Powering Europe’s Cleaner Future:

Beyond Batteries with Alternative Storage Tech Part 02

The first part of this report featured current battery technology and energy storage systems from chemical based batteries for electric vehicles to industrial grid-scale storage. Today, we introduce two additional promising technologies for transportation: flow batteries and fuel cells. Author: Geoff Nairn There are several types of flow batteries but they share a design in which the electrolytes are stored as liquids in separate tanks stored outside the cell where the electricity is generated. As well as not needing solid materials to store energy, the big advantage of a flow battery is that the battery’s power can be decoupled from its capacity. The use of abundant materials for the electrolyte could potentially make flow batteries a highly cost-effective source of power, both for EVs and grid use.

A flow battery stores energy in two soluble redox couples contained in external liquid electro-lyte tanks. These electrolytes can be pumped from the tanks to the cell stack which consists of two electrolyte flow compartments separated by ion selective membranes. The operation is based on reduction-oxidation reactions of the electrolyte solutions. During the charging phase, one electrolyte is oxidized at the anode and another electrolyte is reduced at the cathode, and the electrical energy is converted to the electrolyte chemical energy. The above process is re-versed during the discharging phase. Flow batteries can be classified into the categories of redox flow batteries and hybrid flow batteries, depending on whether all electroactive components can be dissolved in the electrolyte. A crucial advantage of FBES is that the power of a FBES system is independent of its storage ca-pacity. The power of the FBES system is determined by the size of the electrodes and the num-ber of cells in the stack; whereas the storage capacity is determined by the concentration and the amount of electrolyte. Also, the very small self-discharge is an inherent strength of the FBES system due to the electrolytes being stored in separate sealed tanks. Drawbacks of flow batteries include low performance resulting from non-uniform pressure drops and the reactant mass transfer limitation, relatively high manufacturing costs and more complicated system re-quirements compared to traditional batteries. Liechenstein-based start-up nanoFlowcell has developed an electric car that uses a novel water-based flow battery and achieves a range of over 1,000km. Vanadium redox flow batteries (VRFB) and other types of flow batteries are beginning to be used for energy storage. For exam-ple, a 1.26MWh utility-scale energy storage system based on VFRB technology is being built on the Scottish island of Gigha, which has limited connections to the mainland.

Fuel cells to power transport Batteries are not the only candidate for large-scale energy storage applications. For vehicle use, fuel cells are a promising alternative to both ICE and batteries, particularly for public transporta-tion. One of London's bus routes is operated by eight hydrogen fuel-cell powered buses, com-plementing the city's six pure electric buses and more than 800 hybrid diesel-electric vehicles. The big attraction of using fuel cells to power transport is that the hydrogen fuel can be made by electrolysis using surplus electricity that cannot be consumed or stored by other means. However, the development of a hydrogen economy continues to face difficult challenges, not least the cost of the infrastructure for storing and transporting hydrogen.