirt Stove and Open fire .................................................. 73

Figure 4.5 over all Specific fuel consumption of the three tests ......................................................... 73

Figure 4.6 Cooking time taken for Open fire, Mirt and Improved Mirt Stove ................................... 74

Figure 4.7 Temperature Range of the second clay pot till initial star up ............................................ 75

Figure 4.8 Temperature distribution of the second Mitad................................................................... 76

Figure 4.9Temperature distribution alone the flue gas way of second Mitad ..................................... 77

Figure 4.10 Temperature gradients in opposite side of the flue gas way ............................................ 77

Table 4.2 Result of CCT Test Comparing Modified Mirt Stove to Open fire Stove .......................... 79

Drawings of first Stove Scale 1:2 ....................................................................................................... 86



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LIST OF TABLES

Table 1.1 Share of Ethiopian Energy Supply 2009 (%) ........................................................... 1

Table 2.1 Different Factors That Influences Stove Efficiency .............................................. 14

Table 2.2. Typical Property Value of Some Materials at Room Temperature ..................... 17

Table 2.3 Health Affecting Limits According to Who’s Exposure Guidelines ..................... 36

Table 2.4 Improved Household Injera Baking Stoves Trend in Ethiopia [10,41] ................. 38

Table 2.5 Specific Fuel Consumption of Mirt Stove ............................................................. 44

Table 3.1 Selection of Body Materials for Improved Mirt Stove ........................................ 46

Table 3.2 Percent Satisfaction in Achieving The Criteria ..................................................... 46

Table 3.5 Detail Specification and Boq Of Body Construction Material .............................. 47

Table 3.7 Percent Satisfaction in Achieving the Criteria ....................................................... 53

Table 3.8 Best Insulation Material Selected .......................................................................... 54

Table 4.1 Result of CCT Test Comparing Mirt Stove To Open Fire Stove .......................... 79

Table 4.2 Result Of CCT Test Comparing Modified Mirt Stove to Open Fire Stove ........... 79



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BDU, Bahir Dar Institute of Technology, Bahir Dar Energy Center Page 1

CHAPTER 1

1. INTRODUCTION

1.1. BACKGROUND AND JUSTIFICATION

The survival of human being is very much dependent on his interaction with his

environment. This interaction should be observed carefully and checked for its harmony

if the quest for development is to be properly addressed. Human development, along with

other factors, is very much dependent on energy resources. To satisfy the ever-increasing

need for energy, man pushed way beyond what can be carried by the environment and

this has been the cause for many of the problems the world is facing now a days. In

developing countries the low level technology aggravates the problems leading to

unsustainable use of natural resources and environmental degradation.

Ethiopia has one of the lowest rates of access to modern energy services; its energy

supply is primarily based on biomass. With a share of 92.4% (88% according to SREP

investment plan) of Ethiopia’s energy supply, waste and biomass are the country’s

primary energy sources, followed by oil (5.7%) and hydropower (1.6%) [3]

Table 1.1 Share of Ethiopian Energy supply 2009 (%)

Source: [ K. Meder 2011 (design) IEA 2008 data.]

When we see the energy situation, 99% of households, 70% of industries and 94% of

service enterprises use biomass+ as energy source. Households account for 88% of total

energy consumption, industry 4%, transport 3% and services and others 5%. The installed

electricity generating capacity in Ethiopia is about Ethiopia is well endowed with

renewable energy sources. These include first of all hydro, but also wind, geothermal,

solar as well as biomass. As of August 2009, only 7% of the estimated 54 GW

No % type

1 1.6 Hydro Power

2 5.7 Oil

3 92.4 Waste and Biomass

https://energypedia.info/wiki/Ethiopia_Energy_Situation#cite_note-2



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economically exploitable power generation resources was either developed or committed

to be developed [10].

The low technology and the economy of our country forced its people to depend mostly

on forest biomass for energy. Apart from the need for energy, the rapidly growing

population has caused a great depletion of natural forest. Expansions of agricultural land

to feed the large population, urbanization, transport and other development activities have

put pressure on forestland. The over exploitation of forest has led to loss of biodiversity,

deforestation and desertification and ultimately to a climate change, which affected the

other parts of the globe as well.

Our dependence on traditional biomass fuel has been one of the major factors that

aggravated land degradation, soil erosion and other related problems that are the main

contributors to the current draught and food insufficiency in the country.

The use of forest biomass for energy, especially for cooking by traditional stoves takes

significant share in depleting the forest. The preponderate use of household energy in

Ethiopia is for cooking and out of the total biomass used at household level 60 % has

been for baking ”Injera‟ (Oromia Mines and Energy Bureau, 1995). This large share goes

to baking “Injera‟ due to the wide use traditional open-fire stoves that are less efficient

and more fuel consuming. So, technical advances in energy efficiency are critical for

developing countries like Ethiopia whose populations depend primarily on biomass fuels

such as wood, charcoal and agricultural residues. Overuse of these fuels depletes

resources and degrades local environments, multiplies the time needed to collect fuel, and

creates indoor air pollution that threatens the well-being of the most vulnerable members

of households. To address this problem, many efforts have been and are being made by

the government and non-government organizations since the early 1990s. The

development of ‘Mirt’ biomass Injera stove is one of the results of these efforts in the

country.

These days this stove is being widely promoted due to the fact that it can achieve fuel

efficiency up to 50%[15] as compared to the open fire system. It can also improve the

kitchen environment by reducing indoor air pollution and Even if this stove have this

advantages still the thermal Energy efficiency of this stove is form 18%-23% [15] which



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means more than 77%-82% thermal heat produced is lost through the stove wall, flue

gas, cooking pan/mitad/.On other hand the different studies show that initial ignition time

of the Mirt stove is high, stove is heavy (>60kg) [15], and the life time of the stove is five

years i.e. not durable. So, this study is to improve the overall thermal efficiency, weight,

ignition time, fuel saving and durability of the Mirt-Injera baking stove overall system.

1.2. INJERA

From previous researchers and data from GTZ, Injera is flat bread with a unique test;

sour, testy and soft-spongy like structure with a thickness of 2–4 mm [45] circular pan

cake with a diameter of around 58 cm [45]. The major constituents for baking Injera are

teff and also other cereals such as sorghum and barely are also sometimes used. These

days, people tend to add few grams of rice flour for whitening Injera. The knowledge and

skill of baking Injera is well known by Ethiopians and Eritrean. It has been transferred

from generation to generation for a long time. See next picture of Ethiopian Injera.

Figure 1.1 Ethiopian Injera

Majority of Ethiopians still bake Injera using three-stone fire. Since 1980’s [45] efforts

have been undertaken to improve biomass Injera baking stoves and to introduce electric

Injera baking stoves for urban areas. The efficient Injera baking stove has less thermal

efficiency.

1.3. STATEMENT OF THE PROBLEM

Injera baking accounts for over 50% of all energy consumption in Ethiopia, and over

90% of all household energy consumption is for Injera baking. Most of the population

lives where there is no electric access and use inefficient open fired indoor wood based

biomass stoves to fulfill household energy consumption. Due to this much of the thermal

energy comes from the wood lost to the surrounding. Besides Women are the primary



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victims of the indoor air pollution due to excessive wood burning.

Improved fuel saving stoves like Mirt was introduced from 1990s, but the dissemination

to the society is not as expected. Mirt Stove has thermal efficiency of 18-23% [15] which

means most thermal Energy is lost. It saves 33 % [24] house hold fuel wood as compared

to traditional three stone stoves; it takes longer time and relatively high amount of wood

for initial startup of the stove. This is due to heat loss to the surrounding through the wall,

less thermal heat conductivity of clay pan /Mitad/, higher thermal conductivity of the

stove constructing material, which is the composition of sand and cement. Mirt stove has

no fully scientific documented information for further research and improvement. So the

acceptance of mirt stove is less because of the above problems. To maximize the whole

thermal and fuel saving efficiency, it is necessary to maximize heat conductivity by

conduction, convection and radiation; as well increase combustion efficiency and re-use

of wasted heat for further cooking is necessary for more efficiency and acceptance.

1.4. OBJECTIVES

1.4.1. GENERAL OBJECTIVE

The aim of this thesis is to design, manufacture and improve System thermal efficiency

and fuel efficiency of Mirt-Injera baking stove by enhancing heat transfer efficiency from

combustion chamber, recover the wasted heat to use on other stove.

1.4.2. SPECIFIC OBJECTIVES

To achieve the main objective the following specific objectives have been formulated,

To construct and manufacture combustion chamber which has high combustion

efficiency.

To re-use heat loss and heat retained in stove body by re-designing the stove body with

more insulating and less heat retention stove body material.

To use circulating steam cycle and manufacture other 2nd

stove which will use steam and

flue gas heat system for baking other Injera.

To use the cooling system on the steam out let for preparing tea, coffee and wot during

Injera baking

Evaluate the performance of the whole system by using controlled cooking test or water

boiling test and other method for its performance.



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1.5. SCOPE OF STUDY AND LIMITATION

The study of this thesis bounded to Design, Manufacturer and Testing of the new stove

by considering theoretical, practical heat lose through the wall and flue gas as well heat

retained in the stove wall are analyzed. To design the new stove, the size of combustion

chamber, inlet and out let opening of will be sized appropriately using the fuel and air

combustion theory. We use different researcher’s data of Mirt Stove and existing normal

data (Standard data acceptable by community), sizes of Injera as well Mitad size used for

Mirt Injera baking Stove. The system will be designed in such a way that we can bake

Injera using dual stoves in which the second stove will use recovered heat and steam

system. The study is bounded or limited to design of combustion chamber, enhance heat

transfer system and using recover heat disputed by different factors for the second stove.

The system improvement is only limited to the stove body and not concern to the cooking

pan (clay mitad). In general the system will be designed in such a way that fluid will

changed to steam by using partial heat from combustion chamber and flue gas to change

in to steam, transported to second mitad and using copper tube the heat will transported to

the second Mitad.After it gives heat to the baked Injera the fluid will condensed and

recycled for continues use of the fluid.

This thesis is intended to know what is accountable for the thermal Energy loss of 79%-

82% [15] and fuel efficiency of only 33% [24] in comparative to the open fire stove. To

study all this parameters it is difficult as there is no advanced technology or apparatuses

which part is more accountable for heat loss and reason for less fuel efficiency as it is

difficult to study all the parameter at ones. So this study is limited to enhance the wasted

heat re-utilization of the Mirt stove as a system but not focusing on each parts of the

stove system i.e. Mitad. So, it is clear that Mitad (clay) has less heat transfer efficiency

and need more study to enhance its heat transfer efficiency and this study is limited to

improving the efficiency of the thermal heat utilization of the system by focusing on

waste heat recover as consequence improve the fuel efficiency of the Mirt stove.

In addition the water boiling test is more appropriate in conducting Thermal efficiency of

cooking stoves which has less area like rocket stoves. But in this case since there are of



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mitad sit is wider and two stoves are dually working Water boiling test have limitation on

such types of stove which force us for CCT test.

1.6. SIGNIFICANCE OF THE STUDY

The majority of Ethiopia’s population lives in rural areas. Women and children spend much

of their time in collecting fire wood and they are exposed to health problems due to smoke

inhalation from the burning wood while cooking. Even if Mirt stove is efficient when

compared to three stone/open fire/stove still the thermal heat utilization /thermal efficiency/is

very low.

So, this study helps to use more heat generated from combustion chamber as consequence

less fuel usage, less time for baking Injera and new technology (steam) of food cooking will

be invented.



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CHAPTER 2

2. LITERATURE REVIEW

2.1. THEORETICAL BACKGROUND FOR DESIGNING WOOD BURNING

COOK STOVES

An open fire is often 90% efficient at the work of turning wood into energy but only a

small proportion from 10% to 40% of the released energy is reached the pot. Improving

combustion efficiency does not appreciably help the stove to use less fuel but reduce

smoke and harmful emissions that damage health. Improving heat transfer efficiency to

the pot makes significantly reduce fuel use.

Fire is naturally good at its job but Mitad or Pot is not as good at capturing heat because

they are inefficient heat exchangers. In order to reduce emissions and fuel use, the stove

designer’s job is to first clean up the fire and then force as much energy into the pot or

Mitad as possible. Both functions can be accomplished in a well-engineered cooking

stove. It is always best practice to add a chimney to any wood burning cooking or heating

stove.

Additionally, it is preferable to use a cleaner burning stove to protect air quality in and

outside of the house. Chimneys that take smoke and other emissions out of the living

space protect the family by reducing exposure to pollutants and health risks. Even cleaner

burning stoves without a chimney can create unhealthy levels of indoor air pollution. If

possible, all wood burning stoves should always be fitted with a functional chimney. No

energy should not be absorbed into the mass of a stove body because high-mass stoves

can absorb energy that could have gone into the pot. The three stone fires can boil water

fairly quickly. Fire hits the bottom and sometimes the sides of the pot, by exposing a lot

of the pot to the hot gases. Sticks can be fed in at the appropriate rate as the tips burn,

assisting complete combustion. A hot open fire can burn relatively cleanly. Every stove

suffers because it has some mass that absorbs heat. But an improved stove can still

achieve better combustion and fuel efficiency than an open fire.



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2.2. WOOD COMBUSTION

The process of release of thermal energy from fuel is known as combustion. Biomass fuel

is the stored solar energy in the form of chemical energy of its constituents, as a result of

photosynthetic reaction. This energy is released during combustion reaction, in which

oxygen reacts with the chemical constituents of wood to produce carbon dioxide and

water, with the release of heat. Photosynthetic and combustion reactions are reversible

reactions, which can be depicted by the following simplified equation:

Solar Energy

CO + H2O CH2O + O2

Thermal Energy

The physic-chemical processes involved in the storage of chemical energy in the fuel and

the subsequent conversion of this energy into heat are complex in nature. Any

combustion process can be depicted by the fire triangle shown in fig. 2.1. The figure

shows that for self-sustained combustion, three components are essential, namely: fuel,

air and heat. Combustion is a complex process in which processes of devolatilization,

cracking and combustion take place almost simultaneously. The combustion process is

dependent on the physio-chemical properties of the fuel (size, shape, density, moisture

content, fixed carbon content, volatile matter, etc.), quantity and mode of air supply

(primary and secondary air) and the conditions of the surroundings (temperature, wind,

humidity, etc.).The amount of energy released during combustion reaction depends on the

temperature, pressure, the products of reaction and the state of water produced. These last

two factors are important because incomplete combustion will result in the production of

carbon monoxide and other combustible materials, which results in the loss of potential

energy of fuel.

Figure 2.1 Fire Triangle



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Liquid or vapor state of water, produced during combustion of hydrogen in the fuel, will

affect the net heat released. This complex process is depicted in fig. 2.4 and takes place

in three stages, as explained in the following sections

Source:- [Hasan Khan and Verhaart 1992]

Figure 2.2 Processes and temperatures in a burning of wood

2.3. STAGE 1 COMBUSTION

Easily combustible kindling (such as tree leaves, wood shavings, scrap paper and

kerosene) on burning raises the temperature of the spot on which the radiation from the

flame is incident. This heat gets distributed throughout the material due to conductive

heat transfer, thus raising the temperature of the material. When the temperature rises to

100°C, drying of wood takes place due to loss of absorbed and weakly bound water. This

process continues into the deep interior; a part of the heat of combustion is utilized in this

endothermic process (heat consuming). Hence, the higher the moisture content of the

wood, the greater is the loss of energy.

Gaseous Phase Combustion Diffusion Flam, mostly

turbulent a free fire. T>1000 oC probably < 1200

o C

Simultaneous heat and mass transfer with chemical

reaction, surface combustion and slow process

500 < T < 800 o C

Problem same as in zone A but with sources sink due to

pyrrollyses reactions 200 < T < 500 o C

Heat Conduction in a medium with a moving

boundary. Migration of moisture and gases

uncertain properties T< 200 o C



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As the temperature is raised further, the paralytic decomposition of the wood starts. At a

temperature of about 150 °C, the release of volatile matter begins along with the

appearance of semi-liquid tar. In case this stage gets prolonged due to quenching of the

flame, the fuel starts smoldering and dark or gray/blue smoke with a strong smell is given

off. This results in the loss of some useful energy of wood. The tar gets deposited in the

tunnels and chimney resulting in their choking. There is also the danger of fire in the

chimney due to the spontaneous combustion of the deposited tar. Tar also gets deposited

on the cold surface of the pots resulting in their blackening.


