Experimental validation of the optimum design of automotive air

Experimental validation of the optimum design of automotive air-to-air
thermoelectric air conditioner (TEAC)
Alaa Attar [1, 2, 3]
HoSung Lee [1]
Sean Weera [1]
1. Department of Mechanical and Aeronautical Engineering, Western Michigan University, Kalamazoo, MI
49008-5343, USA. 2. Department of Mechanical Engineering, King Abdulaziz University, Rabigh, Saudi
Arabia. 3. —email: alaa.m.attar@wmich.edu
0
Abstract
The optimization of thermoelectric air conditioners (TEAC) has been a challenging topic due to the
multitude of variables needed to be considered. The present work discusses the experimental validation of
the optimum design for automotive air-to-air TEAC. The TEAC optimum design was obtained by using a
new optimal design method with dimensional analysis that has been recently developed. The design
constraints were obtained through a previous analytically study on the same topic. In order to simplify the
problem, a unit cell that represents the entire TEAC system was analytically simulated and was
experimentally tested. Moreover, commercial TEC modules and heat sinks were selected and tested based
on the analytical optimum design results.
Key words: Thermoelectric air conditioner; automotive thermoelectric cooling; thermoelectric automobile
application.
1
Nomenclature
Ae
Ab
Ac
Ah
AUC
COP
cp
𝐺𝑒
H
h
I
L
Lc
Le
Lh
k
cross-sectional area of thermoelement (mm2)
total base area of thermoelectric air conditioner
(mm2)
total fin surface area cold side heat sink (mm2)
T∞c
T∞h
cold fluid temperature (°C)
hot fluid temperature (°C)
∆T
thermoelectric temperature different (∆°C)
total fin surface area hot side heat sink (mm2)
unit cell base area (mm2)
the coefficient of performance
specific heat (J/kg.K)
thermocouple geometric ratio
total height of thermoelectric air conditioner
(mm)
heat transfer coefficient of the fluid (W/m2K)
∆Tcooling cold air temperature inlet – outlet (∆°C)
tc
cold side air fin thickness
th
hot side air fin thickness
Vh
hot fluid volume flow rate (L/min for liquid) or
(CFM for air)
electric current (A)
total length of thermoelectric air conditioner
(mm)
unit cell cold side length (mm)
length of thermoelement (mm)
unit cell hot side length (mm)
thermoelement thermal conductivity (W/m K),
k = kp + kn
W
Z
zc
zh
total width of thermoelectric air conditioner
(mm)
the figure of merit (1/K) = α2 /ρk
fin spacing for the cold side air (mm)
fin spacing for the hot side air (mm)
Greek symbols
α
Seebeck coefficient (V/K), α = αp − αn
ρ
electrical resistivity (Ω cm), ρ = ρp + ρn
𝜑
alimunum block thermal resistance (K/W)
n
the number of thermocouples
ηc
fin efficiency of cold side heat sink
nc
number of fins for the cold side heat sink
ηh
fin efficiency of hot side heat sink
nh
Nk
PD
number of fins for the cold side heat sink
dimensionless thermal conductance, Nk =
n(Ae k/Le )/ηh hh Ah
dimensionless convection, Nh = ηc hc Ac /ηh hh Ah
dimensionless current, NI = αI/(Ae k/Le )
total cooling power from thermoelectric air
conditioner (W)
total input power for from thermoelectric air
conditioner (W)
power density (W/cm2)
R
Re
Nh
NI
Qc
Pin
Subscripts
c
e
h
cold
thermoelement
hot
p
p-type element
n
n-type element
electrical resistance of a thermocouple (Ω)
m
measured
fluid Reynolds’s number
opt.
optimal quantity
Tc
cold junction temperature (°C)
UC
unit cell
Th
hot junction temperature (°C)
∗
dimensionless
2
Introduction
Most automobile air conditioners use refrigerant R-134a, which does not have ozone-depleting
properties like Freon, but is nevertheless a terrible greenhouse gas [1]. Soon enough, this refrigerant will
be banned which means alternative air-conditioning technology is needed [1]. In 2009, the U.S. Department
of Energy (DOE) and the California Energy Commission funded a project to improve the air conditioning
(AC) system of vehicle by developing thermoelectric heating ventilation and air conditioner (TE HVAC)
system which would replace the current conventional AC system [2].
