Energy

Electricity-free radiative system cools buildings and heats water

Electricity-free radiative sys...
An artist's imagining of the new radiative cooling system
An artist's imagining of the new radiative cooling system
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An artist's imagining of the new radiative cooling system
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An artist's imagining of the new radiative cooling system
The new radiative cooling system is composed of two mirrors in a V shape, with a vertical thermal emitter panel in the middle
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The new radiative cooling system is composed of two mirrors in a V shape, with a vertical thermal emitter panel in the middle

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.

The new radiative cooling system is composed of two mirrors in a V shape, with a vertical thermal emitter panel in the middle
The new radiative cooling system is composed of two mirrors in a V shape, with a vertical thermal emitter panel in the middle

“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

10 comments
NMorris
It won't work nearly as well under cloudy skies where the place that it radiates to will not be at 0 K but much closer to 0 C, and if the unit is on a roof there will be significant cleaning issues for the mirrors, unless the whole thing is under glass or can be cleaned easily
paul314
There's a certain irony in something like this being used to counter the greenhouse effect, which is itself about wavelength-selective energy transmission and absorption by the atmosphere. (Scientists have been working on this kind of tech for about 50 years, but it's only in the past decade that wide-area wavelength selective coatings have become cheap enough to make it plausible for practical use.)
EJ222
Sounds expensive, in addition to the issues NMorris mentioned.

Polaris1983
Waits for the issues to be cleared up after r and d info done on cooling/heating climate controls and ai making better smart materials for batteries and pc cases houses cars jets and rooms in GPU/CPU/SSD/HDD/RAM mining situations to cut back in Kw usage on home smart meter and for better cooling in computer parts/pc cases and laptops/smartphone/smart tv designs as I am no fan of this sort of nonsense being a gamer, not a miner post reddit scandal. Id rather still pay in cash for my crap.
Trylon
@NMorris, you don't understand the physics. It's radiative cooling, not conductive. That doesn't rely on a delta-T. The temperature of "the place that it radiates to" is irrelevant. Likewise clouds. As noted by the article, water vapor, as in clouds, is essentially transparent to the wavelengths this system is designed to work at.
Ylime Bangoy
This sounds great -- passive cooling and solar hotwater heating, seems like this has the potential to replace home air conditioning and hot water system, along with swimming pool and hydronic heating boilers.
Username
So it takes ambiant room heat and "shoots" it in outer space. Therefore it needs to be put under a skylight or it will simply heat the ceiling. Additionally, their magic mirrors are actually so inefficient at redirecting the heat that they can be used to heat water. I doubt this contraption, if ever built, will do anything.
ClauS
It seems that some Users did not understand how this system works. First of all, like light, infrared waves are spread over a large range of wavelengths. It seems that the atmosphere is "transparent" for mid-infrared waves, meaning that mid-infrared waves are not heating the atmosphere. The cooling device can emit these mid-infrared waves on both sides, therefore the need for the mirrors to direct the waves to the sky. But the visible and near-infrared waves from the sunlight would heat the device, reducing the efficiency. That's why the mirrors are made to absorb those specific waves. They are heated by sunlight, not by the cooling device. And obvious it will stand outside, probably on the roof, and the heat from the space to be cooled will be transported with a coolant like water.
Username
@ClauS - There is no mention in this article about pumping heat to an exterior unit with a liquid transport. That may be the case but unless you have information from outside this article you are just guessing. That model also leaves something to be desired. You're saying the system will use liquid to absorb interior heat, transport it outside to be cooled while another liquid is being heated by excess energy. So basically a heat exchange between two liquids. Furthermore pumping the liquid would require power, yet the article states this system is passive.
ClauS
@Username, yes, you could say that I have information from outside this article. Please check the first mentioned article or this older one: https://newatlas.com/cooling-system-heat-into-space/51188 . And, as I mentioned before, there is no heat exchange between the liquids. One liquid is cooled by radiating mid-infrared waves, the other is heated by sunlight.