This article presents a comparison study of different aerodynamic models for an X-shaped vertical-axis wind turbine and offers insight into the 3D aerodynamics of this rotor at fixed pitch offsets. The study compares six different numerical models: a double-multiple streamtube (DMS) model, a 2D actuator cylinder (2DAC) model, an inviscid free vortex wake model (from CACTUS), a free vortex wake model with turbulent vorticity (from QBlade), a blade-resolved unsteady Reynolds-averaged Navier–Stokes (URANS) model, and a lattice Boltzmann method (from PowerFLOW). All models, except URANS and PowerFLOW use the same blade element characteristics other than the number of blade elements. This comparison covers the present rotor configuration for several tip-speed ratios and fixed blade pitch offsets without unsteady corrections, except for the URANS and PowerFLOW which cover a single case. The results show that DMS and 2DAC models are inaccurate – especially at highly loaded conditions, are unable to predict the downwind blade vortex interaction, and do not capture the vertical/axial induction this rotor exhibits. The vortex models are consistent with each other, and the differences when compared against the URANS and PowerFLOW mostly arise due to the unsteady and flow curvature effects. Furthermore, the influence of vertical induction is very prominent for this rotor, and this effect becomes more significant with fixed pitch offsets where the flow at the blade root is considerably altered.

This paper introduces the X-Rotor, a hybrid vertical-horizontal axis turbine concept designed to lower the cost of energy in the floating offshore environment. The development of a double multiple streamtube (DMS) simulation tool is presented alongside a thorough discussion of the secondary correction factors included in the model. New corrections for streamline curvature effects applicable to an airfoil where the blade normal plane is not aligned with the rotor plane are derived. The DMS model is successfully validated against experimental data and against higher fidelity lifting line (LLT) simulations. Strong agreement is observed between the LLT simulations and the DMS simulations for both rotor averaged and azimuthally varying outputs, indicating that the DMS simulations can be used for future control simulations.

This work introduces and explores the use of secondary rotors for vertical axis wind turbine power take‐off. A parametric framework based on optimally designed secondary rotors is developed which calculates the maximum achievable efficiency of power conversion between the primary and secondary rotors. It is shown that practicable rotor designs can convert between 87% and 90% of primary rotor power to the secondary rotors whilst facilitating nacelle mass reductions between 85% and 87% compared to traditional reference turbine drivetrains.

Vertical-axis wind turbines (VAWTs) have received renewed research interest in the offshore environment due to a number of design synergies that have the potential to decrease the cost of energy for offshore wind. Many studies have been completed on the rotor design for straight-bladed rotors (H-rotors); however, there is sparse information on the effect of blade inclination angle on VAWT aerodynamic performance and the optimal blade design of VAWTs with inclined blades (V-rotors) for maximum power capture. This paper presents a systematic study into the effect of blade inclination angle and chord distribution on VAWT performance for different aspect ratios using a 3D implementation of the 2D actuator cylinder model which has been previously validated against higher-fidelity methods. A systematic approach based on a grid search is used to allow the wider design space to be studied and trends identified. In the case of fixed-chord-length blades, it is found that significant power gains are available through blade inclination, between 12 % and 71 % depending on blade length. This is driven by the increase in rotor swept area. Further investigation indicates that despite this, under maximum blade stress limitations the most economical solution for fixed-chord-length blades is H-rotors. Optimal chord distributions, which maximise the rotor power coefficient, are then obtained, and a natural blade taper is observed. For rotors with optimal chord distributions, similar power gains are observed through blade inclination, again between 12 % and 71 % depending on blade length. However, rotor configurations with the largest power gains are found to have significantly increased blade mass. For a given power rating, whilst satisfying limitations on maximum blade root bending stress, it is found that blade volume, a proxy for the blade mass, can be reduced between 16 % and 42 % dependent on blade length, and rotor torque can be reduced between 4 % and 9 %. This indicates the potential of V-rotors to reduce the cost of energy compared to H-rotors in traditional VAWT designs. Additionally, inclined blades are shown to increase the operational tip-speed ratio, demonstrating their applicability to turbines using secondary rotors, such as the X-rotor, where high tip-speed ratios are required for efficient power conversion between primary and secondary rotors.

The cost of energy generated by large-scale vertical-axis wind turbines faces great challenges for it to be competitive with conventional horizontal-axis wind turbines for offshore deployment. To become competitive, significant reductions in capital cost and operational costs would be competitive. A novel vertical-axis wind turbine that aims to meet these requirements is proposed: the X-rotor wind turbine. An early-stage feasibility study of exemplary two- and three-bladed 5 MW turbines is reported. The cost savings arising from two aspects of the concept that have the greatest impact, namely the power take-off system and O&M, are quantified. Two other aspects that could have a major impact on the cost of energy are the vertical axis rotor and the jacket. The masses for both are evaluated as proxies for their costs. The former costs are determined to be substantial relative to those of conventional HAWTs. Whereas the latter masses are determined not to be prohibitively greater relative to conventional HAWTs.

This article presents a comparison study of different aerodynamic models for an X-shaped vertical-axis wind turbine and offers insight into the 3D aerodynamics of this rotor at fixed pitch offsets. The study compares six different numerical models: a double-multiple streamtube (DMS) model, a 2D actuator cylinder (2DAC) model, an inviscid free-wake vortex model (from CACTUS), a free-wake vortex model with turbulent vorticity (from QBlade), a blade-resolved URANS model, and a Lattice-Boltzmann method (from PowerFLOW). All models, except URANS and PowerFLOW use the same blade element characteristics other than the number of blade elements. This comparison covers the present rotor configuration for several tip-speed ratios and fixed blade pitch offsets without unsteady corrections, except for the URANS and PowerFLOW which cover a single case. The results show that DMS and 2DAC models are inaccurate – especially at highly loaded conditions, are unable to predict the downwind blade vortex interaction, and do not capture the vertical/axial induction this rotor exhibits. The vortex models are consistent with each other and the differences when compared against the URANs and PowerFLOW mostly arise due to the unsteady and flow curvature effects. Furthermore, the influence of vertical induction is very prominent for this rotor and this effect becomes more significant with fixed pitch offsets where the flow at the blade root is considerably altered.