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Wind turbine systems

Wind turbines use the wind to turn a propeller that generates electricity in an alternator. The efficiency under optimal conditions of a wind generator to convert energy to electricity is about 45%.
They are viable as small-scale generators that can provide electricity to a building or property separately from the mains supply. The main requirement is that the wind generator location is exposed to sufficient intensity and duration of wind.

They are more suitable in remote locations as they can produce noise and may be regarded as unsightly.

Find out about:

wind generator system configuration
wind generator system capacity
wind speed and power
cut-out controls
factors affecting generation capacity
wind generator system installation
electricity supply connection
wind generator pollution.

Wind generator system configuration

: Typical wind turbine for electricity generation

: Wind turbine components

A wind turbine includes:

turbine blades – propellers with two or three blades mounted on the horizontal shaft (this gives higher output than when they are mounted on the vertical shaft) and made of a lightweight material such as carbon fibre, fibreglass or wood, that is strong enough to resist wind forces.
a tail section – generally a fin that rotates the body of the wind generator to turn the turbine into the direction of the wind, with the fin directly downwind
an alternator – AC electricity is generated by rotor windings connected to the shaft from the turbine
a rectifier – converts AC to DC for electricity that is being sent to a battery storage system (the rectifier may be located in the alternator or in a separate control box away from the tower)
electricity cables – transfer the electricity from the generator to the electricity supply or battery storage system
slip rings – stop the cables twisting as they will otherwise twist within the tower as the turbine body rotates
electric element – power is always produced when the turbine spins, so if the power is excess to storage capacity, it must be redirected to a dummy load (generally an electric element that gets very hot) or sold (if permitted under the district plan) to an electricity retailer
tower – the structure (usually steel, concrete or wood) that holds the turbine high in the air, and allows the turbine assembly on top to rotate into the wind – for residential applications, it is typically a mast pole with guy wires
guy ropes – hold the mast pole in operating position
gin pole and winch – allow the turbine to be lowered for maintenance.

Wind generator system capacity

Typical domestic wind generators have a capacity between 0.3–5 kW, but may be 10–20 kW output. The turbines’ actual energy output is typically about 25% to 30% of its rated theoretical maximum output. The output of a wind generator will normally be rated at a specified wind speed, and the rated wind speed may vary between systems and manufacturers.

The electricity generation capacity of wind generator systems is directly proportional to the amount of usable wind, which is itself a function of wind speed and cleanliness.

Wind speed and power

The wind power density is the number of watts of electrical energy produced per square metre of air space (W/m²). This value is normally given at 10 m or 50 m above the ground.

In general, the available wind generation capacity is determined by the average wind speed over the year for each location. Around New Zealand, the average wind speed is typically greater in regions:

along the coasts between the North and South Islands
in the mountain ranges and immediately east of them
towards the tops of ridges or the heads of valleys.

With increase in wind speed, energy output increases to a power of three, i.e. when the wind speed doubles, the energy it can produce increases eight times; if tripled, the wind speed will provide 27 times the power output. This means that sites with short periods of high velocity winds can sometimes produce more power than sites with more constant, lower velocity winds. Clean wind is better for generation.

Wind speed fluctuates, which has an impact on wind electricity generation capacity and operating characteristics. In general, wind speeds are as follows:

8 kph (2 m/s) minimum is required to start rotating (cut-in speed) most small wind turbines – a low cut-in speed is important for turbines in areas with frequent light winds.
10.8–18 kph (3–5 m/s) average for the year is appropriate for a small (residential) wind turbine.
16–19 kph (4.5–5.3 m/s) average for the year is appropriate for a larger (commercial) wind turbine.
36–54 kph (10–15 m/s) produces maximum generation power.
At 90 kph (25 m/s) maximum, the turbine is stopped or braked (cut-out speed).

The wind power at a site can be obtained by a measurement device mounted on a pole at the height of the future wind generator. Collecting data for a whole year is not generally viable, so a couple of months of data can be taken and compared with data from a local weather station and then extrapolated for the year. Devices include:

an anemometer – giving average daily wind speed
a wind totaliser – giving instantaneous wind speed and total wind over an extended period.

Cut-out controls

Cut-out control options are available that:

apply a brake to stop the turbine completely and feather the blades (reduce their angle to the wind) to turn it to face away from the wind
tilt back or lie down the turbine (this is known as ‘tilt-up governing’)
steer the turbine out of the wind through aerodynamics and gravity (this is known as ‘autofurl’)
govern the rotational speed with an air brake to produce constant power
feather the blades (reduce their angle to the wind) to reduce turbine speed.

Factors affecting generation capacity

A system’s generation capacity depends on its effectiveness at converting wind pressure into turbine rotary inertia – data should be available from the system supplier. This increases with:

larger turbine diameter – there is more turbine blade area for the wind to impact on and also greater risk of intrusive noise
appropriate blade profile for the local wind speed – this varies depending on average wind speed and also on whether the wind is constant or comes in short periods of high velocity
lower friction losses in the turbine shaft assembly.

Generation capacity will decrease if the turbine is located:

lower to the ground – wind speed increases with height above the ground, with a minimum of 10 metres recommended
within the turbulent airspace downwind of an obstacle (for example, trees, hills, buildings, structures) – downwind turbulence will extend to twice the obstacle height for a distance around 20 times the obstacle height
a distance from an upwind obstacle of more than 10 times an obstacles height.

: Siting of a wind turbine
Wind turbines work best when a turbulence-free airflow is available to power the turbine blades.

Wind generator system installation

A wind generator system:

will require a building consent and a resource consent
should be installed within 100 m of the electricity supply or storage system, to reduce line losses
must withstand the wind and seismic loads
usually has a concrete footing for the tower (and each guy wire)
must have vibrations in the tower (from turbine rotating forces) dampened if it is connected to a building
must have protection from large animals at ground level – they like scratching themselves on the tower and guy wires
should have lightning arresters to protect electronic components from lightning strikes
needs sufficient area to lower and raise the tower for maintenance and repairs.

Electricity supply connection

Electrical power from the wind generator system may be available at all times of the day, but the output levels will vary according to wind speed. Excess output, generated as AC, is converted to DC by a rectifier for storage in batteries. This will allow for peak demand that is greater than the generator capacity.

Wind generator pollution

Wind generators can produce noise and vibration and have a significant visual impact. Noise can be from the turbine blades, gearbox (if used) and brush gear, as well as from wind moving past the tower and guy wires. Noise and the visual impact may be an issue with neighbours, and vibration may be a problem particularly if a turbine is located on a roof.

These factors should influence decisions about the wind generator location, size and height.