Harvesting blue energy with the breathing cell
Membranes that appear to “breathe” by expanding and compressing like a pair of lungs could better harvest electricity through reverse electrodialysis (RED) stacks, increasing the net power production and bringing blue energy technology a step closer to commercialization.
University of Twente PhD student Jordi Moreno and his Wetsus colleagues are pioneering the breathing cell concept at REDstack—Wetsus’ spin-off company, which is working to harvest the energy available from the salinity difference between seawater and river water.
In the first experimental tests of the breathing cell, the researchers demonstrated that a breathing stack achieves a maximum net power density of 1.3 W/m2 over a broad range of flow rates. This value is close to the maximum net power density ever reported, 1.5 W/m2, which was obtained only for a very narrow flow rate range.
The breathing cell’s potential improvements arise from the fact that it addresses one of the major bottlenecks of current RED stacks, which is that river water (which has a low conductivity) is a major contributor to the overall resistance of the stack. This can be reduced by decreasing the river water channel thickness, but this is at the cost of a higher hydraulic loss.
A conventional RED stack consists of alternating channels of river water and seawater, with each channel separated by alternating cation and anion exchange membranes that allow for the passage of either positive or negative charges, respectively. As the river water and seawater flow through the channels, the chemical potential difference between them generates a voltage over the membranes.
Principle of the breathing cell: In stage 1, both seawater and river water flow through compartments of equal widths. In stage 2, the seawater outlet valves are closed, leading to a pressure build-up in the compartment. In stage 3, the pressure build-up leads to the expansion of the seawater compartment. Because of this expansion, the river water compartment is compressed, leading to a smaller intermembrane distance. In stage 4, the valves open and eventually the pressure decreases and the compartments return to their initial positions.
In current versions of the technology, the membranes are kept at a fixed distance by placing spacers between the membranes. In the breathing cell concept, by contrast, the membrane distance is not fixed by the spacer. The researchers replaced the spacer in the river water compartments by a spacer with an unequal thickness, thicker at the inlet and thinner at the working part of the stack. And by using pressure, the researchers can control the intermembrane distance.
When both river water and seawater flow through the compartments, the membranes are spaced equally as before, with an intermembrane distance of 480 µm. But when the seawater outlet is closed, pressure builds up in the seawater compartments, causing the flexible membranes to expand outward and compress the river water compartments down to a width of 120 µm. The key consequence is that the narrower river water compartments generate less resistance, resulting in a higher power density. (The wider seawater compartment is not a problem since seawater contributes very little to resistance due to its high salinity.) Afterwards, the seawater outlet is opened again, releasing the pressure and expanding the river water compartment again allowing the river water to be refreshed with a lower hydraulic loss. This opening and closing of the seawater outlet is happening with a frequency in the order of seconds.
“The breathing cell is a game changer for RED because it is the first time that we move the membranes inside the stacks,” Moreno said. “We change from using fixed configurations toward dynamic configurations. And it is not only for RED, it is also a game changer for ED stacks. The movement of the membranes give us the freedom to adapt the movement frequency to the fouling conditions of the feed waters. So it can harvest more energy at different flow regimes, and is also a perfect antifouling strategy.”
The breathing cell concept is part of Moreno’s PhD thesis, which is titled “Fouling management and new stack design for RED.” The rest of the thesis is dedicated to fouling—specifically, understanding the interactions of the different foulants with the membranes and studying new antifouling strategies.
“One new antifouling strategy that we studied was the use of CO2 saturated water as a two-phase flow technique,” Moreno said. “What does this mean? That we are using the same effect of opening a bottle of champagne or Coca-Cola to clean the particles deposited in the membranes. We do this by introducing our CO2-saturated water inside the stack and, due to a loss of pressure, the CO2 dissolved in water nucleates, creating really small bubbles and also lowering the pH inside. This method is more efficient, in terms of cleaning, than the current one using air sparging.”
Moreno plans to finish his PhD by the end of the year, and in the meantime, he has started a part-time job at REDstack. Reflecting on the past few years, Moreno explained that his experience at Wetsus has proved to be better than he ever could have expected, both professionally and personally.
“Wetsus gave me the opportunity, besides becoming a better researcher, to do a lot of training about personal development,” Moreno said. “Through these experiences, I learned how to better handle everyday situations and be less stressed and more productive. I am also part of the Blue Energy team, a team very well involved in the project that always gives a lot of freedom for creativity and listens to the opinions of the PhD students. And last but not least, my thesis project is led by Prof. Kitty Nijmeijer. I always enjoy our conversations about the project, where I not only learn about science and research, but also about personal development.”
Jordi Moreno shows his experiments to King Willem-Alexander during the inauguration of REDstack’s Blue Energy Pilot Plant at Afsluitdijk in November 2014.
J. Moreno, E. Slouwerhof, D. A. Vermaas, M. Saakes, and K. Nijmeijer. “The Breathing Cell: Cyclic Intermembrane Distance Variation in Reverse Electrodialysis.” Environ. Sci. Technol., 2016, 50 (20), pp 11386–11393