A rigid spectral selective cover for integrated solar heating and radiative sky cooling system

Solar heating (SH) and Radiative sky cooling (RSC) have been recognized as remarkable renewable energy technologies due to their great energy-saving potential. Recently, the combination of SH and RSC (i.e., SH-RSC) has been proposed and developed since the SH-RSC hybrid utilization can harness heat and cold from the sun and universe directly using a single system. At present, polyethylene (PE) film has been widely used in the SH-RSC system as the wind cover, but it is flimsy and easy to age. Here, we demonstrate a rigid spectral selective cover for the SH-RSC system. The cover consists of bulk zinc sulfide (ZnS) and ZnS/ytterbium fluoride (YbF3) anti reflective multilayer coatings on both sides of the bulk ZnS. Spectral characterization shows that the fabricated ZnS-based cover has high transmissivity of 0.86 and 0.89 within the solar radiation band and the atmospheric window, respectively. Moreover, with the rigid ZnS cover, the gap between the cover and the absorber can be achieved to be vacuum condition to maximally reduce the intrinsic energy loss and improve the performance of the SH-RSC system. The thermal analysis demonstrates that the ZnS cover-based vacuum SH-RSC system shows a thermal efficiency of 43.9% at a temperature of 80 degrees C in SH mode and a maximum temperature reduction of 26 degrees C in RSC mode, which are 17.9% higher and 10.5 degrees C lower than that of the typical PE cover based system, respectively. In summary, this ZnS cover provides a choice for enhancing the performance of the hybrid utilization of daytime SH and nighttime RSC processes.