Waste Heat Converts to Energy

Wednesday, May 25, 2011 @ 03:05 PM gHale


With new technology under development, waste heat from industrial processes may soon be able to convert into electricity.

A high-efficiency thermal waste heat energy converter actively cools electronic devices, photovoltaic cells, computers and large waste heat-producing systems while generating electricity, said Scott Hunter, who leads the development team at the Department of Energy’s Oak Ridge National Laboratory (ORNL). The potential for energy savings is enormous.

“In the United States, more than 50 percent of the energy generated annually from all sources is lost as waste heat,” Hunter said. “So this actually presents us with a great opportunity to save industry money through increased process efficiencies and reduced fuel costs while reducing greenhouse gas emissions.”

Initially, Hunter envisions using the technology for cooling high-performance computer chips, thereby helping solve an enormous problem facing manufacturers of petaflop-scale computers. These mega machines generate massive amounts of heat they have to remove, and the more efficient the process the better. Turning some of that heat into electricity is an added bonus.

Hunter’s technology uses cantilever structures about 1 millimeter square in size. About 1,000 of these energy converters can attach to a 1-inch square surface such as a computer chip, concentrated photovoltaic cell or other devices that generate heat. Although the amount of electricity each device can generate is small – 1 to 10 milliwatts per device – many arrays of these devices can generate sizable amounts of electricity that can power remote sensor systems or assist in the active cooling of the heat generating device, reducing cooling demands.

The underlying concept, pyroelectricity, uses pyroelectric materials which have been around for centuries. First attempts to use this technology to generate electricity began several decades ago, but low thermal to electricity conversion efficiencies – from about 1 to 5 percent have hurt these studies.

This is also the case for techniques using thermoelectric, piezoelectric and conventional pyroelectric platforms. However, using arrays of cantilevered energy converters that feature fast response and cycle times, Hunter’s team wants efficiencies of 10 to 30 percent – depending on the temperature of the waste heat generator – in an inexpensive platform built using standard semiconductor manufacturing technology.

“The fast rate of exchange in the temperature across the pyroelectric material is the key to the energy conversion efficiency and high electrical power generation,” Hunter said, adding ORNL’s energy scavenger technology is able to generate electrical energy from thermal waste streams with temperature gradients of just a few degrees up to several hundred degrees.

The device is an energy harvesting system that features a micro-electro-mechanical (MEMS) pyroelectric capacitor structure that when heated and cooled causes current to flow in alternate directions, which can generate electricity. In this configuration, cantilevers attach to an anchor affixed to a waste heat generator substrate. As this substrate becomes hot, the cantilever also heats and bends because of the bi-material effect, similar in principle to the bimetal switch used in room and oven thermostats.

“The tip of the hot cantilever comes into contact with a cold surface, the heat sink, where it rapidly loses its heat, causing the cantilever to move back and make contact with the hot surface,” Hunter said. “The cantilever then cools and cycles back to the cold heat sink.

“The cantilever continues to oscillate between the heat source and heat sink as long as the temperature difference is maintained between the hot and cold surfaces.”



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