Energy generation from mixing salt water and fresh water: smart flow strategies for reverse electrodialysis

摘要

Renewable energy can be generated from mixing salt water and fresh water, e.g., seawater and river water. This energy is captured in reverse electrodialysis (RED), using ion exchange membranes that are selective for positive or negative ions. This PhD thesis evaluates the current limitations and future opportunities in reverse electrodialysis and investigates the use of novel flow strategies. A gross power density of 2.2 Watt per m2 of membrane is obtained using optimized distance between the ion exchange membrane, which is, to our best knowledge, the highest experimentally obtained value for RED at this scale. However, a major part of this power is lost in conventional RED systems, where spacers are used in each feed water compartment. The use of membranes with integrated spacer functionality (profiled membranes) reduces the pumping power with an order of magnitude, and hence also yields a high net power density. Additional mixing promoters, such as micro-corrugations at the membrane surface, do not enhance the obtained power density significantly, because the effect of the concentration boundary layers is negligible when the water is uniformly distributed over the feed water compartments. Such uniform flow distribution, or prevention of preferential channelling, can be monitored using the response time in a chronopotentiometric series. In practical cases, when using natural seawater and river water, fouling of the membrane stacks occur. The observed fouling is dominated by diatom remnants and clay minerals and contain a minor fraction of biofouling and scaling. These colloids can be removed effectively from stacks with profiled membranes using air sparging. Additionally, the presence of multivalent ions and possibly organic substances in natural feed water decrease the power density. Membranes that are selective for monovalent ions only (such as Na+ and Cl-) would improve the power density in practical applications. When the improvements as proposed in this research are successfully applied at large scale, energy generation from seawater and river water is competitive with other renewable energy sources.