Volatile matter, being at a higher temperature, rises due to the buoyancy force. During

the rise, it mixes with the surrounding air. This mixture of volatile matter may reach the

combustible limit and get ignited, if sufficient heat is available. The flame resulting from

combustion may persist if the heat released from the flame is sufficient for sustained

release of more volatiles from the burning surface. Otherwise, it will flash back to the

surface. Self-sustained combustion commences at around 225 °C and reaches a peak at

about 300 °C. During this stage, heat released by the combustion process is more than the

combined losses and hence there is a net positive release of heat. Thus, it can be

concluded from the above discussion that for the evaluation of the combustion process,

the understanding of the pyrolysis process and the subsequent burning of the released

volatile matter and char is necessary. The second stage determines the extent and nature

of volatiles and the char generated while the third stage determines the extent to which

the potential heat in the volatile matter and char is released. The pyrolytic process

suggests that, for the best design of the stove, the factors temperature, rate of heating and

residence time of biomass in the combustion chamber and physical characteristics of the

fuel such as size and shape (which govern the rate of pyrolysis) must be taken into full

consideration.

Furthermore, an understanding of the heat level required for ignition as well as for the



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maintenance of combustion and their dependence on the thermo-physical properties such

as density, specific heat, thermal conductivity, calorific value and moisture content is

essential.

2.6. COMBUSTION IN SMALL ENCLOSURES

The developmental approach to cook stove design has been shifting from fuel efficient

stoves to emission efficient stoves. A high performance stove should be efficient from

both these fuel perspectives, so as to ensure conservation of the fuel as well as the

environment. This will not only reduce the drudgery of the users but will also save them

from the harmful effects of the pollutants emitted during combustion. One of the

strategies adopted in a large number of designs is to improve thermal efficiency and

provide a chimney for the removal of smoke. Although this strategy helps in improving

the indoor air quality, the quality of combustion is questionable in a number of designs. A

better approach would be to increase the heat transfer as well as the combustion

efficiency. An increase in the heat transfer results from the efficient transfer of heat

produced during combustion and a reduction in the losses from the body of the stove.

Ensuring complete combustion can enhance combustion efficiency. A complete

conversion of chemical energy to heat with a minimum (but sufficient) amount of excess

air can take place if the following conditions are met:

o High temperature in the reaction zone

o A requisite supply of the oxidant (air) and its complete mixing with the fuel

o Adequate residence time of the reactants (air and fuel) under the above

o conditions in the reaction zone



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Source: - [Improved solid biomass burning cook stoves- page 34, Bangkok 1993]

Figure 2.3 Wood Combustion

In case any of these conditions is not met, the combustion reaction will not proceed to

completion, resulting in the emission of pollutants and the loss of potential heat. In

contrast to liquid or gas fuel burners, it is extremely difficult to meet these conditions in

heterogeneous combustion, as is the case in cook stoves using solid biomass fuel.

Whenever there are reducing conditions due to a deficiency of air or an excess amount of

volatile matter in the combustion zone, free carbon and hydrocarbon compounds escape

from the combustion zone without complete combustion, resulting in the formation of



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soot and other toxic poly-aromatic hydrocarbons. Thus the design and operating

parameters are as important as the fuel parameters. There is very little variation in the

chemical composition and energy density (energy/unit mass or volume) in woody

biomass. However, there is a significant variation of other properties among different

types of fuels: wood, agriculture residues, dung cakes, etc. This variation in the properties

has influence on efficiency and hence must be taken into consideration in the design of

cook stoves. In addition to these, there are a number of process factors, which must be

taken into consideration as well, so as to maximize efficiency and minimize emissions.

These can be divided in to three distinct categories, namely - design, fuel and operational

factors.

Fuel factors:- Physical and chemical properties of fuel such as volatile matter, moisture,

ash, etc.

Operational factors:- Burn rate/size of the fuel ratio, volume to surface ratio, mode of

fuel supply, cooking time, etc.

Stove factors:- Fuel/air ratio, temperature of flame and/or envelope, mode of fuel supply,

primary and secondary air, mass of the stove, etc. It is difficult to predict the

quantitatively effect of the variables on the overall efficiency. Qualitative effects of some

of these factors on combustion and thermal efficiency are given in table. 2.1, a critical

examination of the factors given in this table shows that the fuel parameters are

uncontrollable as they depend on the type of fuel. On the other hand, operational

parameters such as fuel size and fuel feeding are user specific, while the stove parameters

are design specific. On a qualitative basis, the stove factors and some of the operational

factors are competing factors. However, the database is inadequate for a quantitative

evaluation of the effect of these parameters on the combustion and heat transfer

efficiency. This is partly due to a lack of proper understanding of various process

principles during which the combustion takes place in small enclosures, and partly due to

the inadequacy of the experimental procedures used



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TABLE 2.1 Different Factors that influences Stove Efficiency

Factors

Action taken to

Minimize Emission Maximize Efficiency

Fuel Factors

Ash content

Volatile content

Moisture content

Minimize

Minimize

Optimize @25%

Minimize

Minimize

Optimize @10%

Operation Factors

Burn rate

Size of Fuel Change

Ratio of Charge size to burn Rate

Volume to surface ratio

Maximize

Minimize

Minimize

Maximize

Minimize

Minimize

Minimize

Maximize

Stove Factors

Combustion confinement

Temperature

Excess Air

Preheat primary Air (Down Draft Stove)

Mass Short Cooking Time

Mass ,Long Cooking Time

Time During Burn

Altitude

Minimize

Maximize

Optimize

Maximize

Minimize

Maximize

High early

-

Maximize

Minimize

Optimize

Maximize

Minimize

Maximize

Low early

-

Source; [Smith 1987]

The overall efficiency, during combustion at a particular value of the burning rate,

depends on the characteristics of the enclosure (semi enclosed combustion chamber,

enclosure without chimney, enclosure with chimney, etc.). With an increase in the

burning rate, the heat transfer efficiency decreases, while the combustion efficiency

increases. The nature of the combustion operation such as steady/unsteady combustion,

short term/intermittent operation, which is controlled by the ratio of the burn size and the

burn rate, has a profound influence on the overall efficiency of the cook stove. Burning of

wood in small enclosures can be classified as controlled combustion in contrast to free

burning of wood in an open fire. However, the operation of the cook stove is dynamic in

nature because of the interdependence between the rate of combustion, rate of induction

mailto:Optimize@25%25

mailto:Optimize@10%25



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of air and the draft.

The rate of combustion is strongly dependent on the manner in which the combustion air

is supplied. In the case of small enclosures, the combustion takes place due to the

pressure field set up as a result of the upward movement of combustion products and

entrainment of air through the firebox opening and the grate, if provided. On the other

hand, combustion in open-fire is maintained through the laminar or turbulent entrainment

of outside air, depending on the size of fire. Hence, apart from combustion, fluid flow

considerations are equally important in the design of efficient stoves

2.7. IMPROVE COMBUSTIONIMPROVING FUEL EFFICIENCY (GET

MORE HEAT INTO THE COOKING FOOD)

Improving the fuel efficiency of a stove thus requires attention to a number of different

factors (parameters). Among these are:-

Combustion Efficiency:- so that as much of the energy stored in the combustible as

possible is released as heat,

Heat Transfer Efficiency:- so that as much of the heat generated as possible is actually

transferred to the contents of the Mitad This includes conductive, convective, and

radiative heat transfer processes,

Control Efficiency:- So that only as much heat as is needed to cook the food is

generated,

Mitad (Pot) Efficiency:- So that as much of the heat that reaches the contents of the

Mitad as possible remains or transfer to Injera to cook.

Cooking Process Efficiency:- So that as little energy as possible is used to cause the

physic-chemical changes occurring in cooking food,

Thermal efficiency:- Combustion and heat transfer efficiencies are often combined for

convenience and are then termed the thermal efficiency of the stove [ The heat transfer efficiency will be discussed first in terms of the conductive, convective,

and radiation processes going on in and around the stove. These processes are sketched as

figure below.



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Figure 2.4. Pictorial representation of heat transfer phenomenon in stove design [44].

a) Conduction

Transfer of energy between objects in physical contact is the transfer of heat from a hot side

to a cooler side through a dividing medium. The hot side heats the molecules in the dividing

medium and causes them to move rapidly, heating the adjacent molecules until the cool side

is heated. In a solid, heat is conducted as more rapidly vibrating atoms excite and speed

up the vibration rate of more slowly moving neighbors. Additionally, in metals heat is

conducted as free electrons with a high velocity move from regions at a high temperature

into regions at a lower temperature where: they collide with and excite atoms. In general,

heat conduction by such electrons is much more effective than that by adjacent atoms

exciting each other. For this reason, metals (which conduct electricity) have much higher

thermal conductivities than electrically insulating solids. A brief table of thermal

conductivities and other factors is presented in Table 2 below. The points just made about

the low conductivity of gases, the high conductivity of metals, and quality insulators

being mostly air (notice the low density) can be clearly seen in this table 2.



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Table 2.2. Typical Pproperty Value of Some Materials at Room Temperature [50]

No Material

Thermal

conductivity

K (W/moC) Density kg/m

3

Specific Heat

(J/kgoC)

1 Metals

1.1 Steel Alloys 359 (10-70) 7700-8000 450-480

2 Nonmetallic solids

2.1 Cement 0.8-1.4 1900-2300 880

2.2 Insulators

2.2.1 Fiber glass 0.04 200 670

3 fluids

3.1 Water 0.597 1000 4180

3,2 Gases

3.4 Air 0.026 1.177 1000

Figure 2.5 Parameter for Conductive Heat Transfer [50]

The transfer of heat stops when the temperature of the hot side equals that ofthe cool side.

In heat transfer by conduction heat flow (q) through the area A in a plane normal to the

direction of heat transfer in time (δt) given by Fourier’s law of conduction as [35] [36]

[37] [38]:

Ǭ = -KA δT ………………………………………………………..………………..…2.1

δX

Conduction through a plane wall: The details of conduction are quite complicated but for

engineering purposes may be handled by a simple equation, usually called Fourier’s

equation. For the steady flow of across a plane wall with the surfaces at temperatures of



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T1 and T2 where: T1 is greater than T2; the heat flow Q per unit area A; (the heat flux) is:

Q = K T1-T2 = K ΔT ……………………………..……...…….……………………….2.2

A X1-X2 ΔX

Written as a differential:

…………………..………………………….….………………..…..………...…………2.3

Where:-

Q Heat flow (W)

K Thermal conductivity (W/m oC)

A Area perpendicular to heat flow (m2)

dT/dX Temperature gradient in plane of heat transfer in the direction of heat

flow (o

C /m)

The negative sign in the equation is introduced to account for the fact that heat is

conducted from a high temperature to a low temperature, so that (dT/dX) inherently

negative; therefore the double negative indicates a positive flow of heat in the direction of

decreasing temperature.

However, using this equation alone for the heat transfer across a stove wall would lead to

values that are many times too large. The heat transfer into and out of an object depends

on the conductivities to and from the surfaces as well as the conductivity within the

object itself .In some cases, dirt or oxide layers may reduce the heat transfer across the

surface; in other cases, the air at the surface itself significantly reduces the heat transfer.

Taking this into account then gives [51]

Q= A(T1-T2) .…………….…..…………..………………..……….……….2.4

1/ [h1] + s/k +1/ [h2]

Where:

[h1] and [h2] Inner and outer surface heat transfer coefficient

1/h and s/k Thermal resistances to heat transfer

Typical values for h are 5 W/ m2 o

C in still air to over 15 W/m2 o

C in a moderate 3 m/s

wind. Thus, it is the surface resistance, not the resistance to heat transfer of the material



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itself that primarily determines the rate of heat loss through the stove wall. This is true

until very low conductivity (high thermal resistance) materials such as fiberglass

insulation are used. Fiberglass, for example, has a thermal resistance (1/k) typically about

25 m2 0

C/W. In this case the insulation, not the resistance of the surface air layers, is the

primary determinant of the stove's rate of heat loss. In controlled cooking tests with

aluminum pots, fuel savings were about 45% [45] compared to using clay pots. Coating

aluminum pots with mud to protect their shine, or allowing a thick layer of soot to build

up on the outside reduce the pots' energy efficiency and should be discouraged. In

addition to their high performance and ease of use cooks prefer aluminum pots because,

unlike traditional fired clay pots, they won't break. Calculating Thermal Storage Another

factor of importance in conductive heat transfer calculations is the ability of a material to

store thermal energy, measured as its specific heat. The specific heat of a material is the

amount of energy required to raise the temperature of 1 kg of its mass by 1o

C. For a

given object, the change in the total heat stored is then given by

dE= M *Cp *dT …………………………….………………………….………………..2.5

Where:

M is the object's mass,

Cp is its specific heat, and

dT is its change in temperature.

Thus, if the wall of a 3 kg metal stove increases by 380 OC during use, the change in

energy stored in its wall is dE = (3 kg) (480 Ws /kg O

C 380 O

C = 547200 Ws or 547.2 kJ

Thus, the thermal conductivity carries thermal energy through a material; the specific

heat and mass of an object store this heat energy. The larger the mass and specific heat of

an object the more energy it can store for a given change in temperature. Thus a

thermally massive (large M*Cp) object warms up slowly; a thermally lightweight (small

M*Cp) object will warm rapidly. This is called the thermal inertia of an object and is an

important design parameter in stoves.

Convection is transfer of energy between an object and its environment, due to fluid

motion. Convection can be forced convection in which the flow is caused by a pump or a

fan or it may be natural convection in which the flow is caused by density differences due



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to differences in temperature. It is found that the heat flux is approximately proportional

to the temperature difference between the wall and the bulk of the fluid [35,38, 39,40,51]

.

……………………… ………………...……..………….………..2.6

This causes to define a constant proportionality called “convection heat transfer

coefficient” denoted by, h

.…………………………………….………….…….……..…..….2.7

Thus the rate of convection heat transfer is given by

……………………………………………………..……….…….………2.8

Where:-

h convection heat transfer coefficient (W/m2 oc)

A Surface area where: convection takes place (m2)

AT Temperature difference between the fluid and the wall surface (oC)

There for to increase the heat transfer Q to the pot there are then, in principle, three things

one can do. First, the temperature T1 of the hot gas can be increased. This can be done

only by closing the stove and controlling the amount of outside air that enters. This is

often impractical as it requires manipulating a door on the wood entry, prevents easy

visual monitoring of fire, and usually requires cutting the wood into small so that the door

can be closed behind them. Further, the user must consistently close the door. Second, as

much of the area A of the pot should be exposed to the hot gas as possible. This is very

important. The pot supports, for example, must be strong enough to support the pot but

should be kept small in area so as not to screen the hot gas from the pot. The gas should

be allowed to rise up around the pot and contact its entire surface. Third, the convective

heat transfer coefficient h should be increased. This can be done by increasing the

velocity of the hot gas as it flows past the pot. In convective heat transfer, the primary

resistance to heat flow is not within the solid object (unless it is a very good insulator),

nor within the flowing hot gas. Instead, the primary resistance is in the "surface boundary

layer" of very slowly moving gas immediately adjacent to a wall. Far from a wall, gas



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flows freely and readily carries heat with it. As the pot wall is approached, friction

between the pot and the gas prevents the gas from flowing easily, within this region, heat

transfer is primarily by conduction and, as previously noted, the conductivity of gases is

quite low. It is this surface boundary layer of stagnant gas that primarily limits heat

transfer from the flowing hot gas to the pot.

To improve the thermal efficiency of a stove, the thermal resistance of this boundary

layer must be reduced. This can be accomplished by (among others) increasing the flow

velocity of the hot gas over the surface of the pot. This rapid flow helps "peel" away

some of this surface boundary layer and, thinner, the boundary layer of stagnant gas then

offers less resistance to conductive heat transfer across it to the pot.

Transfer of energy from or to a body by the emission or absorption of electromagnetic

radiation. All objects with a temperature above absolute zero radiate energy at a rate equal to

their emissivity multiplied by the rate at which energy would radiate from them if they were a

black body. According to Stefan-Boltzmann law, ideal radiators emit energy at a rate

proportional to the fourth power of the absolute temperature [36] [38] [40]. And the net rate

of exchange of energy between two ideal radiators A and B is expressed as:

,,,……………………………………..….………..…..……………….2.9

Where

σ Stefan- Boltzmann constant = 5.67x10-12 w/cm2. k4

TA Temperature of body A (o

C)

TB Temperature of body B (o

C)

In many physical situations, we are interested in radiation heat transfer from the surface of an

object to the surrounding uniform temperature. Thus, the net radiation from a non-black

surface to the surrounding is given by [39].

……………………….…………..………...….……………..………....…2.10

Where:-

Ts Body surface temperature (oC)

T∞ Surrounding or ambient temperature (oC)

ε Emissivity of surface and has a value between 0 to 1 for perfect reflector ε = 0 and

for a perfect emitter a so called “black body”, ε = 1.

