On September 30^th 2024, the UK’s last coal power station closed down. We are now at a point that over reliance on fossil fuels is being taken seriously, and we are truly thinking about the future of our planet.

With the ongoing global revolution of green energy generation, the need for large scale energy storage solutions is on the rise. Given that most green energy generation in the UK relies on the weather (solar power & wind power) it is not possible to reliably increase energy generation when demand is high, as we traditionally would with fossil fuel or nuclear power generation.

It is possible to an extent, as wind farms routinely shut off or activate numbers of wind turbines to align with demand, but doing so both has a more significant lag time than traditional power generation facilities as well as relying on conditions being favourable for wind turbines to operate. Solar farms can undertake a similar process to an extent, however are obviously restricted to day time operation.

As we transition from fossil fuels to renewable energy sources, the need to solve this problem increases by the day – the nation’s energy demands can be tricky to predict, and even harder to keep up with. Anything that changes the behaviour of many people at one time can stress the national grid and cause increased demand.

When is this a problem?

There is a term within the UK energy grid called a ‘TV pickup’. This is when an event on television can cause millions of people either to turn on their TV at a specific time, or to change some other behaviour, like in the following example:

On the 4^th of July 1990, England played a World Cup semi final against West Germany. After being tied in regulation time, the match eventually went to a penalty shootout. England midfielder Chris Waddle stepped up to take the decisive penalty to keep England’s World Cup hopes alive – and fired the ball right over the crossbar. It’s estimated that around 26 million people watched the game in the UK, and this penalty miss was almost immediately followed by around 26 million people finding something else to do.

Kettles, Fridges, ceiling lights, ovens, you name it. This ‘TV pickup’ caused a 2.8 Gigawatt energy surge on the national grid, about 2.5x as much as the average power output of Hunterston Nuclear Power Station in Scotland.

How have we dealt with this in the past?

It is simply not possible at the moment to account for these sort of demand surges using solely wind, solar, and tidal power. What we need is a solution (or multiples solutions) for medium and large-scale energy storage to be used when demand is high. What term do we usually use for things capable of energy storage? Batteries.

The most famous examples of large scale energy storage would be Pumped Hydroelectric Storage (PHES), or dams. The basic principle is that when energy demand is low, these Hydroelectric stations can use electricity to pump water from a lower reservoir up to a higher one, which has been created using a dam. If it then becomes necessary to increase power generation, the water from the higher reservoir (a high energy state) can be allowed to flow down to the lower one(a low energy state), passing through turbines and generating electricity.

This process of converting gravitational potential energy to kinetic energy to electrical energy is great for large-scale, long-duration operations. The downside is that to store a lot of energy, these facilities have to be really, really big. They are extremely expensive and time consuming to build, and can take a long time to recharge.

We now find ourselves in a position where we have a need for more cost-effective solutions for energy storage at various scales. As traditional chemical batteries are extremely resource heavy and environmentally damaging to produce, as well as becoming more and more dangerous the larger they get, we have to look elsewhere to solve this problem.

 

What is Liquid Air Energy Storage?

One such solution that is now being developed is Liquid Air Energy Storage (LAES). The idea was first proposed by researchers at the University of Newcastle back in 1977. Although it was developed further in the 80s and 90s by Hitachi and Mitsubishi in Japan, the idea hasn’t really came to fruition until recently when, following smaller-scale pilots, Highview Power have announced the construction of grid-scale LAES plants across the UK.

The first of these plants, under construction near Manchester, will be capable of storing up to 300MWh (Megawatt-hours) – which could power over 5000 homes for a full week, on average. They have also now submitted planning permission to build a slightly smaller, 250MWh facility on the former Hunterston site near Fairlie, Ayrshire – which is how it came to my attention.

So how does it work?

On the face of it, Liquid Air Energy Storage is very similar to Pumped Hydroelectric Storage – in that it involves using off-peak low-cost energy to move something from a low energy state to a high energy state, where the energy used can later be recovered. Although working on smaller scales than PHES, LAES requires a lot less space and so can be deployed closer to where the energy will be needed.

The basic premise is that the system is charged by drawing in air that is then highly compressed and cooled until it reaches a liquid state. It is stored as a liquid, before being released when energy is needed. This works in a way that is remarkably similar to a refrigerator.

The air used is first compressed at around 15MPa (mega Pascals), which is around 150 atmospheres of pressure. When doing this, the air has the same amount of heat energy it had originally, but is compressed into a much smaller space, effectively heating it up. This heat is removed from the compressed air and stored in a separate part of the system to be used later. Removing the heat allows the air to cool past its boiling point and condense into liquid at -194.5 degrees Celsius, and reducing its volume around 700 times, compared to before it entered the compressor.

To recover energy, the air is allowed to flow into an evaporator where atmospheric heat causes the liquid air to boil, turning back into a gas. The thermal energy stored from the compression process is then added to this gaseous air to superheat it.

This superheated high pressure air is then allowed to flow through a turbine, which is ultimately where the electricity is generated and fed in to the national grid. After passing through the turbine, the air is exhausted back into the atmosphere – remaining just as clean as when it was drawn in.

This cycle is completed purely using electricity, the idea being that this electricity can come from renewable sources alone, without the need to burn any fossil fuels. Given the medium used by the system is air, there would also be no pollutants produced – asides from the obvious benefit this also provides greater freedom of choice when deciding where to build these plants.

Although LAES finally becoming commercially viable represents a great leap forward in the green energy revolution, it’s unlikely that this technology alone will be the only advancement necessary to run a nation purely on renewable energy. There are many other solutions for energy storage under development – such as sand batteries, hydrogen storage, and redox flow batteries. The fact that we’re at a point where significant investment is being put into this research is a major positive for reducing our overall reliance on fossil fuels.