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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Advanced rotor technologies' taking place on Tuesday, 11 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

Guoying Feng Vrije Univesiteit Brussel, Belgium
Co-authors:
Diego Domínguez (1) F P Marco Pröhl (2) Tim De Troyer (1) Marcus Werner (2) Mark Runacres (1)
(1) Vrije Univesiteit Brussel, Brussels, Belgium (2) Fraunhofer IWU, Chemnitz, Germany

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Ms. Guoying has been working in wind industry for almost 7 years. She is currently a researcher at Vrije Universiteit Brussel and involved in the project ‘Hydroformed blades for meshes of vertical axis wind turbines’. She got her Ph.D degree in wind energy technology in Inner Mongolia University of Technology in China. She spent one year studying wind turbine measurement in Risoe National Laboratory in Denmark. Her research is focused on small wind turbine technology and application.

Abstract

REDUCING BLADE MANUFACTURING COST OF VERTICAL AXIS WIND TURBINES WHILE MAINTAINING AERODYNAMIC PERFORMANCE

Introduction

Vertical axis wind turbines (VAWTs) can be built in tight arrays with alternating directions of rotation to increase global performance [1]. However, VAWT farms are economically not yet competitive compared to horizontal ones. This work is focused on ways to improve global profitability by using low-cost large-series production technologies, as well as reviewing the materials used for blade production. Knowing the performance of such designs under operating conditions is crucial. The methodology proposed here, combining structural and aerodynamic analysis, allows to look for new structural designs capable of reducing the manufacturing costs while ensuring performance of the turbine.

Approach

The idea to design a new low cost blade, capable of being manufactured using large-series production technologies, makes sense when it is considered together with the construction of large farms of counter-rotating VAWTs. That is why our approach involves, on the one hand, investigating arrangements of turbines for optimized wind farms, and, on the other hand, the design of a metal blade that is light and stiff enough to compete with its fibre-reinforced counterparts.

The current paper focuses on the engineering process of building a competitive metal blade, where competitive means: equal performance (turbine energy production, emissions), decreased cost (materials, manufacturing process) and increased sustainability (energy for production, recycling, lifetime).

Wind turbine blades are currently often made of fibre-reinforced composites, which are quite costly (up to 30 % of the turbine cost). The use of metal could facilitate the production process of the whole turbine and has an additional advantages in that is can be recycled.

Since the blade’s shape has a great impact on turbine performance, knowing its deformation under operational conditions is paramount. The methodology proposed here, combining structural and aerodynamic analysis, allows to look for new structural designs capable of reducing manufacturing costs while ensuring performance during operation. As the deformation is caused mainly by inertial forces [2], decoupling aerodynamic and structural studies is a feasible and resource-saving option (no Flow-Structure Interaction solver is necessary), with structural analysis only based on blade geometry and operational parameters, e.g. rotational speed, wind speed, blade mass, etc.

This results in the following areas of investigation:
•Structural / manufacturing studies:
•Sheet thicknesses (blade surface, inner structure)
•Inner reinforcements (yes/no, geometry)
•Choice of material (steel, titanium, aluminium and magnesium alloys)
•Load cycle / fatigue calculation (stress variation during one rotation, lifetime)
•Aerodynamic studies:
•CFD analysis for the original non-deformed blade by using a 3D URANS model to define blade geometry (profile choice, chord, span, trailing edge shape)
•CFD analysis for the blade selected by the structural study (3D URANS of the (deformed) geometry in operation).


Main body of abstract

The structural and aerodynamic studies have been conducted in accordance with the following workflow:
1. Global design parameters are initially defined on the basis of known aerodynamic and performance concerns.
2. The structural and manufacturing analysis is carried out in order to define and optimize sheet thicknesses, inner reinforcements, the choice of the materials and fatigue issues.
3. Aerodynamic analyses are conducted again in order to define final design performance under operational conditions.
Non-symmetrical profiles can have advantages for VAWT turbine design compared to the more commonly used symmetrical ones [3]. We have opted for the Selig S2027 profile, based on a literature survey and numerical simulations. Although the profile definition already imposes the geometry of the blade, it is not the case for the trailing edge shape. In theory, this edge is sharp without any radius. But in practice, manufacturing techniques limit the achievable geometries to small radii or - when thinking of metal blanks - to simple cut-offs. Therefore the trailing edge design should be defined as a compromise between manufacturing cost and aerodynamic performance. Four different designs for the trailing edge have been proposed (Figure 1) and studied by means of CFD computations, a trailing edge with a small radius was revealed as the best option.

