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Section 6.3 Evaluation methodology is identical. For the 1000-cm2, 2000-cm2, 3000-cm2 and 4000-cm2 cell, the tank volumes are 250 L, 500 L, 750 L and 1,000 L each, respectively. As the system only exists as a model, there is no exact piping plan available. Orifices and pipe lengths of the estimated external hydraulic circuit are the same for all designs. Per half-side, a total pipe length of 6 m is assumed. Usually, piping is mostly carried out using plastic pipes. This is primarily the case for connecting tanks and pumps. The connection of the stack is usually carried out using flexible tubes. However, pipes and tubes are identically modelled in this work. In single stack test systems, all pipes and tubes have the same diameter. This diameter is equal to stack manifold diameter. Therefore, no expansion or contraction, except for tank inlet and outlet, needs to be considered. In terms of orifices, a total loss coefficient of 5.82 per half-side is assumed. This results from eight 90°-bends (kL=0.3 each), tank inlet (kL=1) and tank outlet (kL=0.42). An additional loss coefficient of two is added per half-side to account for connection resistances and sensors (e.g., temperature and/or flow rate). The pumps are assumed to have nominal capacities that comply with the flow rates according to Table 5-2 on page 80. Further, they have a lower limit of 10 % of nominal capacity, which cannot be undershoot as long as the pump runs. The flow rate dependent pump efficiency is shown in Figure 2-21 on page 55. 6.3 Evaluation methodology 6.3.1 Methodology In the following, the terms round-trip-system-efficiency (RTSE) is used as a synonym for the system efficiency, ηSys. RTSE and discharging capacity of every design are determined using constant current cycles with four different current densities. The cycles are bounded by a combination of voltage and SoC limits. The charging process is finished when a cell voltage of 1.65 V or a tank SoC of 90 % is reached. The discharging process is finished when a cell voltage of1.1V oratankSoCof5%isreached. SoC limits are required for operation with low current densities, to compensate for imbalances in the operation of the individual cells. While for a high current density, overpotentials are high and system operation can be governed by cell voltage limits, it has to be governed by SoC limits for a low current density. In practice, it is most likely that not all cells are equally well supplied with electrolyte. This can be caused by variations in the thickness of the graphite felt electrode, which leads to variations in the compression of the individual electrodes. Hence, the porosity and thus the permeability of the electrodes will vary to a certain extent, which directly influences the flow rate distribution on the individual cells. The cell with the least permeable electrode will suffer from the lowest flow rate. If tank SoC is not limited during the charging process with a low current density, the SoC might reach values close to 100 % in the worst supplied cells, imposing these cells 86PDF Image | Model-based Design Vanadium Redox Flow Batteries
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