HomeMy WebLinkAboutSaint Mary's - Pitka Point Wind Energy Project - Pitka's Point Wind Resource Report - Apr 2012 - REF Grant 7040017Pitka’s Point, Alaska Wind Resource
Report
Pitka’s Point met tower, photo by Doug Vaught
April 25, 2012
Douglas Vaught, P.E.
V3 Energy, LLC
Eagle River, Alaska
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Summary
The wind resource measured at the Pitka’s Point met tower site is outstanding with measured wind
power class 6 by measurement of wind power density and wind speed. Extensive wind resource analysis
has been conducted in the Saint Mary’s region, with met towers at a lower elevation site closer to the
village of Saint Mary’s and near Mountain Village in addition to the Pitka’s Point met tower.
Documented in another report, the wind resource measured at the nearby Saint Mary’s met tower site
is less robust than that measured at Pitka’s Point and appears to experience similar icing problems. The
Mountain Village wind resource classification appears to be between those measured at Pitka’s Point
and Saint Mary’s. Considering the inland location of Saint Mary’s/Pitka’s Point, the wind resource
measure at the Pitka’s Point met tower site is highly unusual, and very favorable, with its combination of
a high annual average wind speed, relatively low elevation, likely good geotechnical conditions, and
proximity to existing roads and infrastructure.
Met tower data synopsis
Data dates October 26, 2007 to February 12, 2009 (16 months)
Wind power class Class 6 (excellent), based on wind power density
Wind power density mean, 38 m 558 W/m2
Wind speed mean, 38 m 7.62 m/s (17.0 mph)
Max. 10-min wind speed 29.5 m/s
Maximum 2-sec. wind gust 26.3 m/s (81.2 mph), January 2008
Weibull distribution parameters k = 1.93, c = 8.63 m/s
Wind shear power law exponent 0.176 (low)
Roughness class 2.09 (description: few trees)
IEC 61400-1, 3rd ed. classification Class II-c (at 38 meters)
Turbulence intensity, mean (at 38 m)0.076 (at 15 m/s)
Calm wind frequency (at 38 m)20% (<4 m/s)(16 mo. measurement period)
Test Site Location
A 40 meter NRG Systems, Inc. tubular-type meteorological (met) tower was installed on Pitka’s Point
Native Corporation land on the bluff immediately above
the Yukon River with excellent exposure to northeasterly
winds down the Andreafsky River, northerly winds from
the mountains and southerly winds from the flat, tundra
plains leading toward Bethel. The met tower site is near
an active rock quarry and visual inspection of that quarry
indicates the likelihood of excellent geotechnical
conditions for wind turbine foundations. Also of
advantage for the site is near proximity of the road
connecting Saint Mary’s to Pitka’s Point, the airport and
Mountain Village. A two-phase power distribution line (connecting the St. Mary’s powerplant to Pitka’s
Point as one phase and to the airport as the second phase) routes on the south side of the road. This
line could be upgraded to three-phase at minimal cost to connect wind turbines to the system.
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Photo of St. Mary’s from Pitka’s Point site, view to NE, Andreafsky River in background
Site information
Site number 0066
Latitude/longitude N 62° 02.252”W 163° 14.820”
Time offset -9 hours from GMT (Yukon/Alaska time zone)
Site elevation 177 meters (580 ft.)
Datalogger type NRG Symphonie, 10 minute time step
Tower type Tubular tall tower, 8-inch diam., 40 meter height
Tower sensor information
Channel Sensor type Height Multiplier Offset Orientation
1 NRG #40C anemometer 38.0 m 0.765 0.35 NNE
2
NRG IceFree III
anemometer 28.2 m 0.572 1.0 WNW
3 NRG #40C anemometer 28.8 m 0.765 0.35 NNE
4 NRG #40C anemometer 21.0 m 0.765 0.35 NNE
7 NRG #200P wind vane 38 m 0.351 260 080° T
8 NRG IceFree III wind vane 29 m 0.351 350 350° T
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9 iPack Voltmeter 0.021 0
10 NRG #110S Temp C 2 m 0.136 -86.383 N/A
12 RH-5 relative humidity 2 m 0.097 0
Google Earth image, Pitka’s Point and Saint Mary’s
Topographic maps
St. Mary’s
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Data Quality Control
Data was filtered to remove presumed icing events that yield false zero wind speed data and non-variant
wind direction data. Data that met criteria listed below were automatically filtered. In addition, data
was manually filtered for obvious icing that the automatic filter didn’t catch, and invalid or low quality
data for situations such as logger initialization and other situations.
