The annual cost of heat-related issues is estimated at about $2.4 trillion, with cooling systems costing an estimated $300 billion and producing 1 Gt of CO2 per year. By 2050, the additional energy needs related to cooling are expected to surpass the total electricity use of China and India today, combined. This is often referred to as one of the most critical blind spots in today’s energy debate, given that the rising demand for cooling will add an enormous strain on the electricity systems of many countries, driving up emissions and triggering a self-aggravating feedback loop.
Passive Radiative Cooling (PRC) materials can dissipate heat through the infrared transparency window (8 μm – 13 μm) without using any electricity, using outer space as a cold and renewable thermal energy sink to reach sub-ambient temperatures even under direct sunlight owing to their tailored optical and infrared photonic properties.
Despite hundreds of promising PRC coatings and devices demonstrated in the literature in the past few years, reliable testing protocols to evaluate their cooling performance have not been established yet, which is a major obstacle hindering the further development and commercialisation of this new technology. Typical tests up to now are limited to measuring either a temperature drop or cooling power with a heater, using improvised testing rigs with inconsistent insulation and shielding properties, unspecified thermal loads and under different atmospheric conditions, altitudes, ambient temperatures, etc.
Defining standardised figures and testing protocols requires the development of a new conceptual framework and a highly multidisciplinary approach improving both the modelling and the characterisation of emissivity and reflectance properties of thin coatings over a broad wavelength range, the realisation of benchmark systems with known properties, the calibration of portable instruments for on-site monitoring, as well as models accounting for the impact of atmospheric and geoclimatic conditions on the expected cooling potential and the design of standardised testing apparatuses with known thermal loads and insulation.