# Geothermal energy **Geothermal energy** is the [[Thermal_energy|thermal energy]] stored in the rock and fluids of the [[Crust_(geology)|Earth's crust]], together with the steady flow of [[Heat|heat]] that resupplies it from the planet's interior. Two numbers describe the resource almost completely: a heat flux of roughly 65 mW/m² crossing the continental surface, and a [[Geothermal_gradient|geothermal gradient]] of 25 to 30 K per kilometre of depth.[^spec-e45][^earle-128] They are not independent. Where [[Thermal_conduction|conduction]] carries the heat — which it does through solid rock, and through soil as well — Fourier's law ties them: `q = k·dT/dz`.[^ochsner-119][^openstax-tc] In the microsim below the reader sets the gradient, the target depth and the flow rate of a two-well doublet, and four readouts follow in order. Fourier's law inverted gives the depth at which the rock reaches 150 °C. A doublet that circulates water through that rock delivers `Q = ṁ·c_p·ΔT` in megawatts of heat. The [[Carnot_cycle|Carnot]] limit, `η = 1 − T_L/T_H` on absolute temperatures, converts that to a ceiling in megawatts of electricity for a flash or binary plant.[^yan-115] And [[Darcy's_law|Darcy's law]], `Q = k·A·Δp/(μ·L)`, decides whether the rock will pass the flow at all — the constraint that in practice decides the project.[^zeidouni-084] A fifth readout, the years until the reservoir cools, is ILLUSTRATIVE: it treats the swept rock as one well-mixed block, which no real field is.[^spec-e45][^derived-ge] On the [[Energy]] flagship this article serves the *Geothermal* section of Part V — Transformation. Its sim is the sibling of [[Thermal_conduction]], which builds the same slab without the well, and it shares the Darcy machinery of [[Groundwater|groundwater]] flow with the hydrology pages. ## History Hot springs have been bathed in, cooked over and washed in for as long as there are records of people living near them, and the first industrial uses — boric acid from the Larderello fumarole field in Tuscany, salt from hot brines — took the chemistry of geothermal fluid rather than its heat. Electricity came in 1904, when Piero Ginori Conti drove a small generator at Larderello with natural steam and lit five lamps; a commercial plant followed at the same field in 1913.[^larderello] District heating from geothermal water began at about the same period, with the Boise, Idaho system of 1892 among the first, and the largest dry-steam field in the world, The Geysers in northern California, began generating in 1960.[^doe-gto] Every one of these is a hydrothermal field: a place where nature had already done the two hard things, heating the water and providing the permeability to move it. ## Resources The planet's heat has two sources: the primordial heat of accretion and differentiation, still leaking out after 4.57 billion years, and the continuing [[Radioactive_decay|radioactive decay]] of long-lived [[Isotope|isotopes]] in the crust and mantle — ⁴⁰[[Potassium|K]] with a [[Half-life|half-life]] of 1.3 Ga, ²³⁸[[Uranium|U]] at 4.47 Ga, ²³⁵U at 0.704 Ga and ²³²[[Thorium|Th]] at 14.0 Ga, each the head of a [[Decay_chain|decay chain]] that deposits its energy locally as heat.[^earle-128][^murphy-nuc] Their half-lives are the reason the supply is steady on any human timescale: the youngest of them takes 700 million years to weaken by half. The flux is huge in total and useless in density. At the sim's 65 mW/m² over the Earth's 5.1×10¹⁴ m² of surface, the throughput is 33 TW, against the roughly 3 TW of primary energy the United States consumes — and yet 65 mW/m² is about one fifteen-thousandth of noon sunlight, so nothing can be collected at the surface.[^derived-ge][^murphy-units] Geothermal energy is therefore not harvested from the flux but mined from the store, by going deep enough that the temperature, not the flux, is the resource. The gradient sets how deep. With a surface temperature of 15 °C, reaching 150 °C takes 135 K of rise: 4.5 km at 30 K/km, 5.4 km at 25 K/km, and only 2.7 km where the gradient is 50 K/km, as it is in thin-crust basins.