How wind turbines work and their limitations

Wind turbines – how they work and their limitations

I have not written much about wind turbines in these posts for two reasons. First, understanding how much energy an wind turbine can deliver is quite complicated, a task not made easier by the varying claims and standards and secondly because wind energy suffers, like all forms of renewable electricity from intermittency, which is a bad thing when you cannot store the electrical energy in a way that makes sense, environmentally or financially.

Wind turbines are an essential ingredient in getting our future energy; like all the other ingredients that will make up the future energy cake, you cannot use it alone. Some of the ingredients are yet to be discovered but with wind turbines the scientists can pretty well tell us what they can do and what they cannot do.

I will concentrate on for now on explaining how wind turbines work. When the wind blows it is full of kinetic energy- the energy of motion. When something is moving it has kinetic energy, whether that movement is vibrational, rotational, or “translational” – which is movement from one place to another, like the wind. The amount of kinetic energy in the wind depends upon its mass and its speed.

A wind turbine’s blades catch the wind and actually remove translational energy from the some of the wind. Other parts of the wind’s translational energy are diverted around the turbine, which cannot, of course, capture all the wind’s kinetic energy (or more precisely the kinetic energy in the moving molecules of air). The energy causes the blades of the turbine to rotate and this converts the kinetic energy of the wind into mechanical energy of electricity. Some energy passes through the blades of the turbine, reducing the wind speed immediately behind the blades.

To find out how much mechanical electrical energy the turbine can produce we have to know the wind speed (which we can measure) and the mass or density of air, as well as the size of the turbine blades. We can then calculate the power that the turbine can produce. (Power is a measurement of energy over a period of time.)

Applying the thermodynamics and physics to wind turbines means:

·         The bigger the turbine the more power you get; double the size of a wind turbine and you get four times the power

·         The faster the wind, the better.

But (there are always buts)

·         It is very expensive to build big turbines that can work in very high winds so the blades are “feathered” to reduce stress on them, so that they do not break off in very high winds; thus you find turbines either stop or slow down in very high winds.

·         You cannot get more than about 59% of the power in the wind into a wind turbine (Beltz’s law)

·         Efficiency of wind turbines depends on the design criteria; efficiency is not that important because kinetic wind energy is free, but if we could make turbines 50% efficient we would not need to build so many of them; the best wind turbines actually operate at around 35% efficiency

·         Wind does not always blow when you need the energy.

·         Generally places where people need the energy are some distance away from places where there is plenty of wind. In the United Kingdom the wind blows best around the coast and just off it. The main wind direction is from the west, so the west coast, particularly in Scotland, gets lots of wind which is some distance away from where the electricity generated will be used.

·         Cities tend to have less wind than open land, because the buildings deflect wind away.

When you have understood this you may wonder

·         Why are small wind turbines marketed in cities for domestic energy generation? David Cameron put one on his home in Notting Hill and there was talk at some stage for one to be placed next to the Houses of Parliament, which probably have more wind inside the building than outside it.

·         Why are wind turbines apparently rated as producing so much power or energy?

The first question is perhaps simple to answer; they are perhaps more about vanity and actuality. The second needs a longer post than this, because there are various different statements, standards and descriptions, which misled the layman into thinking that a turbine is producing more than it actually can.

The biggest problem to solve with turbines is how the grid can be made to cover the intermittency of wind power without creating a large carbon emission problem with the start stop of back up fossil fuel power stations.

Don’t get me wrong. Turbines are an essential ingredient in a sensible energy mix but the way in which they have to work provides limitations. Wind turbines cannot by themselves provide more than a small part of the energy that we use so thoughtlessly and greedily.