The energy payback of offshore wind farms could be increased significantly if manufacturers were to utilize new methods of producing wind turbine towers and foundations, finds a new study conducted at Cambridge University.
Jim Platts, an engineer at Cambridge's Institute for Manufacturing, is encouraging the wind energy industry to conduct more research into the design of wind turbine towers and foundations in order to make offshore wind more cost-effective. Incorporating new methods and designs, he says, could improve offshore wind turbines' efficiency and, therefore, increase offshore wind farms' payback ratios.
One way to achieve this is for wind turbine manufacturers to use guyed towers – which are held in place by steel cables and are made of composite materials – rather than free-standing towers made with conventional steel materials, he explains.
"The development of the wind turbine industry and the way in which it works with the civil engineers who make the heavy supporting towers and foundations – which are not visible out at sea once the turbines are installed – mean that we have ignored something which is almost embarrassingly obvious in our race to meet the targets set for renewable energy production," Platts notes in a statement released by Cambridge.
"We urgently need to reduce the high levels of energy embedded in offshore wind turbines which make them both ineffective in energy payback and costly in financial terms," he continues. "We can do this fairly easily if we invest in more innovative methods for making and installing the towers and foundations that support them."
Harvesting ratio
The effectiveness of wind turbines is determined by the harvesting ratio, a measure of the energy it provides set against the energy embedded in it (i.e., the energy used in manufacturing it).
The energy embedded in the moving parts of an onshore wind turbine represents two-thirds of the total energy invested in the installation, and the supporting structure (tower and foundation) represents the remaining third. Onshore wind turbines usually have a harvesting ratio of about 40:1.
However, offshore wind turbines require taller and heavier foundations, which use up to four times the amount of steel and concrete.
"When you look at offshore wind turbines, you see a series of slim structures," Platts says. "What you don't see are the far heavier supporting structures below the surface that they slot into."
Both steel and concrete are very energy-intensive to produce, so the harvesting ratio of offshore turbines is typically only about 15:1.
Moreover, offshore turbines are subject to corrosion, which reduces the life span of the steel used.
"Steel is prone to corrosion and to fatigue," Platts points out. "This begs the question, could we do better with other materials? The answer is, yes, we can use composites for towers just as we do for blades. They are lighter, stronger, corrosion free and more resilient than steel."
The use of steel cables, which are fixed to the seabed by screw anchors, would allow towers to be significantly slimmer, as the tent-like guyed shape distributes the loads more efficiently to the seabed, the Cambridge study shows. In addition, this would allow foundations to be significantly lighter.
The resulting reduction in the volume of steel and concrete needed would lead to a harvesting ratio of about 25:1, according to the Cambridge study.
However, Platts stresses that it will take a combination of strategies to increase offshore wind's energy payback. In fact, another study suggests that the payback ratios for offshore wind farms could double if the wind industry were to adopt new construction methods.
"The use of guyed towers is just the first step for the industry to take," he says. "The second step would be to make towers in composite materials, which are less energy-intensive to make than steel, which relies on smelting and concrete that also depends on a chemical-reduction process in manufacturing cement.
"Composites also have a longer life than steel, as they stand up to fatigue much better," Platts adds. "Using these new materials could increase the harvesting ratio still further – to 32:1 – and extend the lifetime of a turbine installation from the present 20 years to up to 60 years."
More information on the Cambridge University study can be found here.
