Energy storage

Wind energy and photovoltaics can greatly contribute to the energy supply. The big challenge here, though, is the volatility of these forms of energy. With an increasing share of renewable energies on the grid, therefore, ensuring grid stability is thus a major challenge for electricity distribution. Peak load power stations and pumped-storage power plants come up against their limits. Ideally, large accumulators should be used as network buffering in both directions, thanks to their fast response times. Here we are still facing the challenges of the necessary storage capacities and the economic feasibility of associated storage technologies. The first applications based on Li-ion batteries as well as the first installations using redox flow cell technology are beginning to appear.

The Fraunhofer ISE, for example, has been working on a 1 MWh accumulator based on the redox flow cell since 2010:
www.ise.fraunhofer.de/de/forschungsprojekte/1-mwh-redox-flow-netzspeicher

Japanese operator Sumitomo Electric is the largest (6MWh) commercially operated redox flow cell installation in operation:
global-sei.com/news/press/12/prs069_s.html

The Schwerin-based utility company WEMAG put the largest (5MWh) commercially used battery storage system using Li-ion technology into operation in the autumn of 2014:
www.wemag.com/ueber_die_wemag/presse/pressemeldungen/2014/09_16_Eroeffnung_Batteriespeicher.html

The market thus appears to be reacting; after all, an efficient storage technology is the key for the extensive use of renewable energies in the future. However, both currently used technologies have limitations.

The redox flow cell is limited by its low storage density. The charge carriers are dissolved in an aqueous solution. Once it is saturated it cannot take up any further charge carriers. The most efficient commercially used redox flow cells are currently based on vanadium as an electrolyte, which has an energy density of approximately 25Wh per litre. By way of comparison, the lead acid battery commonly used in cars has an energy density of 42Wh/kg. The electrolyte is also very expensive and highly toxic.

The largest development effort today is without a doubt being invested in Li-battery technology. The Li-polymer battery in particular is currently the largest field of application. With an energy density of 150Wh/kg, it is far more powerful than redox flow cell systems; likewise, the widespread use of this technology has now resulted in a price level of less than € 1000 per kWh.

Even with this technology, the energy density is nevertheless still far from what we are used to from fossil fuels such as diesel fuel with its 10,000 Wh/l. Li polymer batteries as fossil fuels are far more “powerful”, in the negative sense, when it comes to the risk of fire. Mechanical damage suffices to spark a heat generation in the Li polymer cells that leads to an uncontrollable fire. That is also why Li polymer batteries were banned in aviation.

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