Wind power has been used for millennia as a source of power, first as a pumping aid to irrigation then for milling. Wind was first used to generate electricity in Scotland, in 1887. Almost 120 years later, today’s turbines are approaching the technology’s maximum theoretical efficiencies. However, the physics of wind energy cannot change and the importance of location remains key to whether it is a feasible option in any domestic situation. In the right location a wind turbine connected to a building can work well and be cost-effective.
A wind turbine’s rotor blades use aerodynamic lift, like aircraft and bird wings, but to convert airflow into a rotational force which spins a generator to produce an alternating current. This fluctuates with the windspeed so that it is not useful for powering machines or appliances directly. First the energy must either be stored in a battery bank and drawn at the rate required through an inverter, or be grid-connected via another kind of inverter so that the grid can act as a buffer to absorb surplus electricity and balance supply with demand as required. The grid connected system is far more common as battery banks are costly and have a fairly short service life. Grid connect inverters also enable the generator to export power to the grid and receive payment for it.
The output from a turbine is very sensitive to the average windspeed at the site and the steadiness, or turbulence, of that wind. Turbulence is caused by any object raised above the flat ground and it extends to twice the obstacle’s height and 20 times its height horizontally. It reduces the energy in the wind and causes stresses on the machine which can shorten its life considerably. Windspeed is sensitive to topography – surrounding higher ground reducing it significantly – and to altitude and to turbulence, eg. from buildings or trees within several hundred metres. The prevailing wind direction is important because obstacles in a direction from which the wind seldom blows (eg. from the North East in most of Scotland) may have little effect on annual output.
Therefore a good turbine site is on open, elevated land on top of a hill or on a slope which does not rise into the region’s (not the exact site’s) prevailing wind direction, or on an extensive flat area, preferably 200-300m from any buildings or trees.
Turbines placed elsewhere will operate but they are unlikely to be cost-effective and/or will have shortened service lives.
The mean windspeed should be at least 5 metres per second (m/s) and preferably over 6m/s to be profitable. This can be judged by a trained eye in conjunction with published windspeed data, but where there is any doubt it may be worth employing the services of an independent wind monitoring specialist or installing a good quality windspeed data logger on a guyed tower at the exact same spot and height of the proposed turbine for a few months and comparing the resulting mean windspeed readings with the prevailing conditions over the period monitored.
Planning – planning permission must be obtained and will be sensitive to the visual impact of the proposed turbine on the neighbourhood, potentially on effects on air-traffic RADAR equipment and on wildlife in certain situations. There are a growing number of independent small wind planning consultants familiar with the requirements who can be employed if you do not want to tackle an application yourself.
Grid connection – turbines over 3.6kW per phase (eg. 11.8kW if you use a three phase connection) must apply to the local Distribution Network Operator (DNO – SSE or ScottishPower in Scotland) for connection under the G59/2 engineering standard. The DNO’s charge for this connection will depend on the capacity of the power network locally and may be from a few £’00 to a few £’000 or possibly much more if the network is full to capacity. Turbines below 3.6kW per phase can simply be connected using a G83/2-compliant inverter and the appropriate declaration submitted to the DNO for their records.
Reliability – assuming there is sufficient wind, the success of a wind project is more sensitive to reliability than anything else. Some small turbines have a much longer track record of ongoing development than others and maybe expected, as a result, to be more reliable.
Location - The performance of turbines put in turbulent sites will disappoint and wear and tear will cause early failure. The Feed-In Tariff scheme is intended to make economic turbines in sites with good wind resource. If the windspeed is 5m/s or lower, electricity will be generated but probably not enough for the installation to be economic. Some designs are more suited to lower windspeed sites than others.
Where the mean windspeed actually exceeds 6m/s and there is little turbulence, investment in a good quality small turbine can be very cost-effective, giving payback in perhaps 6-9 years. If you are off-grid or very prone to power cuts, a battery bank creating an ‘island’ set up can be used to give you a power supply without regular recourse to a generator.
Effective wind systems are very environment-friendly, saving around 400grams of CO2 for every kWh used, averaged over their lifetime if displacing coal and gas-generated electricity.
Economies of scale make small-scale turbines more cost-effective as they get bigger. They range from £5-6,000 per kW of nominal output, so ranging from £30,000 to £70,000 for systems of 5kW to 15kW, fully installed with planning permission and grid connection. Smaller and larger systems are available.
5kW - £30,000
8kW - £50,000
11kW - £63,000
15kW - £70,000