05766415 Solar energy application

7/29/2019 05766415 Solar energy application

http://slidepdf.com/reader/full/05766415-solar-energy-application 1/8

Thermal Issues in Emerging Technologies, ThETA 3, Cairo, Egypt, Dec 19-22nd 2010

978-1-61284-266-0/10/$26.00 ©2010 IEEE ThETA3_077 331

TECHNICAL AND ECONOMIC ANALYSIS OF A SOLAR –ASSISTEDAIR-CONDITIONING SYSTEM

Z. Sayadi 1, S. El May 1, Mahmoud Bourouis2 , A. Bellagi 1*

1U. R. Thermique et Thermodynamique des Procédes Industriels, UTThPI Ecole Nationale d’Ingénieurs de Monastir, ENIM Avenue Ibn El Jazzar, 5019 Monastir, Tunisie

2Mechanical Engineering Department, Universitat Rovira i Virgili,43006 Tarragona, Spain* [email protected]

Abstract

In this paper, we present an analysis aiming at

assessing the feasibility and economic performance of a solar-assisted air-conditioning system for a middleclass house under the climatic conditions of Tunis City. A single effect water-lithium bromide absorptionmachine, with a cooling capacity of 10 kW isconsidered. Various simulations are carried out using the TRNSYS and EES programs. The calculations show that 30m

2of evacuated tube collector area with a

thermal storage tank of about 1m3 , can cover 87% of

the energy needs of a water-cooled machine with a

maximum driving heat temperature at 95°C. The total annual expenses for the water-cooling are about1608US $.

1. Introduction

During the last few decades, the increased fossil fuelenergy consumption associated with the overloadingof the electricity grid for air-conditioning purposes,especially at peak demand periods in hot summer increased dramatically in several countries,

particularly in hot climate regions. In 1996, about11000GWh primary energy were consumed in Europealone by small air-conditioners up to a coolingcapacity of 12 kW [1]. This value is expected toincrease by a factor of 4 by 2020. Energy conservation

is an approach to reduce the disadvantages of theconstantly growing energy demand of air-conditioningin both an economic and environmental sense.Single effect absorption chillers cooled by either air or water are best adapted for solar-assisted air-conditioning systems with common solar collectors(flat plate collectors and evacuated tube collectors), asthey require a rather low temperature heat input andcan be relatively performent with a COP ranging

between 0.6 and 0.8 [2]. The appropriate workingfluids pair is water-lithium bromide (LiBr). Gas-firedH2O/LiBr chillers are currently used in the lowcooling capacity range starting at about 4 kW. Thus, it

is possible to install these kinds of chillers for several building sizes, from single-family residential to largecommercial buildings. The choice of the working pair

fluids (LiBr/water) is based on the followingconsiderations. For a single-effect chiller, as the one

proposed here, the driving fluid temperature is close to100°C, a temperature range that evacuated tubecollectors can ensure. In addition, it is reported thatLiBr/water machines performance are higher thanthose working with a water/NH3 system [3]. Finally,evaporation temperatures below 0°C are unnecessaryin the air-conditioning sector, which allows the use of water as refrigerant.This paper analyses the technical and economicfeasibility of a solar-powered air-conditioning system

by means of mathematical modeling and numericalsimulation using the software tools EES (EngineeringEquation Solver, Flowchart Engineering) and

TRNSYS (Transient Energy System Simulation program). A single effect water-cooled chiller isdesigned for a typical middle class family house (170m

2) under the climatic conditions of Tunis City. The

solar-assisted air-conditioning system comprises theLiBr/H2O absorption machine, an evacuated tube field,a thermal back-up source provided by a gas heater, ahot water storage tank and fan coils for the house air-conditioning.

2. Lithium bromide-water absorptionmachine

The working principle of an absorption system (Fig. 1)is similar to that of a vapor compression machine withrespect to the key components evaporator andcondenser. The refrigerant (water) evaporates in theevaporator producing the useful cooling effect, Qev.The water vapor (3) flows to the absorber where it isabsorbed by the salt rich solution (LiBr) returningfrom the generator (10). The absorber can be air- or water-cooled; in the latter case a cooling tower isnecessary to keep the cooling process going. Thedilute salt solution exiting the absorber (4) is pumped

(5) through the regenerative solution heat exchanger--where it is pre-heated (6)--in the generator where it isheated above its boiling point temperature, so thatrefrigerant vapor is released (7). The concentrated



Technical and economic analysis of a solar assisted air-conditioning system, Z. Sayadi et al.

