Offshore and nearshore wave energy assessment around the Korean Peninsula

A wave resource assessment is presented for the region around the Korean peninsula. Offshore wave power was obtained from significant wave heights and peak periods, and wave directions hindcast for the period of 1979-2003. The spatial distributions for the seasonal and annual averaged wave power were obtained on a 1/6° grid covering the longitudes of 117-143°E and latitudes of 20-50°N. The highest monthly averaged wave power (25 kW/m) was observed on the southwestern side of the peninsula in winter. In order to obtain the wave power around Hongdo, numerical simulations were performed with respect to the monthly averaged waves. The correlation between the significant wave height and energy period was considered to adjust the nearshore wave power obtained by the numerical simulation. The correction procedure was validated from comparing the simulated data with wave buoy data.     

Further analysis of change in nearshore wave climate due to an offshore wave farm: An enhanced case study for the Wave Hub site

This paper addresses the use of numerical wave models for assessing the impact of offshore wave farms on the nearshore wave climate. Previous studies have investigated the effect of energy extraction by wave energy devices through the use of spectral models such as SWAN, representing a wave farm as one or more barriers within the model domain and applying a constant wave energy transmission percentage across the whole wave spectrum incident at the barrier. However, this is an unrealistic representation of the behaviour of real wave energy converters. These will exhibit frequency-dependent energy absorption characteristics that will correspond to the spectral response of the device, and may reflect its ability to be tuned to extract energy at particular frequencies. This study describes a modification of the SWAN source code to enable frequency-dependent wave energy transmission through a barrier. A detailed analysis of the wave climate at the Wave Hub wave farm site is also presented, with a particular focus on the occurrence of bimodal sea states. The modified SWAN code is used to assess how impact predictions for typically occurring sea states may differ when using frequency-dependent rather than constant wave energy transmission, with reference to a previous study using the unmodified code (Millar, Smith and Reeve, 2007 [1]). The results illustrate the dependence of the magnitude of the impact on both the response function of the devices and the spectral sea state in which they are operating.     

Numerical simulation of a submerged cylindrical wave energy converter

In this study, a numerical model based on the complete solution of the Navier–Stokes equations is proposed to predict the behavior of the submerged circular cylinder wave energy converter (WEC) subjected to highly nonlinear incident waves. The solution is obtained using a control volume approach in conjunction with the fast-fictitious-domain-method for treating the solid objects. To validate the model, the numerical results are compared with the available analytical and experimental data in various scenarios where good agreements are observed. First, the free vibrations of a solid object in different non-dimensional damping ratios and the free decay of a heaving circular cylinder on the free surface of a still water are simulated. Next, the wave energy absorption efficiency of a circular cylinder WEC calculated from the model is compared with that of the available experiments in similar conditions. The results show that tuning the converter based on the linear theory is not satisfactory when subjected to steep incident waves while the numerical wave tank (NWT) developed in the current study can be effectively employed in order to tune the converter in such conditions. The current NWT is able to predict the wave-body interactions as long as the turbulence phenomena are not important which covers a wide range of Reynolds and Keulegan-Carpenter numbers.     

Adaptability of a generic wave energy converter to different climate conditions

This study evaluates the influence of wave climate tunability on the performance of a generic Wave Energy Converter (WEC) for different climate scenarios. The generic WEC is assumed to be composed of an array of heaving, floating cylinders. In this study, two natural periods for the cylinders of 4 s and 8 s (typical of enclosed seas and the mean Atlantic swell, respectively) and a location-tunable cylinder are considered to evaluate the influence of tuning on the power performance of the cylinder. The WEC power matrix is computed using a frequency domain model, and the performance of the WEC is evaluated along the global coasts; the met-ocean data originated from the global reanalysis database (GOW) from Reguero et al. (2012). The performance of the WEC is evaluated using two parameters: the capture width ratio (CWR), which evaluates the efficiency of the converter at each location, and the kW/Ton (KWT) parameter, which evaluates the efficiency of the converter using “economic” terms. Tuning a converter for each location displayed a positive CWR; however, the KWT was low after WEC tuning because of the weight of the structures required to tune the converter that experiences high peak periods.     