Anemometer icing – data filtered if temperature < 1°C, speed SD = 0, and speed changes < 0.25
m/s for minimum 2 hours
Vane icing – data filtered if temperature < 1°C and vane SD = 0 for minimum of 2 hours
Tower shading of 29 meter and 28 meter (IceFree) paired anemometers – refer to graphic below
Because the met tower site is a known rime icing environment, it was thought that installation of a
heated anemometer and wind vane would result in much better data recovery than from standard non-
heated sensors, but that did not prove entirely true. As one can see in the table below, data loss due to
icing was actually higher from the IceFree anemometer than the standard anemometers, although data
loss due to icing from the IceFree wind vane is not quite half that from the standard vane. It is not clear
why data recovery from the heated anemometer was so poor. One possible explanation is excessive
voltage drop from the power line tie-in to the sensor on the met tower. Another explanation is simply
the difficult nature of the rime icing environment at the site.
Note also that all data was lost for the period from December 27, 2008 to January 7, 2009. The tower
itself collapsed during a severe rime icing event on February 12, 2009, although temperature and
relative humidity data collection continued for two additional weeks until March 1, 2009. The February
12 ice storm also resulted in the collapse of the nearby St. Mary’s met tower. The St. Mary’s met tower
was replaced in order to continue that study but with more than one year of Pitka’s Point data obtained,
it was decided not to replace the Pitka’s Point met tower.
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Sensor data recovery table
Sensor
Possible
Records
Valid
Records
<Unflagged
data> Icing Invalid
Low
quality
Tower
shading
Recovery
Rate (%)
Speed 38 m 74,016 52,519 52,519 15,962 2,702 0 0 70.96
Speed 28 m
IceFree 74,016 47,014 47,014 17,676 2,706 648 6,252 63.52
Speed 29 m 74,016 51,775 51,775 14,605 2,702 0 4,294 69.95
Speed 21 m 74,016 54,025 54,025 13,971 2,702 0 0 72.99
Direction 38 m 74,016 51,528 51,528 17,608 2,772 0 0 69.62
Direction 29 m
IceFree 74,016 58,876 58,876 9,803 2,772 0 0 79.54
iPack
Voltmeter 74,016 69,122 69,122 0 245 0 0 93.39
Temperature 74,016 68,694 68,694 0 673 0 0 92.81
RH-5 Relative
Humid.
(installed on
1/8/2009) 74,016 7,344 7,344 0 62,023 0 0 9.92
Sensor data recovery rate by month
anemometers vanes
Year Month 38 m
28 m
IceFree 29 m 21 m 38 m
29 m
IceFree
2007 Oct 86.9 75.4 88.3 88.3 79.2 28.1
2007 Nov 53.7 82.9 51.9 54.0 44.5 100.0
2007 Dec 74.1 69.3 80.1 82.7 77.5 79.0
2008 Jan 28.7 23.8 27.6 43.2 54.8 72.5
2008 Feb 76.3 73.5 79.2 78.5 65.8 86.7
2008 Mar 89.5 78.0 92.7 96.6 77.7 89.6
2008 Apr 39.3 71.1 49.1 42.4 66.9 76.5
2008 May 98.5 85.5 89.5 97.3 96.3 98.2
2008 Jun 100.0 89.8 89.3 100.0 100.0 100.0
2008 Jul 100.0 93.6 93.7 100.0 100.0 100.0
2008 Aug 100.0 92.6 97.0 100.0 100.0 100.0
2008 Sep 100.0 89.6 99.0 100.0 96.7 100.0
2008 Oct 97.9 88.2 98.3 97.9 93.8 95.6
2008 Nov 62.3 28.1 58.0 56.4 54.9 79.0
2008 Dec 65.4 31.9 65.5 64.6 46.6 72.6
2009 Jan 53.6 36.7 50.1 58.0 45.2 51.1
2009 Feb 40.9 22.7 42.3 42.3 37.9 36.6
2009 Mar 0.0 0.0 0.0 0.0 0.0 0.0
All Data 71.0 63.5 70.0 73.0 69.6 79.5
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Data loss due to icing conditions
Tower shading filter plot
Documentation of Icing
Rime icing is more problematic for wind turbine operations than freezing rain (clear ice) given its
tenacity and longevity in certain climatic conditions. For this reason, wind power at the Pitka’s Point site
should be developed with consideration to the possible need for anti-icing and de-icing measures.
