International Journal of Marine Energy最新文献

Horizontal axis marine current turbine is a viable device which can harness kinetic energy from ocean currents. It is the closest concept to be commercialised among other marine turbines. Literature shows that computational fluid dynamics (CFD) models can accurately simulate turbine performance provided appropriate numerical techniques are employed. In this paper, the influence of different numerical approaches on the performance prediction of a two bladed turbine model was assessed by towing tank results from the USNA. Two turbulence models of k-ω SST and BSL EARSM as well as three boundary layer modeling techniques, including wall function, near wall region and transitional Gamma-Theta model, were compared. The effects of using steady state or transient solution methods by applying moving reference frame (MRF) and sliding mesh were investigated. Single blade simulation instead of whole turbine model was also evaluated together with the Reynold number effect. Although Transient solution with sliding mesh method offers a simulation closer to the real condition of turbine operation with accurate results, steady state MRF provides reasonable results while saving a significant computational time as well. Therefore, authors recommend utilising steady MRF simulation of whole turbine model using k-ω SST with wall-function model for performance prediction of horizontal axis marine current turbines in a balance between simulation time and results accuracy.

This paper describes the optimisation of small arrays of Wave Energy Converters (WECs) of point absorber type. The WECs are spherical in shape and operate in heave alone and a linear array of five devices is considered. Previous work is extended by considering the constrained performance of the array members, where an uaniper limit on WEC displacements is enforced. Two opimisations are performed. In each case, the objective function is defined as the mean of the averaged interaction factor ovehe non-dimensional length of the array. The first considers the array layout fixed at a geometry previously identified as optimal in an unconstrained regime and optimises the displacements of the WECs subject to constraints. The second allows both the WEC positions and displacements to vary as optimisation variables. It is shown that the optimal layout of the constrained arrays is different from the unconstrained case. Applying constrained motions results in optimal layouts that are more separated, with less grouping of WECs and this will have practical considerations. The effect of the constraints varies depending on the incident wave angle. In some cases, performance is reduced drastically and stability of performance is improved, while in other cases there is a degradation of performance. Thus, a trade-off between performance and stability of performance is seen when displacement constraints are applied.

This study aims to provide further understanding about the effects that tidal turbines arranged in block farms could have over the water levels when they are deployed in estuaries with shallow bathymetry. As an application of the methodology presented for this purpose, a real case study, the Solway Firth estuary in the UK, has been modelled by means of MIKE21. An idealised model of the estuary has been used to identify the trends in extractable energy and impacts on tidal range for different farm densities and contrasted with a fully detailed model. A spring tide and a coastal flooding scenario have been considered for the detailed model as well as different densities and configurations of the tidal farms. For the densest arrays the extractable energy tends to a limit and the impacts on tidal range become relevant, although they are still lower than the ones resulting from tidal range schemes with similar installed capacities. It has been found that the effects on water levels are not linearly proportional to the increase of the farm density but they seem more closely related to the energy dissipated by the farms. The results of the changes on high and low tide levels over the estuary and time-series at specific locations have been analysed for the detailed models. When comparing between different layouts, changes during low tide levels are slightly higher for the parallel arrays. It can also be concluded that flood risk levels could be potentially reduced in the shoreline whereas some of the intertidal areas and their habitats could disappear as a consequence of significant water level rise at low tides.

This paper presents the influence that ambient turbulence has on a tidal turbine farm. Firstly, the analytical model developed by Bahaj and Myers (2004) is used and modified in order to incorporate the ambient turbulence effects. Ambient turbulence is taken into account via the experiments of Mycek et al. (2014), where two levels of turbulences were tested, namely 3% and 15%. Modifications in wake velocity deficit are treated. However, the influence that ambient turbulence has on the power coefficient of downstream turbine(s), which is usually neglected, is taken into account. For the lower level of turbulence, three scenarios for the downstream turbine(s) behaviour are considered.

This enhanced model is then tested on a given site in the Alderney Race (Raz Blanchard). Yearly energy productions depending on ambient turbulence, turbine layouts and proposed scenarios are evaluated and compared. A technico-economical analysis is also carried out. Finally, the tidal turbine farm profitability highly depends on ambient turbulence and turbines layout.