[^derived-ge] Inverting Fourier's law on the sim's two presets gives the rock's [[Thermal_conductivity_and_resistivity|thermal conductivity]] as a consistency check: `k = q/(dT/dz)` = 0.065/0.025 = 2.6 W/(m·K) at the shallow gradient and 2.2 at the steep one, which is the right range for granite and basalt and confirms that the two sim controls are not free of each other.[^derived-ge][^openstax-tc] ## Geothermal power A power plant is a doublet plus a [[Heat_engine|heat engine]]. Water is produced from a hot well, passed through the plant and returned down an injection well, and the heat it carries is `Q = ṁ·c_p·ΔT`. At 50 kg/s produced at 150 °C and reinjected at 60 °C, with c_p = 4.18 kJ/(kg·K), that is 18.8 MW of heat.[^derived-ge] The [[Second_law_of_thermodynamics|second law]] then takes most of it away. With the resource at 423 K and the condenser at 298 K, the Carnot ceiling is `η = 1 − 298/423` = 29.5 %, so the ideal output is 5.6 MW of electricity; a real binary plant reaching about 40 % of the Carnot value — a typical second-law efficiency for a low-temperature [[Rankine_cycle|Rankine cycle]] — delivers near 2.2 MW.[^yan-115][^derived-ge] Kerlin's rule for thermal plants of every kind applies here in its harshest form: the rejected heat is several times the electrical output, and because T_H is low the multiple is larger than in a fossil or nuclear station.[^kerlin-geo][^yan-115] Three plant types follow from the resource temperature. Dry-steam plants take steam straight from the well to the [[Steam_turbine|turbine]], and need a rare field like Larderello or The Geysers. Flash plants take pressurised brine above about 180 °C, drop its pressure so that part of it flashes to steam, and run the turbine on the steam. Binary plants, which cover everything from roughly 100 °C upward, never expand the brine at all: they pass its heat through an exchanger into a working fluid of low boiling point and run a closed [[Thermodynamic_cycle|cycle]] on that, which is what makes moderate-temperature resources usable and is why the sim's Carnot readout is labelled as a ceiling rather than a prediction.[^kerlin-geo][^yan-115] ## Geothermal heating Used as heat rather than as electricity, the same resource is several times more valuable, because no Carnot factor intervenes: 18.8 MW of 150 °C water is 18.8 MW of district heat, not 2.2 MW of electricity.[^derived-ge] Direct use covers district heating, greenhouses, aquaculture, industrial drying and bathing, and it works at temperatures far below anything a generator could use. A ground-source [[Heat_pump|heat pump]] is a different machine on a different resource. It does not reach the gradient at all; it exchanges heat with ground a few metres down, which sits near the annual mean air temperature and, crucially, stays there. The [[Coefficient_of_performance|coefficient of performance]] of an ideal heat pump is `COP = T_H/(T_H − T_L)`, so delivering 40 °C water from ground at 10 °C has a Carnot COP of 313/30 = 10.4, against 6.3 for an air-source machine on a −10 °C winter day; real units reach 3.5 to 4.5, and the advantage of the ground is not that it is warm but that it does not get cold exactly when the heat is wanted.[^yan-115][^derived-ge] ## Types Fields divide by whether nature supplied the permeability or the engineer has to. Heat at depth is nearly universal; connected pathways for water to carry it are not, and that single asymmetry separates the two families below. ### Hydrothermal systems A hydrothermal system is hot rock that already contains circulating water in connected fractures or porous beds. Production needs only to find it and drill it, and the natural [[Convection|convection]] of the system has usually already concentrated heat far above what conduction alone would give at that depth. These are the fields that carry every plant built before the 1980s, and the reason geothermal electricity is geographically concentrated: the coincidence of heat, water and permeability is rare, and the sim's Darcy readout is the reason. At k = 10⁻¹³ m² (100 millidarcy), a 200 m × 500 m section and 1 km between wells, driving 0.05 m³/s of 150 °C water through the rock costs about 10 bar of pressure drop — a pump's work. Drop the permeability by a factor of ten and the same flow costs 100 bar, which no pump will supply and the rock will not withstand.