Not to be confused with buildings-mounted micro wind turbines, which are
dealt with separately.
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.