Talk about thinking outside the box. American researchers are working on a revolutionary new type of solar panel that produces energy from darkness rather than sunlight.
A team at Stanford University in Palo Alto, California, published their study findings on September 12, 2019. In a nutshell, the scientists have built a thermoelectric generator whose cold side radiates heat to the sky.
The night-time temperature difference between the colder surrounding air and the surface of the new device cools it by emitting infrared radiation into the night sky.
Most people have heard of solar panels that convert sunlight to usable electric power. Most such devices rely on a physical process known as the photovoltaic (PV) effect where light exposed to special materials produces an electrical current.
Other types of solar-powered collectors produce electricity from heat through thermal processes: the difference in temperature between the hotter Sun and the relatively cooler Earth can be translated into usable energy.
The Stanford innovators began with the second type of solar panel technology and inverted it, developing a system where the Earth is the heat source:
“While the solar panel uses the heat difference between the sun and Earth with the Earth being the cooler side – their system makes use of the heat difference between the coolness of the night atmosphere and the Earth with the Earth being the hotter side.”
The device features a thermoelectric generator with one side exposed to the ambient air temperature. The other side contacts an aluminum plate – the anti-solar panel – which stares up into the night sky and radiates thermal energy upward and outward. This radiant energy reduces the plate’s temperature to less than about 35.6 degrees Fahrenheit (2 degrees Celsius) of the lower part of the device which has the same temperature as the surrounding air.
The device was constructed with an 8-inch aluminum disk painted black attached to commercial thermoelectricity generators. The disks emit radiant energy and are typically several degrees cooler than the surrounding air.
Heat (thermal energy) flows from Earth into the air. From there, the air passes through the thermoelectricity generators and into the disk, which, in turn, radiates the heat upward.
Shanhui Fan, one of the study authors and a Stanford electrical engineering professor, said there is plenty of power available for the new radiative cooling panel:
“The amount of power coming in from the Sun has to be approximately equal to the amount going out from the Earth as thermal radiation, in order to keep the Earth at a roughly constant temperature. The amount of power available for harvesting is very large.”
The Stanford team was able to power an LED bulb using only about $30 worth of equipment.
The study, published in Joule magazine, noted that their invention could help the large percentage of the global population that still has no access to electricity, especially after the sun goes down when photovoltaic systems stop working:
“The ability to generate electricity at night could be a fundamentally enabling capability for a wide range of applications, including lighting and low-power sensors.”
The Stanford group’s low-cost solution harnesses the cold of space and generates electricity with an off-the-shelf thermoelectric generator through radiative cooling:
“Unlike traditional thermoelectric generators, our device couples the cold side of the thermoelectric module to a sky-facing surface that radiates heat to the cold of space and has its warm side heated by the surrounding air, enabling electricity generation at night.”
The researchers were able to produce 25t mW/m2 (milliwatts per square meter), enough energy to power one small LED, and validated that their approach is viable:
“Further, we show that the device can directly power a light-emitting diode, thereby generating light from the darkness of space itself.”
During daytime hours, the anti-solar panel could operate in reverse, absorbing sunlight and producing electricity from heat passing from the Sun to the disk and on to the surrounding space.
The new power source is at the proof-of-concept stage. Given more insulation and more optimal conditions – such as a drier climate – the Stanford developers aim to increase the output of their invention up to 0.5 watts per square meter (1.2 square yards) of disk area. Larger disks might be powerful enough to light a home 24/7 – day and night.
Study author Aaswath Raman, Assistant Professor of Materials Science and Engineering at the University of California, Los Angeles, cautioned that the anti-solar panel will never compete with solar panels that can produce up to 100 times more power than their darkness-driven alternative. However, it would be cheap to install and operate, work for a longer time than a battery, and generate electricity when solar panels are inoperable – at night and on overcast days.