The development of wind turbines started with small units for small applications, but as the turbines grew in size, they became less and less attractive as a source of electricity for individual or household consumption. Consequently, almost all of the electricity generated by such plants today is fed into the grid. The output of a wind turbine of typical size is already so high that it exceeds the capacity of the local electricity mains. This is precisely the case in areas along the coast with a good wind regime but often lacking electricity facilities, making it necessary to install new and higher-capacity mains facilities, with the related additional costs. Because the additional expense is not an economically viable venture in the case of individual units, there has been an increasing tendency to install several plants (at least five in most cases) in consolidated areas known as windfarms. The output of several turbines is combined and sold under contract to the utility company.
Starting in the early 1980’s, larger wind turbines were developed for “windfarms” that were being constructed in windy passes in California. In a windfarm a number of large wind turbines, now typically rated between 400-600 kW each, are installed on the same piece of property.
In the USA the windfarms are usually owned by private companies, not by the utilities. Although there were some problems with poorly designed wind turbines and overzealous salesmen at first, windfarms have emerged as the most cost effective way to produce electrical power from wind energy. There are now over 16,000 large wind turbines operating in the California and they produce enough electricity to supply a city the size of San Francisco. Large wind turbine prices are coming down steadily and even conservative utility industry planners project massive growth in windfarm development in the coming decade, most of it occurring outside California. One recent study actually called North Dakota the “Saudi Arabia of wind energy”.

Offshore Wind Turbines
The success story of onshore wind energy created an interest for the exploitation of wind energy at offshore sites since suitable locations on land are becoming scarce or do not have good enough wind conditions. On sea the wind blows harder and a large amount of space in shallow waters not too far from shore is available especially in most states of Northern Europe. Both aspects are essential for a future large scale development.  Firstly, a ten percents increase in the mean wind speed can result potentially in 30% more energy yield.  Secondly, it is generally believed that the continental shelf with water depth up to some 30 m and distance from shore of up to about 30 km offer considerable economic advantages. In the future technological progress, e.g. floating offshore wind farms or HVDC (High Voltage Direct Current) power transmission, may also enable exploitation of deeper water locations as typical for the Mediterranean and many sites outside Europe as well as more remote offshore sites. In a recent study carried out in the scope of the European non nuclear energy research programme JOULE the potential of offshore wind energy in the European Union has been estimated to be nearly two times the total consumption.

In the 1990s first promising steps were taken to develop the required technology and to gain experience. The general feasibility of offshore wind energy was demonstrated and together with the demand for environmentally green technology it was seen as a considerable and renewable contribution to the energy supply in Europe. Utilisation of wind energy offshore has even less environmental constraints than on land due to large available space and relaxed noise limitations. Generally the prospects are assessed quite positively and investment in offshore wind energy today is a preparation for a big market tomorrow. Offshore wind energy is an extremely promising application of wind power, particularly in countries with high population density, and thus difficulties in finding suitable sites on land. Construction costs are much higher at sea, but energy production is also much higher. The Danish electricity companies have announced major plans for installation of up to 4000 MW of wind energy offshore in the years after the year 2000. The 4 000 MW of wind power is expected to produce some 13,5 TWh of electricity, equivalent to 40 % of  Danish electricity consumption. Four possible areas (ranging from 135 to 500 km2, water depths from 5 - 15 m) are designated suitable to erect turbines at sea, with only few conflicting interests (e.g. environment, landscape, fishing, defence, communication, transport and national monuments). Production prices of about USD 0,05/kWh (20 years loan, 5% discount rate) are estimated.

Offshore windfam in the Netherlands.
In spring 1998, five offshore wind farms were realised in Denmark, The Netherlands and Sweden, respectively. These farms are demonstration projects, characterised by medium sized wind turbines of the 500 kW class, moderate farm capacity up to 5 MW, low water depths (less than 10 m) and small distance from shore (between 40 m and 6 km). The energy prices of the pilot plants are considerably higher than onshore wind farms at good coastal sites. Some e.g. the Danish ‘Plan of action for large scale offshore wind farms’, show that the cost of energy for large plants is competitive with onshore wind farms at average sites. Moreover the price of wind energy is close to or in the range of other energy sources.

The world’s first offshore wind farm is located North of the island of Lolland in the Southern part of Denmark Vindeby.  The Vindeby wind farm in the Baltic Sea off the coast of  Denmark was built in 1991 by the utility company SEAS. The wind farm consists of eleven 450 kW wind turbines, and is located between 1,5 and 3 kilometres North of the coast of the island of Lolland near the village of Vindeby. The turbines were modified to allow room for high voltage transformers inside the turbine towers, and entrance doors are located at a higher level than normally. Two anemometer masts were placed at the site to study wind conditions, and turbulence, in particular. The park has been performing flawlessly. Electricity production is about 20 per cent higher than on comparable land sites, although production is somewhat diminished by the wind shade from the island of Lolland to the South.

Vindeby offshore wind farm in Denmark.
The world’s second offshore wind farm is located between the Jutland peninsula and the small island of Tunø in Denmark. The Tunø Knob offshore wind farm in the Kattegat Sea off the Coast of Denmark was built in 1995 by the utility company Midtkraft. The wind farm is situated in an area where the sea depth varies from 3-5 m. The Tunø Knob area is of considerable environmental interest, both as a resting area for birds and as a beautiful part of the coastline and landscape. Furthermore, a careful archaeological investigation of the site has been carried out as part of the off-shore wind farm planning process. The Wind farm consists of ten 500 kW wind turbines. Each turbine is a horizontal axis pitch regulated machine, orientated up-wind with a tubular tower, and a 3-bladed rotor of 39 m diameter. The turbines are mounted on specially-developed, reinforced concrete caisson foundations. The turbines are connected to the national grid via a 6 km submarine cable to the mainland of Jutland. Each turbine is controlled remotely. The production manager can monitor the performance and operation of the wind turbine from an operation centre in Hasle. The control system is continuously collecting all relevant data. The data are transmitted via a radio system from the individual data-collecting unit of each wind turbine to computers at Hasle. On-site maintenance is estimated to be needed only twice a year, when engineers will sail to the wind turbines and carry out the regular scheduled maintenance programme. The turbines were modified for the marine environment, each turbine being equipped with an electrical crane to be able to replace major parts such as generators without the need for a floating crane. In addition, the gearboxes were modified to allow a 10 % higher rotational speed than on the onshore version of the turbine. This will give an additional electricity production of some 5 %. This modification could be carried out because noise emissions are not a concern with a wind park located 3 kilometres offshore from the island of Tunø, and 6 kilometres off the coast of the mainland Jutland peninsula. The park has been performing extremely well, and production results have been substantially higher than expected. In November 1995, its production was 1,3 GWh almost 40% more than originally estimated. The total production price/kWh is expected to be DKr 0,49 with an annual total production of 15 GWh. The entire costs of the off-shore farm are estimated to be about DKr 78 million.
The on-shore noise from the wind turbines has been calculated, at the nearest island of Tunø, to be less than someone whispering [15 dB(A)]. On the mainland it is inaudible.

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