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Once a site has been identified and the decision has been taken to invest in its development, the wind farm design process begins. The fundamental aim is to maximise energy production, minimise capital cost and operating costs, and stay within the constraints imposed by the site. As the constraints and costs are all subject to some level of uncertainty, the optimisation process also seeks to minimise risk.
The first task is to define the constraints on the development:
- Maximum installed capacity (due to grid connection or Power Purchase Agreement terms);
- Site boundary;
- 'Set back’ - distances from roads, dwellings, overhead lines, ownership boundaries and so on;
- Environmental constraints;
- Location of noise-sensitive dwellings, if any, and assessment criteria;
- Location of visually-sensitive viewpoints, if any, and assessment criteria;
- Location of dwellings that may be affected by ‘shadow flicker’ (flickering shadows cast by rotating blades) when the sun is in particular directions, and assessment criteria;
- Turbine minimum spacings, as defined by the turbine supplier (these are affected by turbulence, in particular); and
- Constraints associated with communications signals, for example microwave link corridors or radar.
These constraints may change as discussions and negotiations with various parties progress, so this is inevitably an iterative process.
When the likely constraints are known, a preliminary design of the wind farm can be produced. This will allow the size of the development to be established. As a rough guide, the installed capacity of the wind farm is likely to be of the order of 12 MW per km2, unless there are major restrictions that affect the efficient use of the available land.
For the purpose of defining the preliminary layout, it is necessary to define approximately what sizes of turbine are under consideration for the development, as the installed capacity that can be achieved with different sizes of turbine may vary significantly. The selection of a specific turbine model is often best left to the more detailed design phase when the commercial terms of potential turbine suppliers are known. Therefore at this stage it is either necessary to use a ‘generic’ turbine design, defined in terms of a range of rotor diameters and a range of hub heights, or alternatively to proceed on the basis of two or three layouts, each based on specific wind turbines.
The preliminary layout may show that the available wind speed measurements on the site do not adequately cover all the intended turbine locations. In this case it will be necessary to consider installing additional anemometry equipment.
The preliminary layout can then be used for discussions with the relevant authorities and affected parties. This is an iterative process, and it is common for the layout to be altered at this stage.
The factors most likely to affect turbine location are:
- Optimisation of energy production;
- Visual influence;
- Noise; and
- Turbine loads.
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