This site uses cookies. By continuing to use this site you agree to our use of cookies. To find out more, see our Privacy and Cookies policy.
Skip to the content

[IOP] A community website from IOP Publishing

Tag Archives: intermittency

Beyond pessimism

by Dave Elliott

One of the justifications for the UK government plan to expand the use of nuclear power is the assertion that energy demand will double in the decades ahead. That seem a little odd given that DECCs statistics show that in 2011 electricity consumption went down by 3.3% and gas use fell by almost 20%, while renewable generation expanded, albeit from a low level, by 33%. Renewables seem likely to continue to expand, given that the UK has amongst the EU’s (if not the world’s) best renewable energy resources, even if we have not developed them much yet.  As then Tory Energy minister Charles Hendry said at the opening of the annual All Energy conference in Aberdeen last spring, ‘It is shameful that with some of the strongest winds and highest tidal reaches in Europe, the UK is currently third from bottom in the whole of the EU in its use of renewables.’   He was removed from office a few months later.


Posted in Renew your energy | Tagged , , , | Leave a comment | Permalink
View all posts by this author  | View this author's profile

Variable truths on wind

The debate over how to deal with the variable energy output from wind turbines continues to rumble on. Some say that, when wind availability is low, there will be a need for extensive back up from conventional plant to maintain grid reliability. However, this backup may already exist: we have a lot of gas-fired capacity, much of which is used regularly, on a daily basis, to balance variations in conventional supply and in demand. Balancing wind variations means this will just have to be used a few times more often each year, adding a small cost penalty and undermining the carbon savings from using wind very slightly. But some say we will need much more that that. A report from Parsons Brinckerhoff (PB) claims that “the current mix of generating plant will be unable to ensure reliable electricity supply with significantly more than 10 GW of wind capacity. For larger wind capacity to be managed successfully, up to 10 GW of fast response generating plant or controllable load will be needed to balance the electricity system”.

“Controllable load” includes the idea of having interactive smart grids which can switch off some devices when demand is high or renewable supplies are low.

However even if that option is available, some say that, with more wind on the grid, to meet peak demand, we will still need more backup plants than we have. By contrast, wind energy consultant David Milborrow claims we have enough, and that some fossil-fired plants can actually be retired when wind capacity is added. That depends on the “capacity credit” of wind – how much of the wind plant capacity can be relied on statistically to meet peak demand. Milborrow puts the capacity credit of wind at around 30% with low levels wind on the grid, falling to 15% at high levels (at say 40% wind on the grid). That indicates how much fossil plant can be replaced.

PB see it very differently: “A high penetration of intermittent renewable generation drastically reduces the baseload regime, undermining the economic case for more-efficient plant types with lower carbon emissions.”

Milborow admits that balancing wind variations has the effect of reducing the load factor for thermal plant, but says that this only costs ~£2.5/MWh at 20% wind, or ~ £6/MWh at 40%. PB will have none of this: “Very high early penetration of wind generation is likely to have adverse effects on the rest of the generating fleet, undermining the benefits of an increased contribution of renewable electricity.”

PB also seems to slam the door on a possible way out, importing power from continental Europe, the wider footprint then helping to balance variations across a much larger geographical area. It says: “Electricity interconnection with mainland Europe would offer some fast-response capability, but would be unlikely to offer predictable support. Without additional fast-response balancing facilities, significant numbers of UK electricity consumers could regularly experience interruptions or a drop in voltage.”

Addressing the interconnector issue, among others, TradeWind, a European project funded under the EU’s Intelligent Energy-Europe Programme, looked at the maximal and reliable integration of wind power in Trans European power markets. It used European wind power time series to calculate the effect of geographical aggregation on wind’s contribution to generation. And it looked ahead to a very large future programme, with its 2020 Medium scenario involving 200 GW – a 12% pan-EU wind power penetration. It found that aggregating wind energy production from multiple countries strongly increased the capacity credit.

