Long Term Wind Speed Variability in the UK
14
Long Term Wind Speed Variability in the UK S J Watson and P Kritharas
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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. - PowerPoint PPT Presentation
Transcript of Long Term Wind Speed Variability in the UK
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
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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
1958
1960
1962
1964
1966
1968
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
0.80
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
7 Station Index (1958-2007) height correctedLinear (7 Station Index (1958-2007) height corrected)60 Station Index (1983-2007)Garrad HassanMet O UKCIP (1969-2006)
Index
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UK Regional Indices1
98
19
8
19
8
19
8
19
9
19
9
19
9
19
9
19
9
20
0
20
0
20
0
20
00.80.85
0.90.95
11.05
1.11.15
1.2North West
19
8
19
8
19
8
19
8
19
9
19
9
19
9
19
9
19
9
20
0
20
0
20
0
20
00.80.85
0.90.95
11.05
1.11.15
1.2North East
19
8
19
8
19
8
19
8
19
9
19
9
19
9
19
9
19
9
20
0
20
0
20
0
20
00.80.85
0.90.95
11.05
1.11.15
1.2Centre West
19
8
19
8
19
8
19
8
19
9
19
9
19
9
19
9
19
9
20
0
20
0
20
0
20
00.80.85
0.90.95
11.05
1.11.15
1.2
Centre East
19
8
19
8
19
8
19
8
19
9
19
9
19
9
19
9
19
9
20
0
20
0
20
0
20
00.80.85
0.90.95
11.05
1.11.15
1.2South West
19
83
19
85
19
87
19
89
19
91
19
93
19
95
19
97
19
99
20
01
20
03
20
05
20
070.8
0.9
1
1.1
1.2South East
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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
1950 1960 1970 1980 1990 2000 20106.0
6.5
7.0
7.5
8.0
8.5
9.0 TireeUncorrected
Height corrected
Year
An
nu
al
me
an
win
d s
pe
ed
(m
/s) Pressure tube
EH=16m
Munro MK4
EH=12m
Munro MK4
EH=10m
Vector MK6
EH=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
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