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Organic Rankine Cycle

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Publication Name: FROM WASTE HEAT TO POWER 2008
Original File Name Searched: FROM WASTE HEAT TO POWER.pdf
Page Number: 001

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Closed-loop, organic Rankine-cycle plants are adding multimegawatts —without additional fuel.

By David Engle

Combined heating and power is a mantra of distributed energy efficiency, but how about capturing the heat to yield more power?

When the waste heat itself isn’t directly needed for warmth, using it to boil water for steam-cycle generation does make sense, provided the gain in electricity will be enough—say, 10 MW or more—to justify the capital cost, operational overhead, and maintenance.

But even in this case, suppose the given heat source is in a remote

locale, as undoubtedly many thousands are—for example, at

compressor stations along North America’s natural-gas network?

Although lots of potentially valuable heat is roaring up those

stacks, harnessing it would not be easy, and making steam

wouldn’t often make sense. For one thing, abundant water would be needed—not always easy to find in the boondocks. For another, getting wastewater permits might be tough. And what if a mishap occurs in winter?

“When a steam unit goes down,” observes Ron Rebenitsch, whose job entails evaluating and developing such out-of-the-way energy projects, “parts begin to freeze up, and this will damage a lot of equipment.”

Rebenitsch, of the Basin Electric Power Cooperative (BEPC) in Bismarck, ND, is far more comfortable with four newly arriving power plants being provided by Ormat Technologies’ heat recovery generator: the Ormat Energy Converter, or OEC. Since their development in Israel several decades ago and entry into the US in 1972, OECs now boast a diverse portfolio of successful geothermal and heat-recovery projects worldwide. About 70 patents are applied in this unique system, which optimizes a closed-loop, organic Rankine cycle. The OEC design offers a range of benefits for a select power niche, covering a surprisingly broad range of applications, as the following discussion shows.

No Steam Needed

At BEPC, for example, each of Rebenitsch’s four plants with single-digit megawatt output eliminates the need for steam conversion, thus avoiding attendant water-handling problems.

As a working fluid in lieu of water, several options are available, depending on local conditions and needs. At the BEPC’s plants on the frigid northern plains, the choice was pentane. Aside from its desirable thermodynamic properties, pentane eliminates problems associated with boiler turbine plants, such as corrosion, burdensome water-conditioning, potential freezing, excessive pressures, and operator oversight.

Jan/Feb 2008

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