Flow cell and stack design
The principle of capacitive mixing has been so far explained with two single particles that are sequentially charged and discharged. Of course a single particle has a limited capacity to store charge and thus can generate only a tiny current. To use this principle as a technology with a high performance one can use porous electrodes. Porous electrodes can be considered as a huge number of particles that are clustered together. Between the particles there is a void space in which water is present from which the ions are absorbed or into which the ions are desorbed. Using this type of electrodes several designs can be envisioned that translate the principle of capacitive mixing into a power generating device. A flow cell-based design has been used to give a proof-of-principle for both processes and will therefore be the starting point for the technology development,.
Figure A shows a sketch of a flow cell using the DcCM principle. The main elements of the cell are the water channel in the middle, through which sequentially sea and fresh water is pumped. Next to the channel is a membrane which is either an anion-exchange membrane (AEM) or a cation-exchange membrane (CEM). The next layer is the porous carbon electrode that stores the ions and electrons. This carbon electrode is connected to a current collector to transfer electrons to the external circuit. Figure A illustrates the charging step with sea water in the water channel. The anions can only pass the AEM and the cations can only pass the CEM and in this way charge the porous electrode. When in the next phase the water channel is filled with fresh water the ions will flow in the reverse direction. A similar design can be used for the EcCM principle, with the only difference being that the membranes can be left out and that an external power supply is needed.
(A): A flow cell used to generate electricity from sequentially exposing the cell to sea
and fresh water. The principal elements are the current collector, the porous carbon electrode and the membrane, either AEM or CEM. (B) a bipolar stack of DcCM cells.
To generate a higher potential several cells can be stacked, as illustrated in figure B. This illustration indicates already a number of additional elements that have to be studied: the way of stacking, the distribution of the water and the connection of the cell to an external load that uses the electricity or supplies the power in case of the charging step of the EcCM. Also the operation of the cell in terms of the duration of each phase (frequency of switching) is of importance to consider because too high frequencies lead to losses due to mixing, while for too low frequencies the electrodes reach their adsorption capacity and the process becomes inefficient. A further important point of attention is the necessity of water pre-treatment. As the water channel of the cell is very small, removal of particles from the water will be necessary to prevent clogging of the channels and fouling of the surfaces. However there first needs to be cell design before the requirements of water treatment can be defined.
Also attention must be paid to the hydraulics of the system, the energy needed for pumping and pretreatment should be limited. Other design concepts will be considered during the project whenever the growing insight into the processes shows this to be attractive. One option is to step away from the flow cell design where fresh water and saline water are intermittently fed to the same cell, but instead to remove and dip the electrode-membrane assemblies (which are overall charge-neutral) alternatingly in continuous water streams of low and high salinity.