The Role of Double-Φ Floating Semi-Submersible Vertical Axis Wind Turbines in Suppressing the Gyroscopic Effect

The rapid shrinkage of fossil fuel sources and contrary fast-growing energy needs of social, industrial and technological enhancements, necessitate the need of different approaches to exploit the various renewable energy sources. Among the several technological alternatives, wind energy is one of the most emerging prospective because of its renewable, sustainable and environment friendly nature, especially at its offshore locations. The recent growth of the offshore wind energy market has significantly increased the technological importance of the offshore vertical axis wind turbines, both as floating or fixed installations. Particularly, the class of lift-driven vertical axis wind turbines is very promising; however, the existing design and technology is not competent enough to meet the global need of offshore wind energy. In this context, the project AEROPITCH co-investigated by EOLFI, CORETI and IRPHE aims at the development of a robust and sophisticated offshore vertical axis wind turbine, which would bring decisive competitive advantage in the offshore wind energy market. In this paper, simulations have been performed on the various airfoils of NACA 4-series, 5-series and Selig profiles at different chord Reynolds numbers of 60000, 100000 and 140000 using double multiple streamtube model with tip loss correction. Based on the power coefficient, the best suitable airfoil S1046 has been selected for a 3-bladed vertical axis wind turbine. Besides the blade profile, the turbine design parameters such as aspect ratio and solidity ratio have also been investigated by varying the diameter and chord of the blade. Further, a series of wind tunnel experiments will be performed on the developed wind turbine, and the implementation of active pitch control in the developed turbine will be investigated in future research.

In the present article, progress on the development of an aero-hydro-servo-elastic coupled model of dynamics for floating Vertical Axis Wind Turbines (VAWTs) is presented, called FloVAWT (Floating Vertical Axis Wind Turbine). Aerodynamics is based on Paraschivoiu’s Double-Multiple Streamtube (DMST) model [1] [2], relying on blade element momentum (BEM) theory, but also taking into account three-dimensional effects, dynamic stall, and unsteady wind profiles and platform motions. Hydrodynamics is modelled with a time domain seakeeping model [3], based on hydrodynamic coefficients estimated with a frequency analysis potential method. In this first phase of the research program, the system is considered a rigid body. The mooring system is represented through a user defined force-displacement relationship. Due to the lack of experimental data on offshore floating VAWTs, the model has initially been validated by taking each module separately and comparing it against known experimental data, showing good agreement. The capabilities of the program are illustrated through a case study, giving an insight on the relative importance of aerodynamics loads and gyroscopic effects with respect to hydrodynamic load effects.

The Energy Technologies Institute (ETI, UK) funded project called NOVA (for Novel Vertical Axis wind turbine) examined the feasibility of a large offshore vertical axis wind turbine in the 5 to 10 MW power range, with a view toward 20 MW. The NOVA feasibility study required a methodology to be developed to select the best configuration, based on the system dynamics. Two options have been analysed: fixed and floating support structure, with the floating option is here analysed. The design space has been investigated, ranking the possible options using a multi-criteria decision making (MCDM) method called TOPSIS. Based on this, two configurations have been promoted to the next analysis stage: a barge and a semisubmersible. The evolution of the configuration designs, necessary to optimise and compare these two options, is here presented, taking their dynamics and costs into account. The barge concept evolved to the ‘triple doughnut-Miyagawa’ concept, consisting of an annular cylindrical shape with an inner (to control the damping) and outer (to control added mass) bottom flat plates. The semisubmersible was optimised to obtain the best trade-off between dynamic behaviour and amount of material needed. The main conclusion is that, in general, the requirement driving the design is a good dynamic response to waves. If only basic requirements were taken into account (ability to float and ability to counteract the wind turbine inclining moment), a much smaller, lighter, and cheaper structure would fulfil them.

The design of floating support structures for wind turbines located offshore is a relatively new field. In contrast, the offshore oil and gas industry has been developing its technologies since the mid 1950s. However, the significantly and subtly different requirements of the offshore wind industry call for new methodologies. An Energy Technologies Institute (ETI) funded project called NOVA (for Novel Vertical Axis wind turbine) examined the feasibility of a large offshore vertical axis wind turbine in the 10–20 MW power range. The development of a case study for the NOVA project required a methodology to be developed to select the best configuration, based on the system dynamics. The design space has been investigated, ranking the possible options using a multi-criteria decision making (MCDM) method called TOPSIS. The best ‘class’ or design solution (based on water plane area stability) has been selected for a more detailed analysis. Two configurations are considered: a barge and a semi-submersible. The iterations to optimise and compare these two options are presented here, taking their dynamics and costs into account. The barge concept evolved to the ‘triple doughnut-Miyagawa’ concept, consisting of an annular cylindrical shape with an inner (to control the damping) and outer (to control added mass) bottom flat plates. The semi-submersible was optimised to obtain the best trade-off between dynamic behaviour and amount of material needed. The main conclusion is that the driving requirement is an acceptable response to wave action, not the ability to float or the ability to counteract the wind turbine overturning moment. A simple cost comparison is presented.

