Electricity from Waste Heat  

Technology Review has an article on a new system from Ener-G-Rotors which harvests energy from low temperature waste heat - Electricity from Waste Heat.

Factories, data centers, power plants--even your clothes dryer--throw off waste heat that could be a useful source of energy. But most existing heat-harvesting technologies are efficient only at temperatures above 150 °C, and much waste heat just isn't that hot. Now Ener-G-Rotors, based in Schenectady, NY, is developing technology that can use heat between 65 and 150 °C.

The company replaces the turbine in a typical electrical generator with a device called a gerotor, which it claims to have made "near frictionless." "If this works, it's so huge," says Bob Bechtold, president of Harbec Plastics, one of Ener-G-Rotors' potential customers. "I've been dreaming about the concept of using [low-temperature waste heat] ever since I first knew what it was about ... It's all about using what we have more completely."

Ener-G-Rotors' technology is based on the Rankine cycle, in which heated fluid flowing through a tube heats a pressurized fluid in a second tube via a heat exchanger. The second tube is a closed loop; the so-called working fluid flowing through it (a refrigerant with a low boiling point, in the case of Ener-G-Rotors) vaporizes and travels into a larger space called an expander. There, as the name would imply, it expands, exerting a mechanical force that can be converted into electricity.

Instead of turning a turbine, the expanding vapor in Ener-G-Rotors' system turns the gerotor, which is really two concentric rotors. The inner rotor attaches to an axle, and the outer rotor is a kind of collar around it. The rotors have mismatched gear teeth, and when vapor passing between them forces them apart, the gears mesh, turning the rotor.

The company claims that the rotor design is far simpler than that of a turbine, making it potentially easier and cheaper to manufacture, as well as more durable. And the company says that it has invented a proprietary way of mounting the rotor on rolling bearings that makes its movement nearly frictionless.

Reducing the friction means that the rotor turns more easily, so the gas doesn't need to exert as much force to generate electricity. That's why the system can work at lower temperatures, which impart less energy to the gas.

The company expects to convert 10 to 15 percent of low-temperature waste heat into electricity, delivering a payback in two years or less in most cases, says CEO Michael Newell. Ener-G-Rotors plans to both sell systems to customers outright and operate its own systems and sell power.