Testing and Modeling Passive Daytime Radiative Cooling Devices

Passive Daytime Radiative Cooling (PDRC) is a phenomenon that enables a surface to cool below ambient temperatures when exposed to solar radiation without the input of an external power source. Potential applications include reducing the need for air conditioning in hot climates. This project aims to model, fabricate, and test PDRC devices. These devices are designed to reflect the majority of incoming solar radiation; their emissivity is tailored to emit most of their thermal radiation in the atmospheric window such that the devices exchange heat with deep space. Our initial test devices are fabricated by depositing a thin layer of thermally evaporated silver onto a rigid substrate made of silicon or fused silica. The top layer is a roughly 500 microns thick coat of polydimethylsiloxane (PDMS), for which the emissivity is well matched to the atmospheric window in the infrared. Under summertime rooftop conditions in southern California, we have demonstrated that these devices can cool to 3-degrees Celsius below ambient temperature and to 20-degrees below the temperature of uncoated control samples. The experimental work is accompanied by a power balance model that quantifies the expected cooling of a device, given measured reflection, transmission, and absorption spectra.