Storing energy in undersea rock
In the UK we get about 30% of our energy from renewable sources; this number is rising and will help to make a major contribution to reducing the country's carbon footprint. But the problem is that, unlike a conventional power station, which we can turn on when we need it, we can't make the wind blow or the sun shine on demand. And when both of those things are happening, we might not need the energy that's being produced. Instead what's needed is an ability to store energy on a massive scale. UK scientists have published a report that explores the feasibility of using surplus electricity to compress air into underground rock formations. The idea is that the pressurised gas can then be used later to drive turbines and generate electricity. Chris Smith spoke to Stuart Haszeldine from the University of Edinburgh...
Stuart - In the UK we're accustomed to having electricity 24 hours a day seven days a week. In the past we've done that by storing energy by having a great big pile of coal next to a coal fired power station, and then it's been done for the past 10 or 15 years by having gas pipes coming out of the North Sea gas fields and when we wanted a bit more gas in the winter to provide heat and electricity, we've basically turned the taps on a bit more and produce more gas. But now our gas production from the North Sea is declining. We're not using coal fuel power stations anymore because of the carbon dioxide emissions and we've gone over to building more and more wind power, and more and more solar power and with wind and with solar power you cannot control when the electricity comes.
Chris - So in essence we need some kind of energy sponge where we can shove surplus energy during the good times, so we have a supply for the bad times.
Stuart - That's right and the good times and the bad times worth thinking about particularly the difference between summer and winter. We're thinking about storing energy for months at a time.
Chris - And the mechanism of that storage. What do you have in mind?
Stuart - Well it's well known that the storage is available for very short timescales with batteries for minutes and seconds. You can also do storage for hours and days with pumping water uphill. We're thinking of using the surplus extra electricity from when it's a very windy day or a very sunny day. We turn that into driving a compressor, compress air put that down a borehole and that compressed air spreads out into porous sandstone deep beneath the seabed, so it's filling up like a rigid sponge by compressing more and more air into it.
Chris - And then how do you get the energy back from the gas you've compressed underground?
Stuart - We basically release the valve and the trapped gas under pressure in that rock expands back out up the borehole and can then turn a turbine and turning the turbine generates electricity in a conventional way.
Chris - Now how much energy do you think that we can store?
Stuart - We've looked at how much sandstone and how much microscopic pore space there is all around the UK. So what we're looking at is storing a very large amount of energy and it's equivalent to all of the electricity supply for the UK for about six weeks at a winter rate of usage. So that's a huge amount of energy, equivalent to like 10 or 20 coal fuelled power stations running at the time but of course this would be much much much cleaner energy.
Chris - In terms of practical application though, yes you could store all that gas but what about all the infrastructure needed in order to liberate that energy and get it back into a useful form electricity that can then be brought ashore. Wouldn't that take enormous amounts of infrastructure to do that?
Stuart - Well you're right in that there'll be compressors needed offshore. We think though the smartest place to use this is around the large existing wind farms in the future wind farms which are being built offshore of the UK. So if we develop the storage underneath these wind farms we minimise the transport distance for the compressed air it just goes straight down the boreholes which we can reuse existing boreholes in many cases, but we've also priced in the cost of drilling new boreholes. And so we put the air down there, we bring the air back up, generate the electricity and we use the existing wires from the windfarm to bring the electricity into the national grid.
Chris - Are there any geological consequences to doing this because people will worry because we've seen connections between people doing deep injections for fracking for example and subsequent seismic outcomes. Could the same thing happen under the North Sea?
Stuart - I think that's a good question and that's one of the reasons for going offshore and that is if we create small tremors with this activity then people will not be very worried about those because nobody's living on top, there's no damage to property, but importantly no damage to the site either. We know offshore there are small earth tremors all the time and we know what amounts of rock stress triggers that. So we know how much pressure we can put in till the rock starts to move and how much pressure we can take out. And still the rock starts to move the mechanics of the rocks known quite well so we can control that.
Chris - And obviously the world is bigger than just the UK. Could other people also exploit this?
Stuart - Well one of the great things about this is that the the principles of what we're outlining here around the UK could be transferred and applied anywhere that there are thick sandstones has the opportunity to do this. So it's very very portable type of approach.