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Technology Frontier

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By daniel n. Schochet

ThoUGh GEoThErMaL PoWEr plants have been producing reliable energy for three decades or more, the advances in the power-conversion technology have increased the power-conversion efficiency, lowered the cost of energy generation and improved sustainable management of the geothermal resources.





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of the 8,000 megawatts of geothermal plants installed worldwide, most use steam turbines operating on dry steam or steam produced by single-flash or double-flash systems. about 800 megawatts are ormat binary-based power plants, combinations of steam and air-cooled binary systems. Qspevdujpo Xfmm Jokfdujpo Xfmm Jokfdujpo Qvnq Ipu Hfpuifsnbm Gmvje Dppmfe Hfpuifsnbm Gmvje The combined cycle technology boosts power output at geothermal sites. The steam is derived either directly from the geothermal source or from “flashing” a portion of the pressurized hot geothermal fluid into steam. Source: ormat

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ormat Technologies has announced it has completed four recovered energy generation plants along pipelines in the Dakotas. The ormat-owned units will have a capacity of 22 megawatts, and the power will be sold to Basin Electric in Bismarck.

In a binary plant, the geothermal energy is transferred to an organic working fluid in a vaporizer/boiler. The working fluid characteristics allow it to vaporize at lower temperatures, with vapors that efficiently drive low-speed turbines, which are directly coupled to a generator. The spent organic vapors are condensed back to liquid form an air-cooled condenser, and pumped back into the vaporizer. The cooled geothermal fluid is injected back into the reservoir.

For geothermal resources from 200 to 350 degrees Fahrenheit, binary technology is used to convert the geothermal heat to electrical power. Most of yet undeveloped resources are in this category. Turbine efficiency has been increased by some 20 percent in the past two decades.

The units will not produce greenhouse gases and generate power from hot exhaust gases from compressor stations along the pipeline. For resources with temperatures from 350 to 400 degrees, where the flashed steam produces from 10 to 30 percent of the usable energy, the binary plants are also cost-effective. Separated steam is introduced in the vaporizer to vaporize the organic fluid. The geothermal condensate at the vaporizer exit is then mixed with the hot separated brine to provide for the preheating of the organic fluid. The dilution of the brine with the condensate effectively lowers the precipitation temperature of the silica dissolved in the brine, allowing the fluid to be cooled to a lower temperature and adding 20 percent more heat to the cycle. This additional heat produced an additional 600 kilowatts.

For geothermal resources over 400 degrees, energy is best captured by a geothermal combined cycle unit where the steam first flows through a back-pressure steam turbine and then is condensed in the binary cycle vaporizer. In this cycle, only dry steam contacts the steam turbine train and air-cooled condensers are effectively used. This allows for 100 percent injection of geo fluids, has near zero emissions and is adaptable to resource changes.

In the past two decades, binary combined-cycled geothermal plants have achieved greater efficiency and promise to expand near emissions-free geothermal power production domestically and worldwide.

Daniel N. Schochet is vice president of Ormat Nevada.


the tax credit provides. .a huge incentive for.

.geothermal development.


Continued from page 65

projects such as raft river.

“the tax credit provides a huge incentive for geothermal development,” said Gawell of the Geothermal energy association. “It brings the price of the power plants down to a more acceptable level. It also brings in investors because it provides them a tax advantage. But it is an all-or-nothing deal because the tax credit requires that you have to be online by Jan.1, 2008 to take advantage of the credit. It usually takes two to three years to build a plant.” the Gea and others are pushing Congress for an extension of the federal tax credit.

the move to geothermal is also being aided by a renewed focus on controlling greenhouse gas emis- sions by western states such as California, which earlier this year approved a plan to slash emissions 25 percent by 2020.the plan mandates caps on carbon dioxide and other emissions, which is resulting in a move toward clean alternative energies such as geothermal, wind and biomass. “California has clearly been pushing the envelope in develop- ing an alternative energy future,” Gawell noted. he estimates that state alone has between 5,000 and 25,000 megawatts of undeveloped geothermal resources. If geothermal proponents have their way, all that energy could be headed onto the grid during the next decade.


EnErgyBiz magazinE

November/December 2006

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