Renewable Energy Review – Might and Majesty

Each of the 80 Vestas wind turbines that make up Denmark’s Horns Rev wind farm are now generating electricity and supplying the national grid. PEI visits the world’s largest offshore wind farm and examines this most ambitious of projects.

Nigel Blackaby

The technology and engineering associated with the generation of electricity is often striking in its scale and complexity as well as being impressive in its ingenuity. Rarely though, would it be regarded by anyone outside the industry as aesthetically pleasing. Horns Rev, the world’s largest offshore wind farm, located off the west coast of Jutland, Denmark is a notable exception. Its rows of majestic turbines stretching out towards the horizon make a truly awe-inspiring sight which, were it not for their very practical purpose, would unquestionably merit an artistic prize. But the graceful rotation of the turbine blades does unquestionably have a practical purpose through their ability to generate worthwhile amounts of “green” electricity. As part of a landmark project, they also act as signposts to one of the world’s potentially most important power resources over the coming decades.

The Horns Rev (rev is Danish for reef) Offshore Wind Farm consists of 80 wind turbines positioned over an area of 20 km2 with a total output of 160 MW. Expected annual electricity production is 600 GWh which corresponds to the consumption of 150 000 households. “Horns Rev is not just a few turbines,” said Bjarne Henning Jensen, director of the project’s co-owner Elsam, “It is effectively a new power plant.”

It should come as no surprise that Denmark is the home to an offshore wind project on the scale of Horns Rev. Although a relatively European small country with a population of 5.3 million people, Danish companies are among the world’s leaders in wind generation technology with two of the top five wind turbine suppliers, Vestas and NEG Micon, headquartered there.

Access for maintenance will usually be via helicopter
Click here to enlarge image


Click here to enlarge image

With 2880 MW of wind energy generating capacity, Denmark is the world’s fourth largest wind power generator, according to figures published by the European Wind Energy Association, which rank it behind Germany, Spain and the USA. Europe is leading the way in the drive to develop wind power. Over three quarters of the world’s wind power is generated in Europe and the resource currently meets two per cent of European electricity demand, compared with 0.4 per cent worldwide.

Danish prominence

Denmark’s prominence in the field is partly down to its physical characteristics ” the county being very flat and relatively windy. In addition, the Danish government has adopted stringent national and international environmental targets. The Danish Government’s energy action plan “Energi 21” targets a reduction in CO2 emissions by 50 per cent by 2030 (as compared to 1988). The 1996 plan includes a target of achieving 20 per cent of electricity consumption from renewables by 2003. Denmark is expected to achieve 27 per cent of electricity consumption from renewables this year, with 18 per cent of this coming from wind.

Danish energy policy is therefore very supportive of wind power development but at the same time there is a recognition that the erection of additional land-based wind farms could create unacceptabconcentrations of wind turbines. Power suppliers and developers have therefore been looking increasingly toward offshore locations in order to develop larger projects and unlock the economies of scale that they offer. The Danish government initially authorized the construction of five offshore wind farms, although this was reduced to two in 2002. The first of these was close to Horns Rev in the North Sea, 14-20 km northwest of Esbjerg on Denmark’s west coast. The government plans a further three large-scale demonstration facilities and intends that Danish waters produce 4000 MW of electricity by 2030.

Developing Horns Rev

In 1999, Danish utility Elsam was granted in-principle approval to build and operate Horns Rev, along with Eltra, the independent transmission system operator for western Denmark, Ownership of the project is shared between Elsam, which owns the wind farm and Eltra, which owns the interconnection to shore linking Horns Rev to the main high-voltage grid. Total construction costs for the project are around DKK2 billion ($289million). Elsam is Denmark’s largest producer of heat and electricity as well as having international interests in the environment and energy. The company has been developing wind power for 25 years and today owns and operates 500 wind turbines with capacity in excess of 400 MW.

Among the many remarkable aspects of the Horns Rev project has been its speed of construction. The erection of the wind turbines commenced in March 2002 and was completed within six months. Project manager for Horns Rev is Jens Bonefeld, a civil engineer employed by Elsam subsidiary Tech-wise A/S. Commenting on his project Bonefeld said: “The construction stage has gone very smoothly due to the vast amount of pre-planning that we undertook, plus the fact that we have been fortunate with overall weather conditions throughout the construction period.” As a result of cable damage to one of the main cables in the park early in the installation period and an overall underestimation of the logistics involved, the turbine commissioning stage of the project has overrun by three months. All turbines are now fully operational with the last turbine group almost through their contractual test period. The end of these tests will mark the official handing over of the project to Elsam to operate.

Horns Rev will make a significant environmental contribution
Click here to enlarge image


Even relatively close up, it is hard to gauge the true size of each of the turbine structures. The tip of each rotor blade stands 110 m above sea water level and each of the 80 turbines is supported by a monopile driven 22-24 m into the seabed. The water depth over the firm sand and gravel reef varies between 6.5 m and 13.5 m and each turbine is supported by a 61m tower located on the monopile foundation, which itself is 9 m high. During the construction period, sub-contractors were able to transport and drive one pile per day. The main contractor for the foundations was MT Hàƒ¸jgaard A/S. On average, the erection of the turbines also took one day, although as experience was gained, installation from a special purpose ship was achieved in just four hours. Foundation works were completed two weeks ahead of the agreed milestone.

