Geothermal Flash Plant

What is modelled

Brine in a hot reservoir is liquid because it is under pressure. When it is throttled to a lower pressure in a separator part of it flashes to steam at constant enthalpy. A low separator pressure makes a lot of steam but each kilogram of low-pressure steam can do little work; a high separator pressure makes little steam of high quality. The net power is the product of the two, so it has an optimum.

Equations

• Isenthalpic flash x = (hf,res − hf,s) / hfg,s, steam flow x m.
• Specific work w = η (κ/(κ−1)) R Ts [1 − (Pc/Ps)(κ−1)/κ] with κ = 1.3.
• Net power (1 − parasitic) x m w.
• Saturation data interpolated from steam tables, pressure linear in ln P against 1/T.

Checks run in the page

• The enthalpy of the separator outflow equal to that of the well liquid.
• A golden-section optimum against a brute-force scan.
• The slope of ln P against T compared with the Clausius-Clapeyron estimate.
• The two limits of the power curve.

Honest simplifications

This is a single-flash cycle with no non-condensable gases, no silica scaling limit on the brine temperature, no wetness loss in the turbine and a constant turbine efficiency. The steam is treated as an ideal gas with a fixed exponent for the expansion work, and the parasitic load is a flat fraction. A real plant optimises with much more data.

• Press Pressure too high and Pressure too low, then compare them with the dashed best pressure.
• Press Moderate reservoir: the best pressure drops with the reservoir temperature.
• Raise the condenser pressure and watch the optimum move up.