2. GEOTHERMAL POWER CYCLES

When a geothermal resource has been deemed suitable for power generation, a decision is required to select a suitable power cycle. A power cycle consists of a series of thermodynamic processes that converts heat energy to electricity. The most common geothermal power cycle is the Rankine cycle, consisting of two isothermal and two adiabatic processes. More than one power cycle can be used for a geothermal resource. Indeed, other direct uses can be combined with power generation.

Geothermal power cycles can be classified into five major types: steam Rankine cycle, organic Rankine cycle (ORC), total flow cycle, Kalina cycle system (KCS), and trilateral flash cycle (TFC). ORC, KCS, and TFC are binary cycles that use geothermal fluids as the primary heat source to vaporize a secondary fluid for power generation. Currently, nearly all geothermal power plants use steam or ORC or a combination of the two. There are several total flow machines, but they are not widely used. The first pilot Kalina geothermal power plant was commissioned in Iceland in 2000. TFC is still at the experimental stage.

The following subsections examine the various power cycles based on the types of geothermal resources.

2.1 Dry Steam (Vapor-Dominated) Systems

Dry steam systems are rare, but they have high power capacity and are currently producing appro­
ximately 50% of total world geothermal electrical power.

Conceptually, the power cycle for dry steam geothermal systems is the simplest: saturated or slightly superheated dry geothermal steam from wells is piped directly to condensing steam turbines (Fig. 2). The condensate is reinjected for environmental reasons. Other problems associated with solids and gases carried in the steam have to be addressed. Solids are removed using a trapping device before they reach the turbines, and gases are removed from the condensers by a gas extraction system. The gases are mainly carbon dioxide (>90%) and some hydrogen sulfide. Depending on the environmental protection law, the hydrogen sulfide might need to be treated by a chemical process.

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