geothermy

Geothermy is the extraction and use of thermal energy from within the Earth's crust to generate electricity or provide direct heating. The practice exploits the fact that temperature increases with depth, typically at a rate of 25 to 30 degrees Celsius per kilometer of descent. In geothermally active regions, hot rock and steam reservoirs at accessible depths (often 1 to 3 kilometers) can be drilled and tapped much like oil wells, delivering a reliable, non-intermittent energy source.

There are two broad applications. In geothermal power plants, pressurized hot water or steam drives turbines connected to generators, producing electricity at capacities ranging from a few megawatts to over 100 megawatts per site. Direct-use systems pump hot water to buildings and industrial facilities for space heating, agricultural greenhouse warming, and process heat without the intermediate step of electricity generation. Direct-use applications are significantly more efficient because they avoid thermodynamic losses inherent in the steam cycle.

Resource types and production mechanics

The utility of geothermy depends critically on local geology. High-enthalpy resources, typically found in tectonically active zones (Iceland, New Zealand, Indonesia, the Philippines, the western United States), contain steam or water exceeding 150 degrees Celsius and support electricity generation. Medium-enthalpy fields (100 to 150 degrees Celsius) can drive binary cycles, where geothermal fluid heats a secondary working fluid with a lower boiling point, allowing power generation from cooler reservoirs. Low-enthalpy resources below 100 degrees Celsius are suitable only for direct heating and heat pump applications.

The drilling and completion methods mirror petroleum extraction: slim-hole reconnaissance wells (typically 10 to 25 centimeters diameter) establish temperature and pressure profiles, followed by larger production wells (up to 35 centimeters diameter) that may extend 2 to 4 kilometers vertically or at angles. Casing and cementing prevent cross-flow between thermal zones and protect freshwater aquifers. Many fields operate as doublets or networks, where one well produces hot fluid and another injects the cooled return to sustain pressure and temperature over decades of production.

A critical issue in geothermal operations is reservoir depletion and induced seismicity. Continuous extraction cools the rock matrix; reinjection sustains productivity but can trigger small earthquakes if fluid pressure rises sharply in confined zones. Scaling (mineral precipitation on pipe walls and heat exchanger surfaces) clogs systems, particularly in high-silica or high-salinity brines. Operators address these through careful fluid chemistry management, well spacing optimization, and injection temperature control. The term geothermy encompasses both the geological phenomenon and the engineering discipline needed to harvest it reliably.