ThETA3_077332

solution (8) flows back to the absorber. The desorbedrefrigerant (7) condenses in the air- or water-cooledcondenser. The condensate (1) is passed thereafter through an expansion valve where its pressure is soreduced as to provoke its partial evaporation causing asubstantial decrease of its temperature. The refrigerantflows finally in the evaporator (2) to complete theevaporation.

Figure 1. Solar-assisted air conditioning installation

using LiBr/water absorption machine

In the lithium-bromide water chillers, crystallization of the salt may occur at higher salt concentrations andlower temperatures [2] as it can be the case in the saltrich solution on its way to the absorber after cooling inthe solution HX. As discussed later in this paper, sucha situation can be avoided by finding out theappropriate operating conditions that must prevail inthis machine compartment under which thecrystallization cannot occur.

3. Solar cooling system

A solar-assisted air-conditioning system for a single-family residential building is composed of three major sub-systems [1] (Fig.1):

The load sub-system: This is the distributionsystem for the cold medium (supply andreturn) It is connected to the delivery

terminals located at space to be air-conditioned in the building.

The cold production sub-system. The heatreleased in this unit by the absorber and thecondenser is discharged indirectly to theenvironment through a cooling tower.

The heat production sub-system: It providesthe high temperature heat to the thermallydriven air-conditioning system. Besides the

solar collector field, other key componentsare the thermal storage unit, the pumps andthe thermostat controllers. Furthermore,depending on the system needs (insufficientsolar radiation, inadequate collector area, etc.)a back-up heat source incorporated. In this

paper, it is assumed that a gas heater providesthe auxiliary thermal energy.

4. Solar air-conditioning systemanalysis

The investigations of the solar cooling system consist

of the four following steps: House cooling loads assessment using

TRNBUILD, a component of TRNSYS.

Absorption machine modeling and simulationusing the EES software without consideringthat the driving heat is supplied to thegenerator by the solar components.

Whole system simulation using TRNSYS.

Technical-economical analysis of the solar cooling system to find out the optimalcollector area and system componentsresulting in the best energy cost-performance.

As indicated earlier, a water-cooling scenario with

a cooling-tower is considered. The choice of water-cooling is thermodynamically morefavorable than air-cooling. It presents however thedrawback of using water as cooling medium--rather scarce in arid and semi-arid regions--hencethe need of a cooling-tower to minimize the water consumption.

4.1 TRNSYS program description

TRNSYS is a complete and extensible simulation

environment for the transient simulation of systems,including multi-zone buildings. It is used by engineersand researchers around the world to validate newenergy concepts, from simple domestic hot water systems to the design and simulation of buildings andtheir equipment, including control strategies, occupant

behavior, alternative energy systems (wind, solar, photovoltaic, hydrogen systems) [5]. A TRNSYS project is typically setup by connecting componentsgraphically in the ‘Simulation Studio’. Each ‘Type’ of component is described by a mathematical model inthe TRNSYS simulation engine and has a set matchingPerforma’s in the simulation studio. The Performa has

a block-box description of a component: inputs,outputs, parameters, etc.




ThETA3_077 333

4.2 Cooling loads estimation

The house considered has a 170 m2

floor area and it ismodeled, under the climatic conditions of Tunis city,as a multi-zone building (Type 56 in the TRNSYSlibrary) with three bedrooms, a living room, a dining

room, two bathrooms and a kitchen. The differentzones are specified in TRNBUILD (building visualinterface used to enter input data for multi-zone

buildings). It allows specifying all the buildingstructure details, as well as everything that is needed tosimulate the thermal behavior of the building, such aswindow optical properties, layers and walls type,heating and cooling schedules, etc. The construction isselected from a list with descriptions of standardconstructions and materials [5]. The convective andradiant losses and gains are calculated withinTRNBUILD after selecting the zone construction. Anumber of pre-defined input parameters and variables

(occupancy, internal gain, comfort, infiltration,ventilation, weather data, etc.) have to be specified.

