Performance assessment of a Flettner wind turbine

Abstract

Introduction W the renewed interest in utilizing wind energy has stimulated a number of novel ideas and brought about considerable progress in conventional wind turbine technology, a system invented about 60 years ago which may have a significant economic advantage over propeller-type and Darrius machines has apparently been overlooked by the engineering community. Anton Flettner, who is often cited in fluid mechanics texts for his utilization of the Magnus effect to propel a sailing ship, devised and constructed a wind turbine using spinning rotors for blades. With the Magnus effect providing lift, the turbine functions aerodynamically in the same way as a propeller-type device does, as indicated in Fig. 1. Other than a brief description and photograph of the prototype (the turbine was 20 m in diameter and had four blades) in Flettner's autobiographical account, however, no citation of work on the concept either by Flettner or others could be found in the literature. With the blades spun by d.c. drive motors mounted in the blades, the Flettner turbine could be operated at constant rotational speed to drive an alternator, the mode of operation generally favored now for large conventional wind turbines. Control of the spin rate of the blades would provide simple, effective control of output torque and rotational speed. This study was undertaken to provide a preliminary assessment of the performance of the Flettner wind turbine, primarily from an aerodynamic standpoint. Hopefully, the results obtained can serve as a guide in the formulation of a complete preliminary design study to obtain a quantitative measure of the merits of the device. An analysis of optimum aerodynamic configuration and power output as a function of wind speed using the classical strip formulation is outlined in what follows, together with a limited performance analysis of a hybrid configuration. Aerodynamic aspects of control system and structural design are then discussed, and specific advantages and disadvantages of the Flettner concept which should be quantitatively assessed in a design exercise to determine economic viability are summarized.