Experimental study of closely spaced counter-rotating vertical-axis wind turbines

Over the past decade, there has been a renewed interest in vertical-axis wind turbines (VAWTs). One important driver for this revival has been the discovery that a pair of closely spaced counter-rotating VAWTs perform better than two isolated VAWTs. To date, most studies on the subject have been either computational or experimental in open-field. We introduce a new perspective to the study of paired VAWTs through a series of controlled wind tunnel experiments.
In this study, isolated VAWTs and paired VAWTs were tested in four European wind tunnel facilities. To do so, we developed and built a dedicated experimental setup with H-type Darrieus VAWT scale models. The performance and wake structures were compared for an isolated configuration and three paired configurations with varied inter-turbine distance and direction of rotation. Particular caution was required for the interpretation of aerodynamic torque measurements as they were contaminated by the structural dynamics of the setup and modal vibrations of the rotors. These contaminations were identified through spectral and modal analysis. The wake was measured using hot-wire anemometry and multi-hole pressure probes at locations from one to twenty-one diameters downwind.
The pairing of VAWTs causes the streamlines of the incoming flow to converge between the two VAWTs. This convergence leads to an improved incoming flow angle and an increased velocity. In our experiments, this translates to a power increase of up to 16%. The direction of rotation of paired VAWTs has an important effect on the wake structures. The wake of pairs with inner-downwind moving blades is quite similar to the wake of an isolated VAWT. However, paired VAWTs with inner-upwind moving blades induce a remarkably narrow wake. Thus, thanks to a substantial power increase and a narrow wake, paired VAWTs present advantages for specific applications, e.g. for offshore floating platforms.