Thermophotovoltaic Energy Conversion for Space Applications

Abstract

Thermophotovoltaic (TPV) energy conversion cells have made steady and over the years considerable progress since first evaluated by Lockheed Martin for direct conversion using nuclear power sources in the mid 1980s. The design trades and evaluations for application to the early defensive missile satellites of the Strategic Defense Initiative found the cell technology to be immature with unacceptably low cell efficiencies comparable to thermoelectric of <10%. Rapid advances in the epitaxial growth technology for ternary compound semiconductors, novel double hetero-structure junctions, innovative monolithic integrated cell architecture, and bandpass tandem filter have, in concert, significantly improved cell efficiencies to 25% with the promise of 35% using solar cell like multi-junction approach in the near future. Recent NASA sponsored design and feasibility testing programs have demonstrated the potential for 19% system efficiency for 100 We radioisotopic power sources at an integrated specific power of ~14 We/kg. Current state of TPV cell technology however limits the operating temperature of the converter cells to < 400K due to radiator mass consideration. This limitation imposes no system mass penalty for the low power application for use with radioisotopes power sources because of the high specific power of the TPV cell converters. However, the application of TPV energy conversion for high power sources has been perceived as having a major impediment above 1 kWe due to the relative low waste heat rejection temperature. We explore this limitation and compare the integrated specific power of TPV converters with current and projected TPV cells with other advanced space power conversion technologies. We find that when the redundancy needed required for extended space exploration missions is considered, the TPV converters have a much higher range of applicability then previously understood. Furthermore, we believe that with a relatively modest modifications of the current epitaxial growth in MOCVD, an optimal cell architecture for elevated TPV operation can be found to out-perform the state-of-the-art TPV at an elevated temperature.

Analytical model of solar thermophotovoltaic systems with cylindrical symmetry: Ray tracing approach

“…In thermophotovoltaic (TPV) systems, a photovoltaic (PV) cell converts the radiation emitted by a heated body (also called an emitter) into electricity. 1 The different ways of heating the emitter could lead to the use of these systems in a very broad range of applications: recycling waste energy from industrial processes, 2 co-generation of heat and electricity in domestic boilers, 3,4 make silent and portable generators for the army, 5 power generators for space missions, 6 etc. TPV can be also used in the solar energy field when the sun is used for heating the emitter.…”

Thermophotovoltaic energy in space applications: Review and future potential

“…2). This matches solar powered systems, radioisotope (~100W e ) [12][13][14][15], and perhaps small (kilowatt-class) nuclear reactors [16,17]. Other important concern of TPV is the unproven reliability for long-term missions, especially concerning the possible contamination of the TPV optical elements by the evaporation of materials at very high temperatures.…”

A Review on Thermophotovoltaic Energy Conversions and its Space Power Applications

“…However, sometimes at much lesser temperatures below 1,300°C, the observed efficiencies remain only as high as 32%, in comparison to expectations that TPV efficiencies can approach more than 32% [6,7]. The cells employ band-edge spectroscopic filtering to achieve maximum efficiency, rejecting radiation of extraneous sub bandgap material and return to the source with highly polished back surface reflectors [3,5,8]. In comparison with a photovoltaic cell or solar cell, a TPV device can store and ultimately transform the energy contained inside sub-bandgap photons.…”

“…This can entitle approaches to energy conversion and storage which can use very high temperature thermal sources than the turbines which are present in electrical energy generation now days [1,2,3]. TPV construction and efficacy have improved since the early exhibition of 29% efficient TPV that used a tungsten emitter and a back scattered screen at a temperature of 2,000°C [4,5]. However, sometimes at much lesser temperatures below 1,300°C, the observed efficiencies remain only as high as 32%, in comparison to expectations that TPV efficiencies can approach more than 32% [6,7].…”

“…Cells having lower bandgap are suffered more loss than cells with larger bandgaps because this loss accounts for a lower portion of the voltage in those materials [17]. To keep a high-power density that scales with the emitter while using larger bandgap materials and working at extremely high temperatures [2,5,6].…”