Effect of aging, thickness and color on both the radiative properties of polyethylene films and performance of the nocturnal cooling unit

This study presents the effects of the changing in color due to aging, thickness and the new color of polyethylene films on the radiative properties. Also, the influences of the radiative properties on the performance of the nocturnal cooling unit used to cool flowing water were studied. The measured data of the transmittance of the polyethylene films having a thickness of 50 mu m and having changes in its color by 5, 30 and 100 days aging and also new film data were used to study its effects on the performance of the cooling unit theoretically. Also, two films of thickness 25 and 50 mu m and two films having a thickness of 25 mu m, with colors of light blue and colorless, were used to examine their effects on the performance of the cooling unit experimentally. The results showed that aging of the polyethylene films leads to substantial degradation in its transmissivity from average values of 0.72, 0.69, 0.57 and 0.42 corresponding to a new one, 5, 30 and 100 days aging in the wavelength range of 8-13 mu m. Also, neither thickness of the films nor their new colors have significant effects on the transmittance. It has been found that the effects of aging of the polyethylene film by 100 days led to the reduction in performance of night cooling by 33.3 %. The decrease in thickness of the polyethylene films from 50 to 25 mu m leads to an increase in its radiative properties (transmittance) and the performance of the night sky radiation unit by 8.6%, approximately. (C) 1997 Elsevier Science Ltd.