The Low Carbon Partnership

Site considerations

What is the wind resource at your site?
The amount of energy that can be harnessed by a wind turbine is proportional to the wind speed cubed. This means very small variations in wind speed can significantly increase, or reduce, electricity generation. Wind turbines usually work best in strong, steady winds, though electricity can be generated in an intermittent wind supply.

The suitability of a wind turbine is very specific to the site selected for installation. It is not possible to determine whether a wind turbine is the best option for you without looking at wind speed data for the location of your site. The wind speed at your site can be measured using an anemometer mounted on a mast at the height of the proposed wind turbine. Ideally, data would be collected over a year to take into account nearby obstructions (see below) and variations to the wind resource.

As a general rule, a site with an average annual wind speed of 6m/s or more might well benefit from a wind turbine. For a rough guide to the wind resource at the location of your site, see BERR’s wind speed database which lists figures according to grid reference or post code.

Ground mounted vs building mounted systems.
There are two types of small-scale turbine – mast mounted and building mounted. For a mast mounted turbine the ideal site is a smooth hilltop free from obstructions and with stable ground to set foundations or secure wire guys. This is not the only suitable site, but any factors that might cause turbulence or obstruct the wind supply at any other type of site must be considered carefully (see below).

Some small-scale wind turbines are suitable for mounting on buildings (above roof height) or rooftops. The integrity of a building's structure and/or roof must be examined before installation. A building survey will determine whether your potential site can withstand the weight of the turbine, vibration caused by the motion of the turbine and any intermittent impacts of the wind against the turbine itself, e.g. in high winds or gusts.

Is your site free from nearby obstructions to the wind?
Large obstructions in the vicinity of a wind turbine site, such as buildings, trees and pylons, disrupt airflow and increase turbulence causing a reduction of the wind’s speed at point of contact with the turbine. BERR’s wind speed database does not account for site-specific factors that might cause turbulence and, as such, data collected from a proposed site is preferred when evaluating the suitability of a wind energy system.

When data collection is not possible (e.g. due to constraints of building and/or funding deadlines) the likely effects of surrounding obstacles can be estimated by taking into account their type, height and distance from the proposed turbine site, in conjunction with average wind speed and direction data for the locality.

It is especially important to evaluate site-specific obstructions to the wind when considering a building mounted turbine, in particular at urban sites. Turbulence created by nearby buildings, tall structures, trees, flat roofs and so on can radically disrupt airflow even in areas of persistent strong winds. In these cases, a steady wind resource is not enough to ensure a turbine will smoothly capture the energy in the wind and deliver the optimum rate of electrical output.

What are the potential impacts of a turbine on your site and its surroundings?
Wind speed increases with height and so a wind turbine should ideally stand as high as possible above the ground. With mast mounted turbines this is not always an option and may also be visually intrusive, unappealing to neighbours and affect planning consent.

When a wind turbine rotor is in motion, the wind vortices at the blade tips are audible and so noise emission from your chosen machine needs to be considered. In high winds some turbines will emit sound at a similar level to passing traffic or falling rain, though this is mitigated by the noise of the wind itself. Noise limits may well affect planning consent for your site, but many modern designs are very quiet in operation.

A wind turbine might also create flicker – reflections or shadows from the turning blades – on neighbouring properties. As such, the wind turbine position in relation to surrounding buildings and the movement of the sun must be taken into account.

How far is your potential turbine location from the mains distribution board?
Cable length affects the overall efficiency of your system. Generally it is better to have as short a cable run as possible.

Could you benefit from a stand alone/off-grid Wind Turbine system?
Where connection to the grid is not desirable or possible, a wind turbine can be used to supply a battery store that feeds power on demand. This type of system will still require an inverter to convert DC/direct current electricity to AC/alternating current for use in lights, electrical appliances and so on. A controller is also needed to divert electricity to another useful source (e.g. space or water heaters) when the battery is fully charged.

In off-grid situations in remote areas it is common to incorporate a diesel generator for use during periods of low wind speeds. A combination of wind and diesel gives greater efficiency and flexibility than a diesel only system, and greater reliability than a wind only system.

Will you need planning permission?
With the Low Carbon Buildings: Phase 2 grant programme the government is actively promoting renewable energy installations and permissions for new systems is becoming easier to obtain.

However, local authorities will often insist on planning consent being gained prior to installation, particularly if your potential site is in a protected area or involves a listed building. Always check before going ahead as retrospective permissions can be difficult to acquire.

In the case of a wind turbine, visual impact, noise and conservation criteria are the principal issues to be considered.

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