On improving wave energy conversion,part I: Optimal and control technologies

Extracting wave energy from seas has been proven to be very difficult although various technologies have been developed since 1970s. Among the proposed technologies, only few of them have been actually progressed to the advanced stages such as sea trials or pre-commercial sea trial and engineering. One critical question may be how we can design an efficient wave energy converter or how the efficiency of a wave energy converter can be improved using optimal and control technologies, because higher energy conversion efficiency for a wave energy converter is always pursued and it mainly decides the cost of the wave energy production.In the first part of the investigation, some conventional optimal and control technologies for improving wave energy conversion are examined in a form of more physical meanings, rather than the purely complex mathematical expressions, in which it is hoped to clarify some confusions in the development and the terminologies of the technologies and to help to understand the physics behind the optimal and control technologies. And as a result of the understanding of the physics and the principles of the optima, a new latching technology is proposed, in which the latching duration is simply calculated from the wave period, rather than that based on future information/prediction, hence the technology could remove one of the technical barriers in implementing latching control technology. From the examples given in the context, this new latching control technology can achieve a phase optimum in regular waves, and hence significantly improve wave energy conversion. Further development on these latching control technologies in irregular waves can be found in the second part of the investigation.     

Improving the assessment of wave energy resources by means of coupled wave-ocean numerical modeling

Sea waves energy represents a renewable and sustainable energy resource, that nevertheless needs to be further investigated to make it more cost-effective and economically appealing. A key step in the process of Wave Energy Converters (WEC) deployment is the energy resource assessment at a sea site either measured or obtained through numerical model analysis. In these kind of studies, some approximations are often introduced, especially in the early stages of the process, viz. waves are assumed propagating in deep waters without underneath ocean currents. These aspects are discussed and evaluated in the Adriatic Sea and its northern part (Gulf of Venice) using locally observed and modeled wave data. In particular, to account for a “state of the art” treatment of the Wave–Current Interaction (WCI) we have implemented the Simulating WAves Nearshore (SWAN) model and the Regional Ocean Modeling System (ROMS), fully coupled within the Coupled Ocean Atmosphere Wave Sediment Transport (COAWST) system. COAWST has been applied to a computational grid covering the whole Adriatic Sea and off-line nested to a high-resolution grid in the Gulf of Venice. A 15-year long wave data set collected at the oceanographic tower “Acqua Alta”, located approximately 15 km off the Venice coast, has also been analyzed with the dual purpose of providing a reference to the model estimates and to locally assess the wave energy resource. By using COAWST, we have quantified for the first time to our best knowledge the importance of the WCI effect on wave power estimation. This can vary up to 30% neglecting the current effect. Results also suggest the Gulf of Venice as a suitable testing site for WECs, since it is characterized by periods of calm (optimal for safe installation and maintenance) alternating with severe storms, whose wave energy potentials are comparable to those ordinarily encountered in the energy production sites.     

Experimental study on load characteristics in a floating type pendulum wave energy converter

A floating type pendulum wave energy converter（FPWEC） with a rotary vane pump as the power take-off system was proposed by Watabe et al.in 1998.They showed that this device had high energy conversion efficiency.In the previous research,the authors conducted 2D wave tank tests in regular waves to evaluate the generating efficiency of FPWEC with a power take-off system composed of pulleys,belts and a generator.As a result,the influence of the electrical load on the generating efficiency was shown.Continuously,the load characteristics of FPWEC are pursued experimentally by using the servo motors to change the damping coefficient in this paper.In a later part of this paper,the motions of the model with the servo motors are compared with that of the case with the same power take-off system as the previous research.From the above experiment,it may be concluded that the maximum primary conversion efficiency is achieved as high as 98%at the optimal load.     

