Development of a Novel Eco-Friendly

Thermoelectric Air-Conditioning System

Tiasha Joardar ©

Tiasha Joardar ©

5124 Water Haven Lane

Plano, TX 75093

2

Acknowledgements

First I would like to thank Ms. Deanna Shea for her help and guidance with the entire science

fair process. I would like to thank my father for purchasing all the items required for this project,

for his help with constructing some of the apparatus, for allowing me to use the garage and

laundry room, for explaining several electrical and thermal concepts, and for proof reading and

help with formatting this report. All experimental work was done in our home in Plano, Texas.

3

Abstract

A novel thermoelectric air conditioning system is reported that can remove heat without

requiring the use of energy intensive compressors or environmentally harmful

chlorofluorocarbons (CFCs). Thermoelectric cooling units (TECs) operate on the Peltier Effect

and use electricity to pump heat. A TEC based air conditioning unit is demonstrated which

consumes significantly less energy than a vapor compression unit of similar size when used to

maintain a scaled model home at 8°C below outside temperature under a heat load of 150 W/m2.

In addition, a method for optimization of the currents in each stage of a two-stage cascaded

thermoelectric cooling system is developed theoretically and confirmed experimentally for the

first time.

Since commercially available TECs have low Coefficients of Performance (COP),

several innovative steps are taken to overcome this limitation. First, it is recognized from

theoretical analysis that the energy used by TECs decreases exponentially with heat load. By

using a "divide and conquer" approach where the heat load is shared by multiple TEC modules in

a room, the overall energy efficiency of the system is greatly improved. Second, it is also noted

from theoretical analysis that the COP of thermoelectric cooling systems improves significantly

if its hot side temperature is kept as low as possible. This is accomplished by using a water-

cooled heat sink which efficiently removes heat from the module, thereby keeping its hot side at

a temperature no higher than the outside air temperature. The active heat sink is powered by

solar cells, which have no operating cost. Thirdly, theoretical analysis is used to show that

energy efficiency of a thermoelectric system can be increased further by using a cascaded dual-

stage system if the current in each stage is optimized.

Several experiments are conducted the results from which support the theoretical

findings. A scaled model home is constructed for experiments and fitted with a traditional vapor

compression air conditioner unit on one side and a thermoelectric unit on the other. The walls are

insulated using Styrofoam insulation. A resistor bank driven by a variable power supply is placed

inside the model home. This serves as a controllable heat load. Energy used by the vapor

compression and thermoelectric systems as a function of home indoor temperatures is

investigated. It is found that under identical heat load and temperature conditions, it is possible to

obtain up to 40% savings in energy usage using a dual-stage cascaded thermoelectric system by

optimizing the current driven through the TECs.

The findings of this project provide an opportunity to reduce energy usage in homes and

buildings greatly and open up the possibility of providing air conditioning in homes in less

developed areas. Additional efficiency gains are possible by using real time adaptive control on

the current flowing through the units. This aspect will be studied in future.

4

Table of Contents

1. Introduction ………………………………………………………………………………… 7

1.1 Problem Statement………………………………………………………………........ 8

1.2 Fundamentals of Solid State Thermoelectric Cooling ……………………………..... 8

1.3 Review of Prior Work…………………………………………………….………..... 11

2. Theory…………………………………………………..……………………………………12

2.1 Single Stage Thermoelectric Cooling ………………….................................................14

2.2 Two-stage Cascaded Thermoelectric Cooling.……………………………………..…..14

2.3 Design Innovations Based on Theoretical Analysis ………………………..………… 18

2.4 Theoretical Calculations and Hypothesis ……………………………… ……………19

3. Experimental Setup and Procedures …...……………………………………………….. 20

3.1 Variables……………………………………………………..……………………….. 20

3.2 Apparatus Used………………………………………….……………………………. 20

3.3 Measurement Procedures……………………................................................................ 23

4. Experimental Results……………………………………………………………………… 23

4.1 Energy Consumption of Vapor Compression System………………………………... 23

4.2 Energy Consumption of Single Stage Thermoelectric Systems…… …………......25

4.3 Energy Consumption of Two-Stage Cascaded Thermoelectric Systems………………28

5. Discussion………………...………………………………………………………………….29

6. System Cost Comparisons……………….………………………………………………….30

7. Future Work………………………………………………………………………………... 32

8. Conclusions…………………………………………………………………………………. 32

9. References ………………………………………………………………………………….. 33

5

List of Figures

Fig. 1 Simplified diagram of a TEC showing (a) a single junction and (b) an assembled

view of a TEC module.. 9

Fig. 2 Theoretical dependence of optimum COP of a TEC module on its hot

side temperature. 11

Fig. 3 Theoretically calculated plots of indoor temperature, cooling power, and COP as a

function of current passed through a single-stage thermoelectric

air conditioning system. 14

Fig. 4 Diagram showing a two-stage thermoelectric cooling system. 14

Fig. 5 Theoretically calculated variation of cold side temperature TC with currents in the

upper and lower TECs. (a) is a 3-d surface plot and (b) is a contour plot of the data. 16

Fig. 6 3-d surface plot of COP versus IC and IH for the two-stage system described

in this section. 17

Fig. 7 Theoretically calculated TC as a function of input power for TH = 37°C

and Qgen = 4W. 18

Fig. 8 .Diagram showing how an active water cooled heat sink is used to control the

temperature of the hot side of the TEC. 19

Fig. 9 Diagram showing equipment used to monitor energy usage by different

air conditioning units. 21

Fig. 10 (a) Diagram of the single-stage, quad-module thermoelectric cooling system,

(b) diagram of the two-stage cascaded thermoelectric system, and (c) photograph

of fully assembled cascaded thermoelectric cooling unit used in this project. 22

Fig. 11 Energy consumed as a function of time by the vapor compression air conditioner

over a 20 minute duration for three different T settings, each with a heat

load of 24W. 24

Fig. 12 Temperature inside home as a function of time for the vapor compression

air conditioner for different T values. 24

Fig. 13 Summary of energy used over one hour by the vapor compression system as a

function of indoor temperature setting. 25

6

Fig. 14 Energy consumed as a function of time by the single-stage, single-module,

thermoelectric air conditioner over a 20 minute duration for three different T

settings, each with a heat load of 24W. 26

Fig. 15 Temperature inside home as a function of time for the single-stage, single-module,

thermoelectric air conditioner over a 20 minute duration for different T settings. 26

