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High fuel prices and emissions legislation are driving interest in the development of flywheels for use in public transport, including electric trains and also for energy storage within smart-grid electricity infrastructure. Flywheel energy storage systems offer great potential for improving vehicle efficiency and reducing harmful emissions when employed as part of a kinetic energy recovery system. The ideal flywheel material for use in hybrid vehicles has a high strength and low density. The high strength permits the flywheel to operate safely at very high rotational speeds (more than 40,000 rpm), which also maximises the stored energy. A low density allows the overall vehicle weight to remain low, whilst the flywheel design can optimise the rotational inertia for a given mass.

With modern materials and clever design, lighter, composite flywheels have real potential to deliver improved fuel efficiency in passenger cars too and could become the technology of choice for green vehicles. Composite materials reinforced with carbon fibres offer the optimum combination of properties for this application, but their failure mechanisms at such high speeds are not well understood.

We have been collaborating with other academic and industrial research teams to improve our understanding of the disintegration of such high-speed composite flywheels. Our research has demonstrated the influence of high rotational speeds on the flywheel structure, and that high-speed videos of disintegrating flywheels could be recorded safely and successfully, even without prior knowledge of the exact instant when the fractures would occur.

A Knowledge Transfer Project with Highview Power experimentally characterised material, investigating the relationship between physical and cryogenic materials. This study focused on gravel properties, at both ambient and cryogenic temperatures, to aid in the simulation of large scale thermal packed beds which are used to improve the efficiency of the Liquid Air Energy Storage (LAES) system. Novel methods for measuring material properties were utilised, namely the density, specific heat capacity and thermal conductivity. Multiple packed beds were constructed in the lab to evaluate the thermal performance with different aspect and particle-to-packed bed diameter ratios. Cyclic testing of materials was conducted to evaluate and produce prediction models for gravel fracturing and material degradation. This project resulted in the completion of 1-D and 3-D models, which can be used to predict the thermal performance of a packed bed under varying conditions, over a wide temperature range.

An Innovate UK project, in partnership with Highview Power, focused on the assessment of novel mixtures of molten salts for the high grade thermal storage of an LAES system. A comprehensive literature review was carried out on lower melting point molten salts, resulting in a useful database of the most relevant physical properties, namely melting and degradation temperature, viscosity, specific heat, density, thermal conductivity and costs. With this information, molten salt mixtures were replicated on Aspen+ through a novel hybrid approach, implementing different correlations found in literature as well as Aspen data. Thanks to the numerical simulations, novel alternative mixtures were evaluated in the temperature range suggested by the industrial collaborator. A salt performance index was utilised to compare these alternative mixtures against the original published in literature. The only parameter that the numerical simulation could not capture was the melting temperature, thus an experimental investigation was performed, resulting in two promising alternatives that provide high performance boost despite a small increase in melting temperature. Overall, a methodology to tailor and select novel molten salts mixtures has been developed and proven to be reliable and effective.

The intraday and intraseasonal balancing of power grids requires a low capital cost solution. Liquid Air Energy Storage (LAES) utilises the thermal energy potential stored in a tank of liquid air. In an LAES system, the device is charged using electricity to drive an air liquefaction plant. Thermal energy is stored in a cryogenic tank as liquid air and as thermal energy in sensible heat store. When electricity is required, the cryogenic fluid is compressed, heated and expanded in a turbine to generate work. The process efficiency is almost doubled by capturing the ‘cold’ thermal energy released during power recovery and using the cold to reduce the energy cost to recharge the system. The Advanced Engineering Centre work with Highview Power on the cycle design and various sub systems in the LAES system, such as the thermal stores. Other fundamental work has included research on the separation of the phases in a cryogenic fluid in the storage tank. This can result in a phenomena known as rollover which can cause the tank to fail.