[^zeidouni-084][^derived-ge] ### Engineered geothermal systems An [[Enhanced_geothermal_system|engineered geothermal system]] attacks that constraint directly. Hot rock is available almost everywhere at 4 to 6 km; what is missing is permeability, so the rock is stimulated hydraulically or chemically to open a fracture network between an injection well and one or more production wells, and water is circulated through the engineered reservoir. The prize is that the resource stops being a map of rare places and becomes a function of depth, and the difficulty is that fracture networks are not designed but discovered: flow finds the most permeable path, sweeps a small part of the rock, and cools it while the rest stays hot. That short-circuiting, not the total heat in place, is what limits the life of an engineered field, and it is exactly the effect the sim's well-mixed drawdown model cannot represent.[^spec-e45][^zeidouni-084] ## Economics Geothermal is capital-heavy, fuel-free and site-bound, so its economics are drilling economics. Well cost rises steeply — roughly exponentially — with depth, while the resource temperature rises only linearly with it, so there is an optimum depth for every gradient and the optimum is shallower than the temperature alone would suggest. Against that, the plant runs at a [[Capacity_factor|capacity factor]] above 90 %, higher than any other renewable source and higher than most thermal plants, because the resource neither sets nor blows nor runs dry on a daily cycle; over a thirty-year life that steadiness recovers much of the drilling cost.[^kerlin-geo][^theis-geo] ### Socioeconomic benefits Because the heat cannot be shipped, the value stays where the field is: district heating networks, greenhouse agriculture and industrial process heat cluster around producing fields, and the operating workforce is local and long-term. In places with a steep gradient and no fossil resource, geothermal converts a geological accident into an [[Electrical_grid|energy supply]] that is neither imported nor priced abroad.[^theis-geo] ## Development Developing a field is a sequence of decreasing uncertainty and increasing expense: surface survey, shallow temperature-gradient holes, one deep exploration well, a flow test, and only then the production wells and the plant. The flow test is the decision point, because it measures the one quantity that surface work cannot — the product `k·A` in Darcy's law, which is what sets the pressure drop the pumps will fight for the life of the field.[^zeidouni-084] Reservoir engineering after that is a matter of [[Pressure|pressure]] management: reinjecting the cooled brine both disposes of it and holds the reservoir pressure up, and the placement of injection wells is a compromise between supporting pressure and cooling the producers too soon. ### Precipitate scaling Geothermal brine is a hot, concentrated solution, and anything that changes its temperature or pressure changes what it can hold. [[Silicon_dioxide|Silica]] [[Solubility|solubility]] rises steeply with temperature, so brine cooled in a heat exchanger or concentrated by flashing becomes supersaturated and deposits amorphous silica in pipes, exchangers and the injection well itself; carbonate scale forms by the opposite route, when the release of dissolved gas raises the pH and calcite [[Precipitation_(chemistry)|precipitates]].[citation needed][^scale-cn] The countermeasures are to keep the brine above its saturation temperature all the way back to the injection well, to hold it at pressure so nothing flashes, or to dose it chemically — and scaling in the injection well is the more dangerous case, because it destroys the permeability the project was built on. Corrosion by the same brine, driven by dissolved chloride and hydrogen sulfide, sets the alloys of every wetted surface.