# A Review of Flywheel Energy Storage System Technologies and Their Applications
## 1 Introduction  
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 [3]; however, there are major challenges associated with energy supply coming from renewables, due to their intermittent natureacross a range of timescales [4]. 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 [3]. 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 [5]. 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 [8]. 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 [13]. 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.