The Enercon design differs in three respects from most of the Danish wind turbines operating in Britain: it is variable speed; it uses a "direct drive" concept obviating the need for a gearbox; and the UK turbine sits atop a slender concrete tower, with a base diameter of only 1.7 metres, tapering to 1.3 metres at the top. Steel towers, for wind turbines of similar size, are typically twice this size. All three concepts help reduce noise levels, particularly at the critical "cut-in" speed, when background noise from the wind is low.
At cut-in the Enercon machine is turning at only 18 rpm, about half the speed of its fixed speed cousins. A conservative view is that noise increases with the fifth power of rotational speed (aerodynamic sources vary on this and experts differ as to the exact relationships), so noise at cut-in will be about one thirtieth of that of a fixed speed machine, corresponding to around 5 dB. The absence of a gearbox cuts down mechanical noise, whilst the slender tower reduces "tower shadow" pulsation, caused as blades pass through the diverging airstream in front of the tower. Lastly, a concrete tower cuts down on any resonating vibrations.
The overall effect of these design features is to reduce noise by about 10 dB at cut-in, according to data tabled by Enercon in its environmental assessment report and based on measurements made by the German wind energy institute, DEWI. Based on public reactions so far, the design philosophy has reaped dividends, with the added bonus that the slender concrete tower is seen as visually attractive, says Vince.