🌋 Geothermal: Power from the Planet's Heat

What you’ll learn

1. The Heat Is Not the Scarce PartReframe geothermal from an abundance story into a concentration problem: the Earth's heat is enormous in total and negligible per square metre.Earth loses about 44.2 TW continuously — more than twice all human energy use, replenished by radioactive decay — but spread over the planet that is only about 65 mW/m² through continental crust and 101 mW/m² through oceanic crust, or roughly 13 watts under a 200 m² house. Sunlight delivers about 1,000 W/m², more than 15,000 times as much to the same spot, so geothermal energy is not scarce but dilute, which for a power engineer is nearly the same thing. Heat does concentrate with depth at a gradient of about 25–30 °C/km away from plate boundaries, which is why the industry exists at all — but a hot rock kilometres down is not yet a power plant.
2. Water Is the Whole BusinessEstablish the course's through-line: heat must be carried by a fluid through permeable rock, so geothermal is a plumbing problem rather than a heat-hunting problem.Rock's poor conductivity is what lets heat concentrate with depth and equally what stops it coming out — conduction cannot resupply a borehole at anything like the rate a power plant consumes heat, so the heat must be carried by water rather than waited for. That requires permeability, the interconnectedness of pores and fractures, which is distinct from porosity and which deep crystalline rock like granite essentially lacks. A conventional resource therefore needs heat, water and permeability in one place; where all three coincide, buoyancy organises the reservoir into a self-sustaining convection cell that hauls heat upward for free, and vents at the surface as geysers, fumaroles and hot springs.
3. The Rare Triple CoincidenceExplain why the world map of geothermal power is a map of plate boundaries, and read Iceland as a special case rather than a template.Good heat lives where the crust is thin, young or actively being torn: volcanic arcs around the Pacific Ring of Fire, continental rifts such as East Africa's, and mantle hotspots — settings where the gradient far exceeds the ordinary 25–30 °C/km and 200 °C rock sits at drillable depth. Iceland sits on both the Mid-Atlantic spreading ridge and a mantle plume, with abundant water and highly fractured young volcanic rock, so it holds all three ingredients in extravagant quantity at national scale. The honest lesson from Iceland is therefore not that geothermal works anywhere if you try, but how much of the map must be right before it is easy — which motivates the idea of supplying the missing ingredients ourselves.
4. Three Ways to Turn Steam Into MoneyDistinguish dry steam, flash and binary plants as three answers to what the resource delivers, and understand flashing as a pressure trick that adds no heat.Dry steam fields deliver superheated steam directly to a turbine and are almost nonexistent in nature — Larderello and The Geysers, where PG&E began an 11 MW plant in 1960 and which now has about 1,590 MW of nameplate capacity across 22 units at 18 stations. Flash plants exploit the fact that reservoir water stays liquid at 200 °C only because pressure raises its boiling point: drop the pressure in a flash tank and a fraction boils instantly using heat it already carried, with no energy added by the plant. Binary (Organic Rankine Cycle) plants never send brine to the turbine, instead boiling isobutane or pentane across a heat exchanger, which opens up the far commoner moderate-temperature resources and keeps the brine's silica, salts and hydrogen sulphide inside a closed loop.
5. The Reservoir Is Not a BatteryUnderstand reservoir depletion as a fluid-budget problem rather than a heat problem, using The Geysers' decline and wastewater recharge.By 1999 steam production had begun to deplete The Geysers and output was dropping — not because the rock had cooled, but because decades of production and cooling-tower evaporation had removed fluid faster than nature replaced it. The fix was to pipe in treated municipal wastewater: the Southeast Geysers Effluent Pipeline (1997) delivers about 9 million gallons/day over 40 miles and the Santa Rosa Geysers Recharge Project (2003) about 11 million gallons/day over 42 miles, roughly 20 million gallons daily, producing approximately 77 MW of capacity in 2004. Geothermal is therefore renewable at planetary scale while a specific field has a fluid budget that can be overdrawn — but unlike a depleted oil field the energy remains, so a field can be re-plumbed rather than written off.
6. EGS: Manufacturing the Missing IngredientsExplain Enhanced Geothermal Systems as an attempt to manufacture the two missing ingredients, and state the fracture-slip mechanism precisely.EGS proposes supplying water and permeability to the one ingredient that is genuinely universal — heat — which would turn geothermal from a geological accident into an engineering choice available almost anywhere. The mechanism is specific: injection raises fluid pressure in the rock, triggering shear events that expand pre-existing cracks and enhance permeability, so the water unclamps old fractures and lets them slip rather than smashing new ones into virgin granite; once rough faces have slid past each other they cannot close cleanly, and a sealed crack becomes a path. Fenton Hill, begun by Los Alamos in 1973, proved the principle — heat extracted from stimulated hot crystalline rock at around 185 °C at 2.6 km, reaching roughly 10 MW thermal in a 1986 test — before budget cuts ended it.
7. The Honest ObstacleConfront induced seismicity as the honest obstacle to EGS — the same mechanism that creates permeability — using Basel 2006 and Pohang 2017, attributing contested findings rather than adjudicating them.Because EGS works by making deep rock slip along fracture planes, the permeability and the seismicity are one phenomenon rather than two, and the engineering question is whether you can have as much as you need and no more. At Basel in December 2006, a stimulation at about 5 km beneath the city produced roughly 13,000 microearthquakes in six days and one ML 3.4 event on 8 December — no injuries, around 1,000 emergency calls, about 7 million CHF in insured damage — and the project was cancelled in December 2009 after a three-year risk study. At Pohang, South Korea, a magnitude 5.4 earthquake in 2017 near an EGS site operating since December 2010 was concluded to be an induced event by research published in Science, with all research activities stopped in 2018; the picture that emerged is that injection unclamped a pre-existing tectonically loaded fault, which is what makes the hazard hard to bound, and attribution remains a contested, after-the-fact science.
8. The Idea That Works EverywhereDistinguish ground-source heat pumps from geothermal power, and land the through-line: the heat pump wins by refusing to concentrate dilute heat at all.A ground-source heat pump makes no electricity, reaches metres rather than kilometres, and taps ground at roughly the annual average air temperature — around 10–15 °C in much of the temperate world — which is essentially stored sunshine rather than the Earth's internal heat. It works because a building's misery is its distance from average and the ground is the average: warm at 12 °C in January, cold at 12 °C in July. A heat pump moves heat rather than making it, so a joule of electricity can deliver three or four joules indoors, and since the work scales with the temperature lift, pulling from 12 °C ground beats pulling from −5 °C air on precisely the coldest night — the product is steadiness, not heat.

