Part Two: Wind Turbines

History

Windmills in various shapes and sizes have been around since 200 B.C. grinding wheat in Persia and pumping the dykes out in Holland. It took doddery old Don Quixote to make them really famous when he put the spurs into his gallant steed Rocinante, and charged full tilt at one sitting faithfully doing its work on the dry Spanish Plains of the early 17th century. However, windmills are used to convert the kinetic energy of wind into the mechanical energy required to grind grains or pump water. When kinetic energy is used to produce electricity they are known more correctly as wind turbines and basically only come in two shapes and sizes….horizontal or vertical.

Horizontal Axis Wind Turbines (HAWT)

These are by far the most common. The generator usually is mounted on the same axis as the blades and some form of weather vane keeps the turbines pointing into wind. This can be a simple vane or some electro servo device driven by a motor. The turbines only work really efficiently when pointing into wind and clear of turbulence. Siting is therefore of paramount importance and can be a problem in a domestic situation.

Another difficulty which must be addressed is that of metal fatigue to which the rotor, bearings and blades themselves are subjected. Because of the natural friction which the earth’s surface has, wind strength increases with altitude. Even across a 3 metre turbine this is enough to result in a force differential being applied to the blade as it peaks compared to its opposite number at the nadir of the cycle. For this reason three bladed systems are common in order to reduce the bearing stresses.

Gyroscopic force also comes into play. As a turbine is turned into the wind by the vane an equal and opposite force is applied 90 degrees around and 90 degrees down. In a strong veering or backing wind this force is significant and further stresses the bearings and blades.

Apart from the internal pressures which the system must resist these also impact on the structural side. You must ensure that where you mount your turbine has sufficient structural rigidity to withstand the powerful forces it will be subjected to particularly as wind strengths are increasing with global warming.

Vertical Axis Wind Turbines. (VAWT)

As the name implies the rotor is orientated vertically with the immediate advantage of making it possible for the generator to be placed at the base of the unit. This lowers the overall centre of gravity and so increasing structural stability. Also there is no need for a wind vane to point the turbines into wind simplifying the system.

You never get something for nothing so the disadvantages are that the turbine operates in more turbulent air and so efficiency is lowered. There are also losses in aerodynamic efficiency. As the blades rotate through the wind direction they will be at right angles to the wind and encounter maximum profile drag.
This factor lowers their comparative efficiency to about 50% of the HAWT.

Several manufacturers now produce VAWTs for larger installations up to 16 metres high. At the other end of the scale the Savonius variety has ice cream, style scoops as blades, and these applications are often seen on boats and caravans to charge 12-24Volt batteries.

What Is Their Output?

Use of wind power for the world’s future energy needs is now firmly on the agenda. Led by Denmark which uses wind turbines to supply 25% of the country’s electrical energy needs, followed closely by Germany and many other European countries, wind farms are spreading rapidly across the globe. The U.K. recently crossed the 2 G.W. output barrier. The largest turbines are made in Germany and towering to over 180 metres the Enercon pushes out 6 MW.

On the domestic side outputs typically are between 500watts-1.5kW for a system with a blade diameter of 2.3 metres.

Is It Worth Installing One?

1 Siting.

Obviously your installation will be much more effective if it has maximum exposure to the prevailing wind. In the U.K. this means generally locating the wind turbine where it will benefit from the most common wind direction of south west to west. If you are in a very sheltered position a wind turbine might not be for you.

Locate it in a position where it will be as free from turbulence as possible to achieve maximum aerodynamic efficiency, and as high as possible. Wind speeds increase rapidly with height, and have a more even laminar flow, as ever sailor knows.

2 Structural Considerations.

Generally it is better to locate the unit clear of any buildings but if this is not possible it can be mounted directly onto the house itself. However, be aware that significant vibration can occur and take expert advice to ensure your particular building will be able to withstand the not inconsiderable forces of vibration and torque to which it will be subjected.

3 Noise

Although quieter designs are becoming available, the blades do make a noise, and this has to be considered, especially if the unit is to be mounted directly onto the building.

4 Cost versus Payback

Making some bold assumptions, in the U.K. with electricity at 12 p per kWh and with average annual wind strengths of 8 mph a 1kW unit would push out about 600kWh in a year. This means a £2000 wind turbine installed would take 27 years to pay back. The average wind strength you get affects the mathematics dramatically. This is because electrical output is a cube function of the wind speed. If the wind speed doubles for example, the turbine output goes up by a factor of 8. If your average wind is just 20% stronger at say 10 mph the break even comes down to about 14 years.

Putting it all together

Once the wind turbine and manufacturing costs have been paid, all future electricity is free of both carbon footprint and utility company charges. Although at first glance the payback period looks long you will be reducing your carbon footprint from the moment the manufacturing costs have been offset.

Now here is where it gets interesting. You have seen in the paragraph Cost Versus Payback how the breakeven point is affected by the average wind strength over the year because of the cube function of output. Research at the Global Warming Alliance on hurricanes indicates the intensity and hence wind strength in these super storms is set to increase rapidly. As that is likely a result of the rising thermal stress to which we are subjecting the planet it is logical that wind strengths in normal depressions will also increase as the core pressures lower and the isobars pack more tightly together. Therefore there is every probability that we are in for a windier world. That will make wind turbines a great investment for the future as the payback period will shorten in a disproportionate way.

Additionally, as the world’s oil supplies become exhausted prices for oil and gas can only rise. Obviously, if the cost per kWh the utility companies charge doubles the payback period halves again making the choice of a wind turbine potentially more attractive.

All in all wind turbines look like an investment that can only go one way.
Good for the individual and good for the planet.

An excellent source for further research is the British Wind Energy Association’s site which has a list of products available and a mass of useful background information. Visit www.bwea.com