Solar-Powered, Micron-Gap Thermophotovoltaics for MEO Applications

The proposed innovation is an InGaAs-based, radiation-tolerant, micron-gap thermophotovoltaic (MTPV) technology. The use of a micron wide gap between the radiation source and the photovoltaic substantially increases the efficiency of the thermophotovoltaic. This work will be accomplished by combining MicroLink's state-of-the-art InGaAs-based epitaxial lift-off solar cell structure on an InP substrate, and MTPV, LLC's leading-edge, micron-gap thermophotovoltaic device technology. The relevance of this innovation is that it addresses NASA's ongoing need for high-efficiency, lightweight, compact sources power for space vehicles. TPV is an attractive and technically feasible candidate for space power supplies, but its relatively poor efficiency performance has meant that it has not seen widespread use in space applications. However, MTPV technology, by bringing the emitter into the near field of the photovoltaic, couples an order of magnitude more power across the nano-scale vacuum gap than can be absorbed from the far-field spectrum and does so while maintaining the benefits of conventional TPV. MicroLink's ELO growth technology promises to further increase coupled power relative to the current germanium TPV cells while simultaneously reducing substrate costs. Thus, MicroLink ELO and MTPV technology can offer both high efficiency and reasonable cost TPV systems for the first time.

Non-NASA commercial applications for the MTPV technology are huge. MTPV enables the generation of electricity from waste heat at commercially competitive costs. Fully 57% of all energy generated in the U.S. is rejected, primarily to waste heat. Over recent decades, there has been a large effort to find a technology that can recover waste energy in an efficient and cost-effective manner. A candidate energy recovery technology is thermophotovoltaics. Conventional thermophotovoltaics is limited by the amount of energy contained in the far-field spectrum of the hot emitter. MTPV technology promises to overcome this limitation. By virtue of the nano-scale vacuum gap, MTPV can operate in high temperature environments and do so with high efficiency, at high power density, and with low cost.

The main potential application of MTPV for NASA is as a power sources for space vehicles. MTPV has advantages for near-Earth missions, particularly in the Van Allen belts. Our MTPV uses cells based on InGaAs, which is relatively radiation resistant, and is expected to degrade at a lower rate than conventional triple-junction PV cells. Another advantage of MTPV in Earth orbit missions, where the spacecraft goes into eclipse frequently, is that the use of thermal energy storage in place of batteries may represent an enabling technology. Another potential application for MTPV is as a replacement for radioisotope thermal generators (RTGs) for outer solar system missions. The current generation of RTGs is heavy and has efficiency of only 6.3%. Baseline TPV systems offer the potential for efficiency in excess of 15%.

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