Abstract
Thermoelectric Generators (TEGs) are devices for direct conversion of heat into electrical power and bear a great potential for applications in energy scavenging and green energy harvesting. Given a heat source, the conversion efficiency depends on the available temperature difference, and must be maximized for optimal operation of the TEG. In this frame, the choice of materials with high thermoelectric properties should be accompanied by the identification of criteria for an optimal exploitation of the electrical power output. In this work, we briefly review the main properties of TEGs, focusing on the electrical power output and the thermal-to-electrical conversion efficiency. Besides, we discuss principles of operation of TEGs enabling the optimization of the electrical power output, based on the suitable choice of the electrical load. In particular, we comparatively present and discuss the conditions for matching the electrical load—yielding to maximum power transfer—and those for maximizing the conversion efficiency. We compare the two conditions applying them to the exploitation of a heat reservoir for energy storage and to the recovery of heat from a heat exchanger. We conclude that the difference between the two conditions is not significant enough to justify the complexity required by the implementation of the maximum efficiency. In addition, we consider the effect of the thermal contact resistance on the electrical power output. Using a simple thermal-electrical model, we demonstrate that the equivalent electrical resistance measured between the terminals of the TEG depends on the thermal exchange. Hence, for maximum power transfer, the electrical load of the TEG should not match its parasitic resistance, but the equivalent electrical resistance in each specific operating conditions, which determine the thermal fluxes. The model can be applied for the development of efficient alternative algorithms for maximum power point tracking.
Highlights
Thermoelectric Generators are able to generate electrical power by thermal-to-electrical energy conversion starting from any heat source: they are compact, robust and reliable, and are very attractive for a wide range of applications envisioning the energy recovery of waste heat, ranging from the automotive sector to space missions or other applications in extreme environments.Recently, thermoelectric generators have been proposed for energy scavenging
After a review of the principles of thermoelectric generation, and of the basic equations concerning the electrical power output and the efficiency of a Thermoelectric Generators (TEGs), we discuss the energy that can be extracted with the maximum efficiency or instead the maximum power output strategy, exploiting simple models applied to two practical cases
The generated electrical energy is maximum if the TEG is driven in the condition of the maximum conversion efficiency
Summary
Thermoelectric Generators are able to generate electrical power by thermal-to-electrical energy conversion starting from any heat source: they are compact, robust and reliable, and are very attractive for a wide range of applications envisioning the energy recovery of waste heat, ranging from the automotive sector to space missions or other applications in extreme environments. Available TEGs are usually based on tellurium compounds and display a ZT around 1 at room temperature [5,6] The research in this field is currently focusing on the development of innovative material platforms that assure high values of ZT while at a time are biocompatible, as well as low cost and technologically affordable [7,8,9]. Given the temperatures TH and TC of the hot and cold reservoirs exploited by the TEG, the conversion efficiency depends on the electrical load applied to the generator The latter can be controlled with a suitable driving of the dc-dc converter, necessary to adapt the output voltage of the TEG to the user device. We analyze the impact of the thermal contact resistance between the TEG and the heat sources, and we discuss its effect on the electrical power output, providing guidelines for improving the power management (Section 4)
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