[^kerlin-geo] ## Sustainability Geothermal is renewable at the scale of the planet and minable at the scale of a field. The sim's ILLUSTRATIVE drawdown makes the arithmetic visible: a swept block of 0.5 km³ of rock at 2,700 kg/m³ and 900 J/(kg·K) stores 1.2×10¹⁵ J for every kelvin it is allowed to cool, so 30 K of cooling is 3.6×10¹⁶ J, and at 18.8 MW of heat extraction it lasts 61 years.[^derived-ge] The conductive flux that resupplies the same footprint — 65 mW/m² over 1 km² — is 65 kW, three hundred times less than the extraction. A field is therefore drawn down and then rested, on a cycle far longer than a human career; The Geysers lost steam pressure through the 1980s and was partly restored by injecting treated wastewater, which is a reservoir-management fix rather than a thermal one.[^derived-ge][^doe-gto] The honest statement is that a well-run field is sustainable as a system and depleting as a reservoir. ## Environmental effects Geothermal plants have a small footprint and no combustion, but they are not inert. Geothermal fluid carries dissolved gases, and in a flash or dry-steam plant these leave with the steam: carbon dioxide at levels far below a fossil plant but not zero, and hydrogen sulfide, which is the characteristic nuisance of a producing field and is now usually abated. Binary plants avoid both, because the brine never leaves its loop. Water use is small if the brine is reinjected and large if it is not. The significant hazard belongs to engineered systems: hydraulic stimulation raises fluid pressure on existing faults and can trigger felt earthquakes, which ended a prominent European engineered-system project after a stimulation sequence in 2006 and has shaped stimulation practice and regulation since.[^seismic] Set against these, the land area per megawatt is among the lowest of any generating technology, and the plant runs without fuel transport.[^theis-geo] ## Production Installed capacity follows the map of hydrothermal fields, while direct use follows the map of gradients — two different lists of countries. The three below are chosen to make that split visible: two with volcanic fields and turbines, one with a steep gradient and no turbines at all. ### Philippines The Philippines sits on an active volcanic arc and is among the world's largest geothermal producers, with on the order of 1.9 GW installed and geothermal supplying a substantial share of national electricity — the clearest case of a country in which the resource is a strategic, not a marginal, part of supply.[^ph-doe] ### United States The United States has the largest installed geothermal capacity in the world, on the order of 3.7 GW, and most of it is in California, dominated by The Geysers — the rare dry-steam field that has been producing since 1960. Almost all of it is hydrothermal; the federal programme's interest in engineered systems is precisely an attempt to break the geographic limit that the hydrothermal inventory imposes.[^doe-gto] ### Hungary Hungary generates almost no geothermal electricity and is nonetheless one of Europe's heaviest users of geothermal heat. The Pannonian Basin has thin crust and a gradient near 50 K/km, so 150 °C water sits about 2.7 km down instead of 4.5 km, and the porous sedimentary fill of the basin supplies the permeability that engineered systems elsewhere must manufacture.