Development of a High-Fidelity Model for an Electrically Driven Energy Storage Flywheel Suitable for Small Scale Residential Applications

Mustafa Amiryar, Keith Pullen, Daniel Nankoo
2018 Applied Sciences  
Energy storage systems (ESS) are key elements that can be used to improve electrical system efficiency by contributing to balance of supply and demand. They provide a means for enhancing the power quality and stability of electrical systems. They can enhance electrical system flexibility by mitigating supply intermittency, which has recently become problematic, due to the increased penetration of renewable generation. Flywheel energy storage systems (FESS) are a technology in which there is
more » ... ering interest due to a number of advantages offered over other storage solutions. These technical qualities attributed to flywheels include high power density, low environmental impact, long operational life, high round-trip efficiency and high cycle life. Furthermore, when configured in banks, they can store MJ levels of energy without any upper limit. Flywheels configured for grid connected operation are systems comprising of a mechanical part, the flywheel rotor, bearings and casings, and the electric drive part, inclusive of motor-generator (MG) and power electronics. This contribution focusses on the modelling and simulation of a high inertia FESS for energy storage applications which has the potential for use in the residential sector in more challenging situations, a subject area in which there are few publications. The type of electrical machine employed is a permanent magnet synchronous motor (PMSM) and this, along with the power electronics drive, is simulated in the MATLAB/Simulink environment. A brief description of the flywheel structure and applications are given as a means of providing context for the electrical modelling and simulation reported. The simulated results show that the system run-down losses are 5% per hour, with overall roundtrip efficiency of 88%. The flywheel speed and energy storage pattern comply with the torque variations, whilst the DC-bus voltage remains constant and stable within ±3% of the rated voltage, regardless of load fluctuations. Energy demand continues to increase with high growth rates, which would increase energy prices if there is no change from use of traditional generation methods. Pressures to limit carbon dioxide emissions, market deregulation, as well as power quality problems, in turn add to aggravating the issue of imbalances between generation and demand [2, 3] . Potential distributed generation (DG) and renewable energy sources (RES) can be considered as replacements or supplements for traditional generation methods [3] . Yet there are major obstacles linked to energy being supplied from renewables, due to their intermittent nature across a range of timescales [4] . Their availability is always subject to weather conditions, and there are annual, seasonal, and monthly fluctuations in RES supply which may not match the periods of energy demand [3, 4] . Therefore, ESS are a vital necessity to supplement intermittent RES for integration into the electrical network and meet the excessive demand by aggregating the traditional generating plants [5] . There is a high demand for cost effective, reliable, long lasting, and environmentally friendly energy storage systems to support a variety of applications in modern electrical networks. There have been significant developments in flywheel technology due to advances in materials technology, power electronics, and bearings [6, 7] . High energy efficiency flywheels that also possess high power and energy densities, vie with other electrical power storage solutions, as well as in the transportation, military and space satellite industry sectors [8] . With power ranges from kW to GW, and up to 500 MJ storage capability per machine, they are able to provide solutions for many significant energy storage applications in an electrical power system [8, 9] . In electrical energy storage applications, flywheels are widely used for uninterruptible power supplies, as well as for improving power quality problems [10] [11] [12] . As these applications require a high number of cycles per day, the electrochemical battery suffers from insufficient cycle life and is highly mismatched [13] . There has been a growing need for storing energy in residential premises, which is attractive for providing arbitrage type storage for buildings with solar panels. Most of these are connected to a stable grid and the incentive for the consumer is to have storage behind the meter, which reduces energy bills. The energy storage system of choice for residential photovoltaic (PV) systems is more typically an electrochemical battery, whereas flywheels have not been widely tested nor considered for this application. Such batteries are typically Li-ion or lead acid chemistries promoted very publicly by companies such as Tesla and several others [14] . However, there is an issue of degradation in hotter countries, where the combination of ambient temperature and heat generated within the battery can lead to reduced life guarantees as evidenced by their warranties [15] . In addition, there are many countries where the grid is highly unreliable, so when the grid fails, the