An experimental study on the efficiency of the submerged plate wave energy converter

Several studies have been made using submerged plates for wave-damping purpose. A pulsating flow occurs opposite to the direction of wave propagation below these wave breakers. This water flow can be used for energy production purposes. In this study, the energy efficiency of the plate wave energy converter is determined experimentally. The length of the plate L=1 m, the water depth d=60 cm, the width of the plate b=60 cm and the thickness t=2 cm were held constant through all the experiments. Each experiment set has a total number of 20 different wave properties composed of T=1.16, 1.50, 1.87 and 2.05 s wave periods and H=2, 4, 6, 8 and 10 cm wave height values. The velocity and the wave length of the water flow occuring below the plate were measured for several conditions such as: 1. the plate only, 2. the plate and a triangular structure below it, with five different heights, 3. The plate and a vertical wall below it, with two different heights. In this manner, the submerged plate wave energy converter efficiency values were determined for 20 different conditions. It is understood that the efficiency of the submerged plate wave energy converters can reach up to 60% and the existence of a vertical wall below the plate rather than a triangular form is more efficient.     

Design, simulation, and testing of a novel hydraulic power take-off system for the Pelamis wave energy converter

The economic viability of a wave energy converter depends largely on its power take-off system. Active control of the power take-off is necessary to maximise power capture across a range of sea-states and can also improve survivability. The high force, low speed regime of wave energy conversion makes it a suitable application for high-pressure hydraulics.This paper describes the hydraulic power take-off system employed in the Pelamis wave energy converter. The process of the system's development is presented, including simulation and laboratory tests at 1/7th and fullscale. Results of efficiency measurements are also presented.     

离岸摆式波浪能装置基础适用性分析

文章分析了离岸摆式波浪能装置的工作原理及其适用的环境条件,针对目前海洋石油平台几种主要的基础形式,包括桩基础、重力式基础和桶形基础等,分析研究其特点及其适用的土质条件,并就某一站址海域具体的海洋地质情况进行了分析,提出适用于波浪能装置的海底基础形式,计算基础承载力,为后续的实际应用提供理论支持。     

A novel control method to maximize the energy-harvesting capability of an adjustable slope angle wave energy converter

This paper introduces a novel control approach to maximizing the output energy of an adjustable slope angle wave energy converter (ASAWEC) with oil-hydraulic power take-off. Different from typical floating-buoy WECs, the ASAWEC is capable of capturing wave energy from both heave and surge modes of wave motions. For different waves, online determination of the titling angle plays a significant role in optimizing the overall efficiency of the ASAWEC. To enhance this task, the proposed method was developed based on a learning vector quantitative neural network (LVQNN) algorithm. First, the LVQNN-based supervisor controller detects wave conditions and directly produces the optimal titling angles. Second, a so-called efficiency optimization mechanism (EOM) with a secondary controller was designed to regulate automatically the ASAWEC slope angle to the desired value sent from the supervisor controller. A prototype of the ASAWEC was fabricated and a series of simulations and experiments was performed to train the supervisor controller and validate the effectiveness of the proposed control approach with regular waves. The results indicated that the system could reach the optimal angle within 2s and subsequently, the output energy could be maximized. Compared to the performance of a system with a vertically fixed slope angle, an increase of 5% in the overall efficiency was achieved. In addition, simulations of the controlled system were performed with irregular waves to confirm the applicability of the proposed approach in practice.     

Nonlinear 2D analysis of the efficiency of fixed Oscillating Water Column wave energy converters

This paper reports on the development of a two-dimensional, fully nonlinear Computational Fluid Dynamics (CFD) model to analyse the efficiency of fixed Oscillating Water Column (OWC) Wave Energy Conversion (WEC) devices with linear power take off systems. The model was validated against previous experimental, analytical and numerical results of others. In particular, the simulation results show excellent agreement with the analytical results obtained by Sarmento and Falcão [1] for linear waves in a 2D channel and with previous experiments by others on the interaction between nonlinear waves and a fixed barge. Results are presented for linear waves on the influence of the seaward wall draft and thickness of the OWC device on the resonant frequency and the capture efficiency of the OWC. The key outcome of the present work is that for fully nonlinear waves a substantial decrease in the hydrodynamic capture efficiency of the OWC device was observed with increasing wave height, which represents a significant departure from the linear wave case. The optimal pneumatic damping coefficient for the OWC was also found to be dependent on the wave height. By analysing the magnitude of the first and higher order components of the incident nonlinear waves and the response of the OWC it was found that the first order capture efficiency decreases with increasing wave height, which in turn implies that the OWC hydrodynamic system is fully nonlinear and that the behaviour of an OWC in a nonlinear wave train cannot be accurately represented by the superposition of the linear response to a number of component linear waves. These results have significant implications for the design and operation of practical OWC systems.     