[^derived-ge][^hu-geo] The result is district heating, greenhouses and thermal baths rather than turbines: a demonstration that the [[Exergy|exergy]] of a resource, not its energy, decides what it can be used for. ## See also - [[Geothermal_power]] — the electricity half of the resource - [[Geothermal_gradient]] - [[Enhanced_geothermal_system]] - [[Earth's_internal_heat_budget]] — where the 65 mW/m² comes from - [[Thermal_conduction]] — the sibling sim, the slab without the well - [[Darcy's_law]] — the flow constraint that decides the project - [[Heat_pump]] - [[Rankine_cycle]] ## References [^spec-e45]: Matter & Energy Cluster contract, `_registry/plans/ENERGY_SECTIONS.md` row E45: the sim concept (sibling of `Thermal_conduction`), the crust-as-slab model `q = k·dT/dz` at 65 mW/m² and 25–30 K/km, the doublet heat balance `Q = ṁ·c_p·ΔT`, the Darcy flow constraint `Q = k·A·Δp/(μ·L)`, the Carnot-limited electrical readout, and the reservoir-cooling readout marked ILLUSTRATIVE. [^earle-128]: Earle, Steven (2015). *Physical Geology*. Chapter 8, *Measuring Geological Time* (pp. 222–249): ⁴⁰K half-life of 1.3 Ga and the half-by-half rule, p. 237; Table 8.2 isotope systems and useful ranges, p. 239. Chapter 9, *Earth's Interior* (pp. 250–271), for the internal heat budget (page to pin). https://open.umn.edu/opentextbooks/textbooks/physical-geology [^murphy-nuc]: Murphy, Thomas (2021). *Energy and Human Ambitions on a Finite Planet*. Chapter 15: half-lives of ²³⁵U (0.704 Gyr), ²³⁸U (4.47 Gyr) and ²³²Th (14.0 Gyr), pp. 277, 279. https://open.umn.edu/opentextbooks/textbooks/energy-and-human-ambitions-on-a-finite-planet [^murphy-units]: Murphy, Thomas (2021). *Energy and Human Ambitions on a Finite Planet*. Chapter 5: the unit ladder, and the United States at about 100 quads per year ≈ 3 TW ≈ 10,000 W per person, p. 95. https://open.umn.edu/opentextbooks/textbooks/energy-and-human-ambitions-on-a-finite-planet [^ochsner-119]: Ochsner, Tyson (2019). *Rain or Shine*. Chapter 13, *Soil Temperature* (pp. 297–317): conduction as the dominant heat-transport mechanism in soil, p. 294 as cited by the contract row (page to pin). Chapter 4, *Soil Water Flow* (pp. 109–140), for Darcy's law in the form `q = K·Δψ/L`, p. 119. https://open.umn.edu/opentextbooks/textbooks/rain-or-shine [^openstax-tc]: Sanny, Jeff; Ling, Samuel (2016). *University Physics Volume 2* (OpenStax). Chapter 1 §1.6, Mechanisms of Heat Transfer: Fourier's law `P = k·A·ΔT/d` and Table 1.5 thermal conductivities of common substances (page to pin). https://open.umn.edu/opentextbooks/textbooks/university-physics-volume-2 [^yan-115]: Yan, Claire Yu (2022). *Introduction to Engineering Thermodynamics*. Chapter 6, *Entropy and the Second Law of Thermodynamics* (pp. 239–348): the Carnot efficiency `η = 1 − T_L/T_H`, p. 272; the heat-pump and refrigerator coefficients `COP_HP = T_H/(T_H − T_L)` and `COP_R = T_L/(T_H − T_L)`, p. 277; the rule that no actual device may exceed the Carnot value, p. 272; large steam plants raising T_H with 300–600 °C steam, p. 273. The equation displays were lost in extraction and are supplied here in standard form. https://open.umn.edu/opentextbooks/textbooks/introduction-to-engineering-thermodynamics [^zeidouni-084]: Zeidouni, Mehdi (2025). *Petroleum Reservoir Dynamics*, pp. 2–167 (page to pin): Darcy flow in porous media, absolute and effective permeability, fluid viscosity and the compressibility of reservoir rock — the machinery behind the doublet's pressure drop and the flow-test measurement of `k·A`. https://open.umn.edu/opentextbooks/textbooks/petroleum-reservoir-dynamics [^kerlin-geo]: Kerlin, Thomas (2013). *Future Energy: Opportunities & Challenges*. Chapter 7, pp. 249–266 (page to pin): geothermal steam, reservoir permeability, plant types and field development; Chapter 3, p. 185, for the rule that a thermal plant rejects 2.5–3 times its electrical output. https://open.umn.edu/opentextbooks/textbooks/future-energy-opportunities-challenges [^theis-geo]: Theis, Tom; Tomkin, Jonathan, eds. (2015). *Sustainability: A Comprehensive Foundation*. Chapter 10, *Sustainable Energy Systems* (page to pin): capacity factor, land use and the local economics of site-bound renewable resources. https://open.umn.edu/opentextbooks/textbooks/sustainability-a-comprehensive-foundation [^derived-ge]: Computed for this article from the equations on the page and the cited constants: depth to 150 °C from a 15 °C surface, 135 K ÷ 30 K/km = 4.5 km, ÷ 25 = 5.4 km, ÷ 50 = 2.7 km; `k = q/(dT/dz)` = 0.065/0.025 = 2.6 and 0.065/0.030 = 2.2 W/(m·K); global throughput 0.065 W/m² × 