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.