Passive cooling through the atmospheric window for vehicle temperature control

One way of countering climate change is the efficient use of energy. One of the most energy-intensive activities for a vehicle is the space air conditioning, either for cooling or for heating. In this sense, considerable energy savings can be achieved if air conditioning and cooling can be decoupled from the use of fuel or electricity. The study aims to analyse the opportunities and effectiveness of applying the concept of passive cooling through the atmospheric window (i.e. the 8-14 μm wavelength bandwidth where the atmosphere is transparent for thermal radiation) for the temperature control of vehicles. A recent dr. Thesis work by M Fält at Åbo Akademi (ÅA) has resulted in a skylight (roof window) design for the passive cooling of building space, and this should be applicable to vehicles as well, using the same materials and design concept. An overall cooling effect is obtained if outgoing (long wavelength, > 4 μm) thermal radiation is stronger than the incoming (short wavelength, < 4 μm) thermal radiation. Of particular interest is the passive cooling of a vehicle parked under direct sunlight. The goal is to give engineering designs for passive cooling units for a passenger car or a truck, equipped with a skylight window as designed at ÅA for buildings. The work is done using CFD software implementing (as far as possible) wavelength-dependency of thermal radiation properties of the materials used. For the size of the vehicle, standard dimensions of 4-person family car are considered. The results of this study help in estimating reduced cooling loads for cars. The findings report that by the use of passive cooling, a temperature difference of up to 7-8 oC is obtained with an internal gas flow rate of 0.07 cm/s depending on the configuration and operating conditions. The passive cooling effect of almost 27 W/m2 (COMSOL) from the vehicular skylight is attainable for the summer season in Finland. Comparison of results from ANSYS and COMSOL model shows differences of ~10 W/m2 in the estimation of passive cooling effect for a vehicle skylight.