“The cold darkness of the universe can be used as a renewable thermodynamic resource, even during the hottest hours of the day,” the scientists write in Nature. In tests, the mirror had a cooling power of 40 watts per square metre at ambient temperature. Writing in the journal, Fan puts the installed cost of mirrors at between $20 and $70 per square metre and calculates an annual electricity saving of 100MWh on a three-storey building. Fan said that the mirror could cool buildings – or other objects – simply by putting it in direct contact with them. Coating the roof of a building with the mirror would prevent heating from sunlight but do little to remove heat from its interior. More likely, the mirror would be used to cool water or some other fluid that would then be pumped around the building. He ruled out the idea of using the mirrors to slow down global warming. “Roof space accounts for only a small portion of the Earth’s surface so, at this point, we don’t think this would be a geoengineering solution. Rather, our contribution on the greenhouse-gas-emission issue is simply to reduce electricity consumption,” he said. “I’m really excited by the potential it has and the applications for cooling,” said Marin Soljačić, a physicist at the Massachusetts Institute of Technology. “You could use this on buildings so you have to spend much less on air