| The development of renewable energy has contributed to the research of large-scale energy storage technologies.Redox flow batteries,especially vanadium redox flow batteries,have attracted great attention in engineering and academic fields,due to their independent power and capacity configurations,eco-friendly characteristics,and long operating life spans.Over the past few decades,extensive efforts have been paid on the innovation of the critical components or materials in batteries,and the battery performance has been dramatically improved.However,currently the cost of vanadium redox flow battery system still remains at a high level,which limits the wider application in large-scale energy storage fields.The theoretical energy density of vanadium redox flow battery is 50 W·h/kg,while the effective energy capacity in operation could only reach about 70% of the theoretical value,i.e.,35 W·h/kg,due to the excessive ohmic losses,concentration overpotentials,and concentration discrepancies of active species between tank and stack.Considering the expensive cost of electrolytes in vanadium redox flow batteries and the great importance of enhancing effective energy capacities to reduce the battery costs and electrolyte volume per application,one of the key objectives of the current research is to improve the electrolyte utilization.The system configuration design and operation strategy of redox flow battery system is a relatively convenient way to improve the effective energy capacity,which usually does not need much innovation or improvement costs of the system.The system optimization also has guiding significance in the field of engineering application.In this thesis,the existing system configuration design and operation strategy of redox flow batteries were comprehensively summarized and compared.It is found that the current research still has the following shortcomings.Firstly,less attention was paid to reduce the concentration discrepancies of active species between tank and stack,and there is still a lack of effective means to reduce the concentration discrepancies and increase the effective energy capacities.Secondly,there is a lack of attention on the tank in the field of system configuration design.Lastly,at present the current optimization strategy for redox flow batteries still needs further investigations on dynamic control methods and combination of electrolyte flow rate.In view of the shortcomings of previous studies,firstly,this thesis proposed a no-mixing redox flow battery design with the aim of reducing the concentration discrepancies of active species between tank and stack.This objective is achieved through the separate storage of charged and uncharged electrolytes in the external tanks and the design of electrolyte circulation structure.Secondly,this thesis proposed a dynamic current optimization strategy based on the mass transfer balance equations with the aim of reducing the ohmic losses at the end of the(dis-)charge process.The applied current of the battery system is adjusted in real time according to the state-of-charge of electrolytes in stack.The governing equation can be further adjusted according to the choice of electrolyte flow rate.Through the combination of simulations and experiments,it is shown that the proposed design and strategy in this thesis could effectively reduce the concentration discrepancies of active species between tank and stack.And both of them could enhance the effective energy capacity by more than 10%.In the first part of this thesis,a no-mixing flow battery was designed.The effectiveness of the no-mixing design was investigated under varying electrolyte flow rates,electrolyte volumes in tank,and current densities through simulations and experiments.The results were compared with those of the traditional two-tank system,which show that when the electrolyte volume in tank is relatively large,the electrolyte utilization enhancement of no-mixing design could be extended,and the voltage efficiency could be improved by about 2%.Through mathematical analysis,the maximum electrolyte utilization enhancement could be predicted under given parameters.The relationship between the concentration discrepancies of active species between tank and stack,and the electrolyte flow rate,electrolyte volume,and current density was derived.In addition,economic analysis and practical discussion were also conducted.In the second part of this thesis,a novel current optimization strategy was proposed.The effects of operation or design parameters on this strategy were investigated through simulations,while the operability was verified by experiments.The practicability of this strategy was also discussed.The results show that under proper operation,the ohmic losses and concentration overpotentials at the end of(dis-)charge process could be effectively reduced.The electrolyte utilization could be enhanced under the condition of ensuring required average power output and satisfactory system efficiency.The current optimization strategy could be combined with the electrolyte flow rate optimization strategy to further improve effective energy capacity.The experimental and simulation results,as well as the economic and practical analysis show that the no-mixing design is more suitable to be adopted in large-scale energy storage scenario when the electrolyte volume in tank is large,while the dynamic current optimization strategy is more suitable for the off-grid scenario,where there is no great limitation on the power variations.Since the utilization enhancement of no-mixing design is not affected by the applied current density,and the dynamic current optimization strategy mainly adjust the applied current values,the no-mixing design and the dynamic current optimization strategy could be jointly adopted in redox flow battery system to achieve higher effective energy capacity.The results obtained in this thesis have instructional significance as well as practical application values in engineering fields of redox flow battery system. |
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