Fig. 16 Measured variation of indoor temperature versus electrical energy used over one

hour by a single-stage single-module thermoelectric system. 27

Fig. 17 Measured variation of indoor temperature (blue) and energy consumed in one

hour versus electrical current input for a single-stage quad-module thermoelectric

system. 28

Fig. 18 Measured variation of indoor temperature versus (a) electrical current IH, and

(b) versus hourly energy use for a dual-stage cascaded thermoelectric system. 29

Fig. 19 Comparison of energy usage of vapor compression and thermoelectric air

conditioning systems as a function of T with a heat load of 24W. 30

Fig. 20 Dependence of purchase price of vapor compression air conditioners on

cooling power found from a market survey (dots). Dashed line represents

estimated dependence of cost per cooling watt for TEC systems based on

current market prices. 31

7

1. Introduction

From an environmental sustainability viewpoint it has become critical that global energy use be

brought under control. One of the largest sources of energy consumption is air conditioning

systems. According to the American Council for an Energy-Efficient Economy, home air

conditioning consumes 5% of the total electricity produced in the US [1, 2]. Energy use from

residential air conditioning is also increasing rapidly in Brazil, China, and India as their

economies expand. Reducing this energy usage will not only provide economic relief to residents

but, on a larger scale, it will help protect our environment by lowering pollution from fossil fuel

based power plants. Further, the refrigerants used in today's air conditioners are

hydrochlorofluorocarbons (HCFCs) which are known to be harmful to the environment.

Therefore, it is worth investigating alternative energy efficient and eco-friendly ways of heat

pumping.

One possibility is the use of the thermoelectric effect. Thermoelectric cooling modules

(TECs) pump heat from a high temperature region to a low temperature region by passing an

electric current through a thermoelectric material. Since this does not require compressors or

refrigerants, and since its cooling power can be electrically controlled, it may be possible to

develop thermoelectric air conditioners without the drawbacks of traditional vapor compression

units. Thermoelectric systems may be most suitable as small room air conditioners that are

popular in Asia, Europe, and South America.

In this project, a thermoelectric based room air conditioning system is developed. Since

the practical use of such a system will depend entirely on how well it compares against a vapor

compression air conditioner, this aspect will be the main theme of this project. The basis for

comparing a thermoelectric and a vapor compression system will be the energy consumed by

each system operating under identical environmental conditions. Energy consumption of an air

conditioning system is normally stated in terms of its Coefficient of Performance (COP) which is

defined as the ratio of the amount of heat energy pumped by the system to the amount of electric

energy required to do so. The higher the COP of a system, the lower its energy consumption

under constant heat loads.

UsedyElectricit

PumpedHeatCOP

_

_ . (1)

8

1.1 Problem Statement

The question addressed in this project can be stated quantitatively as follows.

Is it possible to demonstrate a thermoelectric air conditioning system, using

commercially available parts, that consumes 20% less energy when compared to a

similarly sized conventional vapor compression air conditioning system while

maintaining a scaled model home at 8°C below outside air temperature, with an

internal heat load of 150 W/m2?

The choice of an 8°C temperature difference is explained as follows. The industry standard

conditions for stating the COP or energy efficiency ratings (EER) of air conditioners require an

indoor temperature of 80°F (27°C) and outside temperature of 95°F (35°C), which is a difference

of about 8°C. Regarding the target of 20% reduction in energy use, it is noted that if air

conditioners would consume about 30% less energy than they do today, it would become

practical to operate them off solar panels, a green source of energy [3]. If the objective of this

project can be met, then the results could be a useful step towards controlling and sustaining the

rapid growth in worldwide energy usage. Lastly, regarding the use of 150 W/m2 as internal heat

load, the reasons for this are explained in Section 2.4.

1.2 Fundamentals of Solid State Thermoelectric Cooling

Thermoelectric cooling is based on the Peltier effect. This effect, discovered by Jean-Charles

Peltier in 1834, states that when an electric current is driven through a junction between two

different conductors heat is absorbed or generated, depending on the direction of current flow.

The amount of heat transferred depends on the materials used to form the junction [4, 5].

Extensive research has been conducted in the last few decades to develop materials that exhibit

thermoelectric properties strong enough to be of practical value [6]. Today's commercially

available thermoelectric modules use tightly arranged pellets of bismuth telluride, and are

capable of pumping almost 10 watts/cm2. Fig. 1 shows a simplified diagram of such a TEC

module. The figure on the left, Fig. 1(a), represents a thermoelectric single junction and shows

the flow of electric current and heat. The figure on the right, Fig. 1(b), shows how several of

these thermoelectric junctions are assembled to make a TEC module.

9

Conductor

Semiconductor

pelletp n

Heat sink

HEAT

HEAT

(a)

(b)

Fig. 1. Simplified diagram of a TEC showing (a) a single junction and (b) an assembled view of a TEC module.

The Coefficient of Performance (COP) of TEC modules depends on a "figure of merit" of

the thermoelectric material used. This figure is denoted as ZT and is given as [6]:

K

TSZT

2 , (2)

where is the electrical conductivity of the thermoelectric material, S is its Seebeck coefficient,

is its thermal conductivity, and T is the average temperature of the TEC. For commonly used

bismuth antimony telluride alloys the highest ZT is about 1. New superlattice materials can have

10

ZTs as high as 3.5 but these are not commercially available [7]. The dependence of maximum

achievable COP of a TEC module on ZT is given by [8]:

11

1

maxZT

TT

ZT

TT

TCOP C

H

CH

C , (3)

where TH is the hot side temperature and TC is the cold side temperature in Kelvin (degrees

Celcius + 273). Using equation (3) it is found that for a hot side temperature of 37°C (outside

temperature) and a cold side temperature of 27°C (temperature in home) a maximum COP of

about 5 is possible with bismuth telluride TECs (ZT = 1). In comparison, vapor compression air

conditioners have COPs between 2 and 3. This may suggest that TEC based air conditioners will

almost always consume less energy than vapor compression systems. Unfortunately, there are

additional considerations that pose significant challenges when using TEC based systems.

