Wave energy considerations for predicting average overtopping discharges of depth-limited waves breaking onto rock-armoured coastal revetments

Many countries have enacted laws requiring the increased use of and research into renewable energy sources. The push to increase the use of renewable energy is directly related to the push to decrease the amount of greenhouse gases being released into the atmosphere. Further, there is a desire to increase the amount of renewable energy and rely less on imported energy, such oil and gas. Wave energy has been identified as a potential source for this renewable energy. The development of wave energy converters (WEC) is still in its infancy. There are over 60 concepts that are in different stages of development. These range from concepts to full-size prototype devices and early commercial units. Each WEC has a different method of operation, resulting in different amounts of power production in different wave patterns. The establishment of standards is important to industry development. Different sectors of the industry need a uniform method to evaluate both a WEC and the location for installation. Investors are interested in the power capabilities, power utilities need to look at the grid connection and interface, and insurance companies have to evaluate safe operation. To address these needs, the wave energy industry approached the International Electrotechnical Commission (IEC) to develop a uniform set of standards for marine energy converters. Wave and tidal converters standards are currently being written, although other forms of renewable energy, such as ocean thermal energy conversion (OTEC), could be included within these standards. The standards under development are for the appraisal of converters, resource evaluation, and design. The first standard1 under development is the Appraisal of Wave Energy Converters. The Project Team 62600–100 is chaired by the United Kingdom and has ten member-countries supporting the development of the standard. Each country has several experts as part of the working group. The standard has been divided into sections, with each country responsible for a section. The United States is supported through the American National Standards Institute (ANSI). Multiple companies, government agencies, and academic partners are voluntarily assisting with their time and expert knowledge in the development of this standard for the United States. The process of developing the first standard is well underway and the issue of this initial standard is anticipated in 2011. The purpose of the standard is to provide a standard methodology of appraising a post-prototype WEC with the creating of a power matrix. The power matrix will be based on wave height and period at the site. These parameters are critical since electrical power from the WEC is measured in terms of each wave height and period. The standard will be written to support testing at various locations. This requires the definitions of test sites and measurement systems. Critical information for testing a WEC, such as wave patterns, wave height, wave period, bottom features, and bathymetrie data, will be clearly defined within the standard so comparisons can be readily made across the wide range of devices. The uniform understanding of the wave pattern, including the wave front direction, height, and period, are needed to predict the available wave power and the subsequent development of an accurate power matrix. This paper looks at the development of a methodology for appraising a WEC using the IEC standard. To date, the WEC standard has gone through several revisions for both methodology and prescriptive changes. Each change requires the experts to understand and agree to the change. This process of standard development and other considerations are discussed.

The average overtopping discharge is an important parameter for the design of flood defences. Several empirical formulas are available for predicting the overtopping discharge at dikes. However, these empirical formulas often have their specific applicable conditions. To complement with the empirical methods, a numerical model has been developed using the open source CFD package OpenFOAM to model the wave overtopping at dikes. Systematic calibration and validation of the numerical model are performed. The influences of the mesh, solver, turbulence model and roughness height on the modelled results of the average overtopping discharge have been investigated during the model calibration. The simulations show that the turbulence model increases the accuracy of the numerical model for predicting the average overtopping discharge under wave breaking conditions. The calibrated model is then validated by comparing the modelled average overtopping discharges with the measured ones from the physical model tests. Results show that the OpenFOAM model is capable of predicting the average overtopping discharge accurately at dikes that have a smooth straight waterside slope.

Hybrid deep learning model for wave height prediction in Australia's wave energy region

Many sea defense structures need to be adapted to the rising sea water level and changing wave climate due to global warming. The accordingly required investments open perspectives for wave energy converters (WECs) — that are built as part of the sea defense structures — to become economically viable. In this paper the average overtopping discharges q of overtopping wave energy converters built in sea defense structures are studied. Physical model tests with this type of devices have been carried out in a wave flume leading to experimentally determined values for the average overtopping discharge q. These experimental data are compared with predicted average overtopping discharges using existing empirical formulae from literature — derived mainly for sea defense structures. Overtopping WECs have small relative crest freeboard heights and smooth slopes to maximize overtopping, which is contradictive to the basic role of sea defense structures. As a consequence, the experimentally achieved average overtopping discharges are situated in a range that is not well covered by the existing traditional prediction formulae. The presented results for linear-slope overtopping WECs fill the gap between those for smooth dikes and those for plain vertical walls. The overtopping behavior in that particular range is discussed in this paper.

