Energy Systems and Technologies for the Coming Century ...

Figure 1: Using experience curves to forecast wind energy economics up to 2015. Thecosts shown are for an average 2 MW turbine with a present-day production cost of euro¢6.1/kWh in a medium wind regime (from [3])3 Technology trendsMainstream technologyThe 30-year development of wind energy technology, with its focus on reducing the costof energy, has seen the size of the largest turbines increase by a factor of 100, fromroughly 50 kW to 5 MW.This is in spite of a theoretical limit to the maximum size of a wind turbine. As a windturbine increases in size (while keeping the same proportions) its energy output increasesas the square of the rotor diameter, but its mass increases roughly as the cube of the rotordiameter (the “square-cube law”). As the mass increases, the mechanical loads imposedby gravity increase even faster, until the point where the materials available are notstrong enough to withstand the stresses on the turbine.So far, engineers have avoided the limits of the square-cube law by avoiding directgeometrical similarity, using materials more efficiently, and using stronger materials.Perhaps most importantly, designers have tailored the responses of turbines ever morecarefully to the conditions under which they operate, and this remains one of the mainways to reduce the cost of energy from future turbine designs.Issues of geometry notwithstanding, several factors favour larger turbines. At somepoint, however, it seems fair to assume that at some point the cost of building largerturbines will rise faster than the value of the energy gained. At this point scale-up willbecome a losing economic game.As a result, it is important also to look at other ways to cut costs. This can be done, forinstance, by introducing cheaper technology or by increasing the amount of energycaptured by a rotor.Conventional wind turbines use gears to match the slow speeds of the blades and hub tothe higher speeds required to drive a standard induction generator. It has been known formany years that a multi-pole generator, which can run at slower speeds, offer the chanceto eliminate the gearbox. Early multi-pole generators were large and heavy, but newerpermanent-magnet designs, in which the rotor spins outside the stator, are compact,efficient and relatively lightweight. The next generation of multi-MW gearless windRisø International Energy Conference 2011 Proceedings Page 207