Researchers in Denmark and the UK have modelled the fluid dynamics of multi-rotor wind turbines, and how they interact in wind farms. The research demonstrates a clear advantage for a wind turbine with four rotors.
Despite new turbines at the proposed Dogger Bank wind farm having 220m diameter blades, the researchers found that bigger is not necessarily better.
Researchers from Aarhus University and Durham University modelled the fluid dynamics of multi-rotor wind turbines via high-resolution numerical simulations, and it turns out that a wind turbine with four rotors on one foundation has a number of advantages.
A wind turbine harvests energy from the incoming wind, but when the wind passes through the blades of the turbine, a region with lower wind speeds and higher turbulence is created called wind turbine wake. A second wind turbine downstream is affected by this turbulence in several ways. First of all, it produces less energy, and secondly, the structural load is increased.
“In the study, we found that turbulence and currents in the wake of the turbines recover much faster with multi-rotor turbines. This means that, with multi-rotors, a second turbine downstream will produce more energy and will be subjected to less load and stress, because the turbulence is correspondingly smaller,” said Mahdi Abkar, assistant professor at the Department of Engineering, Aarhus University and an expert in flow physics and turbulence.
Using more than one rotor creates less turbulence, and the wind is “restored” faster, which means a higher energy output. “You can always increase your energy output by increasing the diameter of the rotor blades, but there are major structural challenges in building these massive constructions with diameters exceeding 150 metres. The material requirements increase, the transport of the structures is cumbersome and expensive, and it becomes more costly to maintain the wind turbines,” he said.
A turbine with four rotors costs around 15 percent less to build than a turbine with one rotor, even though the blades cover the same area in total. At the same time, a construction with four rotors is much lighter and therefore easier to transport. And if one of the rotors stops working, the rest of the turbine will still produce energy, unlike ordinary wind turbines. The researchers also found that individual multi-rotor turbines actually produce slightly more energy, two percent more, than single-rotor turbines.
“We’ve explored several different geometries and dynamics of multi-rotor turbines and have found that the optimum construction is a turbine with four rotors as far apart as possible. The latter results in less downstream turbulence and a faster stabilisation of the wake behind the wind turbines,” he said.
This approach has already been tested out in Denmark in a prototype 4-rotor concept turbine (above) from Vestas at the Risø Campus of the Danish Technical Univesity (DTU). Researchers from DTU Wind Energy contributed with counselling and metrological services, and the project looked at reducing the consumption of materials and using smaller components for the turbine. The smaller parts were visible at the small blades of the turbine. In spite of the height of the concept turbine of just 74 meters, it had a capacity of 900 kW.