Bigger is... better?

As I mentioned in my previous post, there has been a large growth in turbine size in terms of rotor diameter and rated capacity - the last five years in particular has seen a remarkable growth. Here is a quick plot of . The general parabolic shape is what one would expect when considering that power in the wind is proportional to the frontal surface area i.e., the square of the radius. This has resulted in some very large increases in rotor diameter to incrementally increase the rated power; however as the 'power law' kicks in, smaller and smaller increases in diameter will result in the same relative increase in rated power.

Extrapolating this simplified argument, this relationship would predict a 10MW turbine with a rotor diameter of around 160m. This isn't a large change in blade length (an approximate 20% increase), however is a doubling of the rated capacity. This therefore suggests that for future turbines the technology focus will be on developing the higher capacity drive-trains: the gearboxes, generators, and associated power electronics required for power generation.

Graceful giants

I recently had the opportunity to visit Cuxhaven in northern Germany, and saw the two largest turbines in the world - the new Repower and Enercon 5 MW + turbines. Graceful giants? or intruding monsters? you be the judge!

Cuxhaven is often used by turbine manufacturers for power curve and loads development due to the higher average wind speeds which are important for this type of testing.

Here you can see the new prototype Enercon E-126 6 MW nacelle being fitted to an existing E-112 tower. Enercon state on their website that the production turbine will have a 135m hub height (HH) segmented concrete tower. You can infer from the very large signature 'tear-drop' nacelle the size of the underlying direct drive system. The larger nacelle weight has also resulted in a much thicker tower, especially at the base, which is an impressive engineering feat.

In the background you can see two REpower 5M turbines, with a 126m rotor and rated power of 5 MW. Due to the conventional asynchronous generator configuration used by REpower, the nacelle is much smaller in contrast to the Enercon design, and the thinner tower becomes quite obvious. This turbine is stated by REpower on their web-site to be developed particularly for offshore use.

With this latest generation of turbines exceeding 5 MW now, and rotor diameters exceeding 120m, where will these turbines be installed? It is expected that the new Repower turbines will be utilised for the offshore market, with the turbine already being installed on several offshore projects. It would definitely be a challenge to install the larger Enercon turbines offshore, as they would by deduction have a much larger system weight and footprint area.

How much bigger will they get? I read a magazine article from the 1990s recently, and the author quite profoundly stated that 'the 50-60m rotor diameters was expected as the technical limit for wind turbine development'. The question of the 'optimal' turbine aside (as this is a very detailed economic question), future turbine sizes are limited only by our own innovation and considered applications.