Velocity plots for the different trailing edge shapes at angle of attack 8°

Further analyses about remaining turbine design parameters were conducted following standard procedures for VAWTs [4].
Although inertial forces will be the dominant factor for the structural design of the blade/rotor, aerodynamic forces (which vary over every rotation) have to be considered in load cycle/fatigue calculations. Since the number of rotations during the lifetime of the turbine can add up to more than 108, the number of rotation-caused stress variations is huge, possibly resulting in considerable fatigue.
Each blade design has been simulated under operating conditions (10 m/s wind speed, resulting in about 240 rpm with a turbine diameter of 2.4 m). Figure 2 shows for each blade the maximum blade deformations and equivalent stresses.

Equivalent stresses obtained for a magnesium blade during the structural analysis

The major results of the structural/manufacturing studies are:
a) The blade span should be limited to 1.5 m to limit the increase of the equivalent stress. A blade chord length of 180 mm is a good starting point that limits the equivalent stress in the blade. A sheet thickness of 1.0 mm seems to be a good compromise between blade mass and deformation / equivalent stress, even when combined with a perpendicular sheet as inner reinforcement for reducing the deformation differences between upper and lower profile surface. Using different metal materials will have almost no influence on the maximum deformation of the blade (Figure 3). The maximum equivalent stress decreases nearly in the same way as the E-modulus of the material does.
b) From the point of view of blade stability, it does not matter which material is used, questions of production (price, formability) are more important. Using stainless steel would have the advantages of a high corrosion resistance, availability and relatively low costs, combined with a well examined forming behaviour and a good fatigue durability. First estimations show that with the positive effects of a mass production (economies of scale), such metal blades potentially have a 90 % reduction of production costs compared to fibre-reinforced ones.

Influence of material choice and sheet thickness on maximum blade deformation and equivalent stress

Results from the structural analysis should be checked again from the aerodynamic point of view, as it is not easy to predict how the deformation in the blade affects the aerodynamic characteristics. Having this in mind a new set of CFD simulations have been performed with the aim of measuring the negative (or possibly positive) effect of the blade deformation, calculating the new lift and drag coefficients for the deformed shape at different angles of attack (3D URANS simulations).

Pressure coefficient in the surface of the blade

This analysis has shown that the deformed blade retains good aerodynamic coefficients, ensuring that turbine performance is not degraded.

Conclusion

In the present work we have shown that it is possible to build a competitive metal blade for small to medium size VAWTs. The parameters that influence the blade’s performance and stability (operational parameters, profile geometry, sheet thickness, inner reinforcements, materials, fatigue) have been identified and varied to find the best compromise. The results did not only suggest a good starting point for a first demonstrator but also showed a strong potential for further investigations (e.g. the use of magnesium, when its availability and level of examination increases and price decreases in the coming years).

The summarised results from the structural and manufacturing analysis suggest that the best a priori option could be a blade with 1.5 m span and a chord of 180 mm, made of stainless steel in 1.0 mm sheet thickness and with a perpendicular sheet as inner reinforcement. Such a blade has a moderate deformation and a tolerable stress level. It requires a minimal amount of material and associated manufacturing costs. First estimations show that with the positive effects of large-series production (economies of scale), such metal blades might reduce production costs to about 10% of the cost to manufacture fibre-reinforced composite blades.

Using the techniques here proposed –mass production techniques and performance enhancement using farms of counter-rotating turbines– it is possible to maintain the same turbine performance that can be achieved with traditional composite blades, as well as a significant improvement in global profitability. By building a demonstrator, these theoretical results will be verified in practice in the next step.



Learning objectives
We present a new field for industrial development. We show how we can improve the design and the manufacturing process of low-cost blades to be used in farms of vertical axis wind turbines.


References
[1] Dabiri, John O. “Potential order-of-magnitude enhancement of wind farm power density via counter-rotating vertical-axis wind turbine arrays.” Journal of renewable and sustainable energy 3, no. 4 (2011).
[2] Castelli, Marco Raciti, Andrea Dal Monte, Marino Quaresimin, and Ernesto Benini. “Numerical evaluation of aerodynamic and intertial contributions to Darrieus wind turbine blade deformation.” Renewable Energy, no. 51 (2013): 101-112.
[3] Mohamed, M. H. “Performance investigation of H-rotor Darrieus turbine with new airfoil shapes.” Energy, no. 47 (2012): 522-530.
[4] Paraschivoiu, Ion. Wind Turbine Design: With Emphasis on Darrieus Concept. Montreal: Presses inter Polytechnique, 2002.