Fig. 2 shows a plot of COP as a function of the high side temperature TH of a TEC

module, obtained using equation (3). The low side temperature TC is assumed to be 27°C and ZT

is assumed to be 1. It can be seen that the COP drops very quickly as TH increases. For TH larger

than about 45°C the COP drops below 2, the COP of many vapor compression systems of good

quality. In theory, TH of a TEC based air conditioner can be as low as the outside air temperature.

In reality, though, it could be several degrees higher. This is because TH is determined by how

effectively the heat being pumped by the TEC, plus the heat it generates internally, can be

carried away from its hot side. If the heat energy accumulates on the hot side of the TEC, TH will

continue to increase and, as a result, its COP will drop. An effective heat sink is therefore critical

to the success of a TEC based cooling system. Similarly, due to inefficiencies in heat transfer, TC

needs to be a few degrees lower than the desired room temperature. This also leads to lower

COP. Finally, the promisingly high COP values calculated from (3) occur at very low cooling

powers, meaning a large number of TECs will be required to attain reasonable cooling.

11

0

1

2

3

4

5

6

7

8

9

10

30 32.5 35 37.5 40 42.5 45 47.5 50

TEC Hot Side Temperature, TH (C)

CO

P

Fig. 2. Theoretical dependence of optimum COP of a TEC module on its hot side temperature. The red dots are

COPs reported by the US Department of Energy from tests on a high quality Mitsubishi vapor compression air

conditioning system (model FE12NA). Both data sets are at cold side temperature of 27°C [9].

For purposes of comparison, the measured variation of COP with TH of a very high

quality Mitsubishi vapor compression system is also shown in Fig. 2 (red dots). In theory, from

Fig. 2 it can be assumed that it is feasible to design a thermoelectric air conditioning system that

will outperform typical vapor compression systems of similar size on the basis of COP.

1.3 Review of Prior Work

Since Goldsmid first demonstrated in 1954 the possibility of producing large Ts using

semiconductor thermoelectrics like bismuth telluride, there have been many attempts at

developing a commercial thermoelectric unit [10]. Unfortunately these systems were not

commercially successful because they consumed large amounts of energy. The general opinion

appears to be that until thermoelectric materials with high ZT can be developed, vapor

compression systems will be more energy efficient and practical. However, it is also recognized

that thermoelectric cooling is likely to be effective (i) under relatively low heat loads, (ii) where

the heat load shows large variation, and (iii) where large temperature differences are not required

[11]. Since room air conditioning satisfies these conditions there is reason to continue to pursue

the development of a thermoelectric system for such use.

A review of technical literature revealed only one prior technical report on thermoelectric

air conditioning where the installation of a TEC based air conditioner in a studio size room with

12

a heat load of 960 watts is described [12]. A total of 48 TEC modules were required. However,

no quantitative comparison with traditional vapor compression systems is provided.

A major aspect of this work involves the use of two-stage cascaded thermoelectric

modules. Cascaded thermoelectric cooling systems have been studied in the past, but all of the

existing literature is theoretical with almost no experimental confirmation of results [13, 14]. In

addition, the majority of existing work on cascaded thermoelectric cooling systems is centered

on optimizing the geometry ratio of the different stages of the structure. In this project, for the

first time, a method for optimization of the currents in each layer of a two-stage cascaded

thermoelectric cooling system is developed theoretically and confirmed experimentally.

2. Theory

2.1 Single Stage Thermoelectric Cooling

The cooling power at the cold end of a single stage thermoelectric cooling module is given by

[10]:

L

ATK

A

LISITNQ Cp

2

2

12 , (4)

where N is the number of thermoelectric junction pairs, S is the Seebeck coefficient of the

thermoelectric material, I is the current through the thermoelectric module, TC is the cold side

temperature, is the resistivity of the thermoelectric material, A is the cross-sectional area of

each thermoelectric pellet, K is the thermal conductivity of the thermoelectric material, and T is

the temperature difference between the hot and cold sides of the TEC. The first term inside

braces is the heat removed by the Peltier effect. The second and third terms represent,

respectively, the heat injected into the cold side due to the electrical resistance of the TEC and

heat conduction through the TEC from the hot side. These two latter effects tend to reduce the

cooling power of the TEC.

Under steady temperature conditions the above cooling power is balanced by the heat

load, which consists of any internal sources of heat in the room being cooled, plus conduction of

heat through the walls and ceiling of the room. This can be expressed as:

TKQQ Wgenload , (5)

13

where Qgen represents heat generated by any internal or external heat sources such as appliances

or sunlight coming in through windows. KW is the effective thermal conductivity of the walls and

ceiling of the room.

From equations (4) and (5) the temperature in the room being cooled can be solved as:

)}/(2{2

)}/(2{)/( 2

LANKKNSI

TLANKKIALNQT

W

HWgen

C

. (6)

The total rate of energy consumption by the TEC (QTE) is the sum of Qp and the rate at

which heat is generated within the TEC due to its electrical resistance. It is given by [10]:

TSI

A

LINQTE

22 . (7)

Lastly, the COP is given by the ratio of Qp/QTE. Using equations (4) and (7) the COP is

expressed as follows:

A

LITSI

L

ATK

A

LISIT

COPC

2

2

2

1

. (8)

Fig. 3 shows plots of TC, Qp, and COP as a function of current I, as computed from the

equations above. The following values, which are typical of commercially available TECs, were

used for the various quantities in the equations: S = 2x10-4

V/K, K = 3 W/mK, N = 127, =

2.5x10-4

ohm-cm. TH was taken to be 37°C (i.e. 310K) and a 4W internal heat source was

assumed.

