| name | geothermal-systems |
| description | Geothermal energy systems — flash/dry steam/binary cycle, ground source heat pumps (GSHP), geothermal gradient, borehole heat exchangers, COP, resource assessment, IGA standards. |
| metadata | {"priority":7,"promptSignals":{"phrases":["geothermal energy","geothermal system","ground source heat pump","GSHP","flash steam geothermal","binary cycle geothermal","borehole heat exchanger"],"minScore":3}} |
Geothermal Systems — Complete Skill
Geothermal Resource Types
| Resource | Temperature | Type | Application |
|---|
| High-enthalpy | > 150°C | Hydrothermal | Electricity (flash/dry steam) |
| Medium-enthalpy | 100–150°C | Hydrothermal/EGS | Electricity (binary ORC) |
| Low-enthalpy | 50–100°C | Hydrothermal | Direct use (heating, spa) |
| Very low | 10–30°C | Ground-source | GSHP |
Geothermal gradient: typical 25–30°C/km; volcanic areas 80–150°C/km
Bottom hole temperature (BHT): T_BHT = T_surface + gradient × depth
High-Enthalpy Electricity Generation
Dry Steam Plants (Larderello, The Geysers)
Resource: pure steam at wellhead (rare); T = 150–250°C; P = 5–8 bar
Direct to steam turbine; no separator needed
Efficiency: η_th ≈ 20–25%; T_cool = ambient; T_hot = steam temperature
Single-Flash Steam Plants
Most common; liquid-dominated reservoir
- Wellhead fluid: mixed water + steam; P_wellhead = 20–30 bar, T = 200–250°C
- Throttle to P_sep = 5–10 bar → partial flashing → steam separated
- Steam → turbine; brine → reinjection
Steam fraction (Rankine flash):
x = (h_f,wellhead - h_f,sep) / h_fg,sep
Plant efficiency:
η = W_net / (ṁ_total × h_wellhead - h_reinjection)
Typical: 12–17%
Double-Flash Plants
Second flash of separated brine at lower pressure → additional steam for turbine
Increases generation 15–25% vs. single-flash for same resource
Binary Cycle (Organic Rankine Cycle / ORC)
Used for lower-temperature resources (100–150°C)
Working fluid: isopentane, isobutane, R134a, ammonia
Heat exchanger: geothermal brine → evaporate ORC fluid → turbine → condenser → pump
ORC efficiency:
η_ORC = W_turbine / Q_geothermal = (h₃ - h₄)/(h₃ - h₁) × η_pump_turbine
Typical η_ORC = 10–15%; lower absolute efficiency but uses lower-temperature resource
Kalina cycle variant: ammonia-water mixture; variable boiling T → better match to geothermal brine temperature profile
Enhanced Geothermal Systems (EGS)
Create permeability in hot dry rock (HDR) by hydraulic fracturing
Inject cold water → flows through fractures → heats up → produce hot water
T_rock > 200°C at 4–6 km depth; granite or basement rock
Heat extraction rate:
Q = ṁ_fluid × C_p × (T_out - T_in)
Typical: ṁ = 50–100 kg/s; ΔT = 20–30°C; Q = 4–12 MW_thermal per well doublet
Challenges: induced seismicity (M < 3 target); water loss; scaling; corrosion
Ground Source Heat Pumps (GSHP)
Heat Pump Coefficient of Performance
Heating COP:
COP_H = Q_H / W_electrical = Q_H / (Q_H - Q_L)
For Carnot (ideal): COP_H,max = T_H / (T_H - T_L) [temperatures in K]
Real GSHP: COP_H = 3.5–5.0 (vs. COP_H = 1.0 for electric resistance)
Cooling COP (EER):
COP_C = Q_L / W = Q_L / (Q_H - Q_L) = COP_H - 1
Annual efficiency:
HSPF (Heating Season Performance Factor) [BTU/Wh]; SEER (cooling); COP varies with ground T
Borehole Heat Exchangers (BHE)
Vertical loops in 100–200 m boreholes; U-tube or coaxial pipe
Grout: thermally enhanced bentonite; k = 1.5–2.5 W/mK
Ground thermal conductivity test (TRT): required for > 25 kW systems; measure k_ground from temperature response
Borehole thermal resistance:
R_b = 1/(2πk_grout) × [ln(r_b/r_pipe) + σ] + R_pipe_wall + R_fluid
Superposition for borehole array:
Line source model (Kelvin): ΔT = Q/(4πk_ground) × Ei(-r²/(4αt))
α = k/(ρc_p) = thermal diffusivity [m²/s]
Required borehole length:
L_BHE = Q / (UPW × ΔT_design)
UPW (unit power per unit length) ≈ 50–70 W/m for typical European ground
ASHRAE Borehole Length Formula
L = (q_a × R_ga + q_max × (R_b + R_gs × F_sc)) / (T_g - (T_w,lm + Δt_p))
q_a = average annual ground load [W]; q_max = design peak load [W]
R_ga = effective ground resistance for annual pulse; R_gs = ground resistance for short pulse
F_sc = short-circuit factor; T_g = undisturbed ground temperature; T_w,lm = min/max fluid T
Horizontal Ground Loop
Horizontal trenches at 1.5–2 m depth; cheaper installation
T_ground varies seasonally → worse performance vs. vertical
Needs large land area (2–3× floor area of building)
Design: 100–200 W/m in moderate climate; pipe spacing 0.6–1.2 m
Direct Use Applications
District heating; agriculture greenhouse; fish farming; industrial process heat (food processing, wood drying)
Lindal diagram: temperature-application matching:
< 50°C: space heating, swimming pool, soil warming
50–80°C: greenhouse heating, timber drying, industrial process
80–120°C: fish drying, refrigeration (absorption), pasteurization
120–150°C: evaporation, distillation, drying
Geothermal Well Design
Drilling costs: $1,000–3,000/m for geothermal wells
Casing program: surface, intermediate, production/injection strings
Completion: production well 7–9" casing; injection 5–7"
Well productivity index (PI):
PI = Q / (P_static - P_wellhead) = ṁ / ΔP_drawdown [kg/s/bar]
Typical: PI = 5–30 kg/s/bar for good geothermal wells
Reservoir modeling: numerical simulation (TOUGH2, CMG, FEFLOW); pressure/temperature history matching
Scaling and Corrosion
Silica scaling: SiO₂ precipitates as brine cools below solubility limit
→ reinjection before silica supersaturation; or silica inhibitors
Calcite scaling (CaCO₃): from CO₂ degassing in flash plants
→ CO₂ pressure maintained; scale inhibitors; mechanical cleaning
H₂S: toxic; corrosive; address with H₂S abatement scrubber
Corrosion: low-pH condensate attacks steel; use stainless or coatings
Output
Provide: resource temperature and depth [°C, m], plant type (dry steam/single-flash/double-flash/binary ORC), generated power [MW_e] and efficiency [%], GSHP COP_H and COP_C, required borehole length [m] and number of boreholes, ground thermal conductivity k [W/mK], UPW [W/m], seasonal energy savings vs. conventional HVAC [%], applicable standard (ASHRAE 90.1 for GSHP; IGA International Geothermal Association guidelines), CO₂ intensity [gCO₂/kWh] vs. grid.