These may include redundant control sensors, air-heated rotor blades, leading edge blade heaters, and
active operational intervention during winter months to visually detect and de-ice the turbines.
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
80.0%
90.0%
100.0%Anemometer Icing Data Loss
38 m
28 m IceFree
29 m
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An icing event leading to data recovery loss from the sensors is indicated in the January 15, 2009
photographs below, which clearly indicate the presence of icing conditions. This icing event is also
shown in the data graphs of January 15 below. Note that temperature is below freezing, relative
humidity is high, wind speed standard deviation equals zero, and the wind speeds are stopped at their
offset values of 0.4 m/s. These conditions met the criteria of icing conditions and were automatically
flagged by the wind analysis software.
Pitka’s Point Icing Event Photographs, 1/15/2009
Pitka’s Point Icing Event Data, 1/12/2009 to 1/16/2009
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Wind Speed
Anemometer data obtained from the met tower, from the perspectives of both mean wind speed and
mean wind power density, indicate an outstanding wind resource. Note that cold temperatures
contributed to a higher wind power density than standard conditions would yield for the measured
mean wind speeds.
Anemometer data summary
Variable
Speed 38
m
Speed 29
m
Speed 28
m IceFree
Speed 21
m
Measurement height (m) 38 28.8 28.2 21
Mean wind speed (m/s) 7.68 7.29 7.33 6.83
MoMM wind speed (m/s)7.62 7.24 7.33 6.78
Median wind speed (m/s) 7.20 6.80 7.00 6.40
Max wind speed (m/s) 29.50 29.20 27.50 28.40
Weibull k 1.94 1.89 2.22 1.88
Weibull c (m/s) 8.64 8.20 8.26 7.68
Mean power density (W/m²) 573 502 441 414
MoMM power density (W/m²)559 490 441 404
Mean energy content (kWh/m²/yr) 5,015 4,396 3,861 3,627
MoMM energy content (kWh/m²/yr)4,897 4,294 3,861 3,541
Energy pattern factor 1.95 2.00 1.73 2.01
Frequency of calms (%) (< 4 m/s) 20.4 21.9 17.6 24.7
MoMM = mean of monthly means
Time Series
Time series calculations indicate high mean wind speeds during the winter months with more moderate,
but still relatively high, mean wind speeds during summer months. This correlates well with the Saint
Mary’s/Andreafsky/Pitka’s Point village load profile where winter months see high demand for
electricity and heat and the summer months have lower demand for electricity and heat. The daily wind
profiles indicate relatively even wind speeds throughout the day with slightly higher wind speeds during
night hours.
38 m anemometer data summary
Mean Median
Max 10-
min avg
Max
gust (2
sec)
Std.
Dev.
Weibull
k
Weibull
c
Month (m/s) (m/s) (m/s) (m/s) (m/s) (-) (m/s)
Jan 10.17 10.70 29.5 35.9 5.34 1.97 11.45
Feb 9.21 9.20 20.1 23.3 4.07 2.41 10.36
Mar 8.62 8.50 21.8 26.3 4.33 2.07 9.71
Apr 7.98 7.80 16.9 20.6 2.83 3.05 8.90
May 7.27 6.90 21.8 27.1 3.67 2.06 8.19
Jun 5.70 5.80 13.2 15.3 2.62 2.28 6.40
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Jul 7.98 7.70 21.7 26.3 3.33 2.55 8.99
Aug 5.89 5.70 15.3 17.9 2.95 2.05 6.62
Sep 6.37 6.70 12.5 16.8 2.44 2.85 7.11
Oct 6.80 6.60 20.1 24.8 3.81 1.80 7.62
Nov 7.32 6.40 24.1 29.8 4.48 1.72 8.23
Dec 8.97 8.90 22.9 27.5 4.69 1.95 10.07
Annual 7.62 7.20 29.5 35.9 4.09 1.94 8.64
Monthly time series, mean wind speeds
Daily wind profiles (annual)
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Probability Distribution Function
The probability distribution function (PDF), or histogram, of the Pitka’s Point met tower site wind speed
indicates a shape curve dominated by moderate wind speeds and is reflective of a “normal” shape
curve, known as the Rayleigh distribution (Weibull k = 2.0), which is defined as the standard wind
distribution for wind power analysis. As seen below in the wind speed distribution of the 38 meter
anemometer, the most frequently occurring wind speeds are between 5 and 10 m/s with very few wind
events exceeding 25 m/s (the cutout speed of most wind turbines; see following wind speed statistical
table).
