Binary Geothermal Plant: Evaporator Pinch Point

Working Fluid

Plant Controls

Brine inlet temperature 150 °C

Evaporating temperature 110 °C

Working-fluid flow rate 5.0 kg/s

Cycle Readouts

Net power output

— kW

Cycle efficiency

— %

Evaporator pinch ΔT

— °C

Heat duty Q

— kW

Brine outlet temperature

— °C

Subcooled liquid Boiling / saturated vapor Hot geothermal brine

How it works

Below about 180 °C, geothermal brine is usually too cool to flash into steam efficiently, so a binary (Organic Rankine Cycle) plant instead boils a low-boiling-point working fluid in a heat exchanger, spins a turbine with that fluid's vapor, and the brine is reinjected — never touching the turbine.

Evaporator duty:  Q = ṁ_f·[cp_liq·(T_evap−T_cond) + L]
Brine energy bal: T_brine,out = T_hot,in − Q /(ṁ_b·cp_b)
Cycle efficiency: η = 0.55 · (1 − (T_cond+273)/(T_evap+273))
Net power:        P = η·Q − 0.03·Q  (pump work)

The subtle part is the pinch point. Hot brine cools smoothly along the exchanger, but the working fluid first heats as a liquid (steep temperature rise) and then boils at constant temperature (flat). That "kink" usually creates the tightest brine–fluid temperature gap not at either end of the exchanger, but exactly where boiling begins:

T_brine at boiling start = T_hot,in − ṁ_f·L /(ṁ_b·cp_b)
Pinch ΔT = T_brine(boiling start) − T_evap

Real evaporators are designed to keep this pinch above roughly 3–8 °C. Push the fluid flow rate or evaporating temperature too high and the pinch collapses toward zero — physically that means the heat exchanger would need infinite surface area to move that much heat across a vanishing temperature gap, which is what the warning flags. This exact trade-off (more flow → more power, but a tighter, costlier pinch) is the central sizing decision engineers make for every binary geothermal and waste-heat ORC plant.

Liquid Vapor Brine

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