Development of a Butterfly Wind Turbine with Mechanical Over-Speed Control System

Abstract

Based on a concept of "cost reduction by large rotor and small generator", a kind of small vertical axis wind turbine (VAWT) called a butterfly wind turbine (BWT) has been developed with rotor diameter of 7 m and five looped blades. One of the features is a mechanical system which can twist each blade of a VAWT by using the centrifugal force acting on the blades in order to prevent over-speeding of the rotor. An electric power generation system that combines the BWT utilizing the over-speed control system (OCS) with a coreless generator connected to a three-time increasing gear unit is constructed in this study. The performance of the BWT power-generation system is predicted by the blade element momentum (BEM) theory. The final goal of this research is to show the feasibility of the low-cost VAWT power generation system with the proposed OCS. From the results of a series of forcible rotation experiments of the BWT rotor that is driven by an induction motor in very low wind speed condition, it is shown that the twist movement of the blades is improved by the design of the OCS that considers the observed radial force deprived of the centrifugal force and the pre-compression of springs. In the generation experiments with the final design of the OCS, the expected twists of the blades are observed for the first time in strong natural wind speed over 18 m/s.

Numerical Analysis of Effects of Arms with Different Cross-Sections on Straight-Bladed Vertical Axis Wind Turbine

“…To reduce the influence of the supporting arms on the blades as much as possible, the simple type with the arms installed into the blade tips might be the best choice because the influenced blade-portions of the simple type with two-stage arms are geometrically equivalent to those of the cantilever type with one-stage arms in addition to the possible cancellation of the blade tip loss. As the slant blade parts of the butterfly wind turbine [5], which is explained as "armless", corresponds to the arms of the simple type, the loss caused by the connection between the main vertical blade and the slant blade is expected to be small. However, even in the case of the simple-or the butterfly-type wind turbine, more supporting arms or braces than three stages are needed to restrain the deformation of the blades if the rotor size is large.…”

“…In Figure 8, the total influences of all the arms, which were obtained from the results of CFD analysis by subtracting the armless rotor torque from each arm-equipped rotor torque, are compared with the values estimated using Equation (5). The drag coefficient C D of each arm was assumed to take the value evaluated at the maximum local radius, i.e., the rotor radius R (see the caption of Figure 2).…”

“…As large-scale offshore wind power systems are expected to take advantage of the characteristics of VAWT [1,2] or wind farm with many small-scale VAWTs [3], many studies on VAWT have been conducted [4]. Hara et al [5] have been developing the Butterfly Wind Turbine (BWT), which is a kind of VAWT In the operation of a VAWT, whose rotational axis is perpendicular to the main flow, the direction, or angle of attack, of the relative flow to a blade or an arm changes during rotation; therefore, because the local tangential forces change too, the averaged tangential force coefficients should be different from the drag coefficient obtained experimentally under stable flow. The secondary drag coefficient proposed by Roach and Turner [22] is attributed to a strut-blade connection but includes no influence on blades.…”

Numerical analysis on dynamic interaction between two closely spaced vertical axis wind turbines

“…Recently, Vergaerde et al [14] performed experiments using a closely spaced pair of straight-bladed VAWTs that mutually revolved in opposite directions in a wind tunnel and observed their phase synchronization. The first author of this paper and his colleagues have proposed a concept named "Wind Oasis" [15], which applies a wind farm comprising small-scale vertical-axis-type butterfly wind turbines (BWTs) [16] for agriculture in drylands to pump water for crops and to generate electricity for people. As basic studies for materializing this concept, the authors of the present paper have been conducting wind tunnel experiments using miniature BWT models and performing the 2D-CFD analysis of the corresponding rotors.…”

Numerical Analysis of Dynamic Interaction Between Two Closely Spaced Vertical Axis Wind Turbines

“…Recently, Vergaerde et al [13] performed experiments using a closely spaced pair of straight-bladed VAWTs that mutually revolved in opposite directions in a wind tunnel and observed their phase synchronization. The first author of this paper and his colleagues have proposed a concept named "Wind Oasis" [14], which applies a wind farm comprising small-scale vertical-axis-type butterfly wind turbines (BWTs) [15] for agriculture in drylands to pump water for crops and to generate electricity for people. As basic studies for materializing this concept, the authors of the present paper have been conducting wind tunnel experiments using miniature BWT models and performing the 2D-CFD analysis of the corresponding rotors.…”