Selectively infrared emitting gases for radiative cooling applications

Radiative cooling occurs under clear sky conditions because the atmospheric emittence is low in the wavelength interval 8-13ͧµm particularly if the air is dry. Efficient radiative cooling is feasible with materials which radiate predominantly is feasible with materials which radiate predominantly in the 8-13 µm wavelength range and are non-absorbing elsewhere. This thesis reports on gases which are strongly emitting only in the 8-13 µm wavelength range and which therefore can be used for radiative cooling to low temperatures. We carried out a general discussion of characteristic absorption frequencies due to molecular vibrations and rotations to identify a number of candidate’s gases. Three of the most promising ones-ammonia, ethylene, and ethylene oxide-were studied in detail. Infrared transmittance spectra were recorded for 5-50 µm by spectrophotometry. These data were used to compute the basic cooling parameters and relations between cooling power and temperature difference for pure and mixed gases. Radiative cooling tests were performed by use of simple devices. Cooling to 15◦C and 10◦C below ambient temperature was absolved with ammonia and ethylene gas respectively. The radiative cooling resource was evaluated by means of LOWTRAN computations based on radiosonde data for the dry seasons of 1970 and 1975 in Dar es Salaam, Tanzania under favourable meteorological conditions it is predicted that a cooling power of 20wm² can be achieved for blackbody-like surfaces at ambient temperature, and that a minimum temperature lying 30◦C below that of ambient could be reached with a highly infrared-selective substance incorporated in a thermally well isolated device. Some estimates of radiative cooling resource based on newly developed relations between atmospheric emittance and dew point temperature were also given.