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Energy Efficient Cooling vs Waste Energy Recovery using Thermoelectrics
Keywords: Thermoelectric, energy efficient, optimum design
Thermoelectric (TE) Peltier cooling has been known as the best solution for precise and high speed temperature control of electronics devices. It has not been clear, however, whether the TE as an energy scavenging device in electronic systems could be a useful solution in the future, or if significant advance in material figure-of-merit (ZT) is required. Questions still remain: when TE devices for waste heat recovery make sense? Could we use some of the energy in hot spot in electronic circuits to do useful work or the main emphasis should be selective cooling of the hot spot? Where is the biggest impact of exotic high performance thermoelectric materials? Could latest TE materials really help to reduce electricity bill in data centers, for example? We try to respond to these questions based on physical limits of the thermoelectric materials and devices, and by optimizing the overall integrated system. We analyze examples that range in scale from Micro Electro Mechanical Systems (MEMS) sensors and portable electronics, up to data centers. A key issue is the understanding of available/applicable heat fluxes and that temperature gradient is required to generate work. We show that, especially in waste heat recovery applications, thermal conductivity of the TE material is the most significant component of the thermoelectric properties for an optimum design for maximum power output. If the thermal conductivity is fixed, better ZT improves the performance for any application. The analysis shows, however, that the impact of heat sink performance could be much larger than the ZT improvement. Interestingly, for power generation the material cost of TE material can be quite low if one designs modules with low fractional area coverage of TE legs. It is very important to design the TE module and the heat sink system simultaneously as the heat sink plays a key role in the system cost structure. For the TE cooling, an optimum design for the driving electrical power also exists and the heat sink dominates the performance as well. Concluding the analysis, TE technology can help energy-efficient thermal management in cost efficient manner. To increase the broad impact, simultaneous optimization of the heat sink is required. With improved heat sinks and with robust low fractional area coverage TE modules, energy-efficient TE systems could have big impact even with the currently available TE materials in the market.
Kazuaki Yazawa, Research Associate Professor
Purdue University
West Lafayette, Indiana

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