On the concept of sloped motion for free-floating wave energy converters

A free-floating wave energy converter (WEC) concept whose power take-off (PTO) system reacts against water inertia is investigated herein. The main focus is the impact of inclining the PTO direction on the system performance. The study is based on a numerical model whose formulation is first derived in detail. Hydrodynamics coefficients are obtained using the linear boundary element method package WAMIT. Verification of the model is provided prior to its use for a PTO parametric study and a multi-objective optimization based on a multi-linear regression method. It is found that inclining the direction of the PTO at around 50. to the vertical is highly beneficial for the WEC performance in that it provides a high capture width ratio over a broad region of the wave period range

Operational loads on a tidal turbine due to environmental conditions

Accurate assessment of the fatigue life of tidal stream turbines and components requires prediction of the unsteady loading of turbine components over a wide range of frequencies. This study focuses on the influence of ambient turbulence, velocity shear and the approach taken to model wave kinematics, on the variation of thrust load imposed on the rotor shaft and supporting tower. Load cycles are assessed based on sea-state occurrence data taken over a five month period for a case study site. The influence of each environmental parameter on component loading is evaluated and the impact on material design parameters assessed. Alternative approaches are considered for modelling turbulent loading and wave loading. The frequency variation of loads due to turbulence are scaled from experimental data from trials of a three-bladed horizontal axis turbine of 1.2 m diameter on a bed-mounted supporting structure. Frequency dependent wave loading is estimated by a relative form of the drag term of the widely used equation of Morison et al. (1950), with the depth decay of kinematics modelled by linear wave theory. Over the five month interval considered a ten year design life can be obtained with a lower design load by accounting for variation of turbulence intensity that occurs during each tidal cycle. This is expected to vary further with the approach taken to model the onset turbulence. A component can also be designed for lower loads over the same time period if irregular waves are modelled instead of regular

Variation of loads on a three-bladed horizontal axis tidal turbine with frequency and blade position

Sustainable and cost effective design for tidal current turbines requires knowledge of the complex nature of unsteady loads on turbine components including blades, rotor and support structure. This study investigates experimentally the variation with frequency of rotor thrust and torque loads, of streamwise root bending moment on individual blades and of loads on foundation at the bed. Comparisons between these different load spectra are also established. The impact of absolute rotor angular position on blade and rotor thrust loads is also examined. The study is based on measurements from a 1/15 scale, three-bladed, horizontal axis machine tested in a recirculating flume, in onset flows of 3% and 12% turbulence intensity. It is found that for frequencies below the rotational frequency, load spectra are correlated to spectral density of the onset flow velocity. Above the rotational frequency, loads are mainly affected by turbine operation phenomena. The tower shadowing effect is clearly identified through frequency and angular analysis. Finally, thrust loads as experienced by the rotor alone are for the first time compared with streamwise and transverse foundation loads. Higher frequency loads experienced by the tower are shown to be affected by different vortex shedding regimes associated with different regions of the wake. All the experimental measurements presented in this article can be accessed from http://dx.doi.org/10.7488/ds/2423

Investigation into wave basin calibration based on a focused wave approach

The purpose of this paper is to present a detailed numerical investigation concerning the calibration of force controlled wave generation facilities. The methodology is presented for a 2-dimensional calibration; the findings being equally applicable to the calibration of 3-dimensional wave basins. State-of-the-art force controlled wavemaking facilities comprise sophisticated hardware, software and control systems, commonly incorporating active absorption mechanisms. Such facilities have the potential to reproduce ocean wave of exceptional quality, but poor understanding of accurate calibration processes often hinders full exploitation. A technique based upon the generation of focused wave events may oer a very accurate and time-efficient calibration. However, such a methodology may lead to erroneous results if not employed correctly. The theoretical and statistical analysis presented herein investigates the sensitivity of such method to a number of important parameters. The results obtained are directly applicable to a large number of hydrodynamic facilities

Validation of a control-oriented point vortex model for a cyclorotor-based wave energy device

Recently conducted analytical assessment of the potential performance of cyclorotor wave energy converters (WECs) have shown that such devices offer the best wave absorption behaviour, if energy capture can be optimised through suitable control. Such claims require additional investigation. This article is dedicated to validation of the control-oriented point vortex model of cyclorotor WECs against numerical and experimental assessments conducted by various research groups. The validation is conducted in terms of the traditional metrics for cyclorotor WECs: (a) cancellation of incoming waves; (b) generation of lift and drag forces (c) mechanical power generation. It is shown that the point vortex model generally confirms the previously conducted analytical assessment of device performance. However, accounting for the influence of the hydrofoil induced wakes decreases performance estimates to some extent. It is also shown that, overall, wave cancellation metrics are more optimistic than actual shaft power generation. Analysis of the lift and drag coefficients, which were derived from experimental data, reveal a range of hydrodynamic and mechanic effects which could influence actual device performance. It has been shown that, due to the complexity of hydrodynamic effects, lift and drag coefficients for the control-oriented model should be considered not only as functions of the Reynolds number and angle of attack, but also related to submergence of the foils and direction of their rotation with respect to the free surface. This method allows us to achieve the best validation against experimental results in terms of generation of tangential and radial forces