All objects (materials) continuously emit electromagnetic radiation due to internal



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molecular and atomic motion. The higher the object's temperature, the greater the amount

of energy so radiated. The warmth felt on one's skin when standing near a fire (but not in

the hot gases) is due to infrared radiation from the fire. The temperature of the object can

also be estimated by its color, ranging from 500 OC when glowing dark red to 800

OC

when bright cherry red to 100 O

C when yellow and to 1500 O

C and more energy radiated

by a "black body" (an object that absorbs or emits radiation perfectly regardless of

wavelength) as a function of temperature. Similarly, all objects absorb radiation, exciting

their internal molecular and atomic motion. The ability of a specific material to absorb

radiation is equal to its ability to emit it. Most real materials, however, are not perfect

emitters or absorbers. Metals, for example, are very poor absorbers (emitters) because the

free electrons within them that give rise to large electrical and thermal conductivities also

couple tightly to impinging radiation and screen its penetration into the material causing

it to reflect instead. Gases such as water vapor and carbon dioxide have strongly

frequency-dependent absorption in the infrared corresponding to excitation of vibration

and rotational motion of individual molecules.

Typical emissivity’s range is from 0.05 for well-polished metals to 0.95 for carbon black.

In wood burning cook stoves, radiative heat transfer is an important factor in the transfer

of heat from the fire bed and flames to the pot; from the flames to the fuel to maintain

combustion; from the fire bed and flames to the stove wall; from the stove wall to the pot;

and from the stove wall to ambient. Traditional stoves, typically 10-12 PHU percentage

points (out of perhaps 17 totals) are due to radiative heat transfer directly from the fire

bed to the pot bottom. This is the primary heat transfer mechanism for the traditional

open fire. To calculate the radiative heat transfer from the fire bed to the pot is

determined by the fire bed temperature (Figure 10) and by the view factor between the

fire bed and the pot



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2.8. THERMAL INSULATION SYSTEM

Insulation is defined as a material or combination of materials, which retard the flow or

loss of heat and is used in heating, air conditioning and refrigeration systems to insulate

piping, ducts, vessels, thermal storage devices and equipment in order to conserve

thermal energy, prevent surface condensation and control heat input to the contained

fluids. Generally the thermal insulation can be used to control heat flow in wide

temperature ranges when the appropriate insulating material is selected [42].

2.9. ADVANTAGES OF INSULATION SYSTEMS

The following are the basic advantages of insulating systems [46, 47]

Energy Savings: Substantial quantities of heat energy are wasted because of un-

insulated heated surfaces. Properly designed and installed insulation systems will

immediately reduce the need for energy and benefits to industry include enormous cost

savings, improved productivity, and enhanced environmental quality.

Process Control: By reducing heat loss or gain, insulation can help maintain process

temperature to a pre-determined value or within a predetermined range.

Personnel Protection: Thermal insulation is one of the most effective means of

protecting workers from burns resulting from skin contact with surfaces of hot surface

and equipment operating at higher temperatures. Insulation reduces the surface

temperature of piping or equipment to a safer level, resulting in increased worker safety

and the avoidance of worker downtime due to injury.

Fire Protection: Used in combination with other materials, insulation helps provide fire

protection in fire stop systems designed to provide an effective barrier against the spread

of flame, smoke, and gases at penetrations of fire resistance rated assemblies by ducts,

pipes, and cables.



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2.10. THERMAL CONDUCTIVITY AND THERMAL RESISTANCE OF

INSULATOR

i. Thermal Conductivity (k): Is a specific material property. It represents the heat flow

in watts (W) through a 1 m² surface and 1 m thick flat layer of a material when the

temperature difference between the two surfaces in the direction of heat flow amounts to

1 Kelvin (K). The unit of measurement for thermal conductivity (k) is W/(m.K) [44,42].

ii. Thermal resistance (R): Describes the thermal insulation effect of a constructional

layer. Thermal resistance of a material depends on the geometry and thermal properties of

the insulator. The unit of measurement for thermal resistance (R) is (K/W). [42,43]

……………….…..…………..…………...…………..…...………..…...……..………..2.11

For cylindrical insulation (such as circular pipe insulation) of inner radius r1, outer radius r2,

and length L having average thermal conductivity K; if there is no heat generation in the

layer the thermal resistance can be given by:

…………..……..…..…………..………………...…...…………….…2.12

If the steady heat transfer is through two-layered composite cylinder of length L with

convection on both sides (inner and outer sides of insulator cylinders) can be expressed as:

…………………………………………………....…………..…..…………..2.13

…………………..….…………………………………………………...….………..….2.1

2.11. INSULATING MATERIAL

Most insulation is used to prevent the conduction of heat. In some cases radiation is a

factor. A good insulator is obviously a poor conductor. Less dense materials are better

insulators. The denser the material, the closer its atoms are together. That means the

transfer of energy of one atom to the next is more effective. Thus, gases insulate better

than liquids, which in turn insulate better than solids [35,46].

An interesting fact is that poor conductors of electricity are also poor heat conductors.

Wood is a much better insulator than copper. The reason is that metals that conduct



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electricity allow free electrons to roam through the material. This enhances the transfer of

energy from one area to another in the metal. Without this ability, the material-like wood-

does not conduct heat well.

Here are different types of insulation materials among those; fiber glass, cellular glass,

foamed plastic and calcium silicate etc are the most commonly used materials.

i. Rigid polyurethane foam (PUR/PIR)

Rigid polyurethane foam (PUR/PIR) is a closed-cell plastic. It is used as factory made

thermal insulation material in the form of insulation boards or block foam, and in

combination with various rigid facings as a constructional material or sandwich panel.

Polyurethane in-situ foams are manufactured directly on the building site. In modest

material thicknesses, rigid polyurethane foam (PUR/PIR) offers optimal thermal

insulation coupled with an exceptional space-utility advantage [46].

ii. Fiberglass insulation

Fiberglass insulation is fibrous glass, made either plain or with a heat resistant binder in

order for the fiberglass to hold its shape. Fiberglass is the most popular insulation, and it

comes in many forms. In the form most commonly used for pipe lines, it is molded and

shaped into semicircular sections and into different shapes. The binder is the critical

factor for the ultimate temperature for which it can be used. Fiberglass is recommended

temperatures up to 422oC. A high temperature, flexible blanket can be used with

temperatures up to 530 oC [45, 46].

iii. Ash insulation

Ash is a waste product from the combustion of fire wood especially in the preparation

foods with the largest share in baking Injera. In some cases ash from fire wood is used as

nutrient for plants because it improves the fertility of soil. Particularly the wood ash is

used for insulation system because ash is completely burned material, so it has very low

thermal conductivity; again it is a waste material, so there is no cost for it and available

everywhere: locally and there is no fire hazard and is not toxic, so there is no problem of

safety. In the country side’s where: finding matches is difficult, to start firing early in the

morning what mothers used to do is that they cover the fired charcoal with wood ash

estimated about 3 to 5 cm thickness in the evening. Thus they can get the charcoal with

its fire early in the morning. This shows us that the ash conserves the energy in the



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charcoal, by protecting entering cold air from outside in to the fired charcoal and by

protecting energy loss from the charcoal the outside environment. So the idea of using

ash as insulation material is from this experience [47].

iv. Additive of clay with different material

Since clay is the cheapest material on this planet it is possible to increase and decrease its

heat conductivity to use it for whatever purpose we intended to use different researchers

conclude that we can use it for construction of stove body after we add it.

Figure 2.6 Additive of clay with ash

Figure 2.7 Additive of clay with chip Wood Heat Conduction Property

Insulation from conduction

Conduction occurs when materials, especially solids, are in direct contact with each

other. High kinetic energy atoms and molecules bump into their neighbors, increasing the



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neighbor's energy. This increase in energy can flow through materials and from one

material to another [42].

Solid to Solid: to slow down the transfer of heat by conduction from one solid to another,

materials that are poor conductors are placed in between the solids. Examples include:

Fiberglass is not a good conductor nor is air. That is why bundles of loosely packed

fiberglass strands are often used as insulation between the outer and inner walls of a

house. Conductive heat cannot travel though a vacuum. That is why a thermos bottle has

an evacuated lining. This type of heat cannot be transferred from one layer to the other

through the thermos bottle vacuum.

Gas to Solid: to slow down the heat transfer between air and a solid, a poor conductor of

heat is placed in between. A good example of this is placing a layer of clothing between

us and the cold outside air in the summer. If the cold air was in contact with our skin, it

would lower the skin temperature. The clothing slows down that heat loss. Also, the

clothing prevents body heat from leaving and being lost to the cold air.

Liquid to Solid: likewise, when you swim in water, cold water can lower your body

temperature through conduction. That is why some swimmers wear rubber wet suits to

insulate them from the cold water.

ii) Insulation from convection

Convection is transfer of heat when a fluid is in motion. Since air and water do not

readily conduct heat, they often transfer heat (or cold) through their motion. A fan-driven

furnace is an example of this. Insulation from heat transfer by convection is usually done

by either preventing the motion of the fluid or protecting from the convection. Wearing

protective clothing on a cold, windy day will inhibit the loss of heat due to convection

[42][44][48].

iii) Insulation from Radiation

Hot and even warm objects radiate infra-red electromagnetic waves, which can heat up

objects at a distance, as well as lose energy themselves. Insulation against heat transfer by

radiation is usually done by using reflective materials. A thermos bottle not only has an

evacuated lining to prevent heat transfer by conduction, but it also is made of shiny

material to prevent radiation heat transfer. Radiation from warm food inside the thermos

bottle is reflected back to itself. Radiation from warm outside material is reflected to



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prevent heating cold liquids inside the bottle [41, 44].

2.12. WALL LOSS CALCULATIONS

Reducing the heat loss into and through the stove walls to the outside requires a detailed

analysis of the conduction process. In reviewing these calculations, it is important to note

first that they are based on particular assumed combustion chamber geometry and heat

flux from the fire. When cooking begins, the walls of the stove are cold. With time they

warm up at a rate determined by their mass and specific heat as discussed above.

Lightweight walls have a low thermal inertia and warm quickly. Thick, heavy walls

warm more slowly. Heat loss from the combustion chamber is determined by how

quickly these walls warm and subsequently how much heat the wall loses from its outside

surface. The thicker the wall the more slowly it warms.

Although a thick wall of dense high specific heat material may have slightly lower heat

loss than a thinner wall after several hours .it takes many hours more for the eventual

lower heat loss of the thick wall to compensate for its much greater absorption of heat to

warm up to this state. Thus, it is always preferable to make the solid (non-insulator)

portion of the wall as thin and light as possible. Additionally, the use of lightweight

insulants such as fiberglass or double wall construction can dramatically lower heat loss

Materials such as sand-clay or concrete, which have a high specific heat and density, and

which must be formed in thick sections to be sufficiently strong to support a pot or resist

the fire, should therefore be avoided.

2.13. HEAT RECUPERATION

It has frequently been argued that the large amounts of heat absorbed by the walls of a

massive stove should be utilized by either extinguishing the fire early or using this heat to

complete cooking or by later using it to heat water. Water heating tests on hot massive

stoves, however, have shown that only 0.6-1.3% of the energy released by the fire, of

which perhaps one-third was stored in the massive wall, could be recuperated – heating



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the water by typically 18-19 O

C. What is often thought to be heating or cooking by heat

recuperation is actually done by the remaining coals of the fire. That heat recuperation

from massive walls is so difficult can be easily understood by considering the following.

1) Heat conduction through the wall is slow so that little energy can be transported to the pot

directly.

2) Air is a relatively good insulator. Thus, little heat can be carried from the wall into the air

space inside the stove and then to the pot.

3) Both of these heat paths are further slowed by the relatively small temperature difference

between the wall and the pot. The low temperature of the wall also reduces the radiant

transfer to the pot.

4) The heat stored in the wall tends to equilibrate within the wall and then leak to the

outside. Rather than depending on low efficiency massive stoves for cooking and then

attempting to recuperate heat for hot water, such water heating can be much more

efficiently done directly with a high performance stove. Further, it can then be done when

needed rather than being tied to the cooking schedule. Similarly, using stored heat to

complete cooking is an extremely inefficient technique compared to using a high

efficiency lightweight stove.

If a light weight single wall (metal) stove is heavily tarnished and sooted on the outside

its exterior heat loss can be quite large. This heat loss is due to the emission of radiant

energy and can be reduced by chemically or mechanically polishing or coating the

exterior surface to leave a bright metallic finish. Although such a finish may have

commercial appeal, its effectiveness in reducing heat loss will last only so long as it is

kept relatively clean and free of soot and rust, etc. It should be noted that most paints,

even white paint, will actually increase the radiant heat loss from a stove and should be

avoided; to decrease radiant heat loss, the surface must be metallic. Light weight single

wall stoves are easy to construct, are low cost, and have relatively high performance

when convective heat transfer is optimized. However, during use they can be quite hot on

the outside and can burn the user as well as be uncomfortable to use. To reduce heat loss

and thus reduce this hazard, either double wall construction and/or lightweight insulate

such as fiberglass or vermiculite can be used. Double wall construction with metal alone

can significantly reduce heat loss, user discomfort, and the hazard of burns .The double



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wall serves two functions in reducing heat loss. First, the dead air space between the two

walls is a moderately good insulator. It should be noted, however, that increasing the

thickness of this dead air space does not improve its insulating value. This is due to the

convection currents, which flow more freely the larger the space, carrying heat from one

wall to the other. Second, the inner wall acts as a radiation shield between the fire and the

outer wall. There, the emissivity or, more accurately, the radiant coupling between the

inner and outer walls is the prime determinant of heat loss. The exterior surface

emissivity is less important due to the lower temperature of that wall. As the temperature

of the exterior wall increases due to greater radiant heat transfer from inner to outer wall

єi increasing) the exterior emissivity, є e becomes more important .In practice there are

several potential difficulties:

Although it is preferable to minimize radiant coupling between the two walls by

giving them a bright, long-lasting metallic finish, they will tend to rust, tarnish, and soot

over time. Keeping them clean would be difficult. Even in the worst case є i = .9, є e .9),

however, the double wall still performs better than the best (є e = .9) single metal wall.

The dead air space is a good insulator on its own, but attaching the inner wall to the outer

will tend to short circuit its insulating value due to the high thermal conductivity of metal.

It is necessary that the two walls together be mechanically rigid, but they should not

easily conduct heat from one to the other. This might be done by using nonmetallic

spacers or fasteners, or tack welding the walls together at selected points. Long

continuous welds should be avoided if possible.

The insulating value of the dead air space is reduced if air is allowed to flow through.

Thus, the dead air space should be closed at the top. Double wall metal stoves are now

being developed and commercialized, but yet is to use a high quality insulant such as

fiberglass or vermiculite with the double wall to hold it in place and protect it. Other

lightweight insulants worth investigating include wood ash, diatomaceous earth, and,

possibly, chemically treated (to reduce its flammability) straw or charcoal among others

Just as insulated walls reduce the exterior temperatures, they increase the inner wall

temperature. This can increase heat transfer to the pot by convective heat transfer, by

radiative heat transfer from the inner wall surface, and possibly by improving the quality

of combustion.



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2.14. PHU /PERCENT HEAT UTILIZATION/ OR THE THERMAL

EFFICIENCY/ TESTS METHOD STOVE TEST TYPES

Stove testing is the systematic measuring of the advantages and limitations of a particular

stove model. Its primary aim is to help identify the most effective and desirable stoves for

a specific social and economic context. With ongoing stove production, a testing program

provides essential quality control and may lead to important design modifications. A

group of stove experts by Volunteers in technical Assistance (VITA) introduced a

standardized stove-testing concept prepared from proceedings of a meeting [53]. The

group formulated the following tests:

2.15. WATER BOILING TEST

To measure how much wood is used to boil water under fixed conditions. This is a

laboratory test, to be done both at full heat and at a lower “simmering” level to replicate

the two most common cooking tasks. While it does not necessarily correlate to actual

stove performance when cooking food, it facilitates the comparison of stoves under

controlled conditions with relatively few cultural variables The Water Boiling Test

(WBT) is a relatively short, simple simulation of common cooking procedures. It

measures the fuel consumed for a certain class of tasks. It is used for a quick comparison

of the performance of different stoves. WBT use water to simulate food, the standard

quantity is two-thirds the full pan capacity. The test includes “high power” and “low

power” phases. The high power phase involves heating the standard quantity of water

from the ambient temperature to boiling as rapidly as possible. The lower power phase

follows. The power is reduced to the lowest level needed to keep the water simmering

over a one-hour period. Each WBT should be repeated at least four times. Results may be

averaged and analyzed statistically.



