Thermophotovoltaic (TPV) systems generate electricity using heat drawn from one or more of a wide variety of sources, including sunlight, fossil fuels, and radioisotopes. They function as solid-state devices, in which heat is thermally radiated as photons onto TPV modules that generate electricity using the same mechanism as solar cells. The two key challenges of the field are integrating the heat-generating mechanism with the electricity-generating back end, and achieving high efficiencies at relatively modest temperatures and smaller form factors. Both problems can be addressed by introducing photonic crystals (PhCs), which offer an unprecedented ability to control the emission and flow of light. For example, heat can be harvested from sunlight effectively by 1D PhC selective solar absorbers, which can increase solar absorption up to 98%, while holding spectrally averaged emissivity down to 4%. Additionally, 2D periodic PhCs can be used to enhance thermal radiation of short wavelengths the TPV modules can convert into electricity, while suppressing emission of longer wavelength photons. Furthermore, 1D PhC filters can help recycle the vast majority of the remaining emitted long-wavelength photons. Employing all of these PhC elements and operating at 800-1000 degrees Celsius can potentially result in an array of high-performing devices. Solar TPV can reach up to 45% efficient conversion of sunlight to electricity, an order of magnitude above previous experimental efforts, and well above the Shockley-Queisser limit for single-junction photovoltaic cells. Radioisotope TPV can reach up to 24% efficient conversion of excess heat generated from natural radioactive decay into electricity in a portable form factor comparable to two D batteries. Propane-burning TPV can reach up to 32% efficient conversion of the lower heating value of propane into electricity within a handheld form factor. While many further experiments are needed to approach these theoretical limits, TPV has the potential to achieve much higher energy and power densities and reliability, as well as greater fuel flexibility, than many current energy generation and storage technologies.
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Burton Morgan 121, Purdue University, West Lafayette, IN