Figure 2. Evolution of the cooling needs with theambient air temperature in Tunis City during the

simulation period May-October

The cooling requirement is calculated by setting thedesired indoor temperature at 26°C during the summer

period from May to October.The evolution of the cooling needs during this periodis shown in figure 2.It turns out that the building’s cooling loads in hotsummer are of 11.5 kW. In order to save energy whileensuring almost the same thermal comfort, it is usualto consider that the machine covers the cooling needs

of the house for 95% of the simulation period, whichfixes its nominal capacity to 7 kW.

4.3 Chiller operating conditions

4.3.1 Machine model

In order to assess the performance of the chiller,numerous simulations are carried out using the EES

program [6].The model equations for the various equipmentcomponents (absorber, generator, condenser,

evaporator, solution heat exchanger, solutionexpansion valve, pump and throttle) are given below.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

89 90 91 92 93 94 95 96 97 98

Generator inlet temperature (°C)

C O

P

0

2

4

6

8

10

12

C o o l i n g c a p

a c i t y ( k W )

COP

Qev (kW)

Figure 3. Effect of the hot water inlet temperature

on the performance of the machine

o Absorber

abQhmhmhm

44101033 (4.1)

abQhmhm

13131414 (4.2)

4103

mmm (4.3)

1314

mm (4.4)

ababab LMTD AU Q )()(

(4.5)

o Generator

g Qhmhmhm

668877 (4.6)

g Qhmhm

11111212 (4.7)

1112

mm

(4.8)

g g g LMTD AU Q )()(

(4.9)

o Condenser

cQhmhm

1177 (4.10)

cQhmhm

15151616 (4.11)

17

mm (4.12)




ThETA3_077334

141615

mmm (4.13)

ccc LMTD AU Q )()(

(4.14)

o Evaporator

evQhmhm

2233

(4.15)

evQhmhm

18181717 (4.16)

32

mm (4.17)

1817

mm (4.18)

o Solution HX

55669911 hmhmhmhm

(4.19)

65

mm (4.20)

98

mm (4.21)

65 x x (4.22)

98 x x (4.23)

o Expansion valve

991010 hmhm

(4.24)

910

mm (4.25)

910 x x (4.26)

o Throttle

2211 hmhm

(4.27)

21

mm (4.28)

o Pump

1122 hmhm

(4.29)

12 x x (4.30)

o Chiller performance

g

ev

Q

QC OP

(4.31)

4.3.2 Cycle simulation

The nominal operating conditions and the heatexchange characteristics of the different machinecomponents are presented in table 1.A preliminary study was performed to investigate theeffect of the driving heat temperature and the hot water flow rate on the performance of the machine (Fig.3).These two factors together specify the thermal power and the quality of the driving heat. It is found that theCOP (Eq. 4.31) increases with increasing generator temperature and can reach as a high value as 0.7. It isnoted however that small fluctuations of the hot water temperature can cause a significant decrease of theCOP and consequently, the machine produces far fewer cooling capacity. Figure 3 shows that a hotwater temperature of 95°C providing a coolingcapacity of 7 kW is appropriate for our system.

Table 1. Typical operating parameters for the

water-cooled single effect LiBr/H2O chiller

Cooling capacity (kW) 7

Solution heat exchanger effectiveness 0.64Solution flow rate (kg/s) 0.056

Cooling medium flow rate (kg/s) 1.84

Refrigerant flow rate (kg/s) 0.335

Evaporator outlet temperature (°C) 5

Evaporator pressure (kPa) 0.87

Cooling medium temp. at condenser inlet °C) 32

Cooling medium temp. at absorber inlet (°C) 29

Chilled water temperatures (°C) 12/7

(UA)c (kW K- ) 0.686

(UA)ab (kW K

-1

) 1.457(UA)g (kW K

-1) 2.753

(UA)ev (kW K -

) 1.753

The simulation performance results of the water-cooled LiBr/water chiller are summarized in tables 2and 3.

