Wind Energy Systems and Wind Farms
Components of a Wind Turbine
Most components of a horizontal-axis WEC are contained within the nacelle, which sits on top of the tower:
The Power Curve
The power coefficient measures the percentage of wind power that is transformed onto the shaft into mechanical power. TIf all wind power could be captured by a rotor device, the air would be completely still behind it, so that no more wind could pass. Hence, there is a limit to the rotor efficiency. This theoretical limit, the so-called "Betz-Limit" is cp,max= 59.26%. Modern wind turbines come close to 50%.
The power coefficient is also dependant on the wind speed. Low wind speeds (below the cut-in speed) will not move the blades. In high wind speeds, the efficiency decreases again.
The electric power output from the generator is the wind power multiplied by the power coefficient:
It is zero below the cut-in wind speed, proportional to the cubic wind speed up to the nominal power, PN, at the rated wind speed, vN. The power output is almost constant at speeds above the rated wind speed until the turbine has to be switched off above the cut-out speed.
To estimate the energy yield from a chosen wind turbine at a particular site, multiply the electric power curve with the wind speed distribution for that site, as the wind speed distribution of hourly means wind speeds reflects the probability of wind speeds occurring.
Most energy yield comes from the wind speeds where the power coefficient is maximum, not where the power is maximum (i.e. the rated power). To maximise the energy yield, compromise between high frequency of low wind speeds and low frequency of high wind speeds!
Larger areas can be covered by multiple wind turbines, forming a wind farm. Unfortunately, adding a second or third turbine does not double or triple the energy yield. The wind farm efficiency is
defined as the ratio of actual production of N wind turbines in the farm configuration to the theoretical production of N undistrubed turbines. Good farms should have a farm efficiency of at least 85%.
WECs that operate downwind from another turbine are impacted by the wake effect: Behind the tower and rotor, the cone-shaped core area of the wake has constant but reduced wind speed. The wider area behind the whole area covered by the blades, the peripheral wake area, suffers from high turbulence, which also reduces the energy yield of WECs in the wake.
The reduction from 1st to 2nd WEC is more pronounced than from 3rd to 4th. The wake effect is more severe below the rated power. Also, the wake effect is worse when the wind direction is not in the same line as the turbines, as different air flows cause wear and tear.
To avoid the wake effect: Space turbines in main wind direction with more than 4 times the rotor diameter distance, and perpendicular to main wind direction twice the rotor diameter. Fewer turbines may yield more energy!