Survey of Energy Resources 2007
Geothermal - Technologies for Electricity Generation
Most commonly, electricity generation takes place in conventional steam turbines (Fig. 11-5 ). The steam, typically at a temperature above 150°C, goes directly from dry steam wells or, after separation, from wet wells through a turbine which drives the electric generator. After that it goes to a condenser where vacuum conditions are maintained by cooling water. The unit sizes are commonly 20-50 MW.
Binary plants have been gaining popularity in recent years. They utilise geothermal fluids at lower temperatures than conventional plants, in the range 85-170°C. They use a secondary working fluid, usually an organic fluid, which has a low boiling point and high vapour pressure at low temperatures, compared with steam. The fluid passes through a turbine in a similar way as steam in conventional cycles. Binary plants are usually constructed in small modular units of up to a few MW capacity which are linked together. Kalina is a relatively new binary-fluid cycle which utilises a water-ammonia mixture as working fluid. This increases the efficiency of the cycle compared with other binary cycles. A 2 MW Kalina plant has been in operation in Husavik, North Iceland, since 2000.
The efficiency of geothermal utilisation is considerably enhanced by cogeneration plants, compared with conventional geothermal plants. A cogeneration plant produces both electricity and hot water which can be used for district heating as well as other direct uses. A necessary condition for the operation of a cogeneration power plant is that a relatively large market exists for hot water not too far distant from the plant. Iceland, where three geothermal cogeneration plants are in operation, is an example of this. There, the distance of the plants outside the towns is 12-25 km.