Offshore wind energy: full speed ahead
by S Krohn, Danish Wind Turbine Manufacturers Association

More than 4,000 MW of wind power may be installed offshore in Denmark in the course of the next 30 years. Large wind turbines, cheaper foundations and new knowledge about offshore wind conditions are improving the economics of offshore wind power. Two pilot offshore wind farms of 5 MW each have been built by electric utilities in Denmark using conventional wind turbines: Vindeby in 1991 and Tunų Knob in 1995.

Energy 21
According to The Danish Government's Action Plan for Energy, 'Energy 21', 4,000 MW of offshore wind power should be installed before 2030. With another 1,500 MW installed onshore, Denmark will then be able to cover more than 50% of total electricity consumption by wind energy. In comparison, the current wind power capacity in Denmark is 1,300 MW (end 1998).

A total of 5,500 MW of wind power in the Danish electricity system means that, periodically, the wind turbines will cover more than 100% of Danish electricity demand. Therefore, the future Danish offshore power plants should be considered an integrated part of the Scandinavian electricity system, which is currently based on huge amounts of hydro power.

With a total investment of some DKK 48 billion (where DKK is the Danish krone) for the 4,000 MW offshore capacity, the Danish action plan will be the world's largest ever investment in wind power.

Danish power companies have already applied for planning permission for 750 MW of offshore wind farms. According to their timetable, more than 4,000 MW of wind power will be installed offshore in Denmark before 2027. The first stage is likely to be a 40 MW offshore farm just off the coast of Copenhagen in the year 2000.

Why offshore?
One of the primary reasons for moving wind farm development offshore is the lack of suitable wind turbine sites on land. This is particularly the case in densely populated and relatively flat countryside in countries like Denmark or the Netherlands.

Equally important is the fact that the wind speeds are often significantly higher offshore than onshore. An increase of some 20% at some distance from the shore is quite common. Given the fact that the energy content of the wind increases with the cube (the third power) of the wind speed, the energy yield may be some 73% higher than on land.

In other countries however, like the UK, the difference between good land sites and offshore sites may be smaller or even nil, since turbines on land are often situated on hilltops where the wind speeds up significantly compared to the speed on flat terrain.

Another argument in favour of offshore wind power is the generally smooth surface of the water. This means that wind speeds do not increase as much with the height above sea level as they do on land. This implies that it may be economic to use lower (and thus cheaper) towers for wind turbines located offshore.

Offshore challenges
The primary factor delaying offshore development of wind farms has been cost. Although the price of wind turbines has fallen some 20% over the past five years, offshore installation costs have remained more or less stable.

Foundations and grid connection of large wind farms on land can be a relatively modest cost of less than DKK 450,000 per wind turbine (eg in the case of the Rejsby Hede wind farm in Denmark, consisting of 39 wind turbines of 600 kW each). Foundation costs were 6% of project costs, while grid connection accounted for 3%.

Offshore, however, foundations and cables add significantly to project costs. In the latest Danish offshore wind farm at Tunų Knob (1995), for instance, wind turbines were placed in water 3­5 m deep. Here, foundation costs per turbine were 23% of project costs while grid connection costs were around 14% .

Waves (and, in some areas, pack ice) are the most important factors determining the required strength and weight of offshore foundations. Consequently, it is far more economic to use larger wind turbines, since the size and costs of foundations do not increase in proportion to the size of the wind turbine. As for grid connection, it is far cheaper to attach fewer turbines to the grid for a given wind farm size.

Foundation technology
While economies of scale are important, the most interesting breakthrough in offshore technology has been new engineering studies which indicate a 35% decrease in foundation costs, due to the use of steel rather than concrete foundations. The present offshore wind farms in Denmark are placed on reinforced concrete foundations built onshore and floated out to sea where they are filled with gravel and sand, much like traditional bridge building technology. Such a foundation is known as a 'gravity foundation' since it relies on gravity to keep the turbine in place.

One of the newer technologies adopts a similar method, but uses a cylindrical steel tube placed on a flat steel base on the bottom of the sea. Such a foundation is considerably lighter, allowing barges to transport and install numerous foundations quickly, using the same fairly lightweight crane used for the erection of turbines. These foundations are filled with olivine, a very heavy mineral, which gives the foundation sufficient weight to withstand wave and ice pressure.

Other foundation technologies include 'monopile' foundations, effectively extending the turbine tower under water, and drilling or ramming it into the sea bed. For larger water depths, three-legged steel platforms, similar to offshore oil rigs, are being studied.

Lower maintenance costs
Larger machines save money on maintenance, since the number of units that have to be visited by boat will be smaller. In addition, because the wind is less turbulent on sea than on land, smaller fatigue loads act on the wind turbines, giving less wear and tear, and adding significantly to the estimated lifetime of the turbines. While offshore foundations are built to last 50 years, wind turbines are presently built to last 20 years. With a somewhat longer expected lifetime, of say 25 years offshore (a conservative estimate), the same set of foundations can be used for two successive generations of wind turbines.

More wind at sea
The two Danish pilot offshore schemes have provided very important advances in the knowledge of the offshore environment. Even if higher wind speeds were expected at sea, the latest results from Tunų Knob indicate that offshore wind energy output is 20­30% larger than forecasts made by traditional wind modelling methods.

The Risų National Laboratory in Risų, Denmark, which is known world-wide for its WAsP wind modelling software, is in the process of revising its basic models after the experience gained from Vindeby, Tunų Knob, and four new offshore meteorological masts erected in 1996.

Turbine design
The wind turbines used in the current offshore projects are largely standard machines in the 450­1,000 kW range, usually with some additional corrosion protection. Interesting modifications are gradually beginning to appear, however.

Since the beginning of offshore deployment, high-voltage transformers have, of necessity, been installed inside the turbine towers. As well as better corrosion protection, this gives the additional advantage of heating the equipment, thus avoiding cold starts of the turbine.

At Tunų Knob, special electrical cranes were installed in each turbine to allow replacement of major components, such as rotor blades or generators, without using a large and expensive floating crane. Another interesting modification was a design change allowing a 10% increase in rotor speed, increasing the efficiency of the turbines by some 5­6%. Higher rotational speed always carries a noise penalty, but as the theoretical sound level onshore several kilometres away is extremely low, this is of no concern.

Finally, the turbine manufacturers have taken a leaf out of the Navy's book: the turbines are painted in the standard NATO light grey camouflage colour, and the rotor blades were manufactured in exactly the same colour. The result is that even a slight amount of haze makes the turbines disappear completely when viewed from the shore.

A bright future
The Danish power companies are currently planning very large wind farms of some
300­600 MW each, built in modules of 150 MW, using 1.5 MW wind turbines. Watch the seas around Denmark just after the year 2000 to catch a glimpse of the first large-sized offshore wind farms in Europe.

For more information please contact the CADDET Danish National Team.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.