International Journal of Marine Energy最新文献

The present study aims to predict and optimize the operating range of a Wells turbine that essentially works on the principle of bidirectional flow in an ocean renewable energy system. The turbine operates in a narrow range because of variability in waves, machine geometry and low incidence angle that lead to stumpy performance of the turbine. Hence, a relationship between the fluid velocity and the turbine speed has been established to design a turbine with higher performance. The two different cases, with and without a tip groove, were considered to predict the optimal turbine speed for the different flow velocities. A multiple-surrogate based approach has been used to find correlation between the turbine speed and the air velocity, and a Reynolds-averaged Navier–Stokes equation solver evaluated the turbine performance parameters. Furthermore, several combinations of the variables (flow velocity and turbine speed) along with an objective function (efficiency) were evaluated by the solver. The grooved-casing design performs better than that of the without grooved-casing, and the mid-chord of the blade enhances the exchange of momentum among different directions and suppresses the unsteadiness.

This paper presents a comparison of predicted and measured performance and loading for the Alstom Ocean Energy’s 1 MW tidal turbine, DEEP-Gen IV, which was deployed at the European Marine Energy Centre (EMEC) in Orkney as part of the ReDAPT (Reliable Data Acquisition Platform for Tidal) project. The ReDAPT project was commissioned and co-funded by the Energy Technologies Institute.

Unsteady time domain simulations are conducted using DNV GL’s Tidal Bladed software. The hydrodynamic loads are computed using a blade-element formulation that accounts for flow blockage. The onset flow turbulence is described using a von Kármán velocity spectra and coherence functions. Length scales are determined from a site characterisation study using field measurements.

Machine data is processed and quality control filters applied to obtain measurement ensembles suitable for comparison with simulation outputs. Comparisons are made for electrical power, pitch angle and blade near-root bending moment. Good agreement is found between the simulated and measured flapwise near root-bending damage equivalent loads and load spectra. The stochastic blade load data is further analysed where it is found that the methodologies applied provide accurate predictions of machine fatigue loads due to turbulence.

This paper presents the development of a time-domain simulation model, with experimental verification, for a submerged oscillating water column (OWC). The Stellenbosch Wave Energy Converter (SWEC) makes use of series of these submerged chambers in order to create a rectified flow through a single turbine. The main objective of this research was to produce a verified and validated simulation model for a single chamber of the SWEC. The mathematical model was derived from first principles and then coded in Simulink. The simulation results were verified using measurements from a scale model in a wave tank test. The model provides a better understanding of the hydrodynamics and thermodynamics associated with the submerged chamber. The submerged chamber achieved a peak conversion efficiency of 22%. The device achieved a conversion efficiency of 13% at the expected operating conditions when orientated at 45° with regards to the incident waves. The simulation model predicted the transmissibility of the device with errors which ranged from 0% to 20% with the majority of the errors being less than 5%. The model predicted the conversion efficiency of the device with errors which ranged from 0% to 43% with the majority of the errors being less than 15%.

Several commercial tidal turbine designs feature axial flow rotors within bi-directional ducts. Such devices are typically intended to increase power extraction through a flow-concentrating effect, operating on flood and ebb tides without a yawing mechanism. Research focused on such devices has been limited so far, with available results indicating poor performance relative to bare rotors. This study further investigates the relative performance of bi-directional ducted tidal turbines in confined flow.

Several duct profiles are evaluated relative to unducted rotors using the Reynolds-averaged Navier–Stokes solver ANSYS Fluent. The rotor is represented as an actuator disc, which mimics the streamwise thrust of a real device but does not reproduce its swirl or additional turbulence generation. This idealised model achieves optimal energy extraction and enables fair comparison of duct geometries. Device power is reported relative to total frontal area, reflecting the fact that the overall dimension of the device will be limited by water depth. Comparisons based on rotor area show how the absolute power is increased by a duct, but that this is attributable to an increase in blockage.

The fundamental effect of a duct on a rotor, as well as the secondary effects of duct camber and thickness, are identified by analysing streamwise distributions of velocity, pressure and cross-sectional area along the rotor streamtube. Ducts are found to limit the expansion of the downstream flow, in turn restricting the pressure reduction immediately behind the rotor. This effect, in combination with the reduced volumetric flux through a ducted rotor relative to a bare rotor, results in reduced power extraction.

The effects of duct curvature and thickness on turbine performance are also examined. Where a ducted rotor is desirable, e.g. for the protection of rotor blades, a thick profile with slight curvature performs best.

