Thermal and Photothermal Energy Harvesting

Abstract

In this chapter we focus on the problem of energy harvesting using hot sources. The phenomenon known as thermoelectric power harvesting rests on the fact that current can be generated by using hot sources linked to cold sources, and vice versa, currents can transport heat from cold sources to hot sinks making coolers. The mechanism is reviewed and explained. The efficiency involves electrical and thermal transport properties of materials. In this short chapter, the emphasis is on harvesting not cooling. The theory of electrical conductivity, Seebeck coefficients, Peltier coefficients, and thermal conductivity is considered, and because a low thermal conductivity raises the thermo-harvesting efficiency, disorder can actually play a very useful role in this context. Another question examined here is as to the role of many body interactions. It will be shown that the so-called Kondo insulators, where localized spins bind conduction electrons, exhibit very high Seebeck coefficients (energy transported per carrier), but this exciting phenomenon can at present only be exploited at low temperatures. Photovoltaic cells constitute well-established ways of harvesting solar power, so we also consider the problem of how to efficiently harvest the solar heat, i.e., the energy in the wavelength region not normally addressed by conventional solar technology. This is called “thermophotonic energy harvesting,” and it turns out that some interesting new science has been developed in recent years which has helped to address this question and to raise the efficiency. Thermophotonics is beyond the scope of this book and we refer the reader to the original literature. But one of the basic ideas in this technology is to make devices which strongly absorb radiation in a broadband, the body heats up, and the hot body then emits black body radiation. The short-wavelength components of the reemitted radiation are filtered out by conventional devices leaving the long-wavelength part of the hot body spectrum. The long-wavelength part of the radiation is reflected back into the absorber and recycled back again into the heat using “photonic crystal” mirror technology. If little radiation is allowed to leak out, then the heat absorbed and reabsorbed raises the temperature of the body. In this way the radiation reflected will eventually be radiated out in the allowed higher energy photonic window where photovoltaic cells collect it.