electricity from waste heat
May 23, 2014

New Method May One Day Turn Low-Grade Heat Into Electricity

Brett Smit for redOrbit.com - Your Universe Online

In recent years, engineers have been looking into ways to capture ‘waste heat’ generated as a byproduct of industrial process and convert it into electricity. These efforts have primarily focused on improving the efficiency of thermoelectric devices, but a limited availability of materials has hampered this research.

According to a new report in Nature Communications, Stanford and MIT engineers have devised an alternative method based on temperature differences that are less than 100 degrees Celsius.

"Virtually all power plants and manufacturing processes, like steelmaking and refining, release tremendous amounts of low-grade heat to ambient temperatures," explained Yi Cui, an associate professor of materials science and engineering at Stanford. "Our new battery technology is designed to take advantage of this temperature gradient at the industrial scale."

The novel system leverages a phenomenon called the thermogalvanic effect, which can be observed when temperature changes affect a rechargeable battery.

"To harvest thermal energy, we subject a battery to a four-step process: heating up, charging, cooling down and discharging," said study author Seok Woo Lee, a postdoctoral scholar at Stanford, in a statement.

Capable of harnessing energy from temperature differences as small as a 90 degrees F, the system begins by heating an uncharged battery via waste heat. After the battery is fully charged, it is allowed to cool and due to the thermogalvanic effect, the voltage in the battery rises as the temperature falls. When the battery has cooled, it provides more electricity than was applied to it via heat that was added to the system.

The engineering team said their system attempts to harvest heat at temperatures below 100 C, which makes up a main part of harvestable waste heat.

"One-third of all energy consumption in the United States ends up as low-grade heat," said study author Yuan Yang, a postdoc at MIT.

In the study, researchers heated the battery to over 100 degrees F, then charged and cooled it. The method triggered an electricity-conversion effectiveness of 5.7 percent, nearly twice the efficiency of typical thermoelectric devices.

This procedure was first suggested in the 1950s at temperatures of 900 degrees F or more, said Yang, and the technique does produce the best results with higher temperature differences. However, today’s advanced materials for electrodes were not around at that time.

"This technology has the additional advantage of using low-cost, abundant materials and manufacturing processes that are already widely used in the battery industry," Lee noted.

Although the new system can work at much lower temperature swings, it has a significantly lower power density – meaning the quantity of power that can be delivered for a certain weight. The team said their system also needs further research to guarantee long-term reliability and enhance the speed of battery charging and discharging.

"It will require a lot of work to take the next step,” Chen said.

"This has an efficiency we think is quite attractive,” he added. “There is so much of this low-temperature waste heat, if a technology can be created and deployed to use it."