A single rooftop surface delivered three energy services at once in outdoor daytime tests: cooling to 6.5 °C below ambient temperature, electricity at 60.6 W/m², and heat up to 110.8 °C. Researchers at the Karlsruhe Institute of Technology (KIT) in Germany, working with colleagues at Stanford University, built the device by stacking a transparent radiative-cooling emitter on top of a concentrated photovoltaic-thermal collector. The results appear in Cell Reports Physical Science.

The design tackles a decade-old limitation in passive daytime radiative cooling (PDRC). PDRC materials — multilayer nanophotonic emitters, single-layer polymers on reflectors, porous ceramics — cool by reflecting sunlight strongly across the 0.3–2.5 µm solar spectrum while emitting strongly in the 8–13 µm long-wave infrared band, where the atmosphere is transparent enough to let heat escape toward the cold of outer space. Until now, however, conventional PDRC systems could not harvest solar energy and the coldness of space from the same surface at the same time. A surface either reflected sunlight to stay cool, or absorbed it to make electricity and heat.

The KIT team's device splits the two functions vertically. The top emitter consists of a silica substrate coated with the silicone polymer polydimethylsiloxane. It transmits visible sunlight but radiates heat through the atmospheric infrared window. Below it, a Fresnel lens concentrates the transmitted light onto a gallium-arsenide-based photovoltaic-thermal collector mounted on a two-axis solar tracker, where the electricity and heat are produced.

The geometry solves what principal investigator Gan Huang, head of the Hybrid Solar Technologies lab at KIT, identifies as the core engineering problem: keeping the solar collector hot while the radiative cooler above it stays cold. "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 says.

The measured figures come from daytime outdoor experiments on the prototype. They are single-device results, not system-level performance data, and the team has not yet published cost or scaling figures. For R&D managers evaluating building-integrated energy technologies, the relevant comparison is against compression-based air conditioning, which currently consumes large amounts of grid electricity and carries a corresponding CO₂ footprint — a load that grows as northern-hemisphere summers warm.

Huang frames the applications concretely. "The new system could be installed on the roofs and facades of buildings to provide these three energy services at once," he says. "It could also be attractive for AI data centres that need both intensive power and cooling at the same time." That dual requirement — power and heat rejection at co-located facilities — is exactly the operating point the hybrid collector addresses.

The physics rests on a large temperature differential. Earth's surface sits near 300 K; outer space sits near 3 K. Objects radiate heat toward that sink whenever they emit more energy through the atmospheric window than they absorb in the solar band, and the effect works during the day if solar reflection is strong enough.

"This is exciting, because it shows that the hot Sun and the cold universe can be harvested together, rather than treated as separate resources as has been the case until now," says Huang of the prototype results.

The team is already working on the next iteration. Huang says the priorities are a better optical design, improved thermal management and more efficient solar cells. "The Sun is not the only renewable energy resource in the sky, the coldness of outer space is another," he says. "We believe there are many exciting possibilities for a new generation of energy systems if we learn how to manage both these together."