WIND ENERGY

Wind Turbines (Microwind)

What is a Wind Turbine?
Wind turbines convert the kinetic energy of the wind into electrical energy. Given that the UK has roughly 40% of the entire wind resource of Europe, there is significant potential to extract energy from this source. However, to date this has been under utilised.

Domestic wind turbines are much smaller than those seen in wind farms, but they operate in much the same way, providing output from just a few tens of Watts to several kilowatts for household sized installations.

Of all the domestic renewable energy systems, wind turbines are probably the most controversial, whilst also being more difficult to predict in terms of performance due to the large number of variables involved.

Types of domestic wind turbine
Turbine design depends largely upon whether it is horizontal axis or vertical axis, with horizontal axis machines by far the most common. The axis is defined by the direction of the rotor shaft. They can also be either free standing on a pole or mast, or more recently, building mounted.


Figure 1. Schematic of a pole mounted wind turbine

•Vertical axis machines (VAWT) do not need to point in the direction of the wind and so may be more suitable for areas of turbulent wind conditions. One draw back with vertical axis machines is that they might not be capable of self-starting and are generally less efficient
•Horizontal axis machines (HAWT) are by far the most commonly installed and typically have three or more rotor blades. They automatically orientate themselves with the direction of incoming wind, but turn away from it in times of high wind speed to prevent damage
•Pole mountings consist either of a steel or composite material (tubular tower), a lattice tower, or a guyed mast on top of which the turbine and generator are located
•Building mountings are designed for attaching the machine to a building, such as on the gable end of a house ranging in output from around 0.5kW ? 2.5kW       

How does a Wind Turbine work?
Horizontal axis wind turbines have a plane of rotation perpendicular to the direction of the incoming wind.

When air flows past the blades of a turbine a force is exerted onto the blades and tail fin, pushing the turbine in the direction of the wind. Because of the special shape of the rotor blades, a lift force is also created forcing rotor to revolve about the central axis in the familiar way.

The rotor is connected to a shaft which drives a generator directly, producing the electrical output from the machine. Larger machines can make use of a gearbox to maintain a fixed generator speed for different wind conditions.

Domestic wind turbines tend to produce electricity of variable AC voltage and frequency, which needs to be converted to DC and then AC of the correct characteristics for integration to the local electricity grid and for use with household appliances.

What components comprise a Wind Turbine System?
The main components of a wind turbine are:
1.The turbine and generator
2.Pole/mast/truss/fixings
3.Inverter/power conditioner
4.Battery Bank (optional)
5.Dump Load (optional). This is to dump excess energy production in off grid systems once batteries are full and unable to take more charge, therefore keeping the turbine fully loaded to prevent damage. This often done by an electric heater

How much energy can I expect for a wind turbine?
As is true with many other fluids, for every doubling in wind speed, the energy available from the wind increases eightfold. Therefore, the more exposed the site and the greater the average wind speed obtainable, the greater the potential for harnessing power from the wind.

Furthermore, wind speed increases with height whilst at the same time turbulence reduces, providing more reliable and consistent power output and a safer operating environment for the turbine.

Well sited wind turbines can expect to produce energy equivalent to their rated power for around 30% of the year, according to the Energy Saving Trust. For example: for a turbine rated at four kilowatts this equates to (4kW x 0.3 x 8760 hours in a year) = 10,512 kWh per year. If all this power is used to offset electricity costing 10 pence per kWh, then a saving of £1,051 per year could be possible.

Try to ensure that your proposed site has average wind speeds of at least 5 metres per second, preferably 6 m/s before installing a wind turbine.

Fig 2. 6kW Tubular Tower (Pole) mounted turbine ( © Sustainergy)

What are the benefits of a wind turbine?
•Potential power generation throughout the year, night and day
•They can be installed at a variety of different sites
•They can be grid connected or stand alone
•They can provide a strong visual statement of a commitment to protecting the environment

How are Wind Turbines installed?
Wind turbines can be either building mounted or fitted on top of a tower which is either a solid tube or guyed (see diagram). In the latter case, they may be assembled on the ground and winched into place.

It is recommended that only turbines having a low output rating be building mounted. Additionally a greater blade number will increase performance at lower wind speed, reduce vibration and allow quieter operation. Low power ratings coupled with a reduced rotor diameter lessens the lateral load on the building, since buildings are designed primarily to take compression loads

For sites where obstacles interfere with the air flow (such as trees, buildings, large objects), the flow becomes more turbulent requiring extra control measures to operate the turbine reliably and safely. In any case, turbulence will reduce the operational lifetime of a wind turbine as well as increase the frequency of necessary routine maintenance.

Suitability
More advanced blade designs have reduced noise levels dramatically over the past 20 years or so. However, issues may arise over the visual and audible impact your system may have on you and your neighbouring properties, so it is recommended that you seek advice from your Local Authority Planning Department before committing yourself to any purchases.

You need to consider whether or not you would prefer a building mounted turbine or one that is mounted on a tower. The tower could either be a guyed mast, or of solid tubular construction; the former is less expensive but requires more land area.

There needs to be enough room to raise and lower the mast on a periodic basis for routine maintenance.

Wind speed increases with height, so to get the best from your installation you ideally need a combination of a high mast/tower and an exposed site. Any nearby buildings or large trees cause potentially damaging turbulence and reduce energy output, so installations next to these should be avoided if possible.
The wind resource is usually unpredictable, so it is best to ask your installer for an assessment the potential of the site.

Maintenance
As this is an electrical machine operating in harsh conditions an annual service may be necessary as described in the technical guide of any machine purchased.


Fig 3. Lattice Tower (Truss) with guys

Grid connection and exporting excess energy
Before connecting any electricity generating equipment to the local grid, you should seek advice and approval from your Distribution Network Operator (DNO), and make sure that your equipment satisfies the G83/1 regulations. Your turbine installer should be able to help with this.
In the event of grid failure (power cut), your wind turbine system will automatically shut down in order to ensure the safety of any engineers working on the power lines.

Excess electrical generation can be sold to the national grid; contact your energy supplier for more details of the options available to you. For turbines rated over 6kW, grid connection may become more complex.

Installation Costs
A significant proportion of the cost of a wind turbine system is the turbine itself and the grid-connect inverter. Wind turbines are mechanical machines and operate in harsh conditions; choosing a machine constructed from higher quality components will generally increase its cost and reliability. The installed and commissioned cost of a grid connected wind turbine system should be in the region of:
•Building mounted: £1,500 for one kW rated output
•Tower mounted: £2,500 - £5,000 per kW rated output
•Typical 2.5kW system costs around £11,000 - £12,500
•Typical 6kW system costs around £18,000 - £25,000