What do renewable energies such as solar and wind have in common? Mainly that they are subject to the whims of nature. When there’s no sun or the wind stops blowing, the production of electricity falls steeply. This problem can be solved by using batteries to store electricity and offset the intermittency of these renewable energies or placing them within a wider energy mix that includes hydropower or thermal power plants. There are, however, many areas lacking from any alternative power sources. Also, for the time being, the current battery systems are expensive and not very durable. As Heraclitus the philosopher would say, “change is the only thing that remains unchangeable”, and the variations of temperature between night and day are one of those immutable things. Now, scientists from the Massachusetts Institute of Technology (MIT) have presented a thermoelectric device that harnesses the thermal variations of the day and night cycle to produce energy.
“The thermal resonator harvests energy from temperature variations during the day/night cycle.”
Research on devices that draw power from the difference of temperature between cold and hot sides simultaneously has a long history behind. However, the system devised by the North American researchers can also leverage asynchronous differences of temperature between day and night. The new technology, that goes by the name of “thermal resonator”, can operate under wildly different circumstances, whether in the shade, on cloudy days or even in conditions of fog. To arrive at their conclusions, published in Nature Communications magazine, the scientists carried out a large number of tests in their quest for the optimal materials.
Graphene topping with an octadecane heart
The main obstacle that the researchers had to overcome is thermal effusivity. Basically, this is the ability of a material to store ambient heat and release it later. In turn, thermal effusivity is based on thermal conduction and capacity, i.e., the rate at which heat propagates through a material and the amount of heat that can be stored in a given material. The problem that the scientists faced to leverage these properties was that highly conductive materials are not very good at storing heat and the other way round. In the end, they opted for a combination of materials that include a copper-based metal foam coated with a layer of graphene. Finally, they injected a specific wax called octadecane, which turns solid or liquid according to temperature. The election of each material has a sound logic behind it, as graphene is highly conductive while octadecane stores heat in an extremely efficient way.
The researchers state that the system harnesses daily temperature swings to produce energy but is not limited to the night and day cycle. They also envisage its use in industrial environments, drawing power for instance from the heat released by an engine.
The electricity output of these thermal resonators is comparatively modest, so charging your laptop or electric vehicle is not an option right now. However, the fact that they don’t require batteries or complex electric circuits means they can be an ideal solution for remote regions in need of sensors or communication systems. On top of that, their simple design makes them highly durable, and they could technically remain in operation for decades without human intervention. In the near future, these devices could also allow outer space exploration vehicles to harvest power from the extreme temperature swings in other moons and planets.