The Economics of Renewable Energy - Economic Affairs Committee - Contents

Supplementary memorandum by E.ON UK


  1.  Following the recent oral evidence session this supplementary note provides further explanation for the statement that a high % of wind capacity needs to be "backed up" by thermal plant to meet winter peak demand.

Variability in Wind Generation

  2.  Wind generation and thus its contribution to meeting electricity demand varies with wind speed. A typical single 3MW wind turbine generates no electricity output when wind speed is less than about 3 metres per second (m/s), reaches its maximum output at about 15 m/s, and shuts down when wind speed reaches around 25 m/s to preserve its physical integrity.

  3.  A single wind farm with a number of turbines will smooth or average this effect as wind speed will vary somewhat across the area of the wind farm. For example at an average wind speed of about 3 m/s there will be some output because the wind will be above that speed at some locations and below it at others. On the same basis, it is less likely that the wind speed will be high enough across the entire area to deliver maximum output from all wind turbines. For a small part of the year (less than 1%) an individual wind farm is close to full output, but for much of the year it generates far less and for 15-20% of the year it generates no output.

  4.  The effect of operating a portfolio of wind farms is to smooth this effect further, with less variation in output across the year. The portfolio never reaches full output and peak output is around 80% of full capacity, but there is only a very small portion of the time when there is virtually no output.

Correlation between wind output, season and time of day, and with electricity demand

  5.  The extent to which wind speed, and thus output from wind generation, correlates with periods of high electricity demand is important in assessing the extent to which we can rely on wind generation to meet winter peak electricity demand. Winter is generally windier than the summer, with the median output for a winter day higher than in the summer. However, on the coldest days (with temperatures below zero), there tends to be little to no wind, corresponding to winter anti-cyclones. There is an increased risk of very low wind speeds, with wind generation output less than 10% of theoretical maximum, on high demand days.

  6.  It is also important to recognise that the output of windfarms are correlated with each other, so that if a particular windfarm is suffering a lack of wind it is very likely that those nearby are too, and even the most distant windfarms are less likely to be generating. This is a result of weather systems such as windless anticyclones being large enough to affect all of the UK.

  7.  The precise correlation between UK wind generation and wind speed is complex and needs further analysis but, overall, we conclude that the relationship between the level of UK wind power output and UK electricity demand is very weak and, at best, the availability of wind generation is no better during high demand periods than in periods of lower demand.

Assessment of the extent to which we can rely on wind to generate to meet winter peak demand

  8.  From the point of view of the system operator (National Grid) who must schedule sufficient capacity in order to meet winter peak demand with a very high degree of probability, an assessment must be made of how likely it is that the capacity available will in fact operate at the time required. For this purpose, planned outages and weekend maintenance can be ignored as they are highly predictable and scheduled for low demand periods.

  9.  Excluding these factors, the weekday availability (actual availability/maximum total availability) of thermal (ie burning coal, gas, oil or biomass) plant over the winter period is about 95% with breakdowns accounting for 5% of maximum total availability. Crucially, breakdowns are generally not correlated with each other. There are exceptions and these have to be taken into account (such as loss of gas supply at a number of stations, or type faults), but they are generally a small effect. So, if a unit at Station X is unavailable, there is no reason to suppose that another unit at a different station is going to be unavailable. This means that conventional units are very effective at backing each other up, especially when the portfolio contains a mixture of plant types and fuels.

  10.  To assess the extent to which investment in wind capacity will be able to replace thermal plant on the system while ensuring that peak demand can be met at the same level of reliability, we need to assess how much wind capacity on the system can be relied on to meet peak demand at a dependability of 95%. Our assessment of winter wind generation data in 2007[5] indicates that the system operator could rely on 8% of total UK wind capacity to meet winter peak demand at the same level of dependability as thermal plant. On this basis, if the UK required, say, 40,000MW of wind capacity to meet its renewable target by 2020, only 8% of this renewable capacity (3,600MW) could be relied on to meet winter peak demand. This would avoid the need to build 3,600MW of new thermal plant but the remaining 36,400MW of renewable capacity would need to be "backed-up" by thermal plant to meet winter peak electricity demand in 2020. This effect could be to some extent mitigated by more extensive electricity interconnections with continental Europe (which would enable "back-up" power to be imported), the longer term development of new electricity storage technologies at a significant scale (which would be able to store power from the grid and produce it when required), or more demand side management capability which would enable demand to be varied in relation to the level of wind generation.

  11.  This assessment is consistent with other studies carried out in the UK and Germany[6], where there is extensive experience of operating grids with large volumes of onshore wind. However, for the UK, the calculation should be refined in the light of a more detailed assessment of the actual wind portfolio likely in 2020, further analysis of the correlation between wind speed and demand and an analysis using data over a longer period, but we believe the broad implications for future generation requirements will remain valid. We recommend that the issue is explored in more detail as part of the Government's forthcoming consultation on delivery of the UK's share of its renewable targets.

June 2008

5   Based on E.ON and E.ON contracted windfarms but assumed to be representative of all of UK. Back

6   DENA Grid Study: "Planning of the Grid Integration of Wind Energy in Germany Onshore and Offshore up to the Year 2020" Cologne, February 2005 Back

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