Thermoelectric Multi-Utility Water Heater cum Air-Conditioner

Purdue UniversityPurdue e-PubsInternational Refrigeration and Air ConditioningConference School of Mechanical Engineering

2016

Thermoelectric Multi-Utility Water Heater cumAir-ConditionerMilind Vishwanath RaneIIT Bombay, India, [email protected]

Dinesh B UphadeIIT Bombay, India, [email protected]

Adittya M RaneVishwakarma Institute of Technology, Pune, INDIA, [email protected]

Follow this and additional works at: http://docs.lib.purdue.edu/iracc

This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.html

Rane, Milind Vishwanath; Uphade, Dinesh B; and Rane, Adittya M, "Thermoelectric Multi-Utility Water Heater cum Air-Conditioner" (2016). International Refrigeration and Air Conditioning Conference. Paper 1816.http://docs.lib.purdue.edu/iracc/1816

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Thermoelectric Multi-Utility Water Heater cum Air-Conditioner

Milind V RANE1*, Dinesh B UPHADE1, Adittya M RANE2

1Heat Pump Laboratory, Mechanical Engineering Department,

IIT Bombay, Powai, Mumbai INDIA 400 076

Phone +91 22 2576 7514, Fax +91 22 2572 6875, [email protected]

2Mechanical Engineering Department, Vishwakarma Institute of Technology,

Pune, INDIA 411 037

* Corresponding Author

ABSTRACT

A novel Thermoelectric Multi-Utility Water Heater cum Air-Conditioner, TE_MUWH, with water heating as

primary utility and air-conditioning as bonus utility is presented. It is intended to serve the residential market. In

India, 2 to 3 kW electric instant water heaters are typically deployed in residences. They typically consume ~3

kWh/day of electrical power. Their use contributes significantly to the peaking demand from residential buildings.

Patented TE_MUWH is being developed as a modular scalable unit. It is expected to reduce energy consumption

for residential water heating applications by over 50% and reduce demand by over 95% when coupled with suitable

storage. As it has no moving parts and is expected to operate reliably for 20+ years.

An enhanced surface aluminum extrusion serves as a radiant-cum-convective heat collection surface.

Thermoelectric, TE, heating chips are in thermal contact with the extrusion on one side and a rectangular tube on the

other. Tap water flows through the tube and heats up from 25oC to 50oC. The radiant-cum-convectively cooled

surface of the TE_MUWH dispenses the cooling effect in the conditioned space of the house. It can cool the room

air from 27oC to ~21oC. Judicious deployment of the chips ensures that the lift across the TE heat pump ranges from

10 to 41oC while heating water up to 50oC.

Simulation of air/water flow over/in the TE_MUWH module are presented. Simulation results are compared with

preliminary experimental data. This simple, modular, maintenance free TE_MUWH is expected to pave the way for

future wide spread adoption of this type of energy efficient green technologies in residences.

1. INTRODUCTION

Thermoelectric coolers can be used to transfer heat from cold side to hot side without using moving parts and

refrigerant. Flow of free electrons enables pumping heat from low temperature to high temperature. Novel

Thermoelectric Multi-Utility Water Heater heats tap water from 25oC to 50oC while simultaneously cooling

conditioned space air from 27oC to 21oC. Residential water heaters are commonly used for meeting the hot water

needs for bathing, dish washing, cloth washing, etc. Storage type electric water heaters are having 2 kWe capacity.

While instantaneous or tank less water heaters are typically 3 kWe capacity. They are operated one hour in a day for

3 to 4 person family. Bathing water temperature ranges between 38 to 40oC. Bathing hot water requirement per

person is 20 to 25 liter (Shaban and Sharma, 2007). Total water requirement is about 100 liter per day. Air-

conditioning of room is a co-produced utility. It is utilized for conditioning of living room/bed room. Residential

water heaters are operated using electric resistance heaters, LPG, solar or electricity based heat pumps.

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1.1 Electric resistance water heaters Electric resistance heating are commonly used for water heating from several decades. Storage type water heaters

were used in earlier days. Current market is growing up for instantaneous water heaters (Paul, 2014). One unit of

electricity heats 35 L water from 25oC to 50oC. Operating cost is high as compare to any other system.

