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32456 Paths tSouSsutastianianbalbeleEEnneergrgyy of 0.5. The optimal operating point maximizes the current |Istack| delivered to the stack in order to store the maximum amount of electroactive species at a given power PVRB,re f ; again, the optimal flowrate Qopt increases with the battery power PVRB until it reaches the plateau due to the flow regime transition. Battery power PVRB SoC = 0.5 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 −80 −60 −40 −20 0 20 40 60 current Istack [A] Fig. 21. Battery power PVRB as a function of the charge current Istack and the electrolyte flowrate Q at a state of charge SoC equal to 0.5. The optimal operating points occurs when the current |Istack| is maximal for a given battery power PVRB. Interestingly, we observe in Fig. 21 that the stack current Istack changes its sign at high flowrate Q; in these unacceptable conditions, the stack is discharged while the battery is being charged. During the charge, the stack current |Istack| decreases above the optimal flowrate Qopt to compensate the higher mechanical loss Pmech; in consequence, less power is available to charge the stack (see (42)). Below the optimal flowrate Qopt, the stack current |Istack| also decreases because the stack voltage Ustack increases due the change in electroactive species concentrations within the cells ccell; note that the mechanical power Pmech is also reduced below Qopt. Furthermore, the shape of the curves in Fig 21 might be generalized to other states of charge SoC. 9.1 Charge and discharge cycles A new series of charge and discharge cycles at constant power was performed to determine the energy efficiencies at minimal flowrate Qmin and at the optimal operating point: Iopt and Qopt. This optimal point is constantly determined as a function of the actual conditions. The energy efficiencies are given in Tab. 10. The energy efficiency at optimal flowrate ηenergy,Qopt is increased by 10% at maximal power when compared to battery operations at minimal flowrate Qmin. 10. Epilogue Today, the electricity industries are facing new challenges as the market is being liberalized and deregulated in many countries. Unquestionably, electricity storage will play, in the near future, a major role in the fast developing distributed generations network as it has PVRB = −250 PVRB = −500 PVRB = −750 PVRB = −1000 PVRB = −1250 PVRB = −1500 PVRB = −1750 PVRB = −2000 m Maxi al current flowrate Q [l/s]PDF Image | Understanding the Vanadium Redox Flow Batteries
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