Fluctuations of temperature, atmospheric pressure and humidity may introduce a variation of up to 10% on future wind energy earning estimates.
1) 10% air density error due to Weather (temperature, pressure and humidity) may severely limit accuracy of your wind energy calculations rendering your anemometer accuracy irrelevant.
- Wind energy measurement standards (IEC 61400-121) require recording air temperature and barometric pressure along with wind speed and wind direction.
- Air humidity measurement is highly recommended for locations that regularly have high air temperatures. (Humidity effect on density increases with temperature.)
Air Density Change Examples:
- Relative Humidity change from 30% to 90% can change air density by 1% at 31°C (88°F). At 22°C (72°F) air density change is 0.6% and 0.4% at 15°C (59°F).
- Air temperature change from winter to summer (5°C to 32°C, 41°F to 90°F) can change air density by as much as 10%.
- Elevation height change of only 82 meters (270′ vertical feet) from sea level can change air density by 1%. (101325Pa -100344Pa =981 Pascals)
- Weather front pressure changes range from 980 and 1050 hPa which corresponds to a 7% fluctuation in air density.
(see our Air density calculator for Wind Energy: Impact of Altitude, Temperature, Humidity and Barometric Air Pressure)
2) Altitude, Weather (Barometric Pressure), Temperature and Humidity have important influence on wind energy calculations.
Wind Energy is proportional to air density. According to Betz’ law windpower is:
- Power from Wind = 1/2 *ρ *A * V³ (where ρ = air density, A =wind turbine propeller disk area (propeller radius squared times π (Pi =3.14)), V = wind speed)
Air Density depends on Pressure and Temperature according to:
- Air Density =P / (R * T) (thus, as P = pressure decreases due to weather or elevation so does air density (linearly) and inversely as T = temperature decreases air density linearly increases) (R = Universal Gas Constant = 8.3145 J/mol/K)