1.2 LPG water heaters Instantaneous gas geysers are also popular. There are issues like space for gas storage, higher cost of non-

subsidized gas. It saves about 3 to 5% compared to electric resistance water heating.

1.3 Solar water heaters These are storage type solar energy operated water heaters. Solar water heaters heats up the water using evacuated

glass tubes and stores it in insulated storage tanks. Storage tank is situated above solar thermal collectors. Water is

naturally circulated due to density difference. Hot water temperature depend on incident solar radiation. It varies

from day and month of the year. Initial cost of solar assisted water heater is about INR 15,000/- for 100 L

(www.mnre.gov.in). Due care is required to handle solar collector with evacuated glass tubes. Space requirement is

high. It is major constraint in urban areas. Payback is typically 3 to 4 years, if electrical resistance water heating is

replaced.

1.4 Heat pump assisted water heaters Small capacity heat pump assisted water heaters can provide water along with air-conditioning as bonus utility. Hot

water requirement is about one hour in a day. Hence, refrigerant based heat pumps are not currently serving water

heating for residential purpose. Thermoelectric coolers are small capacity heat pumps. Their cooling coefficient of

performance is about 7 for low lift of 5 K (Winkler et al., 2006). Judicious design of heat exchangers for cold and

hot side gives low temperature lift. This paper investigates the use of thermoelectric coolers for water heating as

primary use and air-conditioning as bonus utility. Simulation of heat pump model is performed in SolidWorks to

predict the temperature lift.

2. THERMOELECTRIC HEATERS

Thermoelectric cooling chips are typically used for meeting cooling needs of small appliances. These cooling chips

can also be used for heating. Performance of TE heaters is mainly affected by temperature across hot and cold side.

It reduces as the temperature rise increases.

Typical heat flux on cold and hot side ranges from 20 to 25 kW/m2. It is very high. It increases temperature

difference across the heat source and heat sink, the contact resistances on the hot and cold sides and reduces the

performance of chip. Hence, heat source and heat sinks need to be developed with increase heat transfer area, and

which offer good heat transfer coefficients.

2.1 Construction Simple thermoelectric refrigerator is represented with a single element. It consists of p and n type semiconductors.

They are commonly made up Bismuth Telluride or Bismuth Selenide (Goldsmid, 1964). Both are electrically

connected in series with thin copper plates. These plates are attached to the electrically insulated and thermally

conductive ceramic substrates. Lead wires are connected to copper plate to provide direct current supply. Elements

are thermally connected in parallel and electrically in series. This is the optimum configuration based on voltage

and current demand of all elements.

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a. Simple Thermoelectric Heater

b. Heat Flow Equations in TE Heater

Figure 1: Simple Thermoelectric Heater and Heat Flow Equations (www.tellurex.com)

These elements pumps heat from cold side to hot side. Quantity of heat pumped is small, hence number of these

elements are sandwiched between ceramic plates to increase heat pumping capacity.

TES1-03139 is a single stage chip consisting of 31 junctions having maximum working current of 3.9 A. These TE

heaters offers high COP at low power and low temperature lift. If temperature lift required is higher, multi-staging

of TE heater can be deployed. But it increases complexity and cost of TE heaters.

2.2 Working When potential difference is applied across TE heater, current passes through the elements. In n-type

semiconductor, electron and current flow in same direction, while in p-type semiconductor, they flow in opposite

directions. Electron and holes moves as voltage is applied across the leads wires. They carry heat from one ceramic

plate and deliver it at the other side ceramic plate (Rowe, 1995 and Rowe and Bhandari 1985). In the present

application conventional TE Cooling chips are deployed for TE Heating of water along with air conditioning. Heat

from hot side is recovered as useful utility while the cooling realized at the cold side is dispensed as cold utility.

According to utilities being generated hot and cold side heat exchangers can be deployed, to enable effective heat

exchange to the water being heated and room air being cooled.

2.3 Heat flow in conventional TE Cooler Heat flow from low temperature region to high temperature region depends on three effects. They are Peltier effect,

Joule effect and Fourier heat transfer (Cheng and Lin, 2005).

