Beyond Batteries with Alternative Storage Tech

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. 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 electrolyte 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 reversed 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 capacity. The power of the FBES system is determined by the size of the electrodes and the number 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 requirements 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 example, 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 transportation. One of London's bus routes is operated by eight hydrogen fuel-cell powered buses, complementing 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. Compressed air storage has long been used for a variety of niche applications, including powering pneumatic drills and fairground rides. It is now experiencing growing interest as a grid-scale technology and 34MW of compressed air storage (CAES) came online in 2014. Classification of energy storage technologies by the form of stored energy. Source: Applied Energy 137 (2015) According to Navigant Research, energy flywheels are another familiar form of energy storage that is being used in grid applications, typically to improve power quality. Storing large quantities of energy with pumped storage hydroelectricity One of the oldest storage technologies is pumped storage hydroelectricity, which has long been used in many parts of Europe to smooth out demand variations. Germany has 7 GW of PSH capacity and plans to add nearly 24 GW. PSH has the big advantage of high dispatchability, so it can literally be turned on and off in a matter of seconds. Wind and solar dams are a modern variation on this theme but they suffer from the big handicap that water can only be pumped if the wind blows or the sun shines. New PSH projects are constrained by environmental issues and the dwindling number of suitable locations in Europe. However, because of its maturity, PSH will remain the dominant means for utilities to store large quantities of electricity, at least in the foreseeable future. Although the potential benefits of energy storage are widely recognised and there are many new and established contenders, there still remain big challenges in developing cost-effective storage systems that can be deployed on a wide scale. But there are also significant opportunities because as Europe embraces renewables and electric vehicles, it’s clear that energy storage is the one technology we cannot afford to do without. Related Stories: Battery Technologies and Energy Storage Solutions - Part 01

Beyond Batteries with Alternative Storage Tech

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