Little red courgette

The answer, my friend, is blowin' in the wind

I want to talk about courgettes and about wind. This isn’t an excuse for some cheap flatulence jokes¹ , but rather for some more numbercrunching about energy policy.

Welcome to readers who joined after my Nate Silver rant last week, which seemed to strike a chord. However, longer-term Logging the World readers might remember that I spent time before the UK election trying to make sense of Labour’s energy policy. For those who weren’t here then, I’m keen to make sense of their second pre-election pledge:

Make Britain a clean energy superpower: to cut bills, create jobs and deliver security with cheaper, zero-carbon electricity by 2030, accelerating to net zero.

Growing pains

But first I’m going to tell you about courgettes². The key thing to know is that they are incredibly easy to grow. You can plant seeds indoors in spring, and when the weather warms up, if you have space you can put the four strongest-looking plants outside in your garden, with generous room between them.

This is a terrible mistake.

At first, it seems under control. The plants grow, and soon you get the novelty of seeing some flowers on them. If you aren’t from Rome, and leave the flowers alone, soon you’ll have one or two little courgettes growing on the plants. You can harvest them. It’s great! You can make ratatouille. You can eat them raw in a salad with mint sauce.

But a week later you go to your garden again, and you discover that there are now six more courgettes, each the size of a baby’s arm. It has started.

For the rest of the summer you are in a race against the courgette plants. Every time you turn your back, every time it rains, more will grow. If you miss harvesting one or dare to take a holiday, you will soon find a green monster the size of the Hindenburg staring up at you. At first you will try innovative recipes. You can make cake, which is fine, except you thought you were gardening to have healthy veg, not to eat more eggs and butter.

You can’t freeze them. They only last so long in the fridge. You can try giving them away, but after a while your friends will mysteriously stop answering your calls, or hide when they see you coming with armfuls of courgettes. After a while you start leaving your crop on walls for people to pick up, or you just give up the struggle, cut out the middle man and slip one or two of them into the compost heap each time you wander past.

But then, just when you are thinking of entering witness protection, the weather gets cold. Your plants die. For the next few months from apparently endless glut, you are into courgette drought.

Where on Earth is he going with this?

Suppose you’d like to have two courgettes each week all year. That’s about a hundred. If your four plants give you a courgette each for the thirteen weeks of the summer, that’s about fifty. But only the most optimistic mathematician or courgette farmer would say that you could satisfy your demand by doubling the number of plants. You’d have an even crazier glut in the summer, and still be courgette-less for the winter.

This analogy may have got out of hand, so just to be clear: Ed Miliband is a courgette farmer.

I’m not necessarily sure it’s his fault, because I think a big part of the problem is the coverage in the press. But any discussion of renewables that focuses on peak or average capacity is like the courgette story above. The key question is intermittency, and I don’t see enough people writing about that in the media.

Take this piece in the Guardian for example. It quite rightly raises questions of price, and whether the UK 60GW capacity target for offshore wind is deliverable by 2030:

Just do the back-of-the-envelope arithmetic: there is about 15 gigawatts of capacity operating today; add the projects commissioned in Tuesday’s auction result plus those already under construction and you still only get to a grand total of about 27 gigawatts. Getting to 60 gigawatts by 2030 – all of it operational – looks a stretch and a half.

But as mathematicians would say, having large installed capacity is a necessary but not a sufficient condition to guarantee energy supply without blackouts. You need to think about how the energy is spread across the year, and I’m not sure enough journalists are doing that.

When you read that wind provided 30% of the UK’s electricity in 2023, that’s great news. But it would be naive to think that (like our courgettes) quadrupling wind capacity takes care of the problem. There are many times already³ when we generate 50% of our electricity from wind, so this capacity increase might give us 200% of our need at that time.

But that’s not necessarily useful! Like the courgettes, we can try to give the surplus away (or better still, sell it) using interconnectors. But these cables too are limited in capacity, and on windy days in Northern Europe our near-neighbours may well have plenty of electricity of their own already. So the only other option seems to be storage: let’s put that electricity somewhere useful where we can get at it when we need it!