Using a thermoelectric air
conditioning (TEAC) system instead of a conventional AC system has two main benefits: it will eliminate
the need of R-134a and it will provide the ability to cool selected zones instead of the entire cabin which in
turn will reduce fuel consumption [3]. On average, 73% of a vehicle’s mileage occurs when the driver is
the only occupant and it is estimated that the total cooling power required to cool the zone of a single
occupant is around 630 W while 3.5 to 4.5 kW is needed to cool the entire cabin [2]. The goal of the DOE
project was to build a TEAC device that could provide the needed 630 W of cooling power for a single
occupant with a coefficient of performance (COP) of 1.3 or higher [4].
Several studies have been conducted on automotive TEAC systems in order to test their
performance and feasibility. Junior et al. [5] compared a thermoelectric liquid-gas heat exchanger system
used for steady state and transient simulation models with the conventional auto HVAC system. For
ambient temperatures of 25°C and 30°C, the conventional auto HVAC system has cooling capacity of five
times higher than the thermoelectric HVAC system at the same input power [5]. Wang et al. [6] designed
and analyzed an air-to-liquid thermoelectric HVAC system for a passenger vehicle using a numerical
model. They also constructed an experiment to validate their model that was able to reach a COP of 1.55
at a cooling power of 1.55 kW with the same air and liquid inlet temperatures of 30°C [6]. The use of the
thermal isolation method allowed COP improvement and determination of the fluid and junction
temperatures [6]. Raut and Walke [7] constructed and tested a thermoelectric cooler system on a small
passenger vehicle where the goal was to remove 222 W of heat from the cabin. They used six TEC modules
3
(each module has a maximum cooling power of 48 W) connected electrically in series and sandwiched
between two heat sinks. Their results showed that the system was able to reduce the cabin temperature to
as low as 4°C [7]. Hsu et al. [8] studied and tested an air-to-liquid TEAC. After optimizing the heat sink,
they analyzed the effects of the figure of merit and thermoelement thickness on the system COP and cooling
performance. They stated that the change of the element thickness can improve the cooling performance
but not the COP. They built and tested an air-to-liquid TEAC installed in a vehicle (Honda Civic Exi). The
experimental results showed that the TEAC performance curves and cooling power trends followed the
results from the basic equations.
The Ford Motor Company, in collaboration with Gentherm, presented their design of a TEAC along
with a performance curve in the 2012 Directions in Engine-Efficiency and Emissions Research (DEER)
Conference [9]. They tested an air-to-liquid TEAC system where it was able to reach a COP of 1.3 at an
input power of 400 W using a cold air flow rate of 60 CFM. They decided on using liquid instead of air
for the hot (waste) side fluid because of many advantages such as higher heat transfer coefficients, higher
power density, and lower operating noise [10]. However, there are significant considerations to account
for such as air-to-liquid coolant leakage as well as the complexity of an additional radiator system [10].
Table I highlights some of the pros and cons for air-to-air vs. air-to-liquid TEAC systems. Attar et al. [11]
applied an optimal design method developed by Lee [12] to design air-to-liquid TEAC based on Gentherm
design and they were able to obtain a COP of 1.68 at the same input power. This optimal design method
used the dimensional analysis in order to optimize current supplied and the geometric ratio of
thermoelement (or number of thermoelement couples) simultaneously for a given set of fixed parameters.
More details of this method are discussed in the next section. Moreover, with use of this optimal design
method, Attar et al. [11] also designed air-to-air TEAC system that has a COP of 1.3 at the same input
power (400W). This performance was obtained from designing a unit cell that can simulate the whole
system in order to simplify the problem. Even though this work is showing the optimized design, it is still
considered to be analytical work and many uncertainties may occur. Therefore, the present work discusses
the experimental validation of this unit cell of air-to-air TEAC design.