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2.16. KITCHEN PERFORMANCE TEST

To measure how much fuel wood is used per person in actual households when cooking

with traditional stove, and when using an experimental stove. The tester simply measures

how much wood the family has at the beginning and at the end of each testing period.

2.17. CONTROLLED COOKING TEST AND PROCEDURE

To serve as a bridge between the water boiling test and the kitchen performance tests.

Trained local cooks prepare pre-determined meals in a specified way, using both

traditional and experimental stoves. The CCT described here is meant primarily to

compare the performance of an improved stove to a traditional stove in a standardized

cooking task. The procedure that follows should be applied to open fire stove as well as

the Mirt Injera Baking stove. Three repetitions of the CCT for each stove that is being

compared are recommended [52].

1. The first step in conducting the CCT is to consult with staff of the MoWIE at

Laboratory and prepare sufficient dough of Teff a head in order to the teff dough

well fermented.

The stove is designed for home use, so we used to prepare Injera from teff because

it is the common food which is baked always at community level. This test is done

depending on the family size mentioned above.

Once a cooking task has been decided on, ensure that sufficient food is available to

conduct the tests.

2. After deciding on a cooking task, the procedure should be described in as much detail

as possible and recorded in a way that both stove users and testers can understand

and follow. This is important to ensure that the cooking task is performed

identically on each stove. If possible, include an objective measure of when the

Injera is “done”. In other words, it is preferable to define the end of the cooking task

by an observable factor. After dough and fuel have been obtained and the steps of the

cooking task are written up and well understood by all participants, the actual testing

can begin. The cooking itself should be done by a local person who is familiar with



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both the Injera that is being cooked and the operation of the stove to be tested. If the

stove is a new design that differs significantly from traditional cooking practices,

some training will probably be required before on conducting the actual tests.

When comparing stoves with the CCT, if more than one cook is used, each cook

should test each stove the same number of times, in order to remove the cook as a

potential source of bias in the tests. In addition, to ensure that the testers have control

over the testing environment, the tests should be conducted in a controllable setting

such as a lab or workshop rather than in a private home.

3. Record local conditions as instructed on the Data and Calculation form.

4. Weigh the predetermined ingredients and do all of the preparations (washing, peeling,

cutting, etc) as described by the cooking directions recorded in step 2 above. To save

time, for non-perishable food, the preparation can be done in bulk, so that food for all

of the tests is prepared at once.

5. Start with a pre-weighed bundle of fuel that is roughly double the amount that local

people consider necessary to complete the cooking task. Record the weight in

appropriate place on the Data and Calculation form.

6. Starting with a cool stove, allow the cook(s) to light the fire in a way that reflects

local practices. Start the timer and record the time on the Data and Calculation form.

7. While the cook performs the cooking task, record any relevant observations and

comments that the cook makes (for example, difficulties that they encounter,

excessive heat, smoke, instability of the stove or pot, etc).

8. When the task is finished, record the time in the Data and Calculation form.

9. Remove the pot(s) of food from the stove and weigh each pot with its food on the

balance. Record the weight in grams on the Data and Calculation form.

10. Remove the unburned wood from the fire and extinguish it. Knock the charcoal from

the ends of the unburned wood. Weigh the unburned wood from the stove with the

remaining wood from the original bundle. Place all of the charcoal in the designated

tray and weigh this too. Record both measurements on the Data and Calculation form.



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2.18. FLUID FLOW

A thorough knowledge of fluid flow principles is also essential for understanding the

flow of air and flue gases through the stove and the chimney, if required as well as for

understanding how these influence the combustion process, the transfer of heat from the

hot flue gases to the pots for by recovery option and the influence of heat transfer to

surrounding air through the enclose of the main combustion chamber and stove wall of

the second stove. The induction of air is required for the combustion of fuel in the main

combustion chamber through the grate and the subsequent flow of the flue gas from the

first combustion chamber and through the second stove and chimney tunnel is governed

by the principles of fluid flow. The second stove is designed in such a way that it will get

heat from both flue gas following through heat and steam circulating system in the flue

gas to bake other Injera at the same time. The suction effect, responsible for the induction

of air into the combustion chamber, is created as a result of the flow of flue gases through

the chimney. The amount can be estimated by the application of principles of fluid flow.

The steady state flow of fluid is governed by the continuity equation, which is based on

the principle of conservation of mass. According to this equation the mass of fluid

passing all sections per unit of time is constant. This can be represented by the following

equation:

ρ1 * A1 *V1 = ρ2 * A2 * V2………………………………….………..………………...2.15

Where:-

Ρ is the density,

A is the area

V is the velocity.

This equation can be used to calculate the velocity of the flue gases and air at different

occasions in the stove. Based on the applications of conservation of energy to the flow of

fluid, an equation known as Bernoulli's equation can be derived. For the steady state flow

of low-pressure gas in which there is a negligible change in internal energy, the equation

can be described as:

Hst + Hdy*Hpo = Htot ………………………..….........…………..………………….…..2.16



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Where:-

Hst is the static head,

Hdy is the dynamic head,

Hpo is the potential head

Htot is the total frictional head.

The head is expressed in meters and is equal to Δ P/ρ where: Δ P is the pressure loss and

ρ is the density. The potential head or Hpo is equal to Hz where: z is the elevation above

any given level in meters. In some cases a sudden expansion and contraction is

encountered during the flow of flue gases through the chimney and the chambers. The

frictional loss Hfr due to the sudden expansion of a duct with a cross sectional area of A1,

where: the mean velocity is V1 into a duct with a larger cross sectional area of A2 where:

the velocity is V2 is given by the expression:

H fr= (V 2 –V 1)……….…..…….……………..…………..…..……………….….........2.17

2 g c

Where:-

gc is a conversion factor having a numerical value equal to the acceleration due to

gravity.

2.19. INDOORS AIR POLLUTION

The physical form and pollutant content are the two characteristics of fuels that most

determine the quality of pollutant emissions when burned. It is generally difficult to

premix solid fuels sufficiently with air to assure good combustion in simple small-scale

devices such as household stoves. Consequently, even though most biomass fuels contain

few noxious contaminants, they are usually burned incompletely in household stoves and

so produce a wide range of health damaging pollutants. Some times as much as one-fifth

of the fuel carbon is diverted to products of incomplete combustion (UNDP, 2001).

Households reliant on biomass generally use the fuel indoors, in open fires or poorly

functioning stoves, and usually with inadequate venting of smoke (WHO, 2000). The

smoke from biomass fuel contains a large number of pollutants that are dangerous to

health, including small particles, carbon monoxide, nitrogen dioxide, formaldehyde, and



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carcinogenic substances such as benzo, pyrene and benzene. Studies from Asia, Africa

and the Americas have shown that indoor air pollution levels in these homes are

extremely high, many times the limits set by WHO (Table 2.8). Typical 24-hour levels of

PM in biomass using homes in Africa, Asia, or Latin America range from 300 to 3000

micrograms per cubic meter (µg/m3)

Table 2.3 Health affecting limits according to WHO’s Exposure Guidelines

Product Concentration Time Limit

Carbon monoxide 100 gm/m3 15 min

60 mg/m3 30 min

30 mg/m3 1 hour

10 mg/m3 8 hour

Formaldehyde 100 mg/m3 30 min

Lead 1 mg/m3 1 year

Nitrogen dioxide 400 mg/m3 1 hour

150 mg/m3 24 hour

Ozone 200 mg/m3 1 hour

120 mg/m3 8 hour

Sulfur dioxide 500 mg/m3 10 min

350 mg/m3 1 hour

125 mg/m3 24 hours

Suspended Particles 120 mg/m3 24 hours

Benzen 2.5 mg/m3 1 year

Source; [Usinger 1996]

Mirt stove in the kitchen it could be possible to reduce CO concentration by 88.8%, PM

by 17.3% when the analysis is made based on the mean 8-hour time frame [54]. The 15-

minute maximum has also showed similar trend, 91.5% for CO and 19.3% for PM levels

[54].

In conclusion from different studies we absorbed that Mirt stove thermal efficiency is

only from 18-21% and heavy in weight as well has long initial startup. So, this work will

give solution for these issues.



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2.20. BIOMASS USAGE IN ETHIOPIA

Biomass resources include wood, agro-industrial residue, and municipal waste and bio

fuels. Wood and agricultural as well as livestock residue are used beyond sustainable

yield with negative environmental impacts.

According to estimates made by a recent study, at the national level, there appears to be a

surplus of woody biomass supply. However, the same study revealed that there is a

severe deficit of supply when the data is disaggregated to lower local levels. According to

this same study, 307 [12] Woredas (districts) out of the total number of 500 [12] Woredas

are consuming woody biomass in excess of sustainable yield.

Among the key issues that characterize the Ethiopian energy sector, the following are

some that stand out:

The Energy sector relies heavily on biomass energy resources,

The household sector is the major consumer of energy (which comes almost

entirely from biomass) and, Biomass energy supplies are coming mainly from

unsustainable resource base (which has catastrophic environmental implications).

Ethiopia’s biomass energy resource potential is considerable. According to estimates by

Woody Biomass Inventory and Strategic Planning Project (WBISPP), national woody

biomass stock was 1,149 million tons with annual yield of 50 [12] million tons in the year

2000. These figures exclude biomass fuels such as branches/leaves/twigs (BLT), dead

wood and homestead tree yields. Owing to rapidly growing population, however, the

nation’s limited biomass energy resource is believed to have been depleting at an

increasingly faster rate. Regarding the regional distribution of biomass energy resources,

the northern highlands and eastern lowlands have lower woody biomass cover [12]. The

spatial distribution of the "deficit" indicated that areas with severe woody biomass deficit

are located in eastern Tigray, East and West Harerghe, East Shewa and East Wellega

Zones of Oromia and Jigjiga Zone of Somali Region. Most of Amhara Region has a

moderate deficit but a small number of Woredas along the crest of the Eastern

Escarpment have a severe deficit [12].

There is however an energy production potential from agro-processing industries

https://energypedia.info/wiki/Ethiopia_Energy_Situation#cite_note-Energy_resource-11



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(processing sugar cane bagasse, cotton stalk, coffee hull and oil seed shells) [10]. Up to

date, no grid-connected biomass power plants exist. Several sugar factories have however

been using sugar cane bagasse for station supply since the 1950s. A total of 30 MW of

capacity surplus could be fed in the grid by sugar factories [22]. Municipal waste and bio

fuels on the other hand are barely used as energy resources. No estimation of municipal

waste power production potential is available at the time; power production potential of

landfill gas is estimated to be 24 MW [10]. The current GTP plans to disseminate 25,000

domestic biogas plants, 10,000 vegetable oil stoves and 9.4 million improved stoves by

2015[21,10].

2.21. STOVE IMPROVEMENTS IN ETHIOPIA

Since 1989, efforts to improve the stoves, mainly by the Ethiopian Energy Studies and

Research Center (EESRC), currently, Ethiopian Rural Energy Development and

Promotion Center) have resulted in the development of three types of improved stoves:

Lakech charcoal stove, Low cost electric Injera stove and Mirt improved biomass Injera

stove, Gonzye stove and Yekum Injera Mitad

The development of the technologies were carried out by the project called Cooking

Efficiency Improvement and New Fuels Marketing Project (CEINFMP) financed by

World Bank and DANIDA between 1989 and 1995. As reported in GTZ (2000), the

main success of CEINFMP project was the Lakech charcoal stove, which is used for non-

Injera cooking where: as Mirt, Gonzye and Yekuma stoves are for Injera baking. All has

efficiency less than Mirt stove which this project emphasized.

Table 2.4 Improved Household Injera Baking Stoves trend in Ethiopia [821]

No

.

Stove

Name Picture Description

1 Mirt

classical

Mirt stove made of cement, with expansion chamber

and pot-rest, 4 cylindrical enclosure parts of 6 cm

thickness, no chimney, developed by the Ministry of

https://energypedia.info/wiki/Ethiopia_Energy_Situation#cite_note-ERG-8


https://energypedia.info/wiki/Ethiopia_Energy_Situation#cite_note-LOY-20

https://energypedia.info/wiki/Ethiopia_Energy_Situation#cite_note-LOY-20



https://energypedia.info/wiki/Ethiopia_Energy_Situation#cite_note-MEDER-7

https://energypedia.info/wiki/Ethiopia_Energy_Situation#cite_note-MEDER-7



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Mirt Classical

Stove

Water and Energy in the mid of 90 ies, for hh, can be

found almost all over Ethiopia in urban and rural

areas mainly for hh, also by commercial Injera

bakers and sometimes by restaurants, price from 100

– 250 Birr. stove-tests done by GIZ, by Aprovecho,

by the Ministry of Water and Energy, by ESD

(company in UK commissioned by WB project

2 Mirt slim

Mirt Slim

Stove

Mirt stove made of cement, with expansion chamber

and pot-rest, 4 cylindrical enclosure parts of 4 cm

thickness, no chimney, modified by GIZ in

collaboration with Aprovecho in 2006, for hh, can be

found in all intervention areas of the project, Addis,

Amhara, Tigray, Oromia, in urban and rural areas

mainly used by hh, also commercial Injera bakers

and sometimes by restaurants, price from 80 – 200

Birr) stove-tests done by GIZ

3 Yekum

Mirt – I

Yekum Mirt

stove

Mirt stove made of cement, cladded with metal, with

pot-rest and chimney

at the back, on 4 legs, can be found only in Amhara

and Tigray regions in few numbers, developed by

GIZ in 2011 to evacuate smoke from the kitchen and

to have a higher stove for

comfort for the urban users, price between 800 –

1000 Birr) stove-test done by GIZ in

cooperation with Ministry of Water and Energy

4

Mirt with

integrated

chimney

Mirt stove of cement, with expansion chamber and

pot rest as well as a chimney added to the expansion

chamber, developed by GIZ in 2011 in order to

satisfy Government request of smoke evacuation, can

https://energypedia.info/wiki/Ethiopia_Energy_Situation



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Mirt with

integrated

chimney

be found in Tigray and Amhara in very few numbers,

costs from 300 – 500 Birr) stove-test done by GIZ

5

Addis/Apr

ovecho

stove

Addis or

Aprovecho

Stove

stove made of cement, a combination of Mirt and

rocket stove, no expansion chamber, no pot-rests but

air inlet, developed by Aprovecho in 2006 on

demand of GTZ because cement price was high and

costs had to bring down by increasing the

performance, can be found in some places in Amhara

region (10 – 20 stoves), price unknown), stove-test

done by. Aprovecho, acceptability test done in Addis

and is currently under way by the project in Tigray,

Amhara, South and Oromia

6

Yekum

Injera

Mitad

Yekum Injera

Mitad

Brick stove cladded with metal, with chimney and on

4 legs, can be

found in major cities, price from 600 – 1.200 Birr,

sometimes called “Laketch”, developed by Ministry

of Water and Energy for institutional application,

nowadays also used in hh and restaurants), pre-test

done by GIZ in cooperation with Ministry of Water

and Energy . Promoted by government, other NGO,

Community or individual stove producers.

7 Yekum

Mirt – II

Yekum Mirt II

Mirt stove of made cement, cladded with metal, no

pot-rest, no expansion

chamber, one chimney, height was reduced from

24cm to 18, can be found around in Adama in

Oromia Region in few numbers, stove was adapted

by a trained stove producer, Meseret, price about

1.000 Birr) pre-test done by GIZ in cooperation with



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2.22. DEVELOPMENT AND DISSEMINATION OF MIRT STOVE

The study made by Ministry of Mines and Energy and Energy for Sustainable

Development (ESD) (1988) showed that Ethiopia had a complex and highly commercial

biomass fuel sector. However, because of using open-hearth method, energy efficiency in

Ministry of Water and Energy and promoted by

government, other NGO, Community or individual

stove producers.

8

Awramba

fixed

stove

Awramba Stove

It is made of mud, stones, ash and placed on a wood

base above the ground, with expansion chamber and

chimney, developed in the 80ies by the Awramba

communist community in Amhara Region near

Bahirdar, was taken over by Amhara Mines and

Energy Bureau for wider dissemination, can be found

in Amhara Region), stove-test done by GIZ in

cooperation with the Ministry of Water and Energy.

9 Gonzye

Gonzye Stove

it is made of burned clay, similar to Mirt, 3 to 4

cylindrical enclosures, no pot-rests, developed by the

Government in 2002, can be found in Amhara,

Oromia, Tigray and Southern area, price from 35 –

100 Birr, cheapest Injera baking ICS stove in

Ethiopia), stove-test by the Ministry of Water and

Energy

10

Electrical

Injera

Mitad

Electrical Injera

Mitad

It is locally made, developed by the Ethiopian

Electrical Power Corporation; can be found all over

Ethiopia, wherever electricity is available.