5.1×10¹⁴ m² = 3.3×10¹³ W = 33 TW, and 1,000/0.065 ≈ 15,000 as the ratio of noon sunlight to surface heat flux; `Q = ṁ·c_p·ΔT` = 50 × 4,180 × 90 = 18.8 MW; `η = 1 − 298/423` = 29.5 %, giving 5.6 MW ideal and 2.2 MW at 40 % of Carnot; `COP = T_H/(T_H − T_L)` = 313/30 = 10.4 and 313/50 = 6.3; Darcy `Δp = Q·μ·L/(k·A)` = 0.05 × 2×10⁻⁴ × 1,000 / (10⁻¹³ × 10⁵) = 1.0×10⁶ Pa = 10 bar, and ten times that at k = 10⁻¹⁴ m²; stored heat `ρ·c·V` = 2,700 × 900 × 5×10⁸ = 1.2×10¹⁵ J/K, × 30 K = 3.6×10¹⁶ J, ÷ 1.88×10⁷ W = 1.9×10⁹ s = 61 years; resupply 0.065 W/m² × 10⁶ m² = 65 kW against 18.8 MW, a ratio near 300. [^larderello]: Enel Green Power, historical documentation for the *Larderello* geothermal field (page to pin): Piero Ginori Conti's 1904 demonstration and the commercial plant of 1913. No Portal Book covers the field's history. [^doe-gto]: United States Department of Energy, Geothermal Technologies Office, and the U.S. Energy Information Administration (page to pin): the Boise district heating system, The Geysers field and its 1960 start, United States installed capacity, and the federal engineered-geothermal programme. No Portal Book covers the programme. [^seismic]: Swiss Seismological Service and the cantonal review of the Basel engineered geothermal project (page to pin): the induced seismicity following the December 2006 stimulation and the project's termination. No Portal Book covers induced seismicity. [^ph-doe]: Philippine Department of Energy, geothermal resource and installed-capacity reporting (page to pin). No Portal Book covers the country's fleet. [^hu-geo]: Hungarian geological and energy-agency reporting on the Pannonian Basin (page to pin): the elevated geothermal gradient, the porous sedimentary reservoirs and the direct-use fleet. No Portal Book covers the basin. [^scale-cn]: Citation needed. The scaling account — amorphous silica supersaturating as brine cools or flashes, calcite precipitating as dissolved CO₂ leaves solution and the pH rises, and the injection well as the critical location — is standard geothermal reservoir-chemistry practice, but no Energy Portal Book states it: Kerlin's Chapter 7 extract covers steam, permeability and reservoirs without the brine chemistry. The record that would settle it is a geothermal-operations handbook, page to pin. ## External links - [*Future Energy: Opportunities & Challenges*](https://open.umn.edu/opentextbooks/textbooks/future-energy-opportunities-challenges) (Kerlin, 2013), Chapter 7 — the geothermal chapter behind this page - [*Petroleum Reservoir Dynamics*](https://open.umn.edu/opentextbooks/textbooks/petroleum-reservoir-dynamics) (Zeidouni, 2025) — Darcy flow, permeability and reservoir rock - [*Introduction to Engineering Thermodynamics*](https://open.umn.edu/opentextbooks/textbooks/introduction-to-engineering-thermodynamics) (Yan, 2022), Chapter 6 — the Carnot and heat-pump limits - [*Physical Geology*](https://open.umn.edu/opentextbooks/textbooks/physical-geology) (Earle, 2015) — the radiogenic heat sources and their half-lives - The Wikipedia pair's *External links* section lists the national agency and operator sites for the fields named above. <!-- MATTERSIM:BEGIN g33 — Matter & Energy Cluster microsim (framework build, specs/sims/Geothermal_energy.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Geothermal energy* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/matter/Geothermal_energy.html" data-title="Geothermal energy"></div> *Built from `MICROSIM_GUIDE/specs/sims/Geothermal_energy.json`; part of the [[PORTAL_Matter|Matter portal]] spine (section sims and See-also variants).* <!-- MATTERSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Geothermal_energy) : [Wikitube](https://en.wikitube.io/wiki/Geothermal_energy) · pinned revision [1370081785](https://en.wikipedia.org/w/index.php?oldid=1370081785) · 2026-09-11 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Energy]]. --- *Matter & Energy Cluster child articles, wave 1 · 2026-09-11 · drafted · Energy row E45 · sim pending (matter/Geothermal_energy).*