Overview of Mobile Flywheel Energy Storage Systems State-Of-The-Art

Downloaded from orbit.dtu.dk on: Oct 05, 2021 Overview of Mobile Flywheel Energy Storage Systems State-Of-The-Art Dagnæs-Hansen, Nikolaj A.; Santos, Ilmar Published in: Proceedings of 13th SIRM: The 13th International Conference on Dynamics of Rotating Machinery Publication date: 2019 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Dagnæs-Hansen, N. A., & Santos, I. (2019). Overview of Mobile Flywheel Energy Storage Systems State-Of- The-Art. In Proceedings of 13th SIRM: The 13th International Conference on Dynamics of Rotating Machinery (pp. 282-294). Technical University of Denmark. General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. SIRM 2019 – 13th International Conference on Dynamics of Rotating Machines, Copenhagen, Denmark, 13th – 15th February 2019 Overview of Mobile Flywheel Energy Storage Systems State-Of-The-Art Nikolaj A. Dagnaes-Hansen 1, Ilmar F. Santos 2 1 Fritz Schur Energy, 2600, Glostrup, Denmark, [email protected] 2 Dep. of Mech. Engineering, Technical University of Denmark, 2800, Kgs. Lyngby, Denmark, [email protected] Abstract The need for low cost reliable energy storage for mobile applications is increasing. One type of battery that can potentially solve this demand is Highspeed Flywheel Energy Storage Systems. These are complex mechatronic systems which can only work reliably if designed and produced based on interdisciplinary knowledge and exper- tise. This paper gives an overview of state-of-the-art flywheel systems through graphs, tables and discussions. Key performance indicators, technologies, manufacturers, and research groups are presented and discussed. The focus is put on energy density and power of the flywheel systems and on the magnetic bearing technology used to obtain the best performance. 1 Motivation A crucial component of any electrical grid is energy storage. It is used to smooth out fluctuations in power demand and supply, especially in the case of renewable energy sources such as solar cells and wind turbines. Smaller electrical grids, called micro-grids are found in vehicles such as cars and ships. Here, the demand for higher capacity energy storage is increasing due to the growth in demand of electric and hybrid vehicles. When dealing with energy storage in transportation, the key performance indicator is the specific energy density J e [ kg ]. If the system is to function, not only for energy storage, but also as peak shaver, the specific power density W p [ kg ] must also be regarded. When it comes to a Flywheel Energy Storage System (FESS), the stored kinetic energy is proportional to flywheel mass moment of inertia and the square of flywheel rotational speed. For a modern high-speed FESS, the energy is sought to be increased by maximising rotational speed rather than flywheel size and mass. In this way, power and energy densities are also maximised. The limitations of rotational speed are related to the following: - For high rotational speeds, the centripetal stresses will at some point cause the flywheel to burst. The speed limit is thus dictated by the maximum tensile strength of the flywheel material. - The high rotational speed also leads to an undesirable large amount of friction between the flywheel surface and the surroundings. This necessitates the use of a vacuum environment which complicates the use of conventional hydrodynamic and ball bearings due to the vaporisation of bearing lubricant. Furthermore, limitations related to stability, damping, and friction make conventional bearings unsuitable if FESS is to compete with electrochemical batteries on energy density as well as efficiency and reliability. 2 Literature Review The above listed limitations have until recently caused FESSs to be inferior to electrochemical batteries. However, recent technological advancements within lightweight fibre composites with large tensile strength, motor/generators, and Active Magnetic Bearings (AMBs) have now enabled an increase in rotational speed and consequently high energy and power densities. A sketch of a modern FESS can be seen in Fig. 1. AMBs first got into the spotlight of FESS applications in the 1960s with the introduction of high-strength fibre composite flywheels that dramatically improved the stress limitations and consequently the limit on rotational speed [15] [32] [71]. This led to the first reports on magnetically suspended flywheels in the start 1970s where aerospace agencies such as NASA among others found them relevant [84] specifically for attitude control [87] [40] [82] [77], energy storage [72], and the combination of the two [62]. Although the above references are primarily