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2013|14 Annual Report Fraunhofer IGB

120 Emission-free energy conversion by osmosis In energy conversion by means of osmosis two water flows with different salt contents are brought into contact via a semipermeable membrane. The membrane is permeable to water but rejects salt. Water is therefore continuously transferred to the water flow with the high salt content. As a result, a hydrostatic pressure builds up there, which is re- laxed via a turbine, thus enabling the generation of electrical energy. Overall, chemical energy is therefore converted into electrical energy. The principle of this Pressure Retarded Osmosis (PRO) has been known since the 1970s. However, because of the high price of membranes, the low water transfer and a high salt leakage rate of the available membranes, the principle has not so far been implemented on a technical scale. Though there is an immense potential at river estuaries for osmotic power plants producing emission-free electrical energy [1] and in times of climate change and increasing energy prices the inter- est in implementing this potential has grown. In addition, Pressure Retarded Osmosis also offers the possibil- ity of using osmotic power in technical processes that result in water flows with a high salt content. An example is desali- nation of seawater, in which a highly concentrated retentate remains after the reverse osmosis or the thermal desalination. Using this retentate, a PRO process could be carried out with seawater as a low-concentration process stream. Energy could be recovered in this way and the overall process could be made more energy-efficient. The challenge of concentration polarization It is well known that the concentration polarization (CP, Fig. 2) in particular is decisive for the performance of a membrane. Here, a distinction can be made between the internal and the external CP. In the case of the internal CP, there is an increase in the salt concentration in the carrier structure of the mem- brane because of the low salt transport via the separating layer, and thus, there is a reduction of the driving force for the osmotic process. In the case of the external CP, the water and/or salt transport results in a reduction of the concen- tration difference in the surface layers on both sides of the membrane. The structure of the membrane in particular plays a role in the internal CP; in the case of the external CP the process parameters are especially important. The objective – optimized membrane structure and process parameters The objective of the Fraunhofer IGB is to develop new types of osmosis membranes by optimizing the inner structure. We measure the performance of the optimized membranes with an automated test facility (Fig. 1). We also determine the influ- ence of important process parameters such as the flow veloc- ity, the saltwater concentration and the temperature. ENERGY CONVERSION BY OSMOSIS Dipl.-Ing. (FH) Christopher Hänel, Dr. rer. nat. Thomas Schiestel ENERGY 21 concentration difference freshwater to sea water effective concentration difference at the selective layer water salt sea water membrane freshwater concentration selective layer carrier structure

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