In RED, the energy of mixing two solutions with different salinity is extracted through the transport of ions (this in contrast to PRO, where the transport of water accounts for the generation of power). Pattle was the first researcher who proved the principle of RED [4]. With his pioneering work, he was the first one to be able to generate power from the mixing of fresh and saltwater through the selective transport of ions. In the 1970s, Weinstein and Leitz [28] investigated the effect of the composition of the salt solutions on the power output. The main conclusion of their work was that large-scale application of RED could become feasible, but only if major improvements regarding the manufacturing of ion exchange membranes and careful optimization of the operating conditions are possible. In the early 1980s, Lacey [29] prepared a comprehensive review on RED and concluded that to make RED economically viable minimization of the internal stack resistance and maximization of the net power output from the cell are a prerequisite for success. The main conclusion of Lacey's work is that membranes for RED should have a low electrical resistance and a high selectivity combined with a long service life time, acceptable strength, dimensional stability, and low costs. In the early 1980s, Audinos [30] compared two different types of electrodialysis membranes for their applicability in RED (one pair of homogeneous and one pair of heterogeneous membranes) and investigated the effect of the type of salt solution (NaCl vs. ZnSO4). The maximum power output obtained was 400MW/m2. In the mid 1980s, Jagur-Grodzinski [31] investigated the effect of hydrodynamics, that is, different salt solution streams and membrane spacer modifications, as a method to increase the power output. Although promising, the number of papers on RED in the 1990s and in the beginning of the 21st century was very limited. However, since a few years RED has been recognized again as a potentially attractive technology for the production of sustainable energy and as such it has regained the interest of many researchers [32-39], industrial partners, and the public. In this part, we first discuss the principle and the fundamentals of RED. It continues with a closer look at the membranes used for RED.

After that we focus on the different elements in RED, which are subsequently the membranes and the feed compartments including spacers. Although electrodes and electrode reactions are also major elements in a RED stack, the available literature and research on this topic is very limited, and therefore this topic will not be addressed here. This part is followed by a paragraph that focuses on process and stack design. This chapter finally ends with a description of the state-of-the-art and current status of RED and also gives a glimpse on pilot testing and upscaling.

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