Thermoelectric devices, which can produce power when one side of the device is a different temperature than the other, have been the subject of a lot of research in recent years. Currently, researchers at MIT have discovered a novel way to transform temperature fluctuations into electrical power. Instead of needing two different temperature inputs all at once, the new system makes the most of the swings in ambient temperature that take place during the day-night cycle.
The new system, called a thermal resonator, could enable nonstop, years-long operation of remote sensing systems, for instance, without necessitating other power sources or batteries, the researchers state.
The findings can be found in the Nature Communications journal, in a paper by graduate student Anton Cottrill, Carbon P. Dubbs Professor of Chemical Engineering Michael Strano, and seven others in MIT’s Department of Chemical Engineering.
We basically invented this concept out of whole cloth. 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.
While the power levels produced by the new system thus far are modest, the benefit of the thermal resonator is that it does not require direct sunlight; it generates energy from ambient temperature variations, even in the shade. That means it is not influenced by short-term changes in wind conditions, cloud cover, or other environmental conditions, and can be positioned anywhere that’s convenient — even beneath a solar panel, in continuous shadow, where it could even allow the solar panel to be more effective by drawing away waste heat, the researchers explain.
The thermal resonator was shown to outclass an identically sized, commercial pyroelectric material — an established technique for converting temperature fluctuations to electricity — by a factor of more than three with regards to power per area, according to Cottrill.
The researchers realized that to create power from temperature cycles, they required a material that is improved for a less known characteristic called thermal effusivity — a property that describes how readily the material can draw heat from its surroundings or discharge it. Thermal effusivity integrates the properties of thermal conduction (how quickly heat can propagate through a material) and thermal capacity (how much heat can be stored in a particular volume of material). In many materials, if one of these properties is high, the other is likely to be low. Ceramics, for instance, possess high thermal capacity but low conduction.
To overcome, the team developed a carefully custom-made combination of materials. The standard structure is a metal foam, made of nickel or copper, which is then coated with a layer of graphene to give an even greater thermal conductivity. Then, the foam is infused with a kind of wax known as octadecane, a phase-change material, which changes between solid and liquid within a specific range of temperatures chosen for a specified application.
A sample of the material made to test the concept revealed that, simply in response to a 10 °C temperature variance between night and day, the miniature sample of material created 350 millivolts of potential and 1.3 mW of power — sufficient to power simple, small communications systems or environmental sensors.
“The phase-change material stores the heat,” says Cottrill, the study’s lead author, “and the graphene gives you very fast conduction” when it comes time to use that heat to produce an electric current.”
Fundamentally, Strano explains, one side of the device captures heat, which then gradually radiates through to the other side. One side always lags behind the other as the system attempts to reach equilibrium. This continuous difference between the two sides can then be harvested through conventional thermoelectrics. The combination of the three materials — graphene, metal foam, and octadecane — makes it “the highest thermal effusivity material in the literature to date,” Strano says.
While the preliminary testing was conducted using the 24-hour daily cycle of ambient air temperature, tweaking the material’s properties could make it possible to harvest other kinds of temperature cycles, such as the heat from the machinery in industrial plants or from on-and-off cycling of motors in a refrigerator.
“We’re surrounded by temperature variations and fluctuations, but they haven’t been well-characterized in the environment,” Strano says. This is partially because there was no known way to harness them.
Other methods have been used to attempt drawing power from thermal cycles, with pyroelectric devices, for instance, but the new system is the first that can be adjusted to respond to specific periods of temperature differences, such as the diurnal cycle, the researchers say.
These temperature differences are “untapped energy,” says Cottrill, and could be a complementary energy source in a hybrid system that, by uniting multiple pathways for producing power, could keep functioning even if individual components stopped. The research was partially funded by a grant from Saudi Arabia’s King Abdullah University of Science and Technology (KAUST), which anticipates using the system as a way of powering networks of sensors that monitor conditions at oil and gas drilling fields, for instance.
“They want orthogonal energy sources,” Cottrill says — that is, ones that are completely independent of each other, such as solar panels, fossil fuel generators, and this new thermal-cycle power device. Therefore, “if one part fails,” for instance if solar panels are left in darkness by a sandstorm, “you’ll have this additional mechanism to give power, even if it’s just enough to send out an emergency message.”
Such systems could also provide low-power but long-lasting energy sources for rovers or landers exploring remote locations, including other planets and moons, says Volodymyr Koman, an MIT postdoc and co-author of the new study. For such usages, much of the system could be made from local materials instead of having to be premade, he says.
This method “is a novel development with a great future,” says Kourosh Kalantar-zadeh, an eminent professor of engineering at RMIT University in Melbourne, Australia, who was not involved in this research. “It can potentially play an unexpected role in complementary energy harvesting units.”
He adds, “To compete with other energy harvesting technologies, always higher voltages and powers are demanded. However, I personally feel that it is quite possible to gain a lot more out of this by investing more into the concept. … It is an attractive technology which will be potentially followed by many others in the near future.”
The research team also included MIT chemical engineering graduate students Albert Tianxiang Liu, Amir Kaplan, and Sayalee Mahajan; visiting scientist Yuichiro Kunai; postdoc Pingwei Liu; and undergraduate Aubrey Toland. It was supported by the Office of Naval Research, KAUST, and the Swiss National Science Foundation.