Some 750 wind turbines stretching 400 ft in height will be constructed across the Bog of Allen, which stretches across five counties in Ireland. Power will be collected at a central point and transmitted via high-voltage direct-current cables to the United Kingdom’s national grid. Element Power, Ireland
Resembling massive propellers unmoored from a gigantic aircraft, some of the world’s tallest wind turbines may soon be spinning above the boggy midlands of Ireland, generating electricity to be delivered via undersea cables to the United Kingdom.
February 12, 2013—The scope and size of a plan to build ultratall wind turbines within the bogs of central Ireland are producing winds of controversy—as if the sheer engineering challenges aren’t daunting enough. For those who have visited the United Kingdom, a stiff breeze off the sea seems the norm. But the proposed wind farm site—five midland counties west of Dublin—is relatively windless, meaning that the turbines will have to reach high into the sky to capture wind sufficient to generate power. And despite some opposition to that plan, that is exactly what is proposed, according to Peter Harte, the chief technology officer for Element Power Ltd., the global renewable power developer, who answered written questions from Civil Engineering online. Harte says that Element’s plan, dubbed Greenwire, calls for turbines that will soar 620 ft above the ground, or roughly the height of a 60-story building.
“We felt it was better to build slightly larger turbines but fewer of them,” Harte said. “That’s the best way to minimize the impact on the local area.” The $10.8-billion plan calls for the construction of 750 wind turbines grouped into 40 clusters in the midlands area known as the Bog of Allen. Ultimately the wind machines are expected to generate 3,000 MW of electricity, or enough to power three million homes.
European wind turbines are categorized by the Geneva-based International Electrotechnical Commission as class 1, 2 or 3, depending on the strength of the turbines and the turbulence and 50-year estimated maximum wind gusts speed that are expected at the locations at which they are built. “The reason we can get a good energy capture from the midlands of Ireland is that we can use class 3 turbines with long blades and high towers, even though the average wind speed is lower than the mountainous areas of Ireland,” said Harte, who added that seismic risk in the region is “negligible.”
According to Harte, the turbine nacelles—the housings that comprise all the generating components, including the generator and gearbox, the drive train, and the brake assembly—will likely be manufactured in Germany or Denmark, where assembly lines and supply chains are already established. Blades made of carbon and glass fiber may be produced locally, Harte said. Depending on the vendor, the tubular towers might be either steel or concrete.
The blades themselves will be in excess of 210 ft long, sweeping a vertical airspace of more than two acres atop a nearly 400 ft tower and weighing close to 500 tons. The towers will be transported by road in four sections and then erected on-site with the aid of enormous cranes.
As anyone who has unexpectedly come upon wind turbines knows, the turning blades slicing the air provide an otherworldly sight. Harte said the blades will operate in wind speeds ranging from 13 to 72 ft per second and will rotate at 9 to 15 RPM. They will be sited far enough from existing homes to comply with noise regulations, he added.
Element Power plans to situate some of the towers in so-called cut-away bogs, areas from which peat has been harvested over the past half-century. “Depending on ground-bearing pressure,” explained Harte, “we may need to pile some foundations. Otherwise, we use a floating pad foundation. Most turbines are on farmland, with mineral soils and relatively high-bearing subsoils.” But in the sea, the steel towers will generally be anchored to platforms made of more than 1,000 tons of concrete and steel rebar, and Harte estimated the foundations will be 65 ft across and more than 6 ft in thickness, in a design resembling the base of a wine glass.
Apart from the challenge of building and assembling the massive turbines, the company will need to lay more 600 mi of underwater and underground high-voltage direct-current cables to collect the power from the 40 farms and transmit it first to a central location and then an additional 300 mi across the Irish Sea to the United Kingdom. All of the cables will be optimized to minimize the combined capital cost of the cable and any additional financial investments through the 25-year estimated lifetime of the development.
After traveling these great distances the cables will resurface at two locations in Wales: one at Pentir near Caernarfon and another at Pembroke. Both locations are near existing power stations, but the project envisions construction of two converter stations, each about the size of a football field, to handle the new lines.
Maintenance of the turbines also presents a unique challenge. Any unexpected faults will be sensed by the onboard computer-operated supervisory control and data acquisition system and flagged to a 24-hour central monitoring unit. Typically, two service engineers with the appropriate spare parts will be dispatched at that point to correct any problems. Otherwise, regular service of the turbines themselves is akin to a car or tractor: gearbox oil changes, brake pad changes, filter changes, sensor replacements, and cooling fan checkups—with the addition of anemometer checkups and visual inspections of any lightning damage to the blades.
Plans call for the wind farm to be producing electricity no later than 2018. Will the whopping windmills be visible from space? Harte is more concerned that they be visible to birds. “Extensive bird surveys are required to determine the flight patterns of sensitive species,” he explained. “These are currently under way.”