It can be seen that the temperature in the room decreases with I and reaches a minimum

value at about I = 6.5 amps. At this current the cooling power of the system reaches its maximum

value of about 20W. If I increases beyond this value the temperature rises as the TEC begins to

lose its ability to pump heat due to increasing resistive heating. The COP is also a function of I,

decreasing rapidly with increasing I. Therefore, from this theory, it is found that in order to

maximize the efficiency of a thermoelectric air conditioner it should be operated at as low a

current as possible. In other words, a high efficiency thermoelectric air conditioner should be

designed to achieve a maximum cooling power much larger than the maximum heat load it is

expected to handle. In the example shown here, in order to maintain an indoor temperature of

27°C (i.e. T = 10°C), a current of 1.3A is needed. At this current, the system's cooling power is

14

about 2W, which is roughly one tenth of its maximum possible value. The associated COP is

about 2.

0

2

4

6

8

0

5

10

15

20

25

30

35

40

45

0 2 4 6 8 10 12

CO

P

Ro

om

Tem

pe

ratu

re, T

c (

C)

&C

oo

lin

g P

ow

er,

Qp

(W

)

Current, I (Amp)

Qp

Tc

COP

Fig. 3. Theoretically calculated plots of indoor temperature (blue line), cooling power (grey, dashed), and COP

(black) as a function of current passed through a single-stage thermoelectric air conditioning system. An indoor heat

load of 4W and an outdoor temperature of 37°C are assumed.

2.2 Two-stage Cascaded Thermoelectric Cooling

In this section a two-stage thermoelectric cooling system is analyzed, and it is shown that such a

system can, when operated optimally, can have a much higher COP than a single stage unit. Fig.

4 shows a diagram of a two stage cascaded thermoelectric cooling module. In this section the

theoretical behavior of such a module is described.

ICIH

TCQgen

QwallTH

Fig. 4 Diagram showing a two-stage thermoelectric cooling system.

The expressions for the various heat flow rates for the structure shown in Fig. 4 are as

follows. Qpc is the heat pumped by the cold side of the upper TEC, Qrc is the heat exiting the

15

bottom surface of the upper TEC, Qph is the heat pumped by the lower TEC, and Qrh is the heat

exiting the bottom surface of the lower TEC.

CMCCCpc TT

L

KA

A

LITSINQ (

2

12 2 , (9)

CMCMCrc TT

L

KA

A

LITSINQ (

2

12 2 , (10)

MHHMHph TT

L

KA

A

LITSIMQ (

2

12 2

, (11)

MHHHHrh TT

L

KA

A

LITSIMQ (

2

12 2

, (12)

where N is the number of thermoelectric pairs in the upper TEC, M is the number of pairs in the

lower TEC, TM is the temperature at the boundary between the upper and lower TECs, IC is the

current through the upper TEC, and IH is the current through the lower TEC. Other symbols have

the same meanings as described in the previous section. By equating Qrc to Qph it is possible to

solve for TM. The result is:

NML

KANIMIS

NTMTL

KA

A

LNI

A

LMI

T

CH

CHCH

M

22

222

. (13)

This expression for TM can be substituted in equation (9) resulting in the following expression for

Qpc:

))(/(22

/422

222

2

NMLKANIMIS

NTMTLKANNIMINKRI

A

LNTNIQ

CH

CHCH

CCCpc

. (14)

Then, by equating this expression for Qph to Qload (equation 5), TC can be solved for. The result

is:

WC

genHWHCHC

CKLKANDKNSNI

QTKDTLKAMNDNIMINKRIA

LN

T

/2/42

//4/2

22

2222

. (15)

Once TC and TM are found, Qpc and Qrh can be evaluated using equations (9) and (12). Finally the

COP of this two-stage cascaded system can be found from:

pcrh

pc

QQ

QCOP

. (16)

16

Fig. 5 shows a 3-d surface and a contour plot of TC as a function of the currents IC and IH

as computed from equation (15). The numerical values of the various quantities were the same as

in the previous section.

0.2

5

1.5

2.7

5

4

5.2

5

6.5

7.7

5

90.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

0.251.5

2.754

5.256.5 Ic (A)

Tc

Ih (A)

(a) 0

.25

1.5

2.7

54

5.2

56

.57

.75

9

0.25 1.25 2.25 3.25 4.25 5.25 6.25

Ih (

A)

Ic (A)

40.0-50.0

30.0-40.0

20.0-30.0

10.0-20.0

0.0-10.0

(b)

Fig. 5. Theoretically calculated variation of cold side temperature TC with currents in the upper and lower TECs. (a)

is a 3-d surface plot and (b) is a contour plot of the data. Outside air temperature of 37°C and a 4W internal heat

source are assumed.

17

As with the single stage case, TC drops as IC and IH increase, reaching a minimum point,

beyond which it begins to increase as resistive heat from the TEC begin to dominate. However,

what is more interesting is that there are now multiple combinations of IC and IH that can result in

the same TC. For example, the blue band on the contour plot of Fig. 5(b) contains all possible

combinations of IC and IH that will result in TC between 20°C and 30°C. It is possible that

amongst all of these possible combinations, some pairs of IC and IH values will result in

significantly higher COP than a single stage system. Fig. 6 shows a 3-d surface plot of the

corresponding COP values as a function of currents IC and IH. As for the single-stage system,

COP drops rapidly with increasing values of current.

A more detailed analysis of COP done using the solver function in Excel shows that

setting IC = 0.92 A and IH = 0.78 A results in the highest COP of 3.72 while maintaining the

internal temperature at 27°C, i.e. 10°C below outside air temperature. This value is considerably

larger than the COP of about 2 that was obtained for a single-stage system under the same

conditions. It should be noted that by optimizing the currents such a thermoelectric air

conditioning system can be operated with its COP constantly maximized, even under varying

values of heat load, TH and TC. Such a control mechanism that adapts with changing thermal

conditions is very complex to implement in vapor compression systems.

0.2

5

0.5

0.7

5

1

1.2

5

1.5

1.7

5

2

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

0.250.5

0.751

1.251.5

1.75

Ic (A)

CO

P

Ih (A)

18.0-20.0

16.0-18.0

14.0-16.0

12.0-14.0

10.0-12.0

8.0-10.0

6.0-8.0

4.0-6.0

2.0-4.0

0.0-2.0

Fig. 6. 3-d surface plot of COP versus IC and IH for the two-stage system described in this section.

