Think of it as the flip side of solar energy: instead of converting the sun’s rays into electricity, researchers from Stanford University and the University of California, Los Angeles (UCLA) are drawing energy from the cold night sky.
If the technology behind their $40 device can be scaled up, it could help off-grid cottagers charge phones or run lights overnight—no batteries required.
The contraption looks like a black vinyl record atop a foil-covered end table. But when the sun sets, it becomes a rooftop “thermoelectric” generator, producing electricity from the temperature difference between the underside of the table (kept warm by the night air), and the black disk (cooling rapidly as it radiates heat into the dark night sky). Couple the warm and cool sides to an off-the shelf thermoelectric module, and, well, let there be light.
The approach is “an ingenious insight,” says Tony Solecki, president of Caframo Ltd., a cottage country firm that makes its own clever thermoelectric devices.
Thermoelectric generation typically relies on combustion to produce the warm side of the process. Caframo’s Ecofan, for example, draws power from the heat of a wood stove to spin a fan, while the Joi lantern fires up LEDs with the heat of a single votive candle.
The California approach goes the opposite route, using nighttime cooling as a passive, combustion-free “source of cold,” Solecki says. “Like solar power, the energy is free,” he adds. “But unlike solar power, the free energy is available at night when it is most needed, avoiding the cost of batteries to store the power.”
Just don’t expect to buy this at your hardware store soon. “There is more research and development work that remains,” warns UCLA engineering professor Aaswath Raman, a Calgary native and member of the research team. The prototype produced just 25 milliwatts per square metre, much less than the 200 watts per square metre possible with a solar panel. Researchers hope to achieve a half a watt per square metre or more, using existing technology.
As a bonus, nighttime cooling works best in dry areas with long nights, including parts of Canada. The system “is potentially attractive in winter months when the sun is low in the sky, or not present if you’re north of the arctic circle,” Raman adds. Solar energy after all, is “not great when the sun isn’t there—or is very low in the sky—for several months.”