Major Common System Components and Features

a. A geothermal hydrothermal reservoir consisting of hot rock with substantial permeability, and aqueous fluid in situ. The temperature of the fluid ranges from 100 oC to 400oC (212oF to 752oF). The fluid may contain substantia l amounts of dissolved solids and non-condensible gases (particularly carbon dioxide and hydrogen sulfide).

b. Wells for production and injection of geothermal fluids. These range in total depth from 200 to 3,500 meters a t producing U.S. hydrothermal reservoirs. The wells are drilled and completed using technology for deep wells that has been incrementally adapted from oil and gas well technology since the 1960's. The produced fluids range from totally liquid to liquid-vapor mixtures (with two-phase flow at the wellhead). In some systems outside the U.S. , the cooled liquid leaving the plant is disposed to the ground surface or streams, rather than injected.

c. An exploration and reservoir confirmation process to identify and characterize the reservoir. This process is usually complex and can add substantial front-end cost to a hydrothermal project. Such costs are usually borne out o f developer's equity and can be a large barrier to exploration projects. Those costs are accounted for in this TC but not represented in the system schematics.

d. A reservoir design and management process whose goal is to optimize the production of electricity from th e reservoir at least cost over the life of the system. Those costs are accounted for in this TC but not represented in the system schematics.

e. Surface piping that transports fluid between the wells and the power plant equipment.

f. A power plant that converts heat (and other energy) from the geothermal fluid into electricity. Power plant s comprise: (a) One or more turbines connected to one or more electric generators. (b) A condenser to convert the vapor exiting from the turbine (water or other working fluid) to a liquid. (c) A heat rejection subsystem to mov e waste heat from the condenser to the atmosphere. Cooling towers (wet or dry) are used for most systems, but cooling ponds are also used. (d) Electrical controls and conditioning equipment, including the step-up transformer to match the transmission line voltage. (e) An injection pump that pressurizes the spent geothermal liquid fro m the power plant to return it to the geothermal reservoir through the injection wells. Representative power -conversion (power plant) technologies are described below.

g. Activities and costs related to the operation and maintenance of the system over a typical 30-year useful life of an individual power plant and a 40- to 100-year production life for the reservoir as a whole.

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