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Section 7.5 Implications for grid connection The system with six parallel stacks has to be connected to the power grid using a PCS with a low input DC voltage. Table 7-5 gives the efficiency of a typical PCS, ηPCS, for the voltage level of a 40-cell stack [92]. The product of the system efficiency of design 2.5 with the PCS efficiency, ηPCS, gives the total efficiency of the single-stack system, ηTotal. If we divide this efficiency by the system efficiency of design 4.6 in a three-stack string, we yield the efficiency of the three-stack PCS which is required to operate the three- stack string as efficiently as the single stack. Table 7-5: Computation of required PCS round-trip efficiency to compensate for the loss in Coulomb efficiency due to the electric series connection of three stacks. Current density in mAcm-2 ηSys, design 2.5, six stacks in parallel ηPCS,EDC=54V ηTotal, design 2.5, six stacks in parallel ηSys, design 4.6, three-stack string Required ηPCS, EDC = 162 V 25 78.2 % 95.0% 74.3 % 74.9 % 99.2 % 50 78.4 % 94.0% 73.7 % 76.3 % 96.6 % 75 100 75.8 % 72.4 % 92.5% 91.5% 70.1 % 74.2 % 94.5 % 66.2 % 70.7 % 93.6 % It is obvious that for an operation with 25 mAcm-2, obtain the same total efficiency than the single stack. efficiency of 99.2 % is rather utopic at this input voltage level. When operating with nominal current density, the PCS for the three-stack string needs to have an additional 2.1 %-points of efficiency compared to the single stack. Considering the three-fold increased input DC voltage, this appears to be possible. Hence, if operated with a sufficiently high current density, it appears to be possible to operate a three-stack string as efficiently as a single stack. However, if a sufficiently high PCS efficiency is available for the low DC voltage of a single VRFB stack, it would no longer be possible to obtain the required efficiency gains due to the higher DC voltage. In addition, resigning from serially connecting several stacks electrically is beneficial for the system lifetime due to reduced corrosion by shunt currents. It thus can be concluded that developing a high efficient PCS with a low input DC voltage is of high interest for the system design of redox flow batteries. It is noteworthy that several companies and research institutes are currently active in the field of high efficient low voltage AC/DC converters [93]. the three-stack string can hardly This is because a PCS round-trip 117PDF Image | Model-based Design Vanadium Redox Flow Batteries
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