Assessments of wave energy in the Bohai Sea,China

Wave fields in the Bohai Sea are continuously simulated by the third-generation wave model SWAN in order to determine the wave energy resources from 1985 to 2010. The wind parameters used to simulate waves are obtained by the Regional Atmospheric Modeling System (RAMS). Comparisons of significant wave heights between simulations and observations show good agreement. The spatial distributions of mean monthly and annual averaged significant wave height and wave power flux are presented. Wave energy roses and temporal variations of average wave power density at five typical points in the Bohai Sea are calculated. Furthermore, the correlations between significant wave height and wave energy period are studied in scatter and energy diagrams.     

The impact of wave energy farms in the shoreline wave climate: Portuguese pilot zone case study using Pelamis energy wave devices

This paper describes the study of the impact of energy absorption by wave farms on the nearshore wave climate and, in special, the influence of the incident wave conditions and the number and position of the wave farms, on the nearshore wave characteristics is studied and discussed. The study was applied to the maritime zone at the West coast off Portugal, namely in front of São Pedro de Moel, where it is foreseen the deployment of offshore wave energy prototypes and farms between the 30 m and 90 m bathymetric lines, with an area of 320 Km². In this study the REFDIF model was adapted in order to model the energy extraction by wave farms. Three different sinusoidal incident wave conditions were considered. Five different wave farm configurations, varying the position of the wave farm, its number and the width of the navigation channels at each wave farm were analysed. The results for each configuration in terms of the change of the wave characteristics (wave height and wave direction) at the nearshore are presented, compared and discussed for three representative wave conditions.     

A comparison of two meshing schemes for CFD analysis of the impulse turbine for wave energy applications

This paper presents the comparison of a three-dimensional Computational Fluid Dynamics (CFD) analysis with empirical performance data of a 0.6 m Impulse Turbine with Fixed Guide Vanes used for wave energy power conversion. Pro-Engineer, Gambit and Fluent 6 were used to create a 3-D model of the turbine. A hybrid meshing scheme was used with hexahedral cells in the near blade region and tetrahedral and pyramid cells in the rest of the domain. The turbine has a hub-to-tip ratio of 0.6 and results were obtained over a wide range of flow coefficients. Satisfactory agreement was obtained with experimental results. The model yielded a maximum efficiency of approximately 54% as compared to a maximum efficiency of around 49% from experiment. A degree of insight into flow behaviour, not possible with experiment, was obtained. Sizeable areas of separation on the pressure side of the rotor blade were identified toward the tip. The aim of the work is to benchmark the CFD results with experimental data and to investigate the performance of the turbine using CFD and to with a view to integrating CFD into the design process.     

Decadal variability of wave power production in the North-East Atlantic and North Sea for the M4 machine

We look at the variability of the power produced by the three-float M4 wave energy converter for locations in the North-East Atlantic and North Sea using the NORA10 hindcast data from 1958−2011. The aim is to investigate whether the produced power is also strongly affected by the climate variability (such as the North Atlantic Oscillations) in the winter, just as the ocean wave power resource as observed in previous studies. In this study, we demonstrate the use of proxy indices in combination with the climate indices to reconstruct a historic practical wave power climate from 1665−2005. We also conduct sensitivity studies to assess the changes in the practical wave power variability in response to perturbing the machine size, the power take-off coefficient, the response bandwidth and the power limit of the power take off. We find that the resultant temporal variation is still dominated by the climate variability. However, the overall variability important for power availability and energy supply economics is smaller than that of the ocean wave power resource because of the finite capture bandwidth of the M4 machine. The statistical methodology presented here is also potentially relevant to other wave energy converters in similar locations.     

