Exergy Analysis and Entropy Generation Minimization of Thermoelectric Waste Heat Recovery for Electronics

Energy recovery from waste heat is attracting more and more attention. All electronic systems consume electricity but only a fraction of it is used for information processing and for human interfaces, such as displays. Lots of energy is dissipated as heat. There are some discussions on waste heat recovery from the electronic systems such as laptop computers. However the efficiency of energy conversion for such utilization is not very attractive due to the maximum allowable temperature of the heat source devices. This leads to very low limits of Carnot efficiency. In contrast to thermodynamic heat engines, Brayton cycle, free piston Stirling engines, etc., authors previously reported that thermoelectric (TE) can be a cost-effective device if the TE and the heat sink are co-optimized, and if some parasitic effects could be reduced. Since the heat already exists and it is free, the additional cost and energy payback time are the key measures to evaluate the value of the energy recovery system. In this report, we will start with the optimum model of the TE power generation system. Then, theoretical maximum output, cost impact and energy payback are evaluated in the examples of electronics system. Entropy Generation Minimization (EGM) is a method already familiar in thermal management of electronics. The optimum thermoelectric waste heat recovery design is compared with the EGM approach. Exergy analysis evaluates the useful energy flow in the optimum TE system. This comprehensive analysis is used to predict the potential future impact of the TE material development, as the dimensionless figure-ofmerit (ZT) is improved. INTRODUCTION Waste heat recovery from electronic systems has been receiving attention but not reported very much relative to energy harvesters such as Piezo electric from vibration [1], powering from human hand winding motion [2], and embedded film solar cells [3]. A few studies on waste heat exist in the literature, such as the waste recovery from a laptop [4]. The reason might be disappointment with insufficient power output due to the small temperature differences available across heat engines, even with high energy conversion efficiency. This is limited by Eq. (1) in a system including thermal dissipation with an ideal heat engine, as found by Curzon and Ahlborn [5]. This is smaller than Carnot efficiency. s a CA T T  1  (1) where Ts is the source temperature and Ta is the ambient temperature. This is also true for the thermoelectric if extremely large ZT is assumed but only for the symmetric external heat dissipation [6]. There are technologies developed as heat engines like the Brayton cycle, free piston Stirling engines, etc. These are actually used in various power plants. The system efficiencies were discussed by Esposito [7]. All these technologies fall under the same energy conversion principle. Authors have looked into thermoelectric direct energy conversion since the optimum design of the thermoelectric was found to be cost-effective [8] if some Proceedings of the ASME 2011 Pacific Rim Technical Conference & Exposition on Packaging and Integration of Electronic and Photonic Systems InterPACK2011 July 6-8, 2011, Portland, Oregon, USA