PDF of 38 m anemometer (17months’ data)
Weibull k shape curve table
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Weibull values table, 38 m anemometer
Weibull Weibull Mean Proportion Power R
k c Above Density Squared
Algorithm (m/s) (m/s) 7.678 m/s (W/m2)
Maximum likelihood 1.940 8.644 7.666 0.452 543.6 0.990
Least squares 1.898 8.692 7.713 0.454 566.8 0.988
WAsP 1.998 8.725 7.732 0.461 541.3 0.990
Actual data 7.678 0.461 541.3
Occurrence by wind speed bin (38 m anemometer)
Bin Endpoints
(m/s) Occurrences
Bin Endpoints
(m/s) Occurrences
Lower Upper No. Percent Cumul. Lower Upper No. Percent Cumul.
0 1 955 1.82% 1.82% 15 16 893 1.70% 96.52%
1 2 1,918 3.65% 5.47% 16 17 615 1.17% 97.69%
2 3 3,409 6.49% 11.96% 17 18 373 0.71% 98.40%
3 4 4,050 7.71% 19.67% 18 19 306 0.58% 98.98%
4 5 4,141 7.88% 27.56% 19 20 181 0.34% 99.33%
5 6 4,982 9.49% 37.04% 20 21 133 0.25% 99.58%
6 7 5,320 10.13% 47.17% 21 22 93 0.18% 99.76%
7 8 4,975 9.47% 56.65% 22 23 58 0.11% 99.87%
8 9 4,911 9.35% 66.00% 23 24 29 0.06% 99.92%
9 10 3,976 7.57% 73.57% 24 25 11 0.02% 99.94%
10 11 3,177 6.05% 79.62% 25 26 7 0.01% 99.96%
11 12 2,681 5.10% 84.72% 26 27 11 0.02% 99.98%
12 13 2,246 4.28% 89.00% 27 28 5 0.01% 99.99%
13 14 1,707 3.25% 92.25% 28 29 5 0.01% 100.00%
14 15 1,349 2.57% 94.82% 29 30 2 0.00% 100.00%
Wind Shear and Roughness
Wind shear at the Pitka’s Point met tower site was calculated with the three standard (non-heated)
anemometers installed on the met tower. The calculated power law exponent of 0.176 indicates
relatively low shear at the site. Calculated surface roughness at the site is 0.11 m (the height above
ground where wind speed would be zero) for a roughness class of 2.08 (description: few trees).
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Vertical wind shear profile
Comparative wind shear profiles
Wind shear by direction sector table
Time Mean Wind Speed (m/s) Best-Fit Surface
Direction
Sector Steps
Speed 38
m
Speed 29
m
Speed 21
m
Power Law
Exp
Roughness
(m)
345° - 15° 7,444 8.55 8.02 7.61 0.197 0.1777
15° - 45° 5,176 7.53 7.08 6.63 0.214 0.2655
45° - 75° 7,501 9.62 9.06 8.66 0.176 0.0973
75° - 105° 5,627 8.93 8.30 7.57 0.280 0.7842
105° - 135° 3,004 7.71 7.38 7.00 0.165 0.0645
135° - 165° 2,779 8.28 8.04 7.71 0.121 0.0070
165° - 195° 2,364 7.42 7.21 6.95 0.111 0.0035
195° - 225° 456 3.83 3.57 3.35 0.224 0.3280
225° - 255° 1,636 4.73 4.45 4.27 0.172 0.0857
255° - 285° 1,478 4.86 4.59 4.42 0.161 0.0572
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285° - 315° 2,821 6.24 6.00 5.88 0.098 0.0011
315° - 345° 4,845 6.92 6.54 6.38 0.136 0.0189
Extreme Winds
A modified Gumbel distribution analysis, based on monthly maximum winds vice annual maximum
winds, was used to predict extreme winds at the Pitka’s Point met tower site. Sixteen months of data
though are minimal at best and hence results should be viewed with caution. Nevertheless, with data
available the predicted Vref (maximum ten-minute average wind speed) in a 50 year return period (in
other words, predicted to occur once every 50 years) is 41.6 m/s. This result classifies the site as Class II
by International Electrotechnical Commission 61400-1, 3
rd edition (IEC3) criteria. IEC extreme wind
probability classification is one criteria – with turbulence the other – that describes a site with respect to
suitability for particular wind turbine models. Note that the IEC3 Class II extreme wind classification,
which clearly applies to the Pitka’s Point met tower site, indicates relatively energetic winds and
turbines installed at this location should be IEC3 Class II rated.
