Finding the perfect site for a wind farm

Paul Boughton

Finding the best possible sites for new wind farms is a complex and challenging task which demands some very sophisticated processes. Samuel Clarke provides an insight into how their experts routinely and successfully tackle the job.

Advanced geographical information system (GIS) methodology which combines database information with mapping and modelling is at the heart of the process. Even where wind farm developers have their own in house GIS capability, it can often be faster and more cost-effective for an external consultancy to carry out in just a few weeks the intensive research programme that's required.

The parameters used to identify sites can be fine-tuned to suit the individual requirements of any developer - such as wind farm and turbine size, minimum wind speed, distance from houses, designated areas, roads, rail and proximity to the electrical distribution grid. The size of overall land areas reviewed can vary widely from 20,000m2 up to 2000 hectares or more.

Projects are typically completed in several key stages, starting with constraint mapping to eliminate unsuitable areas.

Mapping the constraints

Reviewing all the constraints that can limit the best sites to go for can include a long list of considerations including available wind resource, noise, environment, history, landscape, aviation, air defence, electrical connectivity and electro-magnetic interference - as well as the location of roads, railways and rivers, and slope gradients.

First, wind resource. Data is taken from the 45m above ground level (AGL) 1km2 resolution UK Wind Atlas published by BERR and data is adjusted using a standard log law function to represent the annual mean wind speed (AMWS) at 80m AGL - a height chosen to represent a typical 2MW turbine. The data is then processed via GIS to select only those 1km squares that have an AMWS of 6.5m/s or greater - an economic cut off point.

Noise is next. The actual distance necessary to achieve a satisfactory level of detectable noise at a dwelling will depend on the type of wind turbine and the physical environment, but a set back distance of 600m to dwellings and other buildings is typical. Where the occupier has a financial interest in the wind turbine, a higher level of detectable noise may be acceptable. A higher level is also permitted when background noise levels are higher - for example areas of land adjacent to major transport links. 5dB above background noise levels is usually permitted.

Environmental, landscape and historic considerations are often critical and sensitive factors in wind farm site finding. They include Sites of Special Scientific Interest, National Parks, and Scheduled Ancient Monuments which are removed from the search area.

Aviation and air defence issues are also critical, in order to avoid any risk of disruption to key installations responsible for security, air safety and meteorology.

Wind farms are generally connected to the 33-66kV electricity distribution grid. The cost of this connection can have a significant bearing on economic viability, and the distance from the wind farm to the grid can have significant impact on the connection cost. Sites within 10 kilometres of the grid are therefore preferable to keep cabling costs low.

Electromagnetic interference generated by wind turbines can affect wireless communication - for example shadow or digital interference to TV reception. Using a database of all microwave links and television re-broadcast links registered with OfCom, potential problem areas are identified.

After mappping the constraints we have a set of provisional wind farm areas. But not all will make good potential wind farm sites. Normally around 100 sites are selected, with flexibility to enable the developer to make decisions as the project progresses.

At this point it's not only about software and data sets. Experience, expertise and human judgment are essentia. Sites which are accepted are now manually populated within GIS with wind turbines - anything between two and 50 in each of the 100 farms. Assuming a prevailing wind direction of west south west, turbines are placed where the topography allows in NNE-SSE arrays, perpendicular to the prevailing wind - optimising the energy and the revenue that can be generated.

At this point therefore we know how much each of the 100 farms is capable of generating. This allows them to be compared and ranked again in order of generating capability. This critical information is presented in a number of formats, including a map for each of the proposed sites with wide-ranging information including NOABL average annual wind speed, number of turbines, generating capacity, indicative annual energy output and issues identified. The same information is also indexed within a table for easy reference. Each drawing also contains detailed elements such as adjacent potential wind farms, regional boundaries, turbine layout and elliptical buffers, wind speed environment, distribution and national electric grid, microwave links, TV broadcast and rebroadcast links, NATS and weather radars, airfields, planning and regional boundaries, and environmental, landscape and historic designations.

- Samuel Clarke is a geographical information system and renewable energy consultant with independent engineering and environmental consultancy Wardell Armstrong, headquarted in Stoke-on-Trent, Staffs, UK.