In the present paper, a novel Wave Energy Converter (WEC) named hereafter as Water Level Carpet (WLC) is introduced. Its dynamic response and performance are evaluated based on hydroelastic analysis and are presented for both regular and irregular waves. WLC consists of four floating modules inter-connected flexibly in two directions with hinges and Power Take-Off (PTO) mechanisms with known damping characteristics. The dynamic response (e.g. axial load at hinges, motions) and the estimation of the produced power of the WLC are evaluated through linear hydroelastic analysis in frequency domain with the use of a radiation/diffraction 3D hydroelastic model considering the effect of both the flexibility of the WLC as well as the damping forces associated with the energy extraction by the PTO mechanisms. The results that are obtained demonstrate the relationship between the produced power and the dynamic response of the WLC as well as the relationship between the damping coefficients of the PTO with the axial loads of WLC’s hinges. The produced power of WLC obtains its maximum value for wave frequencies close to the resonance of the generalized degrees of freedom irrespectively of the direction that the waves have. Moreover, the results that are obtained emphasize the importance of the hydroelastic analysis for the assessment of the performance of this type of WEC.

Extensive marine growth on man-made structures in the ocean is commonplace, yet there has been limited discussion about the potential implications of marine growth for the wave and tidal energy industry. In response, the Environmental Interactions of Marine Renewables (EIMR) Biofouling Expert Workshop was convened. Discussions involved participants from the marine renewable energy (MRE) industry, anti-fouling industry, academic institutions and regulatory bodies. The workshop aimed to consider both the benefits and negative effects of biofouling from engineering and ecological perspectives. In order to form an agenda for future research in the area of biofouling and the marine renewable energy industry, 119 topics were generated, categorised and prioritised. Identified areas for future focus fell within four overarching categories: operation and maintenance; structured design and engineering; ecology; and knowledge exchange. It is clear that understanding and minimising biofouling impacts on MRE infrastructure will be vital to the successful development of a reliable and cost effective MRE industry.

The slider crank is a proven mechanical linkage system with a long history of successful applications, and the slider-crank ocean wave energy converter (WEC) is a type of WEC that converts linear motion into rotation. This paper presents a control algorithm for a slider-crank WEC. In this study, a time-domain hydrodynamic analysis is adopted, and an AC synchronous machine is used in the power take-off system to achieve relatively high system performance. Also, a rule-based phase control strategy is applied to maximize energy extraction, making the system suitable for not only regular sinusoidal waves but also irregular waves. Simulations are carried out under regular sinusoidal wave and synthetically produced irregular wave conditions; performance validations are also presented with high-precision, real ocean wave surface elevation data. The influences of significant wave height, and peak period upon energy extraction of the system are studied. Energy extraction results using the proposed method are compared to those of the passive loading and complex conjugate control strategies; results show that the level of energy extraction is between those of the passive loading and complex conjugate control strategies, and the suboptimal nature of this control strategy is verified.

The present paper analyzes the convenience of using non-linear methods for hydrostatic force approximations. An alternative methodology is here proposed and validated by laboratory experiments. The coefficients involved in the calculation of the hydrostatic force, submerged volume, and centre of buoyancy are calculated using a panelization of the hull geometry. The presented methodology is included in a time domain model that solves Cummins’ equation, so the instantaneous value of the hydrostatic force is employed at each time step, extending the validity of the approximation.

The improvements of this methodology are compared with the classical approach of using a constant hydrostatic matrix based on precomputed values at equilibrium. A wave energy converter (WEC) designed to extract energy through large rotations is employed to analyze the effect of non-linearizing the hydrostatic term. The results of the methodology for static conditions are validated using commercial software. A sensitivity analysis of the panelization used is also presented. A comparison of the computed results of all the methods using laboratory data shows the improvement of the results when non-linear approximations are used. In the laboratory large amplitude rotations display a different period than small amplitude rotations. This effect cannot be reproduced by the classical linear model but it can be reproduced by non-linear hydrostatic models.