Geothermal Energy is thermal energy originating within the Earth and stored in rocks, fluids and subsurface geological formations. Earth's internal heat comes largely from residual heat associated with planetary formation and radioactive decay of elements within the Earth. Heat moves through the crust by conduction and is transported by fluids through convection and circulation. Geothermal resources can be used directly for heating or converted into electricity when temperatures and reservoir conditions are suitable. Technologies include geothermal heat pumps, direct use systems, conventional hydrothermal power plants and engineered or enhanced geothermal systems. Geothermal Energy Science combines geology, geophysics, thermodynamics, fluid mechanics, drilling engineering, reservoir engineering, chemistry and environmental science to identify, develop and manage subsurface heat resources.
Conventional geothermal electricity generally requires a suitable combination of temperature, permeability and fluid availability. High temperature hydrothermal reservoirs can produce steam naturally or through pressure reduction and heating processes. Dry steam plants use steam directly to drive turbines. Flash steam plants reduce the pressure of hot geothermal water, causing part of the fluid to vaporise into steam that drives a turbine.
Binary cycle plants transfer heat from geothermal fluid to a secondary working fluid with a lower boiling point, allowing electricity generation from resources that are too cool for conventional steam turbines. After heat extraction, geothermal fluids are often reinjected into the reservoir to maintain pressure and manage fluid disposal. The choice of technology depends on temperature, chemistry, pressure, flow rate and geological structure.
Geothermal heat pumps use relatively stable shallow ground temperatures rather than requiring a high temperature geothermal reservoir. A heat pump transfers thermal energy between buildings and the ground using a refrigeration cycle. In heating mode, the system extracts heat from the ground and raises its temperature for indoor use. In cooling mode, heat is transferred from the building into the ground.
Ground source heat pumps can therefore provide heating and cooling with relatively stable performance because shallow ground temperatures vary less than outdoor air temperatures. Their efficiency depends on ground conditions, system design, building demand, fluid temperatures, compressor performance and installation configuration. Closed loop systems circulate a working fluid through buried pipes, whereas open loop systems use groundwater where appropriate conditions and regulations permit.
Geothermal resource assessment uses geological mapping, geochemical analysis, geophysical surveys, temperature measurements, well data and reservoir modelling. Scientists may examine surface manifestations such as hot springs, fumaroles and altered rocks, but economically useful resources can also occur without obvious surface signs. Drilling provides direct information about temperature, pressure, permeability and fluid chemistry, although drilling is expensive and introduces exploration risk. Reservoir engineers model fluid movement and heat extraction to estimate sustainable production.
Geothermal reservoirs can respond differently depending on fracture networks, recharge rates and pressure changes. Long term management is therefore necessary to avoid excessive pressure decline or cooling around production wells. Reinjection can help maintain reservoir pressure and reduce surface disposal requirements, but reinjected fluids must be managed carefully to avoid undesirable thermal or chemical effects.
Geothermal energy can provide relatively continuous output because subsurface heat is not dependent on daylight or short term weather conditions. This characteristic can make geothermal electricity useful for firm generation in regions with suitable resources. Nevertheless, geothermal power is geographically constrained because economically viable high temperature reservoirs are not evenly distributed. Direct heat applications can access a wider range of temperatures than electricity generation because heating does not require conversion through a power cycle.
District heating networks can distribute geothermal heat to multiple buildings where resource conditions and infrastructure permit. Industrial processes, greenhouse heating, aquaculture and bathing are among other direct uses. Geothermal heat pumps have much broader geographical applicability than high temperature electricity projects because they use shallow thermal resources rather than deep high temperature reservoirs.
Geothermal projects can have environmental impacts associated with drilling, land disturbance, fluid chemistry, induced seismicity and emissions from subsurface fluids. Geothermal fluids may contain dissolved minerals and gases, including naturally occurring carbon dioxide and hydrogen sulphide, requiring appropriate treatment and control. Reinjection can reduce surface discharge but may alter subsurface pressure and in some circumstances contribute to induced seismicity. Enhanced geothermal systems deliberately create or stimulate permeability in hot rock, making seismic monitoring particularly important. Land requirements are often comparatively concentrated at the surface relative to some large energy developments, although wells and access infrastructure can extend over significant underground areas. Life cycle assessment considers drilling materials, construction, pumps, pipelines, plant operation and well management.
Geothermal energy has been used for bathing and direct heating for thousands of years, particularly in areas with accessible hot springs. Modern geothermal electricity developed as engineers learned to drill wells and convert high temperature fluids into mechanical and electrical energy. The first commercial geothermal electricity developments occurred in areas with exceptional natural resources, after which technology expanded into flash steam and binary systems. Geothermal heat pumps developed from advances in refrigeration, heat transfer and ground loop engineering. Contemporary research includes enhanced geothermal systems, superhot geothermal resources, improved drilling techniques, advanced reservoir modelling and closed loop concepts. These developments aim to expand geothermal energy beyond naturally permeable high temperature reservoirs and reduce exploration or drilling costs.
Geothermal Energy should not be confused with ground temperature in general, although geothermal heat pumps rely on shallow ground thermal conditions. Geothermal energy is also not simply heat from volcanic activity. The resource occurs across many geological settings, although temperature gradients and reservoir characteristics vary greatly. Geothermal heat is renewable on human timescales when extraction is managed so that natural and engineered heat replenishment remains compatible with the rate of use.
Excessive extraction from a particular reservoir can reduce temperature or pressure locally, so renewability must be considered at the reservoir scale rather than assuming unlimited output. Advantages include potentially high availability, compact surface footprints and useful direct heating applications. Limitations include drilling cost, geological uncertainty, location dependence, mineral scaling, corrosion and possible induced seismicity.
