When two different materials have different temperatures, the temperature gradient causes the charge carriers to move from the hotter material to the cooler material. This conversion of temperature gradient into electricity and vice versa is called thermoelectric effect.
In this era of renewable energy sources, researchers have been developing thermoelectric devices which tap energy from thermal fluctuations. Recently, instead of needing two different temperature inputs simultaneously, MIT has invented a device that takes advantage of changes in ambient temperature during the day-night cycle to generate electrical power.
The New Concept
The thermal resonator, as what the inventors called the new energy-harvesting device, is the first of its kind. “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,” said Michael Strano, one of the inventors.
The results and details of the study were published in the journal Nature Communications by graduate student Anton Cottrill, Carbon P. Dubbs, Professor of Chemical Engineering Michael Strano, and seven others in MIT’s Department of Chemical Engineering. According to the authors, the thermal resonator could supply power to remote sensing systems for years without the need for other power sources or batteries.
Advantages Over Other Energy-harvesting Devices
Although the pilot version of the thermal resonator generates power that is relatively lower than other major renewable sources, researchers say that it has the following advantages:
- It does not need direct sunlight. It generates energy from ambient temperature changes, even in the shade. That means it is unaffected by short-term changes in cloud cover, wind conditions, or other environmental conditions
- Its location and installation are not complicated. It can be situated under a shadow, such as below a solar panel. This allows gathering the energy wasted in solar panels and thus making them more efficient.
- It performs three times better than a commercial pyroelectric material. A thermal resonator was shown to generate three times more power per unit area than a similar sized pyroelectric available in the market. A pyroelectric device is an established way of converting thermal fluctuations into electricity.
The Concept behind Thermal Resonator
The key to the thermoelectric effect of the first ever thermal resonator is the design of the material and a material property called thermal effusivity. The physical meaning of thermal effusivity is how fast a material can gain or lose heat from its environment. It can be thought of as a combination of two other thermal properties of a material – thermal conductivity and heat capacity.
A material’s thermal conductivity describes how fast heat can spread throughout the material, while its heat capacity describes the amount of heat it can store per unit volume. In most cases, these two properties can’t be both high. For instance, ceramic materials can store a high amount of heat, but heat tends to spread slowly through it.
In order to create electricity from temperature fluctuations, the MIT research team decided to optimize thermal effusivity of a material by tweaking its composition or structure. The researchers came up with using a metal foam that is made of copper or nickel. To further increase its thermal conductivity, it was coated with a layer of graphene. Lastly, the metal foam was infused with octadecane, a wax-like phase-change material. That is, at a specific range of temperature, the octadecane solidifies or liquefies.
“The phase-change material stores the heat and the graphene give you very fast conduction,” explains Cottrill, the study’s lead author.
With this structure, the high thermal conductivity part of the thermal resonator gains heat fast. Subsequently, this heat slowly transfers to the phase-change material that stores heat. In this way, one part is always lagging behind the other, creating a perpetual thermal gradient and generate electricity. According to Strano, combining metal foam, graphene, and octadecane makes up “the highest thermal effusivity material in the literature to date.”
The study shows that with just a 10-degree-Celsius temperature difference between night and day, the thermal resonator can generate 350 millivolts of potential and 1.3 milliwatts of power, which is enough to supply small environmental sensors or communications systems.
The thermal resonator is not limited to harnessing energy from fluctuations in ambient temperature during the day-night cycle. With the right tuning in its properties, it could also be possible to harvest other kinds of temperature fluctuations such as that of on-and-off cycles in motors of refrigerators or industrial machines.
It could also be used in landers or rovers to provide low-power but long-lasting energy sources, according to Volodymyr Koman, an MIT postdoc and co-author of the study.
The Untapped Energy
“We’re surrounded by temperature variations and fluctuations, but they haven’t been well-characterized in the environment,” Strano says. These temperature variations are “untapped energy” because there was no known way to harness it until this new invention of MIT. Pyroelectric devices were used to harvesting energy from thermal cycles, but the thermal resonator is the first to be invented that “can be tuned to respond to specific periods of temperature variations, such as the diurnal cycle.”
The thermal resonator could also be used as a complementary energy source so that if one energy source fails, operations of sensor networks will remain running. “They want orthogonal energy sources, if one part fails, you’ll have this additional mechanism to give power, even if it’s just enough to send out an emergency message,” Cottrill says.