Energy calculated efficiencies

The traditional approach to calculating the energy production of a turbine is using the long term frequency distribution of the wind regime that the turbine subject to.

WindFarmer's energy calculation is able to model several efficiencies through its frequency domain calculation. To model each of the efficiencies, the calculation performs several runs. It starts by calculating the full energy production which includes all the effects a turbine is subjected to and subsequently removing each effect to calculate its efficiency.

Each of the efficiencies calculated by the energy model that serve as input to the net energy model is detailed in the following sections. Also see energy calculation in scripting introduction for detail of which models are used in with runs.

Wake effect internal

The wake effect internal efficiency reported by WindFarmer in the net energy calculation and EPA report is the combination of two efficiencies: the internal wake and the internal large wind farm.

Internal wake

The internal wake accounts for the wake effect within the subject wind farm. It is obtained by applying the selected wake model. The result of this efficiency per wind farm can be observed in the Efficiency results task.

Large wind farm model

The large wind farm model efficiency excludes any large wind farm effects from neighbouring wind farms. See large wind farm model for a complete description of the model. The result of this efficiency per wind farm can be observed in the Efficiency results task.

Blockage effect

Calculated using DNV's Blockage Effect Estimation Tool (BEET). See Wind farm blockage for description of the model.

High wind speed hysteresis

All modern turbines have a maximum wind speed above which the turbine is designed to shut down, called the high wind cut-out. When wind speeds exceed cut-out, the turbine shut down and remains offline until wind speeds are reduced to a designated wind speed, the high wind cut-in, below the initial cut-out. This delay in re-starting at high wind speeds is referred to as high wind hysteresis.

Since the re-starting of the turbine is dependent on prior wind speed it is not posible to model through the frequency domain approach or with 10 minute averaged data. WindFarmer uses an adjusted high wind speed cut-out that is between the high wind cut-out and cut-in to model the hysteresis effect.

Wake effect external

The presence of neighboring wind farms adds extra wake losses and can introduce a change in the boundary layer that impacts our subject wind farm.

WindFarmer will account for this efficiency by doing a run where all non-excluded turbines and farms in the workbook are present in the calculation.

Wind sector management

The frequency domain energy calculation method is only dependent on wind speed and wind direction, so only curtailment strategies that depends on these two parameters can be modelled. To simulate any time dependent curtailments you must use the power time series calculation.

The WindFarmer energy calculation is able to derive the energy efficiency of wind sector management strategies that shutdown or change a turbine's operational mode based on incident wind speed and/or wind direction conditions.

Upflow

Energy production for turbines in a flat terrain is not the same as for a hilly one. Several factors contribute to that difference. One of these factors is the fact that wind reaching a given position will have a vertical and a horizontal component.

The energy impact of this split is given by an upflow efficiency which is calculated as follows. Inflow angles are calculated for each turbine in each direction. Downslopes are set to zero, then a single upflow angle is calculated for each turbine weighted by directional frequency. An efficiency factor for each turbine is calculated by taking $$\cos^2(\theta)$$ where θ is the all directional inflow angle. Turbine results are then weighted by energy yield to give an overall upflow efficiency.