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With levels of wind penetration set to rise dramatically when Denmark's mighty offshore wind farms come on-line, new technical demands are being made of the technology to ensure stable supplies of power. But such are the requirements for power control being made by system operator Eltra that there is a risk that the extra output to be gained at an offshore site will be wasted. This appears to be the case with Horns Rev, the first of the five 150-160 MW offshore wind plant to be built by Denmark's utilities for the government, reports the magazine of the Danish wind turbine owners association, Naturlig Energi. Horns Rev, to consist of 80, 2 MW turbines supplied by Vestas, is being built in the North Sea, 14 kilometres off the south west coast.

Unlike power from wind turbines on land, offshore production will not be classified as "priority electricity" and there is no obligation on the system operator to take all the power. Priority output from wind plant on land, along with other renewable energy plant and local combined heat and power stations, already makes up 36.5% of power production in Denmark, limiting market trade. By not classifying offshore wind as priority electricity, Denmark's utilities are free to trade the power on the open market -- and Eltra is free to regulate offshore output in periods of slack demand. But, as reported by Naturlig Energi, the project budget suggests Eltra is already planning to curtail production from Horns Rev.

Although conventional wisdom holds that a turbine at an offshore site will produce 50% more power than the same machine at a good site onshore, says Naturlig Energi, this is not the basis of the projected accounts for Horns Rev. Each Horns Rev turbine is to have an 80 metre diameter rotor and thus a swept area of 5,028 square metres. The projected annual production for the wind plant is 600 million kilowatt hours, or 7.5 million kWh per turbine, corresponding to 1492 kWh per square metre of rotor swept area.

But compared with the production of a well-sited turbine onshore, Eltra's projected output for Horns Rev appears too low. Last year, when the wind resource in Denmark was 5% less than in a statistically normal wind year, the best onshore turbines produced up to 1440 kWh/m2. In a year of normal winds, production would be over 1500 kWh/m2, more than that in the budget for Horns Rev.

Output control

Eltra engineer Aksel Gruelund Sørensen explains that the limited projected output is a factor of the conditions governing the wind plant's connection to the transmission network, conditions that will also apply to other large offshore wind plant. Together with Vestas, which has already ensured that output control is an inbuilt function of its 2 MW offshore turbine, Eltra is striving to make sure the wind farm meets three basic requirements.

First, Horns Rev must be able to produce an uninterrupted power supply, or reduced supply, even if there are certain failures on the transmission net which would normally stop an onshore wind turbine. Second, a power plant as large as 150 MW must have the ability to gently couple in and out of the transmission system. A surge from 0-150 MW over a few minutes is far from desirable. Third, there must be potential for tailoring output to meet reduced demand, for example by requiring the wind turbines to operate at reduced output.

The risk is that the wind plant is prevented from producing as much power as it could. Not that this matters under the new payment regulations for offshore plant, says Gruelund Sørensen. The government has agreed to a minimum guaranteed price for a fixed number of "full load hours" of operation. Whether a plant reaches its allotted number of hours of operation sooner or later will not affect its economics, he points out.