The Powair project, which is funded under the EU’s Seventh Framework Programme (FP7), aims to create a low-cost modular and environmentally sustainable electrical energy storage system with high energy density and fast response by radically extending the performance of zinc–air batteries.
There are currently only a few technologies available for large-scale energy storage systems of MW-scale. These include pumped storage systems and Compressed Air Energy Storage (CAES). However, use of these technologies is restricted by the fact that they require geographical features: large water reservoirs or salt caverns. The attractiveness of conventional battery systems such as lead-acid, nickel-cadmium and lithium-ion batteries for large-scale energy storage systems is reduced by their limited number of cycles, poor efficiency, often high costs per kWh, and system complexity.
The Powair project aims to compensate for these shortcomings by developing a technology that permits large capacities of electrical energy to be stored indefinitely, to be transmitted or distributed as and when required. The battery system being developed can be charged directly from the grid, for peak shaving applications, or from renewable energy installations, thereby providing stability and flexibility to the grid and eliminating the need for fossil fuel powered peaking plants.
To achieve these aims, the project will extend the performance of zinc–air batteries from small-scale single primary cells to rechargeable redox flow battery modules, which at production scale can be stacked to give powers of 20 kW to MWs with several hours of storage. In tandem with the battery system, a novel distributed power converter will be developed to enable the disconnection, replacement and reconnection of a single battery without interrupting the energy storage system’s performance.
This collaborative project is funded under the FP7 energy call “Energy storage systems for power distribution networks.” The project, which began in November 2010, will last for a period of four years. The total project budget is just over €5.13 million, of which the EU will provide €3.56 million. The objective of energy research under FP7 is to aid the creation and establishment of the technologies required to make the European energy system more sustainable, competitive and secure. This project fits the FP7 remit perfectly, as it will also help reduce power system dependence on imported fuels and facilitate the diversification of energy sources.
As regards the technology, zinc-air batteries use oxygen from the atmosphere rather than a metal-dissolved electrolyte, which is more usual. Air reaches the cathode surface, where an active electrocatalyst promotes the reduction of oxygen. Metallic zinc is oxidised on the opposite side of the cell, usually in an alkaline electrolyte. As the cathodic reactant is not packaged within, zinc-air batteries have a higher energy-to-weight ratio than other types, and also since the cathode is very thin, the anode compartment can be packed with more zinc, resulting in a very high energy density. Due to their scalability, flow batteries have a wide power range, from a few kWs to many MWs, and much higher capacity ratings than static batteries. Eliminating the need for a positive electrolyte effectively doubles the energy density of the system. The project takes advantage of the knowledge of electrochemical technologies that exists within the consortium to develop a flow battery that will compete with systems currently under development.
Some of the advantages of zinc-air flow batteries include a much higher power and energy density than vanadium redox flow batteries (VRFB). Their high alkali content and flow prevents zinc passivation and reduces dendrite1 formation and they have a well-known chemistry with fast kinetics. Furthermore, there are additive systems already available from the electroplating industry that can be used to control deposit morphology, which also reduces dendrites. The fact that the batteries contain a liquid electrolyte means they are unlikely to dry out, reducing the need for humidity control. Moreover, thermal management issues are minimal due to the recirculated electrolyte and the air electrode. Finally, production of the batteries involves the use of low-cost, benign and widely available materials.
According to the technology roadmap for the project, the project should currently be at the stage of demonstrating a prototype in a test environment, with prototype demonstration in an operational environment planned for year four. There are major challenges ahead if wholesale centralised and decentralised renewable power generation is to be implemented. That said, Powair’s contribution to grid flexibility and the integration of intermittent renewable energy sources means that it will be a valuable component in the technology basket underpinning the large-scale energy storage installations needed to off-set periods of restricted generation, and is therefore likely to play an ever-increasing role in facilitating the integration of renewables into the European grid.
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1 Dendrites are deposits that form on electrode surfaces while batteries are recharging and may cause batteries to fail or even ignite.