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Summary
Harvesting and directly converting the enormous, yet largely untapped, reservoir of low-grade heat into electricity has the potential for immense economic and environmental impacts. To this end, thermoelectrics (1), thermionic capacitors (2), and thermocells (3) are among the state-of-the-art options. Despite distinct working principles of the internal circuit, the trio are heat engines in a general sense because they convert a temperature difference input ΔT into a device voltage output ΔVD that empowers the electrons in the external circuit to do work. The unit of the ΔVD/ΔT ratio, a numeric performance index of the energy conversion device, is the same as the thermopower. However, it is misleading to equate or compare this ratio to the thermopower, an intrinsic physical property of material described by a rank-2 tensor (2–7).
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