Gravitricity: how mineshafts can store electricity

Renewable energy sources like solar and wind are intermittent - which means they don’t always produce enough energy to power our homes when the conditions aren’t right. To overcome this challenge, industry needs to find ways of storing surplus energy during particularly windy or sunny days. Traditional batteries are one way of storing energy, but they aren’t a silver bullet. That’s why companies like Gravitricity are exploring whether disused mines can solve the problem. Here’s Gravitricity’s Chris Yendell…

Chris Yendell - If you were to look at a mineshaft, the vertical drop there can be substantial, in some cases several kilometres from top to bottom. And this system ultimately stores electricity by raising a weight and converting that into a potential energy. When the system wants to give electricity back to an electricity grid for example, the weight can be lowered. Now it's attached via cables to electric winches. The winches effectively become generators that spin and generate electricity from that gravitational pool to put the electricity back onto the grid.

Chris Smith - It's a bit like, I suppose, if you've got a long case clock and you winch the weights up with those chains, they go up to the top and then they are effectively powering the clock, aren't they? As they fall under gravity, they're making the clock tick. You are doing something sort of similar.

Chris Yendell - That's exactly it. Yeah. It's the same technology that one might find in a very old grandfather clock. It's applying that same physics, that same concept to a challenge that is arising today, which is providing energy storage to sit alongside the likes of intermittent renewable electricity generation technologies.

Chris Smith - That was going to be my question to you, Chris, which is what problem does this actually address? Why do we need this?

Chris Yendell - When I started my working life, I was actually working to deploy wind turbines and I'm fascinated by them, you know, significant structures generating electricity from a free fuel, shall we say. So that's very exciting. But what happens when the wind doesn't blow? There's a gap and I don't know that we are so comfortable, especially in the developed world, with intermittent electricity supplies. So what do we do when the wind doesn't blow or the sun doesn't shine for solar panels? We need to store electricity when we can make lots of it and then use that when there is times of demand and less available clean generating technologies.

Chris Smith - I need you to appeal to my inner geek now and wow me out with the sort of scale that you're talking about. You mentioned some mines are kilometres deep, but how much mass, how heavy are the weights you are deploying and therefore how much energy can one of your systems store?

Chris Yendell - We have found suppliers that are lifting in excess of 10,000 tonnes in a single lift. That's an awful lot of weight. That kind of value would be well over, let's say, 50 blue whales. At this stage as we develop the technology, we're not quite lifting that many, but we are looking at lifting or lowering, one after the other, many weights so that the total mass, if you like in a system could be well in excess of the 50 blue whales that I'm talking about here, electricity grids typically see periods of several hours of demand. So this is a system that caters to that. Any one mine shaft might offer two up to eight megawatts of power. To contextualise that, that's 2000 to 8,000 homes being powered by this repurposed mine site.

Chris Smith - I suppose one of its major strengths is that it's a very simple concept, but it's instantly on. Even with some of these pump storage systems where you let water flow down a turbine, there's still a ramp up time. This must be incredibly responsive, very, very quick. You need that surge and you can switch this on.

Chris Yendell - Absolutely. As a company, we are in the stage of starting to prepare early projects to repurpose real mines that are coming to the end of service life and have that need. In advance of that and a key part of developing any new technology is a demonstration of that. So for us, what that looked like was a smaller grid connected system, which was actually situated down on the docks here in Edinburgh where our headquarters is. The exciting outcome of that was a response time of less than one second. So when some kind of grid control centre thinks, okay, we're light on power here, we need more in the system. And we were able to prove that within one second we could reply to such a signal to be operating at full power and helping to balance that electricity grid.

Chris Smith - Is it also beneficial that a lot of our major cities, certainly in countries like the UK, are sitting on top of mine workings. That's why the city is there in the first place and therefore you are minimising the grid transport cost of sending the electricity over long distances because you're feeding it onto the grid near where it's actually going to be consumed.

Chris Yendell - Yes. Typically mining infrastructure has been set up around towns. So there's that. On the more business side of things, we are starting to think a lot about, well how many mines are there in the world and where are there mines that are looking for something else to do right at this point in time?

Chris Smith - So how big do you think the potential market is for this?

Chris Yendell - Oh, in the UK we spoke with the coal authority, which has the ultimate responsibility to look after a lot of legacy mines in the uk. They have records for over 50,000 vertical shafts in the UK alone. Now that's a phenomenal number and I think our view on that is not all shafts will be suitable for a plug and play gravity based energy storage system, shall we say. But if you think about that number in the UK alone, then you look at other geographical areas. The potential, pardon my pun, feels very great. It is also very interesting to us at what stage a mine is. So in the UK a lot of our mineshafts closed many tens of years ago. And so we're starting to work around, well what do we need to do to open up those mineshafts? A lot have been backfilled, but many have also just had a lid put on top, if you like, a cap. Of importance to our process is how to prepare the mine shaft ready for the installation of such a system that has led us to mine owners who are facing this challenge now where they are approaching the end of service life and think, well, there's a really significant asset that has already been built. Can that be repurposed to provide another use case? So I expect that a lot of our early systems will deploy in mines that are coming out of service right as we speak.

Chris Smith - What's the position on ownership of the electricity? Who owns the underground facility? Because this isn't exploiting a mineral, this is a different use for the mine. So where do we stand legally on doing this?

Chris Yendell - Yeah, that is interesting, isn't it? The way that we are thinking about it at the minute is ultimately the system attaches to and is developed on land at surface. The legal status of it could in theory be relatively straightforward. It might appear something like a land use lease. In terms of who owns the electricity or who's responsible for that. I guess there's two ways in which a system might be deployed. In one sense, the mine owner might have the asset and they would look to trade electricity storage services with the electricity grid operator. The other way of thinking about it is any kind of system could also help to ease the load locally on site. So for example, an industrial process that had a very peaky requirement for power on site might be able to use this storage solution to alleviate any stress or any additional power import requirements that they may have traditionally looked to the wider grid to deliver.

Chris Smith - I was going to say it might make mining itself more viable for the mines because it gives them another revenue stream and offset some of their direct costs, which might make the difference between a mine staying open or not being viable at all.

Chris Yendell - Yeah, absolutely and you can go even further into that when mines come to the end of service life, even though they have finished mining in order to maintain sufficient level of environmental consideration in order to ensure that there's no environmental impacts after the mining has occurred. They may even have requirements to retain a mineshaft in place for many years beyond. So in that case, this is something that can provide a useful service in the energy transition as well as easing the financial headache for the owner of that mineshaft.