Field test and preliminary analysis of a combined diurnal solar heating and nocturnal radiative cooling system

A type of composite surface was manufactured for trial to achieve integrated solar heating and radiative cooling functions. The spectral properties of the composite surface present a relatively clear selectivity in the spectra of solar heating and radiation cooling wavelengths. A combined system for both solar heating and radiative cooling (named SH-RC system) based on the composite surface was mounted together with a traditional flat-plate solar heating system. Comparative experiments were carried out to investigate their thermal performances both at daytime and nighttime. Results showed that the composite surface has a relatively evident spectral selectivity. In diurnal collector testing mode, the thermal efficiency of the SH-RC collector was 62.7% at zero-reduced temperature, which was about 86.4% of that of the traditional flat-plate solar heating collector. In nocturnal collector testing mode, the SH-RC collector had net radiative cooling powers of 50.3 W/m2 on a clear night and 23.4 W/m2 on an overcast night; by contrast, the traditional flat-plate solar heating collector exhibited very little radiative cooling capacity. In diurnal system testing mode, the daily average thermal efficiency of the SH-RC system and the traditional flat-plate solar heating system at zero-reduced temperature was 38.6% and 48.4%, respectively. Based on experimental results, the SH-RC system showed a considerable performance for both diurnal solar heating and nocturnal radiative cooling.