Computed effects of tip clearance on performance of impulse turbine for wave energy conversion

This paper depicts numerical analysis on Impulse turbine with fixed guide vanes for wave energy conversion. From the previous investigations, it is found that one of the reasons for the mismatch between computed and experimental data is due to neglecting tip clearance ef fect. Hence, a 3-D model with tip clearance has been generated to predict the internal flow and performance of the turbine. As a result, it is found that the comparison between computed and experimental data is good, quantitatively and qualitatively. Computation has been carried out for various tip clearances to understand the physics of tip leakage flow and effect of tip clearance on performance of such unconventional turbine. It is predicted that the turbine with 0.25% tip clearance performs almost similar to the case of without tip clearance for the entire flow coefficients. The designed value of 1% tip clearance has been validated numerically and computed that the efficiency of the turbine has been reduced around 4%, due to tip clearance flow at higher flow coefficients.     

Multi-criteria evaluation of wave energy projects on the south-east Australian coast

In the last decade, multiple studies focusing on national-scale assessments of the ocean wave energy resource in Australia identified the Southern Margin to be one of the most energetic areas worldwide suitable for the extraction of wave energy for electricity production. While several companies have deployed single unit devices, the next phase of development will most likely be the deployment of parks with dozens of units, introducing the risk of conflicts within the marine space.This paper presents a geo-spatial multi-criteria evaluation approach to identify optimal locations to deploy a wave energy farm while minimizing potential conflicts with other coastal and offshore users. The methodology presented is based around five major criteria: ocean wave climatology, nature of the seabed, distance to key infrastructure, environmental factors and potential conflict with other users such as shipping and fisheries.A case study is presented for an area off the south-east Australian coast using a total of 18 physical, environmental and socio-economic parameters. The spatial restrictions associated with environmental factors, wave climate, as well as conflict of use, resulted in an overall exclusion of 20% of the study area. Highly suitable areas identified ranged between 11 and 34% of the study area based on scenarios with varying criteria weighting. By spatially comparing different scenarios we identified persistence of a highly suitable area of 700 km² off the coast of Portland across all model domains investigated. We demonstrate the value of incorporation spatial information at the scale relevant to resource exploitation when examining multiple criteria for optimal site selection of Wave Energy Converters over broad geographic regions.     

10-year high resolution study of wind,sea waves and wave energy assessment in the Greek offshore areas

Nowadays, renewable energy resources are one of the top priority issues for the environmental and political community. In particular, wind and wave energy are two of the most promising solutions, with great potential from research and technological point of view. In this work, an integrated high resolution platform, consisting of state-of-the-art wind-wave numerical models, has been utilized and produced a 10-year database containing all the relevant environmental parameters for a detailed resource assessment over the Greek seas. The results of the atmospheric and sea wave numerical models concerning the environmental parameters that directly affect the wave energy potential were evaluated. High resolution maps for the coastal and offshore areas of Greece present sea wave and wind climatological characteristics, as well as the relevant distribution of the wave energy potential. A number of statistical indices have been employed for analyzing the output of the models, including the potential impact of extreme values and the corresponding distribution of the above parameters, which optimally describe the spatial and temporal analysis of the wave power potential over the area of interest. It is shown that the regions with increased wave energy potential are mainly the western and southern seas of Greece, which are usually exposed to swell from central and south Mediterranean Sea.     

Thermodynamic modeling of partially stratified charge engine characteristics for hydrogen-methane blends at ultra-lean conditions

A thermodynamic model considering flame propagation is presented to predict SI engine characteristics for hydrogen-methane blends. The partially charge stratification approach which involves micro direct injection of pure fuel or a fuel–air mixture, to create a rich zone near the spark plug, is proposed as a method to improve engine performance. Presented approach was validated with experimental data for the natural gas at lean condition. The model was generalized to predict the performance of engine for a variety of hydrogen contents in hydrogen-methane blends. Hydrogen molar concentrations of 0%, 15%, 30%, and 45% were used in the simulations. Results showed that partially charge stratification improves engine performance by increasing indicated mean effective pressure and decreasing specific fuel consumption. The results indicated that increasing mole fraction of hydrogen content would improve the PSC effect on engine performance. An advantage of the presented model is the flexibility and simplicity that make it possible to investigate several effects such as mixture distribution and fuel constituents on engine performance more practical than other types of simulation.