Air Conditioning Without Electricity? Beam Heat Into Space, Scientists Say
Scientists at Stanford University have discovered a way to provide cooling sans electricity by beaming heat into space.
The process -- called radiative cooling -- involves beaming heat from Earth's surface into outer space using an experimental thermal emitter, which could give out more heat than it could take in.
The emitter is placed in a vacuum chamber, isolating it from the atmosphere and cutting off almost any heat transfer through conduction or convection, which warms the emitter. The heat coming from the emitter is radiated out of a window on top of the vacuum chamber, which is directed at a clear patch of sky, New Scientist reports.
According to the researchers, Earth's atmosphere allows thermal radiation of wavelengths between 8 and 13 micrometers to pass through and into space. While most objects release heat at different wavelengths, the new emitter was specifically designed to emit heat within that range, allowing it to pass straight into space without being bounced back by the atmosphere on a clear day.
The researchers detailed the results of the experiment in a paper published in the journal Nature Communications. The vacuum chamber pumped air out in about half an hour and the temperature of the emitter dropped to 40 degrees Celsius below that of the surrounding air, the scientists said. Over the next 24 hours, the temperature averaged at 37 degrees Celsius below the air temperature and reached its biggest temperature reduction of 42.2 degrees Celsius when exposed to the peak of the sun's heat.
"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," Zhen Chen, one of the co-authors of the study, told New Scientist.
To address the hurdles of daytime radiative cooling -- such as wavelengths, the atmosphere reflecting light back down or reflectors absorbing too much sunlight -- the research team built upon previous technology called "nighttime cooling" to create the material.
"Basically you have a material that emits very well in the infrared, and that will cool itself down when you send the heat on the outside," Shanhui Fan, co-author of the study, said in a report by FoxNews.com. "What we did was figure out a way to combine [nighttime cooling technology] with a very good reflector for sunlight, so that it doesn't [absorb] heat [from] the sun."
According to Chen, similar technology could be used to refrigerate food and medicine in areas where ambient temperature is high or in air-conditioning units on top of buildings.
The researchers have launched a start-up to explore the commercial use of the technology, but they said the system won't be able to completely replace existing air-conditioning units, as overcast skies almost eliminate the effect of radiative cooling.