Binary Power Plants

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Figure 2 shows a schematic of a geothermal binary power plant [1]. All the geothermal fluid passes through the tub e side of the primary heat exchanger and then is pumped back into the reservoir through injection wells. A hydrocarbon working fluid (e.g., isopentane) on the shell side of the primary heat exchanger is vaporized to a high pressure (HP ) to drive the turbine-generator. Low pressure vapor from the turbine is liquified in the condenser and re-pressurize d by the hydrocarbon pump. Waste heat is ejected to the atmosphere through a condenser and a cooling tower. Makeup water is required for the heat rejection system if wet cooling towers are used, but not if dry cooling towers are used. The binary system characterized here uses dry cooling, but wet cooling could be less expensive where cooling wate r is available. Most geothermal binary plants are constructed from a number of smaller modules, each having a capacity of 1 to 12 MWe net.

Technical descriptions of recently-built binary organic Rankine cycle power systems and other systems proposed fo r moderate-temperature reservoirs can be found in the NGGPP report [1] and others: binary systems [11]; vacuum-flash [12]; ammonia-based cycles [13].

Equipment present in most binary systems includes:

a. Downhole production pumps in the production wells. These keep the geothermal fluid from vaporizing in the wells or in the power plant, and enhance the production well flow rate.

b. A working fluid pump, the "main cycle pump", that pressurizes the low-boiling-temperature liquid working fui d to drive it around the power-conversion loop.

c. A turbine converts energy in the high-temperature high-pressure working fluid vapor to shaft energy. It exhaust s low-temperature low-pressure vapor to a condenser.

System Boundary

Vapor

System Boundary

Production Wells

Generator

HP Turbine

Primary Heat

Exchanger

(Downhole production pumps)

Vapor

Liquid

Primary Heat

Exchanger

Liquid

(Downhole production pumps)

Interconnect

Air-cooled Condenser

Fan i

Brine

Injection

Pump

Injection Wells

Geothermal Reservoir

Cooled fluid

Electricity

Waste heat

Ambient air

Figure 2. Geothermal hydrothermal electric system with binary power plant schematic. 2.0 System Application, Benefits, and Impacts

Application: Traditionally, geothermal systems have been perceived to compete with other baseload generatio n systems. Currently, geothermal el ectric systems compete most directly with gas-fired turbines and cogeneration systems in California, and coal and natural gas plants in Nevada. However, recent experiments have shown that som e geothermal power plants (e.g., the dry steam plants at The Geysers) can be cycled to follow system load in th e intermediate-baseload area of the utility time-demand curve [14], thereby increasing their value in certain applications. It is likely that lo ad-following would be more difficult to do at flash and binary plants than at dry steam plants. Current contract capacity factors are on the order of 80 percent. Experienced capacity factors for many currently operatin g plants are on the order of 100 percent or higher (see discussion in Section 4.2)

Benefits: Typical plant sizes are 5 to 50 MWe net. Once the geothermal reservoir is confirmed, system constructio n time is on the order of a year or less. O&M costs are low compared to fossil-fueled systems because there are no "fuel" costs other than those for the O&M of the field wells and pipes. With appropriate emission control equipment , geothermal-generated electricity provides an environmentally attractive alternative to baseload gas, oil, coal, an d nuclear-fueled electricity. Some in the U.S. geothermal industry have recently indicated interest in using relativel y small geothermal power plants (from 50 kW to 2,000 kW) to supply off-grid or "mini-grid" power in a number o f remote places that are favored with geothermal resources.

Economic Conditions: The recent surge in competition from low-cost electricity from natural gas has broa d implications fo r the economic competitiveness of geothermal electric systems. Approximately 900 MW e of geothermal hydrothermal systems were installed in the western U.S. between 1980 and 1990. However, since about 1990, the advent of cheaper electricity from natural-gas fueled systems and low load growth rates have slowed the pace of U.S . domestic geothermal installation to nearly zero. (One 40 MWe plant was installed at the Salton Sea, California reservoir in 1996, under a high-price-of-power contract that originated in the early 1980's.)

In 1990, geothermal power developers expected to be able to compete easily against 6 to 70/kWh power in 1996. But by about 1993, the developers found themselves competing (not very successfully) against 2.5 to 3.50/kWh power i n western states. However, it was expected that the currently strong overseas markets for these systems, especially i n the Philippines and Indonesia, would continue to provide a strong experiential base for ongoing technolog y improvements. With the large recent decreases in the cost of geothermal flash power plants, U.S. technology for using higher-temperature geothermal resources may be able to again compete for new electricity demand. (See "Special Note on Power Plant Costs," page 3-20, for more details.)

Impacts: All emissions stated in Table 1 are for flashed-steam plants [7]. Emissions for binary plants are essentially nil because the geothermal fluid is never exposed to the atmosphere. The zero value for sludge assumes use o f "pH modification" technology at locations where silica scaling would otherwise be high. By comparison, sludge at 6 kg/MWh has been cited for the previously-used crystallizer/clarifier technology, circa 1985-90 [15].

Table 1. Environmental impacts of geothermal flashed steam plant.

Indicator

Base Year

Name

Units

1997

2000

2005

2010

2020

2030

Gaseous

- Carbon Dioxide

kg/MWh

45

45

45

45

45

45

- Hydrogen Sulfide

kg/MWh

0.015

0.015

0.015

0.015

0.015

0.015

Liquid

kg/MWh

0

0

0

0

0

0

Solid

- Sludge

kg/MWh

0

0

0

0

0

0

Note: Emissions for binary plants are essentially nil because the fulid is never exposed to the atmosphere.

Note: Emissions for binary plants are essentially nil because the fulid is never exposed to the atmosphere.

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