Cool running: Passive radiative cooling to sub-ambient temperatures inside naturally ventilated buildings

Abstract

One priority for avoiding runaway climate change is finding viable alternatives to mechanical air-conditioning. Recent advances in daytime radiative cooling materials are promising. However, researchers have not yet shown how to use them for passive cooling below ambient temperature indoors. Ventilation is challenging in this regard as healthy air changes will heat the sub-ambient interior. We present a field study using analog models to observe how daytime radiative cooling materials can passively reject heat from inside naturally ventilated buildings. We mounted two test boxes on a rooftop in Southern California, replicating in miniature the thermal loads, losses, and air changes for one occupant. The control box represented a reference ‘gold standard’ for passive cooling: internal thermal mass with night ventilation. The test box had an uninsulated metal roof with a top surface for daytime radiative cooling. Both boxes had internal heat sources with ventilation driven not by wind but by buoyancy. Under clear skies, the test box maintained an interior temperature of 3.9 +/- 4.8 °C below the mean prevailing exterior temperature while venting 6.9 +/- 0.3 air changes per hour during the day. In comparison, the control box maintained an interior temperature of 5.0 +/- 1.7 °C above the mean prevailing exterior temperature while venting 8.6 +/- 0.1 air changes per hour during the night. We show with a calibrated model how to improve the sub-ambient temperature stability of the test box with more thermal mass in the roof and how to scale up the results to actual buildings.