Wind Farm Siting and Layout Design

The development of a wind farm is typically initiated by a land owner, a developer or the proactive planning of a local community.  In each case the process starts with finding a site that is suitable for a wind farm development.



1. SITE FINDING

Finding a wind farm site is a juggling act where many, often conflicting, issues need to be considered and balanced before a decision to develop a potential site is taken. At the most basic level, after establishing that the output from a wind power project can be sold at an acceptable price, the following are the acid tests for any potential development:


A good expectation that the answer to all of the above tests will be positive is a pre-requisite for making the investment necessary to realise a project, although inevitably early decisions need to be made on incomplete information.  Each of the issues is considered in a little more detail below:


Grid connection

When looking for a site, proximity to a medium voltage grid is a good initial indicator that an appropriate connection is practical. The next stage of the process is to hold discussions with the appropriate electrical authority.  The results of such discussions will usually indicate a cap, or a series of caps on the maximum installed capacity at a potential site which are associated with progressively more costly grid connection scenarios.  Some detailed analysis by the electrical authority, at the cost of the developer, may be necessary before even approximate figures are available.  The presence of only a high voltage line close to a small or medium wind farm may not be helpful as the cost of connection to such a grid may be prohibitive.



Wind Resource

It is difficult to generalise how best to assess the wind resource at a potential wind farm site when no site wind data are available, as different countries have markedly different wind regimes.  Some general rules, for which there are many notable exceptions, are listed below:



Building permits

Key issues will vary between regions and countries but common sense indicates that areas with special designations are best avoided.  Low visibility from key areas of habitation or recreation is also desirable.  If there are dwellings within a few hundred meters of the wind farm site noise or shadow flicker may prove an insuperable problem in some countries.  Turbines can interfere with electromagnetic telecommunications signals.  The presence of a telecommunications mast at a site or such signals which cross a site may therefore complicate the process of obtaining a building permit.  A check for television communications should also be made which may not be apparent from visual inspection.



Access

The distance to the nearest road access and the complexity of the terrain will substantially influence the capital cost of the project.  



Land availability

Land availability varies from country to country but a potential site where there are relatively few landowners and landowners who can give exclusive rights to the developer is the ideal situation.

The problem of site finding lends itself well to a thorough and detailed Geographical Information System (GIS) based approach where wind atlases, an electrical grid map, roads, environmental designations and other criteria can all be input and the optimal sites defined.  In practice, however, a more pragmatic approach may well prove more appropriate.



2. WIND FARM LAYOUT DESIGN

The wind farm layout is typically designed using a professional wind farm design package.  Such tools allows for an effective iteration and optimisation of the key parameters for the layout.



Preliminary layout design

Once a site has been identified and the decision has been taken to invest in its development the wind farm design procedure commences.  This is inevitably an iterative process.  The first task is to define the constraints on the development:


These constraints may change as discussions and negotiations progress with various parties.

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 achievable 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 the various suppliers are known.



Specification of anemometry

The wind resource at the site is the key parameter in determining its economic viability. To assess the energy for a project it is necessary to obtain data on the local wind regime. Typically this means installing anemometry equipment at the site. The preliminary layout allows the wind measurements to be made in appropriate locations. As a general rule the mast should be at least two thirds of the hub height of the turbines. A useful rule in complex terrain is that no turbine is located more than 1 km from the closest mast. In very severe terrain, the closest mast should be within 500m, but for wind farms located in simple terrain a much lower density of masts over the site may be appropriate. For large developments that require several masts there may be advantages in initially installing just one mast on the site. Once it is confirmed that the wind resource is reasonable, other masts can be installed to confirm the variation in wind speed over the site area. Provided the original mast remains as a constant reference other masts can be moved after, say, six months of operation to reduce the total number of masts required.+



Detailed layout design

A key element of the layout design is the minimum turbine spacing used.  In order to ensure that the turbines are not being used outside their design conditions, the minimum acceptable turbine spacing should be obtained from the turbine supplier and adhered to. The appropriate spacing for turbines is strongly dependent on the nature of the terrain and the wind rose at a site.  If turbines are spaced closer than 5 rotor diameters in a frequent wind direction it is likely that unacceptably high wake losses will result.  For areas with predominantly uni-directional wind roses, such as the San Gorgonio Pass in California, greater distances between turbines in the prevailing wind direction and tighter spacings perpendicular to the prevailing wind direction will prove to be more productive.  Tight spacings require approval by the turbine supplier if warranty arrangements are not to be affected.

With the wind farm constraints defined, the layout of the wind farm can be optimised.  This process is also called wind farm “micrositing”.  The aim of such a process is to maximise the energy production of the wind farm whilst minimising the infrastructure and operating costs and meeting all constraints.  For most projects the economics are substantially more sensitive to changes in energy production than infrastructure costs.  It is therefore appropriate to use the energy production as the dominant layout design parameter.

The detailed design of the wind farm is facilitated by the use of commercially available wind farm design tools.  Once an appropriate analysis of the wind regime at the site has been undertaken, a model is set up which can be used to design the layout, predict the energy production of the wind farm as well as being used to address economic and planning related issues.

For large wind farms it is often difficult to manually derive the most productive layout.  For such sites a computational optimisation using a wind farm design tool may identify a layout for which substantial gains in predicted energy production are achieved.  Even a 1 % gain in energy production from improved micrositing could easily represent an increase in annual revenue of $50,000 to $100,000 for a 50 MW wind farm.  The computational optimisation process will usually involve many thousands of iterations and can include noise and visual constraints.  Wind farm design tools conveniently allow many permutations on wind farm size, turbine type, hub height and layout to be considered quickly and efficiently increasing the likelihood that an optimal project results.  Financial models may be linked to the tool so that returns from different options can be directly calculated, further streamlining the development decision making process.

In many countries the visual influence of a wind farm on the landscape is an important issue.  The use of computational design tools allows the Zone of Visual Influence (ZVI), or visibility footprint, to be calculated to identify from where the wind farm will be visible.  The tools may also be used to provide visualisations, to facilitate the production of photomontages and to predict the noise and shadow flicker which results from a proposed development.  These are often key aspects of the Environmental Assessment for a project.

Figure 1 shows an initial preliminary layout of a wind farm consisting of 26 turbines that meets all site specific constraints.  There are two noise sensitive dwellings west of the proposed wind farm with a defined noise limit that are marked with dwelling icons.  The solid black line represents the site boundary in which the turbines can be placed.



GH WindFarmer
Figure 1


The layout of the wind farm after the optimisation is shown in Figure 2.  Compared with the initial layout the predicted energy production has increased by approximately 3 %.  In the upper section of Figure 2 the optimised layout of the wind farm superimposed with the noise levels predicted for this layout can be seen.  A rendered visualisation of the wind farm appearance from a viewpoint southeast of the wind farm is shown at the bottom.



GH WindFarmer
Figure 2



Conclusion

The design of a wind farm is a compromise between high energy yield, easy access, easy permitting and commercial viability. Careful consideration of a very large number of factors is typically required to reach the best designs and consequently dedicated software tools are used by the majority of wind farm developers.



For more information, see:



Wolfgang Schlez
Andrew Tindal
Garrad Hassan and Partners Ltd
www.garradhassan.com