A system capable of passively cooling water up to 5 °C below ambient air temperature, with the potential to reduce the amount of electricity required to cool commercial buildings by about 20%, is reported in a paper published online in Nature Energy this week.

Air conditioning accounts for a significant portion of building energy consumption. Typical systems use a condenser system to cool circulating air, which consumes electricity and heats the local environment. Finding ways to lower the energy usage of air-conditioning systems could save money, reduce greenhouse gas emissions, and reduce the impact of buildings on the local microclimate.

Eli Goldstein, Aaswath Raman and Shanhui Fan have developed an alternative system that uses the sky to accept heat and induce cooling without requiring energy beyond the circulation system. Their system uses a specially designed panel that emits infrared radiation directly into the atmosphere and outer space through a mechanism known as radiative sky cooling. Exploiting this mechanism, the authors pump fluid - in this case water - through their panels and find that they can cool it by 3-5 °C on a rooftop in Stanford, California. They then model the behaviour of this panel when it is integrated into a typical air-conditioning system. If used to remove heat from the condenser of a vapour-compression-based air-conditioning system in a two-storey commercial building in Las Vegas, the authors show a reduction of 21% in the electricity required for cooling over May to August of a typical year. Further tests are required to determine the commercial potential and to optimize the technology.

In an accompanying News & Views, Geoff Smith and Angus Gentle write: “The dual heat sink in the sky has many attractions for cooling efficiently that can now begin to be exploited following Goldstein and colleagues’ demonstration… “