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Zinc-Air flow batteries for electrical power distribution networks

Periodic Report Summary 3 - POWAIR (Zinc-Air flow batteries for electrical power distribution networks.)

Project Context and Objectives:
The concept of the project is to create a new class of electrical energy storage system with the unique combination of characteristics of high energy density, modularity, fast response and low cost. To achieve these aims, the project will radically extend performance of zinc air batteries from small primary cells into robust and cheap flow batteries suitable for integration into electrical power grids.
At all stages of the project, a system approach will be adopted to develop a working and robust energy store from the individual components (flow battery, power conversion, grid interconnection, control system) which is suitable for industrialisation within a short timescale at the end of the project.
The project will be steered by analysis of the user applications and requirements, leading to system specifications for potential applications, which will in turn define targets for the individual system components. The rechargeable Zn-air battery proposed here will overcome many of the problems of other storage technologies including conventional redox flow cells by the introduction of the following innovations:
1. The use of an air electrode for one half-cell reaction (only half the reactant volume is required), increasing energy density.
2. Use of an alkali electrolyte in which the metal is highly soluble allowing high energy densities with fast electrode kinetics.
3. Fast response due to the fast electrode kinetics and the reactants being already present at the electrodes.
4. Decoupling of power and storage capacity due to the external electrolyte storage.
5. Low cost (due to the cheap electrolyte & simple material requirements).
6. Highly scalable and modular using distributed power electronics.
A 10 kW demonstrator will be built which will be fully tested against worldwide grid connection standards and over a wide range of operating regimes, in particular those associated with renewable generation.

Project Results:
An initial study of the markets for smaller scale electrical energy storage (10 kW to 1 MW) identified that the most promising applications were: electric service reliability; time-of-use energy management; renewable generation integration (particularly solar and wind); transmission and distribution upgrade deferral and demand charge management. Computer simulations were then used to specify the overall system requirements for an energy store to meet those requirements and designs were developed for a modular power converter (and system controller) to allow for modular plug and play expansion. Initial cost estimates were made for the flow battery and power converter and they remain very favourable with respect to current market products.
The development of the battery was split into separate work packages covering the zinc half cell, the air electrode, unit cell development and engineering of a pilot scale flow battery system.
A low cost but high energy density zinc electrolyte has been developed which can operate over 80% of the charge range without dendrite formation using a formulated additive system. The rate of self discharge is exceptionally low and has been engineered to be insensitive to carbon dioxide in the air from the air electrode.
A low cost but stable bi functional catalyst system has been developed with performance far superior to precious metal catalysts. This catalyst has been engineered into a number of air electrode architectures and demonstrated in flow cells. Optimisation is continuing.
The successful integration of the zinc and air half cell reactions into a combined flow battery has demonstrated the compatibility of the approaches to the two electrode reactions and that the battery performance comfortably exceeded the second year targets. The engineering of the air electrodes to A4 size has been more challenging than expected but significant progress is now being made and should offer world class performance in practical, scalable and low cost air electrodes. The engineering of the battery unit cells is currently being optimised and scaled up to A4 size and integrated into short battery stacks.
A novel modular power converter has been developed which is suitable for integration of many kinds of electrical energy sources into the grid while allowing flexible expansions and reconfiguration.
Exploitation planning has commenced. A number of technical presentations have been given on the objectives and none confidential results. Engagement with stakeholders, investors, distribution partners and potential customers had started to occur in years two and will ramp up in year three as the consortium is in a position to disclose more information.
The partners consider that significant progress has been made in the first three years and the project is on track to produce both a novel modular power converter and a low cost flow battery which can be integrated into a low cost energy storage system by the end of the project.

Potential Impact:
The final outcome of the project is expected to be an integrated energy storage system (10 kW and up to 100 kWh) whose performance has been fully characterised on a grid simulator and is suitable for rapid commercialisation.
Impacts achievable from POWAIR for future electricity networks are :
1. Cost-effective energy storage – Indications are that the cost of the energy store should be comparable with lead acid batteries but with much improved performance. This level of price and performance is required for electricity storage technology to succeed in the market place.
2. Increasing the hosting capacity for variable distributed energy resources – The ability to place relatively high density small scale storage deep within the low voltage network allows peak power flows to be limited to the existing network capacity. This allows more micro and mini-generation to be connected than would otherwise be the case.
3. Accelerating the penetration of Electric Vehicles and Heat Pumps – As with distributed generation high density small scale storage connected at substation level or within domestic properties will allow the expansion of these low carbon technologies into streets where the network would not otherwise cope.
4. Improved Reliability – By providing more local sources of energy the addition of storage to a low voltage grid will improve the reliability and security of power supply to the customers.
5. Efficiency – from reduction in transmission losses and displacing fossil fuel generation capacity working inefficiently as spinning reserve or rapid response reserve.
6. Long Term Security – The avoidance of using materials that are in short supply, and the active displacement fossil fuels from outside the EU for some network balancing activities will improve the EU’s long term security of supply.
7. Reduced environmental impact – From enabling the effective integration of variable renewable generation sources within the electricity network, reduced transmission losses and the low environmental impact of this battery compared will almost all the available alternatives.
In addition to the creation of highly skilled jobs and a knowledge based industry for the worldwide supply of energy storage systems and enabling the use of more intermittent electricity generation, the POWAIR storage system supports the reliability of the electricity supply.

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