Hybrid system integrates power, heating and cooling

Photovoltaic panels generate electricity, solar thermal collectors provide heat, while cooling is usually supplied by air-conditioning systems that themselves consume electricity. As a result, buildings require different technologies competing for the limited space available on roofs and facades.

Now, a team of researchers led by Dr Gan Huang at the Karlsruhe Institute of Technology’s (KIT) Institute of Microstructure Technology, has developed a hybrid energy system that simultaneously provides cooling, electricity and heating from a single surface. This hybrid PDRC-solar system combines photovoltaics and solar thermal with passive daytime radiative cooling (PDRC). At the heart of the system is a transparent cooling layer that radiates heat into outer space as infrared radiation while allowing sunlight to pass through. The researchers successfully demonstrated the concept with a prototype under outdoor conditions and see it as a potential building block for multifunctional building envelopes.

The researchers see applications wherever cooling and energy are required simultaneously. “Our vision is that roofs and facades could become active energy surfaces that simultaneously provide electricity, heating and cooling. The concept could also be attractive for energy-intensive applications such as AI data centres which need intensive power and also cooling,” Huang said.

Radiating heat through the atmospheric window

The system features a transparent emitter consisting of a silica substrate coated with the silicone polymer polydimethylsiloxane. The layer allows sunlight to pass through while simultaneously emitting heat as infrared radiation through the atmospheric window — the wavelength range in which the Earth’s atmosphere is highly transparent to thermal radiation. Beneath the transparent emitter, a Fresnel lens concentrates the transmitted sunlight onto a compact photovoltaic-thermal collector, where electricity and heat are generated.

During outdoor experiments, the prototype simultaneously achieved cooling of up to 6.5°C below ambient temperature, an electrical power density of 60.6 W/m², and heating temperatures of up to 110.8°C.

‘Fire’ and ‘ice’ in one system

“We demonstrate that a single system can simultaneously harvest the hot sun and the cold universe,” Huang said.

The greatest challenge was to combine two fundamentally opposing processes. “We are bringing together ‘fire’ and ‘ice’. While the solar collector should absorb as much solar energy as possible, the radiative cooler must remain as cool as possible,” Huang said.

Transparent cooling layer separates hot and cold

“We solved this problem by allowing sunlight to pass through the transparent cooling layer first. Beneath it, a Fresnel lens concentrates the light onto a much smaller solar collector. This keeps the hot solar collector physically separated from the cooling layer,” said Iván Alberto Cruz García from KIT’s Institute of Microstructure Technology and first author of the study published in Cell Reports Physical Science.

Towards multifunctional energy systems

The study builds on the team’s previous work. In 2024, the researchers introduced a transparent material that passively cools buildings while transmitting daylight. Building on this concept, the team has now integrated radiative cooling with a solar energy harvesting component. “Based on this work, we have now combined radiative cooling with solar energy harvesting. This transforms a single surface into a multifunctional energy system,” Huang said. The next steps include improving the optical design, thermal management and solar cells.

For Huang, the study — carried out in collaboration with Professor Shanhui Fan of Stanford University, a leading expert in PDRC — represents a shift in perspective: “The sun is not the only energy resource in the sky. The coldness of outer space is another renewable resource. If we learn to harvest both the hot sun and the cold universe together, we can unlock a new generation of energy systems.”

Image caption: Prototype of the hybrid sun-universe energy harvester for the simultaneous generation of electricity, heating and cooling. Image credit: Gan Huang, KIT.