Coming: More efficient thermoelectric generators, thanks to nanocomposites

You can add nanocomposites to the list of materials that will turn heat into electricity. Researchers at the Massachusetts Institute of Technology will be describing their work in thermoelectric generators based on nanocomposites at the upcoming IEEE International Electron Devices Meeting (IEDM), Dec. 5-7 in Washington, D.C.

The idea behind thermoelectric power is as conceptually simple as its name implies — turning heat into electricity. However,real-world applicationof the idea is anythingbut easy. The conversionof sunlight into electricityis dominated by photovoltaic(PV) and solarthermal power generationusing optical concentratorsand mechanical heat engines.

Taking a place alongside these technologies is a promising flat-panel, solar thermal-to-electric power conversion technique based on the Seebeck effect. Developed by MIT's Dr. Gang Chen and his research team, the work has implications that could potentially be quite broad. The team's solar thermoelectric generators (STEGs) have hit a peak efficiency of 4.6% — a figure seven or eight times higher than the best flat-panel STEG values reported previously.

From one pair of pn junctions, each about 1 mm3 and developing between 0.06 and 0.08 V, researchers say they can get 60 to 80 mW. In real devices, these will go in series to boost output voltage, as typically done in thermoelectric devices. They also expect efficiency improvements will come from three directions: improving the materials' figure of merit; improving surfaces so they better absorb solar radiation and minimize thermal emission; and from combining the devices with some optical concentration.

“The new device employs high-performance nanostructured thermoelectric ma terials and spectrally-selective solar absorbers in a unique design that exploits high thermal concentration in a vacuum environment,” explains Chen. It works by generating a temperature difference of roughly 200°C between the device interior and ambient air. Rather than using movable mirrors that follow the sun and focus its rays on a tiny area, the new system uses stationary panels not unlike traditional solar panels. Doing so eliminates the need for expensive tracking systems. Similar to silicon PV cells, the team's creation is a solid-state device without moving parts.

Inside a glass vacuum chamber sits a thermoelectric generator covered with a black plate of copper, which absorbs sunlight but does not re-radiate it as heat. The generator's other side is exposed to ambient temperatures. When in the sun, the unit quickly heats up. In comparison with conventional PV panels, the economics of the new device look good. It requires less material and would be much cheaper to produce than typical PVs. Another benefit: It could be integrated into existing solar hot water systems, which would boost the bang for the capital expenditure buck. Although solar water heaters aren't widely deployed in the U.S., they're common in residences throughout China and Europe, where they provide affordable hot water.

“Integrating thermoelectric generators into an existing solar hot water system incurs very little additional expense. A fully installed system that uses selective sur faces and vacuum tubes costs around $300 per family in China,” says Chen.

Materials needed to build these STEGs are made by means of a nanostructured process developed a few years ago by Chen's lab at MIT and at researcher Zhifeng Ren's Boston College lab. The teams continue to improve the materials and use them in complete systems. Beyond the idea of piggybacking onto solar hot water heaters, Chen says that the DoE is supporting research to develop better thermoelectric materials for such applications as capturing waste heat from car and truck engines, which could play an important role in reducing carbon emissions.

Besides working on the new STEG application, Chen is also researching the fundamental aspects of electron and phonon transport in thermoelectric materials. (A phonon is basically a special type of vibrational motion within a crystal lattice.) Bismuth telluride-based alloys are among those having the most useful properties, but others hold promise as well.