Power in the darkness from inverted solar cells
It seems counter-intuitive, but the utter coldness of outer space could offer a new source of energy. Scientists have invented a technology that converts heat that naturally flows towards the cold of space into useful power that could light houses at night, and even help us to live on Mars.
Darkness falls on Palo Alto in California, and a starry sky can be observed over Stanford University. All is quiet on the campus, but not everyone has finished their day’s work. On the roof of one of the university’s buildings, a small group of physicists and electrical engineers is carrying out a ground-breaking experiment. In the cool air of the night, they have erected a device which aims to turn on its head everything that we usually think about solar energy. The scientists check their measuring equipment, and confirm their results. It is a breakthrough: the invention successfully generates power – and not in spite of the dark, cold night sky, but because of it.
The scientists have invented an inverted solar cell. Ordinary solar cells generate electricity when they are exposed to sunlight, but this new technology generates power at night, when it harnesses solar heat radiation into space. The ground-breaking technology is known as negative lighting, and it could provide the world with a new and unique type of renewable energy.
The technology could lead to solar cells that operate 24/7, because they can generate power both during the day and at night. Inverted solar cells could also generate electricity from warm smoke that leaves factory chimneys. The cells might even provide the energy source we will need in order to be able to live for long periods on Mars.
Heat cools houses
The precursor of the inverted solar cell is cooling technology. Huge amounts of power is consumed to cool houses – in the US, 15% of all the energy consumed in buildings is related to air conditioning. That energy consumption could be reduced if the heat could flow into space instead of being removed by air-conditioning equipment. In 2014, that idea led the Stanford scientists behind the inverted solar cell to build a radiator that cools the air around it instead of heating it. The inverted radiator absorbs heat from the air below it and directs it upwards towards the cold of space. Importantly the scientists designed the radiator to emit heat – infrared radiation – at specific wavelengths that can pass through the atmosphere’s gases, which would otherwise curb or reflect the radiation.
The radiator successfully cooled the building below it. But then the scientists wondered whether the cold of space could be used not only for cooling, but also as a source of energy.
Old theory revived
The idea of harvesting energy from heat that flows towards cold dates back to 1824, when French physicist Sadi Carnot had an ‘aha’ moment after wondering why steam
engines work the way they do. He realised that temperature differences can be converted into motion because heat always flows from hot towards cold, and the heat flow can set things in motion en route. He also found a formula for calculating the maximum mechanical energy that any given temperature difference can generate.
The Stanford scientists have taken up this almost 200-year-old idea again. The Earth itself is warm compared to the cold of outer space, because it is constantly absorbing energy from sunlight. If a temperature difference can set things in motion, and electricity consists of electrically charged particles – typically electrons – that move, then it should be possible, the scientists thought, to develop an electronic device that can harvest electrical energy from the termperature difference compared with the cold of space, where the temperature is minus 270.42°C, only 2.73 degrees above absolute zero.
Cold can light a bulb
The scientists’ theory for an inverted solar cell is based on an infrared photo diode, such as those normally employed in infrared detectors such as night vision equipment. An infrared photo diode converts the heat radiation from humans and animals into electrical impulses, which become visible light on a display. But the scientists realised that an infrared photo diode generates electricity not only when it is influenced by exterior heat radiation, but also when it is itself warmer than its surroundings – although in these circumstances the electricity flows around the circuit in the opposite direction. The heat radiation towards colder surroundings ‘steals’ its energy from the electrons of the photo diode, and they begin to move more slowly at the coldest end of the diode. This causes a difference in charge between the warm and the cold ends. If the two ends are linked via an electric circuit, electrons will flow though the circuit to reestablish the balance — and power is generated. That was how the Stanford scientists generated electricity with their photo diode on the roof in Palo Alto in 2019.
In that roof experiment, a thermometer measured a temperature of 20°C, and the wattage was measured at a modest 64 billionth of a watt per square metre. However, the scientists calculate that the method can be optimised to generate 4 watts/m – and consequently, an inverted solar cell of 1m can power an LED light bulb that shines with the same intensity as an old-fashioned incandescent bulb of 40 watts.
Lighting up Mars
Since the new technology can generate environmentally-friendly energy at night, it might illuminate the darkness for the 1.1 billion people of the world who are still not on any electricity grid. Most of these people live in warm countries, and the higher the temperature the more efficiently the inverted solar cells will function.
The inverted solar cell can also be used to generate environmentally-friendly electricity from surplus heat, such as from warm smoke rising from factory chimneys, power stations or incineration plants. With the major temperature rises in these environments the solar cell’s performance is able to rise still further: when the scientists heated their photo diode to 96°C, it generated about 80 times more power than it did at 20°C.
Inverted solar cells could even become key to our exploration of the Solar System. A related technology is already used for the Curiosity rover on Mars (see p76). Curiosity is powered by a generator in which the heat from radioactive plutonium is converted into power. The new technology can generate more power per degree of heat than existing methods. And if humans are to survive on other planets such as Mars, we will need renewable energy 24/7. Mars is further away from the Sun than Earth, so solar cells are not quite as efficient in their operation. On the other hand, the new inverted solar cells could provide energy all through the night on Mars, which has a relatively thin and almost cloudless atmosphere that would do little to curb heat emission.