How Geothermal Power Plants Work

Geothermal power plants convert underground heat into electricity through steam turbines. Four main plant types operate globally, each suited to different resource temperatures. This guide walks the technologies in detail with the reservoir management and control systems that make them work.

The basic mechanism

All geothermal plants share the same fundamental process: bring hot water or steam from underground to the surface, convert its thermal energy into rotational mechanical energy at a turbine, and use a generator to produce electricity. The reservoir management, working fluid, and cycle configuration vary by plant type.

The four plant types

Dry steam plants

Reservoirs that produce direct steam are rare and highly valuable. Dry steam plants send reservoir steam directly to a turbine. The Geysers in California and Larderello in Italy are the two main dry steam field examples globally.

Flash steam plants

Most large geothermal fields produce hot pressurised water rather than steam. In a flash plant, this hot water is directed into a lower pressure vessel where a fraction "flashes" (rapidly boils) to steam. The steam drives the turbine; the remaining hot water may go to a second flash or reinjection.

Binary cycle plants

For lower temperature resources (110 to 180 C), reservoir water is not hot enough to flash effectively. Binary plants use a heat exchanger to transfer heat to a secondary working fluid (typically an organic like isobutane) that has a lower boiling point. The secondary fluid vaporises and drives the turbine.

Enhanced geothermal systems

EGS plants inject water into hot dry rock, create or enhance fractures, and circulate water through the fractures. Extracted hot water then feeds either flash or binary generation. EGS uncouples geothermal from natural reservoirs. See geothermal energy explained.

Reservoir management

Sustainable geothermal operations require reinjecting cooled water back to the reservoir. Reinjection maintains reservoir pressure, prevents subsidence, and reduces environmental impact. Modern plants inject 90+ percent of extracted water.

Wells

Production wells extract hot water or steam. Injection wells return cooled water. Wells reach 2 to 4 km depth for typical geothermal; EGS reaches 4 to 8 km. Drilling costs are the largest single capital line, USD 5 to 20 million per well.

Turbines and generators

Geothermal steam turbines are similar to those in fossil steam plants but designed for the lower temperature and pressure of geothermal steam. Binary cycle plants use organic Rankine cycle turbines optimised for the working fluid. Generators are typically 20 to 100 MW per turbine unit.

Cooling systems

Waste heat rejection uses cooling towers or air cooled condensers. Wet cooling is more efficient but requires cooling water; dry cooling is used at arid sites.

Geothermal fluid chemistry

Capacity factor

Control systems

Modern geothermal plants use SCADA for reservoir monitoring, wellhead control, and generation dispatch. Predictive analytics guide well management and reservoir sustainability.

Emissions

Geothermal plants can release reservoir gases (CO2, H2S) and require abatement in some designs. Modern binary plants have virtually no emissions; older flash plants may have significant CO2 emissions. Total lifecycle emissions per kWh are low.

Cost structure

CAPEX dominated by drilling (50 percent) and power plant equipment (30 percent). LCOE USD 60 to 100 per MWh for natural geothermal; USD 100 to 200 for EGS.

Operations workforce

Geothermal plants require reservoir engineers, mechanical engineers, chemists, and operators. Smaller than fossil plants of similar output.

Future technology

• Enhanced geothermal systems commercial scale.
• Supercritical geothermal (very high temperature, very high output).
• Closed loop systems (Eavor design).
• Hybrid solar plus geothermal.
• Direct use for heating alongside power.

Frequently asked questions

Summary

Geothermal plants convert underground heat to electricity through steam turbines. Four main types serve different resource temperatures. All operate as base load with the highest capacity factors of any renewable. EGS technology may expand geographic reach dramatically in the coming decade.

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