Cooling Buildings by Radiating Heat to Outer Space

by Emil Morhardt

Global warming is occurring because there is a slight imbalance in the amount of sunlight striking the earth over the amount of heat being lost. The only way the earth can shed heat is by radiating it into space, and the problem is that with the current concentrations of greenhouse gases in the atmosphere, earth isn’t quite warm enough to radiate enough heat outward to stabilize the temperature. One way to address this imbalance, often considered in proposals for geoengineering, would be to decrease the amount of sunlight captured by the earth, say, for example, by reflecting some of it back into space so it doesn’t have a chance to be absorbed. Another way would be to increase the effectiveness of radiating heat into space, but I haven’t seen any proposals for the latter. Stanford University researchers, however, have just figured out how to accomplish both substantial reflection (97%) of solar radiation and an increase in radiation of heat to space in a single device, such that it can passively cool the air in it by at least 5°C (9°F), and theoretically by almost 20°C (36°F) if protected from convective warming (from the wind or breezes). Their prototype ejects about 40 Watts per square meter, which they figure could be improved to 100 Watts per square meter. This occurs in full sunlight with no energy expenditure whatever, and is the first time a device such as this has been implemented. It won’t solve the global warming problem directly, but once commercialized, it could go a long way toward decreasing the electricity consumption of buildings, much of which is needed only for cooling.

The researchers accomplished this by using what they call photonic techniques; they deposited a series of thin layers of hafnium dioxide (but they could have used the much cheaper titanium dioxide) interspersed with layers of silicon dioxide over a thin layer of silver on a silicon wafer. This carefully engineered sandwich reflects almost all the incoming radiation while simultaneously strongly radiating longwave infrared radiation at just the wavelengths that go straight through the atmosphere into space. This being Stanford, I’ll be on the lookout for a Silicon Valley startup commercializing the amazing technology in the not-too-distant future.

Raman, A.P., Anoma, M.A., Zhu, L., Rephaeli, E., Fan, S., 2014. Passive radiative cooling below ambient air temperature under direct sunlight. Nature 515, 540-544.

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