Wind Turbines

Much like t-shirts, wind turbines come in small, medium and large and not a few sizes in between.  This page contains a lot of generalisations, it is intended a primer for someone about to explore the subject in more depth.  The discussion is limited to machines used to generate electricity.  The range of machines reflects those found in southern England, the data is generalized and does not reflect the offerings of any specific manufacturer.

Small	Medium	Large

Small (less than 1 kw)

These machines are typically used in off-grid applications in conjunction with some form of battery.  They are found on road-signs, track-side data collection, yachts and mobile homes.  The smaller and simpler models may not have any over speed protection, the rotor being small can be made strong enough to survive short periods at several times the rated speed of rotation.  For these designs, it is essential to have a charge controller between the generator and the battery to prevent over charging during periods of high wind.  The rotor diameter is often less than 1m and operational speed can be in the range 500 to 1,500 rpm.  Mounting height is usually related to the structure of the device they are powering which is generally 10m or less.  Because of the low mounting height, the maximum value of the  coefficient of performance is around 5 m/s.  Typically, such devices cost less than 1,000 GBP.  Successful installations tend to be in open and uncluttered locations, anecdotal accounts suggest the they do not work well in urban locations unless mounted well above roof height.

Medium (2.5 - 15 kw)

The prime market for these devices appears to be organisations which want to make a step towards carbon neutrality, but require the security of a grid supply.  There are a wide range of designs on offer.  These larger machines require over speed protection.  The options include, furling in which the rotor is turned out of the wind either mechanically or by a trailing vane.  Some machine have a brake which prevents rotation at high wind speed, in such cases, the output drops to zero in high winds.  The electrical system may include a dump load to ensure that the turbine does not over speed in case of a drop in the demand for electrical power.  These machines are often mounted on poles between 15 and 30m above the ground and the rotor design is optimised for around 7 m/s.  Costs range from 5,000 to 50,000 GBP or more with installation costs being a significant element.

Large (1,000 kw and above)

These are mainly found on wind farms, although there are a few solitary machines dotted around the country (a plan to have one at the east London Olympic site was recently abandoned).  Typically, the rotor diameter can be 100m or more and the mast is often in the range 100 to 120m.  The nacelle which houses the generator also contains yaw and pitch controls.  Because the wind speed at the top of the arc of rotation is higher than at the bottom, many designs have individual pitch controls for each blade.  Over speed protection is a combination of yaw control (similar to furling on small machines) and braking.

Coefficient of Performance

Standards and good practice require that power curves are based on the intended application of the machine.  Thus if the turbine is to be used for battery charging, the power curve will show the charge delivered to the battery.  If the turbine is to be connected to the grid, the power readings are taken at the output terminals of the inverter.  Thus the coefficient of performance is a composite value which is the product of aerodynamic, mechanical and electrical efficiency.  The curves below are typical of the machines described above, for detailed evaluation, data for a specific machine should be obtained.

From a modelling perspective, there are three points on the performance curve which need to be considered.  The first is the cut-in speed, typically this is in the range 2 - 4 m/s, below this, the turbine may turning, but there is no significant output.  The the wind speed at the maximum value of the coefficient of performance is generally lower than the rated wind speed of the turbine.  Behaviour at high wind speeds varies.  The output of a small turbine with no over speed protection, may continue to rise with wind speed (some of these devices have very high survival speeds).  A machine which relies on braking for protection will simply stop at high speeds (typically in the range 15 - 20 m/s).  The behaviour of ones which rely on furling or use of a yawing motor to turn the rotor away from the wind can be complex, fortunately, the number of days in a year when these devices can be expected to be activated is not large, so the effect on the annual yield will not be great, but the supply will be discontinuous.  The output of Large machines with variable geometry rotors and/or variable speed gearboxes becomes constant in the speed range between the rated wind speed and cut-out.

Page Updated: 10-Jun-10

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