Small Wind Factsheet
A small wind turbine has a hub height of between 6 and 25 metres and is rated at between 1 and 25 kilowatts (kW). In comparison, the larger turbines used in wind farm developments frequently top 100 metres and are rated in megawatts (MW). Turbines need to be mounted on a tower of a height that reduces the blocking and eddying effects of buildings and trees. The 6 kW Proven turbine in the picture on the right, at Brill Primary School in Buckinghamshire (9m hub height) is an example of a 6kW turbine on a 9m high mast.
The amount of energy generated is determined by the wind speed and the area swept by the blades. Blade shape and rotation speed determine efficiency. On some turbines a braking system shuts off the turbine if the wind is excessively strong. On others, aerodynamic load shedding (blade furl) is used. Turbine rotor blades are attached to the hub; the generator and associated machinery sit in the nacelle, the box at the top of the tower to which the hub is affixed. The tower is fixed to a reinforced concrete base foundation, and a cable connects the turbine to the site electricity supply.
The electricity generated is DC and is therefore converted to AC by one or more inverters.
The heat source can be the ambient air, water, soil or rock. The outside heat exchanger (the collector) transfers energy as heat to the circuit of working fluid within the heat pump itself.
Grid connection, electricity export and value
Grid operation operates in the following manner; the turbine(s) feeds electricity into the site buildings, offsetting that imported from the national grid. When electricity demand on site is more than the output from the wind turbine(s), the difference is drawn from the grid. Should electricity demand be less than the output from the wind turbine(s), electricity can be exported back into the grid. Depending on negotiations with the electricity supplier or a 'consolidator', this export of electricity could generate a credit on the electricity bill, or a one off annual payment, to also include payment for the Renewable Energy Obligation Certificates (ROCs).
All generators of renewable electricity are entitled to claim ROCs for every 1,000kWhs or units generated. This is currently a flat rate across all the technologies, but will soon be differentiated, with small scale wind remaining at one ROC per 1,000kWhs generated. ROCs have a changeable value established through trading on dedicated markets (£48.06 per ROC on 27th January 2011). They are an additional source of income, separate from the actual price of electricity consumed/exported.
It is recommended that turbines under go an annual maintenance for the first few years of operation and then more intermittently thereafter. Costs for this vary between installers, from £200 - £750, depending on size of turbine and distance of travel required. It is something that a local operative could be trained to undertake at a more reasonable cost. Maintenance will involve the lowering of the tower, so space must be provided for this task.
Inverters would probably need replacing at some stage and a replacement factor of once every ten years would be a reasonable assumption.
Costs for domestic and other small scale applications typically vary depending on individual requirements. A typical 2.5kW installation may cost around £10,000 while a 6kW installation around £18,000. A 15kW-20kW wind turbine may vary from £35,000-£50,000.
Siting and effect on performance
The first thing to consider is wind speeds at the site. These can be estimated from the NOABL wind speed model at http://www.dti.gov.uk/renewables/technologies/windspeed/online.html, which provides data for each grid square in the country. An annual-average wind speed of around 5 metres per second (m/s) and upwards at 10 metres above ground level is normally considered reasonable for a small turbine.
Grid-square wind speed data is, however, only part of the story, and contains inaccuracies due to the averaging involved. The actual wind speed at your site will also be influenced by the local topography and any nearby obstructions, such as trees or buildings. If using general wind speed data, local conditions must be factored in qualitatively. Alternatively, a more accurate is to install a small anemometer on the site for at least 6 months (a year is preferable) to gain site-specific data. Because of the additional expense, this will normally only be justified for larger, more costly projects.
Ideally, the higher you go, the more you will generate and if this is significantly taller than obstructions, so much the better. As a rule of thumb, however, a turbine should be located clear of obstacles by at least ten times the height of the obstruction to mitigate interference with the wind flow.
The other general rule of thumb is to locate a turbine where it has an unimpeded draw on the prevailing south-westerly winds that predominate for much of the year in the Thames Valley area.
Planning permission will be required for any free-standing turbine. Guidance for local planning authorities on how to assess applications is limited, hence the possibility of a range of responses.
The main issue is that of landscape sensitivity, but as these turbines are not particularly tall, their effects are minimal, and easily screened if necessary. TV Energy advises anyone unsure as to the scale of these machines to visit one within the region, and can suggest locations. The image on the right is of a 300kW, 47m high (to blade tip) turbine located in the North Wessex Downs Area of Outstanding Natural Beauty at ¾ mile distance. It has been operational for nearly two decades and is a fine example of a turbine sensitively located in a highly valued landscape.
It can also be said with confidence that small wind turbines are neither a danger to bird/bat life, nor generators of excessive noise. Noise surveys at Thames Valley locations indicate that the noise of the wind on its own will drown out any noise from the rotation of a turbine from 20m distance. Similarly, small turbines do not interfere with television, radio, mobile phone or broadband reception. Evidence exists to back up all of these statements.
Building mounted turbines
Micro-turbines differ from most small wind turbines in that they are usually mounted on a pole attached to a building rather than a ground-based monopole.
Much of what has been said about free-standing turbines also applies to building-mounted turbines, although the following should also be borne in mind if considering this technology.
- The effect of a building (and neighbouring buildings) on the pitch and flow of the wind is highly pronounced and renders any wind speed estimates other than actual site data of little use in assessing how a building mounted turbine will perform at a given location.
- Maintenance of these turbines will involve the expense of hiring scaffolding or a 'cherry-picker'.
- It is vital to check that the construction of the building to host the turbine can accommodate the load and stresses that the turbine will place on it.
- Noise data for these turbines is still only emerging, causing difficulties for planning officers.
- In 2011 planning applications are still needed for building-mounted turbines.
- British Wind Energy Association – www.britishwindenergy.co.uk - Trade and professional body for the UK wind industry
- The Danish Wind Industry Association - www.windpower.org - A really useful site aimed at all levels of interest in wind energy
- Energy4All – www.energy4all.co.uk – Company specialising in facilitating the development of co-operative, community-owned wind energy schemes in the UK