geothermal-systems

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

1. Wellhead fluid: mixed water + steam; P_wellhead = 20–30 bar, T = 200–250°C
2. Throttle to P_sep = 5–10 bar → partial flashing → steam separated
3. 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.