Posts Tagged ‘layout design’

Wind farm layouts are pretty controversial. The bare fact is that putting turbines in the most lucrative positions which catch the most wind generally means putting them on top of hills. Which makes them visible for miles around.

There’s not really much that can be done about this conflict.

Besides, developers of wind farms don’t, as a general rule, actually buy the land they’re building on. Usually they rent it under contract from the landowner. And although the area of the wind farm is usually large, there’s usually a fair bit of spare ground around the turbines which can continue to be used for livestock or crops. In Scotland, we have laws protecting the right to access land; this means that if you want to go mountain biking at an operational wind farm the law is on your side (up to the point where you do any malicious or criminal damage etc obviously).

Once a contract has been drawn up with the landowner or landowners for a particular wind farm, it’s time to design a layout. This remains a challenging issue.

There are a number of criteria which are likely to restrict your options before the wind can be taken into account. These will include bird and wildlife surveys; land use and availability for roads; waterways and steep valleys which restrict access to heavy plant; planning restrictions on tip height; noise considerations; nearby residents; ground suitability; and local considerations such as archaeology, sites of scientific interest, and so on.

From there, the best practice is to use actual wind measurements to model how the wind flow changes across the site. Because you need at least a year’s worth of data from a met mast before you can really use the data (to cover all seasons), the reality of this part will vary substantially depending on how far into the project we are. If the project has two years’ of measurements at one or more masts on site, then great. Otherwise there are other sources of wind information we can use: bought data from a Met office measurement station; a virtual met mast built from a model; reanalysis data based on satellite measurements; extrapolation based on a combination of measurements. If the worst comes to the very worst the rule of thumb that “higher elevation = windier” would provide at least a guide.

Once you have an idea of the wind flow, you need to decide where to put the turbines. There are a number of things to take into consideration when doing this.

Each individual turbine removes a little of the energy from the wind it encounters, resulting in a slower wind speed for those turbines behind it. It also increases the turbulence, which further reduces the effectiveness of the turbines behind: it’s harder to extract energy from turbulent air. The combination of these is called the “wake” effects in the industry. To reduce the impact, it’s considered best practice to leave between 4 and 7 rotor diameters’ worth of gap between the turbines. Larger spacing is generally left in the predominant wind direction so that the overall wake effect is lower. (Offshore the spacing is larger, because wakes travel further offshore for reasons to do with atmospheric effects. Best practice will also vary from region to region based on the appropriate climate drivers.)

Trees and slopes will have several impacts on your positions. The top of a hill will be the windiest location, but steep slopes can provide huge challenges for accessing the turbines for construction or maintenance. Steep slopes also tilt the wind to an angle, and above 17° or so start to cause real problems for accurate wind flow or turbine performance modelling. Forestry increases turbulence directly above the forest, and can have other effects on the wind flow (increased change in wind speed with height, for instance, and boundary effects at the edge of the forest) which reduce the efficiency of the turbines.

Dwellings should generally be avoided as far as possible. I think the guideline in Scotland is 500m (note: there are experts on these constraints, and I’m not one), but a much larger buffer zone is wise. The issues of noise and shadow flicker are only relevant with regards to nearby homes. The danger of ice throw from blades or of blade throw is not thought to be a risk beyond tip height of the turbine (so if the turbine is 160m tall and you’re more than 500m away the risk to your property from these things is vanishingly small). To be honest I think the main driver here is the good will of the community. Big wind farms are generally built by bespoke developers, and there is much to be lost in appearing to trample over communities.

You want to maximise both the number of turbines and their output. Developers (or the banks who lend to them) take on the financial risk of a project when they sink their money into constructing the wind farm; they get nothing back until they start to produce electricity. If the costs of building and maintaining the wind farm turn out to be more than the wind farm can generate, the project is a failure. So the energy output is actually critical to project success.

Ultimately, then, from an industry perspective, the challenges of layouts are as follows:

  • Comply with all planning restrictions
  • Keep the local community on-side as far as possible
  • Space the turbines 5 by 3 rotor diameters, which for an 82m rotor diameter machine (about average for large wind farms at present) is 410m by 246m
  • Keep the turbines away from steep slopes, forestry, and dwellings as far as possible
  • Install as many turbines as you can to increase your maximum production
  • Put your turbines as high up as you can manage

I’ve often seen the accusation “poorly sited” levied at wind farms in newspaper letters. Reading between the lines, I suspect that this is because the writer objects to wind farms on hills where they can be seen, rather than that they know a secret way of establishing the best place to put wind farms that the industry hasn’t stumbled on yet.


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