Here's what you need to know:
- Energy storage has emerged as a critical technology in the UK’s transition to a low carbon energy system.
- As the generation mix shifts towards more renewable sources, energy storage will play an important role in stabilising the grid.
- There are different types of energy storage including mechanical, electrochemical, chemical, thermal and electromagnetic storage.
Inertia – the tendency of moving objects to remain in motion – has long been a critical component of power systems. In the context of the electrical grid, inertia from rotating electrical generators in fossil fuel-fired power plants represents a source of stored energy that can be utilised to provide a time buffer for quick start generators (or load resources) to detect and respond to failure. Alternative energy sources such as wind and solar do not use such generators and therefore, grids with high penetrations of renewable energy can have stunted inertia levels.
Energy storage has emerged as a critical technology to help support the increased uptake of renewables whilst also maintaining the grid’s historic reliability. It works to plug in the gaps where energy demand is high, but generation is low, and absorb surplus energy where generation is high, but demand is low. Storage helps smooth out supply/demand disequilibria, making our grid systems more flexible and, by extension, more reliable.
Energy can be stored in a variety of ways, with the five overarching categories being:
Mechanical storage covers Pumped Hydro Storage (PHS), Compressed Air Energy Storage (CAES) and Flywheel Energy Storage Systems (FESS). PHS utilises the force of gravity to generate energy on demand. Put simply, water is pumped to an elevated reservoir behand a dam until needed, at which point it is then discharged, causing turbines to rotate and generate electricity. When demand (and hence the cost of electricity) is low, the upper reservoir is replenished, meaning the plants can often reach efficiencies greater than 80%. Unsurprisingly, PHS accounts for over 97% of installed global energy storage. CAES plants operate similarly; gas is compressed and stored under pressure in an underground container until electricity is required, at which point the air is heated in an expansion turbine which then drives the power-producing generator. However, it is worth noting that only two commercial CAES plants have ever been built. Finally, when short-term back up is required, FESS spins a rotor inside a nearly frictionless enclosure, converting kinetic energy into electricity. This is best suited to fast response and short duration roles; FESS involve low energy densities and high associated security costs.
Electrochemical energy storage essentially describes the use of batteries as storage. Conventional batteries include lead acid, nickel-cadmium (NiCd) and lithium ion. Lead acid batteries are rechargeable; they submerge lead in sulphuric acid, causing a reversible chemical reaction that produces electricity. Due to the finite number of ions in the acid, the charge level deteriorates over time, meaning this form of storage is best for shorter, more powerful energy applications. Similarly, NiCd batteries utilise the reaction between a cathode (containing nickel oxide-hydroxide) and an anode (containing metallic cadmium) in a reaction that is reversible. Its robustness and low maintenance means it’s often used in harsh environments, but its limited energy and power performance means other technologies are often preferred. Lithium-ion is perhaps the most well-known battery, and again utilises the chemical reaction between a lithium cathode and a carbon anode to store energy. They have become increasingly popular due to decreasing costs of lithium, their relative low weight and the ease at which they can be recharged.
Chemical energy storage uses hydrogen and synthetic natural gas (SNG) to generate fuel for both use in conventional power plants and as a means of transport. It does rely on electrical energy (meaning efficiency is generally lower than PHS or electrochemical storage); however, the use of hydrogen as an energy carrier is generally preferable to traditional fossil fuels.
Thermal energy storage (TES) stores excess heat for later use in heating/cooling applications and power generation. It can commonly be classified within three groups: sensible (where a material’s temperature is raised/lowered), latent (where the phase of a material is changed i.e. solid to liquid) and through thermo-chemical reactions. It is most widely used within buildings and industrial processes.
Electromagnetic energy storage typically comes in the form of electrical double-layer capacitors (EDLCs) or superconducting magnetic energy storage (SMES). EDLCs do not rely on chemical reactions, meaning degradation is minimal, and expected lifetime exceeds 1 million cycles. However, their low energy density (around 1% of a typical Li-ion battery) makes them impractical for high energy applications. SMES uses the flow of direct current through a superconducting coil to store energy within a magnetic field, resulting in a short reaction time and high power provision over short periods. However, this means operating timescales are particularly short, and so their application is not yet suited to large scale commercial use.
Energy storage can come in a variety of forms depending on its purpose, scale and cost-requirements. Worldwide, investment in energy storage is expected to reach USD $23.38 bn in 2026 and, as the market continues to grow, current technologies are likely to become more efficient and accessible.
