Radiative cooling is a passive process where an object loses heat as thermal radiation, requiring no external energy input. This natural phenomenon occurs constantly across various environments, influencing temperatures on Earth and beyond. It relies on emitting infrared radiation, a form of electromagnetic energy, to dissipate heat into the surroundings or directly into the colder expanse of space.
How Radiative Cooling Works
Radiative cooling involves the emission of thermal radiation, primarily infrared light. All physical bodies continuously emit electromagnetic radiation, with warmer objects emitting more and transferring heat away. Unlike conduction or convection, radiative cooling does not require direct contact or a medium, allowing heat to travel through a vacuum.
A crucial aspect of terrestrial radiative cooling is the “atmospheric window,” a specific range of infrared wavelengths where Earth’s atmosphere is largely transparent. This window, typically 8 to 13 micrometers, allows infrared radiation to pass directly through the atmosphere and escape into outer space, which acts as a vast heat sink. Materials can be engineered to be highly emissive within this window, maximizing their heat loss to space.
For effective daytime cooling, materials must also possess high solar reflectance. This property ensures incoming solar radiation is reflected away, preventing heat absorption. By combining high emissivity in the infrared atmospheric window with high solar reflectivity, specialized materials can achieve cooling even under direct sunlight, often reaching temperatures below the ambient air temperature. The ability of materials to emit heat efficiently is quantified by their emissivity, with higher values indicating better radiative cooling performance.
Natural Occurrences of Radiative Cooling
Radiative cooling is a pervasive natural phenomenon. On clear nights, surfaces on Earth efficiently radiate heat directly into the cold, open sky, leading to significant temperature drops. This explains why frost or dew forms on grass and car windshields even when the ambient air temperature remains above freezing, as the surfaces cool locally to the dew point or freezing point.
Desert environments provide another example of natural radiative cooling. During the day, desert surfaces absorb substantial solar energy, but at night, under clear, dry skies, they rapidly lose this accumulated heat through radiation into space. This efficient heat loss causes desert nights to become remarkably cold, often with temperature swings of 15°C to 20°C or more. Ancient civilizations historically leveraged this phenomenon to make ice at night, even when air temperatures were above freezing, by allowing water in shallow containers to radiate heat into the sky.
Some animals utilize principles akin to radiative cooling for thermoregulation. For instance, certain desert ants have specialized hairs that reflect sunlight and enhance thermal radiation, allowing them to offload excess body heat and maintain cooler body temperatures. Similarly, the nanostructured wings of some butterflies maximize heat loss through thermal radiation, helping them cool rapidly even in direct sunlight. These natural adaptations highlight the fundamental role of radiative heat transfer in maintaining thermal balance.
Current and Emerging Applications
The principles of radiative cooling are being harnessed to develop innovative technologies with diverse practical applications. Passive daytime radiative cooling (PDRC) materials are promising for buildings, significantly reducing cooling loads. Applied as reflective paints or coatings on roofs, these materials reflect nearly all sunlight while emitting absorbed heat through the atmospheric window. This leads to cooler indoor temperatures and reduced reliance on air conditioning. Such cool roofs can reflect up to 94% of solar radiation, and have been shown to reduce a building’s cooling load by up to 90% in some cases.
Radiative cooling also enhances the efficiency of solar panels. Solar panels heat up during operation, which can decrease their efficiency and shorten their lifespan. By integrating radiative cooling materials, the operating temperature of photovoltaic modules can be lowered by several degrees Celsius, increasing electricity output. Studies have shown temperature drops of up to 13°C and increased power output by 4.6 W/m² for some systems. This passive cooling method can boost solar cell voltage by as much as 25% and potentially extend the lifespan of concentrated photovoltaic cells by a factor of 4 to 15.
Emerging applications extend to personal cooling and off-grid solutions. Radiative cooling textiles are being developed to create fabrics that keep wearers cooler by reflecting solar radiation and allowing body heat to radiate away. These specialized fabrics can lower skin temperature by 3°C or more compared to conventional textiles, offering potential energy savings by reducing the need for personal cooling. Radiative cooling is also explored for passive refrigeration in remote areas, electronics cooling, and large-scale power generation by cooling power plant condensers, addressing thermal management challenges without consuming additional energy.