ME Thermoelectric -I (Design) Summer - II (2015) Project Report. Topic : Optimal Design of a Thermoelectric Cooling/Heating for Car Seat Comfort

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1 ME Thermoelectric -I (Design) Summer - II (2015) Project Report Topic : Optimal Design of a Thermoelectric Cooling/Heating for Car Seat Comfort Team Members WIN ID Karthik Reddy Peddireddy Ravi Teja Jogiparthi Vikas Reddy Faculty: Dr. HoSung Lee Date of Submission: 19 th Aug, 2015

2 ACKNOWLEDGMENTS I would like to express my sincere thanks to Dr. HoSung Lee, Professor of Mechanical & Aeronautical Engineering and Alaa M. Attar, for their time and guidance throughout the study that is continued in this project. It would not have been possible to make this project a better one without their presences. I would like to thanks my classmates for the moral support. Team Work i

3 TABLE OF CONTENTS ACKNOWLEDGMENT...i LIST OF FIGURES...iii ABSTRACT...iv 1. Introduction Optimum Design Background Optimal Design of Cooling/Heating of a Car Seat Present Study Application Conclusion Reference...10 ii

4 LIST OF FIGURES Fig.1.1 a) TEC module with two heat sinks, (b) Schematic of thermoelectric couple...1 Fig 3.1 Typical thermoelectric cooler module...4 Fig.3.2 Cooling power (W) and COP vs. element length with the current of I = 4 A used...5 Fig 3.3 Fluid outlet temperatures vs. Element length (mm) with the current of I = 4 A used..5 Fig 3.4 Heating power (W) and COP vs. element length (mm)...6 Fig 3.5 Schematic of thermoelectric car seat cooling/heating device...7 Fig. 3.6 Schematic of thermoelectric car seat cooling/heating with a recirculating duct...8 iii

5 Abstract To improve the performance and system design we optimize new method which include a fan, a thermoelectric device, under-seat channels, and an optional recirculating duct enveloped in constant/heating and low power consumption. This work illustrates whether the distributed air is consumed completely at the end of channels or recirculated using a return duct and impermeable materials except the perforated holes. Selecting suitable thermoelectric modules from various commercial modules is quite difficult for the system designers. A typical 1.2 mm thermoelectric cooler module is intended to decrease the Joule heating also concurrently revokes energy backflow by the thermal conduction. By considering five independent dimensionless parameters such as N k, N h, N I, T * and ZT we can predict analysis that indicates the thermoelectric devices must be designed based on the specific cooling/heating system. iv

6 1. INTRODUCTION Moving to the standard vehicle optimal design of car seat is progressively becoming a competitive issue. Since then early 1960s, it was shown that aerated car seats improved human comfort (Johnson, 1964). Recent climate chamber tests of different types of seat indicate that transpiring (perforated) materials with ventilation showed enhanced comfort (Malvicino, 2001). Thermoelectric devices probably first time applied to car seat comfort (Feher, 1990). Later, seat climate control for initial startup warming and cooling using thermoelectric devices was reported (Gallup, 2003). Placing seat temperature control unit in series with automotive HVAC module for considering humidity control increased body comfort (Kadle, 2007). Recently, compactness of a novel ventilation system with thermoelectric devices was reported (Bell, 2013). A simple electrical circuit for thermoelectric cooling (TEC) is shown in Fig.1. The amount of heat absorbed at the cold junction is associated with the Peltier cooling, the half of Joule heating, and the thermal conduction. It is determined by net heat removed the cold junction, such that, = 1 2 Where T = T h T c and is thermal conductance. Fig.1.1. a) TEC module with two heat sinks, (b) Schematic of thermoelectric couple Multiple thermocouples can be used as to increase the cooling capacity and greater temperature difference can be achieved by operating the cooling unit. An electric insulator is usually placed between the electric conductor and the cold plate to avoid short circuit. 1