4
Table I Comparison between air-to-air TEAC and air-to-liquid TEAC [10]
Air Waste Stream
Pros
Liquid Waste Stream
Cons
Pros
Cons
Low weight
Poor heat transfer
Higher power density
More weight
No risk of coolant leaks
Lower power density
Less noise
Risk of leaks
Difficult to vent the
Requires an additional
waste heat
radiator
Waste side temp tied to
Noise at higher flow rate
ambient
Background
In the previous work [11], the air-to-air TEAC model was built and analytically optimized. The
cold air heat sink is sandwiched between two layers of the thermoelectric modules while two layers of hot
air heat sinks are attached to the hot sides of the thermoelectric modules as shown in Fig. 1a. The optimum
design model was built based on a unit cell (Fig. 1b) located at the center of the TEAC system. The unit
cell ambient cold and hot fluids temperatures, T∞c and T∞h, are calculated by assuming linear change of
the cold and hot temperatures along the TEAC system. The design of the heat sinks were optimized (using
optimization method found in Lee [13]) to give the minimum thermal resistances at a provided flow.
Moreover, the Nusselt number correlation devolved by [14] is used to calculate the heat transfer coefficients
for both cold and hot flows.
5
Fig. 1. (a) Schematic of the air-to-air TEAC and (b) unit cell schematic.
6
It is found that in order to obtain the optimum design for the thermoelectric cooling system to
maximize the cooling power 𝑄̇𝑐 , the electrical current I and the thermocouple geometric ratio (𝐺𝑒 = 𝐴𝑒 /𝐿𝑒 )
must be optimized simultaneously. Therefore, the optimum design method using the dimensional analysis
technique that was developed by Lee [12] is used. In this method, dimensionless numbers were defined
under the assumption that the electrical and thermal contact resistances in the TEC are negligible, the
material properties are independent of temperature, the TEC is perfectly insulated, and the p-type and ntype element dimensions are identical. This method converts the four basic heat balance equations (Eqns.
1 to Eqn. 4) into two non-dimensional equations (Eqns. 6 and 7). Fig. 2 (a) and (b) show schematics of a
thermoelectric module with heat sinks and thermoelectric couple, respectively.
Moreover, the two
aluminum blocks, which are sandwiched between the cold and hot sides of the TEC and their respective
heat sinks, needed to be considered in the analysis where 𝜑 is thermal resistance of each block and it is
equal to 0.149 K/W.
7
Fig. 2 (a) thermoelectric cooler module (TEC) with two heat sinks, (b) schematic of thermoelectric
couple.
𝑄̇𝑐 = 𝜂𝑐 ℎ𝑐 𝐴𝑐 [𝑇∞𝑐 − (𝜑𝑄𝑐 − 𝑇𝑐 )]
(1)
1
𝐴𝑒
𝑄̇𝑐 = 𝑛 [𝛼𝐼𝑇𝑐 − 𝐼 2 𝑅 + 𝑘(𝑇𝑐 − 𝑇ℎ )]
2
𝐿𝑒
(2)
1
𝐴𝑒
𝑄̇ℎ = 𝑛 [𝛼𝐼𝑇ℎ + 𝐼 2 𝑅 + 𝑘(𝑇𝑐 − 𝑇ℎ )]
2
𝐿𝑒
(3)
𝑄̇ℎ = 𝜂ℎ ℎℎ 𝐴ℎ [(𝑇ℎ − 𝜑𝑄ℎ ) − 𝑇∞ℎ ]
(4)
𝑃𝑖𝑛 = 𝑄̇ℎ − 𝑄̇𝑐
(5)
8
where 𝜂𝑐 is the fin efficiency, ℎ𝑐 is the convection coefficient, and 𝐴𝑐 is the total surface area of the cold
heat sink.
