The world wastes a lot of heat. Between half and two-thirds of the fuel we burn to create energy is dissipated as heat into the atmosphere. While it has been long known that waste heat can be converted into energy, the low efficiency of early thermoelectric generation systems was such that it limited the process’s usefulness.
Thermoelectric generators (TEGs, and also called thermogenerators) are essentially devices designed to convert heat into electricity, using two materials at different temperatures, directly into electrical energy. The greater the differential (delta temperature, or DT) between the “hot” side and the “cold” side, the more power can be produced.
TEGs can take waste heat from energy generation or industrial processes and convert it into electricity. TEGs can provide electricity to a load directly when a constant heat source is available, or they can be used in combination with batteries if the heat source is not constant. A typical TEG is made of bismuth-telluride semiconductors sandwiched between two metallized ceramic plates.
Because TEGs eliminate the need for wires and batteries, their primary applications have been in remote places where the use (and replacement) of batteries is impractical or impossible, such as in offshore engineering operations, lighthouses, oil pipelines and remote telemetry and data collection in satellites and spacecraft. (NASA’s Curiosity rover uses radioisotope thermoelectric generators that produce power by converting the heat generated by the decay of plutonium-238 fuel into electricity.)
While TEGs will never be major energy sources – they would require insane amounts of heat to make that type of power via the Seebeck effect – they have a number of small but increasingly important applications in manufacturing, data centers, the automotive industry and in military applications.
TEG — The Next Generation
The efficiency of thermoelectric power generation has been traditionally low: between about 5 and 10 percent. But in recent years, advances in technology (particularly nanotechnology) have raised efficiency to 15 to 20 percent. This, along with the emergence of small and targeted applications, has raised interest in TEGs.
Self-powering machine sensors. Manufacturing facilities and data centers run large amounts of equipment that must be kept cool to operate at maximum efficiency. Sensors can help make sure equipment doesn’t overheat, but sensors that, themselves, must be plugged in add to the heat loads. TEG-powered sensors located at machine hot spots can power themselves using ambient heat while monitoring and communicating problems to operations personnel.
The sensors can provide information such as temperature, humidity, wear and tear, and whether parts need maintenance or replacement. If these intelligent network sensors are activated only when sending or receiving data, the amount of energy they require is tiny (on the milliwatt scale), and only the smallest thermoelectric generators/sensors are required.
Printed thermogenerators. While printed electronics, an application of nanotechnology, have the potential to revolutionize the electronics industry, thermogeneration may also a beneficiary. Researchers at Germany’s Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) will soon introduce a printed thermogenerator that can be tailored exactly to technical interfaces, according to Nanowerk News.
In the case of self-powered machine sensors, components often need to be highly customized to particular machines and operations. The new printed thermogenerators ultimately mean that manufacturers, data centers and others that operate complex machinery might literally customize and print, on their own, the sensors they require — sensors that are less susceptible to faults because the energy supply can be adapted directly to the equipment.
“Generative manufacturing processes produce both sensors and sensor networks, as well as the required elements for energy harvesting, such as thermogenerators, by directly depositing functional structures, which have an ink or paste base, using ink-jet, aerosol-jet, screen-printing or dispensing processes,” says Dr. Volker Zöllmer, head of functional structures at IFAM. “Not only can electrical circuit boards and sensor elements be attached to different interfaces but it is also possible to produce structures which harvest energy.”
Automotive. Heat from the exhaust of internal combustion engines can be harvested into energy with the addition of a thermoelectric generator in the vehicle. With car exhaust reaching temperatures of about 1,300 deg F, the enormous delta temperature could be capable of generating between 500 and 750 watts of electricity, which could, for example, charge a battery in a hybrid vehicle or reduce the load on a car’s alternator, improving fuel economy.
Military. Given how enthusiastic the U.S. military is as of late to develop and further advance alternative energy sources, thermogeneration has attracted the attention of military researchers. The U.S. Army Research Laboratory (ARL) is currently looking for ways to harness, package and shrink TEG technology in hopes that it could lead to wearable power sources on soldiers — using the temperature difference between body heat and outside air — or to more efficient military vehicles, according to the U.S. Army’s website.
“Perhaps if the technology is advanced in later years, it will be possible to extend flight times, increase available mission scope and add additional sensors or payloads,” says John Gerdes, mechanical engineer at the Technology Development and Transition Team of ARL’s Vehicle Technology Directorate.
Earlier this year, the ARL teamed with Research Triangle Institute International, General Dynamics Land Systems and Creare Inc. to demonstrate a prototype energy harvesting solution for an M1 Abrams tank. The waste heat recovery system converts the heat from the turbine engine exhaust into electrical power with the help of a thermoelectric generator, and dissipates the heat through a heat-rejection system.
The military is particularly interested in this type of system since one research team has found that prototype waste heat recovery systems, once scaled up, could be retrofitted to existing tanks without modifications.
As the military gear becomes more and more high-tech and require larger amounts of electrical power, TEG systems, scavenging waste heat, have the potential to contribute to electrical power generation and improve fuel efficiency. There is also a tactical advantage: By harvesting the exhaust heat from vehicles, thermogeneration systems reduce their infrared signals and help them stay hidden from enemy surveillance.
Many futurists have declared that large-scale alternative energy will be only part of a wide overall energy mix. What will ultimately fill the gaps where where solar farms, wind farms and fuel cells simply cannot go is distributed microgeneration. Thanks to advancements in nanotechnology, not to mention the escalating need for microgeneration, thermoelectric power generation may be part of that mix.