Two-bladed floating offshore wind turbines with a yaw mechanism inspired by helicopter technology and tilting hinges that disconnect the shaft from the rotor.
A Dutch manufacturer is working on floating offshore wind turbines with a lifetime of 50 years.
Integrated with a floating concrete structure that can be installed at depths of between 60 and 3000 meters and that can withstand winds of up to 325 kilometres per hour, it is not just more flexible than other turbines. It also has a longer durability, says Seawind Ocean Technology, the Netherlands-based manufacturer behind the new floating offshore wind energy system.
The floating concrete foundation, which is expected to be demonstrated with a 6.2 MW turbine in European waters in the first quarter of 2024, is already drawing attention. The turbine has a lifetime of 50 years, twice as long as Danish wind turbines.
The lifetime of our wind turbines with the classic three-bladed rotors is 25 years, but that does not mean that we are lagging behind, according to senior consultant at DTU Flemming Rasmussen.
“No, not necessarily,” says Flemming Rasmussen, senior consultant at DTU Wind, Department of Wind and Energy Systems.
He has been dealing with wind turbines since the 1970s and recognizes that there may be benefits to the two-bladed rotors in the same way as three-bladed and for that matter single-bladed rotors.
The two-blade rotors, for example, not only save one blade but also the total rotor mass approximately corresponding to one blade. On the other hand, stability is better with three blades than with two. The strong forces to which the two-bladed rotor is subjected when it passes the tower place high demands on the design, especially if it is a downwind rotor. In addition, due to design optimization, two-bladed rotors require a higher rotation speed to harness the same wind energy. That means more noise and blade erosion. On the other hand, the two-bladed turbines are easier to handle than the three-bladed ones during assembly and installation.
“In principle, a lot of innovation is possible,” Flemming Rasmussen says.
Seawind Ocean Technology hinges the rotor and attaches it to a shaft that is perpendicular to and rotates together with the main shaft. It is not a new concept. Ultimately, it is about finding the balance between cost and efficiency. The Netherlands-based manufacturer has entered into a partnership with Petrofac, British service provider for the energy industry, to introduce two-bladed 6.2 wind turbines on floating concrete foundations within two years.
A lifetime of 50 years indicates that a design has been found that is judged to be both cost-effective and strong enough to withstand half a century of waves and wind.
According to Flemming Rasmussen, long lifetime of a turbine is definitely an advantage. But long design lifetime does not necessarily in itself make a turbine or a wind power system greener. It is not that simple.
“On the contrary, there may be conditions where it can actually be an advantage to have a design lifetime that is shorter than the 25 years we work with in Denmark, also from a sustainability perspective,” he says.
It primarily depends on the speed of the development of green energy and the development of wind turbine technology compared to the energy used to manufacture the wind turbines.
A wind turbine that has to withstand wind, weather conditions, and maybe waves for 50 years has to be more powerful than one that has to withstand the pressure for 25 years. It goes without saying. The longer one designs a wind turbine to last, the stronger it must be, and the more materials and thus also greater energy consumption are needed to manufacture it.
Although large modern multi-MW wind turbines are significantly more optimized compared to the early turbines, consumption of materials increases relatively with size. The consumption of materials goes up with the third power and the effect only with the second power of the dimensions. This means that relatively more energy is spent on the turbine, and this makes the manufacturing method more important. The goal now is for both the wind turbines and their foundations to be able to be manufactured and installed completely free of fossil fuel.
“In light of the current speed of development of wind power technology, it may be better to minimize energy consumption until fossil fuels are completely phased out, which means that we may move towards producing wind turbines with a shorter design lifetime,” says Flemming Rasmussen, who does not think that there is a risk of creating a large landfill of obsolete wind turbines by doing so.
“Firstly, due to safety factors, most wind turbines are in operation for much longer than they are designed to be,” he says, referring to a statement of operating costs for older wind turbines submitted by EMD International A/S to the Danish Energy Agency in 2019.
The report includes the assessment that towers and foundations have a real lifetime of 50 years, while blades do not constitute “any significant cost in further operation of older turbines”.
In addition, dismantled wind turbines are sold to other countries, which may not have quite as advanced wind power technology as we do in Denmark. And then there are wind turbines that are repowered and operated further. It could be an upgrade of the operating system, installation of new and longer blades, or addition of aerodynamic devices to the existing blades.
Last but not least, the industry has an enormous focus on recycling, not least in the design process.
“The goal is for everything to be recyclable, which is why we invest heavily in modularisation and standardization. It’s a crazy development. Every single part will be recyclable,” he says.
But until that goal is achieved, Flemming Rasmussen believes that, from a sustainability point of view, it may even make sense to use lightweight turbines with a design lifetime of perhaps as little as 15 years under certain conditions.
“It’s also a way to postpone the consumption of fossil fuels,” he says.
Flemming Rasmussen also emphasizes that he has not seen the new design by Ocean Sea Technology, and that his statements should in no way be perceived as a criticism of their turbines.
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