Efficiency, passive, sub-ambient cooling, subcooling, vapor-compression
Radiative sky cooling is a passive process that can be harnessed to subcool refrigerants in air conditioning and refrigeration systems, thereby increasing the cooling capacity of the refrigerant, and improving the underlying efficiency of the base cooling system. Here, we demonstrate for the first time the use of a radiative sky cooling-enabled passive fluid cooling panel to improve the efficiency of an air conditioning system by subcooling. The panel’s passive cooling capability is enabled by a multilayer optical film that enables the sky cooling effect 24-hours a day. The film is simultaneously a good reflector of solar energy and a strong emitter of infrared heat in the 8 to 13 micron wavelength range. Multiple such panels were built and then connected in a closed fluid loop to two 1-ton split air conditioning units in a field trial in Davis, CA. The panels were used to subcool refrigerant out of the condenser by rejecting heat to the sky via a closed fluid loop. Refrigerant R410A was passed through a counterflow plate heat exchanger, where the cold fluid source was the circulating water/glycol solution in the panels. As much as 15˚F of additional subcooling was observed during the hottest time of the day. This resulted in calculated net efficiency improvements up to 8%. The only added operating electricity required was to run a small circulating water pump, which consumed less than < 1% of total compressor power. These results reveal the remarkable ability of radiative sky cooling to markedly improve the efficiency of vapor compression systems.as an add-on technology.