Salinity Gradient Power (Pressure-Retarded Osmosis)

Feed & Draw Conditions

Seawater (draw) salinity 0.60 mol/L

Hydraulic backpressure ΔP 8.0 bar

Temperature 15 °C

Membrane permeability A 1.5 L/m²h/bar

Live Readouts

Regime: —

Osmotic pressure Δπ

0.0 bar

Water flux Jw

0.0 L/m²h

Power density

0.0 W/m²

How it works

Pressure-retarded osmosis (PRO) sandwiches a semi-permeable membrane between fresh river water and pressurized seawater. Fresh water crosses the membrane toward the saltier side by osmosis, and the resulting volume increase spins a turbine — converting the chemical potential of mixing into mechanical work before the streams ever actually mix.

The driving force is the van't Hoff osmotic pressure difference between the two solutions:

Δπ = i · M · R · T
  i = 2   (NaCl dissociates into Na⁺ + Cl⁻)
  M      = draw-side molar salt concentration (mol/L)
  R      = 0.08314 L·bar/(mol·K)
  T      = absolute temperature (K)

Net water flux through the membrane follows the solution-diffusion model, opposed by the applied hydraulic backpressure ΔP:

Jw = A · (Δπ − ΔP)     [L/(m²·h)]
Power density = ΔP · Jw   (converted to W/m²)

ΔP < Δπ — PRO regime: net flow moves into the pressurized draw chamber, turning the turbine and generating power. Power density is maximised near ΔP = Δπ/2.
ΔP = Δπ — no net flow; the membrane is at osmotic equilibrium.
ΔP > Δπ — the applied pressure overwhelms osmosis and the flow reverses (reverse-osmosis regime): the plant now consumes power to push water back across the membrane instead of generating it.

Real-world relevance: this is the mechanism behind salinity-gradient ("blue energy") power plants proposed at river mouths, where the Gibbs free energy of mixing fresh and salt water — theoretically about 1.4 MJ per cubic metre of fresh water — is harvested instead of being lost to uncontrolled mixing.

Fresh water (feed) Dissolved salt ions Membrane / turbine

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