power quality but lack storage capacities to be used in larger applications. These constraints are a natural limitation to the storage's applicability. Several studies have developed interest and investigated the suitability or selection of the optimal energy storage for certain applications. Literature surveys comprise the available information of the state-of-the-art and compare the storage's uses based on current existing projects. Other studies take a step further in evaluating energy storage with each other and rank their fitness based on multiple-criteria decision analysis. Another paper proposed an evaluation scheme through the investigation and modelling of storage as equivalent circuits. An indexing approach has also been suggested in a few studies, but is still in the novel stages. In order to gain increased economic potential of grid connected energy storage systems, it is of interest to consider a portfolio with several services for one or more applications for an energy storage system. By doing so, several revenue streams can be achieved by a single storage and thereby also increasing the degree of utilization. To mention two examples, a combination of frequency response and reserve services is examined in, meanwhile load peak shaving together with power smoothing is considered in. Forms Air Compressed air One grid energy storage method is to use off-peak or renewably generated electricity to compress air, which is usually stored in an old mine or some other kind of geological feature. When electricity demand is high, the compressed air is heated with a small amount of natural gas and then goes through turboexpanders to generate electricity. Compressed air storage is typically around 60–90% efficient. Liquid air Another electricity storage method is to compress and cool air, turning it into liquid air, which can be stored, and expanded when needed, turning a turbine, generating electricity, with a storage efficiency of up to 70%. A commercial liquid-air energy storage plant is under construction in the North of England, with commercial operation planned for 2022. The energy storage capacity of 250MWh of the plant will be nearly twice the capacity of the world's largest existing lithium-ion battery, the Hornsdale Power Reserve in South Australia. Since 2022 the Italian Company Energy Dome has run an almost similar pilot of 4 MWh on Corsica with not the use of (liquid) air but only . While discharging, the is stored in a dome. Batteries Battery storage was used in the early days of direct current