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Cooling systems account for nearly 20% of electricity consumption and 10% of greenhouse gas emissions, globally. With demand for cooling expected to grow tenfold by 2050, and the increasing frequency of extreme heat waves, improving the efficiency of cooling systems plays a critical role in addressing the global climate challenge. Passive Radiative Cooling (PRC) materials that can dissipate heat as infrared radiation, have recently emerged. The overall aim of this project is to define the figures of merit and testing conditions enabling the standardized evaluation of their cooling performance and potential energy saving that could derive from such technologies. To date, the annual cost of heat-related issues is estimated at about $2.4 trillion, with cooling systems costs estimated around $300 billion and producing 1Gt of CO2 per year. Within 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 will add an enormous strain on the electricity systems of many countries, driving up emissions and triggering a self-aggravating feedback loop. PRC materials can dissipate heat through the infrared transparency window (8 - 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 commercialization of this new technology. Typical tests up to now are limited to measuring either a temperature drop or a 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 standardized figures and testing protocols requires a highly multidisciplinary approach improving the characterization of emissivity and reflectance properties of thin coatings over a broad wavelength range, the realization of model systems with known properties, the calibration of portable instruments for on-site monitoring, accounting for the impact of atmospheric and geoclimatic conditions on the expected cooling potential and the design of standardized testing apparatuses with known thermal loads and insulation. The overall goal of this project is to establish a metrological framework for the evaluation of passive radiative cooling technologies in order to enable their comparison. This will require the identification of suitable figures of merit per performance indicators, as well as the definition of standardized testing conditions and protocols. The specific objectives of the project are: 1. To develop a metrological framework for the classification and comparison of passive coolers based on key performance indicators (KPIs) for appropriate categories of passive cooler architectures. For this: a. laboratory test methods should be surveyed, and boundary conditions identified for the measurements to ensure comparability in the determination of the KPIs under appropriate conditions b. benchmark materials need to be selected (e.g. SiO2 microspheres) exhibiting reproducible passive daytime radiative cooling performance, to develop micro and nano-structured model systems with well-defined thermal and optical properties. 2. To develop and validate modelling methods to correlate the cooling performance of model systems with the thermal and optical properties of their components, and to establish the materials’ specifications and associated tolerances for quality control. To carry out the thermal infrared spectral modelling of the transmission and emission of the atmosphere at different zenith angles. Additionally, to evaluate the impact on energy savings and heat-island effect for urban environments in different geographic regions on a year-to-year basis under different atmospheric conditions (e.g. humidity, cloud cover). 3. To perform an interlaboratory characterisation and comparison of the reflectance and emittance of benchmark passive cooling materials over a broad spectral range (200 – 50 000 nm) encompassing the solar spectrum and the infrared transparency window of the atmosphere (8 – 13 µm). To develop and validate methods to convert measured infrared radiometric quantities (e.g. total near-normal or near-grazing emissivity) into a usable form for simulations and heat-balance calculations. In addition, to develop best practice guidelines for the conversion of directional to hemispherical emissivity, based on measurements obtained using commercial instruments. 4. To design a testing setup and validate protocols for testing KPIs (e.g. tracking solar irradiance, humidity and wind speed) of candidate passive cooling materials for both indoor and outdoor use and to perform a systematic error analysis, validating the determination of important KPIs at 10 % uncertainty level. 5. To produce a good practice guide on the on-site determination of the performance of passive cooling solutions and their degradation with time and ageing in terms of the above KPIs. To facilitate the take up of the technology and measurement infrastructure developed in the project by the measurement supply chain (testing laboratories), standards developing organisations (CEN/TC 89) and end users in the commercial, residential, and photovoltaic sectors.