7 The objective of this work is to optimize design (Lee, 2012) of a thermoelectric device such as, element length, cross section and number of thermoelements. This method improves the performance and then an innovative system design a fan, a thermoelectric device, under-seat channels, and an optional recirculating duct with high efficiency (constant cooling/heating and low power consumption). This design includes transient startup warming and cooling before the car (Heating Ventilation and Air-conditioner) HVAC is active in the cabin. Usually the distributed air of the channels are completely consumed through perforated holes and permeable seat materials. This study gives an option whether the distributed air is consumed completely at the end of channels or recirculated using a return duct and impermeable materials except the perforated holes. A thin thermoelectric cooler module is considered with element length 1.2 mm and element area 4.4 mm 2 and there exits two thermal resistance between the two hot and cold junctions and fluids. We perform a non-dimensional analysis for minimizing parameters defining five independent dimensionless parameters, not conflicting with one another, one of which is Nk that includes the most important geometric information such as number of thermoelements, element length and cross section, and thermal conductivity. The next important parameter is dimensionless current NI which is the ratio of the Peltier cooling power to the thermal convection. The third dimensionless parameter is Nh, which is the ratio of the cold convection to the hot convection. The fourth is the ratio of cold inlet fluid temperature to the hot inlet fluid temperature. Lastly the fifth is called the dimensionless figure of merit ZT, which represent the quality of materials, the higher is the better. The performance curves is difficult to predict without the analysis of dimensionless parameters. 2. Optimum Design Background The dimensionless analysis developed by Lee, (2012) obtains the maximum cooling power by simultaneously determining the dimensionless current supplied (N I ) and the ratio of the thermal conductance to the convection conductance (N K ) for a given set of fixed parameters. This method converts the four basic heat balance equations, such as = Ƞ h ( ) (1) = ( ) (2) 2

8 = ( ) (3) = Ƞ h ( ) (4) Q c and Q h are the rate of heat transfer for the cold and hot fluids, and n is the no. of thermoelectric couples. the thermal resistance of the cold heat sink can be expressed by the reciprocal of the convection conductance Ƞ h, where Ƞ is the fin efficiency, h is the convection coefficient, and is the total surface area of the cold heat sink. ( ) = 2 + ( ) (5) 1 = 2 + ( ) (6),,, and are defined as the dimensionless figure of merit, convection ratio, the ratio of thermal conductance to convection conductance, and dimensionless current, respectively. The dimensionless temperatures are then a function of five independent dimensionless parameters as, = (,,,, ) (7) = (,,,, ) (8) Then, the dimensionless cooling power Q C, heat rejection Q h, input power P in, and COP can be defined as, = Ƞ h = Ƞ h (9) (10) = Ƞ h (11) = (12) Using these equations, where,, are set to be inputs, the dimensionless parameters and can be optimized to solve for maximum cooling power. 3

9 3. Optimal Design of Cooling/Heating of a Car Seat A newly developed optimization method is used to improve the performance on the thermoelectric devices which includes a fan, a thermoelectric device, under-seat channels, and an optional recirculating duct. In this the under seat channels contain perforated holes and permeable seat materials which consume the total distributed air of the channels. This optimal design gives an option whether the distributed air is consumed completely at the end of channels or recirculated using a return duct and impermeable materials except the perforated holes which can be verified by measurements. Many manufacturers provide performance curves of the thermoelectric products based on the ideal conditions that assume no thermal resistances between the junctions and medium which is indeed unrealistic. Furthermore, the material properties are unknown and even the thermal and electrical contact resistances. 3.1 Present Study This Present work provides a new design which significantly improves the performance of car seat cooling/heating and indicate that a much lower power consumption almost in half could be achieved with equivalent cooling/heating. Fig 3.1. Typical thermoelectric cooler module The above figure shows a typical thin thermoelectric cooler module. Here an element length of 1.2mm is used as shown to decrease the joule heating. But we cannot say thatt this short length 4

10 is beneficial until optimization is done. The Performance of TED is a function of thermoelectric element length as shown below: Fig.3.2. Cooling power (W) and COP vs. element length (mm) with the current of I = 4 A used. The above figure shows the graph between cooling power and COP vs. element length (mm) which provide solution to determine the element length. From the above figure it can be observed that the optimal element length is near 2.2mm and also the increase of element length from 1.2mm (commercial) to 2..2mm (present) results in 35% increase of cooling power from 28W to 38W with an acceptable decrease in the COP from 1.3 to 1. Fig 3.3 Fluid outlet temperatures vs. Element length (mm) with the current of I = 4 A used. 5

