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Engineering Feat of the Month: Horns Rev Wind Farm

10/05/2016
Horns Rev (aka Horns Reef) is a windfarm 17.9km off the west coast of Jutland, Denmark. Elsam (now DONG energy) was given responsibility for the project, operates the wind farm, and also owns 40% of it. The remaining 60% belongs to Vattenfall, a Swedish power company. In 2002 when the project began it was the world’s first wind power plant and is still thought to be the largest ever offshore wind farm. The project, which cost around €270m, has aided the Danish government’s plan to reduce CO2 emissions at a rate of 229,021 tonnes per year.

The location of Horns Rev was decided upon as it adhered to a number of specifications. Not only is the area not visible from the coastline (to appease those who believe turbines to be an aesthetic nightmare) but the water is also relatively shallow, which is a necessity. The average wind speed for this particular location in the North Sea is 10m per second which is greatly advantageous as it results in 150% more electricity being produced than a land-based turbine farm of the same size.


The Turbines

80 turbines make up Horns Rev, which take up a collective space of 20 square km. There is 560m between each turbine and the farm is divided into 5 sections (16 turbines in each section). Put simply, the reason for this spacing is to ensure that one turbine does not ‘steal’ the wind from another. Each turbine has a hub height of 70m, with each of its 3 blades measuring 39 metres long (total blade span 80m including the rotor). Incredibly, the thinnest point of a blade measures just millimetres.

A floating crane carried the turbines out to the construction site in sections where they were then fitted together. The towers were transported across in two sections and fitted directly into the foundations. Next was the nacelle, which was already been fitted with two out of its three blades, and finally the third blade was carried out to be fitted. Vestas, who supplied and installed the turbines, developed a mechanism specifically to lift the last blade into place for off-shore turbine projects. It was decided that Vestas would both develop and manufacture the turbines for the purpose of quality control.


The Transformer Platform

A 1200 tonne ‘transformer platform’ also sits in the North Sea at Horns Rev. This platform, which is 40m above sea level, collects the output from the 80 turbines, raises the voltage from 33KW to 150 KW, and then transmits the electricity to shore. A floating sheerleg (a specific type of crane), known as The Asian Hercules II, was responsible for towing this substation out to the north east of windfarm site and installing it onto pre-installed piles. This process took no longer than a day.

The on-shore control room, where the wind farm output can be adjusted, recorded the first power from Horns Rev in July 2002, which meant it was just four months between the first power output and all 80 turbines being in operation.


Cables

The power produced by Horns Rev is conducted through a 21km cable which, at 20cm in diameter, is one of the thickest sub-marine cables ever made. It weighs in at 72kg per meter. What makes this cable particularly remarkable is that it was all made in one length.

Horns Rev is also home to 63km of Array Cabling (which carries the output from the turbines to the transformation platform). The implementation of such a huge amount of cable was only made possible by a collection of people with different expertise, including divers who were responsible for placing the cables into protection pipes at the foundation of each turbine.

Although 120,000 tonnes of stone was installed underwater to protect the structures from erosion this protection sank by 1.5m causing some areas of cable to become exposed.

Cable issues are quite common in the world of wind farming, and is most often blamed on a lack of skills. The power cable sector which focusses on underwater cabling does not have enough guidance or documentation on what the best practices are, hence these issues. More detailed guidance and the sharing of best practice would allow standardised approaches to underwater cabling.


Conclusion

The construction and maintenance of Horns Rev has played a huge part in providing the world’s energy sectors with information about wind farms, and Danish companies have since played a part in the operation of almost every offshore wind farm globally. The Danes are well known for their focus on R&D to not only ensure reliable renewable energy production but also reduce the cost of it.

Each of Vestas’ turbines at the Horns Rev plant have a 25 year life span and 2MW of power are produced by each turbine per year. This means that Horns Rev produces enough power to supply 150,000 Danish households per year, roughly 2% of Denmark’s overall energy consumption.

In 2009, 7 years after the installation of Horns Rev, another wind farm, Horns Rev 2, was developed nearby. Another 5 years on from that and a third wind farm, Horns Rev 3, has been commissioned- a project which has yet to be completed. All three of the Horns Rev sites will contribute towards Denmark’s aim of having 50% of its energy coming from wind power by 2020.

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