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Turning waste heat into electricity?twice as efficiently

Researchers double the efficiency of a heat-harvesting thermoelectric material …

Thermoelectric materials have long held the promise of being able to harvest waste heat and turn it into electrical energy. Although useful in applications other than energy generation, thermoelectrics have long suffered from poor efficiency, limiting their wide-spread use in energy generation. Researchers at Ohio State, CalTech, and Osaka University have collaborated to double the efficiency of a commonly used thermoelectric material by doping it with Thallium.

Thermoelectric materials function in a manner analogous to a heat engine that uses the Carnot cycle that every student of thermodynamics is forced to become intimately familiar with. Instead of moving fluids across a temperature gradient, thermoelectric materials move electrons. This has the advantage of taking up much less space than a heat engine, and thermoelectrics have no moving components to wear out. 

A thermoelectric material's measure of efficiency (zT) can be improved in a variety of ways. zT depends on the thermal and electrical conductivity of a material, the temperature of operation, and the Seebeck constant of the material, which deals with the charge carriers and the states they occupy.

There are hard limits on gains that can be made by increasing the zT of a thermoelectric by decreasing thermal conductivity, due to the nature and mechanics of phonons (heat's quantum relative of the photon). This avenue has been explored; fully realizing the gains here involves nanostructuring the material. Nanostructures are difficult and expensive to produce, and they tend to evolve and grow at high temperatures—the sort of temperatures thermoelectrics normally operate at.

The new research involved manipulating the electronic states of a thermoelectric material by doping it with impurities, much in the same way semiconductor materials for transistors are manipulated. The material in question, lead telluride (PbTe), is a workhorse in the thermoelectric world and, like other semiconductors, it will readily incorporate an impurity from a nearby member of the periodic table. In this case, the authors explored the use of Group III elements, which include elements commonly used as dopants in semiconductors, such as indium, gallium, aluminum, and thallium.

Doping alters orbitals in PbTe

In this case, modeling showed thallium to be the optimal choice for engineering the electronic states of the PbTe. Thallium doping creates additional energy levels (called resonance levels) in the valence band, where electrons would reside at room temperature before being stimulated into higher energy bands by thermal excitation. Other Group III elements also create resonance levels, but in positions not advantageous for improving a thermoelectric material.

The  increase in the available electronic states in the valence band contributed to an increase in the Seebeck coefficient. As a result, the zT of the Tl-PbTe rose to 1.5, more than double the zT of 0.71 achieved using the best PbTe alloys currently available. It's important to note that these gains were made without any structural manipulations to reduce the thermal conductivity. Electronic state manipulation and structural manipulations are not mutually exclusive, and could be combined to further increase the zT value. Additional dopants. The authors speculate that additional dopants can be used in conjunction with thallium to manipulate other parts of the energy bands, further increasing the performance.

PbTe is relatively well understood from an industrial standpoint, but it is only used in niche applications due to its low efficiency. Virtually any energy producing or consuming device generates waste heat that could potentially be harvested by thermoelectric materials. This could include harvesting waste heat from solar arrays, car engines, gas turbine engines, and even today's top-of-the-line, groin-immolating laptops, making those once unsightly thigh burns a sign of your commitment to energy efficiency.

Science, 2008. DOI: 10.1126/science.1159725

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