18

2.3 Design Innovations Based on Theoretical Analysis

In this section the main takeaways from the theoretical analysis presented above are summarized.

First, it is noted that the electrical power consumed by a thermoelectric cooling system to

maintain a constant temperature difference, T, is strongly dependent on the heat load, Qload.

With decreasing Qload, the power consumption decreases. In reality, the heat load itself cannot be

controlled, but one way to reduce the effective heat load on each TEC module is to use multiple

modules operating simultaneously. Since each module pumps a fraction of the total heat load, the

overall power consumption, and hence the efficiency of the system, is improved, compared to

having a single module handle the total heat load. Fig. 7 illustrates this "divide and conquer"

approach. In this figure the theoretically calculated variation of TC for with input power is plotted

for the single-stage system described in Section 2.1. TH = 37°C and Qgen = 4W are used as

before. Results from two cases are shown, one for a single TEC module and a second with four

modules connected in series. It is seen that for any value of TC the 4-module system requires

much less input power, i.e. has a much higher COP.

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50

Input Power, Pin (W)

Ro

om

Tem

pera

ture

, T

c (

C) 1 TEC Module

4 TEC Modules

Fig. 7. Theoretically calculated TC as a function of input power for TH = 37C and Qgen = 4W.

A second technique to improve the effective COP of a thermoelectric cooling system is

based on that fact that for a given heat load and cold side temperature, its energy consumption

depends on the temperature of it hot side. Using a water-cooled active heat sink it should be

possible to lower the hot side temperature of the TEC module, thus reducing its energy

consumption. Existing thermoelectric cooling units typically use passive heat sinks such as large

19

metal fins with or without a fan attached to them. These approaches are not very effective in

conducting away all the heat pumped by the TEC from its hot surface. Fig. 8 shows the approach

that is used in this project. It consists of placing a metal block with an inlet and an outlet for

water, between the TEC and a finned metal heat sink. The detailed theory of heat conduction by

water cooled heat sinks is complicated and not covered in this study. However, it is known that

water cooled heat sinks can be ten times more effective than air cooled ones [11].

Water in

Water out

Water cooled

heat sink

TEC

Fan

Hot side

Metal fin passive

heak sink

Water in

Water out

Water cooled

heat sink

TEC

Fan

Hot side

Metal fin passive

heak sink

Fig. 8. Diagram showing how an active water cooled heat sink is used to control the temperature of the hot side of

the TEC.

The third technique to improve the effective COP of a thermoelectric cooling system is to

use TEC modules in cascade. As shown in Section 2.2, such a system has, in theory, a much

higher COP than a single-stage system.

2.4 Theoretical Calculations and Hypothesis

In this section a hypothesis is developed from theory for the objective of this project. Typically

15% of a home's floor area is covered with windows. Normal solar intensity can be taken to be

1000 W/m2. Therefore, the peak total solar power expected to enter through windows of a typical

home is 150 W/m2 of floor area. The model home used in this study has a floor area of 40 x 40

cm2. Thus, the solar heat load on such a home can be estimated to be 24W. In order to maintain a

steady temperature inside the home the air conditioner will have to pump out a total of 24 watts.

This assumes that all of the solar energy entering the room is trapped inside the room.

In order to come up with a reasonable hypothesis regarding the project objective that the

TEC system must have a COP that is 20% higher than that of a similar vapor compression unit,

some additional assumptions are needed. A COP of 2 is assumed for a vapor compression air

conditioner. This is typical of small commercially available units. The TEC system must then

20

have a COP of 2.4 or higher. The equations of Section 2 were set up in an Excel calculator to

help design the kind of thermoelectric cooling system that will be required to pump out 24 watts

and the associated COP. It was found that for a single-stage design with one TEC module, it

would require 15W of input power to attain a T of 8°C. This implies a COP of 1.6 which does

not meet the objective. Next, with a single-stage system consisting of 4 serially connected TEC

modules it was found that 8W would be required to attain a T of 8°C, resulting in a COP of 3,

which meets the project objective. Lastly, using a two-stage system, the theoretical calculations

showed that under optimum current combinations, about 4W of input power would be needed to

attain a T of 8°C, which implies a COP of 6. Therefore, based on the theoretical calculations it

is hypothesized that the project objective can be met by using a two-stage design, if it is ensured

that the TEC's hot side temperature is equal to the outside air temperature. It is noted that the

energy consumed by the pump used in the water cooled heat sink is not considered in these

calculations because it expected to be operated on solar power.

3. Experimental Setup and Procedures

3.1 Variables

The independent variables of this project are the type of air conditioning system used,

(thermoelectric / vapor compression). For the thermoelectric system, the number of modules and

number of stages are also independent variables. The dependent variable is the energy consumed

by each type of air conditioning system under identical heat load conditions. The heat load, the

difference maintained by air conditioners between room and outside air temperature, and the

volume of space being cooled are kept constant, i.e. these are the control variables.

3.2 Apparatus Used

A scale model home with outside dimensions of 60cm x 60cm x 60cm is constructed out of foam

board. 10 cm thick insulation is placed on the inside of its walls, floor, and ceiling. A bank of

resistors with an equivalent resistance of 80 ohms is placed in the center of the model home and

connected to a variable ac power source (variac) to act as the heat load. It is assumed that the

electric power consumed by the resistors is equal to the heat load. The heat load can be changed

21

by changing the variac output voltage. For example, when the variac is set to supply 44 volts, a

power output of 24.2 watts results from the resistors. The exact relation between the heat load

and variac output voltage is given as

80

2

acheat

VQ (17)

where Vac is the variac output voltage. Electronic thermometers are placed both inside and

outside the house. The house is fitted with the thermoelectric cooling unit on one side, and with a

small vapor compression unit on the opposite side. Temperatures inside and outside the house, as

well as the energy consumed by the air conditioners are measured and logged electronically. A

Velleman PCS-10 logger is used for recording the temperatures and a "Watts Up Pro" unit is

used for recording the electric energy used by vapor compression unit. The thermoelectric

system is driven by adjustable DC power supplies and its energy usage is found by monitoring

the voltage and current supplied by the power supplies. Fig. 9 shows a diagram of the setup with

the connections for the thermometers, thermostat, and data loggers.

