Heat at intermediate temperatures (120−220 °C) is in significant demand in both industrial and domestic sectors for applications such as water and space heating, steam generation, sterilization, and other industrial processes. Harnessing heat from solar energy at these temperatures, however, requires costly optical and mechanical components to concentrate the dilute solar flux and suppress heat losses. Thus, achieving high temperatures under unconcentrated sunlight remains a technological challenge as well as an opportunity for utilizing solar thermal energy. In this work, we demonstrate a solar receiver capable of reaching over 265 °C under ambient conditions without optical concentration. The high temperatures are achieved by leveraging an artificial greenhouse effect within an optimized monolithic silica aerogel to reduce heat losses while maintaining high solar transparency. This study demonstrates a viable path to promote cost-effective solar thermal energy at intermediate temperatures.

Solar steam generation at the sterilization condition suffers from low efficiency, especially in passive solar thermal devices. We developed a stationary solar collector with a transparent aerogel layer to achieve efficient solar steam generation via thermal concentration. In field tests performed in Mumbai, India, the device generated steam at 100 C with 56% efficiency and successfully powered a sterilization cycle following the standard sterilization protocol. Our work shows the potential of solar thermal technology in steam generation and other applications.

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Optically transparent, thermally insulating monolithic silica aerogel, with its high solar transmittance and low thermal conductivity, is well-suited for solar thermal applications, particularly concentrated solar power systems. The properties of silica aerogel are directly determined by the structure of the highly porous, interconnected silica network. By using high temperature annealing to control this structure post-synthesis, we were able to optimize the material to increase solar transmittance using an easy and scalable method.The changes caused by annealing were investigated with respect to both temperature and time to relate the structural change to the optical and thermal performance change. The temperaturedependent study samples were annealed for 1 hour at various temperatures ranging from 400-1000 'C. The time-dependent studies used samples made from two silica aerogel chemistries and were annealed at two temperatures (400 'C and 600 C). In general, lower temperatures and times have less overall change (slower change rates) than higher temperature or longer time annealing. Both annealing studies indicate optical performance has an optimum with respect to annealing time, and additional temperature or time negatively affects optical properties due to appreciable structural change.After the temperature annealing studies were used to understand general trends, the timedependent studies were used to maximize the properties of aerogel for CSP applications. The samples showed an increase in solar spectral transmittance of over 3% while the effective thermal 3 conductivity was shown to increase by as much as 40%, indicating a need to optimize the annealing time for maximum performance. The properties of the characterized aerogels were used to demonstrate aerogel annealing optimization in a concentrated solar power receiver model operating at 400 *C. The model predicted a 1% receiver efficiency increase for an operating temperature of 400 *C by annealing for 24 hours, representing a significant gain in overall system efficiency.

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