MIT's thermal resonator generates electricity from the air's daily temperature cycle
Energy is all around us – we just need to work out how to tap into it. Now a team from MIT has developed a device called a thermal resonator, which could essentially pull electricity out of thin air by taking advantage of gradual ambient temperature changes over the course of the day.
Scientists have been experimenting with ways to use temperature fluctuations as a source of energy for years now. Most of these devices work on the thermoelectric principle, meaning they generate electricity by taking advantage of temperature differences between two sides of a material. As heat travels from the hotter side to the cooler side, charge carriers flow with it and create a voltage difference, generating electricity in the process.
Previous work has applied the thermoelectric effect to clothing, paint and cooking pots, and these materials may eventually be built into factories and power plants to recycle waste heat. However, in all of these applications the temperature difference needs to be quite significant. The new technique taps into more gradual fluctuations over longer periods of time, allowing it to work with the natural changes in temperature throughout the day. This is called the pyroelectric effect.
"We basically invented this concept out of whole cloth," says Michael Strano, co-author of the study. "We've built the first thermal resonator. It's something that can sit on a desk and generate energy out of what seems like nothing. We are surrounded by temperature fluctuations of all different frequencies all of the time. These are an untapped source of energy."
The active component of the thermal resonator is a foam made up of copper or nickel that is infused with a phase-changing wax known as octadecane, which liquifies and solidifies at certain temperatures. The foamy mix is coated in a layer of graphene, which is an excellent thermal conductor. Altogether, this specific combination of materials gives the device very high thermal effusivity, meaning it can effectively draw heat from and release heat into its surroundings.
Essentially, heat is captured on one side of the device and slowly radiates through the material to the other and is stored in the phase-changing material in the middle. Since one side of the material will always be cooler than the other, the heat will constantly be moving back and forth as it tries to establish equilibrium. This energy can then be harvested using regular thermoelectric systems.
The researchers tested a sample of the material over 16 days. During that time, temperature swings were up to 10° C (18° F) each day, and the system was able to tap into that to generate 350 millivolts of potential and 1.3 milliwatts of power. It managed to outperform a regular pyroelectric material of the same size.
Its output may be relatively modest, but the researchers say the system would be enough to run low-power, remote sensors and equipment without needing to worry about batteries. And since it makes use of ambient temperature fluctuations, it isn't at the mercy of the elements like solar or wind energy.
This advantage could be key, the researchers say. Being able to operate when other generators can't could make the thermal resonator an important part of a network of "orthogonal energy sources", working together and backing each other up when the conditions aren't right for some of them.
The research was published in the journal Nature Communications.