Ocean energy has many forms, encompassing tides, surface waves, ocean circulation, salinity and thermal gradients. This paper will considers two of these, namely those found in the kinetic energy resource in tidal streams or marine currents, driven by gravitational effects, and the resources in wind-driven waves, derived ultimately from solar energy. There is growing interest around the world in the utilisation of wave energy and marine currents (tidal stream) for the generation of electrical power. Marine currents are predictable and could be utilised without the need for barrages and the impounding of water, whilst wave energy is inherently less predictable, being a consequence of wind energy. The conversion of these resources into sustainable electrical power offers immense opportunities to nations endowed with such resources and this work is partially aimed at addressing such prospects. The research presented conveys the current status of wave and marine current energy conversion technologies addressing issues related to their infancy (only a handful being at the commercial prototype stage) as compared to others such offshore wind. The work establishes a step-by-step approach that could be used in technology and project development, depicting results based on experimental and field observations on device fundamentals, modelling approaches, project development issues. It includes analysis of the various pathways and approaches needed for technology and device or converter deployment issues. As most technology developments are currently UK based, the paper also discusses the UK's financial mechanisms available to support this area of renewable energy, highlighting the needed economic approaches in technology development phases. Examination of future prospects for wave and marine current ocean energy technologies are also discussed.

Generating electricity from the oceans

Experimental study of the turbulence intensity effects on marine current turbines behaviour. Part I: One single turbine

This paper presents our initial work in performing large-eddy simulations of tidal turbine array flows. First, a horizontally periodic precursor simulation is performed to create turbulent flow data. Then those data are used as inflow into a tidal turbine array two rows deep and infinitely wide. The turbines are modelled using rotating actuator lines, and the finite-volume method is used to solve the governing equations. In studying the wakes created by the turbines, we observed that the vertical shear of the inflow combined with wake rotation causes lateral wake asymmetry. Also, various turbine configurations are simulated, and the total power production relative to isolated turbines is examined. We found that staggering consecutive rows of turbines in the simulated configurations allows the greatest efficiency using the least downstream row spacing. Counter-rotating consecutive downstream turbines in a non-staggered array shows a small benefit. This work has identified areas for improvement. For example, using a larger precursor domain would better capture elongated turbulent structures, and including salinity and temperature equations would account for density stratification and its effect on turbulence. Additionally, the wall shear stress modelling could be improved, and more array configurations could be examined.

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A large-eddy simulation study of wake propagation and power production in an array of tidal-current turbines

Interest in the advancement of hydrokinetic energy conversion (HEC) technology has grown substantially in recent years. The hydrokinetic industry has advanced beyond the initial testing phase and will soon install demonstration projects with arrays of full-scale devices. By reviewing the current state of the industry and the cutting edge research this paper identifies the key advancements required for HEC technology to become commercially successful at the utility scale. The primary hurdles are: (i) reducing the cost of energy, (ii) optimizing individual turbines to work in concert considering array and bathymetry effects, (iii) balancing energy extraction with environmental impact, and (iv) addressing socioeconomic concerns. This review is split into three primary sections. The first section provides an overview of the HEC technology systems that are most likely to be installed in commercial arrays. The second section is an in-depth literature review. The literature review is sub-divided into five areas that are positioned to significantly impact the viability of HEC technology: (i) site assessment, (ii) turbine design, (iii) turbine wake modeling, (iv) array performance, and (v) environmental impact. The final section presents an outlook for the HEC industry and future research.

Hydrokinetic energy conversion: Technology, research, and outlook

Experimental study of the turbulence intensity effects on marine current turbines behaviour. Part II: Two interacting turbines

The actuator disc is a useful method for parameterising a tidal stream turbine in a solution of the Reynolds-averaged Navier–Stokes equations. An actuator disc is a region where similar forces are applied to a flow as would be imposed by a turbine. It is useful where large-scale flow characteristics are of interest, such as the far wake, free surface effects, or installation of multi-turbine arrays. This study compares the characteristics of the wake of an actuator disc, modelled using a steady solution to the Reynolds-averaged Navier–Stokes (RANS) simulated equations, with the k–ω shear stress transport (SST) turbulence model, to experimental data measured behind discs of various porosities. The results show that the wake of the experimental and modelled discs has similar characteristics; in both model and experiment, velocity in the near wake decreased as thrust coefficient increased. However, the near wake region in the experiment was shorter than simulated in the model because of near wake turbulence. This, combined with lower ambient turbulence levels in the model, meant that the far wake recovered further downstream, while showing similar overall trends in velocity and turbulence intensity.

