‘Windfarm output is never zero. Sometimes it’s less’
Fresh contenders have entered the UK wind power debate, as a turbines expert funded by the Renewable Energy Foundation publishes an investigation into a hotly-disputed subject – the variability in output to be expected of a large UK windfarm base.
In a just-released article for the journal Energy Policy, titled Will British weather provide reliable electricity?, consulting engineer Jim Oswald and his co-authors model the output to be expected from a large, 25+ gigawatt UK windfarm collection of the type the government says it would like to see in service by 2020. Wind is generally seen as the renewable technology best suited to the UK climate, and so it forms the bulk of most renewables plans for Blighty.
One of the most frequent criticisms levelled at wind power is variability. That is, when the wind drops (or blows too hard) the windmills stop spinning and you get no power. To begin with, Oswald simulates the output rises and falls that might result from a lot of windfarms distributed around the UK by using Met Office archived data from different points up and down the land. Many wind advocates have argued that with enough windfarms, widely enough distributed, you would get more reliable power output as some windmills would always have wind.
Oswald’s analysis says this isn’t true, with calm conditions across pretty much all the UK being fairly regular events.
Analysis from 1996 to 2005 shows similar results: large, rapid, and frequent changes of power output being common occurrences … any national power system has to manage under the worst case conditions likely to occur … These are not extreme cases, whose frequency is so low as to render the events negligible. Rather, these are representative …
If the government succeeds in building its mighty 25 gigawatts of wind base by 2020, according to Oswald’s Met Office data-based model its output will dip to pretty much nothing fairly routinely.
The next line of defence for wind advocates is normally the idea of hooking up the UK’s grid with high-capacity links to those of other European nations, creating a “Supergrid” with wind so widely spread that output would be sure to even out. But Oswald has bad news for that idea, too. He compares his modelled UK big-wind output with that which has been produced in recent times by other European wind bases, particularly the substantial German/Danish one.
Ill winds blow nobody any good.
Not only does the large continental wind base exhibit nasty rollercoaster surges in aggregate output, these surges tend to match those to be expected in the UK. When the wind isn’t blowing across most of the UK, it isn’t blowing in Germany, Denmark etc. either. Worse still, this happens in the dead of winter when electricity demand is highest.
There is good agreement between the model and the [real-world European wind power output] data, which further supports the argument that wind output is controlled by the arrival and dispersal of large low-pressure systems moving over the coasts of Western Europe.
Being an engineer, Oswald examines the worst situations that occurred in his time frame – those that engineers would need to design the system to cope with. The nastiest situation that could happen would be early-evening flat calms in winter.
The relationship between wind power and demand was analysed by considering the moments of peak electrical demand in each of the last 6 years and … the wind output for these moments … each of them occur on a winter’s day between 5 pm and 6 pm, as this is the time when commercial and domestic demand combines into the day’s peak. As can be seen … 16 January 2001 and 2 February 2006 were times of very little wind output … and would likely fall into the category described as ‘low wind cold snap’ … on 2 February 2006 the electricity demand in Britain reached its peak for 2006. The wind power model suggests that the output for the wind farms of Britain at that time would have been zero … wind farm output for neighbouring countries has been determined for the same moment in time … the measured output from NorthWest Germany, Ireland, and Spain was low …
This passage in the report aroused particular ire from the British Wind Energy Association, speaking to the Sunday Telegraph on the matter at the weekend.
“When you look at the UK system as a whole, there is electricity coming from wind 100 per cent of the time,” expostulated a testy spokesperson. “There is no moment in time when the output of the pool falls to zero.”
Oswald’s report partly agrees with this, saying that it is true that aggregate wind output is never exactly zero. Sometimes it’s less than zero, though. It was during the nasty calm of 2006, for instance:
The [output of the UK’s real-world] wind farms monitored by the National Grid [from 5-6 pm on February 2nd 2006] is shown as negative as the consumption of electricity used by these wind farms (to drive auxiliary loads) exceeded the total output.
And a supergrid to Europe wouldn’t have helped, as everyone else had calms and high midwinter early evening demand too. That demand peak in 2006 was especially bad for wind power, but it was far from freakish. Such “low wind cold snaps” are routine, caused by midwinter high-pressure systems lurking on top of northwest Europe.
