The Power in the Wind is also affected by Air Density. The altitude of your Wind Project is vital as you estimate the performance of wind hardware at your site.
The standard density for air is 1.226 Kg/m3 and corresponds to a temperature of 15 degrees C. The Air Density changes with altitude using a formula:
Air Density (p) = 1.226 – (1.194 x 10-4) z
Air Density is in Kg/m3 and z is the altitude of your site in Meters. For example, the average Air Density in Amarillo, Texas is about 1 Kg/m3.
Compared to a site at Sea Level, at constant temperature zero degrees C, a wind generator near Amarillo, Texas would produce nearly 24% less Power for the same equipment used at sea-level. Use a wind resource data logger to record wind zone and wind speeds.
Higher altitudes will lower air pressure reducing the Air Density, which in turn effects the output of your wind generator. An altitude of 1000 meters (approximately 3,280 feet) will reduce the air pressure nearly 10% and therefore the power by nearly 10%. Include this in your planning and site analysis.
The height of your Tower impacts the Power you can extract from the wind. The higher the tower the less ground effects and friction from the earth is experienced by the wind flow and flows at greater speeds the higher you go. This effect is often referred to by the 1/7th law. The exponent á is approximated by 1/7.. The formula to determine the effect of Tower height on wind flow is:
Wind Speed Velocity (m/s) = Vo (H/Ho)á
HINT: If your wind site average wind speed is not high enough to qualify for some State Rebate programs, then increase your tower height until the minimum wind speed is reached – qualifying your project for funding. Use this equation to solve for the new tower height you need to qualify.
NOTE: This equation requires you have a known wind speed at some height Ho. H will be the term for your “new” tower height.
To determine your sites Average Wind Speeds you can look at reference maps (See Resources at the end of the book for a list of these links), or you can make direct measurements. To measure wind speeds (and direction) using an anemometer requires you set up a Meteorological Tower called a Met-Tower. Typically, you’ll choose a height for your Met-Tower to measure as close to the Hub Height of the wind generator you wish to use.
Depending on the hardware you choose this can range from 20 meters to 50 meters in height.
HINT: Use Wind Resource Maps when possible to avoid the cost of setting up a Met-Tower on site. This saves you money.
The rules (standards for wind measurements) state that you should place your Anemometer on an “arm” extending from your Met-Tower at least six (6) tower diameters away from the tower – to insure accurate measurements unaffected by the “wash” that occurs when wind travels around objects. If your Tower is of Lattice design then only four (4) diameters are required.