Table 2. Simulation results

Location T(°C)

H(kJ/kg)

m (kg/s)

P(kPa)

x(%)

144 184.2 0.003 9.1 0

2 4.575 184.2 0.003 0.87 0

3 5 2510 0.003 0.87 0

4 34 82.36 0.056 0.87 55.31

5 34 82.36 0.056 9.1 55.31

6 63.72 143.6 0.056 9.1 55.31

7 87.41 2663 0.003 9.1 0

8 85.21 197.1 0.053 9.1 58.45

9 52.43 132.4 0.053 9.1 58.45

10 40.95 132.4 0.053 0.87 58.45

11 95 2679 0.8




ThETA3_077 335

12 88.32 2666 0.8

13 29 2554 1.84

14 31.88 2560 1.84

15 32 2560 1.84

16

34.16 2564 1.84

17 12 50.36 0.3345

18 7.004 29.43 0.3345

Table 3. Performance of the chiller

Chiller component Energy (kW)

Evaporator

evQ 7

Absorber

abQ

9.958

Generator

g Q 10.42

Condenser

cQ 7.463

4.3.3 Solar cooling simulations

TRNSYS model

Figure 4 presents the simulation model of the solar air-conditioning system under TRNSYS environment.Each component is referred to by a ‘Type’ number (for

example Type 56) and represents a type block programmed in FORTRAN with inputs, outputs and parameters. Each type corresponds to a FORTRANsubroutine.

Figure 4. TRNSYS simulation model of the solar

air-conditioning system

In the following is a brief description of the differentTypes used in Figure 4. The absorption chiller hoststhe chiller programmed in EES. It is a utilitysubroutine which calls EES (external program) wherethe equations can be solved based on the component’s

inputs and sends the results back to TRNSYS. The

necessary inputs from TRNSYS to EES program arethe hot water temperature and flow rate driving themachine generator and coming from the insulatedstorage tank.

Simulation results

The analysis is made for typical meteorological datafor Tunis city in the Mediterranean zone (Fig. 5).The solar sub-system consists of a 1 m

3hot water

storage tank, evacuated tube collector tilted 35° fromthe horizontal and a thermal back-up source provided

by a gas heater. The water flow rate per unit collector is 50 l/h m2. The simulation period extends from Mayto October corresponding to the hot period of the year where air-conditioning is required (Figure 2).We investigate the influence of the collector surfacearea on the solar gain and the auxiliary heat required(Fig. 8). The pressurized water heating the generator temperature is maintained at 95°C for the water-cooledmachine.As expected, the larger the collector area, the more isthe collected solar radiation and hence the producedheat and the lesser the auxiliary energy. The back-upheat power varies between 0.84 kW and 8 kW.

Figure 5. Tunisia climate zoning

ZT1 : Mediterranean zoneZT2 : North zone

ZT3 : South zone




ThETA3_077336

0

1

2

3

4

5

6

7

8

9

10

11

0 5 10 15 20 25 30 35 40 45

Collector surface (m²)

E n e r g

y ( k W )

Auxiliary energy

Solar gain energy

Figure 6. Auxiliary energy needs and solar gain

vs. collector area

Figure 7 shows the evolution of the solar fraction f

buQ

useQ

f

(4.32)

with the collector surface area. As observed, it getshigher with larger collector area, but the auxiliaryenergy is not vanishing even if the solar heat covers100% of the energy needs of the system. Thus, in any

case, fossil fuel make-up energy is necessary to ensurethe required pressurized water temperature of 95°C.The above performed technical analysis is insufficientto find out the optimal collector surface for our solar air-conditioning installation because the economicalaspects of the problem have not yet been considered.

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

5 10 15 20 25 30 35 40


f

( % )

Figure 7. Solar fraction vs. collector area

5. Economical optimization

We focus now attention on the cost performanceanalysis in order to assess the economic viability of the

project.We first calculate the discounted annual total costs

ATd C by summing up all the discounted investment

costs for each component of the entire systemamortized for an expected life time of 15 years and the

discounted annual expenses Ad C including the

maintenance and inspection costs M C estimated as 2%

of the investment charges and the operating energycosts (gas costs).The annual costs are weighed up by the equation:

M E A C C C

(5.1)

The energy costs are calculated as follows:

)1( f C QC kWhbu E

(5.2)

The annual expenses are not constant. They depend ontwo parameters: the Increase in Energy Costs IECestimated at 7.4% and the Discount Rate DR which isthe rate of return required for a project to compensatefor its risk. The DR is estimated at 8.28% for the year 2008.Thus, calculation of the discounted annual costs is

based on the Eq. 5.3.

