Multistage Reverse Electrodialysis for Energy Production

Abstract

The ever-increasing need for sustainable energy sources creates the necessity to research and develop renewable energies. An example of these is the salinity gradient energy (SGE), where power can be harvested from waters of different salinity. One type of SGE technology is Reverse Electrodialysis (RED), which was tested in this thesis in single and multistage (MSRED) setups in order to improve key parameters like power density and energy efficiency. Simulations using a Phyton model developed at Wetsus showed that a multistage setup with 2 stacks in-series using 155 µm thickness spacers is superior to ones with 480 µm. This is due to greater energy efficiencies and gross power densities. A 3-stacks system was also studied for the 155 µm thick spacers, but the increase in efficiency did not compensate the loss in power density. Experimental systems using stacks of dimensions 10 cm x 10 cm and 22 cm x 22 cm, both in a 2-stacks in-series setup were tested to attain the best combination of both net power density and energy efficiency. This was achieved at 20 seconds of residence time, in a multistage arrangement, with 0.821 W/m2 and 31.7 % for the smaller stacks, while the bigger ones achieved the best result in a single stage at a residence time of 44 seconds, with 0.748 W/m2 and 31.9 %, respectively. Testing the 10 cm x 10 cm stacks with only 3 shielding cation exchange membranes, showed a blank resistance of 0.279 Ω, which likely means that it can be underestimating the parameters. Further studies should be performed using this correction or, preferably, employing more cell pairs, since the different possibilities multistage systems offer compared to single stacks allow for a bigger flexibility and thus an easier optimization of the key parameters, leading to a better process performance.