Photovoltaic systems for electricity, solar thermal collectors for heat, and air conditioning for cooling are standard solutions for buildings. Researchers at the Karlsruhe Institute of Technology (KIT) have developed a hybrid system designed to provide all three simultaneously from a single surface. The system combines solar energy harvesting with the cold of outer space through passive daytime radiative cooling (PDRC).
Radiative cooling occurs when the surface of an object absorbs less radiation from the atmosphere and emits more. As a result, the surface loses heat and a cooling effect can be achieved without the need for power.
The hybrid system integrates photovoltaics with PDRC. Its core component is a light-transmitting cooling layer that radiates heat into space as infrared radiation. The transparent layer consists of a silicon dioxide plate coated with the silicone polymer polydimethylsiloxane (PDMS). It allows sunlight to pass through for photovoltaic generation while simultaneously emitting heat into space through the “atmospheric window,” a wavelength range in which the Earth’s atmosphere is particularly transparent to thermal radiation.
Behind the cooling layer, a Fresnel lens concentrates sunlight onto a small photovoltaic-thermal collector, which generates both electricity and heat.
The research team tested the system prototype in outdoor conditions. The device achieved cooling of up to 6.5 C below ambient temperature, an electrical power density of 60.6 W/m², and temperatures of up to 110.8 C for heat utilization.
The main challenge was integrating two competing processes: the solar collector must absorb as much solar energy as possible, while the radiative cooling component needs to remain as cool as possible.
“We solved this problem by having sunlight first pass through the transparent cooling layer, with a Fresnel lens positioned beneath it to focus the light onto a much smaller solar collector,” said Iván Alberto Cruz García, a doctoral researcher and lead author of the study at KIT’s Institute of Microstructure Technology. “This keeps the hot solar collector and the cooling layer physically separated.”
The development builds on research conducted at KIT in 2024, when researchers developed a transparent material capable of passively cooling buildings while allowing daylight to pass through. “Building on that, we have now supplemented the radiative cooling function with a component for solar energy harvesting. This transforms a single surface into a multifunctional energy system,” said co-author Gan Huang.
The researchers presented their findings in the study “Co-harvesting universe coldness and solar energy for tri-generation of cooling, electricity and heating,” published in Cell Reports Physical Science.
The team plans to optimize sunlight guidance, thermal management, and the solar cells. “Our vision is for roofs and facades to become active energy surfaces that simultaneously provide electricity, heat, and cooling,” said Huang. The concept could also be relevant to energy-intensive applications such as AI data centers, which require substantial amounts of electricity and cooling.
Radiative cooling was recently applied to solar panel cooling by researchers from Shanghai Jiao Tong University in China, Purdue University in the United States, the Catalan Institute of Nanoscience and Nanotechnology and the Instituto de Ciencia de Materiales in Spain, and the Jordan University of Science and Technology and the Australian College of Kuwait.
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