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Generator that runs on heat escaping to the sky can charge phones

Technology 12 September 2019 By Alice Klein The new device works at night, when solar panels don’t bjdlzx/GettyA device that makes electricity at night using heat radiating from the ground could be used to power lights and mobile phones in remote locations. Over 1 billion people globally – mostly in poor, rural communities – still don’t…



Technology



12 September 2019

By Alice Klein

Man with tent at night

The new device works at night, when solar panels don’t 

bjdlzx/Getty

A device that makes electricity at night using heat radiating from the ground could be used to power lights and mobile phones in remote locations.

Over 1 billion people globally – mostly in poor, rural communities – still don’t have access to electricity. Cheap solar cells are increasingly used to power lights, mobile phones and home appliances in these communities, but they only work during the day.

Now, Aaswath Raman at the University of California, Los Angeles, and his colleagues have invented a device that makes electricity at night using the thermoelectric effect. This effects allows temperature differences to be converted to electricity.

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Thermoelectric devices have traditionally been used to extract electricity from waste heat from factories and car exhausts, by taking advantage of the temperature difference with the cooler surrounding air.

Raman’s team took a different approach. They created a temperature difference using a mechanism called radiative sky cooling, which causes sky-facing surfaces to become colder than the surrounding air as they naturally radiate heat into the sky. This phenomenon explains, for example, why frost can form on grass even when the air temperature is above zero.

The researchers constructed a polystyrene box with a black disc on the outside facing upwards and an aluminium block on the inside. The black disc was designed to cool down by losing heat to the sky, while the aluminium block was designed to warm up by absorbing heat from the night air. The two were coupled to a commercial thermoelectric generator that converted the temperature difference to electricity.

Light from darkness

The system produced 25 milliwatts of energy per square metre when the team tested it on a rooftop in Stanford, California on a clear night with a midnight temperature of 1 degree Celsius. This was enough to switch on an LED light.

“The energy output could probably be increased 20-fold with improved thermal engineering and operating in hotter climates where the night air is warmer,” says Raman. This would be enough to power lights or recharge a mobile phone, but not to power a cooking stove, he says.

The system cost less than $30 to put together, making it competitive with other technologies for supplying off-grid electricity at night, like batteries that store daytime solar energy for later use, says Raman. However, it may not work as well in cloudy weather or when it is raining, he says.

Thermoelectric generators typically contain toxic materials like lead telluride and bismuth telluride. But with proper packaging, the night-time device should be safe for use for 20 or more years, says Raman. “Especially since we operate it at relatively low temperatures compared to other uses of thermoelectrics,” he says.

Journal reference: Joule, DOI: 10.1016/j.joule.2019.08.009

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