## 1 Introduction  
Energy storage systems (ESS) can provide a viable solution to the problem of balancing electrical energy supply and demand. This is achieved through the process of converting stored electrical energy into one of the forms of thermal, magnetic, mechanical, or chemical energy [1, 2]. ESS enables energy to be stored when the generation exceeds demand, and produced when it is needed. Storage is beneficial when there is low demand, low generation cost, or when available sources of energy are intermittent. Alternatively, the stored energy can be consumed at times of high generation cost, high demand, or when alternative generation is not available [1, 2, 3, 4]. 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, via 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.