Investigation of Superconducting Magnetic Energy Storage as part of Hybrid Energy Storage Systems for Renewable Energy Microgrids

Energy storage systems are ideally placed to harness the intermittent energy from renewable sources which are often connected with weak electricity distribution systems to form local microgrids. Due to the large variation in load/generation balance of microgrids, an energy storage system is required to have a large power density and respond quickly to power fluctuations on a short time scale of seconds to minutes. Additionally it needs to have a large energy density in order to deal with power imbalances on a longer term basis. For microgrids, where power levels are in the range of a few megawatts, there is one technology for the required energy density: electrochemical batteries, and three prime candidate technologies for the required power density: SMES systems, supercapacitors and flywheels. Although the batteries usually have large energy densities, their power densities, life cycles and response speeds are very limited. On the other hand, flywheels, supercapacitors and SMES systems have large power densities, large duty cycles and fast response speeds.

Compared to flywheels and supercapacitors, SMES systems have significantly larger power densities and module power ratings. In addition, they also have advantages including high round-trip efficiencies and solid-state operation. A game-changing energy storage system can thus be achieved by integrating fast-responding SMES systems of high power densities with electrochemical batteries of high energy densities. This system can substantially reduce the number of charge/discharge cycles of batteries and thus extend their life, leading to a significantly reduced volume of usage and hence environmental impact of batteries in renewable energy integration.

There are several fundamental limits that prohibit widespread applications of previous SMES systems using low-temperature superconductors (LTS) and first-generation high-temperature superconductors (1G HTS). LTS require liquid helium as a coolant which is scarce resource and thus expensive. Due to their small operational magnetic field and current density, SMES systems using 1G HTS have a small energy and power density. In addition, their price is prohibitively high. Recently 2G HTS have been significantly enhanced in terms of increased operational magnetic field and current density which enables SMES systems to achieve a substantially higher energy and power density. Predictions indicate the cost of 2G HTS in terms of $/(kA*m) will drop below 1G HTS and copper in the near future. Thus there is now a critical opportunity to develop a SMES system using 2G HTS.

A major technical hurdle for the widespread application of SMES to renewable integration is that its energy storage component, i.e. superconducting coils, needs to be properly modelled and characterised if to be used in hybrid energy storage systems. In this project, SMES systems are proposed for the first time to be used continuously on short time scales of seconds and as part of a hybrid energy storage system. Here, a novel multi-physics model will be created based on numerical methods and at the same time can be utilised in standard electrical circuit analysis tools. A demonstration kiloJoule-range 2G HTS coil will be designed and constructed. The coil will be firstly tested at 77K whilst being stand-alone or connected with laboratory-based microgrid systems. Then the coil will be characterised between 50K and 77K using the facility provided by Florida State University - Centre for Advanced Power System. The experimental results will give us confidence in applying the model and design methodology for future large scale superconducting coil design. Importantly, the results can be used as guidance of future SMES system design at the fabrication stage. Specific recommendations to superconductor manufacturers will be available for further improving the conductors for energy storage application.

The proposed research will be of great help to the government to achieve the 80% CO2 emission reduction plan by 2050 with substantially reduced cost and environmental impacts. Principally through substantially increasing the viability of renewable-based generation (much of which is intermittent in nature) via energy storage; hence a significant impact of this project on the UK's electrical power sectors is envisaged. It will also be of great benefit to the industrial and academic community who are active in the study of and uses for energy storage and applied superconductivity.

There are a wide range of companies in the power industry interested in this technology including National Grid. They will act as industry advisors giving their requirements and expert advice on how the lab demonstrator can be scaled up and adopted by industry partners in the future. This monitoring and feedback process will go on throughout the project, thereby giving confidence in investigating the pathway to scale up the laboratory demonstrator to an industry prototype.

In particular, there is a significant international impact of this project on the electrical power sectors since we are collaborating with the Florida State University - Centre for Advanced Power Systems (FSU-CAPS), which is a leading research centre focused on electrical power systems with advanced simulation and testing capabilities. The research results are expected to showcase the U.K.'s research experience and capability to an international audience. Seminars and discussions will be organised to disseminate the research outcome to the related academic and industry community in the U.S.. This project will improve the U.K.'s visibility and influence abroad, increase the U.K.'s research capability and accelerate the application of the U.K.'s research within the international energy landscape.

This project will also produce a significant impact on the academic and industry community of applied superconductivity. After more than 25 years since the discovery of high-temperature superconductivity (HTS), the conductor is on the edge of penetration into the industrial application market. It is estimated the combined conventional and emerging markets for HTS conductors will be from £8-15 billion. The proposed project will significantly broaden the application area of SMES systems by extending the technology for the first time to the market of renewable energy integration, thereby increasing the profile and attracting vital industry stake holders. It will also give specific recommendations to superconductor manufacturers for further developing their materials to be more suitable for electrical power applications.

As an integral part of the project management, the industrial partners will be invited to attend the project meetings held in Bath every six months. Interested collaborators (the industrial sponsor and other relevant commercial and government bodies) will be supplied with the technical reports about research findings. Informal meetings with potential sponsors to seek a follow-on support of the project will also be held.