Renewable energy plays a key role in the journey to net zero carbon emissions, helping to reduce the demand for fossil fuels by providing cleaner sources of energy.
But as the world derives an increasing amount of its electricity from these renewable energy sources, there’s a growing need for technologies that can capture and store it.
Renewable energy generation mainly relies on naturally-occurring factors – hydroelectric power is dependent on seasonal river flows, solar power on the amount of daylight, wind power on the consistency of the wind – meaning that the amounts being generated will be intermittent.
Similarly, the demand for energy isn’t constant either, as people generally tend to use different amounts of energy at different times of the day and the year.
So, when the amount of renewable energy being generated is greater than what’s needed, it makes sense to store that excess energy so it can be used at a time when the demand exceeds the generation.
Unlike fossil fuels, renewable energy creates clean power without producing greenhouse gases (GHGs) as a waste product. By storing and using renewable energy, the system as a whole can rely less on energy sourced from the more greenhouse-gas emitting fuels like coal, natural gas or oil.
A key benefit of being able to store this energy is that it helps to prevent renewable resources from going to waste.
There are times when the amount of electricity being generated by renewables can exceed the amount that’s needed at the time. When this happens, some renewable generators may need to curtail their outputs in order to help the system remain ‘balanced’ – i.e. when electricity supply meets demand – meaning that an opportunity to generate clean electricity has essentially gone to waste.
Energy storage allows these renewable energy resources to continue to generate electricity even if it’s not needed at that particular time, as it can be stored until a later time when it’s needed.
Energy storage technologies work by converting renewable energy to and from another form of energy.
These are some of the different technologies used to store electrical energy that’s produced from renewable sources:
Pumped hydroelectric energy storage, or pumped hydro, stores energy in the form of gravitational potential energy of water. When demand is low, surplus electricity from the grid is used to pump water up into an elevated reservoir. When demand increases, the water is released to flow down through turbines to a lower reservoir, producing hydroelectric power for the grid as it does so.
Electrochemical batteries store energy by separating positive and negative charges in rechargeable cells. Different types of electrochemical battery storage technology include:
Lithium-ion battery storage
Government and developers are investing substantially in the creation of huge lithium-ion batteries to store energy for times when supply outstrips demand. Lithium battery technologies are diverse to address custom needs for flexibility, modularity, and size, as well as being relatively inexpensive. However these batteries do degrade over time and present unique fire management challenges.
The world’s largest battery energy storage system so far is Moss Landing Energy Storage Facility in California. The first 300-megawatt lithium-ion battery – comprising 4,500 stacked battery racks – became operational at the facility in January 2021.
Flow battery storage
Flow batteries’ cells consist of two charged liquids separated by a membrane. Surplus electrical energy is used to ‘reduce’ the liquid charge state of one and ‘oxidise’ that of the other to efficiently store energy. The process is then reversed to recover electricity with low loss.
This flowing reduction-oxidation operation – known as ‘redox flow’ – allows the batteries to store large amounts of energy for long durations and be cycled many times without degradation. However, they do have a relatively large project footprint.
While not limited to renewable energy, storing excess energy as heat for the longer term is a huge opportunity for industry, where most of the process heat that’s used in food and drink, textiles or pharmaceuticals comes from the burning of fossil fuels.
Liquifying rock or superheating sand and water mixtures can be used to store thermal energy. Thermal energy storage technologies include:
Liquid-to-air transition energy storage
Surplus grid electricity is used to chill ambient air to the point that it liquifies. This ‘liquid air’ is then turned back into gas by exposing it to ambient air or using waste heat to harvest electricity from the system. The expanding gas can then be used to power turbines, creating electricity as needed.
Thermal sand batteries
Finnish researchers have developed and installed the world’s first fully working ‘sand battery’, which can store power for months at a time. Using low-grade sand, the device is charged up with heat made from cheap electricity from solar or wind. The sand stores the heat at around 500°C, which can then warm homes in winter when energy is more expensive.
This type of energy storage converts the potential energy of highly compressed gases, elevated heavy masses or rapidly rotating kinetic equipment.
Different types of mechanical energy storage technology include:
Compressed air energy storage
Compressed air energy storage has been around since the 1870s as an option to deliver energy to cities and industries on demand. The process involves using surplus electricity to compress air, which can then be decompressed and passed through a turbine to generate electricity when needed.
This type of storage system can be used in conjunction with a wind farm, pulling in air and creating a high-pressure system in a series of enormous underground chambers. When wind speeds slow down or demand for electricity increases, the pressurised air is discharged to power turbines or generators.
A ‘gravity battery’ works by using excess electrical energy from the grid to raise a mass, such as a block of concrete, generating gravitational potential energy. When electrical energy is required, the mass is lowered, converting this potential energy into power through an electric generator.
Pumped-storage hydroelectricity is a type of gravity storage, since the water is released from a higher elevation to produce energy.
Flywheel energy storage
Flywheel energy storage devices turn surplus electrical energy into kinetic energy in the form of heavy high-velocity spinning wheels. To avoid energy losses, the wheels are kept in a frictionless vacuum by a magnetic field, allowing the spinning to be managed in a way that creates electricity when required.
This technology has several advantages over conventional energy storage systems, such as direct electrical generation through contactless induction, little maintenance, long life, and few environmental effects.
Pumped heat electrical storage
Pumped heat storage uses surplus electricity to power a heat pump that transports heat from a ‘cold store’ to a ‘hot store’ - similar to how a refrigerator works. The heat pump can then be switched to recover the energy, taking it from the hot store and placing it in the cold store. This produces mechanical work, which is used to power a generator.
One of the benefits of this system is that it reacts considerably faster than other storage systems, taking action within minutes.
Hydrogen electrolysis produces hydrogen gas by passing surplus electrical current through a chemical solution. This hydrogen gas is then compressed to be stored in underground tanks. When needed, this process can be reversed to produce electricity from the stored hydrogen.
Hydrogen can be physically stored as either a gas or liquid and even adhered directly to solids. As a gas, hydrogen storage requires high-pressure tanks, while liquid hydrogen requires storage at cryogenic temperatures to prevent it boiling back into a gas. Hydrogen may also be stored on the surface of solid materials (known as adsorption), or within them (known as absorption).
Underground hydrogen storage technology is also being developed that can re-infuse the geology of the earth to safely store large volumes of green hydrogen.
Last updated: 26 Jun 2023
The information in this article is intended as a factual explainer and does not necessarily reflect National Grid's strategic direction or current business activities.