Assessment of boundary-element method for modelling a free-floating sloped wave energy device. Part 2: experimental validation

The boundary-element method has been widely used as a design tool in the offshore and ship building industry for more than 30 years. Its application to wave energy conversion is, however, more recent. This is the second of two papers on a comparison of numerical and physical modelling of a free-floating sloped wave energy converter. In the first paper the numerical modelling formulation for the power take-off mechanism was derived using the boundary-element method package WAMIT. It was verified against numerical benchmark data. In this paper, the outcome of the modelling of the whole device is compared with experimental measurements obtained from model testing in a wave tank. The agreement is generally good

Experimental evaluation of the wake characteristics of cross flow turbine arrays

One key factor in the exploitation of tidal energy is the study of interactions of turbines when working in tidal turbine farms. The Momentum Reversal and Lift (MRL) turbine is a novel cross flow turbine. The three blades rotate around a common central horizontal axis which is parallel to their own axis and perpendicular to the flow. The novelty of the MRL turbine is that it relies on the combination of both lift and momentum reversal (drag) for energy extraction. Scaled MRL turbine models of 0.164 m in diameter were used to characterise the flow in three different tidal array settings. Detailed maps of axial velocity profiles and velocity deficits downstream of the turbine are presented, enabling the visualisation of characteristic flow patterns. The results show that the MRL generates lower velocity deficits and turbulence intensities in the near wake than those associated with horizontal axis turbines. The downstream wake was not completely symmetrical which was related to the geometry of the device but also due to the flow developed in the flume. Amongst the three array configurations studied, a fence of turbines with the lowest separation provided the highest power output

Design and manufacture of a bed supported tidal turbine model for blade and shaft load measurement in turbulent flow and waves

Laboratory testing of tidal turbine models is an essential tool to investigate hydrodynamic interactions between turbines and the flow. Such tests can be used to calibrate numerical models and to estimate rotor loading and wake development to inform the design of full scale machines and array layout. The details of the design and manufacturing techniques used to develop a highly instrumented turbine model are presented. The model has a 1.2 m diameter, three bladed horizontal axis rotor and is bottom mounted. Particular attention is given to the instrumentation which can measure streamwise root bending moment for each blade and torque and thrust for the overall rotor. The model is mainly designed to investigate blade and shaft loads due to both turbulence and waves. Initial results from tests in a 2 m deep by 4 m wide flume are also presented

The effects of oblique waves and currents on the loadings and performance of tidal turbines

Tidal energy exploitation is at an early deployment stage and costs need to be reduced to improve the long term economic viability of the sector. High costs of tidal turbines are, in part, the result of load uncertainties, which lead to the use of high factors of safety in the design to ensure survival. One of the most important causes of uncertainty is hydrodynamic loadings. To date, most of the scaled model experiments with horizontal axis turbines investigating this issue have been carried out with collinear wave and current directions. To the authors’ knowledge, the work presented herein is the first experimental investigation of a horizontal axis turbine model subjected to combined oblique waves and current. Turbine performance and loading are measured for a 1:15 scale model tested in the FloWave circular, combined wave and current basin at the University of Edinburgh (UK). Three different flow directions were tested and each of them were also combined with regular waves in three different directions non-collinear with the flow. Fifteen physical quantities were measured including flow velocity, rotor and foundation loads and turbine speed. Characterisation of loads and turbine performance in those oblique current and wave conditions are presented. Waves affect means and standard deviation of rotor power and thrust, but off-axis waves are associated with lower thrust loads than head-on waves. Compared to current only, rotor torque and thrust standard deviations are higher in the presence of waves and almost twice as high when the wave crest is parallel to the rotor plan

Influence of resource definition on defining a WEC optimal size

International audienceThis work is a follow up from two previous studies which have been investigating the difference in wave resource between sites and the impact this has on the optimal Wave Energy Converter (WEC) size using scatter diagrams of the sites only. This study expands these works by using omni-directional spectra time series to describe the wave resource instead of scatter diagrams. Two well known wave energy test sites are considered: EMEC (Billia Crew) in the North of Scotland and the SEM-REV on the West coast of France. The sloped IPS buoy is used as a case study, and a succinct description is provided. As in previous work, only the hydrodynamic power capture is considered, and no power-takeoff efficiency or cap are introduced. For both sites, around one full year of data is available. Using both the scatter diagrams and the spectra directly, WEC performance metrics are computed for each site and compared. The results show that using spectral time series instead of the scatter diagrams yield lower annual energy productions, and that the highest average capture width ratio is obtained for larger scale devices. Spectral time series allows also the establishment of a simple O&M model. The effect on device availability of annual planned downtime days, of annual failure rates of 1, 3 and 5 and of operability threshold of 2m and 2.5m are investigated. The results show that larger scales might indeed have higher availability, but are exposed to higher risks