The turbine chosen for the project was the Vestas V.80- 2.0, which has a rotor diameter of 80 m and a generator output of 2 MW. While most of the major wind turbine manufacturers tendered to supply Horns Rev, Jutland-based Vestas was judged to be best able to meet the requirements of the project on competitive terms. The V80-2.0 has pitch-controlled blades, can achieve a greater r/min and is equipped with more monitoring equipment than a standard turbine. Although the wind farm is designed to normally operate at full power (subject to the limitations of current wind conditions), a newly developed remote control system enables electricity production to be varied to suit grid conditions. The turbines can operate in wind conditions up to 24 m/s and wind speeds around Horns Rev exceed this only very infrequently. The wind turbines were mounted by means of large specially built vessels with submersible legs. Cranes on the vessels positioned the turbine components on the foundations.

Click here to enlarge image

The turbines are 560 m apart and in order to manage the project and distances involved, Elsam divided the site into six units which correspond partly to the electrical layout of the wind farm with five independent clusters of 16 turbines each connected to the substation. Work commenced with the substation unit with the main cables being laid and trenched to each of the five turbine units. This section was commissioned in spring 2002, meanwhile work was underway on the erection and commissioning of units two to six.

Eltra placed its order for the submarine cable with Nexans of Norway and the land cable order went to ABB, which also supplied the switchgear. The substation transformer, which steps the power up from 34 kV to 150 kV was supplied by Alstom and HBG won the contract to supply the transformer platform.

Siting of the Horns Rev wind farm required considerable planning. A meteorological mast has been in position for over three years measuring wave and weather conditions and a mean wind speed of 9.7 m/s at a height of 62 m has been recorded. This will produce a power output 50 per cent larger than the equivalent turbine on land. “This is a ‘real’ offshore site with strong winds and a harsh wave climate due to the current and shallow water,” said Jens Bonefeld.

The area is away from shipping channels although, as a precaution, additional piling has been put in place to protect the substation from collision. The project has the support of environmentalists, who recognize the value of its non-polluting power output, and the local tourist industry has been won over and now promotes the wind farm as a feature of the area.

Discussions continue with the local fishing industry over the level of compensation for loss of fishing rights in and around Horns Rev. The effect of the structure on vegetation, birds, seals and fish are to be studied and the knowledge gained can be used in the development of future wind farms. Despite having a reputation for being noisy, the Horns Rev wind turbines emit no noticeable sound above the natural background produced by the wind and waves.

The severity of the weather will have a significant impact on accessibility to the turbines for maintenance and operational purposes. Like any other power plant, it is essential that power output matches requirements whenever possible and therefore Elsam has focused on turbine availability from the very beginning. Given the remote location, the design of turbines has required a balance between state-of-the-art technology, available systems, and a simple well-tested technology. Monitoring is carried out 24 hours a day. It includes high-tech vibration monitoring; temperature; pressure; r/min and voltage measurements; and the use of microphones, which enable experienced engineers to often identify a problem just by the variation in sound. Elsam has awarded Vestas a five-year service contract during which time Elsam will have the opportunity to build up its own expertise. This will afford Elsam the option of carrying out the servicing themselves at the end of the five-year period, should they wish to.

Transportation costs have had a major influence on the finances of the project. Each turbine can be accessed both by boat at the turbine foundation and by lowering engineers from a helicopter onto a platform on the nacelle roof. However, Elsam estimates that the turbines will be inaccessible from the sea 30 per cent of the year. Assuming each turbine will require three or more unscheduled visits, Elsam believes that the use of helicopters will reduce production losses and therefore be more economical than using boats -” despite the additional expense of helicopter transportation. It has concluded that its optimum transport strategy will be to use a large ship for main turbine overhauls and helicopters for smaller planned service works and for small repairs.

Larger turbines lead to lower construction costs ” particularly for foundations and grid connections
Click here to enlarge image


The safety considerations of both methods of transport have been evaluated as well as safety for staff working within the offshore turbines. Should bad weather result in staff becoming stranded, each turbine is equipped with a toilet, sleeping bags, drinking water, 12 freeze dried meals, a first aid kit and a fire extinguisher. There is a lift inside the tower linking the foundation to the hub and an emergency escape mechanism outside the tower to lower staff down from the hub to an awaiting boat.

Commercial challenges

Horns Rev is undoubtedly a well-planned and skilfully engineered project that has been groundbreaking in a number of ways. The disappointment is that this success is not mirrored on the commercial side of the operation.

Location of Horns Rev windfarm turbines, substation and HV cable
Click here to enlarge image


Since the Danish government cleared the way for Horns Rev, the economics of the wind project have changed as the power sector has liberalized.

Since January 2000, Elsam has been transformed into a commercially run operation with outside shareholders. The price for electricity produced by Horns Rev was fixed under specific legislation for the project and assumed that additional value would be available from Renewable Energy Certificates generated by the electricity production.

In the event, Denmark’s ‘green certificate’ system has been abandoned due to the small size of the market and extent of existing subsidies. This has left Elsam dissatisfied with the price it will obtain for power production. While the project is not losing money, it is not achieving a commercial return and is looking at a ten to 20 year payback period.

Wind turbine technology continues to advance with improved levels of reliability and increased output possible. These developments will result in more offshore locations becoming feasible.

Environmental pressures are also likely to add to this trend. A number of plans for offshore wind farms are ready for execution in Northern Europe and North America in 2003. Elsam’s experience at Horns Rev will be valuable as it looks to pursue its strategy of developing operations in Denmark and overseas. “The performance of this project will influence the progress of the planned projects for years to come,” said Jens Bonefeld. “Elsam is ready to pave the way and use its experience in other projects.”

No posts to display