Energy storage as an essential part of sustainable energy systems

Energy supply is an intricate task that provides a reliable energy service to consumers throughout the year. Import dependencies, seasonal differences in energy supply and use, and daily fluctuations in consumption require a sophisticated management of energy resources and conversion, or energy distribution and resource intermittency in order to guarantee continuous energy services throughout all sectors. Therein, energy storage plays a critical role. Energy storage balances the daily fluctuations and seasonal differences of energy resource availability, which results from physical, economical or geo-political constraints. A strongly variable energy demand through day and night also requires energy to be stored in adequate amounts. In particular, short- and mid-term storage levels out or buffers energy output gaps or overflows. Energy is mostly stored in between conversion steps from primary to secondary energy and secondary to final energy. Often rechargeable systems are used to refill the storage capacity when energy demand is low and energy services are not needed. Primary storage such as large crude oil and natural gas storage tanks are essential for the functioning of a country's energy system. Storage of water in reservoirs behind dams is valuable for selling hydropower electricity at the right time or in seasons of increased demand. Secondary or final storage systems, for instance in tanks or in batteries, are crucial for emergency situations, uninterrupted industrial production, long-distance mobility or to secure energy services at home. Storage systems are engineered to hold adequate amounts of mechanical, thermo-physical, electro-chemical or chemical energy for prolonged periods of time. Energy storage systems should be quickly chargeable and should have a large energy storage capacity, but at the same time should also have high rates of recovery and high yields of energy regain. Final energy in factories or households is often stored in tanks as chemical energy in the form of heating oil or natural gas. Thermo-physical energy in the form of steam, hot or cold water, or thermo-oils is also used. For some special applications or for safety reasons energy may be stored electrochemically in batteries or physically in the form of pressurized air. Other storage systems are related to electricity and apply mechanical storage in the form of spinning turbines or flywheels, physical storage in the form of water in reservoirs in highland terrains, or electrostatic storage in super-capacitors. Research is extensive in the area of energy storage since an increase of new renewable energy technologies such as wind and solar is expected to increase fluctuations and deviations from grid parameters. These need too be balanced out using reserve power capacities, grid level power storage capabilities, distributed generation units connected to the grid, and possibly appropriate new grid architectures.