Radiative cooling test facility and performance evaluation of 4-MIL aluminized polyvinyl fluoride and white-paint surfaces

Abstract

Surfaces exposed to the night sky cool to temperatures below that of the air.This cooling is due to the nature of the atmospheric thermal radiation. Materials with dissimilar spectral characteristics behave differently.The purpose of the Radiative Cooling Test Facility is to provide a means for quantitative measurement of the cooling rates of exposed surfaces and assemblies. Emphasis is placed upon assemblies which are specifically designed to produce radiative cooling and which therefore offer promise for the reduction of temperatures and/or humidities in occupied spaces.This report documents the hardware and software used to operate the facility, and presents the results of the first comprehensive experiments.A microcomputer-based control/data acqu~s~t~on system was employed to study the performance of two prototype radiator surfaces: 4 mil aluminized polyvinyl fluoride (PVF) and white painted surfaces set f Current Address:American University, Beirut, Lebanon. Princeton University, Princeton, NJ 08540.below polyethylene windscreens.The tested were determined and can be equation Computer simulations of radiative cooling assemblies were used to confirm that, with proper measurement techniques, the measured values of the constants a and b are nearly independent of the magnitudes of wind speed, air temperature, and sky radiance. Thus the cooling performance equation given in this abstract is applicable to a wide range of conditions, requiring only the sky radiance and the absolute air and surface temperatures as input values.-2-CONTENTS

Thermal performance of radiative cooling panels

“…Equations (11) and (12) are in agreement with our earlier work [28,29] and with that of others [8,10] but disagree with those of Landro and McCormick [17]. It is difficult to assess the acc·uracy of their Eq.…”

“…Our measurements for 12 micron aluminized PVF and white .paint radiators were performed with our radiative cooling panel· test facility [28,29]. The radiators are 53 em wide and 94 em long.…”

Characteristics of infrared sky radiation in the United States

“…As an approximate limiting case of marginal radiative cooling we consider a system using a white painted emitter with a conventional polyethylene glazing in a region having a lOoC (lSoF) average sky temperature depression [13]. If the nighttime ambient air temperature is 26°C (79°F) then the surface can lose heat at a rate of approximately 17 W 1m2 if convective heat intrusion is adequately suppressed and a radiator temperature of 22°C is maintained.…”