Geothermal energy Study Guide

📖 Core Concepts Geothermal energy – heat extracted from the Earth’s crust, sourced from planetary formation (≈ 20 %) and radioactive decay of isotopes (≈ 80 %). Geothermal gradient – average increase of temperature with depth, typically 25–30 °C/km; higher near plate boundaries. Hydrothermal reservoir – natural hot water/steam aquifer; can be vapor‑dominated (super‑heated steam, 240–300 °C) or liquid‑dominated (hot water > 200 °C). Enhanced Geothermal System (EGS) – engineered reservoir created by high‑pressure water injection to fracture hot, low‑permeability rock. Binary‑cycle plant – uses a secondary organic working fluid to generate electricity from low‑temperature (≥ 81 °C) resources. Dry‑steam & Flash‑steam plants – directly use steam from the reservoir (dry‑steam) or flash hot water into steam (flash‑steam) to drive turbines. Heat flow to surface – 44.2 TW conducted upward; ≈ 30 TW replenished by radioactive decay. Renewable status – extraction is negligible vs Earth’s total heat (≈ 100 billion × world 2010 energy use), so geothermal is classified as renewable. Key environmental metrics – CO₂ emissions ≈ 45 g kWh⁻¹ (average) vs > 900 g kWh⁻¹ in the higher reported range; land use ≈ 3.5 km² GW⁻¹; water use ≈ 20 L MWh⁻¹. --- 📌 Must Remember Temperature limits: Binary‑cycle plants work down to 81 °C; vapor‑dominated reservoirs > 240 °C. Global capacity (2019): 15.4 GW electricity; 13.9 GW (earlier figure) and 28 GW thermal for direct use. Cost of electricity (2021 US DOE): ≈ $0.05 kWh⁻¹ for new geothermal plants. Capital cost: €2–5 M per MW (plant + drilling); > $4 M per MW for EGS. Drilling risk: Typical well pair ≈ $10 M; 20 % failure → average $50 M per successful pair. Emissions: Average 45 g CO₂ kWh⁻¹ (≈ 5 % of coal); reported range 900–1300 g kWh⁻¹ for some plants. Land & water: 3.5 km² GW⁻¹ land; 20 L MWh⁻¹ water – orders of magnitude lower than fossil plants. Geothermal gradient: 25–30 °C/km; use to estimate required depth: \( \text{Depth} \approx \frac{T{\text{target}} - T{\text{surface}}}{\text{gradient}} \). --- 🔄 Key Processes Resource Exploration Reconnaissance → Geophysical surveys → Exploratory drilling → Resource confirmation → Plant construction. Binary‑Cycle Heat Transfer Geofluid passes through heat‑exchanger → heats organic working fluid (e.g., Isobutane) → vapor drives turbine → condenses & repeats. Enhanced Geothermal System Creation High‑pressure water injection → hydraulic fracturing of hot rock → proppant placement → circulation of water to absorb heat → production well extracts hot fluid. Ground‑Source Heat Pump Operation Loop buried < 6 m extracts stable ground temperature (≈ annual air mean) → heat pump upgrades low‑grade heat to space heating or cooling. CO₂ Capture Integration Capture CO₂ from plant exhaust (post‑combustion amine absorption) → compress → inject into deep saline aquifer or depleted reservoir. --- 🔍 Key Comparisons Dry‑steam vs Flash‑steam vs Binary‑cycle Dry‑steam: Uses natural steam → high‑temp reservoirs; simplest, but rare. Flash‑steam: Depressurizes hot water → creates steam; works for liquid‑dominated > 200 °C. Binary‑cycle: Heat exchange to secondary fluid → can use resources as low as 81 °C. Hydrothermal vs Engineered (EGS) Hydrothermal: Relies on existing fluid and permeability; limited geographic distribution. EGS: Creates permeability; expands viable locations but adds drilling & seismic risk. Closed‑Loop vs Open‑Loop Closed‑Loop: Sealed pipe network, no contact with formation fluids; no scaling, lower contamination risk. Open‑Loop: Extracts formation fluid, re‑injects; higher risk of mineral scaling and gas emissions. --- ⚠️ Common Misunderstandings “Geothermal is only for volcanic islands.” – While hydrothermal reservoirs cluster near tectonics, EGS and binary plants enable use far from volcanoes. “Geothermal extraction cools the planet quickly.” – Heat removed is negligible vs Earth’s total heat; cooling occurs on geological timescales. “All geothermal plants have high emissions.” – Average emissions are < 5 % of coal; many plants emit < 50 g CO₂ kWh⁻¹. “Drilling is cheap because wells are shallow.” – Deep wells (up to 10 km) and hard igneous rocks drive high capital costs and failure risk. --- 🧠 Mental Models / Intuition Temperature‑Depth Rule: Depth ≈ (Target °C – Surface °C) / (25 °C/km). Helps quickly gauge if a resource is reachable. Heat‑Flow Budget: Earth supplies 44 TW; humanity’s total 2010 energy use ≈ 0.44 TW → geothermal could theoretically cover all energy demand many times over. Cost‑Risk Triangle: Capital cost (drilling) ↔ Failure probability ↔ Electricity price break‑even. Reducing any side improves project viability. --- 🚩 Exceptions & Edge Cases Super‑hot rock systems – Target crystalline rocks at several km depth; require specialized heat exchangers, not covered by typical binary cycles. Scale formation – In carbonate‑rich reservoirs, cooling induces CaCO₃ precipitation, reducing flow; mitigated by chemical inhibitors or temperature control. Induced seismicity – EGS can trigger earthquakes; real‑time monitoring and pressure adjustments are essential safeguards. Carbon capture variance – Reported CO₂ emissions range from 45 g kWh⁻¹ (average) to 900‑1300 g kWh⁻¹ (some plants), reflecting differing gas handling and measurement methods. --- 📍 When to Use Which Resource temperature ≥ 240 °C → Prefer dry‑steam (if steam is present) or flash‑steam. Temperature 120–240 °C with liquid‑dominated fluid → Flash‑steam (if > 200 °C) or binary‑cycle (if ≤ 200 °C). Temperature 81–120 °C → Binary‑cycle (only viable technology). No natural permeability → Enhanced Geothermal System (high‑pressure fracturing) or Closed‑Loop if fluid handling is undesirable. Need for heating only (no electricity) → Direct-use or ground‑source heat pump. High capital budget, low‑risk tolerance → Dry‑steam / Flash‑steam (simpler) vs Binary‑cycle (higher upfront but broader site applicability). --- 👀 Patterns to Recognize High gradient + shallow depth → Likely hydrothermal → consider dry‑steam or flash. Low gradient but high‑temperature rock → EGS candidate → look for binary‑cycle potential. Presence of CO₂, H₂S, mercury in fluids → Expect gas handling and mitigation (scrubbers, reinjection). Scaling minerals (calcite) → Correlates with cooling of fluid below solubility temperature → anticipate maintenance downtime. Land use vs capacity → Small footprint (≈ 3.5 km² GW⁻¹) → advantageous in land‑constrained regions. --- 🗂️ Exam Traps Confusing “geothermal gradient” with “heat flow” – Gradient is °C/km; heat flow is 44 TW (energy per time). Assuming all geothermal plants emit > 900 g CO₂ kWh⁻¹ – Average is much lower; the higher range applies only to poorly managed sites. Mixing EGS costs with conventional hydrothermal costs – EGS typically > $4 M/MW, while conventional plants can be €2–5 M/MW. Thinking binary‑cycle plants need > 200 °C – They operate down to 81 °C. Believing geothermal always requires volcanic activity – Engineered systems expand viable locations far beyond volcanoes. ---