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KWh to several hundred MW/MWh. The applications range from home storage to industrial plants as large storage units in the grid. Several companies sell VRFBs today in different size classes. Commercialisation and ongoing research From the 2000s onwards, the number of scientific publications and commercialisation efforts for RFB types other than those mentioned here increased significantly. Basically, there are many possible combinations for inorganic RFBs. By 2015, there were about 78 different types of RFB, but only a few of them have or will ever have commercial relevance [4]. Of the many different combinations, Pb/Pb- and Zn/ Ce- and H/Br-RFBs were the most studied in addition to the ones mentioned above [24, 25, 26]. The major challenges for new aqueous inorganic RFBs are above all the electrochemical window limited to a maximum of 2.1V, in which the redox pairs can function largely without hydrogen and oxygen formation, side reactions of the redox pairs with the solvent water and costs of the active materials. Especially the limitation of the voltage by the use of water led to investigations on the use of non-aqueous alternative solutions and redox pairs. The maximum possible voltage correlates with the maximum possible energy of an RFB. Doubling the voltage to 4V, as with lithium-ion batteries, doubles the maximum possible energy density. However, it should be noted that the cell resistance significantly determines the real energy density and often only very low energy densities can be achieved with very high cell Tanks of a 2MW/20MWh vanadium redox flow battery at Fraunhofer ICT resistances. Liu et al at the University of Michigan trialled the approach of using organic solvents to increase the energy density for the first time in 2009 with a vanadium-based organic acetylacetonate complex in acetonitrile [27]. Cell voltage versus aqueous inorganic VRFB increased from 1.6V to 2.2V. The same group also showed potential for non-aqueous chromium and manganese-based RFBs [28, 29]. In 2011 also, the concept of a lithium-based RFB attracted attention [30]. As with conventional lithium-ion batteries, solids were used for the anode and cathode. To make the active materials flowable, they were used as suspensions in an organic liquid. Suspensions are mixtures of solid and liquid components. The authors expected energy densities of up to 250Wh/L. In the following years many different concepts of Li-RFBs have been investigated, but it is not known that commercialisation efforts have ever been made. It is likely that the costs associated with low cycle life and low current density are the reasons why there has been little work in the field in recent years. The first completely organic RFB, i.e. organic redox pairs and organic solvent, was presented in 2011 by Li et al [31]. The advantage of a fully organic battery lies in the potentially low cost of organic active materials, their high availability and ease of disposal. However, organic solvents have an increased risk potential due to their flammability, and only low power densities can be achieved due to their low conductivity. These reasons in turn led to a focus on aqueous organic RFBs. The first organic RFB based on water as solvent and organic redox pairs was published in 2014 by Yang et al [32]. The authors used modified quinone and anthraquinone as active materials, substance classes that also occur as natural dyes. In the following years the research activity in the field of organic, especially aqueous RFBs increased significantly. The multitude of possibilities for organic active materials is considerably higher than for inorganic RFBs. However, there are also limits, so that the molecules must not be arbitrarily large, because otherwise they would have a too high mass and thus low energy density. However, organic molecules also offer the possibility of several electron transitions. In the case of inorganic active materials, one electron transition or two electron transitions are usually used. A doubling of the number of electron transitions leads to a doubling of the capacity (Ah) and with the same properties a doubling of the energy density. However, organic RFBs can potentially have up to six and perhaps more electron transitions. However, the reactions of organic active materials are often very complex and side reactions can lead to a small but continuous loss of capacity. The transfer of the results from research is not easy, since often only small concentrations and quantities of active materials are used and the properties in real batteries can be completely different. These can be e.g. deposition effects on electrodes which reduce the power density or limit the capacity or a gradual destruction of the active materials. Although the advantages are clear, there is still a long way to go for a practicable use of organic RFBs. Sunlight can be stored directly in chemical energy by means of photocatalytic reactions. The best-known processes are natural photosynthesis and the artificial photolysis of water into hydrogen and oxygen. The objectives are to reduce the number of conversions through more compact systems and to increase the compactness of the systems. In 2014, Liu et. al. showed an approach in which a VRFB with a suitable catalyst and a transparent positive electrode can use light with a yield of up to 12% directly to charge the battery [33]. At present, however, these investigations are still very much basic research. RFBs have experienced highs and lows throughout their history. The reason for this is that the idea was usually ahead of its time and the intended use, i.e. the stationary storage of energy, was not Technical Briefing www.pv-tech.org | November 2019 | 111 Storage & smart power Credit: Fraunhofer ICTPDF Image | Redox flow batteries for renewable energy storage
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