Electricity Storage via Mechanical Technologies

Production and Distribution of Electricity and Heat

Renewable energy sources such as wind and solar energy are not always available (dispatchable). Therefore, renewable electricity and heat need to be stored to meet the end-user demand. Energy storage is currently a key technology cluster because enables electricity and heat to be stored (electricity storage and heat storage, respectively) and made available on demand. However, electricity cannot be stored as it is and needs to be transformed in another form of energy to be stored. Accordingly, energy (electricity) storage systems (ESS) are usually referred to as to the form in which energy is stored (chemical, electrochemical, mechanical, thermal storage). A wide range of energy storage technologies exists, but no technology is suitable for all applications. ESS range from large-size, long-term storage (e.g., pumped-hydro storage, compressed-air or gas storage) to small-size, short-term storage (e.g. flywheels, electrochemical capacitors). With the sole exception of pumped hydro storage, all ESS technologies still have modest deployment and require additional R&D to reduce costs and exploit their full potential. Nevertheless, ESS are set to play a growing role in the transition to a sustainable energy systems as they allow renewable electricity and heat to meet the variable end-use demand, add flexibility to the energy system, facilitate the integration of distributed generation and electro-mobility in the electrical grids, and support regulation and reserve capacity; not least, they may be used in combined devices (e.g. heat pumps with heat storage for residential service). In Italy, investors and consumers interest in small-size ESS is constantly increasing while large-size ESS (i.e., a significant pumped hydro power capacity) is traditionally used by utilities for power grid regulation and management (i.e., electricity supply-demand balance). Key parameters for ESS are energy storage capacity (size) and energy density, charge and discharge power and time, allowable storage time, energy losses during charging, storage and discharging phased (storage cycle efficiency), life span (allowable number of charge/discharge cycles), and costs (investment and operation costs). For instance, ESS with high storage loss (parasitic loss) are suitable to short-term storage (e.g. power quality and regulation), while ESS with low parasitic loss are suitable to long-term storage. This paper deals with the Mechanical ESS such as pumped hydro storage, compressed (or liquid) air energy storage, and flywheels. Pumped hydro is currently the most used and mature electricity storage technology in which the electricity that exceeds demand (e.g. overnight wind electricity) is used to pump water from a downstream basin up to an upstream reservoir. It is mostly used in regions with proper orography (e.g., Italy, France) and significant hydro-power capacity. Pumped hydro currently accounts for 94% of the total electricity storage capacity worldwide and is suitable for large energy storage, as well as for reserve and modulation purposes. Compressed air energy storage (CAES) also allow for large-size energy storage but are much less used while flywheels are mostly used for small- size, quick-response systems. In Italy, the pumped hydro systems account for about 6.3% of the installed power capacity and for about 32% of the hydropower capacity.

25-07-2022

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