Wind Energy Reference Manual Part 1: Wind Energy Concepts Unit Abbreviations m = metre = 3.28 ft.s = secondh = hourN = NewtonW = WattHP = horsepower J = Joule cal = calorie toe = tonnes of oil equivalent Hz= Hertz (cycles per second) 10 -12 = p pico = 1/1000,000,000,000 10 -9 = n nano = 1/1000,000,000 10 -6 = µ micro = 1/1000,000 10 -3 = m milli = 1/1000 10 3 = k kilo = 1,000 = thousands 10 6 = M mega = 1,000,000 = millions 10 9 = G giga = 1,000,000,000 10 12 = T tera = 1,000,000,000,000 10 15 = P peta = 1,000,000,000,000,000 Wind Speeds 1 m/s = 3.6 km/h = 2.237 mph = 1.944 knots 1 knot = 1 nautical mile per hour = 0.5144 m/s = 1.852 km/h = 1.125 mph
 Wind Speed Scale Wind Speed at 10 m height Beaufort Scale (outdated) Wind m/s knots 0.0-0.4 0.0-0.9 0 Calm 0.4-1.8 0.9-3.5 1 Light 1.8-3.6 3.5-7.0 2 3.6-5.8 7-11 3 5.8-8.5 11-17 4 Moderate 8.5-11 17-22 5 Fresh 11-14 22-28 6 Strong 14-17 28-34 7 17-21 34-41 8 Gale 21-25 41-48 9 25-29 48-56 10 Strong Gale 29-34 56-65 11 >34 >65 12 Hurricane
 Roughness Classes and Roughness Lengths The roughness class is defined in the European Wind Atlas on the basis of the roughness length in metres z 0 , i.e. the height above ground level where the wind speed is theoretically zero. ln is the natural logarithm function. if (length <= 0.03) class = 1.699823015 + ln(length)/ln(150) if (length > 0.03) class = 3.912489289 + ln(length)/ln(3.3333333) You may use the calculator below and enter either the roughness length or the roughness class. This calculator requires a Netscape 3, IE 4, or later browser to work. If you are using Navigator 3, IE 4, or later and you see this message, you need to enable JavaScript. In Netscape, choose Edit | Preferences | Advanced, and click Enable JavaScript. Then click reload on your browser. In Internet Explorer, choose Edit | Preferences | Web Browser | Java and click Enable Java. Then click reload on your browser. Do not use the calculator until this page and its programme have loaded completely. CALCULATOR Roughness Class Calculator Roughness length in m = Roughness class Roughness Classes and Roughness Length Table Rough- ness Class Roughness Length m Energy Index (per cent) Landscape Type 0 0.0002 100 Water surface 0.5 0.0024 73 Completely open terrain with a smooth surface, e.g.concrete runways in airports, mowed grass, etc. 1 0.03 52 Open agricultural area without fences and hedgerows and very scattered buildings. Only softly rounded hills 1.5 0.055 45 Agricultural land with some houses and 8 metre tall sheltering hedgerows with a distance of approx. 1250 metres 2 0.1 39 Agricultural land with some houses and 8 metre tall sheltering hedgerows with a distance of approx. 500 metres 2.5 0.2 31 Agricultural land with many houses, shrubs and plants, or 8 metre tall sheltering hedgerows with a distance of approx. 250 metres 3 0.4 24 Villages, small towns, agricultural land with many or tall sheltering hedgerows, forests and very rough and uneven terrain 3.5 0.8 18 Larger cities with tall buildings 4 1.6 13 Very large cities with tall buildings and skycrapers Definitions according to the European Wind Atlas, WAsP. For practical examples, see the Guided Tour section on Wind Speed Calculation.

 Density of Air at Standard Atmospheric Pressure Temperature °Celsius Temperature ° Farenheit Density, i.e. mass of dry air kg/m 3 Max. water content kg/m 3 -25 -13 1,423 -20 -4 1,395 -15 5 1,368 -10 14 1,342 -5 23 1,317 0 32 1,292 0,005 5 41 1,269 0,007 10 50 1,247 0,009 15 59 1,225 *) 0,013 20 68 1,204 0,017 25 77 1,184 0,023 30 86 1,165 0,030 35 95 1,146 0,039 40 104 1,127 0,051 *) The density of dry air at standard atmospheric pressure at sea level at 15° C is used as a standard in the wind industry.

 Viscosity of Atmospheric Air Temperature ° Celsius μ 20 1.80 E -5 1.50 E -5 50 1.95 E -5 1.79 E -5 Note: E -5 means exponential notation, i.e. the number should be multiplied by 0.00001
 Power of the Wind **) m/s W/m 2 m/s W/m 2 m/s W/m 2 0 0 8 313.6 16 2508.8 1 0.6 9 446.5 17 3009.2 2 4.9 10 612.5 18 3572.1 3 16.5 11 815.2 19 4201.1 4 39.2 12 1058.4 20 4900.0 5 76.2 13 1345.7 21 5672.4 6 132.3 14 1680.7 22 6521.9 7 210.1 15 2067.2 23 7452.3 **) For air density of 1.225 kg/m 3 , corresponding to dry air at standard atmospheric pressure at sea level at 15° C. The formula for the power per m 2 in Watts = 0.5 * 1.225 * v 3 , where v is the wind speed in m/s. Warning: Although the power of the wind at a wind speed of e.g. 7 m/s is 210 W/m 2 , you should note, that the average power of the wind at a site with an average wind speed of 7 m/s typically is about twice as large. To understand this, you should read the pages in the Guided Tour beginning with the Weibull Dustribution and ending with the Power Density Function.
 Standard Wind Class Definitions (Used in the U.S.) Class 30 m height 50 m height Wind speed m/s Wind power W/m 2 Wind speed m/s Wind power W/m 2 1 0-5.1 0-160 0-5.6 0-200 2 5.1-5.9 160-240 5.6-6.4 200-300 3 5.9-6.5 240-320 6.4-7.0 300-400 4 6.5-7.0 320-400 7.0-7.5 400-500 5 7.0-7.4 400-480 7.5-8.0 500-600 6 7.4-8.2 480-640 8.0-8.8 600-800 7 8.2-11.0 640-1600 8.8-11.9 800-2000 © Copyright 1997-2003 Danish Wind Industry AssociationUpdated 12 May 2003 http://www.windpower.org/en/stat/unitsw.htm