UK power demand, according to National Grid, has a typical summer daytime peak of around 30 GW, but at night can fall to 17 GW. At present there is around 42 GW of renewable capacity installed, supplying nearly 32% of UK electricity, and more is on the way; by 2020 there should be 46 GW and by around 2027 maybe 60 GW, on current plans.

Not all this renewable capacity will deliver full power all the time, or at any one time. Over the last year, the average load factor for onshore wind turbines was 30%. For offshore projects it was 50%, while for PV solar it was just 10%. Even so, during summer nights there may soon, at times, be more output than is needed. Solar will, of course, be zero at night but it will peak in the daytime, so that renewables may soon sometimes supply a large part of daytime summer demand.

Certainly by 2027, which is when the Hinkley Point C nuclear plant may start up, it is hard to see what will be done with the output from its 3.2 GW capacity at times, especially since demand for electricity is falling (by about 2% in 2016-17 and 15% in the last decade) and could fall even more if energy efficiency were taken seriously.

The real problem with renewables will not be shortfalls, but regular excesses

Dave Elliott

Something on the supply side will have to give way. Nuclear plants must usually be kept running 24/7 to recoup their large investment costs, but it would be odd to shut down wind and solar projects to allow that to continue when wind and solar are able to deliver much cheaper power. Though they are sometimes being “curtailed” at present, in part due to grid congestion problems, but also due to there being excess generating capacity over demand. Curtailment is wasteful and provocative, since so-called constraint payments have been negotiated to compensate generators for when their contracted power is not needed.

Summer nights

There are, as far as I know, no plans to vary the output of the UK’s proposed new nuclear plants to compensate, so at some point — maybe in the mid-2030s — we may have around 16 GW of fixed inflexible nuclear capacity to deal with. That’s enough roughly to run the whole system on a summer night, without using any of the 65 GW or so of renewables that could be available by then. That would be pretty odd. And what about at other times? One justification sometimes made for large nuclear expansion is that nuclear plants can back up renewables when they are not available and electricity demand is high. There may well be some periods like this each year, with wind and/or solar lulls for days or even weeks. So is that what the nuclear plants will mainly be used for?

It is hard to see how that would work. For one thing, 16 GW isn’t enough to back up 65 GW of renewables. For another, could the very expensive nuclear plants mostly be left off-line the rest of the time, just waiting to ramp up for these rare events? No one is actually suggesting that. When pressed, nuclear proponents talk about making nuclear plants more flexible, so they can balance varying renewable supplies and varying demand all the time. It is true that some designs of nuclear plant can, and do, ramp up and down slowly to match daily demand rise and fall cycles (as they do in France), but it would be hard for them to do this fast and repeatedly to match renewable variations. Cheap, fast-start-up gas turbines are far better at that.

Storage systems like batteries, although costly, can also provide fast short-term balancing. So can pumped hydro reservoirs. Demand can be managed to reduce (delay) peaks, for example, by charging more for power at peak times. Extra power can be imported when demand is high, balanced by exports when there is too much available. With 65 GW of renewables and 16 GW of nuclear on the UK grid, there would certainly be times when large-scale interconnector exports would be possible, needed and lucrative, although some of this excess could go into longer-term storage when there are lulls in renewable availability, perhaps converted into hydrogen for use to make power again. We would certainly have plenty to spare. Even without nuclear.

Winter time

Ah, but what about winter? Isn’t that when things get difficult? Actually, it’s not too much of an issue for electricity — demand for that does increase in winter, but the UK’s maximum wintertime demand is typically well under 60 GW. On top of the current 8.8 GW of nuclear and 42 GW of renewable capacity, the UK also has about 30 GW of gas-fired generation capacity, and some other odds and ends (though soon no coal plants). In all, the total is near 100 GW, including some standby capacity and storage. That provides enough to ensure power-demand peaks can always be met, even when it’s very cold and renewables are at a low ebb. Solar output will be low, but wind plant output is highest in the winter. At present, gas plants do most of the flexible balancing. Nuclear is run continually to supply so-called baseload.

In the newly emerging system, the pattern is likely to change. There will be more storage, more demand management, more interconnectors, all helping to balance the increasing amount of variable renewable capacity. Gas plants will still be needed, including some fast-start-up open cycle plants, but they can increasingly be run on biogas or hydrogen produced and stored during periods when there is excess renewable output. The nuclear plants’ traditional selling point, that they can provide baseload, now becomes a problem: it’s not needed, or at least only, at most, at a low background level. In theory, some of the nuclear plants’ output could be stored when not needed, but it would be very odd to build large new expensive inflexible plants to be run like this — against type. What is needed is flexibility. Some say small modular reactors may be able to offer that, and perhaps some local heat as well, operating in Combined Heat and Power (CHP) mode. But so can local biomass or biomass-fired CHP plants. I know which I would rather live near.

The above analysis should illustrate that, although there will be balancing issues, the real problem with renewables will not be shortfalls, but regular excesses — if we install enough renewable capacity to be able to meet demand most of the time. But that “excess capacity” problem can be turned into a solution, if we can convert the excess output to storable hydrogen to meet power needs when there is a lull in renewables. Some storable heat would be useful too – heat demand is what peaks most in winter.

The big issue is how much excess do we want or need, and how do we handle the economics? At present, the rush to install renewables has led to a surplus of low-cost electricity in some countries, destabilising electricity markets with, for example, PV challenging gas plants in daytime peak markets in Germany. That’s a problem since these gas turbines will be needed at other times for balancing. So some sort of support for balancing capacity is needed. The UK capacity market was one attempt but has had some issues. Hopefully it can be improved. It does seem odd that it has been used to support nearly 8 GW of nuclear in each annual contract round so far.

The UK has very large renewable resources, which look likely to be able to supply over 50%, and even, some say, over 60% of the nation’s power by 2030, and much more later on. But the UK is not unique: many other countries can get high percentages of renewables. Some have already achieved over 50%. In my next few posts, I will look at the debate on whether a target of near-100% renewables by 2050 is possible globally.