IoT Devices Could Tap Ambient Heat for Power

Modern technology emits huge amounts of energy as waste heat. Researchers in Japan have now significantly improved the efficiency with which this heat can be converted back into useful electricity, which they say could help power the Internet of Things (IoT).

More ambitious visions for the IoT imagine almost every device around us being imbued with sensing and computing capabilities. But current battery technology is simply not compact enough or long-lasting enough to power the countless tiny remote devices that would make this possible without regular recharging or replacement.

An alternative is finding ways to scavenge energy from the devices’ environment, including the large amount of waste heat generated by machines and electronics. One of the most promising approaches involves using the thermoelectric effect, which directly converts a temperature difference into an electrical voltage. The problem is that only certain materials exhibit the effect, and in most cases conversion is very inefficient.

However, previous research has shown that some ultrathin 2D materials could present a workaround, doubling performance compared with that of bulk 3D thermoelectric materials. And now, a new fabrication approach, outlined in a paper in Nature Communications on 16 January, has gone a step further, quadrupling the performance of 2D thermoelectric materials. The authors say the breakthrough brings the dream of powering the IoT on little more than ambient heat a step closer.

“All over the world there are many waste heat sources,” says Yoshiaki Nakamura, a professor of engineering at Osaka University. “The harvested energy is very small, for example 10 percent or less, but IoT sensors can work on very small amounts of energy.”

How to Improve Thermoelectric Efficiency

To improve a material’s thermoelectric efficiency, you need to decrease its thermal conductivity while increasing its electrical conductivity and its thermoelectric power—a measure of how much voltage is generated for each unit of temperature difference. But doing so is tricky because these attributes are interrelated, and after a certain point improving one has a negative impact on others.

While this holds true for conventional 3D materials, in 1993 MIT materials scientists Lyndon Hicks and Mildred Dresselhaus showed that it was theoretically possible to boost efficiencies by using materials that confine electrons to just two dimensions. This can be done using quantum wells, which feature a thin layer of semiconductor sandwiched between two layers of another semiconductor with a different bandgap. This creates an energy barrier that traps electrons in the thin middle layer, where they can move in only two dimensions.

This results in a phenomenon known as “quantum confinement,” whereby the electrons can exist only at specific, discrete energy levels rather than on a continuum, as is normally the case. Hicks and Dresselhaus showed that this effect could boost a material’s thermoelectric power without reducing electrical conductivity and therefore improve the efficiency of energy conversion by a significant margin. In 2018, researchers provided the first experimental proof of the idea, achieving twice the thermoelectric power of conventional bulk materials.

New Quantum-Well Shape Quadruples Performance

In their new study, Nakamura and his colleagues take the idea a step further by tweaking the design of these quantum wells. At a conceptual level, a typical quantum well can be thought of as having square sides. This is because the energy barrier abruptly goes from low potential energy at the base of the well to very high potential energy either side. Nakamura’s team instead designed triangular quantum wells, in which the energy barriers slope up gradually from the base of the well.

This shape significantly alters the distribution of energy levels inside the well, says Nakamura. While square-sided wells typically have only one or two energy levels, triangular wells can have many. The more energy levels available to electrons, the better the electron mobility and therefore the thermoelectric power, he says. What’s more, the energy levels in a triangular well are clustered at higher energies, which further increases the thermoelectric power.

The shape isn’t the only important factor though, Nakamura says. How thin the middle layer of a quantum well can be and still achieve quantum confinement varies between materials. The choice of material also impacts how many energy levels can be squeezed into the well. Nakamura’s team chose gallium arsenide, which has good properties for building quantum wells but is also in commercial use, and so can be precisely fabricated using mature processing technologies.

In their paper, the researchers show that they are able to create a material that achieves four times the thermoelectric power compared with that of the next best 2D materials. As impressive a jump as that is, Nakamura says that when it comes to practical applications this would probably translate to generating power only on the order of 100 microwatts. Nonetheless, he says that would be more than enough to scavenge waste heat from microelectronics to power small IoT sensors at no extra cost.

Nguyen Tuan Hung, an assistant professor of physics at Tohoku University, in Japan, who has worked with MIT’s Dresselhaus on the theoretical underpinnings of thermoelectric conversion in 2D systems, says the new paper is another big step in efforts to boost thermoelectric power.

He points out that the complexity and cost of the kind of 2D systems the researchers are working with means they’re a long way from industrial-scale production. “Nevertheless, this paper showed a marked performance improvement, and the 2D system is being developed very rapidly today,” says Hung. “So this is a very promising approach for the future, when thermoelectricity becomes important in IoT devices.”