It also noted that “load” and wind energy are positively correlated – improving the capacity factor – the degree to which energy output matches energy demand. For the 2020 Medium scenario the countries studied showed an average annual wind capacity factor of 23–25 %, rising to 30–40 %, when considering power production during the 100 highest peak load situations – in almost all the cases studied, it was found that wind generation produces more than average during peak load hours.

Given that “the effect of windpower aggregation is the strongest when wind power is shared between all European countries”, cross-EU grid links were seen as vital. If no wind energy is exchanged between European countries, the capacity credit in Europe is 8%, which corresponds to only 16 GW for the assumed 200 GW installed capacity. But since “the wider the countries are geographically distributed, the higher the resulting capacity credit” if Europe is calculated as one wind energy production system and wind energy is distributed across many countries according to individual load profiles, the capacity credit almost doubles to a level of 14%, which it says corresponds to approximately 27 GW of firm power in the system.

Clearly then, with very large wind programmes you do get diminishing returns and need more backup, but it seems that can be offset to some extent by wider interconnectivity – the supergrid idea, linking up renewables sources across the EU.

That is already underway. The UK’s National Grid has agreed with its Norwegian counterpart Statnett to draw up proposals for a £1 bn grid-interconnector grid link-up, to be funded on a 50:50 basis, which could help solve the problem of winds intermittency, given that Norwegian hydro could act as back-up for the UK, in return for electricity from the UK on windy days. As yet no UK landfall site has been indicated, but it could include connection nodes along the route with spurs taking power from offshore wind farms and become the backbone of a new North Sea “supergrid”: the UK and eight other North West EU countries have now agreed to explore interconnector links across the North sea and Irish sea. National Grid said: “Greater interconnection with Europe will be an important tool to help us balance the system with large quantities of variable wind generation in the UK.”

Posted in Renew your energy | Tagged , , | Leave a comment | Permalink
View all posts by this author  | View this author's profile

Can wind work?

Critics say that wind turbines can’t be relied on to provide secure grid power because of the variability of the winds, and will have to be backed up by conventional power plants. So no conventional plants will actually be replaced, costs will be excessive and there will be very few emissions saved net.

Views like this, regularly expressed by groups ranging from Country Guardian to the Renewable Energy Foundation, have just as regularly been challenged by detailed academic and agency studies- with for example a major review of the various studies being produced in 2006 by the UK Energy Research Centre and an overview Earthscan book Renewable Energy and the Grid in 2007. The debate nevertheless continues.

The latest batch of reports includes one entitled ‘Managing Variability’ for Greenpeace, Friends of the Earth, RSPB and WWF by energy consultant David Milborrow. He concludes forcefully that there are no major technological problems with dealing with variable wind inputs to the grid, just minor economic costs: ‘If wind provides 22% of electricity by 2020 (as modelling for Government suggests), variability costs would increase the domestic electricity price by about 2%’.

Basically, this is to pay for the fact that some fossil fuelled plant would have to be run a bit more each year to balance out low wind periods. These ‘standby’ plants already exist- they are used to run up and down to full power on a daily basis to meet the standard demand peaks, and can also be used to cope when some other plant (e.g. a nuclear plant) shuts down unexpectedly. So there’s very little extra cost in using this existing standby capacity a bit more, occasionally, to back up wind plant. And very few extra emissions would be involved by the extra use of these plants. Milborrow says that it would reduce the carbon emissions saved from having 20% of UK electricity supplied by wind by about 1%.

This issue, once sometimes cited by critics as a major problem for wind, now seems to have finally been resolved. As the House of Lord Select Committee on Economic Affairs put it in its review of the Economics of Renewable last year ‘The need to part-load conventional plant to balance the fluctuations in wind output does not have a significant impact on the net carbon savings from wind generation’- a view subsequently accepted by the government.