Site extreme wind probability table, 38 m data
Vref Gust IEC 61400-1, 3rd ed.
Period (years) (m/s) (m/s) Class Vref, m/s
3 29.2 35.5 I 50.0
10 35.4 43.1 II 42.5
20 37.0 45.0 III 37.5
30 39.6 48.2 S designer-
specified5041.6 50.6
100 44.2 53.8
average gust factor:1.22
Extreme wind graph, by annual method
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Temperature, Density, and Relative Humidity
The Pitka’s Point met tower site experiences cool summers and cold winters with resulting higher than
standard air density. Calculated mean-of-monthly-mean (or annual) air density during the met tower
test period exceeds the 1.204 kg/m
3 standard air density for a 177 meter elevation by 5.7 percent. This
is advantageous in wind power operations as wind turbines produce more power at low temperatures
(high air density) than at standard temperature and density.
Temperature and density table
Temperature Air Density
Mean Min Max Mean Min Max Mean Min Max
Month (°F) (°F) (°F) (°C) (°C) (°C) (kg/m³) (kg/m³) (kg/m³)
Jan 4.7 -20.2 39.0 -15.1 -29.0 3.9 1.325 1.204 1.416
Feb 4.1 -24.7 32.4 -15.5 -31.5 0.2 1.343 1.264 1.430
Mar 11.0 -14.3 38.8 -11.7 -25.7 3.8 1.275 1.204 1.397
Apr 19.5 -6.3 44.2 -7.0 -21.3 6.8 1.299 1.235 1.372
May 39.4 13.8 65.5 4.1 -10.1 18.6 1.247 1.185 1.314
Jun 49.2 29.5 70.2 9.5 -1.4 21.2 1.223 1.174 1.272
Jul 50.5 37.9 81.9 10.3 3.3 27.7 1.220 1.149 1.250
Aug 51.3 33.1 70.9 10.7 0.6 21.6 1.218 1.173 1.263
Sep 45.1 30.0 64.6 7.3 -1.1 18.1 1.233 1.187 1.270
Oct 22.7 5.0 37.2 -5.2 -15.0 2.9 1.290 1.252 1.339
Nov 16.3 -14.6 44.6 -8.7 -25.9 7.0 1.308 1.234 1.398
Dec 13.9 -16.2 45.0 -10.1 -26.8 7.2 1.307 1.204 1.403
Annual 27.4 -24.7 81.9 -2.5 -31.5 27.7 1.273 1.149 1.430
Pitka’s Point temperature boxplot graph
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Wind Speed Scatterplot
The wind speed versus temperature scatterplot below indicates cold temperatures at the Pitka’s Point
met tower site with a preponderance of below freezing temperatures. During the met tower test
periods, temperatures were often below -20° C (-4° F), the minimum operating temperature for most
standard-environment wind turbines. Note that arctic-capable (ratings to -40°C) wind turbines would be
required at Pitka’s Point.
Wind speed/temperature
Wind Direction
Wind frequency rose data indicates that winds at the Pitka’s Point met tower site are primarily bi-
directional, with northerly and east-northeasterly winds predominating. The mean value rose indicates
that east-northeasterly winds are of higher intensity than northerly winds, but interesting, the
infrequent south-southeasterly winds, when they do occur, are highly energetic and likely indicative of
storm winds.
