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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Floating wind turbines' taking place on Wednesday, 12 March 2014 at 16:30 -18:00. The meet-the-authors will take place in the poster area.

Alice POURTIER IFP Energies nouvelles, France
Co-authors:
Alice POURTIER (1) F P Timothée PERDRIZET (1)
(1) IFP Energies nouvelles, SOLAIZE, France

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Abstract

A critical review of the floating vertical axis wind turbines presumed benefits

Introduction

The development of floating wind turbine technology to exploit wind power potential in deeper water locations requires economic optimisations to ensure competitive cost of energy. In that context, Vertical Axis Wind Turbine (VAWT) technology, that was not competitive enough compared to Horizontal Axis Wind Turbine (HAWT) layout in onshore configurations, must be reconsidered, and may become a good challenger thanks to some presumed advantages.

Approach

This paper presents a comparison between horizontal and vertical axis wind turbines on floating offshore platform, in term of levelized cost of energy (LCOE). Bibliography states that the VAWT has some advantages. The masses, the vertical position of the centre of gravity and of the rotor aerodynamic centre are presumed to be considerably lower, which can improve stability of the floating platform and so on reduce the size and the cost of the floater. As the global mass is reduced, it can be thought that the VAWT cost is also decreased. This study has for objectives to compare some mechanical and economical quantitative data based on simplified approaches or hypotheses for Vertical Axis and Horizontal Axis Wind Turbines. This work does not include the evaluation of the floater cost reduction which will be performed in a future step. A 5MW horizontal wind turbine was considered as a reference, since such turbines are relatively well documented in terms of technical and economical data. Since, to the authors knowledge, no comparative VAWT design exists in the public domain, the first step consisted in pre-designing a 5MW vertical axis wind turbine. This design mainly concerned aerodynamics and structural mechanics in order to compare the cost of turbines producing the same annual energy submitted to the same statistical wind data. Two kinds of VAWT have been evaluated: a double rotor VAWT with straight blades and a three-bladed troposkian rotor. Some parameters, such as aspect ratio, solidity, mechanical properties of several materials, costs functions have been optimized jointly with the rotor control strategy to obtain an optimal VAWT design.

Main body of abstract

The first step of the study aimed to design vertical wind turbines having the same annual energy production than the HAWT taken as reference. Two VAWT configurations have been investigated: a double rotor VAWT, composed of 2 superposed rotors with straight blades and a troposkien rotor. Setting two rotors one above the other permits to reduce variability of torque and thrust against rotation and consequently to decrease fatigue with smaller blades than a six-straight-blades rotor. This is also the advantage of helicoidal rotor shape, not considered in this work because of its more complex design. However, the major drawback of this rotor is the need of struts to support the shorter blades. Another disadvantage of straight blade rotor is the need to increase the tower height to keep a large enough airgap between the blades and the sea water surface, when the rotor is submitted to platform pitch motion.
The troposkien rotor solves the main disadvantages of the previous design but, consequently, its blades are much longer and more complex to set up.
To design these concepts, the power (Cp), thrust and torque coefficients curves for different tip speed ratio are computed with single streamtube method. The variation of the forces are also included in the procedure to compute the rotor fatigue life.
The surface of the VAWT rotor is jointly optimized with the control strategy to produce the same annual energy as the HAWT, evaluated on the basis of the rotor Cp curve. The limitation of the rotor thrust above rated is also taken into account in the design criteria.
The forces coefficient curves, computed with the streamtube model, and the control strategy are used to calculate the aerodynamic loads applied on the blades and transmitted to the struts and the tower. Pre-design of blades and struts are then performed using classic resistance of material theory and considering the section of the blades as a rectangular box.
Steel, carbon-fibre composite and glass-fibre composite materials have been considered for the sizing of both blades and struts. This gives various weights and costs for the two options of rotors. The tower has been designed using steel material properties and its life time was assessed. Several iterations were necessary to evaluate the mass and the cost of the components. Impacts of some parameters such as solidity, aspect ratio, number of struts per blade and functions of component costs have been investigated to optimized the VAWT design.
The second step of the work is to compare the concepts with the reference HAWT using the same floating platform.
Then, the costs of the two configurations of VAWT have been evaluated according to some hypotheses and compared to the 5 MW HAWT reference. Uncertainties about component cost functions and the evaluation of the manufacturing costs of blades and struts are the main difficulties encountered to accomplish this work. Several ways to evaluate the strut and blade costs have been tested in order to get down the cost of the VAWT concepts, without enough success.
Comparisons also concerned technical points, in particular related to global mass or position of centre of gravity, that are more favourable to the VAWT. Discussion is opened on the potential weight saving if floating support for VAWT.


Conclusion

The results of this comparison show that the usual benefits of floating VAWT seem to be overrated. Depending on the hypotheses, VAWT mass and cost evaluations are quite variable but, even in the most favourable case, VAWT concept is still too expensive compared to the reference HAWT. The generator represents the main part of the global cost, followed by the blades and struts.
Cp curves of Vertical Axis Wind Turbines show that their optimal tip speed ratio are lower than those of the reference HAWT. This implies a higher torque of the rotor. Due to a generator cost function taken into account, only dependent of the torque, the generator cost is much higher than those of the reference HAWT.
Some technical advantages, such as a lower mass and a lower centre of gravity, improve the stability of the floating platform. Displacement and consequently cost of the floater may be drastically reduced. The impact on the design of floating platforms needs then to be addressed.
However, some hypotheses need some verifications or some modifications, especially for the evaluation of generator, blades and struts costs. The impact of the reduction of the floater size has not been considered in this study: it could be a way to reduce the gap between the floating VAWT and HAWT costs. The better capacities for upscaling, the potential increased VAWT performance when operating with a tilt angle, the use of suitable blade profiles and the potential lower aerodynamic impact of VAWTs operating in a wind farm may also strongly modify the conclusions of this study. These potential effects will be investigated in the future and could mitigate or inverse this conclusion.



Learning objectives
The first part of the study shows that the benefits of vertical axis rotors on floating platform are not so clear. Based on public information, costs of the VAWT are larger than the reference HAWT. Getting better component cost functions, reducing floater cost using stability advantages of VAWT, among other perspectives, could inverse this conclusion.




References
Paraschivoiu I., Wind Turbine Design with emphasis on Darrieus Concept. Polytechnic International Press, 2002
Paulsen, 1st DeepWind 5 MW baseline design , 9th Deep Sea Offshore Wind R&D seminar, 2012
Blonk, Conceptual Design and Evaluation of Economic Feasibility of Floating Vertical Axis Wind Turbines, Master of Sciences Thesis, TU Deft, 2010
Sutherland, Berg, and Ashwill, A Retrospective of VAWT Technology, SANDIA Report, 2012,
Paquette and Barone, Innovative Offshore Vertical-Axis Wind Turbine Rotor Project, EWEA 2012