The Power tab (figure 96) contains information specific to the turbine power curve. Users can specify a value of turbulence intensity. This value is used in estimating losses due to increased turbulence intensity. By default, when switched on, turbulence intensity above this value results in 1% energy loss for every 3% of TI above the specified value. The turbulence loss can be turned on in the Settings->Energy Losses menu.
This gridded interface allows users to input one or more power curves, each of which corresponds to a value of air density. The input of more than one power curve allows the energy capture routine to interpolate between power curves. If the IEC adjustment is checked, the nearest power curve will be adjusted in line with IEC 61400-1(ed3) whenever the air density at a turbine is outside the range of air densities for which there are power curves. When the air density at a turbine is between two power curves, the IEC adjustment is not applied regardless of whether the IEC adjustment is checked. Clicking OK causes Openwind to sort the power curves by air density in addition to committing the changes made.
Add TI Range allows the addition of multiple families of power curves by wind velocity and air density - one for each range of TI defined. The standard energy capture uses this information to choose the correct turbine power curve based on the total TI and air density at the turbine location for the currently considered wind speed and direction step. The time series energy capture uses this information to select the correct power curve for each time step based on the air density and TI for this time-step.
Openwind can switch between families of power curves based on the total TI at 15m/s and assuming the normal turbulence model or just based on the total TI and assuming constant TI across all wind speeds. Users can set constant or normal turbulence model in dialog shown in figure 97.
If you only have power curves for a limited range of TI then you can use Openwind to adjust the power curves outside of this range. Figure 97 shows the dialog accessed by pressing the Settings button in figure 96.The TI adjustment method is very closely modelled on the consensus method from the power curve. In practice the TI adjustment is complicated by the fact that some manufacturers provide power curves valid at a fixed TI value over all wind speeds whereas other manufacturers provide power curves valid for a TI value which is the TI at 15m/s and assuming the normal turbulence model (NTM) at other wind speeds.
In principle, the method can be applied for any turbulence distribution and so it is only necessary to know what the turbine manufacturer is providing. It is possible to convert from fixed TI curves to NTM but it probably makes more sense to use the same TI distribution for both input and adjusted power curves. The normal turbulence model used here is defined in IEC61400-1 edition 3 and is the same as IEC61400-1 edition 2 for class A.
The idea of an inner and outer range for a turbine power curve is used such that if a family of power curves is valid for a range of TI then the power curves are considered equally valid for all TI within that range and so for values of TI outside of that range the power curves are adjusted up and down from the higher and lower ends of the range respectively.
The effects of the TI adjustments can be seen by going to File->Reports->TI adjusted power curves or by running a time series energy capture and plotting power production against wake-affected wind speed.