power ramp up must be in milliseconds, effectively similar to the requirements of a UPS system. Diesel generators are also commonly installed in residences in countries with intermittent grids, and this changes the storage system requirements to one of increased number of cycles. A battery is hence less favourable for these applications as compared to their implementation in developed countries. Of course, the batteries are mass produced, and are currently more cost effective than flywheels, but flywheels will still be a viable choice when it comes to numerous cycles per day. A battery will be more comfortable providing storage for only 1 cycle per day if it is managed carefully, both electrically and thermally, so that its depth of discharge is kept low. This can be done by oversizing the storage, so it will be able to last for 10 years (3650 cycles). However, this would require energy storage to be sized at 2-5 times the required capacity, to reduce depth of discharge leading to higher costs. In examples where batteries are installed for solar arbitrage, cycles are, on average, less than 1 per day, and depth of discharge may be low, since not all days are fully sunny. Supercapacitors, with a capital cost more or less the same as flywheels [1], have also been tested for the same applications. However, they have a relatively low lifetime, reaching a maximum 12 years [3] . To reduce cost and minimise capacity, it will be advantageous to use the storage system many times in a day. The flywheel system can also take part in the time shifting of electrical demand, and even fed into the grid when there is a high demand. This paradigm of energy storage will find more interest and will be greatly enhanced when time of use (ToU) tariffs are applied. Appl. Sci. 2018, 8, 453 4 of 29 rotational stress [24] . In 1960s and 1970s, FESS was proposed for electric vehicles, stationary power backup, and space missions [9, 10] . In the following immediate years, fibre composite rotors were built and tested. In 1980s, relatively low speed magnetic bearings started to appear [25] . Although flywheels have been improved for use in different areas, their utilisation as energy storage systems deteriorated with the expansion and improvements of the electrical grid. However, recent major developments in materials, bearing systems, power electronics, and development of high speed electric machines have enabled flywheels to perform energy storage applications and emerge as a promising technology competing with many other storage systems [7] [8] [9] 26, 27] . A flywheel is a mechanical storage system operating on the principal of a rotating mass to store energy. It converts the electrical energy into mechanical energy and stores it as rotational kinetic energy. The flywheel is charged by extracting electrical energy from an available source to accelerate the rotor speed and accumulate energy. It is discharged by delivering the stored energy back into its electrical form. The operation of the flywheel is controlled by an electrical machine functioning as a motor-generator (MG) to perform the energy conversion between electrical and mechanical forms [28, 29] . The flywheel and the MG are coupled on the same shaft, which enables the FESS to be controlled by the operation of the MG [30-33]. Structure and Components of FESS: Priciples and Components of FESS FESS consists of a spinning rotor, MG, bearings, power electronics interface, and containment or housing. A schematic of a typical flywheel system suitable for ground based power applications is shown in Figure 1 . Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 27 composite rotors were built and tested. In 1980s, relatively low speed magnetic bearings started to appear [25] . Although flywheels have been improved for use in different areas, their utilisation as energy storage systems deteriorated with the expansion and improvements of the electrical grid. However, recent major developments in materials, bearing systems, power electronics, and development of high speed electric machines have enabled flywheels to perform energy storage applications and emerge as a promising technology competing with many other storage systems [7] [8] [9] 26, 27] . A flywheel is a mechanical storage system operating on the principal of a rotating mass to store energy. It converts the electrical energy into mechanical energy and stores it as rotational kinetic energy. The flywheel is charged by extracting electrical energy from an available source to accelerate the rotor speed and accumulate energy. It is discharged by delivering the stored energy back into its electrical form. The operation of the flywheel is controlled by an electrical machine functioning as a motor-generator (MG) to perform the energy conversion between electrical and mechanical forms [28, 29] . The flywheel and the MG are coupled on the same shaft, which enables the FESS to be controlled by the operation of the MG [30-33]. Structure and Components of FESS: Priciples and Components of FESS FESS consists of a spinning rotor, MG, bearings, power electronics interface, and containment or housing. A schematic of a typical flywheel system suitable for ground based power applications is shown in Figure 1 .
doi:10.3390/app8030453 fatcat:7jxxdukzgvehdlhtuteuw5rf4y