Turboexpanders in Geothermal Applications

The flashed steam method is less efficient and its requirements for steam properties—cleanliness, high temperature, and high pressure— are usually unavailable in most geothermal fields. The situation is different with the binary cycle system, which is quite efficient and widely used. This wet system involves the transfer of heat from the hot well stream into a more manageable boiling fluid to generate power through a turboexpander. 

A very sueeessful projeet began in the early 1980s when the Ben Holt Company (Pasadena, California) designed and eonstrueted a 10 MW air-eooled, elosed-loop binary eyele at their Mammoth Geothermal Plant, loeated on the eastern slope of the Sierra Nevada mountain range in California. 

In the primary binary loop, production wells recover 170°C hot water from the geothermal reservoir and deliver 3,036 m /lir at 14 bar to heat exchangers in the power plant. Water leaves the heat exchangers at 70-90°C and is recycled to the ground through a re-injection well at a depth of 400-600 m. In the secondary binary loop, the heat from the hot water evaporates the isobutane working fluid at 36 bar and 150°C (Table 4-2). 

Power is generated by the pressurized gas expanding through an 11,000 rpm single-stage, radial-inflow turbine expander, which drives a synchronous generator. Exhaust gas from the expander is liquified by air-cooled condensers and is pumped back to the heat exchangers to repeat the cycle. 

Unique Design Features for Geothermal Energy Recovery Applications 

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