Storing energy on the UK electricity grid
With a pretty serious helping hand from the gas grid
As I discussed in a previous article, energy storage is critical to ensure that we have electricity available when we need it. Today in the UK, storage is primarily provided by fossil gas (and we have storage capacity of 16,500 GWh, according to National Grid). Gas can be kept on hand until it’s needed, and then can be used to generate electricity in a power plant or used to generate heat in gas boilers.
In the future we’re not going to be able to rely on gas (unless carbon capture comes a long way, something I’m sceptical of). Instead we’re going to need ways to store the electricity generated by renewables for times when we aren’t generating enough to meet our needs. And this will only get harder as our electricity needs will increase as we switch from petrol to electric cars, and from gas boilers to electric heat pumps.
It’s hard to put a figure on exactly how much storage we’ll need in a net zero energy system. David Cebon has come up with a very rough estimate of 16,300 GWh, which is very close to the amount of gas storage we have available to us today (coincidence?). National Grid suggests that we’ll have 113–195 GWh of electricity storage by 2050 but that we’ll also need between 11,000 and 56,000 GWh of hydrogen storage. This report from BEIS suggests we’ll need 12,000 to 17,000 GWh by 2050 and that the majority of this should be provided by hydrogen. Finally, Tony Onodi has written an excellent, and interactive, article that allows you to set the key parameters of the energy system of the future and then determine for yourself how much electricity storage we’ll need.
I’m going to refer to the types of storage technologies that we’ll need in the future as ‘electricity storage’. These technologies take electricity as an input and produce electricity as an output, allowing us to save electricity for use at a later point in time. What these technologies do in between (i.e. during the actual storage stage) will vary from technology to technology.
Before we get into specific types of storage, it’s worth highlighting two key components of any electricity storage system:
- Energy conversion — this is the component that converts electricity into something that can be stored, and back again as needed. The scale of this is measured in MW or GW and determines the rate at which electricity can be fed into, or extracted from, the system.
- Storage — this stores the something that was created above until it’s needed. The size of this is measured in MWh or GWh and determines the total amount of electricity that can be stored.
The word something is doing a lot of work here. It can mean many different things, depending on the specific storage technology in question. For example…
- For gravitational storage, that something is a heavy object that is now higher than it was previously
- For chemical storage, that something is a new substance (e.g. hydrogen) that has been created
- For electrochemical storage, that something refers to ions that are now on the other side of a battery cell
The other key concept to be aware of is the round trip efficiency of a storage system. This can be thought of as the amount of electricity that can be extracted from a storage system for every unit that is initially stored in the system. Technologies lose energy in the conversion and/or storage steps, reducing their roundtrip efficiency below 100%. The roundtrip efficiency data included below is taken from the Grantham Institute at Imperial College London.
Electricity storage today
There are two forms of electricity storage deployed at meaningful scale on the UK grid today. One is crude, the other is more complex; both are effective.
Pumped hydro (roundtrip efficiency = 70–85%)
This is the crude one. Pumped hydro systems typically consist of two reservoirs or lakes, one above the other — i.e. one at the top of a hill, the other at the bottom. When electricity is abundant and cheap, pumps run to move water from the lower reservoir to the upper reservoir. When electricity is needed, water is released from the upper reservoir, flows through a turbine and returns to the lower reservoir. As the water flows through the turbine it generates electricity.
Pumped hydro is cheap. And it has the benefit that you can decouple the energy conversion and storage components of the system. For example if you want more rapid conversion, you can add more pumps and turbines. If you want more storage capacity you can make your reservoirs bigger. The net result is that each additional unit of storage capacity is likely to be cheaper than the last.
At the end of 2021, there was 2.4 GW of pumped hydro capacity on the UK grid, roughly 20% of the capacity of all the UK’s offshore wind. All of this pumped hydro is capable of storing 26 GWh of electricity, enough to power ~3m homes for a single day. This might seem like a lot of storage, but in reality it’s a rounding error when compared to the UK’s gas storage capacity of 16,500 GWh.
Building new pumped hydro is hard because it requires specific geographical / topographical features (i.e. reservoirs at the top and bottom of hills), meaning it can only be developed in a limited number of locations. The projects are also vast, meaning that huge investment is needed up front to get the project built. That isn’t to say that people aren’t trying to develop new pumped hydro facilities — for example, SSE has proposed a 1.5 GW / 30 GWh pumped hydro system at the brilliantly named ‘Loch Lochy’ in Scotland. But as far as I know they’re yet to work out how to finance it.
In its most optimistic scenario, National Grid predicts that the total amount of pumped hydro in the UK could more than treble to 84 GWh by 2050. This is optimistic, and in National Grid’s most pessimistic scenario it only increases by a third to 34 GWh.
Companies like RheEnergise are aiming to remove some of the barriers to building pumped hydro, by making pumped hydro systems smaller, more modular and capable of being deployed in areas without large hills. More on this in my next post.
Lithium ion batteries (roundtrip efficiency = 80–90%)
Batteries need no introduction. But the type of batteries I’m referring to here are bigger than the batteries you’ll find in your iPhone, walkman or boombox. These sorts of batteries come in many forms, from a box the size of a household appliance, to the battery in your electric car, to a power-plant-sized battery deployed in a field (see below).
Batteries have the benefit of being modular (i.e. to increase the capacity of the battery above, you can just add more containers). Batteries can also be deployed just about anywhere and are very quick to turn up and down, meaning they complement variable renewables well.
However batteries are expensive, meaning it will be challenging to deploy them at the scale required in the future. In addition, the conversion and storage components are closely coupled, meaning that you can’t add additional storage capacity without significantly increasing costs. Finally, batteries degrade as they’re used, meaning they have a limited lifetime of 10–15 years (although the rate of degradation is improving all the time).
At the end of 2021, the UK had 1.6 GW of batteries that were capable of storing 1.6 GWh of electricity. The vast majority of these batteries were built in the last five years. National Grid expects the total non-EV battery capacity in the UK to increase by 20–40 times to 26–63 GWh by 2050. Whilst a significant increase, this still leaves batteries far short of the 16,500 GWh in gas storage capacity.
The future
So, how are we going to store all the electricity that we’ll need for a net zero energy system? And how can we do it in a way that doesn’t rely on gas?
There are a number of new electricity storage technologies being developed that use different mechanisms to store electricity. These technologies include:
- Flow batteries
- Other non-lithium batteries
- Liquid air storage
- Compressed air storage
- Hydrogen storage (which I wrote about in detail here)
- Gravity storage
I cover these technologies in my latest article.