t

T

T

Ad DR

DR D

C C

)1)1(

)1(R

(5.3)with

)1()1(15

IEC DRC C t T

t A

(5.4)

After estimating the discounted investment costs Id C

1)1(

)1(R 0

T

T

I Id DR

DR DC C

(5.5)

for a life-time period T =15 years, the discountedannual total costs are calculated using the followingequation

Ad Id ATd C C C

(5.6)




ThETA3_077 337

5

505

1005

1505

2005

2505

3005

5 10 15 20 25 30 35 40 45


C o s t s ( U S

$ )

Total costs

Annual costs

Investment

costs

O p t i m a l c o l l e c t o r

3 0 m

2

s u r f a c e

Figure 8. Economical analysis

The results of the economical analysis are representedin figure 8. Because the evolution of the discountedannual and the investment costs with the collector areaare opposed, the discounted total fees exhibits aminimum.We find that the minimum the total costs of about

1608 US $ per year, corresponding to a collector areaof 30 m

2.

6. CONCLUSION

In this paper, we have investigated the technical andeconomical feasibility of a solar air-conditioningsystem using a single effect lithium bromide water-cooled machine with a nominal capacity of 7 kW. Thesystem is optimized for a middle typical house of 170m2. It is found that 30m2 evacuated tube collector areaassociated to a 1m

3hot water storage tank can covers

87% of the heat needs of the water-cooled H2O/LiBr chiller. is by The annual total costs of evaluated to1608 $.These costs are rather high compared to those of acommon air-conditioning system using vapor compression technique. But we should not forget thatthe absorption technique is yet suffering from a scalefactor: If the absorption chillers were as widespread asthe usual air-conditioner, their price would be surelycomparable. On the other hand, the fossil fuel-basedenergy is expected to increase steadily in the future, sothat the solar assisted air-conditioning system would

become economically more viable. Further advantages

of the absorption chilling are: reduced electricitydemand and elimination of the use of harmful workingfluids (CFCs, CHFC’s, etc.).

It can also be noted that the costs of the solar systemcan be further optimized by exploiting the solar collector field to produce solar heat to match other loads as space heating or domestic hot water

production during winter.

7. References

[1] H.M. Henning, 2004-2007, Solar-Assisted Air

conditioning in Buildings, Springer Wien New York.

[2] D.S. Kim & C.A. Infante Ferreira, 2009, “ Air-cooled LiBr-

water absorption chillers for solar air-conditioning inextremely hot weathers”, Energy Conversion andManagement, In press.

[3] J. Castaing-Lasvignottes, , 2004, Aspects

Thermodynamiques et Technico-économiques desSystèmes

à absorption liquide, Institut Français du Froid Industriel. [4] R.A. Zogg & M.Y. Feng.; D. Westphalen, 2005, Guide to

Developing Air-Cooled LiBr Absorption for Combined

Heat and Power Applications, Distributed EnergyProgram

Report; U.S Department of Energy.[5] S .A. Klein, J.A. Duffie, J.C. Mitchell, JP. Kummer,W.A.

Bechkman, N.A. Duffie & Al, 1994, TRNSYS- a

Transient Simulation Program user’s manual , University of Wisconsin-Madison.

[6] S.A. Klein, 1992-2008, EES- Engineering EquationSolver ,

User ’s manual and program documentation.

Nomenclature

C Coefficient of performanceP Pressure, mBar Q Heat flow rate, kWT Temperature, °CUA Thermal conductance, kW K-1f Solar fraction, %Quse useful solar heat, kWQbuLMTD

heat from the back-up, kWLogarithm temperature difference

h Specific enthalpy, kJ kg-1x % mass fraction of LiBr in salt

solution

m

mass flow rate, kg s-1

AC Annual costs, US $

E C

Energy costs, US $

M C

Maintenance costs, US $

0 I C

Initial investment, US $

ATd C

Discounted annual costs, US $

IEC Increasing Energy Cost, %

DR Discounting rate, %

kWhC

Cost of kWh of gas, US $

Ad C Discounted annual costs, US $

Id C

Discounted investment, US $ costs

T Expected life-time, year




ThETA3_077338

Subscripts

c Condenser ev Evaporator g Generator ab Absorber US $ United States Dollar

05766415 Solar energy application

Documents