Energy generation from salinity gradients with reverse electrodialysis - Fouling management and process design
Jordi Moreno Domingo is a PhD student in the research group Membrane, Science & Technology. His supervisor is prof.dr.ir. D.C. Nijmeijer from the Faculty of Science and Technology.
Please note: this PhD Defense takes place at Wetsus, Leeuwarden, Oostergoweg 9
Salinity gradient energy is the energy that can be harvested from mixing two solutions with different salinities. In the Netherlands, this energy is popularly known as ‘Blue Energy’. Reverse electrodialysis (RED) is used to harvest this clean, sustainable and renewable source of energy. A RED stack comprises of an alternating series of cation exchange membranes (CEMs) and anion exchange membranes (AEMs), with seawater and river water in compartments between these membranes. The salinity gradient over each ion exchange membrane creates a voltage difference. The membranes allow the selective transport of cations (CEM) and anions (AEM) from seawater to river water side. At the electrodes, redox reactions are used to convert this ionic transport into electrical transport to power a device.
Fouling on ion exchange membranes and spacers causes a major problem and decreases the performance of the RED stack. In Chapter 2, CO2 saturated water has been used as two-phase flow antifouling strategy against colloidal fouling for the first time, during a period of 60 days using natural feed waters. The CO2 nucleation effect, i.e. the spontaneous formation of bubbles, occurring at the spacer filaments due to depressurization of CO2 saturated water improves the cleaning process. Moreover, the introduction of CO2 saturated water in the feed water introduces a pH decrease in the system (carbonated solution) adding an additional cleaning effect in the system. This antifouling strategy, a combination of mechanical and chemical cleaning-in-place (CIP), avoids the use of environmentally unwanted cleaning chemicals.
Humic acids are the major cause of fouling on AEMs in RED. The effect of AEM fouling on the decrease in total performance in RED is studied in detail using natural river water and seawater (chapter 3). The membrane chemistry and the water content of the membranes are key parameters defining fouling due to humic acids. Nevertheless, the largest decrease in power density does not originate from AEM fouling, as this only counts for 2 - 4% of the total loss. The major cause is the occurrence of fouling on the spacers that keep the intermembrane distance in the fluid compartments, which disturbs the flow distribution (chapter 2)
The presence of multivalent ions (i.e Mg2+) and their associated uphill transport, induces an increase in CEM resistance, which significantly reduces the power density output obtainable in RED stacks, when using natural feed waters. The application of multivalent ion permeable membranes, with a more ‘open’ structure allowing the free movement of both sodium and magnesium ions through the membrane, is proven to be the best long-term strategy to harvest salinity gradient energy when using RED, especially at high magnesium concentrations (chapter 4).
The Breathing cell (chapter 5) is a new design that operates a RED stack by varying the intermembrane distance in time. By compressing the river water compartment, the internal resistance of the stack decreases and higher net power outputs can be achieved, despite the momentarily increase in pressure drop along the compartment. This design introduces a paradigm change from a static stack to a dynamic stack. The breathing cell offers the possibility to adapt the operation (high or low flow rate or breathing frequency) to the water parameters and stack performance characteristics, e.g. fouling.
Scaling-up of reverse electrodialysis is proven to be possible and beneficial to increase the net energy efficiency of RED stacks. The influence of stack size on the power density, energy efficiency and pumping power density can be directly related to the residence time of the feed water in the stack. Membrane characteristics, such as water permeability and permselectivity are key parameters when measuring the thermodynamic efficiency. Chapter 6 demonstrates that to improve the energy efficiency of the stacks, improved membrane characteristics are a key parameter towards achieving the final crucial steps towards commercialization.
The last chapter of this thesis, chapter 7, gives the main conclusions of this work and indicates further research directions. Two directions are proposed: First, RED membrane development is discussed wherein new ideas are proposed to increase the power density outputs. Second, an approach for RED multi-stage is envisaged to increase the energy efficiency of the process with a close look towards fouling development.
