Waste heat has been a very tempting energy source, simply because there's so much of it around. Lots of the heat from burning gas and coal can be harvested, but there's always a substantial remainder that's simply not different enough from the environmental temperature to make obtaining any more electricity feasible. Everything from car engines to computer chips need to be actively cooled to prevent the build up of waste heat from damaging the hardware. If we could effectively harvest this waste heat, it would open the door to a far more efficient energy economy.

Given the small temperature differences between these heat sources and their environment, the only realistic option for converting it to electricity is the thermoelectric effect, where some metals and semiconductors form a current along the direction of a heat gradient. Right now, the current generation of thermoelectric materials is almost, but not quite, efficient enough to make economic sense. Two papers released this week, however, describe materials that push us closer to having viable devices.

The utility of thermoelectric materials is usually measured by what's called a "figure of merit," a unit-less value abbreviated as zT. To make sense in specialized applications, we'd want a zT above two; the higher above that and/or the cheaper the material, the more use cases become possible. Right now, some of the best materials we know about have a zT of about one, and some of these involve the use of toxic metals. So we're close, but not quite where we want to be.

Thermoelectric performance can be increased by a variety of methods, and a new paper makes use of one that sounds like it's straight out of California: valley degeneracy. As far as I can work out, this involves creating a material that has multiple locations in its electron orbitals that are all equivalent energy; aligning the crystal properly puts these orbitals in close proximity. Typically, this increases the effective mobility of electrons in the material, allowing the same temperature difference to liberate more current. Highly symmetric semiconductor crystals tend to produce a decent valley degeneracy.

The authors of the paper have created a model that allows them to calculate how valley degeneracy and orbital structure will contribute to the thermoelectric properties of various semiconductors, and spend a fair bit of time testing their model's predictions against real-world behavior. Based on their calculations, they produced a Pb/Te/Se material that was doped with sodium impurities, and measured its zT as 1.8 at 850K. Those sorts of temperatures will limit its use to major industrial and power facilities, but the material is just about there when it comes to efficient thermoelectric production.

The second paper starts from the perspective that this sort of temperature is much too high for many applications, and the toxic metals involved limit its appeal. Closer to room temperature (200°C or 473K), the best semiconductors we have turn in a zT of about one. But the authors try a completely different track, skipping the metal-based semiconductors and turning to an organic conductor, poly(3,4-ethylenedioxythiophene) or PEDOT.

Conducting polymers tend to have properties that make them a poor fit for thermoelectric devices, but they have one thing in their favor: they conduct heat poorly, which helps keep the device from reaching thermal equilibrium. Organic conductors are also cheap, flexible, and easy to process. PEDOT makes a reasonable conductor at room temperature, but oxidizing it reduces its conductivity while enhancing its thermoelectric production. The authors simply tried various degrees of oxidation until they had optimized the thermoelectric effect, finding they could get a zT as high as 0.25 at room temperature. Not as good as the semiconductor, but not so far off.

To demonstrate the value of the organic material, they fabricated several devices—using an inkjet printer. The end result was able to produce over 40 nanowatts/cm2 at a temperature difference of only 10°C, and the authors suggest it could reach the microwatt/cm2 range once the difference reaches 30°C.

Neither of these materials look quite ready for commercial deployment, but both seem like they're right on the verge, and a bit of improved process technology and/or a close chemical cousin might be all that's needed. But the important thing may be that they work very well in very different temperature ranges, meaning that both could find applications in which they'd be useful. Which implies that we're probably not looking for a single killer thermoelectric material as a series of really good application-specific ones.

Nature, 2011. DOI: 10.1038/nature09996, Nature Materials, 2011. DOI: 10.1038/NMAT3012 (About DOIs).