Solar panels work great during the day as a viable alternative energy source to fossil fuels. But once the sun sets, and photovoltaic cells can no longer convert photons into electricity, how will we power our cities at night? Better batteries, solar fuels, and other energy storage solutions are the conventional approach to tackling intermittency in energy sources. But what if there was another solution that was literally right under our noses?
The Earth emits 1017 watts of energy every night in the form infrared radiation (IR) it absorbed from the sun during the day. It’s not at all obvious how one would even begin to tap into this invisible energy source. Fortunately, co-inventor of the infrared quantum-cascade laser Federico Capasso is perfectly comfortable with the unconventional, especially when it comes to IR, optics, and electronics.
To tackle the challenge of harnessing infrared energy, Capasso and his team of researchers at the Harvard School of Engineering and Applied Sciences (SEAS) first looked at two traditional types of solar electricity generation:
Solar Thermal, which involves heating an object with sunlight and using the temperature difference to spin a turbine.
Photovoltaics, which involves absorbing sunlight and converting it into DC current.
They then asked themselves what might happen if they “reversed” the idea behind these applications to design something more suitable for working with IR:
Radiative cooling, which involves cooling an object by radiating IR heat into the sky, and using the temperature difference to spin a turbine.
A novel optoelectronic approach which involve emitting infrared light to generate DC current.
They took this thought experiment further and came up with the emissive energy harvester (EEH). Let’s take a closer look at the two versions of EEH proposed by Capasso and his team.
Emissive Energy Harvester (Thermal)
At the macroscale, an EEH would run like a solar thermal power plant run in reverse. Compared to the vacuum of space, the surface of the Earth is very warm, even at night. A thermal EEH design involves stacking a “hot” plate at the ambient temperature of Earth with a cold plate on top of it. The cold plate would be coated in a highly emissive material that cools efficiently by radiating heat into the sky. Using basic thermodynamic principles, a heat engine could be created that takes advantage of the temperature differential to produce work.
Emissive Energy Harvester (Optoelectronic)
The second type of EEH device also relies on a radiative cooling induced temperature differential, only this time between nanoscale electronic components. The mechanism by which electricity is generated in this application however is a bit more complicated.
J.B. Gunn, the physicist best known for inventing the Gunn diode used in police radars and microwaves, demonstrated that when a diode in a circuit is at a higher temperature than a resistor it can induce unidirectional current flow which produces a positive voltage. A microscopic antenna could serve as the resistor in Gunn’s circuit, radiating IR towards the night sky while producing a positive voltage.
In the optoelectronic approach, an EEH would use a plate coated in microscopic rectennas (a circuit consisting of an antenna and a diode) to generate power directly from the radiating process, removing the intermediate step of having to cool a macroscale physical object.
There is still a lot of work to be done on the EEH, before a working prototype can be built and tested, and Capasso and his team only unveiled the idea in 2014. Still, the ability to generate energy at night would make it a nice companion to solar cells. With new advances in nanomaterials, small scale electronics, and fabrication processes, it’s only a matter of time before someone can bring an EEH device to life.