The project

what is passive radiative cooling?

Passive Radiative Cooling is a renewable cooling method by which an object on Earth can cool below ambient temperature by radiating its thermal energy through the atmosphere, to outer space.

This is possible due to the existence of a so-called "Atmospheric Transparency Window" in the thermal infrared range between 8 and 13 μm, through which there are no significant absorption bands of atmospheric gases. This wavelength range coincides with the peak of the thermal (black-body) emission of an object at ambient temperature (300 K), which can therefore dump part of its thermal energy to the cold sink of outer space (3 K). This is a well-known phenomenon which explains why, after a clear sky night, we sometimes find a thin layer of ice over our car windshield or over grass blades, even when the ambient temperature remained above zero (silica and cellulose are both strong emitters in the thermal infrared).

During the day, the down-welling intensity from the Sun is too intense to observe this effect due to the fact that most common materials absorb some fraction of the solar wavelengths, which obliterates the radiative cooling effect.

This limitation can be overcome by engineering the spectral properties of materials so that they exhibit a negligible absorption at solar wavelengths, and emit as much infrared radiation as possible inside the atmospheric transparency window. By doing so, even during the day and underneath direct sunlight illumination, a material can lose more thermal energy than it absorbs from the Sun, thus cooling itself and its substrate to a new equilibrium condition below the ambient temperature.

Atmospheric transparency windows and passive radiative cooling. Absorption and scattering from gas molecules in the atmosphere determine its transparency or opacity in different wavelength ranges.(adapted from Wikipedia). A black dashed line is drawn on top of the spectra, representing the emissivity of an ideal Passive Radiative Cooling sample, with near-zero emissivity over the solar spectrum, and near-unity emissivity inside the thermal transparency window of the atmosphere.

Project NEED

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.


  1. To develop the conceptual framework for comparable performance assessments of passive radiative cooling technologies. This will include the preparation of candidate benchmark PRC materials and preliminary characterisation of their spectral responses, with a view to selecting a subset. Additionally, to define one or more figures of merit to assess the performance of PRC materials.

  2. To develop and validate numerical models to correlate the cooling performance of PRC materials with the thermal and optical properties of their components, and thus to establish their specifications and associated tolerances. This will include carrying out the thermal infrared spectral modelling of the radiative exchanges between PRC materials, the atmosphere and space at different zenith angles for calculations of the net cooling power of the materials. Additionally, to evaluate the potential impact of PRC materials on energy savings and heat-island effect for urban environments in different climatic regions of Europe.

  3. To develop accurate and traceable approaches for determining the thermophysical properties and thermal conductivity of PRC materials, and for converting measured radiometric quantities into a usable form for heat balance calculations. The reflectance and emittance will cover a broad spectral range (0.25 µm – 50 µm) encompassing the solar spectrum and the infrared transparency window of the atmosphere (8 μm – 13 μm). The target uncertainties will be below 3% for emissivity and absorptivity, 5% for the total hemispherical emissivity and below 10% for the thermal conductivity.

  4. To develop setups and protocols for on-site testing of PRC materials, with a target uncertainty below 10% for the figures of merit.

  5. 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 and residential sectors.

work Packages at a glance