11 The above figure shows the cold and hot fluid outlet temperatures along with the element length. A temperature differencee of T=11 is obtained from the cooling power of 38W and COP of about 1.0 for cooling in summer. Fig 3.4 Heating power (W) and COP vs. element length (mm) The above figure shows the Heating power and COP vs. element length. Here a temperature difference of = 21 obtained from cooling power of 80 W and COP of about 2.0 for heating in winter. The below diagram in CATIA shows the schematic of commercial model of thermoelectric car seat cooling/heating. It consists of thermoelectric device sandwiched between Heat sink 1 which is a cold heat sink, Heat sink 2 which is a hot heat sink, where each has 20 fins with 1 mm space between the fins. This fin design is based on the optimization of heat sink. Conditioned air is supplied from fan with volume flow rate of 6.3 CFM to the under seat channels (5 channels) of thickness 0.3 cm which covers occupant seat area of 25cm 20cm. Each channel contain perforated holes which helps to dissipate hot air or cold air from the heat sinks through seat channel holes to the seat occupant. 6

12 Fig 3.5 Schematic of thermoelectric car seat cooling/heating device. One of the main advantage of using thermoelectric device is heating or cooling can be achieved just only by changing the polarity of electricity. During cooling the hot air is vented to the cabin which will be somewhat trouble to occupants but compared to the HVAC the flow rate is very small. This below diagram in CATIA shows a design which is modified to utilize the waste conditioned air by recirculating the air to the inlet of the fan. In this the permeable materials on the seat channels are replaced with impermeable materials to decrease the air consumption. With the back flow of air to the fan, part of the total volume flow rate is consumed through the perforated holes and remaining flow rate is vented to cabin for hot air. 7

13 Fig. 3.6 Schematic of thermoelectric car seat cooling/heating with a recirculating duct. 4. Applications Thermoelectric coolers are advantageous than the traditional cooling devices in terms of compact size, no moving parts and working fluid, compatible with automobile electrical system voltage, and easily switching between heating and cooling modes. Therefore, thermoelectric cooler appears to be especially favorable for automotive application. Applications for thermoelectric modules cover a wide spectrum of product areas. These include equipment used by military, medical, industrial, consumer, scientific/laboratory, and telecommunications organizations. Uses range from simple food and beverage coolers for an afternoon picnic to extremely sophisticated temperature control systems in missiles and space vehicles. 8

14 5. Conclusion The presented work show an option whether the distributed air is consumed completely at the end of channels or recirculated using a return duct and impermeable materials except the perforated holes. Now an innovative design on top of the design showed in the preceding paragraphs is implemented into car seat cooling/heating. Suppose that the design is modified to utilize the waste conditioned air by recirculating the air to the inlet of the fan. In this new design, the permeable materials should be replaced by impermeable materials to reduce the air consumption while still having the small holes, where the size can be optimized to be large enough to provide comfort but small enough to minimize the conditioned air consumption. In this way, the unused conditioned air is returned to the fan, so that a portion of the total volume flow rate is consumed through the small holes and simultaneously a portion of the flow rate is vented to cabin for hot side air. For that reason, the fan has to draw more air than the air recirculated through the opening (gap) between the return duct and the fan inlet, where the gap can be determined by experiments. This new design significantly improves the performance of car seat cooling/heating. 9

15 6. References [1] Alaa M. Attar, "Optimal Design of Automotive Thermoelectric Air Conditioner (TEAC)". Journal of Electronic Materials, Vol. 43, No. 26, [2] HoSung Lee, "Optimal Design of thermoelectric devices with dimensional analysis". Applied Energy, Vol. 106, pp , [3] HoSung Lee, "The Thomson effect and the ideal equation on thermoelectric coolers". Energy, Vol. 56, pp ,