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Long Term Wind Speed Variability in the UK

Long Term Wind Speed Variability in the UK. S J Watson and P Kritharas. Introduction. The need for an index Creation of a wind index using observed wind speed data Other wind indices Results Discussion Conclusions. The Need for a Wind Index.

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Long Term Wind Speed Variability in the UK

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  1. Long Term Wind Speed Variability in the UK S J Watson and P Kritharas

  2. Introduction The need for an index Creation of a wind index using observed wind speed data Other wind indices Results Discussion Conclusions

  3. The Need for a Wind Index How far in the past do you need to go to get a reasonable estimate of the long term mean wind speed? How much inter-annual variability in wind speed is to be expected? Can we detect climate change influences? Bottom line: how will wind variability affect wind farm yield over lifetime?

  4. Construction of an Index Need long term reliable measurements of wind speed Well exposed sites Not too much missing data over period of interest Confidence in the instrumentation

  5. A Good Geographical Spread • 60 stations with ~25 years of data • 7 stations with ~50 years of data • Hourly data

  6. Seven Station Index

  7. Calculating an Index Average wind speed for all sites by year Calculate long term average of all sites over all years Divide annual average by long term average to create an index for the period Index created for UK and sub-divided by six UK regions

  8. Other Indices Compared GL-Garrad Hassan Index based on ERA-40 reanalysis data (1 degree data) Index based on UK Met. Office gridded dataset (5km x 5km grid) – created for climate change research (UKCIP project)

  9. The UK-Wide Indices Compared

  10. UK Regional Indices

  11. Discussion Three UK indices broadly the same variation but trends are different Met O UKCIP index shows significant decline Our 7-station index shows very small decline ERA-40 shows slightly increasing trend 7-station index indicates variability of +/-8% about long term average (95% confidence) Decadal swings common Regional indices show evidence of decrease in NW and increase in SE but only slight – not inconsistent with regional climate change model predictions…

  12. Some Points to Note Increased urbanisation may reduce wind speeds at urban meteorological sites – this may be an issue for the Met O UKCIP generated index Instruments are changed over the years (technology changes) Wind speed measuring heights have changed over the years (generally decreased – 10m is now used as standard)

  13. Example: height correction to 10m Height correction assuming log profile and short grass

  14. Conclusions 25-year wind inter-annual variability +/-8% Worse case swing in wind farm yield ~32% Indices show large decadal swings No firm evidence of a long term decline/increase in average UK wind speeds Possible regional trends (NW decline/SE increase) – but tenuous Difference in trends between indices: exposure, instrument changes, measurement height changes, locations/grid points, resolution

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