Questions this course answers

Grounded in trusted sources

• Wikipedia — Geothermal gradient (total heat loss 44.2 TW; mean heat flow 65 mW/m² continental, 101 mW/m² oceanic; gradient 25–30 °C/km): https://en.wikipedia.org/wiki/Geothermal_gradient
• Wikipedia — The Geysers (dry steam field; PG&E's 11 MW plant, 1960; 1,590 MW nameplate across 22 units at 18 stations; decline from 1999; Southeast Geysers Effluent Pipeline 1997, ~9 Mgal/day, 40 miles; Santa Rosa Geysers Recharge Project 2003, ~11 Mgal/day, 42 miles; ~77 MW of capacity in 2004): https://en.wikipedia.org/wiki/The_Geysers
• Wikipedia — Enhanced geothermal system (injection raises fluid pressure, triggering shear events that expand pre-existing cracks; Fenton Hill from 1973, ~185 °C at 2.6 km, ~10 MW thermal in 1986; Pohang M5.4 in 2017 concluded induced by research in Science, research stopped 2018; Basel suspended and cancelled): https://en.wikipedia.org/wiki/Enhanced_geothermal_system
• Wikipedia — Induced seismicity in Basel (ML 3.4 on 8 December 2006 at ~5 km; ~13,000 microearthquakes in six days; ~7 million CHF in insured damage; ~1,000 emergency calls; cancelled December 2009): https://en.wikipedia.org/wiki/Induced_seismicity_in_Basel
• Wikipedia — Permeability (Earth sciences) (permeability vs porosity; granite ~0.001–0.01 millidarcy): https://en.wikipedia.org/wiki/Permeability_(Earth_sciences)
• Wikipedia — Geothermal power (dry steam, flash and binary cycle plants; reinjection and reservoir management): https://en.wikipedia.org/wiki/Geothermal_power
• Wikipedia — Organic Rankine cycle (low-boiling-point working fluids): https://en.wikipedia.org/wiki/Organic_Rankine_cycle
• Wikipedia — Ground source heat pump (shallow ground near annual average air temperature; coefficient of performance): https://en.wikipedia.org/wiki/Ground_source_heat_pump

Every Wunder lesson is built from real, reputable sources — never invented.

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