However some of the other issues are still fiercely debated. The critics insist that there will be times when there is no wind at all over the whole of the UK. In which case, as consultant Denis Stephens put it in a critical review of a recent Carbon Trust report on wind, in which he drew on a study produced by Oswald for the Renewable Energy Foundation, ‘for every megawatt of output from wind turbines there has to be an equivalent backup facility of conventional power generation’.

Milborrow by contrast insists that ‘numerous studies have shown that, statistically, wind can be expected to contribute to peak demands’, although he accepts that the amounts it can reliably supply then (its capacity credit) will be much less than the full rated capacity. However he notes that ‘system operators do not rely on the rated power of all the installed wind farms being available at the times of peak demand, but a lower amount- roughly 30% of the rated capacity at low penetration levels, falling to about 15% at high penetration levels’. He does accept that wind variation adds an extra uncertainty into the management of the grid, although this ‘ is not equal to the uncertainty of the wind generation, but to the combined uncertainty of wind, demand and thermal generation’, which is already dealt with by existing balancing measures. It simply adds a bit to the costs, and these can, if necessary, be reduced by a range of new measures.

He notes that ‘ Improved methods of wind prediction are under development worldwide and could potentially reduce the costs of additional reserve by around 30%. Most other mitigation measures reduce the costs of managing the electricity network as a whole. ‘Smart grids’, for example, cover a range of technologies that may reduce the costs of short-term reserves; additional interconnections with continental Europe, including ‘Supergrids’ also deliver system-wide benefits and aid the assimilation of variable renewables. Electric cars hold out the prospect of reduced emissions for the transport network as a whole and could act as a form of storage for the electricity network – for which the electricity generator would not have to pay.’

This view is shared by National Grid, who have produced a new consultative report, which includes a look at some of these ameliorative balancing options. Overall they seem to think that there should be no major problems in balancing the grid: ‘As wind generation increases, so does geographic dispersion of the wind farms and we believe that this combined with ongoing improvements in wind forecasting will allow us to minimise the reserve requirements for wind going forward.’ They admit that ‘The need to carry operating reserve means the effective ‘capacity credit’ for wind output of 15% of capacity will therefore be less than 15%, but say that ‘National Grid’s view at this stage is that for 2020, a wind generation output assumption of up to 15% of capacity at times of peak demand is reasonable’.

The debate continues, although the emphasis now seems more on the costs rather than the technical viability. For example, in a new report on the ‘Impact of Intermittency’ in the UK and Ireland , Poyry Energy Consulting have opened up a new issue, claiming the problem is not so much the familiar short term variations in wind availability, but annual variations. It looked at the period 2000-2007 and found that annual levels of wind generation output varied by almost 25% in the Irish market and 13% in the British market. As a result there could be significant economic problems facing fossil fuel back-up capacity: ‘plant may only operate for a few hours one year and then hundreds of hours the next year’, making revenue planning hard.

Overall they say that grid interconnectors will be important for grid balancing, especially for Ireland, but as National Grid and Miborrow argue, there are also other technological adjustment that could change the situation- not just interconnectors but also pumped hydro storage and load management techniques like smart metering.

In addition we could use other non-variable renewable plants for balancing, not just hydro plants, but also biomass fired plants and possibly geothermal generation as well. A recent German study showed that it was possible to use biomass generation to balance wind and PV solar over the year on a national basis, despite weather cycles and demand cycles, and new enhanced geothermal systems are being developed in Germany and elsewhere which can provide firm power outputs.

What’s interesting for the present is that the Poyry study shows that, given some inter-connection, Ireland can cope reasonably well without nuclear power. Indeed Poyry note that their assumption that there would be new EPR nuclear plants in the UK ‘had the expected impact of increasing response requirements’ i.e. from backup fossil plant, given expected nuclear plant fault levels. So having nuclear on the grid makes it even harder to balance wind!

Milborrow report:

National Grid report:

Pöyry report : http://

Oswald et al. 2008 Energy Policy Vol 36 Issue 8 Aug pp3202–3215.

Posted in Renew your energy | Tagged , , | Leave a comment | Permalink
View all posts by this author  | View this author's profile