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Section 1.2 Fundamentals Also, most companies only buy small quantities of the required materials from their suppliers. Hence, the stack-related costs are rather high. In [8], stack specific costs of more than 8,000 €(kW)-1 are calculated. The costs for the pumps, the piping and the power conditioning system (PCS) further increase the power-related costs. The PCS for a flow battery is usually more complex than for other types of batteries. Reasons are the low cell voltage, the limitation in the series connection of the cells and large voltage variations with state-of-charge (SoC) and charging/discharging currents. Finally, flow batteries suffer from a lower efficiency than lithium-based batteries. A comparatively high internal resistance, which also limits the power density, and additional loss mechanisms, such as the pump power demand, the crossover of vanadium ions across the membrane, as well as the shunt currents, reduce the VRFB efficiency. 1.2.3 Composition of a flow cell In a flow cell, the electrode should provide a large contact area with the electrolyte. Therefore, porous graphite felts are predominantly used as electrodes. Between both electrodes, the ion exchange membrane is placed. As a single flow cell usually provides a low voltage of less than 2 V, a larger number of cells is connected in series electrically, as shown in Figure 1-2. This is efficiently realized by using so-called bipolar plates, made of a composite graphite material. These plates provide the planar electric connection of the electrode and simultaneously seal each positive half-cell from the successive negative half-cell. On top of the first and last bipolar plates, current collectors made from copper are placed. In the conventional stack design, massive aluminum endplates are used combination with threaded bolts, nuts and strong springs to clamp the stack. This is to obtain a high compact pressure all over the flow cells. The compact pressure reduces the contact resistances and improves the sealing. Finally, to electrically isolate the endplates from the stack, a plastic layer is placed between the current collector and the endplate. Note that the current collectors, the isolating plastic layer, as well as gaskets and bolts, used for the clamping, are not displayed in Figure 1-2. The so-called frame, usually made from plastics using injection molding, provides the outer support. The frame also includes the inlet and outlet channels, which are required to partly decouple the electrolyte supply paths of the flow cells from each other. Internally, the positive and the negative electrolytes are supplied and re-collected via four manifolds, commonly used by all flow cells of the stack. 1.2.4 Components of a flow battery system A flow battery comprises two main units. The electrochemical active energy conversion unit, which is an arbitrary number of flow cells, and at least one pair of tanks, as shown in Figure 1-3. Figure 1-4 shows a universal flow battery design, in which six 30-cell stacks are connected to one pair of tanks with a volume of 8,000 L each. The system, rated 54 kW/216 kWh, is used in a model-based study [9]. 7PDF Image | Model-based Design Vanadium Redox Flow Batteries
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