Mechanical energy storage (or electromechanical energy storage) systems are devices which convert electrical energy into kinetic or potential energy which can be reconverted into electricity at a later stage. Mechanical energy storage systems can be used in the grid to balance peak periods and to provide ancillary services including frequency, primary and voltage control to the power grid. The main technologies include pumped hydro energy storage (PES), flywheels, compressed air energy storage (CAES), and liquid air energy storage (LAES).
PES: This technology comprises two reservoirs at different elevations connected by either pipes or tunnels and exploits the potential energy of water to store energy. In those systems, water is pumped to the upper reservoir during periods of low electricity demand when there is an excess of electricity available and released to power turbines when more electricity is needed. The efficiency of this system is estimated to be between 70-85% .
Flywheels: they store kinetic rotational energy through a rotor which is charged and discharged electrically using a dual-purpose motor/generator. Flywheels are characterized by fast response times, long cycle life, and high power density. The most common applications include transportation, frequency regulation and stability in electric grids, balancing fluctuations in renewable generation and peak management. The storage capacity of the flywheel is proportional to the mass of the rotor, which requires making these systems very heavy (e.g., metallic or composite materials) .
CAES: this technology stores electrical energy in the form of compressed air during periods of excess electricity or low energy demand. In these systems, air is stored in underground mines or caves created inside salt rocks. To reconvert the pressurized air into electricity two main technologies can be used which differentiate the two main CAES typologies: diabatic CAES (D-CAES) and adiabatic CAES (D-CAES) . In systems belonging to the first typology, the pressurized air is fed into a combustion chamber using natural gas or fuel and then expanded into a turbine to generate electricity. The efficiency of these systems can be estimated around 55% . On the other hand, in A-CAES systems no combustion chamber is used. In this case, the heat produced during the compression is recovered and stored as thermal energy and used to increase the temperature of the air before the expansion in the turbine. Because of the heat recovered, the efficiency of these systems can exceed 70%. CAES is a technology suitable for peak shaving, load shifting, or for stabilising the grid through frequency regulation, contingency reserves, voltage support, or black start .
LAES: this technology is similar to the concept of an adiabatic CAES yet, in this case, the pressurized air (or nitrogen) is liquefied using a Linde or Claude process and then stored in an unpressurized vessel. The main advantage compared to CAES is the higher energy density of the liquefied air (or nitrogen) which is then evaporated and expanded in a turbine when electric power is needed. LAES can be also combined with waste heat and waste cold sources to increase the process efficiency. However, also in a stand-alone configuration, the heat of compression and the cold released before the expansion in the turbine can be recovered to improve both the liquefaction and discharging process reaching a round trip efficiency of around 60%. LAES can be integrated into the grid with the same functionality as CAES with the ability to provide also cold thermal energy in a co-generation asset .
Source Borri E, Tafone A, Zsembinszki G, Comodi G, Romagnoli A, Cabeza LF. Recent Trends on Liquid Air Energy Storage: A Bibliometric Analysis. Applied Sciences. 2020; 10(8):2773. https://doi.org/10.3390/app10082773
Another technology is pumped heat electrical storage (PHES) which is similar to PES, but in this case, heat is pumped between two thermal storages at different temperatures using a reversible heat pump/heat engine. However, this technology is in the development stage.
Each technology has a different level of maturity [1,3,5]:
PES is already a mature technology with TRL9 and more than 170 GW already installed worldwide.
Flywheel is a technology already available on the market (TRL9). Nevertheless, some improvements are needed to reduce the cost of manufacturing and be competitive with other storage solutions.
CAES: Diabatic CAES can be considered mature at TRL9 with two plants already operative, one located in McIntosh, US (110 MW) commissioned in 1991, and another in Huntorf, Germany (320 MW) commissioned in 1978. Regarding adiabatic CAES, two commercial facilities were launched in 2019 in Feicheng, Shandong, China (1250 MW) and in Goderich Ontario Canada (1.75 MW).
LAES: unlike the CAES, this technology is at commercial/demonstration stage (TRL 7-8). In 2018, Highview Power  started to operate a grid-scale 5MWe/15 MWh LAES plant located in Bury in the UK. Nevertheless, commercial plants are currently being developed by the company in UK and other countries.