Conventional Geothermal

Iceland generates 25% of its electricity from geothermal plants that tap reservoirs of superheated water and steam below the earth's surface. These plants run 24/7 at capacity factors of 85-95%, making geothermal one of the most reliable renewable sources. The catch: you need the right geology.

Conventional geothermal extracts heat from underground reservoirs where hot rock has naturally heated water to 150-300+ degrees C. Three plant types handle different reservoir conditions. Dry steam plants use reservoirs that produce steam directly; steam goes straight to the turbine. Rare but simplest (The Geysers in California). Flash steam plants pump high-pressure hot water to the surface, where the pressure drop "flashes" it into steam. Most common type globally. Binary cycle plants pass moderate-temperature water (100-180 degrees C) through a heat exchanger to vaporize a secondary fluid with a lower boiling point (like isobutane), which drives the turbine.

Apply the Carnot constraint. A flash plant with 200 degrees C (473 K) reservoir and 30 degrees C (303 K) cooling. Carnot limit: 1 - 303/473 = 36%. Actual efficiency: 10-15%.

Explain the efficiency gap. Geothermal's source temperature is far below fossil plants (600-1,600 degrees C). Lower T_hot means a lower Carnot ceiling, so geothermal electricity is thermodynamically expensive.

If geothermal is so inefficient, why is it still economical?

Zero fuel cost. The heat is free and continuous. Low efficiency matters less when there is no fuel bill. High capacity factor (85-95%) spreads capital cost over many kilowatt-hours.

Geothermal is limited to volcanic and tectonic zones today, which is why enhanced geothermal systems aim to create artificial reservoirs anywhere.

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