Net cooling effect produced due to direct current flow in N number of elements is as follow,

2

h c

c  c c

k.A. T  - TIQ = N  µ.I.T -  ρ.L+ 2r  - 

2A L

…(1)

Net heating duty due to direct current flow and cooling is as follow,

2

h  c

h  h c

k.A. T - TIQ = N  µ.I.T +  ρ.L+ 2r  - 

2A L

…(2)

Coefficient of performance is given by following equation,

c c 

h  c 

QCOP = 

Q - Q

…(3)

2.4 TE Cooler: TES1-03139 Based on the cooling capacity and cost comparison between TE coolers from different manufacturer, TES1-03139

was selected and procured. Its size is 15 mm x 15 mm x 3.2 mm thick. Smaller size chips are preferred as they help

diffuse the heat to be transferred to and from the chip. Using multiple small chips in place of a single large chip

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helps distribute the heat load on the cold and hot heat exchange elements which are deployed to exchange heat with

the cold and hot utility streams.

Detailed operating curves of TES1-3139 from catalogue are listed for th 27 and 50oC. In the present application

average hot side temperature is close to 50oC, hence these performance curves are used for design and optimization.

Maximum temperature difference that the TEC chip can handle across hot and cold side is 76oC. Maximum cooling

capacity is 12.4 W at zero temperature difference, maximum recommended voltage, current and current is 4.23 V

DC and 4.7 A. Chip resistance is 0.77 W. Cost of TEC chip is about 100 INR for TES1-03139. Cooling capacity vs

temperature difference at constant current are plotted with digitized data. On same plot, voltage vs temperature

difference at constant current are superimposed.

Figure 2 a and b represent the performance curves of TES1-03139 at th 50oC. Error in parameter is within the limit

of ±5% except at zero values. TE coolers are operated at three different voltage ranges shown in Figure 2b using

three different power sources.

a. dthc vs Qc and Vdc

b. Vdc vs COPc and Qc

Figure 2: Performance Curves of TES1-03139, COPc and Cooling Capacity at Different Voltage

Section I

Section II

Section III

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Above plot indicates that at low voltage and low temperature difference across hot and cold side leads to high

coefficient of performance. Low temperature difference across hot and cold side can be ensured by reducing the

thermal resistances along the heat flow path. Overall thermal resistance can be reduced by minimizing individual

resistances. Thermal resistances at the source and sink heat exchangers can be reduced by using heat exchange

elements with high thermal conductivity and liberally sizing the heat exchange surfaces. Contact resistances at the

interfaces can be reduced by minimizing bonding/interface material thickness and using high thermal conductivity

bonding material. Optimally sizing the fin structure to minimize weight while ensuring high fin efficiency for

conduction of heat form the heat source/sink to the base of the TE chips is important to ensure cost effectiveness.

Optimal placement for TE chips on the heat exchange surfaces also places an important role in overall optimization.

3. WATER HEATING USING TE HEAT PUMP

Figure 3a shows the cross sectional view of the specially designed aluminum extrusion used to configure the

patented Multi-Utility Heat Pump (Rane et al., 2014). Significantly enhanced fin area helps reduce the thermal

resistances on cold side of TE chips exchanging heat with the air in the conditioned space. Flat region is provided

on one face to accommodate the cold side of TE chip. The small size, 15 mm x 15 mm TEC chips are placed in

staggered arrangement with pitch of 52 mm to help distribute the cooling load. Length of each of the two extruded

heat sources is 500 mm. It provided air side surface area of 0.568 m2/m length of extrusion. Rectangular brass tube

with 20 mm x 5 mm width x 0.5 thickness is used to heat tap water by thermally bonding it to the hot sides of the

TEC chips.

a. Arrangement of Heat Sources, TE chips and Tube b. Experimental Test Setup

Figure 3: Cross Sectional View of Assembly of TE chips, Brass Tube and Heat Sinks

Six chips are electrically connected in series, three from each of the hot water tube, and three such sets are deployed

in the test module. The total of 18 TE chips are deployed to give a nominal heating capacity of 90 W, 5 W/chip x 18

chips. All three sets are electrically connected in parallel. This allows operation of each set at a different

temperature lift. These three sections are marked in Figure 3b. SolidWorks 2015 software package flow simulation

is used to predict the hot and cold side temperature, as well as estimate the rise water temperature in storage tank

and drop in room temperature.