Comparison between CFD simulations and experiments for predicting the far wake of horizontal axis tidal turbines

The actuator disc-RANS model has widely been used in wind and tidal energy to predict the wake of a horizontal axis turbine. The model is appropriate where large-scale effects of the turbine on a flow are of interest, for example, when considering environmental impacts, or arrays of devices. The accuracy of the model for modelling the wake of tidal stream turbines has not been demonstrated, and flow predictions presented in the literature for similar modelled scenarios vary significantly. This paper compares the results of the actuator disc-RANS model, where the turbine forces have been derived using a blade-element approach, to experimental data measured in the wake of a scaled turbine. It also compares the results with those of a simpler uniform actuator disc model. The comparisons show that the model is accurate and can predict up to 94 per cent of the variation in the experimental velocity data measured on the centreline of the wake, therefore demonstrating that the actuator disc-RANS model is an accurate approach for modelling a turbine wake, and a conservative approach to predict performance and loads. It can therefore be applied to similar scenarios with confidence.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.2012.0293

Accuracy of the actuator disc-RANS approach for predicting the performance and wake of tidal turbines.

At a commercial scale tidal stream turbines are likely to be installed in multi-device arrays to maximise power output from a tidal site. This paper demonstrates a computationally efficient method for predicting the performance of an array. The approach parameterises the turbine by deriving momentum source terms using blade element (BE) theory. The source terms are added to Reynolds-averaged Navier-Stokes (RANS) momentum equations. The RANS+BE method is similar to blade element momentum (BEM) theory but can predict the flow field as well as the performance of the rotor. This investigation first verifies the RANS+BE method against BEM theory and shows good agreement, although tip-losses still need to be included. Secondly, it compares the RANS+BE approach to a parameterisation based on a uniform resistance coefficient, over a range of ambient turbulence values and thrust coefficients. The RANS+BE method generates increased azimuthal velocities in the near wake compared to the uniform approach. Finally the investigation compares results of a model of an infinitely wide array of turbines with five rows. The RANS+BE model can predict the performance of each turbine. and shows more rapid wake velocity recovery within the array. The investigation provides a detailed insight into the applicability of wake modelling techniques for the configuration of tidal stream turbine arrays.

A blade element actuator disc approach applied to tidal stream turbines

It has previously been shown that ambient turbulence affects the results from computational fluid dynamics (CFD) models when using an actuator disc to simulate marine current turbines. The turbulence parameters are often estimated using an empirical equation that is dependent on a turbulence length scale. In most literature this length scale is commonly highlighted as the authors ‘best guess’ with little scientific reasoning. The work presented here investigates the effects of using different length scales on the development of a flow in a circulating water channel. The results showed that the best agreement is achieved with a length scale of one third the channel depth. The obtained turbulence parameters were then used with an actuator disc model. Agreement with experimental data was initially poor as the velocity deficit was severely under predicted. The addition of a turbulence source at the disc improved the agreement with experimental data significantly. It was found that the length scale of the disc turbulence should be the diameter of the holes used on the porous discs for experiments. However, there were still discrepancies between the experimental and model turbulence intensities. A possible cause of this may be that the turbulence intensity added at the disc was under predicted. Further work is needed to establish if better agreement can be achieved by increasing the turbulence at the disc.

M. E. Harrison

Papers

Comparison between CFD simulations and experiments for predicting the far wake of horizontal axis tidal turbines

Accuracy of the actuator disc-RANS approach for predicting the performance and wake of tidal turbines.

A blade element actuator disc approach applied to tidal stream turbines

The sensitivity of actuator-disc RANS simulations to turbulence length scale assumptions

Comparisons of a large tidal turbine array using the boundary layer and field wake interaction models