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Soft cell

When the wind blows, we can get energy from it relatively easily, and the UK is often a blowy place. However, sometimes it isn’t, often for long chunks of time. For example, the entire period of two weeks at the start of December 2023 saw very little wind:

The UK’s carbon emissions were high at that time, because we had gas and coal to call on. But, under the terms of the pledge card above, that’s not really an option. The Labour manifesto hedged around this by keeping gas power stations in reserve - but that’s not exactly consistent with the spirit of the “zero-carbon electricity” in the pledge. So the question is what happens when the same thing happens again in 2030?

I posted some calculations about this on Twitter before the election. In a thread there I tried to understand whether we could bridge such periods of low wind using installed battery capacity.

It’s a relatively straightforward calculation: the key unit is a terawatt hour (TWh)⁴ and as I described in that thread, the UK uses about a terawatt hour of electricity per day. (As I advocated in Numbercrunch, it’s useful to have this kind of round number in your mind as a point of comparison).

As I described in the thread, we can extrapolate the current rate of growth in installed battery storage. It’s growing exponentially, which is always nice in this kind of context. But the figure I came up with in that thread is that the installed battery capacity worldwide might be about 7.5TWh of energy in 2030. So, the UK alone would burn through that in about a week -- or more realistically, even if we had access to 5-10% of that storage, then we could hope to cover less than a day’s needs.

At that stage, people started telling me I was missing the point, that good intentions were the main thing, and that I was missing out the contribution from other forms of energy storage. Batteries might seem a bit concerning anyway, because as

Ed Conway

readers will know, it’s going to require us to dig a lot of materials out of the ground. So is there a better way?

Running up that hill

The history of energy storage is really interesting. As long ago as 1726, Part 3 of Gulliver’s 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, but a short walk from my office there is a building dating from 1888 which smooths out the intermittent effort of pumps by using them to lift up water, which is gradually released to operate swing bridges and cranes by hydraulic power.

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 at the time of power demand surges (half time in football matches and so on). It’s a truly magnificent piece of engineering (I remember visiting it on a school trip and being amazed by its scale), but it’s not a solution on its own.

Wikipedia tells me that Dinorwig can store 9.1GWh of energy, which is about a hundredth of our daily terawatt hour, or about 15 minutes worth of UK electricity demand. It’s handy to have, but it’s not even going to get us through the night, let alone a long windless run.

But we can imagine scaling up the system, to try to see the limits of these schemes using some simple maths and physics. Consider the biggest such power station we can construct in the UK. As a thought experiment, imagine using excess electricity to pump the entire contents of our biggest lake up to the top of our highest mountain, ready to be released as and when it’s needed.

Loch Ness is massive, Ben Nevis is tall, so what would that get us? Well (and my A-Level physics is a bit rusty, so you might want to check my sums):

• There are apparently 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 energy⁵.

• So lifting all that mass to the top of the mountain would store (wait for it) 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.

Even if you could get planning permission to build a giant⁶ tank at the top of Ben Nevis, crack the engineering problem of constructing it, and pump the necessary water to the top of it⁷, then you would only store about a month’s worth of UK electricity supply.

The pumped-storage systems which actually exist are more modest: Wikipedia tells me that the largest power station of this kind in the world is in China, 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. However you slice the sums, in my view it’s hard to escape the view that this is extremely cool engineering, but 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 have it by 2030.

There are other storage possibilities which may well work eventually. The Royal Society has some great reports on technologies like ammonia or hydrogen storage that could work at scale to deliver Net Zero by 2050. But that remains a long way from the 2030 that we were promised in the pledge card above, so I’m still waiting to hear how that’s going to be possible.

1. I’d have gone with Jerusalem artichokes if I was going for that.

2. Zucchini, if you are American

3. It happened on Tuesday and Wednesday this week for example, and those weren’t even particularly memorably windy days.

4. You have to be careful here. Energy-related things are often quoted in megawatts, which is a unit of power. But we want to think about volume of storage, which is a question of energy. For example a megawatt hour is the amount of energy it would take to deliver a megawatt of power for an hour. Coverage of systems like Dinorwig which I write about later tends to (rightly) wax lyrical about the power (megawatts) they can produce, but talks less about the energy (megawatt hours) they can store.

5. Yeah, I know it’s 9.8-something at sea level, and slightly lower higher up. I’m Fermi estimating here.

6. Remember we’re talking about a cubic tank more than a mile on each side. If you prefer that in standard units, it’s about three million Olympic swimming pools.

7. Presumably leaving a bemused-looking Loch Ness Monster lying on the mud.