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In economies around the globe, solar power is increasing at breakneck velocity and is already cheaper than the average wholesale price of electricity. This is encouraging considering the emergency of our climate.

However, the thing about solar energy is that it operates only when the sun is up. But at night, though in comparatively minute quantities, it is also feasible to draw power.

Researchers show an innovative tool in recent research that harnesses the distinction in temperature between radiative bodies and the night atmosphere. The power was sufficient to switch on a tiny LED light, making it appropriate for distant location apps and just about anywhere that requires some power at night.

“Remarkably, the device can generate electricity at night, when solar cells don’t work,” says lead author Aaswath Raman, who works as an assistant professor of materials science and engineering at the University of California, Los Angeles.

electricity from the cold dark night

So how exactly is all of this possible?

Solar cells produce electricity by absorbing photons through a semiconducting material that releases electrons collected on the back of the cell by electrodes fitted. When there is no sunlight, it is still possible to use solar power by storing it for subsequent use in batteries.

Batteries, however, can be costly, so it doesn’t make sense to employ them in specific applications, such as in very remote neighborhoods where you only need a bit of electricity to control some sensors, antennas, or small lights.

Rather than harnessing photons, the researchers exploited radiative cooling, the process by which a body loses heat by thermal radiation. Any sky-facing surface will lose heat to the atmosphere, shooting thermal radiation into space, eventually approaching a cooler temperature than the surrounding air. This is why, for example, you will see frost form on vegetation during any cold nights, even though the temperature outside is above water’s freezing point. By controlling this temperature difference, it is possible to generate electricity.

Raman and collaborators, including Stanford University scientists, tested a machine under a clear December sky that harnesses radiative cooling on a rooftop. The low-cost device comprises of a polystyrene enclosure covered in very lightweight aluminized mylar that minimizes the quantity of escaping thermal radiation.

The device was then placed on a desk one meter right above the surface, drawing heat from the surrounding air and releasing it into the night’s sky through a black emitter.

When the thermoelectric module was connected to a voltage boost converter, it was effectively capable of turning on a low-power white LED. Over six hours, the researchers estimated the power output of 25 milliwatts per square meter.

For illustration, a typical solar cell will generate about 150 watts per square meter in peak conditions, almost 10,000 more than the thermal radiative cooling device.

Raman says that the amount of electricity that can be generated per unit area during the night can be primarily increased by order of magnitude with some upgrades. And since it is made from elementary components that can be purchased off the shelf, the researchers understand there are many applications for which their device can find practical use. It can, for instance, operate in scorching, dry climates and could also act as a radiative cooling component.

Journal Reference: Joule



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