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the country is very low. Because of this ESD found that improving household cooking

efficiency is the most important area in Ethiopia. Based on this fact Lakech (‘excellent’,

‘good’) improved charcoal stove could be developed in 1991. According to the ESD

report, commercial production of Lakech (improved charcoal stove) was started in early

1992 in Addis Ababa. To date, millions of these improved stoves, which save over 25%

charcoal relative to the traditional stove, are being in use. This has resulted in the saving

of hundreds of hectares of ecologically and economically important dry land forest in

Ethiopia. Each Lakech stove saves an average of 75 kg of charcoal per household per

year (Bess 1998). This led to savings of over 20,000 tons of charcoal in 1996, worth over

£4 million alone. More importantly, the forest savings from the use of Lakech was equal

to the equivalent of over 2,000 hectares of important dry land forest in Ethiopia.

However, from the earliest days of ESD's involvement with household energy in

Ethiopia, its international and local team realized that the single most significant

household energy demand side intervention was not in household charcoals, important as

this has been. Rather, the most crucial area for energy savings in Ethiopia’s in household

bread, or "Injera" baking. ESD worked with John Parry Workshops of the UK to develop

a modular, prefabricated stove from cement and local construction materials that could be

produced relatively easily, in large quantities, by a range of different scale producers, at

inexpensive prices. While this proved difficult, the team was finally able to meet this

challenge in early- 1994 with what was soon to be called the "Mirt" ("best") improved

biomass Injera stove. The whole process of designing and dissemination of the Mirt stove

followed series of steps. Large-scale dissemination was supported by media

advertisement and cooking demonstrations. Intensive training for the private sector was

provided in the process [12].

2.23. DESCRIPTION OF MIRT STOVE

The Mirt Stove has been specifically designed to cook Injera. The basic design of Mirt

stove was adopted from the traditional Ambo & Burayu enclosed Injera stoves and

optimized to handle different types of biomass fuels. The Mirt stove is a multi-section

stove (figure 1) made by molds. One mold is used to construct the four pcs of the

combustion chamber and two molds are used for the chimney rest. The combustion



2019


chamber is made with height of about 24 cm and diameter of about 60 cm. The pot rest or

the chimney has inner diameter of around 14 cm. The fuel and air inlet is made with

height of about 11 cm and width of around 24 cm. Total weight of the stove is nearly 60

kg [15].

Figure 2.8 Pictorial Description of Mirt-Injera Stove [15]

The Mirt stove was originally designed using lightweight raw pumice with Portland

pozzolana (cement) in a ratio of 5:115 Although pumice is a major source for

constructing building materials in Addis Ababa and other areas in the Rift Valley, it is not

found everywhere: in Ethiopia. Another common material that is more widely found than

pumice, and is used extensively in the building materials industry is scoria or red ash. In

areas where: no pumice or scoria is found, sand and cement is used.

Figure 2.9 Assembled Whole Picture of Mirt Stove [15]

Chimney and Pot rest

Smoke Out let

Wall of Stove

Fuel and Air in late

Mitad Rest



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2.24. FUEL CONSUMPTION OF MIRT STOVE

The specific fuel consumption of Mirt Stove has been determined by a number of

researchers and developers. The average specific fuel consumption of Mirt Stove is 535 g

of wood per kg of Injera see table (1) and Mirt Stove was tested at Approvecho Research

Center. The test was conducted using water boiling test procedure where: the time to boil

is 35.8 min and, 6407 g fuel was used. The CO (g) observed was 192 and PM (g) was

5322. The improved design brought about percentage reduction of 18% (time to boil), 81

% (fuel use), 90 % (CO), and 83 % (PM (Hatfield et al. 2006).

Table 2.5 Specific fuel consumption of Mirt Stove

No. Specific fuel consumption

(g/kg of Injera)

Injera baked per

session

Reference

1 460 - (Workeneh 2005)

2 524 - (Yosef 2007)

3 596 - (Alemayehu et al. 2012)

4 528 23-25 (Anteneh &

Walelign2011)

5 575 25-30 (Alemayehu et al. 2012)

6 667 23-25 (Alemayehu et al. 2012)

7 393 23-25 (Dresen et al. 2014)

Average 535 -



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CHAPTER THREE

3. MATERIALS AND METHOD

3.1. INTRODUCTION

This chapter discusses how the newly efficient stove has been designed and the causes for

less thermal efficiency of Mirt Injera Baking Stove. The chapter explains where we

should focus to improve (change) existing Mirt Injera Baking Stove and re-design it.

Many researchers have reported their findings on efficiency of Mirt Injera baking Stove

and we can use these data as our input. In selection of material depending on local

availability, our society culture, heat transfer property, and depending on durability of

material; we can select construction materials for body of the stove.

3.2. MATERIAL SELECTED FOR CONSTRUCTION OF MODIFIED MIRT

STOVE

For selection of the construction material weighting digital logic approach method is used

[34,59]. That is each property is listed and compared in every combination taken two at a

time. To make the comparison, the property that is considered to be the more important

of the two is given one (1) and the less important is given zero (0). The total possible

alterative combinations can be determined by the following formula [34,60]

N = -1 x n …………………………………………….………………..….…….…..3.1

2

Therefore, properties of the material used for comparison are five, and per the calculation

the possible alternative combinations are 10.

Procedure:

Establish the weighting factor for each of the design criteria

Establish or estimate the level of satisfaction for each criteria for each of the

alternatives

Multiply the level of satisfaction by the weighting factor and sum these for the

overall satisfaction for each alternative



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Depending on the availability of the material in the local market and to design dual

working stoves the following material are selected for construction.

Table 3.1 Selection of Body Materials for Improved Mirt Stove

No Property

1

(1)(2)

2

(1)(3)

3

(1)(4)

4

(1)(5)

5

(2)(3)

6

(2)(4)

7

(2)(5)

8

(3)(4)

9

(3)(5)

10

(4)(5)

po

siti

ve

dec

isio

n

wei

gh

tin

g f

act

or

1

Local

availability 0 1 1 1 3 0.3

2 Weight 0 0 1 0 1 0.1

3

Thermal

Conductivity 1 0 1 0 2 0.2

4 Cost 1 0 0 1 2 0.2

5

Ease of

Manufacturing 1 0 0 1 2 0.2

Total 10 1

The weighting factor for each criterion was determined as shown in table 3.1. The next

step was preparing decision matrix for selecting the best body construction material from

Concert cement, Sheet metal and clay.

Table 3.2 Percent Satisfaction in achieving the criteria

Percent

(Satisfaction)% Description

100 Complete satisfaction, objective satisfied in every aspect

90 Extensive satisfaction, objective satisfied in all of important aspect

75 Considerable satisfaction, objective satisfied in the majority of aspects

50

Moderate satisfaction, a middle point between complete and no

satisfaction

25

Minor satisfaction, objective satisfied in some but less than half of the

aspect

10 Minimal satisfaction ,objective satisfied to very small extent

0 No satisfaction, objective is not satisfied in any aspect



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Table.3.3 Selection of the Best Alternative Body Constructing Material

No Material Lo

cal

av

ail

ab

ilit

y

Wei

gh

t

Th

erm

al

Co

nd

uct

ivit

y

Co

st

Ea

se o

f

Ma

nu

fact

uri

n

g

tota

l

Rem

ark

1 weight 0.3 0.1 0.2 0.2 0.2

2

Concert Mold %

acceptance 50 50 90 75 75

Weight * % satisfactory 15 5 18 15 15 68

3

Sheet metal % acceptance 90 90 75 75 100

Weight * % satisfactory 27 9 15 15 20 86 Selected

4

Clay % acceptance 75 75 75 90 75

Weight * % satisfactory 22.5 7.5 15 18 15 78

Table 3.5 Detail Specification and BOQ of Body Construction Material

No Description Unit Size QTY Local Rem

ark

1 Sheet metal for body construction mm3 1 x 1000 x 2000 2 Available

Copper 10 mm tube for steam system mm2 8000 1 Available

3 Oxygen welding machine set 1 1 Available

4 Grooving metal material No 1 1 Available

5 Welding Machine set 1 1 Available

6 cutter No 1 1 Available

7 Angle Iron for Meted Stand Pcs 58 and 56 2 Available

8 Stand Steel mm3 10 x 10 x 70 14 Available

9 Steam Controlling Valve pcs 20 ,length 70, 2 Available

10 Screws mm2 10 2 Available

11 Fiber glass for Insulation kg 1.5 2 Available

12 Extruder for tea and coffee making mm2 10 1 Available

13 Portland cement for joining kg - 2 Available

14 Welding arc kg - 2 Available

15 Welding metal for oxygen kg - 1 Available

16 Grinder pcs - 1 Available



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3.3. DESIGN OF COMBUSTION CHAMBER AND MATERIAL SELECTION

Thermal efficiency is a combination of combustion and heat transfer efficiencies. So,

improving both these efficiencies will improve the thermal efficiency and fuel efficiency

of Mirt Stove which currently has 18%-23% [15,44] percent. The way of design and

material to be selected will be presented in next section.

3.4. DESIGN OF COMBUSTION CHAMBER AND STOVE WALL

One of the main problems of Mirt Stove is its long time to initially startup of fire. It may

be due to having similar primary air and wood entrance hole. In Mirt Stove wood is rest

on the floor of combustion chamber and cooled air is just pass over the top part of the

wood and fire which even cools down the fire itself. So, to address this problem grate is

proposed in such a way that air to pass under the burning stick (wood) and go up

through the wood which adds effect of more complete combustion of wood. For grate

construction we can select material made of clay or the same as the stove body material

i.e metal sheet.

To design the combustion chamber, we should consider the dry heat content or caloric

value of biomass which is 15.00MJ - 17.51MJ [45]. The Flame temperature of green

wood containing 50% wt moisture is about 980 °C and flame temperature of dried wood

is 1260 to 1370 °C [45]. Maximum moisture content is between 55 to 65 wet percent

[46]. From this data and principle taking the minimum inside temperature of the

combustion chamber as 980 °C [46] for design purpose. Since the objective of this paper

is to bake Injera with more fuel efficient and thermal efficient dual stove, considering

Mirt Injera stove which by now the best fuel efficient stove in Ethiopia and already

known and accepted by community. Increasing its efficiency by adding more innovative

idea and technology improvement increases more acceptances in the community. So

focusing and discussing about Mirt- Injera baking stove data and using data of Injera are

very important.



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3.5. HEAT REQUIRED FOR INJERA BAKING

The heat used to bake Injera can be defined as the energy necessary to raise the batter to a

particular temperature, and evaporate the amount of water that is observed to be lost

during the baking process. To measure the energy utilized in cooking Injera, the initial

mass of batter, and the total amount of Injera produced from this batter were measured.

Thus the mass of water vapor can be obtained by subtracting the mass of the Injera

produced from the initial mass of batter.

It is assumed that the energy utilized in cooking the Injera is the energy required in

raising the batter from room temperature to the boiling point of water which is called

sensible heat, plus the energy required to evaporate the water which is called latent heat. It

is also assumed that the heat capacity of Injera batter is the same as water in order to

calculate the energy required to raise the batter temperature to boiling point of water [17].

Therefore, the utilized heat energy is

E utilized = m batter x CP water x (T boil − T room) + (m batter - m Injera) x h vaporization……..…..…3.1

Where

m batter Mass of the batter = 400 g

T boil Boiling temperature of water in Addis Ababa = 92 OC

T room Room temperature in the baking room (Addis Ababa) = 20 0

C

Cp Heat capacity of water = 4.187 kJ/kg .K

m Injera is the mass of the Injera produced = 320 g

h vaporization Heat of vaporization of water h fg = 2260 kJ/kg

Thus: -

E utilized = 0.4 kg × 4.187 kJ/kg × (92 − 20) k + (0.4 − 0.32) kg × 226 kJ/kg = 120.6 kJ +

180.8 kJ

E utilized = 301.38 kJ

By considering the energy loss in baking the required total energy can be calculated by

assuming safety factor of 1.2 [31, 55, 56]. Thus the total energy required becomes 361.6

kJ. The time taken for cooking of Injera is about 3 minutes on average. The heat transfer

rate (power) required for Injera baking can be calculated as [31, 55, 56].



2019


Ǭ= E utilized ………..…………….………………………………….…………………...19

ΔT

So,

Ǭ = 361.6KJ

3*60 sec

Thus heat transfer rate required for Injera baking is:

Ǭ =2 kw

3.6. STANDARD SIZES OF INJERA AND CLAY MITAD

The size of clay Mitad is 60 cm [15] and thickness of 2.3 cm [15] on average that used on

three stone Stewart and Getachew (1962).But the Mirt stove uses 56 cm [15]. The

standard Injera weighs 320 gm [47] and it is 52 cm in diameter. The clay Mitad thick ness

is from 2.3 to 2.5 cm [15] and average diameters of 58 cm [15].So the data used for design

purpose are these standards.

3.7. THE HOUSE HOLD FAMILY SIZE AND NUMBER OF INJERA BAKED

WITH FREQUENCY OF BAKING PER MONTH

An average family size of 4.8 (CSA, 2007) take 5 for this study is estimated to bake 30

Injera per day, two days a week [15].

3.8. MATERIAL SELECTION FOR STOVE BODY CONSTRUCTION

Under section 2.3.5 we discussed that the stove body should not be massive and heat

transfer property should be less in addition to local availability. The overall body heat

loss should be minimized. In case of Mirt stove the body is bulk which absorbs much

heat as well heat loss to surrounding is high because of high heat conduction property of

cement and sand mixture (mortar).

From the above equation (1) the heat lost from the concert wall is

Q = A*K* (∆T)………………………..…………………………………………..…….3.3

∆x

Where:

A Area of concert wall

K Heat transfer coefficient of mortar

∆T Change in temperature



2019


∆x Thickness of body

Therefore, Heat Loss from the body is

Q = 2πrLk (∆T)

∆x

Q = 2πrLk (∆T) = 2 x 3.14 x 0.35 x .24 x 1.8 x 500 = 527.52 Joules.

∆x 0.9

New design = 2 x 3.14 x 0.24 x 0.08 x 500 = 301.44

0.2

In this case the wall thickness and the value of K of the fiber glass has been changed

The reduced in present of the new stove is 527.52-301.44/527.52 =43 % from stove wall

by conduction.

= 0.27*23=10 % percent increase in thermal efficiency even if we consider the

Convection heat transfer.

Example if we using body of stove which is from mixture of clay and additive clay, heat

absorbed by mass of stove even to boil water is given by

From Equation (5) above

dE= M*Cp *dT……………………………………..……..……..………………….….. 3.4

Where:

M is the stove mass,

Cp is its specific heat, and

dT is its change in temperature.

So,

dE= 60kg*1kJ/kg.k*100oc= 6000kJ. This means heat retained in stove body of Mirt stove

and new to be designed stove. This much heat is retained in stove body of Mirt Stove. For

the same newly invent mass will be measured has h value of 0.1 kJ/kg.k of fiber glass

means 90% more efficient in storing energy or reduce the heat stored in body of stove by

90% .It is possible to take as first priority clay with additive of chip wood for mass

production of the newly stove body, because it has k value of 0.06 W/mK (from table

2.6). On other hand when we consider or to address the radiation heat transfer, sheet

metal with shiny or polished double sheet will be selected with fiber glass in as

insulating material because it has K value 0.08 W/m K, available in the market, easy

weight. Strong polished sheet metal is selected to produce prototype of both stoves

because of ease of production.



2019


3.9. SIZE DETERMINATION OF STOVE WALL, PRIMARY AIR INLET,

WOOD INSERT AND FLUE GAS EXHAUST HOLES

To size the stove wall, cylindrical shape is selected because of shape of cooking pan or

clay Mitad. The internal diameter is 60 cm in diameter i.e internal diameter should be in

the manner that accommodates the Mitad size with clearance of 1 cm. The height above

the grate should be 0.4 times Mitad diameter (0.4x 58cm) which is 24 cm. To use great

additional wall size of 6 cm added to the bottom of the great, so total height is 30 cm.

3.10. SELECTION OF THE BEST INSULATION MATERIAL

The basic requirement for thermal insulation is to provide a significant resistance path to

the flow of heat through the insulating material. To accomplish this, the insulating

material must reduce the rate of heat transfer by conduction of these mechanisms. The

insulating material should have as much as possible a property of low thermal

conductivity, fire resistance, light weight and durability. As injera cooking needs

temperature range of 1800 C to 2200C it is possibleto use medium temperature insulators,

which have temperature range from 900C to 3250C [34].The insulation materials are

selected and prioritized based on their thermal Conductivity, Heat Resistance, Weight,

Cost and Durability by giving weighting factor for each property comparing two

properties at a time by digital logic approach [61]

DESIGN, DEVELOPMENT, EXPERIMENTAL INVESTIGATION AND SYSTEM IMPROVEMENT OF MIRT-INJERA BAKING

STOVE. 2019

Table 3.6 Possible Design Pairs for Selection of Insulating Materials

The weighting factor for each criterion was determined as shown in table 3.6. The next step was preparing decision matrix for

selecting the best insulating material from Wood ash, Fiber glass, Portland cement and Air.

