در رفتار سازه ای خط سیر بیضی متغیر هندسی توربین های بادی Darrieus

We developed a computational model based on a finite-element mixed formulation with quadratic isoparametric beam elements. We applied this model to the analysis of a blade-wagon: a novel structure characteristic of an innovative concept in wind-power called VGOT Darrieus turbine. We studied the structural behaviour of its main components: chassis, suspension and blade, using combinations of beam/bar elements in an appropriate assembling. We defined a set of parameters to characterize the structural behaviour which help to understand the contribution of the different components and assist the process of redesign.

It is a fact that due to economics-of-scale reasons cost effectiveness of wind turbines increases with size. During the last 25 years the size of the state-of-the-art wind machine has been increasing systematically but the actual technology of horizontal-axis wind turbines would ultimately reach its limits. Very large sizes would create a number of gigantism problems in rotor design and low rotational speed associated with large radii would complicate the coupling with the electrical generator. Besides, there are geographical regions (like Patagonia in Argentina) characterized by a vast wind resource. Mean speeds in some areas double those recorded at European locations for which commercially available high-power wind turbines were designed. Regarding that the energy carried by a wind stream depends on the cube of its speed, those regions offer an enormous potential in terms of energy resources. Hence, it is worthwhile to explore innovative concepts in extra-large wind-power plants to overcome the size limits of the actual wind-power technology and being able to exploit the renewable energetic potential that high-wind-speed regions offer. To this end, an innovative concept of wind turbine based on the Darrieus-type rotor had been introduced [1]. In a traditional Darrieus turbine, the blades rotate around a central vertical axis. In the VGOT (variable-geometry oval-trajectory) concept proposed in Ref. [1], each blade instead of rotating around a central vertical axis slides over rails mounted on a wagon formed by a reticulated structure supported by standard train bogies (see Fig. 1 and Fig. 2). Each wagon contains its own electrical generation system coupled to the power-wheels and the electricity is collected by a classical third rail system [2] and [3]. With the VGOT design, if we kept constant the velocity of the wagons (i.e. the tangential speed of the blades), we can increase the area swept by the blades (and hence the rated power of the plant) without the low-rotational-speed problems associated to a classical Darrieus rotor of large diameter. The blade-wagon elements of a VGOT Darrieus, not being solidly affixed to a central axis, could move following a non-circular trajectory (see Fig. 3). For certain locations where the compass rose shows a preferential direction it is possible to optimize the energy-conversion efficiency of the entire plant by increasing the portion of transit perpendicular to the direction of the incoming wind. Along the perpendicular tracks, the blade generates the higher output-power, while along the portions where the trajectory is in-line with the incoming wind the blade-wagon not only does not produce energy but also even consumes it taking power from the rest of the plant to keep on moving. Thus, extending those portions of the path perpendicular to the wind by the addition of straight tracks the overall energy-conversion efficiency of the plant increases. The idea of mounting a blade on a wagon with the aim of generating electricity has been proposed before (see for example Fig. 1.6, Section 1.1.3 in Ref. [4], among others), but with the VGOT concept we intend to carry on a systematic study of the real feasibility of such a design, being the analysis of the structural behaviour a fundamental step.When undertaking the study of the structural behaviour of the blade-wagons of a VGOT Darrieus several particularities arise that make it different from other studies in beam-reticulated structures. The analysis of the blade offers some particular features too. It has a variable section of complex shape and the aerodynamic loads depend on both, the position of the blade-wagon along the path and the height from the ground. In that sense, computation of the aerodynamic loads is not trivial since, due to the interaction of the adjacent blades, the plant works as a whole and it is not possible to analyze the aerodynamic behaviour of each blade-wagon separately. A special code was created to model this complex aerodynamic behaviour [5], [6], [7], [8] and [9] and its results were used as input loads for the present study. When studying the three-dimensional reticulated structure of the wagon chassis, the effects of its coupling with the blade and the suspension system should be considered, together with the effects of the added mass of the components and the ballast placed to improve the stability of the wagon. The suspension system should absorb the fluctuating aerodynamic loads and eventual imperfections in the layout of the rails, for if they were transmitted directly to the bogies and the rails, they would compromise the operational life of the plant. In the following sections we shall describe the development of the computational model that we used to simulate the behaviour of this unique structure.

We developed a computational model based on a finite-element mixed formulation with quadratic isoparametric beam elements. We applied this model to the analysis of a novel structure characteristic of the innovative concept of the VGOT Darrieus wind turbine. The results of this analysis were used to evolve the structural design of the basic module of the VGOT: the blade-wagon. We studied the contribution of each one of its main components: chassis, suspension and blade, using combinations of isoparametric beam/bar elements in an appropriate assembling. A convenient way to estimate the effects on the structure of eventual imperfections in the layout of the railroad was devised. We defined a set of parameters to characterize the structural behaviour which help to understand the contribution of the different components and assist the process of redesign. As it can be observed in all the figures depicting the different parameters along the path, transition from the straight tracks to the curved ones and vice versa produces a jump in the plots. This jump is due to the sudden passage from a state of load with constant centrifugal force to a state with zero centrifugal force and vice versa. This is so because the path was assumed as a concatenation of straight tracks and curved tracks with constant radii. As a matter of fact, this phenomenon is avoided in normal outline of standard railroads by the use of a transition track where curvature changes progressively where the curved track connects the straight one. This phenomenon does not affect the ultimate value of the results shown here, but it has to be taken into account in a future study which considers dynamical effects. We intend to continue our work with a non-linear analysis considering large displacements and including dynamical effects. Besides, the mechanical design of the suspension system and the blade-positioning device are now under development and we intend to include their response in the incoming study.