Nocturnal Radiation Cooling Tests

Abstract The sun radiates heat to earth in the day. At night the reverse happens when the warmer earth radiates heat to the cold night sky. Roofs of buildings radiate heat day and night at a rate of up to 75 watts per square meter. During the day, this is offset by solar radiation gains on the roof, however, at night, this heat loss has the ability to cool air as roofs can experience a temperature drop of 6 to 20 °C below ambient. Cooling a building by long-wave radiation to the night sky has long been identified as a potentially productive means to reduce space cooling energy in buildings but the technology has not been commercially available. A series of tests have been completed at Canada's National Solar Test Facility and data shows that it is possible cool air a few degrees below ambient from sunset to sunrise using a perforated metal panel system mounted onto a sky facing surface. This paper will summarize the night cooling tests, compare the data with published material from ASHRAE and suggest methods for utilizing nocturnal radiation to cool buildings.