Thermoelectric devices transform waste heat into electricity and can one day provide increased efficiency for everything from small gadgets to power plants. Scott Hunter, working at the Oak Ridge National Laboratory (ORNL) hopes his new heat-recovering invention will scavenge lost heat with an efficiency of up to 30 percent.
So far, the best experimental thermoelectric devices have provided a 14 percent efficiency at converting heat into electricity. Hunter’s device uses small, 1 square millimeter-sized cantilever structures to build a device that is not thermoelectric at its core but whose basic elements can produce 1 to 10 milliwatts each.
If 1,000 of these devices are stacked onto a 1-inch square surface, the results are obviously scaled up and the electricity obtained can be used to drive other processes.
Hunter uses pyroelectricity, a phenomenon that harvests the temporary temperature difference between two sides of a material. Unlike thermoelectric devices, which use a constant temperature difference to generate a constant voltage, pyroelectrics only generate that voltage for a short amount of time, for as long as the electrons in the crystalline material leak from one end to the other. Pyroelectric devices have, however, suffered from a kind of marginalization, due to the low efficiency they can do their job – only up to 5 percent.
“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 that 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.
A micro-electro-mechanical (MEMS) device lies at the core of Hunter’s technology. When heated and cooled, it causes current to flow in alternate directions. The cantilevers are attached to a point that is liked to the heat source, and bend when heat reaches them, because of the bi-meterial effect.
“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,” explains Hunter. “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.”
Thermal energy recovery systems could save billions of dollars yearly, if their conversion and price efficiency are well-suited. They can also be used in cars, to recover heat from gasoline-burning engines, and feed the electricity to a battery, in the case of hybrid cars, or save the effort exerted by the alternator.