Offshore wind conditions
Wind conditions at sea
The surfaces of seas and lakes are obviously very smooth, thus the roughness of a seascape is very low (at constant wind speeds). With increasing wind speeds some of the energy in the wind is used to build waves, i.e. the roughness increases. Once the waves have been built up, the roughness decreases again. Thus, there is a surface with varying roughness, just like in areas covered with more or less snow.
Generally speaking, however, the roughness of the water surface is very low and obstacles to the wind are few. When doing wind calculations we have to account for islands, lighthouses etc. just like you would account for upwind obstacles or changes in roughness on land.
Low wind shear means lower hub height
With low roughness, wind shear at sea is very low, i.e. the wind speed does not change very much with changes in the hub height of wind turbines. It may therefore be most economic to use fairly low towers of perhaps 0.75 times the rotor diameter for wind turbines located at sea, depending upon local conditions. Note that typically towers on land sites are about the size of the rotor diameter or taller.
Lower turbulence intensity means longer lifetime for turbines
The wind at sea is generally less turbulent than on land. Wind turbines located at sea may therefore be expected to have a longer lifetime than land based turbines.
The low turbulence at sea is primarily due to the fact that temperature variations between different altitudes in the atmosphere above the sea are smaller than above land. Sunlight will penetrate several metres below the sea surface, whereas on land the radiation from the sun only heats the uppermost layer of the soil, which thus becomes much warmer.
Consequently the temperature difference between the surface and the air will be smaller above sea than above land. This is the reason for lower turbulence.
Wind shade conditions at sea
The conventional WAsP model used for onshore wind modelling is in the process of being modified for offshore wind conditions, according to its developer, Risų National Laboratory.
The different production results obtained from the experience of the first major offshore wind park at Vindeby, Denmark, and the subsequently built wind park at Tunų Knob, Denmark, has led to new investigations with anemometer masts being placed offshore in a number of locations in Danish waters since 1996. The preliminary results indicate that wind shade effects from land may be more important, even at distances up to 20 km, than was previously thought. On the other hand, it appear that the offshore wind resource may be some 5 to 10% higher than was previously estimated.