Emissivity and Reflectivity Measurements for Passive Radiative Cooling Technologies

Due to their optical properties, passive radiative cooling (PRC) materials can effectively reflect solar radiation while simultaneously dissipating heat through the infrared transparency windows using outer space as a cold and renewable heat sink. This makes it possible to achieve sub-ambient temperatures even in direct sunlight without using any electricity for cooling or air-conditioning. However, the accurate determination of these peculiar optical properties is challenging and subject to high uncertainty levels when using commercial instruments available to industrial end users and research laboratories. Within the EU project PaRaMetriC, aiming at establishing a metrological framework for the comparable performance evaluation of PRC technologies, the Physikalisch-Technische Bundesanstalt is leading a work package dedicated to the development of accurate and traceable methods to determine the infrared optical and thermophysical properties of PRC materials. These include reflectivity and emissivity in the broad spectral range from 250 nm to 50 µm, encompassing both, the solar spectrum (250 nm–2500 nm) and the infrared transparency window of the atmosphere (7.1 μm–13 μm) with a target absolute uncertainty of less than 0.03. For this purpose, several candidate benchmark passive cooling materials have been characterized by PTB in the wavelength range between 1.4 µm and 50 µm. The range 250 nm to 1.4 µm will be covered in an upcoming paper. Characterizations of, and comparisons between, reference and end-user measurement techniques applied for the measurements of selected PRC materials will not only allow accurate determination of the thermophysical properties, but also identification of measurement problems and suitable approaches in this rapidly expanding field.