Feasibility Study on the Use of Wintry-Rainwater for Radiative Cooling

The holistic aim of this study is to make clearthe possibility of passive cooling systems usingnatural cool sources produced by cyclic weatherchanges. Here in this study, we focused on apossible use of rainwater for cooling. First, weset up two rooms in an experimental apartmentbuilding: one has radiative cooling panels andexternal shading; and the other internal shadingalone. The panel surface temperature of theradiative cooling was controlled atapproximately 24 C and the water to besupplied in the panel was produced by a heatpump system. We measured the indoor thermalenvironment of these two room spaces.We then made a numerical analysis on theuse of rainwater as a source of coolness, whichmay contribute to more efficient use of heatpumps. We estimated the accumulated watervolume and its temperature from winter tosummer using annual weather data of Tokyo,and calculated the amount of cool exergy.The rainwater was assumed to be collected ina tank underground. The rainwater collection ofeight months from October 1st to March 31st allows 1.7 MJ/m3 of cool exergy to be generated.The exergy of the rainwater increases graduallywith the rise of outdoor temperature towards thesummer. If this rainwater with cool exergy iscirculated in the radiative cooling panel of3.2m2 in a room of 20m2 of the floor area, therainwater can supply cool exergy to theradiative cooling panel for 40 days. We compare the exergy consumption of a radiative cooling system using the rainwaterwith that of a heat pump for a convectivecooling system. In the radiative cooling systemusing rainwater, 24 W of exergy is supplied tothe power plant to deliver 2 W for a circulatingpump of cool exergy contained in the waterfrom the tank to the radiative cooling panel. Onthe other hand, in the case of convective coolingsystem with a heat pump, 472 W of exergy issupplied to the power plant and 60 W of coolexergy is produced. The input exergy to thepower plant is very much different from eachother case. In the case of convective coolingsystem, the input exergy is twentyfold largerthan that in the case of radiative cooling withthe use of rainwater.The result obtained from this calculationclarified the value of coolness of wintryrainwater, namely the cool exergy, which is tobe produced by a smart use of the yearly cyclein the nature.