Figure 2b can indicates that when the TE chip is operated with 2.2 V DC at temperature lift, dthc, of 30oC cooling

COPc achievable is 0.9 and cooling wattage is about 5 W. Similarly performance of other chip sets can also be

evaluated.

Section III

Section II

Section I

TE Chip

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3.1 Working Principle Aluminum heat source is used to receive heat from the room, which is to be conditioned. Heat from the room is

utilized to heat the water. Thermoelectric chips are place on the flat surface of brass tube in staggered order. Then,

aluminum heat sources are placed on chips using thermal bonding material. Flow of water is maintained using

natural circulation in vertical brass tube. Cold air drops down as it gets cooled. It operates both flows of fluids in

counter flow manner. Heat from room is added together with electrical energy and pumped into water.

Figure 4: Heat Flow in TE Heat Pump

Heat flow in TE heat pump is shown in Figure 4. It shows that heat flux on hot side is always higher. Hot side heat

is addition of heat from source and electrical energy input. Heat transfer coefficient on water side is about 450

W/m2.K. Surface temperature of hot side depends on heat flux, pitch length and fin efficiency of tube length.

Figure 5: Fin Efficiency of Tube at Pitch along Tube Length

Pitch of 25 mm can be selected based on the efficiency 92.5%, TE chips 20 #/ m of tube that can be used in

TE_MUWH.

3.2 Thermal resistances Test setup is divided into symmetrical about vertical axis through tube center. It is only considered to show points

in heat flow. They are marked in the half symmetrical section with one heat sink. Thermal resistance diagram is

drawn below.

Qc

Pe.i

Qh

TEC

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Thermal resistances in heat flow from cold water to the heat sinks are as follows.

1. Bulk temperature of water

2. Inside surface temperature of tube

3. Outside surface temperature of tube

4. Temperature at tube and TBM contact

5. Temperature at TBM and TE chip contact

6. Temperature at TBM and heat sink contact

7. Outside temperature at heat sink base surface

8. Room air

Figure 6: Thermal Resistances in Heat Flow

Figure 7: Thermal Resistances in Heat Flow from Room Air to Water

Thermal resistances are calculated as below.

Convection resistance due to water flow

It can be found out for known temperature of water and calculated as follows.

Rconv.hw = 1 /(hhw . Ahe.brs.t.i) = (t1 - t2)/Qh …(4)

Here, hhw for laminar flow is 5.35 for uniform heat flux (ASHRAE HBF 2013). Hence, temperature at the surface of

brass tube over which water is flowing can be calculated.

Rconv.hw = 1 /(Øs.brs.t . hhw . Ahe.brs.t.i) = (t1 - t2)/Qc

hhw = Nu . khw / dh.brs.t …(5)

Here, hhw for laminar flow is 5.35 for uniform heat flux (Bejan and Kraus, 2003).

Conduction resistance due to brass tube

It can be found out for known temperature of water and calculated as follows.

Rcond.brs.t = thkbrs.t /(kbrs . Ahe.brs.t.i) = (t2 - t3)/Qc …(6)

Here, kbrs , thermal conductivity of brass material is known. It is 120 W/m.K (ASHRAE HBF 2013). Hence,

temperature at the contact between brass tube and thermal bonding material is calculated.

Conduction resistance due to thermal bonding material

It can be found out for known temperature of water and calculated as follows.

Rcond.tbm = thktbm /(ktbm . Ahe.tec) = (t3 - t4)/Qc …(7)

Here, ktbm , thermal conductivity of bonding material and t4 are unknowns and required to calculate.

Convection resistance due to air

Ambient air temperature is measured. Convention resistance can be found out for known temperature of air,

Rconv.a = 1 /(ha . Ahe.al.hs) = (t7 - t8)/Qh …(8)

ha for air flow over heat sink is known for still/moving air. Temperature t7 is unknown and required to calculate.

Room Air

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Conduction resistance due to aluminium heat sources,

It can be found out for known temperature of water and calculated as follows.

Rcond.brs.t = thkal.hs.b /(kal.hs . Ahe.al.hs) = (t6 - t7)/Qh …(9)

Here, kal , thermal conductivity of aluminium material is known. It is 210 W/m.K. Hence, temperature at the contact

between heat sink and thermal bonding material is calculated.