The paper proposes a novel algorithm to simulate Vertical Axis Wind Turbines in floating motion based on a low-fidelity approach. The conventional Double Multiple Stream Tube Model is formulated as a steady state model to deal with the inherent unsteady aerodynamics of VAWTs. The Double Multiple Stream Tube Model makes use of an average contribution of the blade passages over a revolution to solve the Blade Element Momentum equation. The model reduces the dependence of the solution (the induction field) to a space variable. Floating Vertical Axis Wind Turbines undergoes a variation of aerodynamic quantities according to wave periods, also different from the rotor period. This paper provides a new formulation of the Double Multiple Stream Tube Model to solve the turbine rotation in time and space, introducing the variation of the platform velocity and a revised approach to the downstream actuator disc. The most impactful parameter is found to be the wave frequency: wave periods at full-scale are lower than the ones of the turbine rotor, featuring optimal operating points at low tip speed ratios. The increase of the wave frequency highlights the differences of the average torque prediction for a single blade up to 10% between the unsteady and the standard formulation. In this context, the development of fast low-fidelity computational tools play a key role in the assessment of the preliminary designs of Floating Offshore Vertical Axis Wind Turbines and it will be used to optimise the aerodynamic design of the laboratory scaled model under surge motion.

A process for optimizing both the design and operation of the generator for a large offshore vertical axis wind turbine (VAWT) is developed. The objectives of the optimization process are to minimize additional costs and losses in the generator to allow for a fair evaluation of the impact of the VAWT environment on the powertrain. A spectrum of torque control strategies was tested based on the ratio, q, of the allowed electrical torque variation to the inherent mechanical torque variation. Equations relating q to the generator losses were established. The effect of q on the energy extracted by the rotor was also investigated and incorporated into the optimization process. This work shows that a variable q strategy with respect to wind speed can improve turbine performance across the range of operational wind speeds depending on the torque loading from the rotor blades. In turn, this also allows for the torque rating of the generator to be reduced from the peak torque rating that would otherwise be expected, creating an opportunity to downscale the generator size, reducing costs. The optimization of powertrain design and operation should be carried out at as high level as is possible, ideally using the fully factored cost of energy (COE) to guard against unexpected losses because of excessive focus in one COE factor (for example reducing upfront cost but in turn reducing availability).

In this study, the performance of offshore wind turbines at low tip speed ratio (TSR) is studied using computational fluid dynamics (CFD), and the performance of offshore wind turbines at low tip speed ratio (TSR) is improved by revising the blade structure. First, the parameters of vertical axis offshore wind turbine are designed based on the compactness iteration, a CFD simulation model is established, and the turbulence model is selected through simulation analysis to verify the independence of grid and time step. Compared with previous experimental results, it is shown that the two-dimensional simulation only considers the plane turbulence effect, and the simulation turbulence effect performs more obviously at a high tip ratio, while the three-dimensional simulation turbulence effect has well-fitting performance at high tip ratio. Second, a J-shaped blade with optimized lower surface is proposed. The study showed that the optimized J-shaped blade significantly improved its upwind torque and wind energy capture rate. Finally, the performance of the optimized J-blade offshore wind turbine is analyzed.

The offshore wind industry is undergoing a rapid development due to its advantage over the onshore wind farm. The vertical axis wind turbine (VAWT) is deemed to be potential in offshore wind energy utilization. A design of the offshore vertical axis wind turbine with a deflector is proposed and studied in this paper. Two-dimensional computational fluid dynamics (CFD) simulation is employed to investigate the aerodynamic performance of wind turbine. An effective method of obtaining the blade’s angle of attack (AoA) is introduced in CFD simulation to help analyze the blade aerodynamic torque variation. The numerical simulations are validated against the measured torque and wake velocity, and the results show a good agreement with the experiment. It is found that the blade instantaneous torque is correlated with the local AoA. Among the three deflector configurations, the front deflector leads to favorable local flow for the blade, which is responsible for the improved performance.

This paper is a state-of-the-art that aims to highlight all the new developments for wind renewable energy sources, wind turbine system, vertical axis wind turbine and the included components to find the best solution for an offshore vertical axis wind turbine that supplies ships with energy in the outer harbor. This review could help to understand the potential future choices in the design of vertical-axes wind turbine in order to reduce pollution in marine environment.