Like all such high-pressure systems they can easily hang about for a few days – sometimes longer – and according to Oswald this means you aren’t going to cope with them by using pumped-storage kit. Such systems, typified by the Dinorwig installation in Scotland, could lift water uphill using spare wind power on breezy mornings and let it flow down through hydropower turbines on calm evenings. They are much fancied by wind power advocates.
But speaking to the Reg yesterday, Oswald said that a realistically feasible UK pumped-storage base would only cope with one or two days of low winds at best. He said that there’s a five-day calm most winters; and every twenty years or so you can expect a ten-dayer.
“I’m quite rude about pumped storage,” he told us.
In his view, all this means that – certainly in a 2020 timeframe – the only feasible backup for the planned 25-gig wind base will be good old gas turbines. These would have to be built even if pumped storage existed, to deal with long-duration calms; and the expense of a triplefold wind, gas and pumped storage solution would be ridiculous. At present, gas turbine installations provide much of the grid’s ability to deal with demand changes through the day.
The trouble is, according to Oswald, that human demand variance is predictable and smooth compared to wind output variance. Coping with the sudden ups and downs of wind is going to mean a lot more gas turbines – ones which will be thrashed especially hard as wind output surges up and down, and which will be fired up for less of the time.
Oswald is an expert on gas turbines, having worked for many years at Rolls Royce*. He says that most people, in allowing for gas backup to wind farms, assume that the current situation of gas-turbine usage applies. Not so, he says. Gas turbines used to compensate for wind will need to be cheap (as they won’t be on and earning money as often as today’s) and resilient (to cope with being throttled up and down so much). Even though the hardware will be cheap and tough, it will break often under such treatment; meaning increased maintenance costs and a need for even more backup plants to cover busted backup plants. Thus, the scheme overall will be more expensive than the current gas sector. And since people won’t want to thrash expensive, efficient combined-cycle kit like this, less fuel-efficient gear will be used – emitting more carbon than people now assume.
High-efficiency base load plant is not designed or developed for load cycling … Load cycling CCGT plant will induce thermal stress cracking in hot components … The other impact on the individual plant is a reduction in the plant’s utilisation. This has an economic consequence, which will encourage operators of generation plants to buy cheaper, lower-efficiency and therefore higher carbon emission plants … Reduced reliability will require more thermal plant to be installed …
And it gets worse. All this will hammer the gas grid’s pipeline networks and storage hardware too, costing the end consumer even more money – again, something that isn’t currently accounted for in wind power schemes.
Power swings from wind will need to be compensated for by power swings from gas-powered plants, which in turn will induce comparable power swings on the gas network as plant ramps up and down. This will have a cost implication for the gas network, an implication that does not seem to have been included in cost of wind calculations …
In essence, wind plans aren’t actually wind plans, according to Oswald. They’re gas plans with windfarms used to reduce the amount of gas actually burned in the plants. But he thinks the assumptions now made on costs and emissions reductions to be anticipated are unduly optimistic.
From one perspective, one might argue that this is the exact purpose of renewable plants, namely to reduce fossil fuel burning. However, it does this not by obviating the need for that plant, but instead by reducing the utilisation of power plants which continue to be indispensable. Electricity operators will respond to the reduced utilisation … high capital [cleaner gas] plant is not justified under low utilisation regimes … it is critically important that the carbon saving achieved by the whole system is known, understood, and achieved in practice. The effect of this higher carbon calculation does not appear to be mentioned …
There was one little ray of light for wind power lovers, however. When we asked Oswald for his views on plans to deal with wind variation using car batteries plugged into the grid for charging, he said he hadn’t so far factored that into his plans. There are those – Google, the Danes – who think this might be seriously useful if a large amount of road transport went electric. Obviously, that doesn’t seem especially likely in a 2020 UK timeframe, but it might not take that much longer if oil prices stay high.
Of course, that in turn would mean a lot more electricity production required – perhaps magnifying the wind variation problem, if the increased ‘leccy demand was met with windfarms. And the calm or windy periods might not come just when the electric car users wanted them to.
“It’s an interesting dance,” says Oswald.
His article (Oswald, J., et al., Will British weather provide reliable electricity? Energy Policy (2008), doi:10.1016/ j.enpol.2008.04.0330) can be downloaded here, though you have to pay for it. ®
*The jet-engine firm, not the unimportant car badge.
By Lewis Page
3 July 2008
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