For my sins, I’ve been trying to make sense of the UK Government’s ambition to de-carbonise the domestic electric grid by 2030. Because the wind doesn’t always blow and the sun doesn’t always shine, a key part of this needs to be energy storage, to overcome problems of intermittent supply. Now the history of energy storage is really interesting. As long ago as 1726, Part 3 of Gullivers Travels was describing an early Net Zero scheme:
“He had been eight years upon a project for extracting sunbeams out of cucumbers, which were to be put in vials hermetically sealed, and let out to warm the air in raw inclement summers. He told me, he did not doubt in eight years more, that he should be able to supply the Governor’s gardens with sunshine at a reasonable rate ”
That was satirical of course, & the first recorded sighting of climate grifting (the banner of which has been picked up – along with stupendous taxpayers subsidies – by Dale Vince) but it’s a very simple idea that you can convert the output of an intermittent energy source into potential energy, by using it to lift up heavy objects that you can drop later. To take advantage of this effect, in 1984 the UK opened the Dinorwig Power Station which stores excess energy at scale by pumping large quantities of water up a Welsh mountain, ready to release it to drive turbines & produce leccy at the time of power demand surges (half time in football matches and so on). It’s a truly magnificent piece of engineering but it’s never going to be a solution on its own. Wiki tells me that Dinorwig can store 9.1GWh of energy, or about 15 minutes worth of UK electricity demand…………
Denis Egan, CC BY 2.0, via Wikimedia Commons
The power station is comprised of 16km of underground tunnels below Elidir Mountain. Its construction took ten years to complete, and required 1,000,000t of concrete, 200,000t of cement and 4,500t of steel. Pumped storage works in a similar fashion to conventional hydro, with water turning the turbines to generate electricity. The plant uses the different altitudes of its two reservoirs created by the mountain to move the water. When the water is released from the upper reservoir, energy is created by the downflow, which is directed through high-pressure shafts linked to turbines. By the time the valves are halfway open, 65m3 of water per second will already be coursing through the system. The force of this water turns the turbines at 500rpm. The six generating units can achieve maximum output of 1,728MW, from zero, within 16 seconds. The units are fixed vertically within the mountain, where the colossal generators – weighing at 445t each – can maintain maximum output for up to five hours. These units are situated within the main cavern, the largest man-made cavern in Europe, which is big enough to contain St Paul’s Cathedral. Turning on the power generation system is streamlined by the ‘spinning-in-air’ function. This allows the plant to be placed on standby; a small amount of energy is then used to pump compressed air into the turbine casing. This ensures that when the inlet valves are opened, the water flow can take over from the air in just five seconds. It’s handy to have, but it’s not even going to get us through the night, let alone a long windless run.
But what about scaling up the system, using some simple maths and physics ?
Consider the biggest such power station we can construct in the UK. Suspend disbelief (briefly) and imagine using excess electricity to pump the entire contents of our biggest lake (Loch Ness) up to the top of our highest mountain (Ben Nevis) , ready to be released as and when it’s needed. What would that get us? Well, (you might want to check my sums) :
- There are approximately 7.5 cubic kilometres of water in Loch Ness. A cubic kilometer is (1000)^3 metres cubed, or 10-to-the-9. A cubic metre is 1000kg of water, so there are 7.5 times 10-to-the-12 kilograms of water in Loch Ness.
- Ben Nevis is 1,345 metres high
- Lifting a kilogram mass one metre takes 10 Joules of energy.
- So lifting all that mass to the top of the mountain would store (Drum Roll) about 10-to-the-17 Joules of potential energy (or 100 PetaJoules if you think that way)
- A terawatt hour is 3.6 times 10-to-the-15 Joules.
- In other words, we’ve constructed a 30TWh storage system.
Impressive, but………………
Even if you could get planning permission to build a giant storage tank at the top of Ben Nevis, crack the engineering problem of constructing it, pump the necessary water to the top of it, then you would only store about a month’s worth of electricity supply for the UK ……………… The pumped-storage systems which actually exist are more modest. I believe that the largest power station of this kind in the world is in Chyyyna, and has about four times the capacity of Dinorwig. But even that can only store enough for about an hour of UK demand. It’s handy to ponder what multiple of our Loch Ness-Ben Nevis system we could potentially build, and convert that into hours of electricity demand, but however you slice & dice the sums, it’s hard to escape the view that whilst this is extremely smart engineering, it’s not an energy storage mechanism that can save the world by itself. These things take a while to build, and while we might be about to approve a system three times the size of Dinorwig, at the moment that’s just an exploratory tunnel, and we won’t even have that by 2030…..
© DJM 2024