To control the vapor compression unit, a programmable thermostat is used. The

temperatures sensed inside and outside the house are inputted to the thermostat, which turns off

the system when the inside temperature drops to a preset value below the outside temperature.

For the thermoelectric unit, the input electrical power is adjusted manually till the desired room

temperature is attained.

Insulated

walls, roof,

and floor

Thermometer

(outside air)

AC (VC)

Thermometer

(inside air)

Data logger (temperature) Thermostat Data logger (energy)

AC (TE)

Heate

r

VVariac

Insulated

walls, roof,

and floor

Thermometer

(outside air)

AC (VC)

Thermometer

(inside air)

Data logger (temperature) Thermostat Data logger (energy)

AC (TE)

Heate

r

VVVVariac

Fig. 9. Diagram showing equipment used to monitor energy usage by different air conditioning units.

22

Details of the thermoelectric cooling unit are shown in Fig. 10. Fig. 10(a) is a diagram of

the single stage design. Four model TEC modules, each rated at 72 watts maximum cooling

power, are connected in series as shown. They are placed on a water cooled heat sink and held in

place by two 1/8 inch thick aluminum plates. Nylon sleeves and washers are used with the

clamping screws to ensure that the top and bottom plates are thermally insulated from each other.

A fan is attached to the top plate to help circulate cold air (not shown in the diagram to keep

clarity). Fig. 10(b) is shows the design of the two-stage cascaded thermoelectric unit. A fifth

TEC module is used here. It is placed on top of the upper aluminum plate and held in place by

the fan. Fig. 10(c) shows a photograph of the final assembled unit.

Top Al plate

Active heat sink

TEC module

Clamping screw

Bottom Al plate

(a)

2nd stage

TEC module

(b) (c)

Fig. 10. (a) Diagram of the single-stage, quad-module thermoelectric cooling system, (b) diagram of the two-stage

cascaded thermoelectric system, and (c) photograph of fully assembled cascaded thermoelectric cooling unit used in

this project.

23

3.3 Measurement Procedures

The heating resistors and air conditioning unit are turned on and the outside and inside

temperatures are monitored till they become stable. After that the data loggers are turned on and

temperature and energy usage are logged for a period of 20 minutes. After twenty minutes the

data from the loggers are transferred to a computer and saved for analysis later.

4. Experimental Results

4.1 Energy Consumption of Vapor Compression System

In this set of measurements, the amount of energy consumed by a traditional vapor compression

air conditioner to cool the scale model home described previously is measured under different

T. Although the project objectives are for T = 8°C, measurements are done for several T

values to get a more complete picture of the performance of the system around the target

operating conditions.

Fig. 11 shows the power and energy consumed over time by the vapor compression

system under a 24W internal heat load while maintaining an average T of 10°C, 5.9°C, and

0.7°C. The step-like shape seen in the graph of energy versus time is because the air-conditioner

turns off when the home has cooled to the desired temperature and turns back on when the

temperature rises above the set point temperature. Whenever the air conditioner is off, energy

consumption also stops increasing, resulting in the flat portions of the "steps" in Fig. 11.

Fig. 12 shows the temperature variation inside the home as a function of time under the

same conditions as described above. It is seen that the temperature varies above and below some

average value as the thermostat turns the system on and off. The T values used here are based

on the difference between the outside temperature and the average inside temperature over the 20

minute measurement duration (which is the reason why they are somewhat irregular).

24

0

5

10

15

20

25

30

0 2 4 6 8 10 12 14 16 18 20

time (min)

En

erg

y U

sed

(W

h)

DT = 0.7C

DT = 5.9C

DT = 10C

Fig. 11. Energy consumed as a function of time by the vapor compression air conditioner over a 20 minute duration

for three different T settings, each with a heat load of 24W.

8.0

13.0

18.0

23.0

0 2 4 6 8 10 12 14 16 18 20

Time (min)

Tem

pera

ture

(C

)

DT = 0.7C

DT = 5.9C

DT = 10C

Fig. 12. Temperature inside home as a function of time for the vapor compression air conditioner for different T

values.

Fig. 13 is a graph that shows a summary of results from all experiments of this set. The

graph shows electric energy used as a function of indoor temperature for an internal heat load of

24W and an outdoor temperature of 21°C. For convenience, the results obtained from the 20

minute measurements are multiplied by 3 and presented as energy consumed in an hour. The

gray line denotes the best linear fitting to the measured data. As expected, the energy consumed

by the system decreases as T decreases. For an indoor temperature of 13°C, corresponding to a

T of 8°C as required by the project objective, the vapor compression system consumes 70Wh of

energy in one hour.

25

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

4 6 8 10 12 14 16 18 20 22

Temperature in Room (C)

En

erg

y U

sed

in

On

e H

ou

r (W

h)

Qgen = 24W

Fig. 13. Summary of energy used over one hour by the vapor compression system as a function of indoor

temperature setting. Outside temperature = 21°C. The gray line is a linear equation fitted to the measured data. The

vertical dotted line is at 13°C which is the target indoor temperature for energy usage comparison.

4.2 Energy Consumption of Single Stage Thermoelectric Systems

In this set of measurements the energy consumed by single stage thermoelectric air conditioners

to cool the scale model home is measured. As with the vapor compression system, measurements

are done at several indoor temperatures and each measurement is done over a 20 minute time

span. The energy consumed during this period is multiplied by three to estimate the hourly

energy usage of the system.

Fig. 14 shows the energy consumed over time by a single-stage, single-module

thermoelectric system under a 24W internal heat load while maintaining a T of about 10°C,

5°C, and 0°C. Unlike in the vapor compression system an on-off type thermostat is not used in

this case. The current through the system is adjusted till the desired temperature is reached inside

the home. Once the desired temperature is achieved the system runs on its own while the

temperatures and power consumption information are logged. Unlike in the vapor compression

system, the graph of energy versus time is smooth, i.e. not step-like. Fig. 15 shows the

temperature variation inside the home when cooled by a thermoelectric system.