Calm frequency (the percent of time that winds at the 38 meter level are less than 4 m/s, a typical cut-in
speed of larger wind turbines) was a very low 20 percent during the 16 month test period.
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Wind frequency rose (38 m vane) Mean value rose (38 m anem.)
Wind energy rose (38 m anem.) Scatterplot rose of 38 m wind power density
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Wind density (38 meter height) roses by month (common scale)
Turbulence
The turbulence intensity (TI) is acceptable with a mean turbulence intensity of 0.076 and a
representative turbulence intensity of 0.105 at 15 m/s wind speed, indicating quite smooth air for wind
turbine operations. This equates to an International Electrotechnical Commission (IEC) 3rd Edition (2005)
turbulence category C, which is the lowest defined category. These data are shown in the turbulence
intensity graph below. As seen, representative TI (90th percentile of the turbulence intensity values,
assuming a normal distribution) at 15 m/s is well under IEC Category C criteria at the Pitka’s Point met
tower site.
Turbulence synopsis
38 m anem. 29 m anem. Legend
Sector
Mean TI
at 15
m/s
Repres.
TI at 15
m/s
IEC3
Category
Mean TI
at 15
m/s
Repres.
TI at 15
m/s
IEC3
Category
IEC3
Categ.
Mean TI at
15 m/s
all 0.076 0.105 C 0.088 0.117 C S >0.16
315° to 045° 0.060 0.084 C 0.067 0.094 C A 0.14-0.16
045° to 135° 0.079 0.105 C 0.093 0.119 C B 0.12-0.14
135° to 225° 0.089 0.119 C 0.093 0.117 C C 0-0.12
045° to 135° 0.074 0.099 C 0.071 0.084 C
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Turbulence rose, 38 m anemometer, 38 m vane
Turbulence rose, 29 m anemometer, 29 m vane
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Turbulence intensity, 38 m, all direction sectors
Turbulence intensity, 29 m, all direction sectors
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Turbulence table, 38 m data, all sectors
Bin Bin Endpoints Records Standard
RepresentativeMidpoint Lower Upper In Mean Deviation Peak
(m/s) (m/s) (m/s) Bin TI of TI TI TI
1 0.5 1.5 1,336 0.403 0.185 0.640 1.833
2 1.5 2.5 2,692 0.197 0.105 0.331 1.111
3 2.5 3.5 3,834 0.139 0.070 0.228 0.600
4 3.5 4.5 4,076 0.114 0.059 0.190 0.917
5 4.5 5.5 4,528 0.100 0.051 0.165 0.827
6 5.5 6.5 5,278 0.091 0.041 0.144 0.364
7 6.5 7.5 5,126 0.085 0.039 0.135 1.169
8 7.5 8.5 5,027 0.079 0.034 0.122 0.637
9 8.5 9.5 4,538 0.073 0.030 0.112 0.449
10 9.5 10.5 3,503 0.074 0.028 0.110 0.255
11 10.5 11.5 2,881 0.074 0.026 0.108 0.229
12 11.5 12.5 2,488 0.074 0.024 0.105 0.271
13 12.5 13.5 1,966 0.075 0.023 0.105 0.197
14 13.5 14.5 1,519 0.075 0.022 0.104 0.191
15 14.5 15.5 1,054 0.076 0.022 0.105 0.241
16 15.5 16.5 777 0.079 0.022 0.107 0.177
17 16.5 17.5 484 0.082 0.022 0.111 0.163
18 17.5 18.5 322 0.089 0.023 0.118 0.203
19 18.5 19.5 260 0.086 0.020 0.112 0.144
20 19.5 20.5 148 0.085 0.018 0.109 0.138
21 20.5 21.5 113 0.088 0.012 0.103 0.130
22 21.5 22.5 75 0.087 0.013 0.104 0.112
23 22.5 23.5 49 0.085 0.010 0.098 0.107
24 23.5 24.5 11 0.092 0.010 0.105 0.105
25 24.5 25.5 8 0.097 0.014 0.114 0.127
26 25.5 26.5 7 0.089 0.024 0.119 0.137
27 26.5 27.5 10 0.081 0.013 0.098 0.104
28 27.5 28.5 7 0.075 0.013 0.091 0.100
29 28.5 29.5 2 0.071 0.013 0.087 0.080
30 29.5 30.5 1 0.085 0.000 0.085 0.085