What will a site assessor look for during an assessment?

Altitude

As distance above sea level increases the speed and the power of the wind also increases. Although affected by local conditions a general rule is that the higher the site is above sea level the more likely it is to have a good wind resource. Altitudes of 200m or more above sea level are most desirable. Furthermore the higher the site is above the immediate surrounding area the better the wind exposure is likely to be at that location.

Aspect

In Ireland the prevailing winds arrive from a south westerly direction. If there are obstructions in this direction the productivity of the turbine will be reduced. The obstructions, depending on their nature, will either slow down or cause turbulence in the air flow. If a large proportion of the wind during a year is coming from this direction it is clear that the turbine will not be exposed to ideal conditions. If there are similar obstructions in other directions (other than the prevailing wind direction) they would not have as great an impact on the output of the turbine. However it is preferable to have no obstructions close to the turbine in any direction.

The prevailing wind is categorised as a global wind. There may be conditions which create local winds such as sea/land breezes and mountain winds, which may be as important as the prevailing wind in terms of turbine output. Local winds are created by temperature differences between sea and land or highlands and lowlands at different times of the day and night.

Obstructions

It should be clear that the more exposed a site is to the wind the cleaner the air flow over the blades and the more productive the turbine will be. The turbine will also demand less maintenance if the air flow is less turbulent. The least desirable location for obstructions is close to the turbine in the direction of the prevailing wind.

Obstructions can be individual trees, houses, out-buildings, ditches, forests, or hills. It should of course be remembered that trees grow so obstructions can develop over time after the turbine is installed.

Obstructions affect the airflow in a number of ways. Firstly the airflow is disturbed leading up to the obstruction. This disturbed flow zone stretches outwards for twice the height of the obstruction before the obstruction. Imagine water flowing in a river and hitting a rock. The water is backed up and disturbed before it even hits the rock as the fluid (air is a fluid too) moves against, up and around or over the obstruction. For this reason it is undesirable to have a turbine within twice the height of an obstruction in any direction.

Downwind from an obstruction the zone of disturbed airflow stretches outward for up to 20 times the height of the obstruction before the wake settles.
The industry norm is to try to site the turbine at least 10m above any obstruction within 100m. However achieving this is quite difficult in reality.
Obstructions not only create disturbed flow before and after their position but also above. The disturbed airflow hits the obstruction and part of it is thrown up and over. This area of disturbed flow can stretch upwards to twice the height of the obstruction. One of the key reasons why roof mounted turbines perform poorly.

It is worth noting that when they are assessing a possible site for a wind farm, developers consider all obstructions and land use within 100's of metres of a development, even though they might be installing turbines with hub heights of 80m. Micro-scale turbines might only be 10m above ground level.

Space

The more space there is in a site the more options there are for constructing the turbine tower away from obstructions. Planning regulations state the distances the turbines must be from party boundaries and the noise levels allowable at premises. One of the requirements for a turbine in a domestic setting to be exempt from planning requirements is that it must be the total height of the turbine (uppermost point of blade tip) plus 1m from a boundary.

In order for a commercial application to be eligible for exemptions from planning requirements the turbine must be its total height plus 5m from the nearest party boundary.

In an agricultural setting the regulation states that the distance required is one and a half times the total height of the turbine plus 1m.

So a turbine which is 13m in total height must be 14m from the boundary in a domestic setting, 18m from the boundary in a commercial or industrial setting and 20.5m from a boundary in an agricultural application. You should contact your local authority to ensure that the exemptions apply in your situation.

Ample space is also required for the tower structure depending on the type recommended or supplied by the manufacturer. Guyed poles require a large area to accommodate the anchor points for the guy wires. Lattice towers and tubular steel poles can require considerable space in two directions in order to be hoisted up or tilted down either by a winch or crane. Guyed tower wires need to be protected from larger livestock and so may require a parcel of land which should be put to other safe use if currently used for grazing. Once free standing lattice towers or tubular steel towers are erected the land immediately around it can be used safely as normal.

During the site assessment it may be discovered by a competent trained assessor that the options for siting the wind turbine are limited by planning exemptions, safety considerations, obstructions and cost of cabling back to the premises. So a substantial parcel of land might just provide one or two small viable areas for construction.

Vegetation

Trees, shrubs and ditches can have an effect on small scale wind turbines. A field which one year is used for grazing, and so has short grass, and the next year grows maize, and so has tall planting, will affect airflow to varying degrees.

Vegetation can be used during a site assessment to point the direction of the prevailing winds as branches and even whole trees can be permanently deformed by the wind as it prevails in one direction. It is easier to see this effect during the winter when the leaves reveal the deformed limbs and branches.

Proximity to dwellings, neighbours, power lines, airports and other wind turbines

Careful consideration must be given to the location of the turbine for safety and planning reasons. The planning requirements outline the limitations with regard distances from dwellings, noise levels, siting near airports and power lines.

A balance must be found between siting the turbine as far away as possible from the house and minimising cabling costs and line losses in connecting to the house. The site assessment should encapsulate all of these considerations when choosing an optimal turbine location.

While the site might appear to have ample space for a turbine it may be discovered following the application of the various planning requirements that only a small area of the space remains eligible for an exemption from planning within reasonable distance from the property.

Demand profile

The assessment should also consider the dwelling or premises' consumption and the nature and timing of that consumption. Over-sizing a turbine may mean payback is affected in the long term and under-sizing a turbine may mean an opportunity is missed to maximize the potential of the site for reducing the customer's bill. Under-sizing a unit is preferable to over-sizing an installation however as it prevents an inefficient system being installed.

The installation should be looking to meet a steady demand for electricity throughout the day and night rather than peak demand for small periods. Ideally the site will have demand throughout the day and night so that the power generated has a use on-site, especially if it is not possible to avail of an export tariff equal to or greater than the import tariff. While 19c/kWh is currently available to ESB Customer Supply customers it is only for the first 3,000kWh exported per annum. The export value falls to 9c/kWh thereafter.

Refrigeration systems, heat pumps, existing storage heating, heat lamps and other appliances which require demand throughout the day and night improve the viability of the site with regard microgeneration. The demand should also be throughout the year and not just seasonal. Some sites may have demand for cooling in the summer and heating in the winter.

In their guided tour, www.windpower.org has free online tools which a site assessor or customer can use to try and put some numbers on the effect of various characteristics of the site.