Table 3.7 Percent Satisfaction in achieving the criteria

Percent

(Satisfaction)% Description

100 Complete satisfaction, objective satisfied in every aspect

90 Extensive satisfaction, objective satisfied in all of important aspect

75 Considerable satisfaction, objective satisfied in the majority of aspects

50 Moderate satisfaction, a middle point between complete and no satisfaction

25 Minor satisfaction, objective satisfied in some but less than half of the aspect

10 Minimal satisfaction ,objective satisfied to very small extent

0 No satisfaction, objective is not satisfied in any aspect

No Property 1

(1)(2)

2

(1)(3)

3

(1)(4)

4

(1)(5)

5

(2)(3)

6

(2)(4)

7

(2)(5)

8

(3)(4)

9

(3)(5)

10

(4)(5)

posi

tive

dec

isio

n

wei

gh

tin

g

fact

or

1 Thermal Conductivity 1 1 1 1 4 0.3

2 Heat Resistance 0 1 1 0 0 2 0.2

3 Weight 0 0 1 0 1 0.1

4 Cost 0 1 0 0 1 0.1

5 Durability 0 1 0 1 2 0.2

Total 10 1

DESIGN, DEVELOPMENT, EXPERIMENTAL INVESTIGATION AND SYSTEM IMPROVEMENT OF MIRT-INJERA BAKING

STOVE. 2019

Table 3.8 Best Insulation Material Selected

No

Material

Ther

mal

Conduct

ivit

y

Hea

t R

esis

tance

Wei

ght

Cost

Dura

bil

ity

Tota

l

Rem

ark

weight 0.4 0.2 0.1 0.1 0.2

1

Wood Ash % Acceptance 75 75 90 100 75

Weight * % Satisfactory 30 15 9 10 15 79 Second selected

2

Fiber Glass % acceptance 90 90 100 90 100

Weight * % satisfactory 36 18 10 9 20 93 First Selected

3 Port Land cement % acceptance 75 75 50 75 75

3.11. CRITICAL INSULATION (OPTIMIZATION OF CRITICAL INSULATION)

We know that adding more insulation to a wall or to the attic always decreases heat transfer. The

thicker the insulation, the lower the heat transfer rate. This is expected, since the heat transfer

area A is constant, and adding insulation always increases the thermal resistance of the wall

without increasing the convection resistance. Adding insulation to a cylindrical pipe or a

spherical shell, however, is a different matter. The additional insulation increases the conduction

resistance of the insulation layer but decreases the convection resistance of the surface because

of the increase in the outer surface area for convection. The heat transfer from the pipe may

increase or decrease, depending on which effect dominates. Whose outer surface temperature T2

Consider a cylindrical stove wall of outer radius r2 is maintained constant (Fig. 3.1). The pipe is

now insulated with a material whose thermal conductivity is k and outer radius is r2. Heat is lost from

the inner stove to the surrounding medium at temperature T , with a convection heat transfer coefficient h.

The rate of heat transfer from the insulated stove to the surrounding air can be expressed as equation 3.5

The critical thickness of the stove wall (insulation) is determined using equation below based on

the thermal conductivity (k) of the material and the convective heat transfer coefficient (h),

between exposed surface of insulation and its surroundings. [38]



2019


Figure 3.1 Critical Insulation [38]

Rcr, cylinder = k ……………………….…………..…………..……………….…………….3.5

h

Or

Ǭ = T∞ = T1 - T∞ ……………………...…………….……..……..……………3.6

Rins + Rconv ln (r2/r1) + 1

2πLk h (2πr2L)

Typical values for h are 5 W/ m2

oC in still air to over 15 W/m

2 0 C in a moderate 3 m/s wind

[44] at addis Ababa (work shop area)

The insulation material selected is, fiber glass with k value of 0.04 W/m2 o

C .Therefore, the

minimum thickness of insulation required is 0.04/5= 0.008 m. So, the Critical insulation

(minimum insulation) required is equivalent to one centimeter, so for design purpose and for

manufacturing simplicity let us choose 1 cm.

To design the primary air inlet, wood insert and flue gas holes on combustion wall of first stove;

Assumptions,

The principle should be used for these hole design are

1. The amount of air + excess air used for complete combustion of 1 kg of wood to produce

15.00-17.51MJ is 7 m 3 [15]

2. The air velocity around the stove is 1 m/s [15].

To design the combustion chamber and related holes (air + fuel inlet, flue gas out let, height of

stove etc); We should know the power of the stove, in addition to the above principles.

From different experiments and researchers as shown in table 2.7, under chapter 2 above, we

observed that;

Mirt Injera Baking Stove requires 535 g of wood to cook 1 kg of Injera, one dry Injera has 320

gram and the dough has 400 g [31, 55, 56]. In addition the time needed to bake 20-27 Injera is



2019


about 83 minutes.

So, the specific fuel required to bake one Injera is calculated to be,

= 535 g/kg /320g

= 171.2 gram specific fuel required.

Time taken to bake one Injera is; - The total time needed to bake one season an average of 20-23

Injera is 83 minutes, the minimum number of Injera baked to be 20. So,

T= 83 Minutes/20 in number

Time =4.15 minutes is taken to bake one Injera on average

From this conclusion,

We know that 1 kg of wood produce 1500-1751 KJ of Energy and to bake one Injera 171.2g is

needed,

So, to calculate amount of Energy needed to bake one Injera;-

= (171.2*15000)/1000 KJ

= 2568 KJ energy has been used

To get the power of stove used

Power needed to bake one Injera =2568 KJ/(4.15*60) seconds

= 10.31325 KW Stove Power is used

Therefore, theoretically to burn one kg of fuel 7 m3 airs is needed, therefore, the air used for 10

k w stove is assume;

From Mirt Stove Design,

If 5 kw stove consume 1 kg wood [15] therefore, this stove uses 1500 seconds to burn 1 kg of

wood which is 0.42 hours.

I. To calculate the area of air entrance

Figure 3.2 Air Entrance Hole

A1



2019


7 m3

/ (1 m/s*35000 sec) is 0.00220 m2 this is for 5 kw stove and it is two time for 10KW stove

.Therefore, a*b=44 cm2.

Since rectangular a=7 cm, b=6 cm is an approximate of area of air in

late.

To solve the problem of initial delay of firing it is possible to introduce grate system at this point,

so;

II) Sizing of Wood Insertion

Figure. 3.3 Wood Insertion Hole

Assumption;

Area of of wood in late should be 70% of Air in late area,

So let us say A2 for wood insertion Area,

A=0.7A1 a*c = 0.7*42=29.4cm2

Height c =29.4/7=4.2 cm

So, 29.4 cm 2=7 cm x 4 cm. for wood insertion above grate and install great at height

of 6 cm above rear of stove body edge.

III) Design of flue gas out late area (A3)

Figure 3.4 Flue Gas Exhaust Hole

The area of out let for removal of flue gas should be 30 %- 40 % of air in late area. So take an

average of 35%, it becomes (e*d) =56.7cm so choose, d=56cm and e= 7 cm is the minimum.

Using safety factor, use d=7 cm and e= 20 cm.

A2



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IV) Sizing of Rear Out Let Flue Gas Pipe

Diameter of flue gas pipe is equal to 2* the square root of 56 / π which is equal to 2 x the square

root of 28/π.

It is better to erect it 6 cm above the Stove.

Mitad diameter is equal to 2* the square root of 28/π,

So Stove thickness 1 cm, Mitad thickness 2.5 cm.

Diagrammatically,

Figure 3.5 Drawings of first Stove Scale 1:2

The second stove is designed in such a way that it connected by an exhaust channel to the main

stove in order to gate the flue gas from the first chamber and go through it

in order to use the heat going to disputed.



2019


Figure 3.6 Drawing of Second Small Stove Scale 1:2

Figure 3.5 Assembled Stove Systems during Injera Baking

2ND

STOVE

1ST

STOVE

COOLING

WATER

FUEL

INLET

COMBUSTION

CHAMBERTIO

N CHAMER

AIR INLET

COVER

BAKED

INJERA



2019


Figure 3.7. The Photo of Newly Designed Mirt Stove

3.12. RECOVERING THE HEAT LOSS OF THE FLUE GAS

Even if correct figure is not sort out for Mirt stove some portion of heat loss to environment. For

more efficient stove more than 26% [44] has lost through flue gas. In case of Mirt stove it is

more. So to recover this heat the stove should be designed in such a way that the flue gas going

to out should be used for boiling of water which to be changed in to dry steam and use this

steam by heat change, condensation and recalculate mechanism to bake Injera or bread on the

second stove . So the second stove is designed on one hand the flue gas going through it and

again the steam circulate through it for more heat transfer and maintain temperature of baking

Injera. To change the water to dry steam, in addition of the heat recovered from the flue gas

portion of heat we use small amount of heat from combustion chamber. we use one litter water

capacity for circulation and changed to dry steam, heat transfer, cool down ,return back in the

form of liquid to recalculate. After it changed to dry steam it will transported to the second stove

where, it circulates through newly designed Injera Mitad and it gives heat for baking Injera.

To make more heat transfer process metal fibers will be also used as required. In addition to add

Chimney

2nd

Mitad

Stand

1st Stove

wall Steam fluid

return tube



2019


more thermal efficiency of overall stove, heat of steam from 1st stove will be injected on second

stove as it complete its baking process of the first Injera as it required. It is the 3rd

heat recovery

option we have which is to be wasted. It can be quantified as it indicated below which mass of 60

gram of vapor has. The steam from the second stove will be recycled after it partially condensed.

It should fully condensed in outer in air as well can be used for by boiling tea, coffee as well

making wot during baking before returning back to boiling apparatus by gravity .

Mathematically the heat added to the water is calculated as follows:-The heat transfer to boil

litter of water from flue gas and additional from combustion chamber is

Q w=C w x M WB x (TB-Ti) +M evap x H evap ………………………………………….…………………….………………..3.7

Where:-

C w = (4.18 kJ/kg*K) is the specific heat capacity of water,

M WB= is the amount of water after boiling,

TB =is the temperature of the boiling water,

Ti = is the temperature of the water at the beginning (starting),

Mevap= is the amount of evaporated water and

Hevap= (2260 kJ/kg) is the evaporation enthalpy of the water.

To calculate amount of heat content in one litter of water.

Q w = 4.18*1 kg*(100-25) oc +2260 kJ/kg*1kg

= 2573.5 kJ is available

If heat conversion efficiency of circulating copper tube is

Su= 60% then 2573.5*.6= 1544.1 kJ is available

Therefore 1544.1 kJ/361.6 kJ=4.27 or 4 Injera can be baked by the circulating steam at one

cycle i.e. if the wall liquid changed to wet steam. This steam is transported by coil of copper

where heat transfer would take place from the copper tube to the second Mitad for baking Injera.

The heat for evaporation of a litter of water is by the flue gas heat recovery and small portion by

the fire of combustion chamber.

It is possible to recover heat from the steam baked Injera which is calculated as follows;

0.060 kg * 2260 kJ/kg=135 kJ where 0.060kg [47] is the mass of evaporated steam from one

baked Injera during baking and 2260 kJ/kg is the Energy content of Kg of water.

So, if this steam is designed to be injected directly on Injera to be baked on the second stove it is



2019


additional heat recovery system since this thesis is intended to use all possible waste heat

recovery.

3.13. DESIGN OF SECOND STOVE SIZING AND ARRANGEMENTS OF STEAM

CIRCULATING TUBES

As standard form Mirt stove the clay Mitad has 2.5 cm thickness .The heat conductivity of clay

is poor which 0.25 W/m K. So we can add heat transfer efficiency by decreasing the thickness of

heat transfer in such a way that we can make grove at half of the thickness and insert steam

circulated copper tube. At this point the flue gas gets a chance to hit this thickness and transfer

additional heat to the Mitad. So, for this thesis the second Mitad is designed in such a way that it

will get heat from three sources

i) From direct contact of flue gas flows through it.

ii) Through the heat from coiled copper tube carrying steam.

iii) Through direct injection of vapor from cooked Injera of 1st stove as the need arise.

Figure 3.8 Picture of Circulating Copper Coil

We use in the 1st stove the conventional Mitad without any modification. For design of the steam

circulation system to the second stove, one litter fluid is boiled in combustion chamber by partial

heat from fire and flue gas. After it changed to steam it is transported by pressure difference

created in tubes and heat transfer mechanism. After it loses its heat to the baked Injera in the

second method, it will cool down by means of external coolant may be Wot, Tea and Coffee

used as coolant. After changed to liquid form it backs to the boiling utensil by gravity. For the

purpose of heat transfer enhancement the copper tube is placed in the grooved of the second

Circulating

Steam in

groove of

Mitad

Outer condenser



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Mitad which is grooved at half thickness of the Mitad. Heat transferred by the law of

Temperature difference between the Copper tube and the surface of Mitad. Heat transfer is by

conduction is depend on the shape factors (S).The configurations is;

Q/t= - k S (T1 _ T2) ……………………….…….….………………………..……..……………3.7

Where:

k is thermal conductivity coefficient

S Shape factor

T1 Temperature of the cylindrical tube

T2 Temperature of surface

T time

To determine the steady rate of heat transfer through a medium of thermal conductivity k

between the surfaces at temperatures T1 and T2.

For A row of equally spaced parallel isothermal cylinders buried in a semi-infinite medium (L

>>D, z and w >1.5D), [38]

S= 2 * π * L

ln (2W sinh 2πz) Per Cylinder…………….…………….…………………….……………3.8

ΠD W

Where

s Shape Factor

L length of cylindrical tube

W distance of one cylinder from other

T1 Temperature at mid distance of Mitad

T2 Surface temperature of Mitad

Z Distance from Surface

Figure 3.9 Shape Factors [38]

To determine heat transfer of each segment at width w= 2 cm already average length is known,



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diameter is known thickness is known so,

S=2x π x1.16/ ln (2*0.02/ π*0.01 sin h 2*π*sin h 2* π*0.0125/0.02),S=5.58

Each segment can transfer

Q/t= - k S (T1 _ T2) =0.25*5.6*(20-10)

Q/t=110 watt

No of coils =23

So 23*110w= 2534.6 watt =2.5346 Kw to bake Injera 2 kW is enough so by this system we can

bake Injera.

Figure 3.10 View of Newly Mitad after Assembly of Coil.

3.14. USING METAL CHIPS IN THE GAP OF SECOND STOVE

It is possible to increase the heat conductivity of the second stove by adding metal chips or red

soil which contains high iron content in the second stove free space. It enhances the heat

conduction of the second stove because increase surface area of heat conduction. For less k value

and availability in local market a fiber glass material is selected for insulation.

3.15. STOVE MANUFACTURING

Auto CAD 2007 software is used to develop the working drawing of the dual stove.

The overall system is assembled in such a way that the fuel is inserted at gate one to be on grate

system. The air is get in to the system through the hole under the get and up lifted by the pressure

difference at the chimney and with flue gas. The system is assembled in such a way that the fire

Mitad to be grooved by this shape



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burned in the combustion chamber and the flue gas pass through the second assembled Mitad

and hits both the steam circulation copper tube and the second mitad. Then it passes to the

surrounding through the chimney found at the rare end of the second Mitad. The second Mitad is

working by the heat it gets from the flue gas and circulated copper tube carrying steam.

For the manufacturing of the stove trained crafts man and simple shop is sufficient enough for

mass production.

Figure 3.11 AutoCAD Three Dimensional View of Improved Mirt Stove

1. Fuel inlet

2. Stand

3. Stove wall

4. Steam carrying tube

5. Stoves

6. Condensation utensil and Cooking Pan

7. Chimney

8. Saturation steam returning tube

1

7 4

2

3

6

5

5

8



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Figure 3.12 on construction with steam and flue gas system for second stove

3.16. PERFORMANCE AND THERMAL EFFICIENCY TESTING OF IMPROVED

STOVES

The controlled cooking test is (CCT) is a stove testing protocol that is designed to assess the

performance of the improved stove i.e newly designed stove relative to the common ( Mirt Injera

Baking Stove ) or traditional stoves (open fire) that the improved model is meant to replace.

