Pressure-retarded osmosis

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, 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 has become highly topical.

Membrane development

Commercially available membranes for reverse-osmosis (desalination of sea water) are not suitable for osmosis power plants because of their propensity for concentration polarization. This phenomenon is mainly observed in the porous carrier structure of such membranes, it results in a significant reduction of the effective concentration difference. The best commercially available membranes (forward osmosis membrane) tested so far show an energy yield of 1.3 W/m2. We are developing a new customized membrane for pressure-retarded osmosis (PRO) both based on cellulose acetate and thin film composite membranes.

Concentration polarization in an osmosis membrane.
© Fraunhofer IGB
Concentration polarization in an osmosis membrane.
Simulation of PRO performance as a function of membrane geometry.
© Fraunhofer IGB
Simulation of PRO performance as a function of membrane geometry.
Power density as a function of flow velocity of a cellulose acetate membrane developed at IGB.
© Fraunhofer IGB
Power density as a function of flow velocity of a cellulose acetate membrane developed at IGB.

Outlook

The membrane developed for PRO will be further optimized for forward osmosis applications. In an ongoing project we use such membranes to enrich aqueous biobutanol solutions up to phase separation concentrations.

Publications

  • Touati, K., et al. (2015). "Effect of the feed and draw solution temperatures on PRO performance: Theoretical and experimental study." Desalination 365: 182-195.
  • Touati, K., et al. (2014). "Impact of Temperature on Power Recovery in Osmotic Power Production by Pressure Retarded Osmosis." Energy Procedia 50: 960-969.
  • Touati, K. and T. Schiestel (2013). "Evaluation of the Potential of Osmotic Energy as Renewable Energy Source in Realistic Conditions." Energy Procedia 42: 261-269.

Reference projects

H2Ocean – Development of a wind-wave power open-sea platform

 

Development of an offshore platform which will harvest wind and wave power, using part of the energy on-site for multiple applications – including a multi-trophic aquaculture farm, and convert on-site the excess energy into hydrogen that can be stored and shipped to shore as green energy carrier.

 

Duration: January 2012 – December 2014