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Section 3.4 Efficiency definitions 3.4.2 Challenges in determining the round-trip efficiency During the determination of the round-trip system efficiency (RTSE), variations in the tank SoC at the beginning and the end of the conducted cycle introduce large errors. This is in particular important when cell voltage limits are used to determine the end of the charging and discharging process. The following example demonstrates the origin of undesired variations in the RTSE and their prevention. Figure 3-1: Tank SoC and cell voltage over time for a sample cycle with 200 A Three sample cycles with a stack consisting of 40 2000-cm2 cells are simulated. A charging/discharging current of 200 A is applied. At the beginning of the first cycle, the tanks have an initial SoC, SoC0T, of 5 %. For the second cycle, it is assumed that the initial tank SoC is 20 %. The third cycle starts with a so-called pre-discharging process from an initial tank SoC of 50 %. Table 3-2: Efficiencies and discharge capacities of the three sample cycles shown in Figure 3-1 Coulomb efficiency Voltage efficiency Energy efficiency System efficiency Discharge capacity SoC0T = 5 % 87.4 % 78.6 % 68.7 % 67.6 % 3.90 kAh 4.70 kWh SoC0T = 20 % 108.4 % 77.7 % 84.2 % 82.7 % 3.90 kAh 4.70 kWh Pre-discharging 95.3 % 78.2 % 74.5 % 73.2 % 3.90 kAh 4.70 kWh Without additional measures, all efficiencies, except for the voltage efficiency, strongly depend on the initial tank SoC, as demonstrated in Table 3-2. The reason for these variations is the tank energy balance. The tank SoC at the end of the discharging process is independent of the initial value. If the tank SoC at the end of the discharging process is higher than the initial one, a certain part of the energy, fed to the tank during the associated charging process, stays in the reservoir. If the tank SoC at the end of the discharging process is lower than the initial one, energy, which we did 60PDF Image | Model-based Design Vanadium Redox Flow Batteries
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