Thermodynamic Efficiency, Power Output and Performance Indices of Classic and Nanostructured Thermoelectric Materials

This paper will address the use of the thermoelectric figure of merit ZT and the related concept of efficiency in low temperature heat conversion. It will be shown that, while ZT is an appropriate performance index when optimizing the rate of heat conversion, it may be misleading for actual applications. This is of special relevance when related to the recent surge of research effort in the area of low-dimensionality semiconductors (e.g., Si nanowires) where the increase of the figure of merit is mostly due to their lowered thermal conductivity κ. Under fixed heat flow conditions the optimization of ZT may actually proceed only by maximizing the power factor, since materials with low κ may be unable to duly dissipate heat. Also when operating between sources at fixed temperature it will be shown that at any given value of power factor the highest output can be obtained by increasing κ, not decreasing it. A brief analysis of the economic factors concurring to define mature scenarios of application will also be provided.