Stoves are compared as they perform a standard cooking task that is closer to the actual cooking

that local people do every day. However, the tests are designed in a way that minimizes the

influence of other factors and allows for the test conditions to be reproduced [51].

In this study we use CCT performance measuring protocol rather than Water boiling test

(WBT) which is more appropriate to measure the thermal efficiency of small cooking stoves,

because it is easy to use for Injera baking stove, rather than WBT which requires boiling of water

in big apparatus which is not appropriate in this stove performance test. In addition it is difficult

to get diameter of 56-58 water boiling apparatus capacity which should be filled with ¾ water

[52]. So, to calculate thermal efficiency of stove, we should use indirectly method from the CCT

test. CCT is a laboratory or a field test that evaluates the performance of the cooking stoves using

a standardized local cooking task(s) control kitchen.

Similarly, in this work it was designed to use CCT for testing the performance of improved

Injera baking stove by comparing it to open fire stove as well comparing Mirt Injera baking stove



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with open fire stove and lastly comparing Mirt Injera baking stove to newly developed stove.

Both Mirt and new improved Injera baking stove base is open stove as base in controlled kitchen

i.e in laboratory of Energy center in Ministry of Water Irrigation and Electric found at Gurd

Shola. The test was conducted done by trained and experienced staff of the ministry with the

researcher.

We used Eucalyptus Camaldulensis (Red River Gum, Red Gum) which has average size of

40x2x5 shapes and 8% moisture content, because of its accessibility.For this work a household

with an average family size of 4.8 (CSA, 2007) take 5 for this study is estimated to bake 30

Injera per day, two days a week [15]. Therefore, during the test each stove is arranged to cook 30

Injera.

3.17. EQUIPMENT

Radiation to the following equipment a sufficient quantity of food will be needed to conduct all

of the tests. This is discussed in more detail below.

Fuel: A homogeneous mix of air-dried fuel wood should be procured. Sufficient wood for all of

the CCTs should be obtained ahead of time. For each stove three tests are to be conducted.

Figure 3.13 Fuel

Dough and water: Testers should be sure they have sufficient dough of teff and water for the

entire range of tests. Like fuel, the food should be homogenous so that variability in food does

not bias the results of the test.



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Figure 3.14 Dough for Cooking

Cooking Clay Mitad or pot (s): it is recommended to use the standard pots supplied with the

testing kits. If the standard pots do not fit one or more of the stoves being tested, use the most

appropriate similar Clay Mitad and be sure to record the specifications in the Data and

Calculation form. If possible, the same type (size, shape, and material) of clay Mitad should be

used to test each stove. We should use Cover (Akumbalo) since local cookers use it.

Scale: weighing scale should be available.

Figure 3.15 weighting scale

Heat resistant pad: to protect scale when weighing hot charcoal.

Wood moisture meter

Timer stop watch was used to record starting and ending time of cooking and initial start up

heating time of Mitad

Thermometer: this is only for recording ambient temperature and surface temperature of the

stove. It is infrared types.



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Small shovel: spatula to remove charcoal from stove for weighing.

Dust pan: for transferring charcoal

Metal tray: to hold charcoal for weighing.

Heat resistant: gloves protect hands from burning during duty

3.18. DATA ANALYSIS

In CCT there are a number of variables that are directly measured. These include environmental

variables and physical test parameters. The environmental variables may vary slightly from one

test to another, but should nearly constant. The physical test parameters should be constant for all

tests.

Environmental variables:

Wind conditions

Air temperature

Physical test parameters:

Avg. dimensions of wood (centimeters)

Wood moisture content (% - wet basis) m

Empty weight of Pot # 1 (grams) P1



Weight of container for char (grams) k

Local boiling point of water (°C) Tb

3.19. MEASUREMENTS AND CALCULATIONS

Upon finishing the test, a number of measurements are taken. These include:

Initial weight of fuel wood (wet basis) (grams) …………….…………….…..……………..……fi

Final weight of fuel wood (wet basis) (grams)………………………….….…...........................ff

Weight of charcoal with container (grams) ……………………………………..…..….………...cc

The weight of each pot with cooked food (grams)…………………………….....………….….Pjf

Where;- (j is an index for the cooking pot ranging from 1–4 depending on the number of



2019


Pots used for cooking)

Start and finish times of cooking (minutes)……………………………………………..… ti and tf

These measurements are then used to calculate the following indicators of stove performance:

i) Total weight of food cooked (Wf) – this is the final weight of all food cooked; it is simply

calculated by subtracting the weight of the empty pots from the pots and food after the

cooking task is complete:

4

Wf=∑ (pj r – p j) ………………………………………………………….…...………………… 3.9

j=1

Where j is an index for each pot (up to four).

ii) Specific fuel consumption (SC): is used a principal indicator of stove performance for the

CCT. It measures the amount of wood used per kg of food. It is calculated as a impels ratio of

fuel to food:

SC= f d / W x * 1000……………………..………………………………….……..…………… 3.10

The number 1000 is a conversion factor for grams of fuel per kg of food cooked.

The variables f d and W f are calculated as follows:

fd = (f i-f f) x (1-(1.12 x m))-1.5xDCc…….…………………………………..…...…………..3.11

iii) Total cooking time (Dt): – This is also an important indicator of stove performance in the

CCT. Depending on local conditions and individual preferences, stove users may value this

indicator more or less than the fuel consumption indicator. This is calculated as a simple clock

difference:

t=tf - ti……………………………………………………………..…………..………………3.12



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CHAPTER FOUR

4. RESULT AND DISCUSSION

4.1. EQUIVALENT DRY WOOD CONSUMED

A comparison of the three stoves namely open fire Stove, Mirt Stove and Improved Mirt Stove

(new) have been made with their respective average equivalent dry wood consumed during the

test. The results obtained are presented as shown in figure 4.1 below. The equivalent dry wood

consumed in gram by open fire stove is 7,890 gram, Mirt Stove (old) averagely is 3,988 g. and

Newly Improved Mirt Stove is 3,832g on average. From this result the newly developed Improved Mirt

stove as a system saves 157 gram of equivalent dry wood when compared to previous developed Mirt

Injera baking stove.

4.2. SPECIFIC FUEL CONSUMPTION

A comparison of the three Stoves namely Open fire Stove, Mirt Stove (old) , Improved Mirt

Stove (new) have been made with their respective Specific Fuel Consumption during the test.

The results obtained as in figure 4.2-4.5 shows that the average specific fuel consumption in

gram of fuel wood per kg of food cooked for Mirt Stove is less by 367 than open fire stove with

standard deviation deference of 48. So, the specific fuel consumption of Mirt stove is decreased by 367

g/kg whereas the average specific fuel consumption of newly improved Mirt Stove and open fire

stove has decreased by average of 570 g/kg and standard deviation deference by 68. Therefore, the

difference between Mirt stove (old) and Newly Improved Mirt Stove is has decreased on average by 203

g/kg of Specific fuel consumption.



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Figure 4.2 Specific fuel consumption of improved Mirt stove and open fire

Figure 4.3 Specific fuel consumption of Mirt stove and Improved Mirt Stove



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Figure 4.4 Specific Fuel Consumption of Mirt Stove and Open fire

Figure 4.5 over all Specific fuel consumption of the three tests

4.3. COOKING TIME

This is also an important indicator of stove performance in the CCT. Depending on local

conditions and individual preferences, stove users value this indicator more or equal to fuel

consumption indicator. By this study ,the environmental condition where the test was done is

controlled kitchen condition which is represented by Laboratory of the ministry .As the result of

the test indicated in figure 4.6 and tables 4.1-4.2 ,the average time taken to cook 32kg dough of

Teff is 123 minutes for open fire ,102 for Mirt stove and 98 minutes for improved Mirt Stove.

2 3



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This indicates that Mirt Injera baking stove has an improvement of 17% over open Fire stove

whereas the newly improved Mirt stove has percentage of 28 %.This shows that the newly

developed Mirt stove has improvement of 11%.These results has been found out that with the

many draw backs. We can conclude that this study has a cooking time significant at 95%

confidence interval between stoves is as shown in the CCT Table 4.1, and 4.2.

Figure 4.6 Cooking time taken for Open fire, Mirt and Improved Mirt Stove

4.4. INITIAL START UP

Using the timer it was recorded that the first stove is ready to bake Injera in 10 minutes after fire

ignited under the combustion chamber. As discussed in previous sections Mirt stove has

drawback of longer Initial startup time and this newly improved stove has less initial startup

time. As this test indicates, Mirt stove requires 23 minute to initially starting up while improved

mirt stove needs 10 minutes for initial startup, it means reduce the initial startup time by 57%

and it is good result. This result is due to the addition of great structure (which designed in such a

way that the fuel and air entrances) are separated. The second stove ready for baking after total

of 25 minutes from initial ignition of fire under combustion chamber of the first stove because

change of liquid to steam and flue gas needs time.



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4.5. TEMPERATURE GRADIENT WITH 2ND

STOVE

The second stove has room temperature of 23 oC at initial time. It is increased in progress to 25

minute to temperature of 180 and above after 25 minutes because till the correct flue gas and

steam produced. After 25 minutes we can start baking Injera. This data is still with the

drawbacks of leakage of steam.

Figure 4.7 Temperature Range of the second clay pot till initial star up

4.6. THERMAL EFFICIENCY IMPROVEMENT OF IMPROVED MIRT STOVE

Water boiling test is very appropriate to determine the thermal efficiency of any stove especially

for small stoves using standard pots. But in this case it is not much important to determine the

overall thermal efficiency of Injera baking stove because we use clay Mitad instead of steel pots

with less heat transfer coefficients. So to determine the thermal efficiency of this study we

should use indirect method from CCT result. Accordingly, we can use findings depending on the

specific fuel consumption and previously researches data done on Mirt Stove. The Mirt Stove

has thermal efficiency 18-23% [15] for which 371g /kg from table 4.1 and 4.2 of cooked food

whereas for Improved Mirt stove has 273g /Kg. So it increased the thermal efficiency by 6.08%.

It is clearly seen that about 27% fuel is saved per kg of food cooked

.

Temperature

Temperature



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4.7. TEMPERATURE DISTRIBUTION ON SECOND MITAD

As it is discussed before the second Mitad is getting heat from Flue gas passing under it. It also

get heat from the heat transfer of the steam inserted in its groove at 1.25cm or mid thickness of

the Mitad. The metal tube selected is copper tube of 8mm OD because it has high melting point

and can be bended easily in the shape we required..

Figure 4.8 Temperature distribution of the second Mitad

From the above figure 4.8 the temperature gradients at each point is presented as the following chart.

A

l

o

n

g

fl

u

e

g

a

s

2

0

0

o

c

Temperature Along the flue Gas Way

Chord



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Figure 4.9Temperature distribution alone the flue gas way of second Mitad

The difference in temperature is due to not uniform grooving of Mitad and irregular distribution of steam

copper tube .But this result is change from time to time.

Figure 4.10 Temperature gradients in opposite side of the flue gas way

As figure above the Temperature is fluctuated between 178 oc and 181

o c. This is due to not

appropriate fixation of copper tube to the grooved Mitad and leakage of steam because of arc

weld.

Temperatur

e

Chord

Diameter

1:2 Ratio

Temperature

Tem

per

atu

res

Gra

die

nt



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Results of CCT comparing two stoves

Summary of comments on

stove 1

Stove type/model:

Stove 1 open fire

Stove type/model:

Stove 2 Mirt Injera Baking Stove

Location Addis Ababa

Wood species Eucalyptus Camaldulensis (Red River Gum, Red Gum)

1. CCT results:

Stove 1 units Test 1 Test 2 Test 3 Mean St Dev

Total weight of

food cooked g ###### ###### ###### ###### 361

Weight of char

remaining g 473 507 830 603 197

Equivalent dry

wood consumed g 8,259 8,208 7,204 7,890 595

Specific fuel

consumption g/kg 802 746 667 738 68

Total cooking

time min 125 121 123 123 2

2. CCT results:

Stove 2 units Test 1 Test 2 Test 3 Mean St Dev

Total weight of

food cooked g ###### ###### ###### ###### 183

Weight of char

remaining g 490 225 330 348 133

Equivalent dry

wood consumed g 3,797 4,062 3,847 3,902 140

Specific fuel

consumption g/kg 355 393 366 371 20

Total cooking

time min 104 100 102 102 2

Comparison of Stove 1 and

Stove 2 % difference T-test Sig @ 95% ?

Specific fuel g/kg 50% 9.00 YES



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Table 4.1 Result of CCT Test Comparing Mirt stove to open fire stove

Table 4.2 Result of CCT Test Comparing Modified Mirt Stove to Open fire Stove


Summary of

comments on stove

Stove type/model: Stove 1 open fire

Stove type/model: Stove 2 Improved New Mirt Stove

Location Addis Ababa

Wood species Eucalyptus Camaldulensis (Red River Gum, Red Gum)

1. CCT results: Stove 1 units Test 1 Test 2 Test 3 Mean

St

Dev

Total weight of food cooked g 8,600 9,300 9,105 9,002

361

Weight of char remaining g 473 507 830 603

197

Equivalent dry wood

consumed g 7,939 7,888 6,903 7,577

584

Specific fuel consumption g/kg 923 848 758 843

83

Total cooking time min 125 121 123 123

2

2. CCT results: Stove 2 units Test 1 Test 2 Test 3 Mean

St

Dev

1. Between improved

Stove and open fire

stove the second

Total weight of food cooked g ###### ###### ###### ######

256

steamed stove will start functioning after the

main stove

Weight of char remaining g 490 225 330 348

133

Baked 3 Injera. This may be due to till

evaporation will takes

place

Equivalent dry wood

consumed g 3,636 3,905 3,693 3,745

142

2.the second stove

which is operated by

steam

Specific fuel consumption g/kg 260 290 270 273

15

can use other working fluid media to enhance

its efficiency

Total cooking time min 92 90 85 89

4

3.It has leakage which should be kept

maintained because

it should be weld by

oxygen method not arc.

Comparison of Stove 1 and Stove 2 % difference T-test Sig @ 95% ?

4.the combustion

chamber looks long in

height and should

Specific fuel consumption g/kg 68% 11.74 YES

be re-adjusted

Total cooking time min 28% 14.40 YES

5. on body due to the

out let welded on outer

body high tem.

consumption

Total cooking time min 17% 13.10 YES



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CHAPTER 5

5. CONCLUSION AND RECOMMENDATION

Mirt stove is the most widely fuel efficient stove disseminated in Ethiopia because of the ability

of fuel saving when compared to the open fire stove. But as it discussed before due to different

reasons it has thermal efficiency of 18% - 21% i.e. the heat utilized by the clay Mitad for baking

Injera.More than 79 % of heat produced in combustion chamber is wasted due to heat retained by

the mass of concert, heat wasted through the flue gas and heat wasted by three mode of heat

transfer to the surrounding. So the main focus of this study is to re-use this wasted heat by using

different mechanism from widely accepted and efficient stove in Ethiopia. We focused here

because Injera baking is the main Energy consuming at house level in Ethiopia for now and

future.

Therefore this study addresses how to recover the heat to be disputed from Mirt Injera baking

stove and re-use it for baking Injera and other food like bread. For study of this research we used

data from different previous researchers and existing community norms for design of newly dual

stove. We used as in put the existing diameter of the stove and Mitad diameter which is

commonly from 56-60 cm in diameter. To select the material for body construction the drawback

of existing Mirt stove is analyzed and use all possible options for re-use of wasted heat and

finally we designed two stoves working at the same time which works dually with one source of

fuel from the first stove and transport the heat disputed from the combustion chamber by means

of working fluid (water, oil etc) and re-use flue gas as second option for heating the second clay

pot directly. Finally dual stoves are manufactured, tested and analyzed using the collected data

with CCT software to check their performance. The result demonstrated that the improved Mirt

stove has better efficiency as compared to Mirt stove. The CCT test result shows the improved

Mirt stove saves up to 18 percent fuel wood, 13 percent cooking time and increase Thermal

Efficiency of Mirt stove by 6.08 percent. The newly designed Improved Mirt stove has 95%

significance level over Mirt stove. This outcome is met without function of full capacity of the

newly designed Mirt stove because of leakage of steam which contributes to the small

performance of the second stove. The great thing is possibility of baking Injera at the same time



2019


using one combustion chamber and the core of this finding. If all things are available means

work shop, oxygen welding machine , all necessary material , enough budget and enough time

the result even may be two folds in any parameter. Even by so far the test result the study is

useful for changing the life of our community, impact it has on climate means of income

generation for youth and women starting from manufacturing of the body, dissemination and end

user.

The following recommendation can be made from the study:

1. The study is promising and if sophisticated work shop and enough budgets is there it is

possible to get more result.