𝑁ℎ (𝑇∞∗ − 𝑇𝑐∗ )
𝑁𝐼2
∗
(𝜂
= 𝑐 ℎ𝑐 𝐴𝑐 𝜑 + 1) [𝑁𝐼 𝑇𝑐 −
+ (𝑇𝑐∗ − 𝑇ℎ∗ )]
𝑁𝑘
2𝑍𝑇∞ℎ
(6)
(𝑇ℎ∗ − 1)
𝑁𝐼2
= (𝜂ℎ ℎℎ 𝐴ℎ 𝜑 + 1) [𝑁𝐼 𝑇ℎ∗ −
+ (𝑇𝑐∗ − 𝑇ℎ∗ )]
𝑁𝑘
2𝑍𝑇∞ℎ
(7)
ZT∞h , Nh , Nk , and NI are defined as the dimensionless figure of merit, convection ratio, the ratio
of thermal conductance to the convection conductance, and dimensionless current, respectively, and can be
defined as
𝛼2
=
𝑇
𝜌𝑘 ∞ℎ
(8)
𝜂𝑐 ℎ𝑐 𝐴𝑐
𝜂ℎ ℎℎ 𝐴ℎ
(9)
𝑛(𝐴𝑒 𝑘/𝐿𝑒 )
𝜂ℎ ℎℎ 𝐴ℎ
(10)
𝛼𝐼
𝐴𝑒 𝑘/𝐿𝑒
(11)
𝑍𝑇∞ℎ
𝑁ℎ =
𝑁𝑘 =
𝑁𝐼 =
∗
Tc∗, Th∗ and T∞
are the dimensionless cold junction temperature, the dimensionless hot junction temperature,
and the fluid temperature ratio, respectively, and are defined as
𝑇𝑐∗ =
𝑇𝑐
𝑇∞ℎ
𝑇ℎ∗ =
𝑇ℎ
𝑇∞ℎ
(12)
(13)
9
𝑇∞∗ =
𝑇∞𝑐
𝑇∞ℎ
(14)
the dimensionless temperatures are then a function of five independent dimensionless parameters as
𝑇𝑐∗ = 𝑓(𝑁𝑘 , 𝑁ℎ , 𝑁𝐼 , 𝑇∞∗ , 𝑍𝑇∞ℎ )
(15)
𝑇ℎ∗ = 𝑓(𝑁𝑘 , 𝑁ℎ , 𝑁𝐼 , 𝑇∞∗ , 𝑍𝑇∞ℎ )
(16)
setting 𝑍𝑇∞ℎ , 𝑇∞∗ and 𝑁ℎ to be the inputs, the dimensionless parameters 𝑁𝑘 and 𝑁𝐼 can be optimized to
solve Eqns. 6 and 7 for the maximum cooling power. The design requirements were to have an input power
of 400W and COP of 1.3 (or input power of 4.5W and COP of 1.3 for the unit cell). Therefore, for the
current experimental analysis, the input power is fixed at 4.5W and 𝐶𝑂𝑃 can be maximized.
Experimental Setup
In order to conduct an experiment that can simulate the unit cell of air-to-air TEAC system, a
commercial thermoelectric cooler module is sandwiched between two heat sinks (for hot and cold air). The
selection of the TEC and the heat sink were based on the optimized analytical design that was built
previously. Since the optimum heat sinks are not commercially available, closer heat sinks were selected.
Therefore, two heat sinks ALPAH UB30-20B and ALPAH UB30-25B were used for the cold and hot sides,
respectively. Fig. 3a shows the overall experimental setup while Fig. 3b shows the details of the test section.
Moreover, two aluminum blocks (30 × 30 × 19.1 𝑚𝑚3 ) were fabricated and inserted between the TEC
module and each of the heat sink. Two parallel (5mm apart) K-type thermocouples were drilled to the
center of each block where the average hot and cold blocks temperatures occurred. Moreover, the cold and
hot airs were driven by variable speed centrifugal blowers where a temperature bath controller and a heater
are used to control the inlet cold and hot air temperatures, respectively. The air speed was obtained by
measuring the dynamic pressure (the difference between the total pressure and static pressure) using a pitot
tube that was connected to a manometer. The blowers were set to have mass flow rates for cold and hot air
to be 3.21CFM and 6.1CFM, respectively. E-type thermocouples were installed at the air inlet and exit for
10
both cold and hot air in order to measure the air temperatures (𝑇∞𝑐,𝑖𝑛 , 𝑇∞𝑐,𝑜𝑢𝑡 , 𝑇∞ℎ,𝑖𝑛 , and 𝑇∞ℎ,𝑜𝑢𝑡 ) so that
1
the average ambient temperature at the heat sinks could be averaged such that 𝑇∞𝑐 = (𝑇∞𝑐,𝑖𝑛 + 𝑇∞𝑐,𝑜𝑢𝑡 )
2
1
and 𝑇∞ℎ = 2 (𝑇∞ℎ,𝑖𝑛 + 𝑇∞ℎ,𝑜𝑢𝑡 ). On the other hand, the TEC input power was controlled by a variable
DC power supply which allowed controlling the input voltage.
