Energy storage systems (ESS) can be used to balance electrical energy supply and demand. The process involves converting and storing electrical energy from an available source into another form of energy, which can be converted back into electrical energy when needed. The forms of energy storage conversion can be chemical, mechanical, thermal, or magnetic [1,2]. ESS enable electricity to be produced when it is needed and stored when the generation exceeds the demand. Storage is beneficial when there is a low demand, low generation cost, or when the available energy sources are intermittent. At the same time, stored energy can be consumed at times of high demand, high generation cost, or when no alternative generation is available [1,2,3,4].
Energy demand continues to increase, as demanded by the households and industries with high growth rates in BRIC and developing countries. This has led to increases in energy prices and traditional energy generation methods are less able to adapt, exacerbating the issues due to market deregulation, power quality problems, and pressures to limit carbon dioxide emissions [2,3]. Renewable energy sources (RES) and potential distributed generation (DG) are considered as supplements or replacements for traditional generation methods ; however, there are major challenges associated with energy supply coming from renewables, due to their intermittent nature across a range of timescales . At a time when RES are supplying energy, there may be low demand, but when the energy is demanded, it may exceed RES energy production . Also, there are monthly, seasonal, and annual fluctuations in RES supply, as their availability is always subject to weather conditions. On the other hand, the energy demand differs from time to time, which does not necessarily match the intermittences of RES, thus creating reliability problems [3,4]. Therefore, ESS are a vital necessity to aggregate traditional generating plants in order to meet an excessive demand, and supplement intermittent RES for their integration into the electrical network .
As a counterpart to today’s electrical network, there is a high demand for reliable, cost-effective, long lasting, and environmentally sound energy storage systems to support a variety of energy storage applications. With advances in materials technology, bearings, and power electronics, the technology of flywheels for energy storage has significantly developed [6,7]. Flywheels with the main attributes of high energy efficiency, and high power and energy density, compete with other storage technologies in electrical energy storage applications, as well as in transportation, military services, and space satellites . With storage capabilities of up to 500 MJ and power ranges from kW to GW, they perform a variety of important energy storage applications in a power system [8,9]. The most common applications of flywheels in electrical energy storage are for uninterruptible power supplies (UPS) and power quality improvement [10,11,12]. For these applications, the electrochemical battery is highly mismatched and suffers from an insufficient cycle life, since the number of cycles per day is usually too high . The authors note that this is not necessarily true for some UPS with highly reliable grids, so storage is seldom called upon. Particularly for power quality improvement, electrical disturbances are frequent but short, with the vast majority of them lasting for less than 5 s. Such disturbances are effectively managed by flywheels and offer an improvement over batteries considering the instantaneous response time and longer life cycle of the former. Even with one cycle a day, an electrochemical battery is unlikely to last for even 10 years under these circumstances (3650 cycles). This can only be achieved if the depth of discharge is kept low and the battery is carefully managed, both electrically and thermally. It also requires specifying an energy storage capacity two to five times the required capacity, to reduce the depth of discharge, thus leading to a higher cost. Supercapacitors have been tested for these types of applications; however, with more or less the same capital cost as flywheels , their operational lifetime is relatively low (reaching up to 12 years) . To make more use of such a system and minimise its capacity in order to reduce the cost, it is more useful for the storage system to be used many times a day, to allow for the time shifting of demand and to feed into the grid at times of high demand. Interest in this new paradigm of how energy is used will be greatly enhanced once Time of Use (ToU) tariffs are in place.
A number of reviews of flywheel storage systems have been presented by several papers in the literature. A comparison of energy storage technologies is made in , where a numerical and graphical review demonstrates the improvements and problems associated with FESS. A comparative analysis of energy storage technologies for high power applications is carried out in  and a survey of FESS for power system applications is provided in . The control of high speed FESS in space applications is discussed in . FESS is briefly reviewed in  and an overview of some previous projects is presented in ; however, such sources offer a scarcity of information. The authors of  focus on the developments of motor-generator (MG) for FESS, where the common electrical machines used with flywheels, along with their control, is reported in . A review and simulation of FESS for an isolated wind power system is presented in . This review takes a different approach from earlier work and particularly picks up on very recent literature in what is a rapidly developing subject.
This paper focuses on the description and applications of FESS, providing an overview of some commercial projects for each application. Many of the above papers have provided reviews of FESS, but what is missing in the literature is a comprehensive review of FESS with a description of the applications which are commercially available. Following the introduction, a description of FESS is presented. The main components of FESS, including the rotor, electrical machine, bearings, and flywheel containment are discussed in detail in Section 2. A flywheel’s main characteristics are stated in Section 3 and its applications are described in Section 4. The paper concludes with recommendations for future research in Section 5.
2. Description of Flywheel Energy Storage System
The flywheel as a means of energy storage has existed for thousands of years as one of the earliest mechanical energy storage systems. For example, the potter’s wheel was used as a rotatory object using the flywheel effect to maintain its energy under its own inertia . Flywheel applications were performed by similar rotary objects, such as the water wheel, lathe, hand mills, and other rotary objects operated by people and animals. These spinning wheels from the middle ages do not differ from those used in the 19th or even 20th centuries. In the 18th century, the two major developments were metals replacing wood in machine constructions and the use of flywheels in steam engines. Developments in cast iron and the production of iron resulted in the production of flywheels in one complete piece, with greater moment of inertia for the same space . The word ‘flywheel’ appeared at the beginning of the industrial revolution (namely in 1784). At the time, flywheels were used on steam engine boats and trains and as energy accumulators in factories . In the middle of 19th century, as a result of the developments in cast iron and cast steel, very large flywheels with curved spokes were built. The first three-wheeled vehicle was built by Benz in 1885 and can be named as an example . Over time, several shapes and designs have been implemented, but major developments came in the early 20th century, when rotor shapes and rotational stresses were thoroughly analysed, and flywheels were considered as potential energy storage systems . An early example of a flywheel system used in transport was the Gyrobus, powered by a 1500 kg flywheel, produced in Switzerland during the 1950s . In the 1960s and 1970s, FESS were proposed for electric vehicles, stationary power back up, and space missions [9,10]. In the following years, fibre composite rotors were built and tested. In the 1980s, relatively low-speed magnetic bearings started to appear .
Despite major developments during their early stages, the utilization of flywheels has not been significant and has declined with the development of the electric grid. However, due to the recent improvements in materials, magnetic bearings, power electronics, and the introduction of high speed electric machines, FESS have been established as a solid option for energy storage applications [7,8,9,26,27].
A flywheel stores energy that is based on the rotating mass principle. It is a mechanical storage device which emulates the storage of electrical energy by converting it to mechanical energy. The energy in a flywheel is stored in the form of rotational kinetic energy. The input energy to the FESS is usually drawn from an electrical source coming from the grid or any other source of electrical energy. The flywheel speeds up as it stores energy and slows down when it is discharging, to deliver the accumulated energy. The rotating flywheel is driven by an electrical motor-generator (MG) performing the interchange of electrical energy to mechanical energy, and vice versa [28,29]. The flywheel and MG are coaxially connected, indicating that controlling the MG enables control of the flywheel [30,31,32,33].
2.2. Structure and Components of FESS
FESS consist of a spinning rotor, MG, bearings, a power electronics interface, and containment or housing, which are discussed in detail in the following subsections. A typical flywheel system suitable for ground-based power is schematically shown in Figure 1.