Conduction resistance due to thermal bonding material

It can be found out for known temperature of water and calculated as follows.

Rcond.tbm = thktbm /(ktbm . Ahe.tec) = (t5 - t6)/Qh …(10)

Here, ktbm , thermal conductivity of bonding material is measured using Tube-Tube Heat Exchanger in previous

year’s experiments and t5 is unknown and required to calculate.

Overall Heat Transfer Coefficient

Cold side heat transfer is calculated as follows,

Uc.ra = Qc.ra / (Ahe.al.hs.o . LMTDcs) …(11)

4. RESULTS AND DISCUSSIONS

Cold and hot side surface temperature of TE heat pump is obtained using the simulation in SolidWorks 2014, as

shown in Figure . It indicates that cold side temperature is almost 19.8oC. Hot side temperature rises from 42.2 to

60.7oC. Overall dthc increases from 22.6oC to 41.1oC. Chips in section I, II and III operates at average 26.8oC, 32.8

oC and 39.1 oC dthc respectively. Weighted average cooling capacity is 4.7 W and COPc is 1.0. It causes the natural

circulation of air. Water get heated from 27oC to 46oC in single pass with natural convection.

Figure 8: Variation in Cold and Hot Side Temperature of TE Heat Pump

Transient analysis of conditioned space is shown in Figure 8, from time 10 s, 100 s, 500 s, 1000 s and 1110 s. Flow

trajectories with fluid temperature vectors shows that at 10 s time, natural circulation of air is just started. About

50% of room space is under the flow of air at 0.09 m/s. Room temperature dropped from 27oC to 19.4oC in 1110 s.

Room temperature causes the changes in density of air and cold air displaces the hot air. Overall heat transfer

coefficient on air side is 9.7 W/m2.K. Velocity vectors indicates that air gets circulated throughout the room within

initial 10 s duration. Experimental cooling COP obtained was 0.92 with 12 V DC supply and 5.4 A current input.

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a. Physical Time 10 s

b. Physical Time 100 s

c. Physical Time 500 s

d. Physical Time 1000 s

e. Physical Time 1110 s

f. Physical Time 1110 s

Figure 9: Temperature Distribution in Conditioned Space at Different Time

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5. CONCLUSIONS

A novel Thermoelectric Multi-Utility Water Heater cum Air-Conditioner, TE_MUWH, provides hot water as

primary utility with air-conditioning as bonus utility. It replaces the storage type 2 kW electric water heater, which

is commonly used in residential buildings. Judicious design of heat source and heat sink as heat exchangers

maintains the hot and cold side temperature difference in the range of 11 to 42oC. It is validated by the simulation

results using SolidWorks flow simulation. The radiant-cum-convective cool surface of the TE_MUWH distributed

the cooling in the house and recover hot water. Thermoelectric Multi-Utility Water Heater cum Air-Conditioner

seems to be a viable option delivering heating COP of 2 while simultaneously offering cooling COP of 1.0. This

reduces energy consumption for residential water heating applications by over 50% and reduce demand by over 95%

when coupled with suitable storage. As it has no moving parts and is expected to operate reliably for 20+ years.

NOMENCLATURE

cp specific heat at constant pressure (kJ/kg.K)

d diameter (m)

dt temperature difference (K)

I current (A)

k thermal conductivity (W/m.K)

N number of thermocouples (#)

P power (kW)

Q heat duty (kW)

T / t temperature (K / oC)

thk thickness (m)

v velocity (m/s)

W power (kW)

Subscripts

a air

av average

c cold/cooling

e electrical

f fin

hw hot water

r room

th thermal

TE Thermoelectric

TBM Thermal Bonding Material

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American Society of Heating Refrigerating and Air conditioning Engineers Handbook – Fundamentals 2013, p 33.3.

Bejan, A and Kraus, D P, 2003, Heat transfer handbook, published by John Wiley & Sons, Inc., Hoboken, New Jersey, pp 994-

1091.

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Goldsmid, H J (1964), “Thermoelectric Refrigeration”, Springer Science and Business Media, New York, p 1-5.

Paul, R (2014), “Scoping Study for Residential Water Heaters Mapping and Benchmarking Project”, Technical Report by Weide

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