Pilot testing and scaleup2

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Wetsus - Center for Sustainable Water Technology in the Netherlands-started with the "Blue Energy'' project in 2005 with a focus on RED. At that time, only a few scientific papers were published [4,28,31] about

2The information given in this paragraph section is provided by and property of the company REDstack B.V., The the Netherlands, and is used with permission. The authors would like to acknowledge REDstack B.V. for the contribution.

results of the RED system in a period of 50 years. Over the past few years, the performance of RED on laboratory scale has improved considerably. However, thus far, RED experiments have typically been performed on a small scale, varying from current-passing areas of just a few square centimeters [32] to hundreds of square centimeters [36] and from four cell-pairs [33] to fifty cell-pairs [38]. State-of-the-art is a stack with an active membrane area of 25 x 75 cm2 and 50 cell-pairs with a power output of about 16W (Fig. 14; drawing prepared by REDstack B.V., the Netherlands, and belongs to the company; used with permission).

To achieve practical implementation, RED still needs to be scaled up by several orders of magnitude. This upscaling and practical implementation is beyond the academic expertise and needs to be done in close cooperation with industry. For this reason, REDstack B.V. was founded by Magneto Special Anodes B.V., the Netherlands and Landustrie/Hubert, two industrial companies participating within the Blue Energy research of Wetsus. The challenges still faced by REDstack B.V., concerning the economics, technological feasibility, and the developing path of RED, are the development of low-cost membranes, the pretreatment in relation to stack design and operation, and the upscaling.

Figure 14 Reverse electrodialysis stack for research purposes with a total membrane area of 25 x 75 cm2 and 50 cell-pairs as manufactured by REDstack B.V., the Netherlands.

Although the technical requirements are already met by currently available membranes, the cost prices are out of range to make RED affordable. According to Turek and Bandura [32], it is hard to believe that the price of low-resistance ion exchange membranes may be reduced 100 times, which seems to be the desired cost level [32]. Nevertheless, for several reasons, REDstack B.V. is more optimistic that membrane prices for (reverse) electrodialysis can be reduced tremendously [31]. This is because of the fact that electrodialysis membranes have never had a considerable market share. Even then, on the global market, heterogeneous ion exchange membranes can be found with very low cost prices (<5 US$/m2). Of course, low-resistance ion exchange membranes command higher prices of 100 US$/m2 or more [32], but these prices can also be expected to fall, as manufacturing techniques improve, and the range of applications expands. Market research for related membrane applications show unit prices of installed membranes falling by an order of magnitude in 10 years, and this made Sutherland [54] to predict that the 1 US$/m of installed membrane is not far off. Second, it should be noticed that - apart from different technical requirements — the current membrane market would never be able to match the demand of required membrane area for power production. This implicates that besides the expertise in manufacturing of membranes the expertise of bulk production is also needed. While at the start of the membrane development, REDstack B.V. was dedicated to the technical requirements (as described previously) and cost prices of base materials, nowadays REDstack B.V. focuses on the scalability of the production process with focus on labor-extensive reel-to-reel production lines operating at high speeds.

Although addressed in scientific papers, challenges often not considered are the pretreatment of river water and seawater [34] and the hydrodynamic aspects of RED [55]. The required water quality parameters are still unknown. It is not likely to look at experiences in desalination stacks because the usually applied pretreatment steps [56] would be too capital-intensive to be viable for RED. Nevertheless, RED would require an extensive pretreatment as the distance between the membranes is less than in conventional flat-sheet membrane systems. It requires a more robust system design using the developed CFD model for flat-sheet membrane configurations [55]. Besides the cost aspect, also the footprint, energy consumption, and use of chemicals should be taken into account regarding the feasibility of RED.

The promising results raised the interest of different industrial and power supply companies and water authorities to invest in pilot tests. At this

Figure 15 Artist impression of a salinity-gradient power plant at the IJsselmeer (by Rijkswaterstaat); inset top right: reverse electrodialysis pilot in Harlingen; inset bottom right: pretreatment pilot (REDstack B.V., The Netherlands).

stage of the project, focus is on consortium building, with customers entering into technical development agreements with suppliers, joint designs, and test programs. Parties agreed on the following development path for scale-up of the system (Fig. 15):

• Industrial pilot (kW-scale) on saline flows in a salt factory (financially supported by SenterNovem, Innowator project; 2008-2010).

• Feasibility study and definition of requirements for a communal power plant of 200 MW at the Afsluitdijk, The Netherlands (private funding, 2008).

• Communal pilot (10-40 kW) on seawater and river water (2009-2010) at the Afsluitdijk, The Netherlands.

• Communal demonstration plant (1 MW) on seawater and river water (2010-2012) at the Afsluitdijk, The Netherlands.

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