The use of direct energy conversion devices to convert waste heat into useful power has been the focus of extensive research for many years. Optimization of the performance of power harvesting systems has led to various models of the irreversibilities occurring in these devices. The majority of these models are based on dimensionless parameter referred to as the irreversibility factor to account for the entropy generating effects occurring within a generic heat engine. The purpose of this paper is to describe the use of the exergetic or second law efficiency to characterize these non-ideal effects and to describe how this approach may be used in the design and optimization of power harvesting or waste heat recovery systems. The use of a thermoelectric generator in a power harvesting system is considered to illustrate the proposed model.

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