Organic Rankine Cycle

The Organic Rankine Cycle (ORC) is a technology that I recently heard about so I thought I'd share it with everyone else...

First off, since there are alot of non-engineers who will read this, the Rankine cycle is the name of the steam power cycle. It's used worldwide to generate electricity by boiling water to create steam and running that steam through a turbine. Normal efficiencies that a non-regenerative Rankine cylce will reach is around 40-45%.

Now, an Organic Rankine Cycle is the same thing, but instead of using water as the working fluid it uses substances such as pentane, butane, or refrigerants (think of a refrigeration cycle in reverse). By using these other working fluids the organic rankine cycle is able to operate a low temperatures ~100-500 C. This low temperature power cycle is useful because it can be employed as a waste heat recovery system. Large powerplants dump a significant portion of their available energy into the atmosphere by using a tall condenser stack simply because that energy is not cost effective to extract.

This is one example of where the ORC could be employed to utilize this energy that would be otherwise wasted to generate more electricity and boost a plant's overall efficiency.

A company called United Technologies currently manufactures a 225kW ORC system that is used in geothermal power plants. This unit was used by the geothermal plant in Chena, Alaska. You can check out info about it here. It operates at 165 F, and when it started in 2006 was the lowest temperature geothermal resource to be used for power production in the world.
The possible applications for such low temperature power production are vast, but just the capability for waste heat recovery from such a system is intriguing to me.