Flash-steam geothermal plants

In the case of "flash steam" power plants, the steam is either generated directly by the production wells or the wells produce hot water from which steam can be separated to drive conventional steam turbine generators. The size of these plants ranges from 100 kW to 150 mW. Figure 2.104 illustrates the optimizing control system for such a geothermal facility. 

Geothermal flash steam power plant controls. 

In this demonstration plant, the boiler feedwater can be preheated by geothermal heat if available. Depending on the availability and temperature of the geothermal heat in the area, the geothermal plant s design would vary. If the groundwater temperature exceeds 150°C (302°F), a "flash steam" power plant would be used, and if it is between 100 and 150°C (212 and 302°F), a "binary cycle" power plant would be used. 

Flash steam power plants Geothermal fluids above 360°F (182°C) can be flashed ina tank at low pressure causing some of the fluid to rapidly vaporize. The vapor then expands in a turbine. 

Flash steam is the steam produced when the pressure on a geothermal fluid is reduced. A flash steam cycle for a high-temperature liquid-dominated resource is shown in Fig. 5. This dual-flash cycle is typical of most larger flash steam geothermal power plants. Singleflash cycles are frequently selected for smaller facilities. 

More commonly than it puts out dry steam, a geothermal well puts out a mixture of steam and water above 130°C, or just hot water. A separator is needed in a hot water steam mixture geothermal power plant to separate the flashing steam from the hot water, as shown in Figure 2.23. An additional throttling valve is required to generate saturated steam in a hot-water geothermal power plant. 

Resources with such favourable characteristics are rare and most of them already discovered. Water-dominated fields with lower temperatures and enthalpies are much more abundant. For these, flash-steam plants at one or more pressure stages are used to flash the geothermal water to steam, commonly in the capacity range 10-50 MWe. 

The characteristics of the hydrothermal resource determine the power cycle of the geothermal power plant. A resource that produces dry steam uses a direct steam cycle. A power plant for a liquid-dominated resource with a temperature above 165°C typically uses a flash steam cycle. For liquid-dominated resources with temperatures below 165°C, a binary cycle is the best choice for power generation. Power plants on liquid-dominated resources often benefit from combined cycles, using both flash and binary energy-conversion cycles. 

Geothermal brine or a mixture of brine and steam is delivered to a flash vessel at the power plant by either natural circulation or pumps in the production wells. At the entrance to the flash vessel, the pressure is reduced to produce flash steam. The steam is delivered to the high-pressure inlet to the turbine. The remaining brine drains to another flash vessel where the pressure is again reduced to produce low-pressure flash steam. 

Fig. 5 Double-flash steam cycle geothermal power plant. (View this art in color at www.dekker.com.)...

There are two main types of geothermal power plants flash steam type and binary cycle. The flash steam plant is driven by pressurized, hot (300-700°F) steam brought up from depth of as much as more than 10,000 feet. When this pressure is reduced at the surface by about a third or so, the water flashes —explosively boils—into steam which drives the turbine and generator. Binary plants are driven... 

Some geothermal power plants use a combination of flash and binai y cycles to increase the efficiency of electricity production. An initial flash creates steam that drives a turbine then the binai y cycle is run, using either the hot water remaining after the initial flash or the hot exliaust from the turbine. 

A proposal is made to use a geothermal supply of hot water at 1500 kPa and 180°C to operate a steam turbine. The high-pressure water is throttled into a flash evaporator chamber, which forms liquid and vapor at a lower pressure of 400 kPa. The liquid is discarded while the saturated vapor feeds the turbine and exits at lOkPa. Cooling water is available at 15°C. Find the turbine power per unit geothermal hot-water mass flow rate. The turbine efficiency is 88%. Find the power produced by the geothermal power plant, and find the optimized flash pressure that will give the most turbine power per unit geothermal hot water mass flow rate. 

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