Thermoelectric devices could save energy by tapping waste heat

Energy lost from hot engines and combustion systems could save billions of dollars if it could be captured and converted into electricity via thermoelectric devices, Clemson University physicist Terry Tritt told scientists gathered in Dallas for the NanoTX ’07 conference. Tritt delivered an address at the Alan MacDairmid Memorial Nano Energy Summit on challenges in alternative energy, specifically thermoelectricity used to generate electrical energy from waste heat.

Thermoelectric generators are based on materials that are special types of semiconductors. When coupled, they function as a heat pump: a temperature gradient is applied across a sample, electrons diffuse from the hot to the cold part due to the larger thermal speed of the electrons in the hot region, a charge difference then builds up between the hot and cold region, creating a voltage and producing an electric current (schematic, click to enlarge).

Thermoelectric materials can be used for either cooling or power generation. Although current devices have a low conversion efficiency of around 10 per cent, they are strongly advantageous as compared to conventional energy technologies. The converters have no moving parts and are therefore both reliable and durable.

Such waste heat recovery technologies could increase the efficiency of small bioenergy power systems and even of ordinary biomass cooking stoves (an example of research in this context). Large biomass power systems allow for polygeneration and the use of heat in distributed (district) heating and cooling systems. But small biomass power systems generate equally large amounts of waste heat that can not always be used in such a straightforward way. Thermoelectric generators could recover this waste heat and convert it into more electricity.

Many more applications can be envisioned. One of the more interesting ones involves capturing waste heat from cars' internal combustion engines.

Thermoelectric generators are currently used in NASA’s deep-space probes to convert the heat of radioactive elements to electrical energy, powering these systems for over 30 years. Thermoelectric energy conversion is a solid-state technology that is environmentally friendly. One of the more promising ‘down-to-earth’ applications lies in waste-heat recovery in cars. - Terry Tritt, Clemson University

More than 60 percent of the energy that goes into an automotive combustion cycle is lost, primarily to waste heat through the exhaust or radiator system. Even at the current efficiencies of thermoelectric devices, 7 to 8 percent, more than 1.5 billion gallons of diesel could be saved each year in the U.S. if thermoelectric generators were used on the exhaust of heavy trucks. That translates into billions of dollars saved:
energy :: sustainability :: biomass :: bioenergy :: biofuels :: combustion :: heat recovery :: thermoelectricity :: nanotechnology ::

Clemson research focuses on developing higher-efficiency thermoelectric materials that could increase savings significantly. Research on the electrical and thermal properties of new materials could reduce the world’s reliance on fossil fuels and has shown promise with two classes of materials: low-dimensional systems for enhanced electrical properties and increased phonon scattering that leads to inherently low thermal conductivity.

Tritt heads up the Department of Energy’s Center of Excellence in Thermoelectric Materials Research at Clemson, one of the leading laboratories for thermoelectric materials in the world. The national center focuses on the next generation of thermoelectric materials for power conversion and refrigeration. Researchers in physics, materials science and chemistry screen promising new classes of materials in order to achieve higher-performance thermoelectric materials. DOE recently renewed the program with more than $1 million a year in research funding for the next three years.

NanoTX, presented by Semiconductor Industry Association, highlights advances in nanoscience and explains how nanotechnology is being used today and how it will impact a broad range of industries tomorrow, including electronics, energy, aerospace, defense, biomedicine, robotics, chemicals and more.

References:
Clemson University: Clemson physicist addresses international forum on thermoelectric energy - October 4, 2007.

Thermoelectric News: US DoE awards $3 Million to Clemson - September 29, 2004.

NanoTx '07 conference & expo.

An older but good introduction to the topic of thermoelectric material science can be found in: Terry M. Tritt, "Thermoelectric materials: Holey and Unholey Semiconductors", Science 5 February 1999: Vol. 283. no. 5403, pp. 804 - 805, DOI: 10.1126/science.283.5403.804

C. Lertsatitthanakorn, "Electrical performance analysis and economic evaluation of combined biomass cook stove thermoelectric (BITE) generator", Bioresource Technology, Volume 98, Issue 8, May 2007, Pages 1670-1674, doi:10.1016/j.biortech.2006.05.048