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FOCUS Technology Snapshot - MIT's Stacked Hybrid Solar Converter

The joint MIT/UH research team is working on a three-year development project to demonstrate a high-performance stacked thermal and photovoltaic (PV) solar receiver comprising of a hot thermal absorber and a near ambient temperature PV cell (Figure. 1) – dubbed the Hybrid-Electric-And-Thermal Solar (HEATS) receiver. The innovative stack comprises, from top to bottom, a top aerogel layer that is optically transparent and thermally insulating (OTTI); a spectrally selective thermal absorber (SSTC) in which the working fluid is heated; a bottom OTTI layer; and a PV cell.

The transparent aerogels provide the desired optical transparency and thermal insulation. The top OTTI layer serves to reduce the radiation and convection heat losses from the internally hot region to the ambient, and the bottom OTTI layer keeps the PV cell operating near ambient temperature. The SSTC captures infrared solar radiation below the bandgap of the PV cell, as well as the short wavelength photons that are not efficient for PV energy generation. The absorber is made of a high thermal conductivity material and is thermally linked with the heat exchanger, which efficiently transfers the absorbed solar heat to the pumped fluid.

The stacked configuration enables the sharing of the same concentrating optics mechanism for both thermal and PV components, reducing the complexity and cost of the system. Furthermore, the splitting of the solar spectrum minimizes thermalization losses in the PV cell, thus helps to avoid excessive cell heat up. Based on these innovations and preliminary modeling, the proposed receiver has the ability to achieve maximum solar-to-exergy conversion efficiencies of 36.9% at an optical concentration of 20 suns, providing both highly dispatchable thermal energy at a heat fraction fth of 0.62 from the thermal absorber and highly efficient variable electricity from the PV cell, at a reduced total system cost per unit exergy of $0.9/Wx, and an estimated lifetime of > 25 years.