This study used computational fluid dynamics to investigate the effect of waves and a velocity profile on the performance of a tidal stream turbine (TST). A full scale TST was transiently modelled operating near its maximum power point, and then subjected to waves both in and out of phase with its period of rotation. A profile was then added to one of the wave models. For this set of conditions it was found that the longer period and in-phase wave had a significant effect on the power range fluctuations, with more modest variations for thrust and the average values, although this is dependent on the turbine tip speed ratio. The addition of the profile had a strong effect on the bending moment. It has been concluded that a naturally varying sea state may yield a smoothing effect in this turbine response, but that with further structural investigation it may be that some measuring and mitigation techniques are required in the event of a predominantly single long period, in-phase wave.

Recent research has shown that when constrained to causality, the optimal feedback controller for an ocean wave energy converter (WEC) subjected to stochastic waves can be solved as a non-standard Linear Quadratic-Gaussian (LQG) optimal control problem. In this paper, we present a relaxation to the modeling assumptions that must be made to apply this theory. Specifically, we propose a technique that uses the principle of Gaussian Closure to accommodate nonlinear WEC dynamics in the synthesis of the optimal feedback law. The technique is approximate, in the sense that it arrives at a computationally efficient control synthesis technique through a Gaussian approximation of the stationary stochastic response of the system. This approach allows for a wide range of nonlinear dynamical models to be considered, and also accommodates many complex loss mechanisms in the power transmission system. The technique is demonstrated through simulation examples pertaining to a flap-type WEC with a hydraulic power train.

Results from laboratory experiments on a pre-tensioned heaving buoy of 8.4 m diameter, tested at scale 1:16 is presented. The wave energy converter, which is under development by the Swedish company CorPower Ocean, is designed with a passive pneumatic machinery component referred to as WaveSpring, invented at NTNU. The negative spring arrangement inherently provides phase control. The power take off system and the effect of the phase-enhancement component was represented by a motor rig run with a force-feedback controller. Responses with and without the WaveSpring unit were measured in order to compare performance in terms of motions, loads and power absorption.

The experiments included decay tests, radiation tests, irregular wave tests with the system in normal operation, as well as extreme wave tests with the system in survival mode. The power was extracted through a linear damping force, where the damping coefficients were set close to their theoretical optimum for the heave mode.

The results show that with the WaveSpring component, the system is able to absorb three times more power in realistic sea conditions than without it. This is achieved without increasing the damping force, thus giving a three times higher ratio of absorbed energy to PTO force. The maximum mooring line tension in storm conditions is found to be less than 2.5 times the pretension force.

In the development of tidal current turbines there are two common approaches regarding the required level of detail for load simulations. Those two are either to simulate the pressure field in detail with computational fluid dynamics (CFD) and assume a rigid geometry or to use a high fidelity structural model and simulate the hydrodynamic blade loads with the semi-empirical blade element momentum theory.

Within the present research this simplification and the impact of fluid–structure-interaction (FSI) on the loads on tidal current turbines are analysed. Based on coupled CFD and multibody simulations the FSI is simulated for the Voith HyTide®1000-13 turbine. This method allows taking the detailed structure of the full turbine into account, while also simulating the detailed pressure field.

Transient simulations of a representative point of operation are performed considering the structural flexibility of the tower, rotor blades, drivetrain and other components. This comparison is used to quantify the individual and combined effect of flexibilities on the loads and performance. Therefore, the Voith HyTide®1000-13 turbine is simulated within this research in varying levels of detail to analyse the required level of modelling detail for load simulations of tidal current turbines and increases the understanding of fluid–structure-interaction in tidal current turbine applications.

The range and variability of flow velocities in which horizontal axis tidal stream turbines operate introduces the requirement for a power regulation method in the system. Overspeed power regulation (OSPR) has the potential to improve the structural robustness and decrease the complexity associated with active pitch power regulation methods, while removing the difficulties of operating in stalled flow. This paper presents the development of a methodology for the design of blades to be used in such systems. The method requires a site depth, maximum flow velocity and rated power or flow speed as input parameters. The pitch setting, twist and chord distribution were set as input parameters, variable through the use of alteration functions. Rotor performance has been broken down into OSPR performance metrics which consider coefficients of power and thrust, and cavitation inception. Three visual-numerical tools have been developed: the OSPR performance metrics were used in conjunction with a one-at-a-time sensitivity analysis approach to develop a design space; cavitation inception analyses gave plots of converging cavitation and pressure terms for each blade section; the local angle of attack and torque distribution across the blade designs were plotted at key turbine operation states. Alterations to pitch setting and twist distribution are shown to have most impact upon the design requirement of increased gradient in the rotor speed-efficiency relationship in the overspeed region; coupled with such alterations, targeted changes to the chord distribution have been shown to increase the maximum efficiency. The prevention of cavitation has been highlighted as a driver for speed-limiting design alterations. While facilitating blade design, the methodology also produces experiential knowledge which can be stored, and shared in graphical format.