Despite several potential advantages, relatively few studies and design support tools have been developed for floating vertical axis wind turbines. Due to the substantial aerodynamics differences, the analyses of vertical axis wind turbine on floating structures cannot be easily extended from what have been already done for horizontal axis wind turbines. Therefore, the main aim of the present work is to compare the dynamic response of the floating offshore wind turbine system adopting two different mooring dynamics approaches. Two versions of the in-house aero-hydro-mooring coupled model of dynamics for floating vertical axis wind turbine (FloVAWT) have been used, employing a mooring quasi-static model, which solves the equations using an energetic approach, and a modified version of floating vertical axis wind turbine, which instead couples with the lumped mass mooring line model MoorDyn. The results, in terms of mooring line tension, fatigue and response in frequency have been obtained and analysed, based on a 5 MW Darrieus type rotor supported by the OC4-DeepCwind semisubmersible.

Although the onshore and offshore wind energy market is dominated by horizontal axis wind turbines, there is an increasing interest in the use offshore vertical axis wind turbines due to their potential advantages. At the same time, the gearbox problems encountered in wind turbines have increased the interest held towards direct-drive generator systems. The high torque requirements of direct-drive systems have increased both generator dimensions and mass. Permanent magnet C-GEN generators can be a good candidate for vertical axis wind turbines due to their lightweight structure, easy production and simple installation. In this study, a 6-rpm 5MW multi-stage air-cored generator model has been developed for direct-drive vertical axis wind turbines and an automatic a 2D finite element analysis software algorithm has been improved for generator design and optimization.

In recent decades, wind turbines have emerged as substantial renewable energy technologies, particularly for offshore applications. However, the utilisation of conventional wind turbine in regions characterised by a low wind speed profile, such as Malaysia, remains limited. To address this gap, this study develops an unconventional small-scale Vertical Axis Wind Turbine (VAWT) provision for offshore application in Malaysia’s low wind speed regions. The study commenced with a wind feasibility analysis which aimed to determine the viable range of offshore wind speeds prior conducting wind tunnel tests. The VAWT model was designed using SolidWorks software, fabricated using a 3D printer and tested in the National Defence University of Malaysia (UPNM) wind tunnel lab facilities. Through a systematic analysis and optimisation of blade diameters, stages and angles using Taguchi method, this study aimed to enhance the wind turbine performance in terms of power coefficient (Cp) and tip speed ratio. The result highlighted the pivotal role of blade diameter and the number of stages as a key design factor for VAWT, with the optimal configuration of blade diameter of 16 cm, a single stage and Angle of Attack (AoA) of 7°, resulted in a Cp value of 0.622. It is important to note that this Cp value exceeds the typical wind turbine power coefficient benchmarks, which range from 0.3 to 0.45, indicating that the optimised design offers a higher-than-average efficiency for small-scale VAWTs. Additionally, variations in AoA were found to be the least significant design factor, as all models demonstrated peak Cp values corresponding to wind speeds. Although tested in a controlled wind tunnel environment, these results suggest that the optimum design could perform efficiently under Malaysia's real-world offshore conditions, where wind speeds are typically low. This study provides key design considerations for small-scale VAWTs in low wind profile regions, contributing to the sustainable development of wind energy in Malaysia.

Parameter optimization method of contra-rotating vertical axis wind turbine: Based on numerical simulation and response surface

Effect of Turbulence Intensity on the Performance of an Offshore Vertical Axis Wind Turbine

The paper deals with the dynamic analysis of a tri-floater with a 1 MW offshore Vertical Axis Wind Turbine (VAWT) placed at its centroid. Six line catenary mooring system was used for controlling the horizontal movement of the floater. The floater was modeled as a rigid body with six degrees of freedom. Mass, damping and hydrostatic stiffness were calculated by using hydrostatic stability condition. The aerodynamic load on Vertical Axis Wind Turbine was calculated via the stream tube theory. Wave profile was calculated using Airy’s wave theory followed by the use of Morison’s equation to determine the inertial and drag forces on the floater. A computer program was developed by using the MatLab package for force calculation including wind and wave excitations as a dynamic analysis. The Newmark - beta method was performed for these analyses. The equation of motion for the floater was solved in time domain. Restoring force by mooring lines at each instant of time was calculated based on the cable profile. Responses of the tri-floater with VAWT in different sea conditions were analyzed. It was proven that surge, heave and pitch are the predominant motions for a straight (00) wave. These motions were also analyzed for the waves with different inclinations and their responses were also considered and compared.