Researchers at the Karlsruhe Institute of Technology in Germany and Stanford University in the United States have developed a hybrid energy device that can simultaneously cool, generate electricity and produce heat from a single surface. The prototype combines passive daytime radiative cooling with a photovoltaic-thermal collector, achieving cooling of up to 6.5 °C below ambient temperature, an electrical power density of 60.6 W/m² and heating up to nearly 111 °C.
The work, detailed in Cell Reports Physical Science, addresses a longstanding limitation in radiative cooling: conventional systems cannot harvest the coldness of outer space and solar energy from the same surface at the same time. The new design overcomes this by separating the hot and cold functions within a single layered structure.
At the top of the device, a transparent emitting layer made of a silica substrate coated with the silicone polymer polydimethylsiloxane allows sunlight to pass through while emitting heat as long-wave infrared radiation through the atmospheric window. Beneath this layer, a Fresnel lens concentrates the transmitted sunlight onto a gallium arsenide-based photovoltaic-thermal collector mounted on a two-axis solar tracker, where electricity is generated.
«The new system could be installed on the roofs and facades of buildings to provide these three energy services at once,» said Gan Huang, head of the Hybrid Solar Technologies lab at KIT. «It could also be attractive for AI data centres that need both intensive power and cooling at the same time.»
Passive daytime radiative cooling works by exchanging heat between objects at different temperatures through absorption and emission of thermal radiation. When a material absorbs less heat than it radiates to outer space, it can cool down during the day without electricity. The effect relies on strong sunlight reflection at solar spectrum wavelengths and strong heat emission in the long-wave infrared range where the atmosphere is transparent.
Over the past decade, researchers have developed a range of radiative cooling materials, including multilayer nanophotonic emitters, single-layer polymers on reflectors and porous ceramics. However, none had combined cooling with solar harvesting on the same surface until now.
The main challenge, Huang explained, was ensuring that the solar collector became hot while the radiative cooler stayed cool. «We achieved this by concentrating the transmitted sunlight onto a much smaller solar collector underneath the transparent cooling layer, so reducing thermal interference between the hot and cold parts in the device,» he said.
The team is now working to improve the efficiency and practical design of the system. «We are already working on a better optical design, improved thermal management and more efficient solar cells,» Huang revealed.
The development comes as demand for air conditioning rises with global warming. Current compression-based cooling systems consume large amounts of electricity and produce significant carbon dioxide emissions. A device that provides cooling, heating and power simultaneously could reduce both energy demand and emissions if deployed widely.
Huang framed the advance as a shift in how renewable energy resources are viewed. «The Sun is not the only renewable energy resource in the sky, the coldness of outer space is another,» he said. «We believe there are many exciting possibilities for a new generation of energy systems if we learn how to manage both these together.»
