Everyone knows that it’s possible to harvest electricity from the sun, but in many locations the sun’s rays are a notoriously temperamental source of energy. So, what if it was possible to create energy from something much more predictable? Say, the night sky.
A group of international scientists have created an experimental device, which uses the same optoelectronic physics used to harness solar energy, that does just this. The first of its kind, the infrared semiconductor device creates a measurable amount of electricity when pointed at the night sky. It works through a process called the negative illumination effect. Unlike a solar cell which works due to an oversupply of photons beyond what the background environment supplies, this effect generates electricity through an undersupply of photons below that provided by the environment. One crucial part of this process is the temperature difference between Earth and space. When the device is pointed at the night sky the coldness of the universe compared to Earth means that heat outflows from the device. The negative illumination effect allows electrical energy to be harvested as this heat outflow happens.
At the moment, the device is a proof of concept and the negative illumination diode can only generate a very small amount of electricity – about 64 nanowatts per square metre – but the team say that the theoretical output is much higher than this. Shanhui Fan, one of the device’s creators and professor at Stanford University, explains that improvements to the device, which he believes are possible to implement, would increase this output. ‘In this paper we provided a calculation of how much one can expect, given the atmosphere, with this kind of technique. If we have an ideal device this comes to about four watts per square metre. I think we can improve the device quite a bit with optical techniques and that’s something we’re interested in developing.’
This theoretical output is roughly one million times what the group’s initial device generated and could be enough to help power machinery that runs at night. That said, it is still much lower than the electricity generated by a solar cell. ‘To put this in context, a solar cell generates a couple of hundred watts per square metre. So, it is small compared to a solar cell,’ says Fan. ‘However, in the absence of the sun, this actually is a pretty large power density compared with other techniques.’ He adds that because an ideal device of this nature would be very similar to a solar panel, the technique could be used to compliment the generation of solar electricity.
This new device is just one strand of the work done by Fan and his team, which largely focuses on the potential to harvest energy from radiative cooling – the process by which bodies on Earth, and the planet itself, radiate heat out into the universe. If this heat loss could be harnessed to produce energy then waste heat, such as that from machines, could be utilised much more effectively than it is today.
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