"Synthetic shroud" concept for wind turbine performance optimization

This paper provides an explanation on why the power output from laterally closely (less than 2 diameter) spaced wind turbines is larger than for single wind turbines in isolation. First, the theory of wind turbines is reviewed, with special focus on the effects which maximize power output. Then, elements of the physical explanation are formulated in three hypotheses, which are proved via data from scaled wind tunnel experiments and CFD simulations. The wind tunnel tests involved a set of scaled wind turbines tested in an open loop atmospheric boundary layer wind tunnel. Then, CFD simulations were performed of the same cases by representing the turbines via porous actuator discs. The theory, experiment and CFD results were used in concert to explain the physical mechanism of the power increase, namely that power (and efficiency) can only be maximized if the streamtube generated by the wind turbine has a special, "ideal" shape. It is demonstrated that this can be achieved without employing a solid structure such as a classical shroud, - but by using the neighbouring turbines' wakes to control the shape of the wind turbine wake, thus creating a virtual, or "synthetic shroud" effect. The results lead to a 7% increase of power in comparison to a single wind turbine in isolation.