The objective of this study is to investigate the influence of damping from the PTO device on the cumulative output of the oscillating water column wave energy converter under real sea conditions which encompasses wide range of wave period and height. In this presentation, a time domain dynamic simulation model of OWC motion is developed and validated with water tank test results. Then the model is used to calculate a wide range of wave period and height. Finally, annual cumulative air power output of OWC is calculated with different damping values. In the water tank experiment, a cylindrical oscillating water column with a diameter of 0.3 m and submerged depth of 0.2 m is tested. PTO damping was emulated by using several orifice plates. Since the orifice pressure is proportional to square of flow-rate of the orifice, In the simulation, a model was constructed to solve the dynamic equation of motion assuming water column as a rigid equivalent floating body. Validation showed the model captures the influence of PTO damping as observed in the water tank testing. Using simulation, output air power from real scale OWC was evaluated under wide range of inlet wave period and height. With the output power database from dynamic simulation and frequency of wave height and period at specific sites in Japan (Fukui and Kamaishi), annual cumulative output power is calculated. From the results, it was confirmed that a higher output can be obtained with higher energy in high waves by adopting a damping characteristic that increases the efficiency under a wide wave condition rather than a nozzle characteristic that achieves maximum efficiency in the resonance period. Furthermore, it became clear that the damping that increases the maximum efficiency does not necessarily increase the accumulated energy. When considering operation in real sea condition in the future, it is not always effective to select PTO damping that maximizes the output at specific wave height or period. And it is important to adopt a method that can estimate wide range of wave condition and evaluate the cumulative output power.

In order to develop green energy, reduce carbon emissions, and alleviate global warming and the green energy crisis, many researchers focus on wave energy, using a device to convert wave energy into electricity. The three main types of wave energy converters are the overtopping type, the oscillating water column type, and the oscillating body type, and for most of them, the power generation efficiency is low. The research team in this paper proposed a wave energy converter for a wave-induced oscillation heave plate. The plate vibrates up and down under the action of waves, and the captured energy of the vibrating plate transfers the energy to the generator, so as to generate electricity. There is electricity only when there is vibration; therefore, the vibration characteristic of the converter is crucial to power generation. So, the vibration characteristics of the energy capture structure of the converter were studied experimentally. The test results show that the energy harvesting device can vibrate, and the vibration effect is good, which further indicates that the device can generate electricity. The effects of different wave conditions and system stiffnesses on amplitude and corresponding amplitude were studied, and the amplitude increases with the increase in wave height and period and decreases with the increase in system stiffness. The amplitude response decreases with the increase in wave height and system stiffness. Under the test conditions, the maximum amplitude of the system is 6.23 cm (when the wave period is 1.40 s, the wave height is 0.25 m, and the system stiffness is 1735.62 N/m), and the maximum amplitude ratio is 0.34 (when the wave period is 1.1 s, the wave height is 0.10 m, and the system stiffness is 1735.62 N/m).

The average overtopping discharge is an important parameter for the design and reinforcement of dikes. Rock armour on the waterside slopes and berms of dikes is widely used to reduce the wave overtopping discharge by introducing slope roughness and dissipation of energy in the permeable armour layer. However, methods for estimating the influence of a rock berm and roughness of rock armour at dikes on the average overtopping discharge still need to be developed and/or validated. Therefore, this study aims to develop empirical equations to quantify the reductive influence of rock armour on wave overtopping at dikes. Empirical equations for estimating the effects of rock berms and roughness are derived based on the analysis of experimental data from new physical model tests. The influence of roughness of the rock armour applied on parts of waterside slopes is estimated by introducing the location weighting coefficients. Results show that the newly derived equations to predict the average overtopping discharge at dikes lead to a significantly better performance within the tested ranges compared to existing empirical equations.

