If you eyeballed a block diagram of the heat recovery system devised by Echogen Power Systems, Inc., you wouldn't notice anything special. It looks like a conventional setup that routes water through a heat exchanger in a waste stack, then uses the resulting steam to drive a turbine.
What the diagram doesn't show is that the working fluid is CO2. This subtle difference makes it possible for the system to capture as much heat energy as ordinary systems twice or three times its size.
Echogen, Akron, Ohio, has built a prototype of its system capable of producing 250 kW of electricity and installed it at a research center run by American Electric Power Co. Echogen officials say one of their waste heat systems capable of generating 8 MW would be about the size of a standard desk. In contrast, a standard steam system able to generate the same amount of power would be about the size of a standard room.
The key development making this kind of performance possible is the system's use of supercritical CO2 (ScCO2) as the working fluid. Heat energy transfers to the ScCO2 working fluid through a waste heat exchanger installed into a smokestack, boiler or turbine exhaust duct, hot process gas or liquid line, or solar thermal concentrator. The heated ScCO2 passes through a turbo-expander where enthalpy gain from heating gets converted into mechanical shaftwork to produce electricity. The regenerator recycles residual heat while unconverted heat is discharged from the system through a water or air-cooled condenser.
Liquid CO2 is pumped to supercritical pressure, accepts cycle heat at recuperator and waste heat at the waste heat exchanger. High energy ScCO2 is expanded at the turbo-alternator producing high frequency electrical power. Power electronics condition power to pecifications. Expanded ScCO2 is cooled at recuperator and condensed to liquid at condenser. The condensing medium can be water or air.
The CO2 that the system employs as working fluid is the same stuff the beverage industry uses for propelling soft drinks into glasses at soda fountains. But it must be kept at a higher pressure than other more conventional refrigerants. The reason Echogen uses ScCO2 is that its smaller molecules can easily travel through smaller heat exchanger channels to allow greater heat exchange per square inch of available space.
The trick, say Echogen personnel, is in keeping the ScCO2 in liquid form -- at atmospheric pressure, it wants to be a vapor. Echogen has IP that concerns how to operate the system while keeping the ScCO2 in the right state.
In operation, Echogen's system can harvest waste heat at 400°F, temperatures well below those at which steam-cycle systems operate efficiently. The company says its 250 kW engine is practical at air volumes as low as 85,000 lb/hr at 450°F. "If you have a stack that is four-feet in diameter, you probably have that much volume," says Echogen application engineer Alex Kacludis. At higher temperatures, it becomes practical to use the system with less air volume. He figures half that flow rate would suffice if stack temperatures are around 650°F.
More info: http://www.echogen.com/index.aspx