Reverse solar cells generate power at night
The icy outer space can become a new energy source. Scientists have found a technology that turns the heat that rises up towards the universe into electricity. The technology can both light up houses and make it possible to live on other planets.
Reverse solar cell is based on steam engine theory
In 1824, physicist Sadi Carnot discovered that a steam engine works because heat always flows against cold and that this heat stream is converted into motion. In the steam engine, the heat causes the steam to expand so that it bumps into a piston, after which the steam releases the heat again. The principle can be transferred to any machine driven by temperature differences. In a gasoline engine, movement is created when air heated during the combustion of gasoline is expanded and bumps into a piston. Heat can also cause electrons to move, which researchers are using with the reverse solar cell they have recently developed.
Darkness falls over Palo Alto, California, and a starry sky appears over Stanford University, USA.
Everything is peaceful, but not everyone is done with today’s job. On the roof of one of the university buildings, a small group of physicists and electrical engineers are fully occupied with a groundbreaking experiment.
In the cool evening air, they have set up an appliance that can turn upside down on everything we previously knew about solar energy.
The researchers check their measuring equipment and see their big breakthrough: The invention generates power – not despite, but because of the dark, cold night sky.
The researchers have invented a reverse solar cell. Ordinary solar cells generate electricity when the sun’s rays hit them, but this technology generates electricity when it sends out the sun’s heat radiation to the cold space at night.
The pioneering technology is called negative lighting, and with it, the world has been given a new and unique type of sustainable energy.
The technology can lead to solar cells that work around the clock as they can generate power both day and night. Also, reverse solar cells can generate electricity from the hot smoke emanating from the chimneys of the factories.
The cells can prove to be the energy source we lack to be able to settle on Mars.
Heat cools down houses
Cooling technology is the precursor to the reverse solar cell. Lots of power is used to cool houses – for example, in the United States, where 15 percent of all energy consumed in buildings goes to air conditioning.
Electricity consumption can be reduced if the heat could flow out into space instead of being removed by air conditioning systems.
Standford researchers behind the inverted solar cell got the idea in 2014 to build a radiator that cools the air in its surroundings instead of heating it.
The inverted radiator absorbs heat from the air beneath it and sends it up to the cold of space.
The researchers designed the radiator to emit heat – that is, infrared radiation – at certain specific wavelengths that can pass through the gases of the atmosphere (which normally slows the radiation).
The radiator cooled down the building below, which led the researchers to ask: What if space’s cooling can not only be used for cooling but also as an energy source?
The old theory is revived
The idea of gathering energy from heat flowing against cold goes as far back as 1824. Then the French physicist Sadi Carnot got an aha-moment after thinking about why steamers work as they do.
It became clear to him that differences in temperature can generally be turned into motion since heat always flows towards cold. And the heat stream can move objects on the road.
He came up with a formula for how much mechanical energy a given temperature difference can maximize.
The Stanford researchers have now re-embraced the almost 200-year-old idea. The earth is warm compared to the cold outer space because it is constantly hit by the sun’s rays.
The temperature difference can set objects in motion and electricity consists of electrically charged particles – usually electrons – that move.
Therefore, it must be possible, scientists believed, to construct an electronics detail that can collect electrical energy from space’s cold, where the temperature reaches as low as minus 270.42 ° C, just 2.73 degrees above the absolute zero.
Heat radiation rises upward from the earth
The earth is warmed by the sun during the day, but at night the heat leaves the earth again – in the form of infrared radiation that we cannot see.
Heat current generates electricity
The heat passes through an inverted solar cell where the heat current sets electrons in motion, which generates an electric current.
The cold of space “attracts” heat
In space, it is minus 270.42 ° C, close to the absolute zero. Heat always seeks to cool and thus the earth’s heat seeks for space.
Cooling can make a light bulb glow
The researchers’ solution to a reverse solar cell is based on an infrared photodiode, which is usually used for infrared detectors and is often used in, for example, night binoculars.
There, an infrared photodiode converts heat radiation from humans and animals into electrical impulses that then become visible light on a monitor.
The researchers realized that an infrared photodiode can not only produce electricity when it is hit by external heat radiation, but also when it is warm and emits the heat radiation to cooler environments – the current only takes the opposite path around the circuit.
The heat radiation towards cooler environments “steals” its energy from the electrons in the photodiode, and therefore begins to move more slowly at the coldest end of the diode.
It creates a difference in electrical charge between the hot and the cold end.
If the two ends are connected in an electrical circuit, the electrons will run through the circuit to restore balance and tilt: Current.
And that’s how the Stanford researchers created power with their photodiode on the roof of Palo Alto in 2019.
In the researchers’ experiments on the ceiling, the thermometer measured 20 ° C, and the effect measured up to a modest 64 billion parts of one watt per square meter.
But the researchers have calculated that the technology can be optimized and reach up to four watts per square meter – and then a reverse solar cell can, for example, operate an LED lamp that glows with the same strength as an old-fashioned light bulb with a power of 40 watts.
In 2018, researchers Shanhui Fan and Wei Li developed a solar cell that cools down the building it is connected to.
Mars bases can use the technology
The new technology can produce environmentally friendly energy at night – for example, it can give light in the dark to the 1.1 billion people in the world who are not connected to the electricity grid.
Most of them live in warm countries and the reverse solar cell works better the warmer the earth becomes in relation to space.
The inverted solar cell can also be used to extract environmentally-friendly electricity from excess heat, for example from hot smoke from the chimney of a factory, a power plant or a combustion plant.
This is where the solar cell comes into its own because it works better with large temperature differences.
When the scientists behind the technology heated their photodiode to 96 ° C, it gave about 80 times as much current as at 20 ° C.
Reverse solar cells can also be crucial for our exploration of the solar system.
A related technology is already being used for the Curiosity robot car on Mars, which receives power from a generator in which the heat from radioactive plutonium is converted into electricity.
The new technology can generate more electricity per degree of heat than existing methods. And if humans are to survive in settlements on other planets, such as Mars, we need sustainable energy both day and night.
Mars is farther from the sun than the Earth does, so solar cells are not as efficient.
By contrast, the new, inverted solar cells can provide energy throughout the night on Mars, which has a very thin atmosphere almost free of clouds that slows the radiation of heat.