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A crane lifts a weight for energy storage

Using a crane to lift weights and then release them to generate energy later is one novel approach being tried for energy storage.

Energy Vault

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On Monday, Scotland awarded licences for 17 gargantuan wind farms off its coast. Many of them will be in deep water thanks to innovative technology to float the turbines rather than fix them to the sea bed. The sheer size of the schemes ­– 25 gigawatts of capacity on top of the 10.4 GW the whole UK has in offshore wind today – is noteworthy. So is the involvement of two big oil firms, BP and Shell.

Today’s newsletter looks at the emerging technologies for storing some of the electricity those wind farms produce, to help the UK meet its target of a fully decarbonised power grid by 2035. Other countries will also need new ways of storing energy, such as the US, which sees solar power as the cornerstone of a zero-carbon grid.

From 500-tonne weights dropped down mine shafts to cooling air, there are plenty of energy storage ideas jockeying to help deliver a renewable-powered future.

Remind me why we need more storage?

“With the rise of renewables in the energy system, there’s a lot of periods where we’ve got a lot of generation coming from low-carbon sources. But when it’s not windy, when the sun’s not shining, we are still using a lot of gas for electricity,” says Emma Woodward at UK-based analyst Aurora Energy Research. Long-duration storage, which is usually considered a technology that can provide energy for 4 hours or more, can use low-carbon electricity when it is abundant to create a store of energy and release it later.

Another reason we need long-duration storage is those Scottish wind farms mentioned at the outset. Traditionally, big power plants are built near the cities that consume most of their electricity, such as in the Midlands. “We are getting an awful lot of capacity being built a long way from demand, which the grid struggles to deal with,” says Woodward. Only about 5.5 GW of energy can get over the Scottish border to England at any one time, so you can see how 25 GW of new wind farms off Scotland could lead to bottlenecks. More storage in Scotland could overcome those.

Overall, there is about 3 GW of long-duration storage in the UK today. Almost all of it is pumped hydro, or “electric mountains”, where water is pumped up a big hill when electricity is abundant and released later to turn turbines. But Woodward and colleagues reckon the UK will need more like 38 GW by 2035 to meet all its energy and climate goals.

Why not just use batteries?

Rows and rows of lithium-ion batteries packed inside shipping containers have already been deployed across electricity grids, with more than 1 GW installed in the UK already and much more due to be added this year alone. Lithium-ion batteries are useful for companies that run grids because they are extremely fast to respond when called on. But they have their limits and their issues. One is that they are generally only economic and useful for short durations, up to an hour or two at most, says Woodward. They also can’t provide some services that electricity grids require, such as spinning parts that help balance supply and demand, known as inertia. Then there is the increasingly intense demand for batteries from other sectors, particularly makers of electrical vehicles. That isn’t a theoretical issue: after years of falling battery prices, they recently rose because demand is outpacing supply.

If not batteries, then what?

The most obvious idea is to use a mature technology like pumped hydro. That is what some big energy companies, namely Drax and SSE in the UK, are advocating. But no new pumped hydro projects have been built in the country for more than three decades. The main barriers are the sheer capital costs of these projects – they are inherently big, costing billions of pounds – and insufficient incentives to make sure firms get a return on such investments. The good news is the UK government is mulling new incentives or mechanisms to help. But if new pumped hydro does get built, there are only so many places with sufficiently high mountains – in the Scottish Highlands and perhaps Wales – that are suitable for new schemes.

That is where a fleet of competing new technologies come in, which Woodward cautions are much more immature. One is a twist on pumping water up a hill, by adding suspended solids to make a fluid more than twice as dense as water. RheEnergise, the start-up developing the technology, thinks it will unlock about 9500 sites in the UK, as it can be used on even small hills, not just mountains.

The pitch from chief executive Stephen Crosher is that his supply chain already exists – “people know how to make pumps”. “The thing that is not understood in the general public is just the scale of the [storage] challenge,” he says. “The gigawatt scale needed is less of a technological problem than a supply chain one.” The company plans to have a small (1 megawatt) scheme operational in Devon, UK, by the end of 2023, before moving on to 10 MW schemes. Getting planning permission in some areas may prove a challenge, but Crosher say he is optimistic because the equipment can be buried.

Others are looking at exploiting gravity in a different way. In July 2020, Switzerland-based Energy Vault completed a demo project of its crane that lifts blocks of concrete when energy is abundant and drops them to generate energy when it is needed. UK-based Gravitricity is taking a similar approach, and has built a small demo project in Edinburgh.

However, the future for Gravitricity isn’t above our heads, but beneath our feet. The company is currently picking a site in a former coal mine in the Czech Republic and another location in Europe to build its first megawatt-scale schemes. “All of our future projects will be underground,” says managing director Charlie Blair. Building cranes isn’t cost effective, in his opinion. While his technology could be used for long-duration storage, he sees it initially being deployed for short-term storage as there is a ready market for that. He argues gravity storage has a longer shelf-life than batteries and can operate for decades without degrading as lithium-ion batteries gradually do. The initial plan is to use existing mine shafts, before sinking new ones if the technology goes global as Blair hopes.

Another approach is to compress or liquefy air when there is plenty of renewable electricity, store it in tanks, and then expand or heat it to turn a turbine and release energy later. UK-based Highview Power has already built a small demo liquid air plant, and should have a 50 MW one operational near Manchester later this year.

Another idea is the vanadium flow battery, which consist of two tanks of electrolytes separated by a membrane. Using excess renewable electricity to produce hydrogen for energy storage is a further option, but Woodward notes it is expensive and the hydrogen would be better used elsewhere, such as for decarbonising industry.

They’re all very exciting, but what are the pros and cons?

Being cheap enough to deploy is obviously key for all the technologies. Other factors include how location-dependent they are, what services they can offer grids beyond just supplying electricity, how many hours they can provide energy for and how efficient they are (how much energy you get out as a percentage of what you put in). For example, liquid air can go in many places, provide long duration storage and several services, but is less efficient than some alternatives, says Woodward. Gravity-based ideas are an earlier stage technology and less flexible on location, but can offer long durations, she adds. “All of them have their advantages and disadvantages. I don’t necessarily think one technology will win out; I think there is a use for all of them.”

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