2. The study has major strengths like focusing on global concern (addressing the impact of climate

change), focuses in women and children and opens opportunity for women, youth empowerment

and child wellbeing through income generation

3. Water boiling test was the best to know its thermal efficiency. But because of constraint time

and some modification on calibration of water container to fit with the wide stove, further

test can be done.

4. Further study can be done on the Environmental impact like particulate emission

5. The effect of chimney on dual stove performance can further investigated

6. To increase its efficiency many further options can be used (using different fluids for

circulation, addition of fuel like charcoal in the second stove) etc can enhance the stove

performance.

7. Usage of the clay mixture with sawdust and ash for construction of stove body is very

promising

8. Clay Mitad still contributes high impact on the performance of overall efficiency of the

stove. So further research needed to enhance its efficiency.



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6. REFERENCES

1. List_of_Stoves_in_Ethiopia. (2008). Retrieved from https://energypedia.info/wiki/

2. UNDP (2010b) . Retrieved from http://hdrstats.undp.org/en/countries/profiles/ETH.html

3. IEA.(2008). Retrieved from http://www.iea.org/country/n_country.asp,COUNTRY_CODE=ET

4. K.Meder (2011).Design and IEA data, derived from EGGER 2010 and MAPLIBRARY 2011.

5. World Bank .2008. Retrieved from http://siteresources.worldbank.org/INTSTATINAFR/

Resources/LDB-Africa-12-2-08.pdf

6. World Bank Report. (2008). EEPCo “electric energy access”.

7. UN Data.(2007). Retrieved from http://data.un.org/CountryProfile.aspx?crName=Ethiopia

8. Hathaway, T. (2008): What costs Ethiopia’s Dam Boom? A look inside the Expansion of

Ethiopia’s Energy Sector.

9. Meder1,K. (2011): Application of Environment Assessment related to GIZ ECO Micro

Hydropower Plants in the Sidama zone/Ethiopia (unpublished Diploma thesis).

10. Ministerial Conference on Water for Agriculture and Energy in Africa (2008): The Challenges of

Climate Change: Hydropower Resource Assessment of Africa.

11. GTZ (2007): _ Eastern Africa Resource Base: GTZ Online Regional Energy Resource Base:

Regional and Country Specific Energy Resource Database: II - Energy Resource.

12. EEA (2003): Energy Access Project: Baseline Survey, Monitoring and Evaluation Framework

and Hydro Market Development Strategy.

13. Gross, T. et al. (2009): The economic potential of small and micro hydropower projects in

Ethiopia: A situation analysis for national and international investors (AMES-E).

14. D. (2006). Off-grid rural electrification master plan study.

15. www.giz,de/hera Mirt stove user manual (HERA –Poverty-oriented basic energy services)

16. GTZ .(2009).Target Market Analysis, Ethiopia’s Solar Energy Market.

17. GTZ. (2007). Eastern Africa Resource Base, GTZ Online Regional Energy Resource Base:

Regional and Country Specific Energy Resource database: VII - Best Practice Case Studies.

18. Walelu, K. (2006): EEPCo Wind energy projects in Ethiopia

19. Walelu, K. et al.(2011). Ethiopia Wind Profile.

20. EEPCo .(2011). http://www.eepco.gov.et/eepco.php

http://www.giz,de/HERA



2019


21. Ministry of Finance and Economic Development (MofED) .(2010). Growth and Transformation

Plan (GTP) 2010/11-2014/15

22. GTZ. (2007). Eastern Africa Resource Base, Regional and Country Specific Energy Resource

Database: IV - Energy Policy.

23. Ministry of Water and Energy. (2012) . Scaling - Up Renewable Energy Program. Ethiopia

Investment Plan

24. Dagninet Amare, Asmamaw Endeblhatu, Awole Muhabaw .2015. Enhancing Biomass Energy

Efficiency in Rural Households of Ethiopia, Journal of Energy and Natural Resources,Vol. 4,

No. 2, 2015, pp. 27-33. doi11648/j.jenr0402.11

25. University of Oldenburg / Germany .(2002). Efficiency test for a biomass cooking stove ,Marco

Peter, PPRE

26. Biomass Stoves Engineering Design. retrived from http://www .cd3wd.com/cd3wd_40

/vita/biostov/e /biostov.htm

27. Dr. Mark Bryden . Design Principles for Wood Burning, Cook Stoves, Dean Still, Peter Scott,

Geoff Hoffa, Damon Ogle,Rob Bailis, Ken Goyer

28. Bejan Adrian, Kraus Allan D., 2003. “Heat Transfer Handbook” New Jersey

29. Bjorn P., Kungliga T., Hogskolan K., 1993. “Short notes on heat transfer and exchanger”; Royal

Institute of Technology, Department of Energy.

30. F. P. and DeWitt, D.P., 2002. “Fundamentals of Heat and Mass Transfer” 5th ed.; Wiley, New

York.

31. Ercan Ataer. Ö, Gazi. Energy Storage systems-Vol.I, University, Mechanical Engineering

Department, Malteps, 06570, Ankara Turkey

32. Ercan Ataer, Ö., Gazi .(2000 ). “Energy Storage systems”, Vol., I Mechanical Engineering

Department, University Malteps, Ankara Turkey

33. Edwards, D.E., Denny V.E., Mills, A.f, 1979. “Transfer Process, An Introduction to Diffusion

Convection, and Radiation”, 2nd ed.; McGraw-hill book company Washington, London, New

York.

34. Negese Yayu Mekonnen.(2016). Design and Manufacturing of Biomass based Energy Saving Injera-

Baking Stove’’

35. American Thermal Insulation Manufacturers Association (ATIMA). (2005). “Energy Saving



2019


Guide,”

36. Georgia State University.( 2005). “Hyper physics Insulating Material Thickness”

37. PUR/PIR Manufactures.(2006). “Thermal insulation materials made of rigid polyurethane foam

Properties” Report N°1; London

38. Yunus Cengel, A., 2003. “Heat transfer: A practical Approach” 2nd Ed ; Tata McGraw-Hill

39. Delaney, D.(2003). “Scheffler‟s community solar cooker” http://www.solar-bruecke.org

40. Rainer Aringhoff, et al. (2005). “ Exploiting The heat from the sun to combat climate change”;

Greenpeace

41. Swisher, J.H. (2003). “Division of Energy Storage Systems”; ERDA Washington, D.C.,20545.

42. The Thermal Insulation Association of Southern Africa.(2001). “Thermal Insulation Handbook”

43. Joshua FOLARANMI .(2009). “Effect of Additives on the Thermal Conductivity of Clay”

Leonardo Journal of Sciences ISSN 1583-0233 ,Issue 14, January - June

44. Baldwin, Samuel F.(1952).“ Biomass stoves Princeton University”.

45. Assefa Ayalew.( 2011). “Transient Heat Transfer Analysis of Injera Baking Pan (“Mittad”) by

Finite Element Method”; Research report, Addis Ababa University

46. Dr.Reza Fakhrai, KTH Kungliga Tekniska Högskolan Institutionen för Energiteknik Stockholm,

2008

47. Melkamu Yaye .(2013). “Integration of Scheffler Concentrator and Thermal Storage Device for

Indoor Injera” Addis Ababa university

48. Ezana Negusse, .(1996). “Research report: Electric Injera Cooker (mogogo) efficiency”; Asmara,

Eritrea (Injera Baking mitad‟ http://afrofoodtv.com)

49. DANAS Electrical Engineering .(2015) . “ Project document on Electric Injera Mitad energy

Efficiency Standards and Labeling”

50. Mekonnen Melese.(2011). “Design and Manufacture of Laboratory Model for Solar Powered

Injera Baking Oven”; Addis Ababa University, Addia Ababa,

51. Bailis, Rob.(2004). “Controlled Cooking Test”, Version 2.0. University of California-

Berkley, Prepared for Household Energy and Health Program of

Shell Foundation.

52. Walter Kipruto, Intern, SDM/SSU,” A review of the cook stove test methods

and their applicability in small scale CDM cook stove projects”

http://www.solar-bruecke.org/

http://afrofoodtv.com/



2019


53. Assistance (VITA),” introduced a standardized stove-testing concept prepared from proceedings

of a meeting convened 1982 in Arlington 13’’

54. Yosef Assefa Feleke, 2007 “ assessing environmental benefits of mirt stove with particular

reference to indoor air pollution (carbon monoxide & suspended particulate matter) and energy

conservation)”

55. Mekonnen Melese .( 2011) . “Design and Manufacture of Laboratory Model for Solar Powered

Injera Baking Oven”; Addis Ababa University, Addia Ababa,

56. Melkamu Yayeh .(2013). ”integration of scheffler concentrator and thermal storage device for

indoor injera baking, Addis Ababa, Ethiopia

57. www.engineeringtoolbox.com/thermal conductivity- d-429.html, cited on April 4,2016.

58. Problem Solving and Decision Making, lecture note compiled by Belachew Negash, 2009, Bahir

Dar Institute of Technology, Bahir Dar University

59. John Wiley and Sons, Inc., Mechanical Engineers‟ Handbook: Materials and Mechanical design,

volume 1, Third edition, 2006

60. George E. Dieter and Linda C. Schmidt, Engineering Design, Fourth edition,McGraw-Hill

companies, 2009

61. John Wiley and Sons, Inc., Mechanical Engineers‟ Handbook: Materials and Mechanical

design, volume 1, Third edition, 2006



2019


7. ANNEX

The combustion Chamber of first stove

Drawings of first Stove Scale 1:2



2019


Qualitative data

Name(s) of Tester(s) Type of stove: Stove 1

Type of stove: Stove 2

Test Number Location

Date Wood species 42

Quantitative testing conditions data units variable data units variable

Avg dimensions of wood (length x width x height) 40x2x5 cm -- 1,700 g P1

Wood moisture content (% - wet basis) 8% % m 1,700 g P2

92 ºC Tb g P3

(default value is 100 ºC - correct if local value differs) g P4

1,570 g k

SHELL FOUNDATION HEH PROJECT CONTROLLED COOKING TEST

Empty weight of Pot # 4


Local boiling point of water


DATA AND CALCULATION FORM

Shaded cells require user input; unshaded cells automatically display outputs

Modfied New Mirt Stove


Jagama and Negussie

1

May 18 ,2018

open fire

Addis Ababa

Weight of container for char

Other comments on test conditions



2019


CCT-3 for the Wind conditions 1

Shaded cells require user input; unshaded cells automatically display outputs Air temperature 23 ºC

To be filled in after cooking task is complete (as defined by the directions on the "Description" worksheet)

MEASUREMENTS Units data label data label Comments about cooking process (smokiness, ease of use, etc)

Weight of wood used for cooking g 11450 fi 2500 ff

Weight of charcoal+container g 2400 cc

Weight of Pot # 1 with cooked food g 12505 P1f

Weight of Pot # 2 with cooked food g P2f



Time min ti 123 tf

CALCULATIONS CALCULATIONS Formula

Total weight of food cooked g 9105 Specific fuel consumption g/kg 758

Weight of char remaining g 830 Total cooking time min 123 t = tf - ti

Equivalent dry wood consumed g 6903

presence of insulation, chimney, workspace, etc):

cc = k – cc

open fire

weight of butter plus container =16,640gm

Weight of butter container =640gm

weight of Injera container =1700gm

Initial

measurements

Final

measurements

Description of stove (indicate the construction material of the stove, the way that the pot(s) fits in the stove, and the

weight of charcoal container =1570gm

Formula

4

1j

ff PjPjW

cifd Δc1.5m1.121fff

1000f

d

W

fSC



2019




2019






Weight of wood used for cooking g 5885 fi 1225 ff weight of butter plus container =33,280gm

Weight of charcoal+container g 1795 cc Weight of butter container =1280gm

Weight of Pot # 1 with cooked food g 16856 P1f weight of Injera container =1700gm

Weight of Pot # 2 with cooked food g P2f weight of charcoal container =1570gm



Time min ti 92 tf






Modfied Mirt Stove

Initial

measurements

Final

measurements


cc = k – cc

Formula

4

1j

ff PjPjW


1000f

d

W

fSC



2019



Summary of comments on stove 1

Stove type/model: Stove 1


Location

Wood species

1. CCT results: Stove 1 units Test 1 Test 2 Test 3 Mean St Dev

Total weight of food cooked g 8,600 9,300 9,105 9,002 361

Weight of char remaining g 473 507 830 603 197

Equivalent dry wood consumed g 7,939 7,888 6,903 7,577 584

Specific fuel consumption g/kg 923 848 758 843 83

Total cooking time min 125 121 123 123 2



Total weight of food cooked g ##### ##### ##### ##### 256






Specific fuel consumption g/kg 11.74

Total cooking time min 10.44

can use other working fluid media to enhance its efficiency

5. on body due to the out let welded on outer body high tem.

3.It has leakage which should be kept maintened becouse

it should be weld by oxgen methode not arc.

4.the combustion chamber looks long in height and should

be re-adjusted

baked 3 Injera.this may be due to till evaporation will takes place

2.the seconed stove which is operated by steam

steamed stove will start functioning after the main stove

1. Between Modfied Stove and open fire stove the seconed

68%

24%

YES

YES

open fire


Addis Ababa

Eucalyptus Camaldulensis (Red River Gum, Red Gum)

Qualitative data

Name(s) of Tester(s) Type of stove: Stove 1

Type of stove: Stove 2

Test Number Location

Date Wood species 42

Quantitative testing conditions data units variable data units variable

Avg dimensions of wood (length x width x height) 40x2x5 cm -- 1,700 g P1

Wood moisture content (% - wet basis) 8% % m 1,700 g P2

92 ºC Tb g P3

(default value is 100 ºC - correct if local value differs) g P4

1,570 g k

SHELL FOUNDATION HEH PROJECT CONTROLLED COOKING TEST



Local boiling point of water


DATA AND CALCULATION FORM

Shaded cells require user input; unshaded cells automatically display outputs



Jagama and Negussie

1

May 18 ,2018

open fire

Addis Ababa

Weight of container for char

Other comments on test conditions



2019


The Standardized Cooking Task

Ingredient Name Amount (g) Step Directions

1 Teff Butter 32000gram 1

2

3 2

4

5 3

6

7 4

8

9 5

10

11 6

12

13 7

14

15 8

16

17 9

18

19 10

20

Use this space to describe the standardized cooking process that forms the basis of this test. Describe each step with enough detail so that an

experienced cook from the area where the test is performed could follow them easily. If more space is needed, extend the description below the

space provided.


Shaded cells require user input; unshaded cells automatically display outputs Air temperature 23.8 ºC









Time min 0 ti 125 tf






Initial

measurements

Final

measurements



open fire

Formula

cc = k – cc




4

1j

ff PjPjW


1000f

d

W

fSC



2019












Time min ti 121 tf







No smoke the stove constructed 1 meter above the ground

cc = k – cc





Formula

open fire

Initial

measurements

Final

measurements

4

1j

ff PjPjW


1000f

d

W

fSC

CCT-1 for the Modfied Mirt Stove Wind conditions 1










Time min ti 98 tf






cc = k – cc




Initial

measurements

Final

measurements



Formula

4

1j

ff PjPjW


1000f

d

W

fSC



2019












Time min ti 123 tf






cc = k – cc

open fire




Initial

measurements

Final

measurements



Formula

4

1j

ff PjPjW


1000f

d

W

fSC










Time min ti 92 tf






Initial

measurements

Final

measurements


cc = k – cc

Formula

4

1j

ff PjPjW


1000f

d

W

fSC



2019












Time min ti 89 tf






Initial

measurements

Final

measurements


cc = k – cc

Formula

4

1j

ff PjPjW


1000f

d

W

fSC





Location

Wood species


Total weight of food cooked g 8,600 9,300 9,105 9,002 361







Total weight of food cooked g ##### ##### ##### ##### 256






Specific fuel consumption g/kg 11.74

Total cooking time min 10.44

can use other working fluid media to enhance its efficiency

5. on body due to the out let welded on outer body high tem.

3.It has leakage which should be kept maintened becouse

it should be weld by oxgen methode not arc.

4.the combustion chamber looks long in height and should

be re-adjusted

baked 3 Injera.this may be due to till evaporation will takes place

2.the seconed stove which is operated by steam

steamed stove will start functioning after the main stove

1. Between Modfied Stove and open fire stove the seconed

68%

24%

YES

YES

open fire


Addis Ababa

Eucalyptus Camaldulensis (Red River Gum, Red Gum)



2019












Time min ti 98 tf






cc = k – cc




Initial

measurements

Final

measurements



Formula

4

1j

ff PjPjW


1000f

d

W

fSC

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