The role of membranes in the use of natural salinity gradients for reverse electrodialysis

In this thesis, harvesting of natural salinity gradients with reverse electrodialysis and the role of membranes is studied.
There is an urgent need to limit our CO2 emissions to keep global warming within acceptable levels. To achieve this, the use of renewable energies needs to show a major increase. One of these renewable energies is reverse electrodialysis (RED), where energy is harvested from a salinity gradient. For RED, two streams with a difference in salinity are used in combination with ion exchange membranes. These charge-selective membranes allow either the transport of cations (for cation exchange membranes, CEMs) or anions (for anion exchange membranes, AEMs). Using these membranes, the salinity gradient can be used to facilitate directional charge transport that can be converted into electrical energy. Anticipated feed streams for the salinity gradient can be natural waters, for example, river and seawater. However, the contents of these waters pose challenges to the process. Divalent ions, such as Mg2+ and Ca2+, have several negative effects (which will be discussed in Chapter 2 – 4). Moreover, dissolved organic matter such as humic acids, is expected to foul AEMs (Chapter 6). Furthermore, the use of spacers in these stacks has drawbacks (such as partially blocking the membrane surface for ion transport) and can be replaced by using profiled membranes. Their design is discussed in Chapter 5.