Waste Heat to Electricity: New Material Promising May 19, 2014

Christian Science Monitor:

Researchers looking for better ways to convert waste heat into electricity have stumbled across a simple material that is smashing records for making that conversion efficiently. This new material – a semiconductor made by blending tin and selenium – promises to convert heat to energy more efficiently than current technologies and with relatively accessible, inexpensive elements. More than 90 percent of the energy produced to generate electricity, propel vehicles, or dry bricks requires a heat source, researchers say. Yet only 30 to 40 percent of the heat produced actually does the work. Most of the heat is wasted.

In principle, much of that heat could be recovered, using thermoelectric generators made from materials capable of turning a difference in temperature into electricity. Indeed, the “holy grail” in this field is to find materials that can act as efficient thermoelectric generators from room temperature up to perhaps 1,000 degrees F. or more – a span that would significantly broaden the range of sources from which they could draw heat. Such a development could have as big an impact on energy use as another “holy grail” – the quest for materials that conduct electricity with no resistance at room temperatures, says Mercouri Kanatzidis, a solid-state chemist at Northwestern University in Evanston, Ill., and a lead member of the research team that is reporting the results in the current issue of the journal Nature. The nine-member team also included scientists from the University of Michigan in Ann Arbor. In its simplest form, a thermoelectric generator consists of two slabs of semiconductors with different electrical properties and joined at one end by a heated plate that can conduct electricity. At the other end, which must be kept colder, the semiconductor slabs are not connected. The temperature difference between the hot and cold ends of the semiconductors allows voltage to build up at the unconnected ends. In effect, the unconnected ends act like the terminals of a battery. As long as the temperature difference is maintained, the generator continues to produce the voltage needed to allow current to flow through whatever is connected to the terminals. The challenge: These semiconductors must be inefficient conductors of heat, to maintain the temperature difference, while at the same time being efficient electrical conductors. Typically, the less heat a semiconductor can conduct, the less efficient it is at conducting electricity, researchers say. The tin selenide semiconductor must reach temperatures of just over 1,660 degrees F. in order to achieve its maximum efficiency. Even so, it can be used immediately, Dr. Kanatzidis says.

Northwestern University:

EVANSTON, Ill. — One strategy for addressing the world’s energy crisis is to stop wasting so much energy when producing and using it, which can happen in coal-fired power plants or transportation. Nearly two-thirds of energy input is lost as waste heat. Now Northwestern University scientists have discovered a surprising material that is the best in the world at converting waste heat to useful electricity. This outstanding property could be exploited in solid-state thermoelectric devices in a variety of industries, with potentially enormous energy savings. An interdisciplinary team led by inorganic chemist Mercouri G. Kanatzidis found the crystal form of the chemical compound tin selenide conducts heat so poorly through its lattice structure that it is the most efficient thermoelectric material known. Unlike most thermoelectric materials, tin selenide has a simple structure, much like that of an accordion, which provides the key to its exceptional properties. The efficiency of waste heat conversion in thermoelectrics is reflected by its figure of merit, called ZT. Tin selenide exhibits a ZT of 2.6, the highest reported to date at around 650 degrees Celsius. The material’s extremely low thermal conductivity boosts the ZT to this high level, while still retaining good electrical conductivity. The ZT metric represents a ratio of electrical conductivity and thermoelectric power in the numerator (which needs to be high) and thermal conductivity in the denominator (which needs to be low). Potential areas of application for the high-temperature thermoelectric material include the automobile industry (a significant amount of gasoline’s potential energy goes out of a vehicle’s tailpipe), heavy manufacturing industries (such as glass and brick making, refineries, coal- and gas-fired power plants) and places where large combustion engines operate continuously (such as in large ships and tankers). “A good thermoelectric material is a business proposition — as much commercial as it is scientific,” said Vinayak P. Dravid, a senior researcher on the team. “You don’t have to convert much of the world’s wasted energy into useful energy to make a material very exciting. We need a portfolio of solutions to the energy problem, and thermoelectric materials can play an important role.” Dravid is the Abraham Harris Professor of Materials Science and Engineering at the McCormick School of Engineering and Applied Science. Details of tin selenide, probably among the world’s least thermally conductive crystalline materials, are published today (April 17) by the journal Nature. – See more at: http://www.northwestern.edu/newscenter/stories/2014/04/surprising-material-could-play-role-in-saving-energy.html#sthash.0sbPDHHH.dpuf