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The gross energy production is the energy production of the wind farm obtained by calculating the predicted free stream hub height wind speed distribution at each turbine location and the manufacturer’s supplied turbine power curve. In defining the gross energy output, it is assumed that there are no wake interactions between the turbines and none of the loss factors listed in the remainder of the energy table are applied. This result includes adjustments to the power curve to account for differences between the predicted long-term annual site air density and the air density to which the power curve is referenced. It also includes the effect of the terrain on the flow.
Wind turbines extract energy from the wind and downstream there is a wake from the wind turbine, where wind speed is reduced. As the flow proceeds downstream, there is a spreading of the wake and the wake recovers towards free stream conditions. The wake effect is the aggregated influence on the energy production of the wind farm, which results from the changes in wind speed caused by the impact of the turbines on each other. It is important to consider wake effects from neighbouring wind farms and the possible impact of wind farms which will be built in the future.
This factor defines the expected average turbine availability of the wind farm over the life of the project. It represents, as a percentage, the factor which needs to be applied to the gross energy to account for the loss of energy associated with the amount of time the turbines are unavailable to produce electricity. Similar factors are needed for the 'Balance of Plant' availability, which relates to the electrical infrastructure of the site and 'Grid Availability', which relates to the availability of the grid over which power can be exported.
Electrical Transmission Efficiency
This will include the electrical losses encountered when the wind farm is operational and which will be manifested as a reduction in the energy measured by an export meter at the point of connection. This is presented as an overall electrical efficiency, and is based on the long-term average expected production pattern of the wind farm. It is also necessary to consider the power that the wind farm consumes when the wind farm is not operational.
There will be electrical losses experienced between the low voltage terminals of each of the wind turbines and the wind farm Point of Connection, which is usually located within a wind farm switching station.
In an energy production calculation, a power curve supplied by the turbine supplier is used within the analysis.
Most wind turbines will shut down when the wind speed exceeds a certain limit. High wind speed shutdown events can cause significant fatigue loading. Therefore, to prevent repeated start up and shut down of the turbine when winds are close to the shutdown threshold, hysteresis is commonly introduced into the turbine control algorithm. Where a detailed description of the wind turbine cut-in and cut-out parameters are available, this is used to estimate the loss of production due to high wind hysteresis, by repeating the analysis using a power curve with a reduced cut-out wind speed. It is also necessary to adjust for any generic or site specific issues, which may mean that for a specific site the wind turbine will not perform in accordance with the supplied power curve.
In certain conditions, dirt can form on the blades or, over time, the surface of the blade may degrade. Also, ice can build up on a wind turbine. These influences can affect the energy production of a wind farm in the ways described below. Extremes of weather can also affect the energy production of a wind farm; as can the growth or felling of nearby trees.
For wind farm sites located within or close to forests or other areas of trees, the impact of how the trees may change over time and the effect that this will have on the wind flow over the site, and consequently the energy production of the wind farm, must be considered. The impact of the future felling of trees, if known, may also need to be assessed.
Some or all of the turbines within a wind farm may need to be shut down to mitigate issues associated with turbine loading, export to the grid, or certain planning conditions.
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