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Advantages of Wind Energy
Why Wind Energy?
The main drivers for wind power:
- the primary energy (wind) is cost-free
- the primary energy never runs out
- wind turbines cause no carbon, air emissions nor hazardous waste
- there is an abundant resource; creating power independence in many regions of the world
- fossil fuels are neither renewable nor clean
- Wind Energy creates know-how and human labor
Wind power is a proven technology:
It’s promoting energy independency
Power in the wind:
One of the most important tools in working with the wind, is a firm understanding of what factors influence the power in the wind.
Performance and siting of Small Wind Turbines:
The amount of energy in the wind is a function of its speed and mass. At higher speed more energy is available. This relationship among mass, speed and energy is given by the equation for kinetic energy where (m) represent the air’s mass and (V) is its velocity, or speed in common parlance.
Kinetic Energy = 1/2 mV2
The air’s mass can be derived from the product of its density (ρ)* and its volume. The volume must be found by multiplying the wind speed (V) by the area (A) through which it passes during a given period of time (t).
m = ρAVt
When we substitute this value for mass into the earlier equation, we can find the kinetic energy in the wind:
Wind Energy = P= ½ ρAVtV² = ½ ρAtV³
Power (P) is the rate at which energy is available, or the rate at which energy passes through an area per unit of time.
We have learned that Power is depending on air density, the area intercepting the wind and wind speed. Increase any one of these and you increase the power available from wind. But most importantly, slight changes in wind speed produce significant effects on the power available.
Air density: air density decreases with increasing temperature. Air is less dense in summer than in winter, varying 10 to 15% from one season to the next. Change in elevation can produce severe changes in air density. Air density at sea level and at 15°C is 1,225 kg/M³.
Swept Area: power is direct related to the area intercepting the wind. Doubling the area swept by a wind turbine rotor will double the power available to it. Knowing this, you can quickly size up any wind machine by noting the dimensions of its rotor. The rotor sweeps a disk the area of a circle A=πr²
Where (A) is the area and (r) is the radius of the rotor. Swept area is proportional to the square of the rotor’s radius. Compare the area swept by one wind turbine with a rotor diameter of 10 m and that of another with a rotor diameter of 12 m the increase of capture are is 44%.
Wind speed: no other factor is more important to the amount of wind power available to a wind turbine than the speed of the wind. Because the power in the wind is a cubic function of wind speed, changes in speed produce a profound effect on power. If the wind speed increase from 10 to 12 m/sec (is 20%) there is a 73% more power available.
Cp value: the Cp value is a number for the efficiency of the wind turbine design and the type of wind turbine. Savonius rotor has a very low Cp value of 0,1 but a modern MW turbine (horizontal axis and 3 blades ) reach a value of 0,5. You can’t extracting for 100% the energy from de wind. Theoretical max power you can extract from wind is 0,6 (Betz factor). Small Wind Turbines have a Cp value of 0,3 to 0,4 in general.
Power density (P/A) the rate at which energy passes through one square meter rotor area for an annual average wind speed. Power density is given in Watts/square meter (W/M²)
P/A = ½ ρV³
To make this easy to understand, see table below, the Power density for different wind speeds. (In the book of Paul Gipe# you can study how we get to these values).
Annual average wind speed: 4 5 6 7 8 m/sec
Annual Energy Density: 656 1,281 2,214 3,515 5,247 kWh/M²
Estimating the amount of energy available annually to the wind turbine becomes simply the product of energy density (E/A) and the turbine’s swept area (A) in square meters. For example, the Fortis Passaat intercepts 7,64 m² of the wind stream. At a site with a 4 m/sec average wind speed it will intercept about 5000 kWh per year. It won’t actually capture that much because of the Cp value and system losses.
Wind speed, power and height: Wind speed and hence power, varies with height above the ground. Wind moving across the earth’s surface encounters friction caused by the turbulent flow over and around mountains, hills, trees, buildings and other obstruction in its path. These effects decrease with increasing height above the surface until unhindered air flow is restored. Consequently, as a friction and turbulence decrease, wind speed increase.
Windspeeds increase with height, therefore it benefits of using a tall tower. But in general we are restricted by the local rules.