From the analytical model at the required unit cell input power (𝑃𝑖𝑛,𝑈𝐶 = 𝐼𝑚 × 𝑉𝑖𝑛 = 4.5𝑊), the
average ambient cold and hot temperatures were required to be at 21.6 oC and 33.6 oC, respectively.
Therefore, the TEC supplied voltage, the cold air inlet temperature 𝑇∞𝑐,𝑖𝑛 , and the hot air inlet
temperature 𝑇∞ℎ,𝑖𝑛 were adjusted accordingly until the input power and the average ambient temperatures
match the above values. After that, the measurements were taken under steady state conditions for each
input voltage (with increment of 1V) until reaching maximum voltage, 𝑉𝑚𝑎𝑥 provided by the manufacturer
as shown in Fig. 4.
The objective was to measure the cooling power 𝑄𝑐 , the heat rejection 𝑄ℎ , the cold junction
temperature 𝑇𝑐 and the hot junction temperatures 𝑇ℎ . 𝑇𝑐 and 𝑇ℎ can be obtained by extrapolating the two
measured temperatures of each block (𝑇𝑐1 & 𝑇𝑐2 for cold side and 𝑇ℎ1 & 𝑇ℎ2 for hot side) assuming the
temperatures change linearly across the aluminum blocks. For 𝑄𝑐 and 𝑄ℎ , the thermoelectric ideal Eqs.
2 and 3 were used where the electrical current and junction temperatures are experimentally obtained and
thermoelectric effective material properties are used for the values of 𝛼, 𝜌, and 𝑘 [15]. The effective
material properties technique is based on the maximum thermoelectric module parameters (typically
measurements), which were provided by the manufacturer, in order to calculate the material properties.
This technique enables to reduce the errors associated with the assumption of neglecting the contact
resistances and to provide more practical module properties.
The test was done for three TEC modules (module 1: Tellurex C2-30-1503, module 2: Tellurex C2-300904, and module 3: Marlow RC12-4) in order to explore the effect of 𝑁𝑘 on the TEAC performance. All
three modules had the same base area (30 × 30 𝑚𝑚2 ) but different number of couples and/or
thermoelement geometric ratio, 𝐺𝑒 .
11
Fig. 3 (a) Experimental setup, (b) test section
12
Fig. 4 Flowchart of the experimental procedure
Results & Discussion
A comparison has been made between experimental and analytical junction temperatures where the
cold and hot ambient temperatures, electrical current, and air flow rates were the inputs. Then, the
experimental junction temperatures were obtained by extrapolating the temperature readings from the
aluminum blocks while the analytical junction temperatures were obtained by using the four basic heat
13
balance equations (Eqs. 1, 2, 3, and 4). The results show very good agreements as shown in Fig. 5. From
these junction temperatures, the TEAC experimental performances were compared with the predicted
results for the three modules. Fig. 6 also shows a good agreement between experimental and analytical
𝐶𝑂𝑃 for the three tested modules. These results shown in Figs 5 and 6 indicate that the basic heat balance
equations with the effective material properties predict very well the measurements.
(a)
(b)
(c)
Fig. 5 Comparison between experimental and analytical junction temperatures vs. input current for
(a) module 1, (b) module 2, and (c) module 3
(a)
(b)
(c)
14
Fig. 6 Comparison between experimental and analytical COP vs. input power for (a) module 1, (b)
module 2, and (c) module 3
The results from the modified analytical model show the maximum possible 𝐶𝑂𝑃, at given
conditions, and it is equal to 1.09. This result came from the optimization of the two dimensionless values
𝑁𝑘 and 𝑁𝐼 simultaneously. Moreover, the values of 𝑁𝑘 and 𝑁𝐼 for the TEC modules number 1, 2, and 3
are obtained by using Eqs. 10, and 11 where the effective material properties is also been applied. Table II
shows a comparison between the three modules’ performances at the same input power (𝑃𝑖𝑛 = 4.5𝑊).
