By Dave Elliott
If the use of renewables is to expand further, ways have to be found of compensating for their variability. Fortunately there are many, as I have outlined in a new book ‘Balancing green power’, produced for the Institute of Physics. It sets out to show how, taken together, they can help balance grid systems as increasing amounts of renewable capacity is added, helping to avoid wasteful curtailment of excess output and minimising the cost of grid balancing. The options include flexible generation plants, energy storage systems, smart grid demand management and supergrid imports and exports.
By Dave Elliott
Germany is pushing rapidly ahead with renewables, aiming to get 80% of its electricity from them by 2050. The nation briefly obtained 78% of its electricity from renewables in the summer. But that was obviously a one-off event. Even so, averaged annually, it’s over 30%: http://energytransition.de/2015/07/renewables-covered-78percent-of-german-electricity/
By Dave Elliott
In addition to its large-scale grid balancing role, which I looked at in my last post, energy storage may also play a role at the consumer level, with batteries allowing solar PV-using ‘prosumers’ to provide their own backup. Some see this as a possible new type of distributed storage capacity and also, more radically, as further challenging the market power of the big utilities (much of the 75GW of wind and PV in Germany is now owned by local consumers and energy co-ops), even to the point when grid systems are redundant. This may be overstated, but some more movement in that direction may be occurring in Germany and the US as batteries get cheaper. (more…)
By Dave Elliott
There is still large reserve capacity, but with some large old power plants closing, like the Didcot ‘A’ coal plant, the UK has found it a bit more challenging to meet demand when there are unexpected plant outages and cold weather. So will we make it through this winter? (more…)
by Dave Elliott
Earlier this year (26/6/13) Daily Telegraph columnist Geoffrey Lean said that ‘it has become an article of popular faith that building wind farms also involves constructing fossil-fuelled power stations for back‑up when the weather is calm. As a result, some opponents go on to say, wind turbines do little or nothing to cut carbon dioxide emissions’. But he reported that the National Grid says that, although between April 2011 and September 2012 wind produced some 23,700 gigawatt hours of electricity, only 22GWh of power from fossil fuels was needed to fill the gaps when the wind didn’t blow- under 0.1%. Moreover this standby burning of fossil fuels only reduced the emission saving from having wind on the grid by 0.081%. He commented ‘not surprisingly, given these figures, no new fossil‑fuel power station has been built to provide back‑up for wind farms, and none is in prospect’.
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!
National Grid report: www.nationalgrid.com/uk/Electricity/Operating+in+2020
Pöyry report : http:// www.ilexenergy.com.
Oswald et al. 2008 Energy Policy Vol 36 Issue 8 Aug pp3202–3215.