MINI WIND

 

 

 

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A brief introduction to microgeneration, what it is and what it can do for you and the planet

MICROGENERATION TECHNOLOGIES

An overview of different electricity and heat producing microgeneration technologies with links to further details on each

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Before you look for ways to produce your own energy, it makes sense to minimise your energy needs.  An outline of some energy efficiency measures you can take.

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Free-standing mini wind turbines generate electricity from wind striking the turbine.  When installed in rural locations with high average wind speeds (>5 m/sec), a 5kWe turbine can produce around 10,000kWh annually

 

 
Pro
Can generate significant amounts of zero carbon electricity in suitable locations
Con
Needs open location, remote from buildings or other obstructions
High capital cost
Verdict
Cost effective zero carbon electricity production makes this on of the best electric microgeneration technologies
Not to be confused with buildings-mounted micro wind turbines, which are dealt with separately.

The mini wind turbines now being deployed in the UK have been developed for both off-grid and grid-parallel operation.  This section deals only with grid-parallel systems which represent the majority of UK applications.

Generally speaking, larger wind turbines tend to be more efficient at capturing wind energy, but they still require unobstructed sites in locations with a high average wind speed.  For this reason they are not suited to urban locations and are unlikely to be suitable for normal homes.  They are well suited, however, to larger homes, schools, farms and similar buildings in rural locations with a good wind resource.  Caution should be applied to the wind speeds given by the UK Government official NOABL database, which tends to be very optimistic and may give disappointing results in practice. 

At this scale, manufacturers have concentrated on constructing products which are robust enough to withstand the relevant wind conditions, and have invested in electronics and other components which make best use of the available resource.  Key issues include:

  • Grid synchronisation.  It is a requirement of the connection standard G83, that grid parallel generators must have a stable output and the grid must be stable for 90 seconds before connection of the turbine.  Power electronic components are required to avoid the turbine spinning uncontrollably (and precessing out of the wind) before connection (and on-load conditions) are established.  The absence of these components in micro wind turbines results in their continually "hunting" for the wind, such that they rarely synchronise for long enough to deliver any useful output.
  • Overspeed control.  In order to prevent damage to the turbine and uncontrolled output which may stress the components, it is necessary with all wind turbines to be able to manage the operation of turbines in very high wind speeds.  This can be achieved by braking the turbine (electronically or mechanically) or by turning the rotor away from the prevailing wind; some systems simply become more turbulent and are self-limiting, although this can lead to excessive noise and vibration. 
  • Maintaining peak output.  The more efficient turbines incorporate some mechanism to capture the energy under conditions of low wind speed, but which also maintain efficient output as speeds increase.  In the Iskra unit, for example, there is a system of springs which limits the pitch of the rotor so that peak output is quickly established and maintained.  The Eoltec unit, on the other hand, uses centrifugal pitch control.  Unlike larger turbines, active pitch control is not practical at this scale.
  • Making best use of available resource.  A higher hub height (i.e. the height of the tower on which the turbine is mounted) will result in higher and generally more consistent wind speeds. 
  • Vertical axis wind turbines are faced with somewhat different challenges in terms of control and efficiency.  Whilst they are generally better able to withstand turbulent conditions and high wind speeds, the more efficient "lift" products need to be run up to a certain speed (using their generator as a motor) before they can generate power.  One model, the XCO2, carries out scheduled run-ups to test for available wind.  This is clearly wasteful for prolonged periods of little or no wind.

 

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Jeremy Harrison 2008  Last update 20th November 2008