focused on experimentally demonstrating the concept, it was from the beginning acknowledged that the 282 Axial magnetic bearing Radial magnetic bearing Motor/generator Rotor Rotor fibre composite hub Vacuum enclosure Figure 1: Modern high-speed FESS. whole system dynamics must be accounted for when designing AMBs: ”Unlike ball bearings, magnetic bearings are an intimate part of the overall design, and cannot be specified in terms of simple mechanical interfaces” [77]. This is because AMBs use closed-loop control of electromagnetic coils which, as opposed to conventional bearings, introduce additional degrees of freedom into the system. To begin with, however, simple spring-mass-damper models were used to design the AMBs. Through the years, the models improved to more advanced rotor-bearings models accounting for flexible modes and more advanced AMB controls [23]. Consequently, FESS performance has been continuously increasing to a point where too high stresses in the flywheel material again becomes the limiting factor. Experts are looking towards the potential of using new flywheel materials such as graphite in order to further increase rotational speed [10] [75]. Currently, FESSs with energy densities up to 80 Wh/kg have been manufactured [20]. In comparison, modern lithium-ion batteries can contain up to approximately 180 Wh/kg [6]. An overview of the specific energy and power density in different FESSs found in the literature can be seen in Table 1 accompanied by Fig. 2. Items 1 and 2 in the table are both Integrated Power and Attitude Control Systems (IPACSs) from NASA, which function as combined energy storage and attitude control for satellites. Items 3-15 are university prototypes (PT). Items 16-22 are commercially available Uninterruptible Power Sources (UPS) and large scale energy storage systems. Items 23-26 are Kinetic Energy Recovery Systems (KERS) for race cars. Item 27 is also aimed for vehicle applications with a highly repetitive duty-cycle such as excavators and commuter trains or buses. As seen, Li-Ion batteries still have higher energy and power densities than FESSs. However, the flywheels have now reached levels of same orders of magnitude as the electrochemical batteries, which make them a lucrative alternative due to numerous advantages which are listed by the FESS vendor Calnetix in [41] and included here as Table 2. Successful tests of 112,000 discharge cycles are carried out in [8] and the round-trip efficiency is reported as high as 85-95% [14]. In [31], the self-discharge time constant for a 500 Wh FESS is reported to be several days. The NASA G3 IPACS from Table 1 has a total parasitic loss of around 80 W when going full speed storing 2136 Wh. The FESS examples from Table 1 form only a fraction of all the actors currently involved with the technology – a list of 27 different manufacturers and 28 different research groups are given in [39]. The applications are both stationary systems such as the UPS systems seen in Table 1, storage for renewable energy sources, and mobile systems. The mobile systems are in particular challenging to design due to vehicle movements, especially because they cause large gyroscopic effects. The gyroscopic loads are proportional to the mass moment of inertia, the rotational speed, and the tilting rate of the flywheel rotational axis, and they will thus inherently be large when a high speed, high inertia flywheel is subject to movements. The flywheel will strongly resist any tilt motions and will reciprocate large forces through the bearings and the rest of the system. This is utilised for satellite attitude control in the IPACS shown in Table 1, but are otherwise regarded as unwanted and are significantly increasing the required load capacity of the bearings. This means that mobile FESSs usually comprise a suspension made of conventional bearings. This is, for example, the case for the KERSs seen in Table 1. Magnetic suspension is, however, needed if FESSs are to compete on life-time, efficiency, and energy density. As seen for stationary and satellite FESS systems, the proper design of AMBs is based on proper determination of forces and dynamics through rotor-bearing models of varying complexity. This is also the case for a mobile FESS although here, the movement of the foundation (base motions) might have to be accounted for. This includes dynamics of bodies such as the housing as well as accelerations of the vehicle/vessel from e.g. manoeuvring or outer perturbations. The following gives an overview of previous work related to this modelling problem. 283 Wh Wh W W Name Manufacturer e kg er kg p kg pr kg Ref.

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