The most traditional of all energy storage devices for power systems is electrochemical energy storage ees which can be classified into three categories primary batteries secondary batteries and fuel cells the common feature of these devices is primarily that stored chemical energy is converted to electrical energy the main attraction of the process is that its efficiency is not carnot limited unlike thermal processes primary and secondary batteries utilise the chemical components . Iet digital library this title is available electronically in the iet digital library energy storage for power systems 2nd edition by ag ter gazarian the supply of energy from primary sources is not constant and rarely matches the pattern of demand from consumers electricity is also difficult to store in significant quantities therefore secondary storage of energy is essential to . Secondary energy storage systems can accept energy generated by a power system convert it to a form suitable for storage keep it for a certain time and then convert it into the form required by the consumer when it is needed these systems are an essential tool in managing energy supplies. Electrical energy storage systems eess for electrical installations are becoming more prevalent eess provide storage of electrical energy so that it can be used later the approach is not new eess in the form of battery backed uninterruptible power supplies ups have been used for many years eess are starting to be used for other purposes. Energy storage for power systems 3rd edition by andrei g ter gazarian unregulated distributed energy sources such as solar roofs and windmills and electric vehicle requirements for intermittent battery charging are variable sources either of electricity generation or demand