Wind Power basics

Wind Power complements solar power fairly well, in worse weather; when solar doesn’t work so well; it tends to be windy.

However Wind has a number of drawbacks that must be accepted:

• The turbine needs linear non-turbulent wind. In praxctice that means on a tower, ideally 10m above the nearest object for 100m
• The roof of a building does not produce good linear wind, turbulent wind reduces the efficiency of the turbine and exposes it to increased vibration, building roofs are also not designed for the vibration stresses of a turbine.
• A wind turbine up a tower is the opposite of subtle (poor OPSEC)
• Turbines require maintenance, particularly bearings and blades. Each turbine in an offshore wind farm requires an average of four visits a year for routine maintenance, as well as unexpected downtime to replace components.

There are a lot of manufacturers out there who claim the impossible for their turbines, there are some laws of physics that can’t be avoided.

• Even a ‘perfect’ wind turbine can’t extract all the energy from the wind (if it could the air would ‘stop’ behind the turbine forming a high pressure pocket which the wind would have to go round). 59% is the theoretical maximum amount of energy a ‘perfect’ turbine can extract.
• Energy available in the wind is proportional to the swept area of the blades, thus doubling the diameter of the turbine means four times the available energy.
• Energy in the wind is proportional to the cube of the wind speed. Double the wind speed means 8 times the available energy.
• More blades mean more torque (not more energy) high torque is great for pumping water but not for generating electricity. Electrical turbines typical have three blades (because this is slightly easier to balance) although two to six blades are not too unusual.
• These same rules apply whether you have a horizontal Axis Wind Turbine (HAWT) or a Vertical Axis Wind Turbine (VAWT). Some manufacturers claim that VAWTs can do the impossible, don’t be fooled.
• Real systems have inefficiencies in the blades and the turbine. Typically a turbine can generate about 25% of the energy in the wind.

Wind obviously varies both day-to-day and also by location, the good news is that the UK tends to be windy.

Wind distribution looks something like this:

I must confess I’ve not managed to build a wind turbine yet so unlike my solar article these figures are theoretical rather than being backed up by real measurements but I believe they are typical rather than manufacturer data-sheet hype.

• Modal wind (most common wind speed is ~12 mph)
• Average wind speed is ~15 mph

Unfortunately turbines tend to be rated at 30mph which is an uncommon wind speed.

When the wind exceeds a certain maximum speed the wind turbine will be damaged this occurs due to overheating of the windings in the rotor or from physical damage to the bearings. To avoid this; a turbine will ‘furl’ whereby as the wind increases it turns out of direct line of the wind. This is usually achieved by having the turbine offset slightly from the bearing on which it turns to face the wind and having a spring that holds the tail straight. As the force increases the spring begins to allow the tail to turn and the force of the wind offset from the bearing causes the turbine to turn progressively further away from the direction of the wind.

Even so in extreme winds turbines can be damaged and it’s advisable to either take them down when wind is forecast or stop them rotating. Turbines can be discouraged from rotating by shorting the output of the rotor, this acts like an electrical break on the turbine.

I would recommend building your own turbine if you are capable of a little construction work, that way you will be better prepared to do the repair and maintenance work that is required. A purchased turbine may last longer before it needs repaired but at that point you probably can’t fabricate the replacement part.

Even if you don’t want to build the turbine now (before an event) you can consider ensuring you have the plans and the difficult to source components. These components will certainly include the enamelled wire for the stator windings and the rotor magnets.

Wind turbines tend to generate low Voltage A.C. which needs rectified to D.C. and regulated before it can be used either immediately or to charge a battery bank. Matching the blades of the turbine to the stator coils to the battery bank is something of a black art, however don’t be too concerned about optimising everything, something that works fairly well is sufficient.

A good further resource for home made wind turbines is: OtherPower.com.

Lastly a vague rule of thumb for kWh per year from a wind turbine would be 0.19 * Blade diameter (in meters) squared * wind speed (in mph) cubed.

So for our 1m wind turbine and 12 mph winds we can expect 0.19 * 1 * 1 * 12 * 12 * 12 = 328 kWh/year or about 900Wh/day roughly comparable to 900W of solar panel (Given the low daylight availability in the UK a 100W solar panel actually produces on average about 100Wh a day rather than 100Wh per hour.)