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What happens when the wind stops blowing?

from the British wind Energy Association
Much of the following information is taken from an article by DJ Milborrow in WindDirections Volume XIV, No.3 April 1995

Wind turbines generate electricity from a fuel that is free and will never run out, but which isn’t available all the time. This factor of ‘non-firm’ or intermittent’ generation is often cited as a detriment of wind energy, with a popular question being ‘what happens when the wind stops blowing’. Not a lot really, as electricity continues to be provided by other forms of generation, such as gas or nuclear. Our electricity system is mostly made up of large power stations, and the system has to be able to cope if one of these large plants goes out of action. Equally, the system is well used to dealing with fluctuations in demand, such as millions of people putting the kettle on during commercial breaks of a popular soap on TV!

The fluctuations caused by non-firm generation of electricity from wind turbines are not noticeable above the normal rises and falls in demand on the system. In fact, it is possible to have up to 10% of the country’s needs met by intermittent energy sources such as wind energy without having to make any significant changes to the way the system operates.

For more detailed information on how the system operates and what happens when the wind stops blowing, read on.

Is it a problem?

The UK’s power system relies on a diverse collection of different types of generation. No individual power plant is 100% reliable, but the system as a whole is very reliable.

There are numerous power stations in the UK. There are around 23 large coal fired plant (some of which also burn some gas or oil), 17 nuclear plants, 8 large oil plants and 11 new combined cycle gas turbines (CCGTs) and several others. Large in this context means over 100MW. (The percentage of generation by fuel type in 1995 was 48% coal, 23% nuclear, 17% CCGT and open cycle gas turbines, 9% interconnectors and 1% oil and the remainder hydro and the new renewables).

Whilst wind still makes up a very small proportion of our total electricity generating capacity coping with the intermittent nature of the wind poses no problem in relation to the other fluctuations in supply and demand which the system copes with. It is very small in comparison with the problems of meeting demand if one large power station is suddenly put out of action.

Even if wind energy capacity rises to 15,000 MW, i.e. enough to meet 13% of the UK’s electricity demand during 1994, it would still be a smaller threat than one conventional power station being unexpectedly unavailable.

Putting it into perspective

The other threats to the system, which far outweigh the variability of wind are:

  • Failure of the cross channel link. The UK is connected to France by two 1000MW circuits which periodically fail. Loss of one circuit occurs more frequently than the loss of both, but neither occurrence causes any significant upset to system operations.
  • Steam turbine trips – these occur for many reasons, including false alarms. The largest stets have a rating of 660 MW and, again, the system is managed so that these cause no problems.
  • Transformer failures – when these occur on the national grid, significant imbalances can occur and load sometimes needs to be shed as a result.
  • Thunderstorms – National Grid network circuits can trip out if struck by lightning.
  • Unexpected increases in demand – e.g. dark storm clouds can cause a sudden increase in lighting demand. Most increases in demand are predictable and so pose less of a problem.

The imagined threat due to wind generation is simply not in the same league as any of these occurrences.

The output from a wind farm is smoother than the output from a single machine, and the output from a dispersed wind system, with wind farms spread over the county is smoother still. Sudden large changes in output are very unlikely.

Various studies have enabled the probability of significant changes in the output of the whole wind energy system to be assessed. A sudden upsurge in wind strong enough to cause every wind farm in the country to shut down is extremely unlikely to occur, but as the amount of wind plant rises, the generation swings may be discernible above the ‘normal’ load fluctuation. A detailed assessment by the CEGB, just before privatisation, showed that the system could happily cope with 15% of wind energy – possibly more. At around 10% of wind energy penetration, it may be necessary to reject a small amount of wind energy at certain times, but there was no ‘ceiling’.

What actually happens?

When the system has to cope with a sudden shortfalls in supply (or a sudden increase in demand) the following help to protect the system against failure:

  • The inertia of generating plant initially keeps the system smooth.
  • The voltage can be reduced slightly.
  • Pumped storage. The pumped storage systems at Dinorwig and Ffestiniog can respond within 15 seconds or so and can provide a maximum of 2160MW.
  • Spinning reserve. Some plant is kept on part load, and its output can be increased relatively rapidly.

When there is a loss of demand, these things work in the opposite direction. For example the output of reserve plant is reduced and the turbines at the pumped storage reservoirs pump water back up to the reservoir.

Conclusion

No problems arise when the wind stops blowing. If nothing else, it is highly unlikely to have stopped blowing all over the country at the same time. At the present level of wind energy penetration (and even at penetration levels of up to 15% of energy demand) fluctuations in the output from wind farms can be accommodated within normal operating strategies.

Wind farms tend to be operating at high output when demand is most needed. Over the whole of 1995 (which was not a very windy year) the average capacity factor of UK wind farms was 0.313 (i.e. they produced 31.3% of their theoretical maximum). Over the summer the capacity factor was 0.167 but during the winter quarter, when electricity demand is higher, it was 0.445.

Experience gained from UK wind farms suggests that during the triad periods (the three half hour periods with the maximum electricity demand in the year) wind farms tend to be operating at around two thirds capacity factor. (This is no surprise, because electricity demand increases on very cold days, and the ‘wind-chill’ factor increases the heat loss from homes, hence increasing their demand for heating).

One of the criticisms people make about wind energy is that extra spinning reserve is required to cater for the variability in the wind, and that because spinning reserve is an inefficient use of fuel, wind energy does not lead to emission savings. At the moment it is the ‘lumpiness’ of the electricity system which is the main cause of the requirement for spinning reserve. ‘Lumpiness’ in this context meaning the concentration of large power stations; a system with a small number of large power stations is more ‘lumpy’ than a system with a large number of small power stations. Spinning reserve will always be needed to cater for unexpected unavailability of the largest single power source and to cope with the other threats discussed earlier.

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