Lower Cost and Longer Life Flow Batteries for Grid Scale Energy Storage

Flow batteries are a form of electrochemical energy storage in which electrical energy is stored via the generation of a physically separated reductant and oxidant, and electrical energy generated when required by the re-combination of this redox couple. Unlike other forms of electrochemical storage, flow batteries are characterised by the ability to de-couple power and energy, allowing significant cost savings as energy requirements increase, and offering the potential for MW/MWhr scale storage. Considerable progress has been made on this technology recent years, especially within China and the UK, but challenges remain to understand and improve lifetime and performance in the currently used all vanadium approach, and to explore novel approaches which offer significantly reduced cost. This proposal addresses the issue of both cost, performance and lifetime within flow batteries, to develop significantly improved all vanadium systems, and to explore novel approaches.

Flow batteries are a potentially valuable energy storage technology for grid applications, because they decouple power and energy. This becomes particularly important when storage times of several hours are required. They can also operate over a wide range of states of charge, meaning that they impose less constraint on the grid in terms of the required control strategy. Work by Strbac et al [Strategic Assessment of the Role and Value of Energy Storage Systems in the UK Low Carbon Energy Future, Strbac et al, Energy Futures Lab, Imperial College 2012] has shown the need for such forms of energy storage for future low carbon energy systems, and has quantified the value to the overall electricity system in terms of reduced investment and operating costs, with system savings of £10bn/year possible through the application of storage technologies for some high renewable scenarios by 2050. But this work has also shown that if grid scale storage technologies are to address this opportunity, in competition with other measures to deliver flexibility such as demand side management, then a reduction in the capital costs of current grid scale storage technologies are necessary, coupled with much longer lifetimes than can currently be delivered - greater than 25 years.

Flow battery technologies are commercially available, in particular based around the all-vanadium chemistry. But the costs in general remain relatively high, being reported to be for example, 3000-3700 $/kW and 620-830 $/kWhr, for vanadium flow battery technologies, [EPRI, Electricity Energy Storage Technology Options: A White Paper Primer on Applications, Costs and Benefits, 2010]. Here we note that our partner DICP, working with Rongke Power, has reduced the costs of state of the art vanadium systems to 1500 $/kW. However all the available vanadium flow batteries suffer lifetime challenges, especially capacity fade over time, and electrode degradation. Therefore at the heart of this project is the development of improved and lower cost flow batteries for grid scale storage applications. We will work on vanadium flow batteries where we seek to understand and hence improve on lifetime issues whilst also reducing cost (e.g. through the improvement of the currently widely used Nafion type membranes which suffer vanadium cross-over and are expensive at around $1000 m-2, and through the development of higher performance and more durable electrode structures and materials. We are particularly motivated to do this following the recent work by the Zawodsinski group in the USA who has highlighted the untapped potential of vanadium systems, with cell power densities of over 1 W cm-2 being demonstrated under laboratory conditions, compared to the current industry standard of around 0.1 W cm-2, hence continued attention on this system is appropriate. In addition, we will also look to explore some novel approaches which aim to further lower cost, based around the flexiplanar stack concept, and vanadium-hydrogen and polysulphide-air chemistries.

Hence our work in this project will result in improved flow battery membranes, electrodes, cells, and stacks, all of which will contribute to lower cost flow battery systems once the arising innovations and understanding is fed into manufacturing and engineering scale up programmes, and subsequent demonstration and deployment.