WIND TURBINES

Another tried and tested technology derived from windmills but greatly improved by the application of design technology.

In relative monetary terms WTs probably have the shortest theoretical payback time of any renewable energy system, but their performance depends on mean wind speed, positioning and quality of manufacture. Too many people who installed small WTs in the early days have been disappointed by the amount of power produced.

Two basic styles of Wind Turbine exist:

• Horizontal Axis - HAWT -, this may be 'Upwind' with the blades on the windward side of the supporting structure OR 'Downwind' with the blades trailing behind the mounting point. In either case the shaped blades are arranged vertically as in a prop.driven aircraft. Some makers claim the downwind design needs less stabilising in order to keep the blades pointing towards the wind. This type is the most common and has been in use for a long time. Capital costs (ex-installation) are less than for VAWT designs, from around £700 to upwards of £3500.
• Vertical Axis - VAWT - with designs which resemble a circular cage or a futuristic work of art. In both cases the 'blades' are carefully shaped for efficiency and are fitted into a structure which sits vertically on top of the pylon, tower or mount. There is no requirement to hold the turbine into the wind since it rotates around a vertical axis and in theory presents the same driven face to wind from any direction. The now obsolete 'half oil drum' designs were VAWTs. Capital costs (ex-installation) are higher than for HAWT - from around £3500 up to £25,000 but in theory they may perform more efficiently. In my view they are more pleasing aesthetically and much of the new thinking about WTs is being devoted to VAWT designs.

All WTs suffer from the variability of wind speed. Most have a 'cut-in' rotation speed equivalent to about 3metres per second wind speed, below which no power is produced. Horizontal axis designs are susceptible to damage at very high wind speeds above about 50m/s but this is rare and would cause structural damage to buildings as well. The majority are designed to feather or turn out of the wind above such wind speeds. Thus the optimum power production occurs between about 5m/s and 20m/s wind speed.

Vertical axis designs may not be so prone to damage at very high speeds but their bearings and mountings are unlikely to survive hurricane conditions.

The database of mean UK wind speeds in any kilometre square is held at www.berr.gov.uk/energy/sources/renewables/explained/wind   Enter your postcode and judge which square your house sits in, then read off the data. You may be disappointed even if you feel that you live in a very windy area. I live on a ridge directly exposed to prevailing south-westerly winds and subjectively it seems very windy, especially in the spring and autumn. But I experience many days in the year when there is no appreciable wind at all and the database shows a mean of 4.4m/s at a height of 15metres.

The height figure is important: a tower 30m in height would enable a WT to produce far more power than if it were roof-mounted or on a 10m. pole, but this is impracticable for most people. Planning controls will present real obstacles, even well-funded and eminently viable commercial wind farm proposals get stuck in the planning process.

Technical Data for WTs does seem more readily accessible than for Solar Thermal or Photo-Voltaic panels. Most makers produce graphs showing theoretical output against wind speed. The flatter or less steep the line the better, and the 'peak' should not be too pronounced. It's rather like torque production in a car engine - you want at least 75% of maximum torque produced between 1500 and 3500 rpm. Some also offer a measure of 'conversion efficiency' which may indicate the quality of construction. What manufacturers seem unable or unwilling to provide is data relating to noise and vibration - I have only seen one 'polar map' of noise on a maker's website. Erecting a WT on a domestic building or nearby is not desirable if it causes worry to the residents because of vibration, or to the neighbours because of noise. When, as seems likely, WTs become 'permitted development' there will probably be conditions attached and these are likely to include noise and vibration limits. Vertical Axis turbines do appear to have an advantage here, some makers claiming that they are "noiseless" but without providing any supporting evidence. The most likely reasoning is that the noise of the WT rotating cannot be heard above the noise of the wind.

SUMMARY

• WTs can be very efficient at power production on the best sites with strong consistent wind patterns
• Capital cost may not be excessive but maintenance is required
• Power produced is usually DC for smaller units but some have an AC option. Transmission losses need considering
• Erecting a WT still requires planning consent in 2008 but this is expected to change
• WTs are more visually obtrusive than other renewable systems, fitting them on roofs is still only to be done with caution
• Payback time may be less than for other renewable energies but basic costs vary greatly
• The Vertical axis designs will in my view become the type of choice in future

LINKS

Wind Turbine Reviews:    www.bettergeneration.co.uk/wind-turbines/

Warwick Wind Trials (very salutary reading):     www.warwickwindtrials.org.uk

Proven Energy:    www.provenenergy.co.uk

Quiet Revolution (is this the future?):    www.quietrevolution.co.uk

Solar & Wind Electric Power:    www.aagswepco.com

Future Energy:    www.futurenergy.co.uk

Zephyr Air Dolphin:    www.semplice.co.uk

Marlec Engineering (small-scale WTs):    www.marlec.co.uk

Pacwind (from USA but innovative):    www.pacwind.net

Windterra (from Canada and equally innovative):     www.windterra.com

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