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Air conditioners and other cooling systems are among our biggest consumers of electricity, so finding ways to passively cool buildings will be important in our increasingly warmer future. Now, researchers at the University at Buffalo have developed a prototype hybrid device that can not only cool buildings drastically without using electricity, it can capture solar energy to heat water.

Created in many forms over the years, radiative cooling systems absorb heat from inside a room or building, and emit it in infrared waves towards the sky. At those wavelengths, the Earth’s atmosphere is “invisible” to the radiation, meaning there’s nothing stopping the heat from venting directly into the cold of outer space.

These devices use panels made of materials that can absorb and emit the heat. The logical way to orient these thermal emitter panels is to have one face pointing towards the sky, like a solar panel, but the team on the new study says that’s not the most efficient method. The panels emit heat from both sides, so in that position some of the heat is being emitted back towards the ground.

So for the new design, the Buffalo researchers moved the thermal emitter so that heat could be collected from both sides and transmitted into space. To do so, they positioned the thermal emitter vertically, between a pair of mirrors arranged in a V shape. Those mirrors then reflect the infrared waves up into the sky.

“Since the thermal emission from both surfaces of the central thermal emitter is reflected to the sky, the local cooling power density on this emitter is doubled, resulting in a record high temperature reduction,” says Qiaoqiang Gan, lead author of the study.

In experiments, the team showed that the device was able to lower the temperature inside a test unit by more than 12 °C (22 °F) under direct sunlight, and by more than 14 °C (25 °F) in a simulated nighttime test.

The mirrors are more advanced than they might sound, too. Made with 10 thin layers of silver and silicon dioxide, they’re designed to be selective in how they handle different wavelengths. They reflect the mid-infrared waves from the emitter while absorbing the visible and near-infrared waves from the sunlight. That prevents the Sun’s warmth from cancelling out the cooling effect, improving the efficiency.

As an added extra, the heat absorbed by the mirrors can be put to good use – in this test, the team used it to heat water to 60 °C (140 °F).

“Most radiative cooling systems scatter the solar energy, which limits the system’s cooling capabilities,” says Gan. “Even with a perfect spectral selection, the upper limit for the cooling power with an ambient temperature of 25 °C (77 °F) is about 160 watts per square meter. In contrast, the solar energy of about 1000 watts per square meter on top of those systems was simply wasted.”

The researchers say the device could help reduce the costs and environmental burden of cooling, which remains one of the biggest drains of energy. For now though, the focus will be on scaling it up to rooftop-sized – the test model only measured 70 cm² (27.5 in²).

The research was published in the journal Cell Reports Physical Science.

Source: University at Buffalo