Fig. 16 is a graphical summary of results from all experiments on a single-stage single-

module system. The graph shows measured indoor temperature as a function energy input.

Outdoor temperature is kept constant at 21°C and internal heat load is 24W. The gray lines

26

denote the best linear fitting to the measured data. As expected, as the energy input into the

system increases, the indoor temperature decreases. In theory, the inside temperature should

increase if input power is increased beyond a point, but the system was not pushed to that limit.

[Note: Results from thermoelectric air conditioning systems are plotted differently than those

from vapor compression system because of the difference in how the two systems are controlled.

While in the vapor compression system the desired indoor temperature is set on the thermostat

and the energy usage is observed, in the thermoelectric systems the input current, which is

equivalent to the input power, is set to a certain value and the resulting indoor temperature is

observed. Thus it is more convenient to plot the input power on the x-axes when dealing with

thermoelectric systems.]

0

5

10

15

20

25

30

35

40

0 2 4 6 8 10 12 14 16 18 20

Time (min)

En

erg

y U

sed

(W

h)

DT = 5C

DT = 10C

DT = 0C

Fig. 14. Energy consumed as a function of time by the single-stage, single-module, thermoelectric air conditioner

over a 20 minute duration for three different T settings, each with a heat load of 24W.

8.0

13.0

18.0

23.0

0 2 4 6 8 10 12 14 16 18 20

Time (min)

Tem

pera

ture

(C

)

DT = 5C

DT = 10C

DT = 0C

Fig. 15. Temperature inside home as a function of time for the single-stage, single-module, thermoelectric air

conditioner over a 20 minute duration for different T settings.

27

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

22.0

0.0 20.0 40.0 60.0 80.0 100.0 120.0

Energy Used (Wh)

Tem

pera

ture

in

Ro

om

(C

)

Qgen = 24W

Fig. 16. Measured variation of indoor temperature versus electrical energy used over one hour by a single-stage

single-module thermoelectric system. Outside temperature = 21°C. The gray line is a quadratic equation fitted to the

measured data. The horizontal dotted line at 13°C represents the target indoor temperature.

It is found from the data of Fig. 16 that about 90Wh of energy is consumed by the single-

stage, single-module thermoelectric system to keep the indoor temperature of the model house at

8°C below outside air temperature. This is significantly higher than the 70Wh consumed by the

vapor compression system under the same conditions. Obviously, such a system does not meet

the objectives of this project. However, it was observed in Section 2.3 that, in theory, by using

multiple TEC modules in series, the energy usage of a thermoelectric system can be reduced

significantly as it reduces the heat load on each module. This concept was tested next using four

TEC modules (quad-module) in series.

Fig. 17 shows the summary results obtained from the quad-module, single-stage system

shown previously in Fig. 10(a). It is seen that to maintain an indoor temperature of 13°C, about

0.9A of current needs to be passed through the TEC modules. The energy consumed over one

hour at this current is about 50W. This meets the project objective of developing a

thermoelectric system that uses 20% less energy than a similar vapor compression system under

the same operating conditions.

28

4

6

8

10

12

14

16

18

20

22

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Current, I (A)

Tem

pera

ture

in

Ro

om

(C

)

0

10

20

30

40

50

60

70

80

90

En

erg

y U

sed

(W

h)

Temp in Room

Energy Used

Fig. 17. Measured variation of indoor temperature (blue) and energy consumed in one hour (red) versus electrical

current input for a single-stage quad-module thermoelectric system. Outside temperature = 21°C. The horizontal

dotted line at 13°C represents the target indoor temperature.

4.3 Energy Consumption of Two-Stage Cascaded Thermoelectric Systems

Although the project objective was met with a single-stage, quad-module thermoelectric system,

additional experiments were done using a dual-stage cascaded system to confirm if additional

gains in efficiency would be possible as indicated by theory. The system shown earlier in Fig.

10(b) and 10(c) was assembled and tested under identical conditions of 21°C outside temperature

and 24W heat load.

Fig. 18(a) shows plots of indoor temperature as a function of current IH in the lower layer

of the cascaded arrangement for several values of current IC in the upper layer (IH and IC refer to

the currents shown earlier in Fig. 4). As expected from theory, the plots have a parabolic shape;

once the current exceeds a critical value, the room temperature begins to rise due in increased

resistive losses in the TEC. But, more importantly, it can be seen by following the dotted line

that the target temperature of 13°C can be obtained by several possible combinations of IC and

IH.

Fig. 18(b) shows the associated temperature versus energy consumption data. The shapes

of the plots are as expected, and, again, it is seen that the target temperature can be attained by

multiple combinations of IC and IH. From the available data, the minimum energy required to

obtain the target 13C inside temperature is at IC = 0.5A and IH = 0.8A. At these currents the

29

hourly energy used is about 40Wh. This represents a 40% lower energy usage than the vapor

compression system.

4

6

8

10

12

14

16

18

20

22

0 0.5 1 1.5 2 2.5

Current Ih (A)

Tem

pera

ture

in

Ro

om

, T

c (

C)

Ic = 0.25A

Ic = 0.5A

Ic = 1A

Ic = 2A

(a)

4

6

8

10

12

14

16

18

20

22

0 40 80 120 160 200 240 280 320 360

Energy Used (Wh)

Tem

pera

ture

in

Ro

om

, T

c (

C)

Ic = 0.25A

Ic = 0.5A

Ic = 1A

Ic = 2A

(b)

Fig. 18. Measured variation of indoor temperature versus (a) electrical current IH, and (b) versus hourly energy use

for a dual-stage cascaded thermoelectric system. Outside temperature = 21°C. The horizontal dotted line at 13°C

represents the target indoor temperature.

5. Discussion

It was seen in the previous section that a dual-stage cascaded thermoelectric air conditioning

system can be significantly more energy efficient than a similarly sized vapor compression

30

system. The experimentally observed behaviors showed trends similar to what was predicted by

theory, but a closer comparison would be useful in validating the results.