Among the three modules, module number 2 has the closest values of 𝑁𝑘 and 𝑁𝐼 to the optimum analytical
model to give the maximum COP at a given conditions. The table also shows that the performances of the
modules are degraded by the presence of the aluminum blocks. In addition, if the optimum heat sinks
(found in [11]) are used instead of the commercial ones, the system will converge to the optimum design
at the given conditions as shown in the last row of the table.
Table II Comparison between the three tested modules, optimized design without the aluminum
blocks, and the optimum design using the optimized heat sink
Module
1
2
3
Analytical model
w/o blocks
𝑵𝑰
𝑰
(Amp)
𝑵𝒌
𝒏 × 𝑮𝒆
(𝒄𝒎)
𝑷𝒊𝒏 (W)
COP
0.162
0.92
0.258
9.22
4.5
1.044
0.199
1.42
0.18
7.08
4.5
1.085
0.155
0.96
0.28
11.42
4.5
1.025
0.204
1.46
0.172
6.77
4.5
1.09
0.219
2.9
0.154
11.21
4.5
1.16
2.98
0.119
17.94
4.5
1.38
w/o blocks & w/ 0.189
optimal heat sinks
15
One of the experiment goals was to study the effect of the thermoelement geometric ratio (or
thermoelement number of couples) at the optimum input electrical current. This can be analyzed by
studying the ratio of thermal conductance to the convection conductance, Nk , and its relationship with the
optimum design. Testing three different modules validated the goal and gave the ability to view the closest
module to the optimum design. Fig. 1 shows a prediction and comparison of COP vs. 𝑁𝑘 between the three
modules at fixed input power (𝑃𝑖𝑛 = 4.5𝑊). The predicted curve was obtained by fixing 𝑃𝑖𝑛 and assuming
a constant temperature difference at the junctions (for a short range) and then resolved for 𝑁𝐼 to be only as
a function of 𝑁𝑘 . This assumption allows expressing COP independently from 𝑁𝐼 for the range where all
of the three tested modules can be included. It can be seen from the figure that module 2 is the best module
among three at the given conditions because it has the closest values of 𝑁𝑘 and 𝑁𝐼 to the analytical optimal
design.
Fig. 7 COP vs. Nk at Pin = 4.5W
Conclusion
16
The present work discusses the experimental validation of the optimum design for automotive airto-air TEAC. The TEAC optimum design was obtained in a previous work [11] by using a new optimal
design method with dimensional analysis that has been recently developed by Lee [12]. The dimensional
analysis method obtains the maximum cooling power by simultaneously determining the dimensionless
current supplied NI and the ratio of the thermal conductance to the convection conductance Nk for a given
set of fixed parameters. In order to simplify the problem, a unit cell of the TEAC system was used instead,
which is considered to be located at the center of the whole TEAC system.
The experimental set up was designed to obtain the TEC module junction temperatures by
extrapolating two temperatures from thermocouples inserted into an aluminum block sandwiched between
the heat sink and the module. The results from the experiments showed a good agreement with the
analytical model that uses the four thermoelectric basic equations. These basic equations were solved at
the same input parameters from the experiment and by using the effective material properties [15] instead
of the intrinsic properties. Moreover, three TEC modules were tested in order to study the optimum
𝑁𝑘 value in addition to the optimized current. Therefore, module number 2 was the optimum module for
the given conditions.
Even though the use of the aluminum blocks was a necessity, they add extra thermal resistances
which act negatively on the TEAC performance. The use of commercial heat sinks instead of the optimized
designs also limits the reach of the optimum design of the air-to-air TEAC. Once these two constrains are
removed, the optimum design can be obtained with 20% improvement from module 2 performance.
17
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