In Europe, the emphasis is on offshore turbines because of the lack of space and objections to the noise and aesthetics of onshore wind farms. It is more expensive to build towers in the sea, and there are problems with storms, icebergs, and salt water, raising the cost of operation and maintenance. However, the wind can be steadier and stronger at low altitudes so that the towers do not have to be quite so high. Denmark had the most installed offshore wind power as of 2005 (Fig. 3.11) and has led in the development of the technology.

As Fig. 3.12 shows, there are different ways to mount the towers in the sea depending on the depth of the water. If the installation is kilometers offshore, the turbines have to be floated and tethered to the bottom. This is much harder than for floating oil rigs because the towers have to be kept from turning, leaning, or tipping over. Except for experimental trials, no floating turbines have yet been installed, though Germany envisions placing them as far as 40 km offshore.9 In September 2009, Vestas Wind Systems of Denmark announced its V112-3.0MW turbine specifically designed for offshore use.14 This turbine incorporates new technology for increased efficiency, reduced noise, and resistance to the severe conditions, including a heating system to keep the parts from freezing. The power curve for the V112 is shown in Fig. 3.13. The turbine cuts in at a wind speed of

Denmark

53%

Fig. 3.12 Methods for installing offshore turbines (Energy from Offshore Wind, US National Renewable Energy Laboratory, NREL/CP 500-39450, February 2006. Engineering Challenges for Floating Offshore Wind Turbines, NREL/CP 500-38776, September 2007)

3 m/s and achieves its maximum output at 12 m/s. It can maintain this output up to 25 m/s. The steep dependence of power on wind velocity can be seen from this curve.