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The previous section presented wind maps for Europe and considered the wind resource at a strategic level. The purpose of this section is to consider the resource assessment and modelling at a local, wind farm, level. To the wind farm developer, the regional wind maps are valuable tools for site finding, but are not accurate enough to justify the financing of the development. Here, it will be shown that the single most important characteristic of a site is its wind speed, and that the performance of a wind farm is very sensitive to uncertainties and errors in the basic wind speed estimate.
For the majority of prospective wind farms, the developer must undertake a wind resource measurement and analysis programme. This must provide a robust prediction of the expected energy production over its lifetime. This section discusses the issues that are pertinent to recording an appropriate set of site wind data, and the methodologies that can be used to predict the expected long-term energy production of a project. It is noted that a prediction of the energy production of a wind farm is possible using methods such as the Wind Atlas Methodology within WAsP, using only off-site data from nearby meteorological stations. However, where the meteorological stations used have only data from low elevations, such as 10 m height and/or the stations are located far from the site, such analyses are generally used only to assess the initial feasibility of wind farm sites. It is also possible to make predictions of the wind speed at a site using a numerical Wind Atlas Methodology, based on a data source such as the “reanalysis” Numerical Weather Model data sets. Again, such data are usually used more for feasibility studies than final analyses. The text below describes an analysis where on-site wind speed and direction measurements from a relatively tall mast are available.
Figure I.2.2 provides an overview of the whole process. The sections below describe this process step by step. Appendix C provides a worked example of a real wind farm, for which these techniques were used to estimate long-term energy production forecast and compares this pre-construction production estimate with the actual production of the wind farm over the first year of operation. It is noted that this example is from a wind farm constructed several years ago, so the turbines are relatively modest in size compared with typical current norms. Also, some elements of the analysis methods have altered a little - for example a more detailed definition of wind farm loss factor is now commonly used.
Figure I.2.2 represents a simplification of the process. In reality it will be necessary to also iterate the turbine selection and layout design process, based on environmental conditions such as turbine noise, compliance with electrical grid requirements, commercial considerations associated with contracting for the supply of the turbines and detailed turbine loading considerations.
Figure I.2.2: Overview of the Energy Prediction Process
Source: Garrad Hassan
Note: For some sites no suitable reference station is available. In such cases, only site data is used and longer on-site data sets are desirable.
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