New membranes for pressure-retarded osmosis power plants

In an osmosis power plant, two water flows with different salt contents are separated by a semi-permeable membrane. The membrane is permeable for water while salt is rejected, with the effect that water continuously transfers to the high salt content side, causing an increase in pressure. The solution can then be depressurized via a turbine, producing electrical energy.

This principle has been known since the early 1970s [1], but until now has not found commercial application, due to the lack of suitable membranes. The membranes tested to date have been too expensive, their water transfer too low and salt leakage too high. However, against a background of rising energy prices and climate change considerations, the carbon-emission-free generation of energy using osmotic power plants (e.g. 70-MW-power plant [2]) has become highly topical.


Figure 1: Concentration profile in the osmosis process. The decrease of the concentration difference in the membrane and in the hydrodynamic boundary layers is called concentration polarisation. Commercial reverse osmosis membranes with a pronounced supporting layer are not suitable as osmosis membranes because of this concentration polarisation, which reduces the effectively usable concentration difference.

The Fraunhofer IGB is developing a new customized membrane for pressure-retarded osmosis (PRO). A project with Voith Siemens Hydro Power Generation as the industrial partner, which was supported by the Baden-Württemberg Ministry of the Environment, included the designing and testing of membranes, the planning of membrane modules, the calculation of energy fluxes in the power plant and the economic evaluation of such a plant.


Figure 2: Scanning electron microscope image of a commercial reverse osmosis membrane. The upper selective layer is supported by a porous substructure. A concentration polarisation is formed in this support layer.

Commercially available membranes for reverse-osmosis (desalination of sea water) are not suitable for osmosis power plants because of their propensity for concentration polarization (Fig. 1). This phenomenon is mainly observed in the porous carrier structure of such membranes (Fig. 2), it results in a significant reduction of the effective concentration difference (Fig. 3). The best commercially available membranes (forward osmosis membrane) tested so far show an energy yield of 1.3 W/m2.


To improve the performance of membranes for PRO application, the Fraunhofer IGB is currently investigating both commercially available composite membranes and carrier-free hydrophilic mixed-matrix membranes. Surface modification (e.g. by plasma techniques) will be used to minimize concentration polarization and to maximize salt rejection of the membranes.

Project information

Project title

New membranes for pressure-retarded osmosis power plants

Project duration

January 2007 – December 2008


Baden-Württemberg Ministry of the Environment


[1] Loeb, S: Osmotic power plants (1975), Science 189: 654-655

[2] Loeb, S.: Energy production at the Dead Sea by pressure-retarded osmosis: challenge or chimera? (1998) Desalination 120: 247-262