Solar and wind energy are described as variable forms of renewable energy, variable because the sun doesn't always shine, the wind doesn't always blow. There are also times when there's a gale blowing or lengthy spells of sunshine but electricity demand falls, usually at night. This presents the renewable energy industry with two problems: not enough and too much. Now, however, there are five ingenious ways of storing the excess energy during times of plenty. This stored energy can then be released into the grid when demand is high.
Title photo: The upper reservoir of Ben Cruachan's pump storage scheme in Scotland.
[1] Pump hydroelectricity storage.
We'll begin high on the slopes of Ben Cruachan, a 1,126m high mountain in Argyll, Scotland (see title photo above). It's 17:30 on February 22, 2025. It's half time in the Calcutta Cup rugby match between Scotland and England. Hundreds of thousands of homes are about to turn on kettles to make a cup of tea. The electricity demand on the National Grid is about to go through the roof. At this very moment, the lake pictured behind the dam in the photo above begins to shrink. Someone has pulled the plug at the bottom of the lake, releasing water which plunges down pipes leading through the mountain to an enormous turbine hall hidden in a cavern at the bottom of the mountain.
In the space of just two minutes, the water is spinning four turbines which generate 440 megawatts of electricity which are sent on their way to the grid. No need to turn up the gas-fired power stations output. Cruachan has everything under control. The water leaves the turbine hall to enter Loch Awe, a sea loch adjacent to Ben Cruachan. When the top reservoir is full, Cruachan can operate for 22 hours before the supply of water is exhausted.
10% of the water in the top reservoir is rain water. That won't replenish the reservoir any time soon. Instead, in the dead of night when the rugby celebrations are over and the United Kingdom sleeps, the turbines are put into reverse. Using cheap surplus electricity from the grid, the turbines pump water from Loch Awe back up into the reservoir ready for the next surge in demand. As the UK National Grid becomes more and more dependent on renewable sources of electricity, so pump storage schemes like Cruachan will assume a more vital role than ever.
There are four such schemes in the UK and around 400 worldwide. They account for 90% of long-duration energy storage with a combined installed capacity of 200GW. To store all of the world's surplus renewable energy as more and more solar and wind farms come on stream, nearer 4000 pump storage schemes are going to be needed. The problem is building reservoirs and associated infrastructure is expensive. Add to that the scarcity of locations which are suitable - high reservoir, low lake. But technology is fast coming up with alternatives.
[2] Electrochemical battery energy storage.
Batteries were always going to play some part in energy storage. The challenge has been to scale them up to store huge quantities of surplus electricity from solar and wind farms. Lithium-ion batteries are most commonly used in large scale storage systems with the largest being the Moss Landing Energy Storage Facility in California. This 300-megawatt lithium-ion battery – comprising 4,500 stacked battery racks – became operational at the facility in January 2021.
Intelligent battery software uses algorithms to coordinate energy production and computerised control systems are used to decide when to store energy or to release it to the grid. Energy is released from the battery storage system during times of peak demand, keeping costs down and electricity flowing. There is little doubt that large-scale battery storage will play a vital role in the energy provision to every community across the globe.
Flow batteries offer a viable alternative to lithium-ion. In these batteries, which are essentially rechargeable fuel cells, chemical energy is provided by two chemical components dissolved in liquids contained within the system and separated by a membrane.
[3] Thermal and Phase Transition energy storage.
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. 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.
[4] Mechanical energy storage.
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.
Gravity storage
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.
[5] Hydrogen electrolysis.
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.
This section of the website offers the brightest ray of hope that, employing the power of human ingenuity, we might stem the rise in global temperature before it reaches +3°C. The technology exists. All that's needed is the political will to invest huge sums in scaling up renewable energy and the associated energy storage capacity whilst at the same time shutting down fossil fuel power plants.