December 28, 2016

Cooling by radiating heat into space

Physicists have achieved record levels of temperature reduction using the process of radiative cooling, by which heat is beamed from Earth’s surface into outer space. Zhen Chen and his colleagues at Stanford University lowered the temperature of a thermal emitter – a device designed to give out more heat than it takes in – to 42.2 °C below that of the surrounding air

“To achieve high-performance cooling, the key is to couple whatever object you want to cool with outer space and to decouple it from the ambient environment,” says Chen. The researchers placed the emitter in a vacuum chamber, isolating it from the atmosphere and cutting off almost any heat transfer through conduction or convection, which could cause the emitter to warm up. Heat from the emitter was radiated out of a specially designed window on top of the vacuum chamber, which was directed at a clear patch of sky.

Earth’s atmosphere allows thermal radiation of wavelengths between 8 and 13 micrometres to pass through it into outer space – but most objects release heat at different wavelengths. The Stanford emitter, however, was specifically designed so that most of the heat it emits falls within that range, meaning that on a clear day it will pass straight out into space without being bounced back by the atmosphere

Within half an hour of pumping air out of the vacuum chamber, the temperature of the emitter plummeted to 40 °C below that of the surrounding air. Over the next 24 hours, it averaged 37 °C below the air temperature, and reached its biggest reduction of 42.2 °C when exposed to the peak of the sun’s heat.

Bigger reduction
Previous attempts at radiative cooling have achieved maximum temperature reductions of up to 20 °C, unless they are at high altitude and with very low humidity.

Jeremy Munday at the University of Maryland in College Park, says the team’s record-breaking results are down to their use of vacuum chambers and sun shades, which prevent sunlight from directly hitting the emitter. “They’re getting significantly below water-freezing temperature during daylight by improving their set-up,” he says.

Nature Communications - Radiative cooling to deep sub-freezing temperatures through a 24-h day–night cycle

Radiative cooling technology utilizes the atmospheric transparency window (8–13 μm) to passively dissipate heat from Earth into outer space (3 K). This technology has attracted broad interests from both fundamental sciences and real world applications, ranging from passive building cooling, renewable energy harvesting and passive refrigeration in arid regions. However, the temperature reduction experimentally demonstrated, thus far, has been relatively modest. Here we theoretically show that ultra-large temperature reduction for as much as 60 °C from ambient is achievable by using a selective thermal emitter and by eliminating parasitic thermal load, and experimentally demonstrate a temperature reduction that far exceeds previous works. In a populous area at sea level, we have achieved an average temperature reduction of 37 °C from the ambient air temperature through a 24-h day–night cycle, with a maximal reduction of 42 °C that occurs when the experimental set-up enclosing the emitter is exposed to peak solar irradiance.

SOURCES- New Scientist, Nature Communications

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