Solar Thermoradiative-Photovoltaic Energy Conversion

            In this presentation, we describe a solid-state solar thermal energy conversion system for low optical concentration that can be paired with thermal storage. To obtain continuous electricity from intermittent sunlight, some form of energy storage must be utilized. Modern concentrating solar power plants can achieve this by storing solar energy as heat, but these plants must be very large and operate at high optical concentration to be cost-competitive. This results from their use of mechanical heat engines with large turbomachinery. Solid-state alternatives such as thermoelectric generators suffer from low efficiency, and thermophotovoltaics also favor high temperatures and high optical concentration, leaving a need for efficient and scalable alternatives.

            To fill this gap, we propose a system consisting of a selective solar absorber, a thermoradiative cell, and a photovoltaic cell. The absorber is heated under direct or concentrated sunlight, and its spectral selectivity suppresses infrared emission losses. Heat is passed from the absorber to the thermoradiative cell directly through thermal coupling or indirectly via thermal storage. The thermoradiative cell operates as a photodiode under negative illumination, generating electricity from a thermally driven radiative recombination current. Its emitted light is absorbed by the photovoltaic cell, generating electricity from an additional photocurrent. Both the thermoradiative and photovoltaic cell can be engineered to have spectral selectivity to minimize losses associated with below-bandgap emission/absorption

            Using the principle of detailed balance, we find that an ideal, one-sun system with equal absorber, thermoradiative, and photovoltaic areas has a limiting solar conversion efficiency of 45%. Ideal and nonideal solar thermoradiative-photovoltaic converters exhibit significant performance enhancements over solar thermophotovoltaics at lower hot-side temperatures and for low to moderate bandgap materials. We analyze a system with all major loss mechanism under optical concentration and find that a solar thermoradiative-photovoltaic converter can achieve up to a 7.9% absolute increase in efficiency compared to a solar thermophotovoltaic system with the same loss mechanisms. These improvements in performance result from a high tolerance to nonradiative losses and an ability to utilize the benefits of both the thermoradiative cell and the photovoltaic cell.

            Our results suggest that this type of energy conversion system could enable cost-competitive electricity production from low cost single axis solar tracking systems. These are very scalable in comparison to modern concentrating solar power plants and could thus allow for more widespread installations. When paired with thermal storage, these systems could provide continuous electricity generation despite solar intermittency, allowing solar thermoradiative-photovoltaic converters to fill the gap between modern concentrating solar power plants and traditional solar photovoltaics.