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. โemail: [email protected] 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 ๐บ๐ 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) Tโc Tโh cold fluid temperature (°C) hot fluid temperature (°C) โT thermoelectric temperature different (โ°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) โTcooling cold air temperature inlet โ outlet (โ°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) = ฮฑ2 /ฯk fin spacing for the cold side air (mm) fin spacing for the hot side air (mm) Greek symbols ฮฑ Seebeck coefficient (V/K), ฮฑ = ฮฑp โ ฮฑn ฯ electrical resistivity (ฮฉ cm), ฯ = ฯp + ฯn ๐ alimunum block thermal resistance (K/W) n the number of thermocouples ฮทc fin efficiency of cold side heat sink nc number of fins for the cold side heat sink ฮทh 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 )/ฮทh hh Ah dimensionless convection, Nh = ฮทc hc Ac /ฮทh hh Ah dimensionless current, NI = ฮฑI/(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 (ฮฉ) m measured fluid Reynoldsโs number opt. optimal quantity Tc cold junction temperature (°C) UC unit cell Th hot junction temperature (°C) โ 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 vehicleโs 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, Tโc and Tโh, 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 ๐ฬ๐ , the electrical current I and the thermocouple geometric ratio (๐บ๐ = ๐ด๐ /๐ฟ๐ ) 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 ๐ 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. ๐ฬ๐ = ๐๐ โ๐ ๐ด๐ [๐โ๐ โ (๐๐๐ โ ๐๐ )] (1) 1 ๐ด๐ ๐ฬ๐ = ๐ [๐ผ๐ผ๐๐ โ ๐ผ 2 ๐ + ๐(๐๐ โ ๐โ )] 2 ๐ฟ๐ (2) 1 ๐ด๐ ๐ฬโ = ๐ [๐ผ๐ผ๐โ + ๐ผ 2 ๐ + ๐(๐๐ โ ๐โ )] 2 ๐ฟ๐ (3) ๐ฬโ = ๐โ โโ ๐ดโ [(๐โ โ ๐๐โ ) โ ๐โโ ] (4) ๐๐๐ = ๐ฬโ โ ๐ฬ๐ (5) 8 where ๐๐ is the fin efficiency, โ๐ is the convection coefficient, and ๐ด๐ is the total surface area of the cold heat sink. ๐โ (๐โโ โ ๐๐โ ) ๐๐ผ2 โ (๐ = ๐ โ๐ ๐ด๐ ๐ + 1) [๐๐ผ ๐๐ โ + (๐๐โ โ ๐โโ )] ๐๐ 2๐๐โโ (6) (๐โโ โ 1) ๐๐ผ2 = (๐โ โโ ๐ดโ ๐ + 1) [๐๐ผ ๐โโ โ + (๐๐โ โ ๐โโ )] ๐๐ 2๐๐โโ (7) ZTโh , 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 ๐ผ2 = ๐ ๐๐ โโ (8) ๐๐ โ๐ ๐ด๐ ๐โ โโ ๐ดโ (9) ๐(๐ด๐ ๐/๐ฟ๐ ) ๐โ โโ ๐ดโ (10) ๐ผ๐ผ ๐ด๐ ๐/๐ฟ๐ (11) ๐๐โโ ๐โ = ๐๐ = ๐๐ผ = โ Tcโ, Thโ and Tโ are the dimensionless cold junction temperature, the dimensionless hot junction temperature, and the fluid temperature ratio, respectively, and are defined as ๐๐โ = ๐๐ ๐โโ ๐โโ = ๐โ ๐โโ (12) (13) 9 ๐โโ = ๐โ๐ ๐โโ (14) the dimensionless temperatures are then a function of five independent dimensionless parameters as ๐๐โ = ๐(๐๐ , ๐โ , ๐๐ผ , ๐โโ , ๐๐โโ ) (15) ๐โโ = ๐(๐๐ , ๐โ , ๐๐ผ , ๐โโ , ๐๐โโ ) (16) setting ๐๐โโ , ๐โโ and ๐โ to be the inputs, the dimensionless parameters ๐๐ and ๐๐ผ 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 ๐ถ๐๐ 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 × 19.1 ๐๐3 ) 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 (๐โ๐,๐๐ , ๐โ๐,๐๐ข๐ก , ๐โโ,๐๐ , and ๐โโ,๐๐ข๐ก ) so that 1 the average ambient temperature at the heat sinks could be averaged such that ๐โ๐ = (๐โ๐,๐๐ + ๐โ๐,๐๐ข๐ก ) 2 1 and ๐โโ = 2 (๐โโ,๐๐ + ๐โโ,๐๐ข๐ก ). 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 (๐๐๐,๐๐ถ = ๐ผ๐ × ๐๐๐ = 4.5๐), 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 ๐โ๐,๐๐ , and the hot air inlet temperature ๐โโ,๐๐ 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, ๐๐๐๐ฅ provided by the manufacturer as shown in Fig. 4. The objective was to measure the cooling power ๐๐ , the heat rejection ๐โ , the cold junction temperature ๐๐ and the hot junction temperatures ๐โ . ๐๐ and ๐โ can be obtained by extrapolating the two measured temperatures of each block (๐๐1 & ๐๐2 for cold side and ๐โ1 & ๐โ2 for hot side) assuming the temperatures change linearly across the aluminum blocks. For ๐๐ and ๐โ , 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 ๐ผ, ๐, and ๐ [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 ๐๐ on the TEAC performance. All three modules had the same base area (30 × 30 ๐๐2 ) but different number of couples and/or thermoelement geometric ratio, ๐บ๐ . 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 ๐ถ๐๐ 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 ๐ถ๐๐, at given conditions, and it is equal to 1.09. This result came from the optimization of the two dimensionless values ๐๐ and ๐๐ผ simultaneously. Moreover, the values of ๐๐ and ๐๐ผ 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 modulesโ performances at the same input power (๐๐๐ = 4.5๐). Among the three modules, module number 2 has the closest values of ๐๐ and ๐๐ผ 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 ๐ต๐ฐ ๐ฐ (Amp) ๐ต๐ ๐ × ๐ฎ๐ (๐๐) ๐ท๐๐ (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. ๐๐ between the three modules at fixed input power (๐๐๐ = 4.5๐). The predicted curve was obtained by fixing ๐๐๐ and assuming a constant temperature difference at the junctions (for a short range) and then resolved for ๐๐ผ to be only as a function of ๐๐ . This assumption allows expressing COP independently from ๐๐ผ 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 ๐๐ and ๐๐ผ 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. 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