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.

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 and could achieve a current efficiency above 85%. It 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. The spinel catalyst has been engineered into a number of different carbon free air electrode architectures and demonstrated in flow cells. These carbon free air electrodes are radically different to conventional gas diffusion electrodes and the development of air electrodes with good performance was a long process which held back development of the integrated flow battery. The most durable and best performing air electrode was a catalyst powder loaded nickel foam structure. The catalyst and air electrode manufacturing routes were scaled up to be capable of producing low hundreds of A4 electrodes.

The 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 at laboratory scale. The continued scale up to A4 battery stacks was delayed by the limited availability of large air electrodes until the final year of the project. An A4 (active electrode area) battery stack was designed to take the powder / foam electrodes but be flexible enough to incorporate modifications as the air electrodes and stack operating experience evolved. The battery stack was commissioned step wise from 2 to 5 to 10 frames incorporating many lessons along the way. By the end of the project a 10 frame zinc air battery stack was fully commissioned and characterised. The round trip electrical energy efficiency of the battery was around 45 to 50% at a current density of 50 mA/cm2.

Analysis estimated that with further improvements a zinc air flow battery could be produced which had around a 30% lower capital cost than a vanadium redox flow battery but is likely to have a lower energy efficiency and further development may not be economic even with the energy efficiency improved to over 60% at high current density.

Exploitation of the zinc air flow battery may be directly as a battery but the partners are also developing the sub components (catalyst, air electrode and gas diffusion battery stack) for none energy storage applications.

A new multi-modular power converter was developed offering potential benefits for battery and high power applications > ~2 to 12 MVA of : (1) Provide higher flexibility to connect different batteries to a grid; (2) Meet technical requirements according to relevant codes and standards; (3) Maximise availability and (4) Minimise costs. It is suitable for integration of many kinds of electrical energy sources into the grid while allowing flexible expansions and reconfiguration.

The converter has reached a 40 kW prototype level stage with most functions tested successfully at the DNV GL Flex power Grid Laboratory.

Originally the battery and power converter were to have been tested together as a modular energy store but the power converter progressed faster than the zinc air battery and integrated testing was no longer sensible. Consequently both were tested separately by DNV GL.


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