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Although spectrally selective materials play a key role in existing and emerging solar thermal technologies, temperature-related degradation currently limits their use to below 700C in vacuum, and even lower temperatures in air. Here we demonstrate a solar-transparent refractory aerogel that offers stable performance up to 800C in air, which is significantly greater than its state-of-the-art silica counterpart. We attribute this improved stability to the formation of a refractory aluminum silicate phase, which is synthesized using a conformal single-cycle of atomic layer deposition within the high-aspect-ratio pores of silica aerogels. The transparent refractory aerogel achieves a record-high receiver efficiency of 77% at 100 suns and an absorber temperature of 700C based on direct heat loss measurements at this temperature. Such performance and stability can enable the use of advanced supercritical CO2 power cycles and lead to a ~10% (absolute) improvement in solar-to-electrical conversion efficiency. Transparent refractory aerogels may also find widespread applicability in solar thermal technologies by enabling the use of lower-cost optical focusing systems and eliminating the need for highly evacuated receivers.

The choice of heat transfer fluids has significant effects on the performance, cost, and reliability of solar thermal systems. In this chapter, we evaluate existing heat transfer fluids such as oils and molten salts based on a new figure of merit capturing the combined effects of thermal storage capacity, convective heat transfer characteristics, and hydraulic performance of the fluids. Thermal stability, freezing point, and safety issues are also discussed. Through a comparative analysis, we examine alternative options for solar thermal heat transfer fluids including water−steam mixtures (direct steam), ionic liquids/melts, and suspensions of nanoparticles (nanofluids), focusing on the benefits and technical challenges., Center for Clean Water and Clean Energy at MIT and KFUPM (Project 6918351), United States. Dept. of Energy. Office of Science (Solid-State Solar-Thermal Energy Conversion Center Award DE-SC0001299)

To bridge the gap between theoretically predicted and experimentally demonstrated efficiencies of solar thermophotovoltaics (STPVs), we consider the impact of spectral non-idealities on the efficiency and the optimal design of STPVs over a range of PV bandgaps (0.45-0.80 eV) and optical concentrations (1-3,000x). On the emitter side, we show that suppressing or recycling sub-bandgap radiation is critical. On the absorber side, the relative importance of high solar absorptance versus low thermal emittance depends on the energy balance. Both results are well-described using dimensionless parameters weighting the relative power density above and below the cutoff wavelength. This framework can be used as a guide for materials selection and targeted spectral engineering in STPVs., United States. Dept. of Energy. Office of Basic Energy Sciences (DE-FG02-09ER46577)