It is extraordinary to think that only a very few years ago, the concept of the 5 MW wind turbine was considered somewhat exotic. A press release put out by German gear maker Flender in September 1999, for example, announced it was “seeking partners to develop an advanced wind turbine in the 4-5 MW capacity range” and wanted “to partner with US companies in development of next generation turbines.” But, as several presentations at the recent EWEA’s European Wind Energy Conference and Exhibition (Wembley, UK, 22-25 November) made clear, the 4.5-5 MW class wind turbine is now very much with us, the key challenge currently being to demonstrate that such machines can be built and run reliably in offshore environments.

When it comes to scaling up the unit size of wind turbines, the undoubted pioneer was German company Enercon, several of whose E112 machines, rated at 4.5 MW, with a THM* of around 500t, have entered operation over the past couple of years.

In November another German company REpower announced connection to the grid of what it described as the “world’s largest wind turbine”, its 5 MW 5M machine, with a THM of about 400t – at Brunsbüttel (close to the nuclear power station there). At the same time REpower also announced that it has been awarded one of the five sites up for tender in the Cuxhaven test field, which is primarily intended for demonstrating offshore wind turbines. The company has signed a contract with DEWI-OCC Offshore and Certification (an entity owned by the German Wind Energy Institute (DEWI), the federal state of Lower Saxony and the City of Cuxhaven) under which a 5M machine will be constructed by the end of 2005.

Meanwhile, in Bremerhaven, Germany, a 5 MW prototype using Multibrid technology (now owned by Prokon Nord) was expected to achieve grid connection before the end of 2004. With a THM of a mere 310t, installation was reported to be complete on 1 December, with fitting of the hub, together with the 56.5 m blades, to the nacelle, which itself had been mounted to the top of the tower in one lift.

Not to be outdone, Vestas announced at the Wembley conference the launch of its V120, a 4.5 MW turbine with technologies combining the best of Vestas and the recently merged Micon, with an expected THM of as little as 220t.

But the jury is still out, and will remain so for a good few years yet, on the best technology for these large offshore wind turbines. As you might expect for what is a very immature technology, the range of approaches is still wide, from the gearless direct drive, but rather large, E112 (which however is thought to be capable of producing much more power than its current rating of 4.5 MW) to the more conventional, but extraordinarily compact, V120. And then somewhere in between there is the Multibrid concept, which combines a single stage gearbox with multipole generator.

Ease of installation (with the minimum of lifts), reliability in operation (particularly in view of the Horns Rev fiasco), and maintainability are going to be key concerns. But minimising THM – perhaps the key determinant of costs – looks like being a primary consideration as we move towards 5MW offshore machines, and inevitably, well beyond.