Fig. 19 is a comparison of theoretical and experimentally measured values of indoor

temperature as a function of energy input for the dual-stage thermoelectric system. The

experimental data are the same as shown in Fig. 18 and the theoretical data is obtained from the

equations of section 2.2 with the same inputs as used in the experiments. It can be seen that

although there is reasonable similarity in trends between experiment and theory, there is a large

difference between the numerical values. Two main reasons for this are (a) while the theory is

based on 1-d cascade structure and heat flow, in reality it is a 3-d effect, and (b) while the theory

assumed that the TEC cold side is at the same temperature as the indoor air, in reality the air

temperature is likely to be significantly higher.

0

5

10

15

20

25

0 25 50 75 100 125 150

Energy Used (Wh)

Tem

pera

ture

in

Ro

om

(C

) Theory (Ic = 0.5A)

Expt (Ic = 0.5A)

Theory Ic = 1A

Expt Ic = 1A

Fig. 19. Comparison of energy usage of vapor compression and thermoelectric air conditioning systems as a

function of T with a heat load of 24W.

6. System Cost Comparisons

In real-world use, cost is a very important factor. Even though it is found that thermoelectric air

conditioners used with IR filtering and active heat sinks can be made to consume less energy and

are therefore less expensive to operate, if the cost of purchasing and installing such a system is

high then it is unlikely to be accepted widely.

Table 2 is a summary of the costs involved in the two types of systems considered in this

project. It is seen that the thermoelectric system is about 40% less expensive than the vapor

compression system.

The system costs given above are for a very small model home. To be of practical value it

is important to consider how the costs mentioned above would increase as the system is scaled to

31

a larger size cooling area. This is discussed next. The scope is limited to window mounted room

air conditioners. The heat pumping power of commercially available vapor compression window

air conditioners range from 1500 W to 7500 W. Purchase prices of several of these units are

obtained from a market survey. Fig. 18 shows the dependence of purchase price on the cooling

power. It is observed that the price increases roughly linearly at the rate of 8.5 cents per watt. A

similar cost per watt of cooling power can be estimated for TEC based systems as follows. At

wholesale rates, TEC modules such as the one used in this project cost $3 each. These modules

have an optimum heat pumping power of about 20 watts. The cost of the active heat sink used in

this project is mainly due to its motor. Since the motor is large enough to cool 100 TEC modules

the heat sink cost can be reduced to $0.2 per TEC. From these assumptions it is estimated that at

current market prices for TEC modules, practical TEC based air conditioners will cost about 16

to 20 cents per watt of cooling power.

Table 2. Comparison on system costs.

Vapor Compression Air Conditioner Thermoelectric Air Conditioner

A/C unit $55.00 TEC module $5 x 4

Water cooled heat sink $20 Passive heat sink $2

TOTAL $55.00 TOTAL $42.00

0

200

400

600

800

1000

1200

0 1000 2000 3000 4000 5000 6000 7000 8000

Cooling Power Qp (W)

Co

st

($)

Fig. 20. Dependence of purchase price of vapor compression air conditioners on cooling power found from a market

survey (dots). Dashed line represents estimated dependence of cost per cooling watt for TEC systems based on

current market prices.

32

7. Future Work

The primary advantage of a thermoelectric air conditioning system is that its cooling power can

be adapted to the existing heat load and temperatures by controlling the current through the

modules. In case of a dual stage cascaded system there is even more flexibility because the

current in each stage can be varied separately. One of the main items that will be addressed in

future is a control system that can automatically take into consideration the existing heat load

and temperatures and set the currents such that energy efficiency of the system is maximized.

8. Conclusions

In conclusion, a novel thermoelectric based air conditioning system was demonstrated using

commercially available components. The objectives of the project were successfully met. It was

found that with a dual-stage cascaded design it is possible to construct a thermoelectric air

conditioner that is significantly more energy efficient than a similarly sized vapor compression

system. At current market prices the purchase cost of a practical thermoelectric air conditioner is

expected to be a little more than a comparable vapor compression unit.

33

9. References

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conditioners, 2012 [Oct. 01, 2012].

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Buildings," International Building Performance Simulation Association SimBuild Conference.

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4. Rowe, D. M., Editor, CRC Handbook of Thermoelectrics, CRC Press, 1995

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Pumps," Internet: www.nrel.gov/docs/fy11osti/52175.pdf, Sep. 2011 [Oct. 01, 2012].

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British Journal of Applied Physics, vol. 5, pp. 386 - 390, 1954.

11. Brown, D. R., Dirks, J. A., Fernandez, N., Stout, T. B., " Prospects of Alternatives to Vapor

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2012].

12. A.Melero, D.Astrain, J.G.Viin, L.Aldave, J.Albizua, and C.Costa, "Application of Thermoelectricity

and Photovoltaic Energy to Air Conditioning," Proc. 22nd International Conference on

Thermoelectrics, pp. 627-630, 2003.

13. H. Lai, Y. Pan, and J. Chen, "Optimum Design on the Performance Parameters of a Two-Stage

Combined Semiconductor Thermoelectric Heat Pump," Semiconductor Sci. Technol., vol. 19, pp. 17

– 22, 2004.

34

14. M. Olivares-Robles, F. Vazquez, and C. Ramirez-Lopez, "Optimization of Two-Stage Peltier

Modules: Structure and Exergetic Efficiency," Entropy, vol. 14, pp. 1539 – 1552, 2012.

15. R.E. Simons and R. C. Chu, "Application of Thermoelectric Cooling to Electronic Equipment: A

Review and Analysis," Proc. 16th IEEE SEMI-THERM Symposium, pp. 1-4, 2000.

16. Karim, O., Creiber, J. C., Gillot, C., Schaffer, C., Mallet, B., Gimet, E., "Heat Transfer Coefficient for

Water Cooled Heat Sink: Application for Standard Power Modules Cooling at High Temperature,"

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17. O'Brien, B. J., Wallace, C. S., Landecker, K., "Cascading of Peltier Couples for Thermoelectric

Cooling," J. Applied Physics, 27, pp. 820 - 824, 1956.