Performance of a CPC inserted in a high vacuum envelope

Evacuated flat thermal systems equipped with a Compound Parabolic Concentrator (CPC) represent a solar thermal architecture useful to meet the growing world demand for mid-temperature heating. The performance of such a system could be improved by coating the inner side of the encapsulating glass with a selective InfraRed (IR) mirror. The underlying physical mechanism is the cold-side external photon recycling, which allows to recover the power radiated by the absorber and, consequently, to increase the solar thermal efficiency. By means of a thermal model and numerical simulations, here we demonstrate how the mirror sensibly influences the panel performance: despite a transmittance drop in solar wavelengths that affects the efficiency below 250 °C, efficiency increases are achieved at higher temperatures.