Can we use old coal mines for energy storage?

Renewable energy is doing its best to put coal mines out of action. But the need for energy storage could give these shafts a new lease of lease of life.

That’s the hope of Scottish firm Gravitricity, which wants to use disused mine shafts for utility-scale gravity-based storage. Its system uses excess renewable energy to raise up to 12,000 tonnes of weights in underground shafts, and then lower them to produce electricity when needed. You can see more here.

But Gravitricity isn’t building in old mine shafts yet.

Last month, it signed a deal with Forth Ports in Scotland to build a £1m demonstration project in the Port of Leith. Construction is due to start in October with operations due by the end of 2020. It is partnering with Dutch winch manufacturer Huisman on the scheme.

This will be smaller than its full-scale projects. The scheme will be fitted with one 50-tonne weight, rather than 24 weights of up to 500 tonnes each in the full system. Each unit can be configured to produce between 1 and 20MW peak power, with output duration of up to eight hours.

This demonstration project is one-sixteenth the size of its anticipated first full-scale project (4MW), and is being funded with £1.5m Gravitricity has raised in two crowdfunding rounds via Crowdcube in the last nine months, as well as additional government grant funding.

We spoke to Gravitricity managing director Charlie Blair about its commercial strategy, and where gravity-based storage fits in the energy and storage mix.

Unearthing an opportunity

The company’s roots can be tracked back to 2011 when engineer Peter Fraenkel, an early pioneer in sectors including wave power, started to file patents for a system of energy storage based on gravity.

Fraenkel is the firm's technical director, and worked on that early feasibility work with Gravitricity chairman Martin Wright.

The commercialisation plan started to take off when Blair, the former head of marine energy at The Carbon Trust, joined in 2015. In his early days, the firm was funded with a series of UK and Scottish government grants. The most recent of these was a £650,000 grant from Innovate UK secured in 2018.

The demonstration will be an important step in that journey.

In addition to the grant from Innovate UK, the company will fund the current project with the proceeds from two crowdfunding rounds of £750,000 each in the last nine months. The first closed in late 2019 with investors including around 15 angel investors, and the second was in May.

Blair says that Covid-19 influenced its decision to launch the second round.

“The likelihood of potential delay, and the very high likelihood of global recession, meant we wanted a bit more runway,” he says. The company is looking to raise “serious money” for commercial projects soon, and wants to do that from a position of strength in early 2021 rather than a position of weakness in late 2020.

Blair says the interest from angel and other investors shows that people are seeing the market potential and financial value in its technology.

The company’s plan is to build its first commercial projects in locations with good grid connections, and in markets where it can provide grid balancing. It is currently looking at mine shafts in the UK, Europe – especially eastern Europe – and South Africa, but that there may be other opportunities elsewhere in Africa and in southeast Asia too. Blair says there is no shortage of shafts.

“You’d be dumbfounded how many existing pinprick holes there are in the ground around the world. You’re talking hundreds of thousands of shafts... although only a relatively small proportion are interesting to us.”

Blair says a key benefit of the gravity-based system is it's reliability as it should last for up to 50 years. He adds that it could be used in settings other than mine shafts, including the in the foundation shafts of skyscrapers.

But while Gravitricity is a UK-based company, Blair says the storage market isn’t as interesting for it in the UK as in other countries.

This is partly because the frequency response market has been designed for batteries, although he clarifies that current support mechanisms aren’t the primary concern: “The big question for us is what the storage market is going to look like in 10 years’ time or 20 years’ time,” he says.

To get the storage that countries need, they must look beyond just batteries. Gravity-based systems could help when they’re mining for opportunities.