Long Term Wind Speed Variability in the UK

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Long Term Wind Speed Variability in the UK

• S J Watson and P Kritharas

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Introduction

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

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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?

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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

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A Good Geographical Spread

• 60 stations with 25 years of data
• 7 stations with 50 years of data
• Hourly data

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Seven Station Index
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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

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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)

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The UK-Wide Indices Compared
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UK Regional Indices
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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

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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)

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