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Thermophotovoltaic Energy Conversion
Thermophotovoltaic (TPV) energy converters are passive heat engines that effectively transform the thermal energy emitted by a hot emitter into electrical energy by harnessing the principle of the photovoltaic effect. The emitter can be heated by a variety of thermal sources, including as concentrated solar radiation, fuels, and industrial waste heat.
Near-Field Thermal Radiation
The conventional thermal radiation governed by Planck's law is transferred by the propagating waves. Thus, it is independent of the size of the gap between two surfaces. However, when the gap between the thermal emitter and receiver is less than the characteristic wavelength determined by Wien's displacement law, evanescent waves confined at each surface can be coupled. This allows radiative energy to be transferred to a different medium through evanescent waves. In other words, evanescent waves as well as propagating waves can contribute to thermal radiation. This phenomenon is called near-field radiation. It is possible to exceed the blackbody radiation using near-field radiation.
Solar Energy Control and Harvesting
The solar energy reaching the earth is 1 kW/m2 due to the small solid angle from the sun to the earth. Conventional solar cells convert this amount of solar energy into electricity without particular tuning. However, the solar thermophotovoltaic (STPV) system uses an optical concentrator to exploit high-density solar energy and heat the emitter. The radiation emitted by the emitter is converted into electricity in the TPV cell with a large solid angle. If the emitter temperature is 1300 K, 162 kW/m2 of radiative energy can be transferred, which is much larger than that used by conventional solar cells.
In addition, a critical intermittent problem of solar cells can be suppressed with a thermal storage integrated STPV system by storing solar energy in a phase change material like molten silicon. This technology can also be applied to store other renewable energy in the form of thermal energy. Then, the thermal energy will be reused on-demand as electricity through TPV converters.
In terms of radiation control, radiative cooling can be achieved by maximizing the IR emission through the air transparent window among the spectral region corresponding to Planck distribution. The radiative cooling concept can be applied to building energy management by cooling water. Also, a radiative cooling panel also can be applied to solar PV cell thermal management.
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