Engineering aspects

of wind turbines

There is a fair amount of energy in the wind. That is a fact which we all recognise form everyday experience. The energy in wind is inherently more concentrated, and therefore easier to collect, than that in raw sunshine. The technology of 'windmills' is fairly simple and readily understood, and that should in principle make construction costs less than those of hi-tech devices like solar photovoltaic panels. Britain, being surrounded by ocean, is a relatively windy part of the world. This all sounds very promising.

Wind speed is very dependent on height above ground. In particular, most locations experience a 'wind shear' or 'wind gradient' where as you ascend from the ground, the wind becomes substantially stronger at around 100m (300 feet) height. This arises due to the 'friction' of moving air on the ground itself, and on tall obstructions.

This tends to predispose towards the construction of very large, very tall turbines, with those under 300ft height being unable to get-at the best of the energy in the wind. Builders of small-scale turbines often overlook this consideration, and find that the output is far less than anticipated owing to the turbine always operating below the height of the wind gradient. Whilst it would be logistically impractical to place a two-metre turbine on a 100m tower, there are workarounds, such as siting a smaller turbine on a hill edge or high building. However, in general the effect is that in windpower, Big is Best.

The energy in wind increases as the cube of the windspeed. Thus, a 20mph wind contains eight times the energy of a 10mph wind, and a 40mph wind eight times the energy of a 20mph wind.  Thus, it can be seen that the typical light-wind conditions which prevail most of the time in the UK provide only tiny amounts of energy to a turbine. With brisk breeze, there is plenty energy to harvest. However, get towards storm conditions, and the energy in the wind becomes sufficient to wreck any large object which dares to obstruct its passage. Including our turbine, if its blades have not been feathered.

Thus, because of the enormous increase in the power of the wind with its speed, any given turbine design can only offer optimum power output over a restricted range of windspeeds. This in turn has the effect that windturbine output is a highly intermittent, or variable, resource.

Winds at altitude tend to be more constant than those at ground level, and generally vary in strength over periods of a few hours rather than the minute-by-minute changes in wind strength we are accustomed-to on the ground. Thus the output of large turbines tends to be sufficiently predictable to allow synchronisation with other electricity sources, whilst that might not be the case for smaller units. The consequence of these engineering parameters is that turbines tend to be very large devices. Smaller designs cannot yield the same return per materials input.

An oft-repeated claim of the early wind promoters was that 'The wind always blows somewhere' -Implying that a nationwide deployment of interconnected turbines would overcome the intermittency problems associated with single sites. Surprisingly, this claim was accepted without much question. I guess nobody thought to ask  a weather forecaster or aviator if it was true. If they had done so, they would have been told, No. In high-pressure conditions it is common for the entire UK landmass to be a light-wind zone. Sometimes the light-wind zone can extend over the whole of Europe. As we have seen, light winds provide next to nothing in terms of energy owing to the cube law.

Whilst output tends to be reasonably predictable over an hour or two, output tends to be very variable over longer periods. A case in point was a 20-day interval in July 2013, during which UK windfarm output remained at very low levels.

Because of this intermittency, wind energy will always require backup generating plant of some kind. As we have seen, wind output tends to very over a period of hours. Thus, backup plant must be able to come online within that timeframe if it is to take over seamlessly when the wind drops. Coal and nuclear both have an issue here, in that their startup times are far too long for them to serve as backup for wind. Biomass likewise if that uses the same boiler technology as coal, which it generally does. That basically leaves gas turbine fired generators as the only large-scale backup capacity for wind.

Now, here is a paradox. The organisations promoting wind power are also strongly against (OK, fanatically against) shale gas development. Yet, UK gas reserves are dwindling, and the only two other options are to import gas from Russia, or to exploit our own shale gas. Depending too heavily on Russian supplies is probably none too wise in view of the political instability in that region. Supplies might be cut off, or made very expensive, at any time. So, basically, that leaves shale gas as the backup for wind power. No shale gas, no use having windturbines.

This I think underlines the lack of forethought among those who ecstatically promote renewable energy. They seem to think that another form of backup for wind will appear, like a rabbit out of a hat.  Those of us with a more down-to-earth viewpoint see that as unlikely. Thus, the rational way forward is to either go with wind AND shale gas, or neither.