## 1 Introduction
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. Due to the advantage of being able to deal with a high number of daily cycles, flywheels offer a much improved lifespan compared to their chemical storage alternatives. Further, for the same rated power, a flywheel will take up much less space than a battery for applications as described where the C rating (power to energy ratios) is high. In addition, flywheels are less sensitive to ambient temperature, and do not need to be in a cooled or heated enclosure. Finally, there is a growing environmental concern when disposing of spent chemical batteries, particularly more difficult in less developed countries. Flywheels do not have this issue. There are several advantages of flywheel energy storage systems (FESS) over chemical storage systems, but a flywheel is likely to incur a higher initial capital cost, although with high longevity, lifetime costs will be lower. As a means of reducing flywheel costs, what is being proposed here is FESS based on laminated steel with a view to overcoming the drawback of the aforementioned high initial costs of a high speed composite based system. Many publications have addressed the mechanical aspects, such as the bearing systems and rotor dynamics of the flywheels. To name a few, rotor design and analysis of composite rotor FESS is discussed in [16]. Authors in [17] present modelling and simulation of the rotor of a composite flywheel at different speeds. Dynamic analysis and control of an active magnetic bearing FESS is studied in [18], and its vibration control for high speed applications is discussed in [19]. A model of a FESS with integrated bearing system is proposed in [20], and a thorough analysis, design, and control of a novel flywheel assisted by hybrid mechanical-magnetic bearings is presented in [21]. A subject area in which there is a scarcity of publications is the modelling and simulation of the electrical characteristics of flywheel systems when used for ground power applications. Particularly, for small scale customer level where flywheels can support residential PV systems, there is not enough information in the literature and there are no commercially available flywheel storage systems for these applications. Most flywheel manufacturers are providing high power fast-response flywheels for power quality and UPS applications. Filling this gap in the literature is necessary, but remains as a challenging task, since availability of practical data for FESS suitable for energy storage applications is very limited.