Cooling performance and efficiency of night sky radiators

Abstract

A theoretical equation was derived to predict the surface temperature of night sky radiators as a function of power, Q, from radiator properties and sky conditions. The power of an ideal radiator, R I , which is perfectly black in the 8–14 μm atmospheric window, perfectly reflective outside the window and has a transparent cover was used to define radiator efficiency as Q R I . Plots of Q R I against (T a − T s) R I , where T a and T s are the air and radiator surface temperatures, were primarily dependent on radiator properties and only slightly on environmental conditions. These curves provide a means to compare different radiators and to aid in the design and prediction of performance of night sky radiators. Performance measurements were obtained with three night sky radiators constructed with surfaces of aluminum, white TiO 2 paint, and black paint covered with polyethylene. Similar measurements were also obtained with a fourth radiator that had an uncovered black paint surface. Depressions below air temperature for Q = 0 of 6 and 2.5°C were observed with the aluminum and the black-uncovered radiators at vapor pressures of 0.5 and 2 kPa, respectively. Depressions of the white and black paint covered radiators were about 11 and 6°C at vapor pressures of 0.5 and 2 kPa. Fair agreement with theory was achieved. Calculations of cooling losses from various radiators for the Phoenix, AZ, climate were made. Generally the losses were too small for practical use in July and August, but had potential for other months. The procedure presented can be used to predict the feasibility of radiator use for other application temperatures, climates and radiator properties.