In the framework of the ComCoast project, the concept of the Crest Drainage Dike has been studied regarding the reduction of wave overtopping. This study only focuses on the average wave overtopping discharge. The basic concept of the Crest Drainage Dike is a basin, integrated in the crest of the dike, that collects overtopping water and thus reduces the load on the inner slope of the dike. The collected water in the crest basin is drained landward or seaward through pipes. The main goal of this report is to identify the physical background of the concept of the Crest Drainage Dike and to predict the wave overtopping discharge as a function of hydraulic and geometric boundary conditions. Therefore two different types of theoretical studies have been executed. The first study is process-based and serves as a basis for the numerical program that has been developed. Since this model is partly based on several assumptions, several physical model tests have been executed to verify or reject the stated hypotheses. In the physical model tests, several hydraulic and geometric boundary conditions, such as the wave height, the crest freeboard, the use of berms, the wave spectra, the wave steepness and the drain layouts, have been varied. Since the predictions of the numerical program are well in line with the measured wave overtopping discharges, the numerical program is used to investigate the use of a Crest Drainage Dike in two case studies. The case studies are the Hondsbossche Sea Defence and the Perkpolder Sea Defence. Both dikes are located in the Netherlands The use of this numerical program gives a better insight in the physical background of the Crest Drainage Dike. The description of these physics is the second part of the theoretical study. For dikes with severe wave attack, such as the Hondsbossche Sea Defence, only a small fraction of the waves is reaching the crest of the dike. However, the waves that do reach the crest of the dike have a relatively large volume and the buffer capacity of the Crest Drainage Dike limits the effectiveness of the Crest Drainage Dike. Besides this, there is a high statistical uncertainty since the average wave overtopping discharges are determined by only a couple of waves. For dikes with a lower wave attack, such as the Perkpolder Sea Defence, more waves with a lower volume per wave are overtopping and therefore the concept of the Crest Drainage Dike works well. However, the crest freeboard reduction with the use of the Crest Drainage Dike is in these specific cases is only minor. Based on the numerical studies and the current Dutch overtopping criteria, the reduction of the crest freeboard with the use of the Crest Drainage Dike is determined and is significantly lower then the assumed reductions in earlier studies.

Wave energy potential along the southern coast of the Caspian Sea

The study aims to assess the feasibility of harnessing wave energy from the low‐energy waves prevalent along the east coast of India, where wave energy converters (WECs) may not produce sufficient power economically. Vishakhapatnam port, located on the east coast, experiences such waves (period, Tp = 5 to 10 s, and wave height, Hs = 0.07 to 2.61 m). This study analyzes the performance of a two‐body point absorber WEC and estimates the wave power using the parameters Hs and Tp. The hydrodynamic parameters were assessed in the frequency domain using the boundary element method, while the output power was estimated in the time domain using the open‐source code WEC‐Sim. The harvested power initially increased with the wave period, peaked at Tp = 7 s and Hs = 2 m, and subsequently decreased. This study highlights the potential of wave energy as a renewable resource for India, given its extensive coastline and untapped wave energy potential. It suggests that green energy firms should explore opportunities in regions abundant with wave energy to promote sustainable energy production and drive innovation in wave energy technology, thereby indicating avenues for future research in this domain.

Mid-twenty-first century global wave climate projections: Results from a dynamic CMIP5 based ensemble

Although theoretical available wave energy is higher than most of ocean energy sources, the commercial utilization of wave energy is much slower than other ocean energy sources. The difficulty of integration with the electrical grid system and the challenges of the installation, operation and maintenance of large energy generation and transmission systems are the major reasons. Even though there are successfully tested models of wave energy converters, the fact that wave energy is directly affected by wave height and wave period makes the actual wave energy output with high variation and difficult to be predicted. And most of the previous studies on wave energy and its utilization have focused on the large scale energy production that can be integrated into a power grid system. In this paper, the authors identify and discuss stand-alone wave energy converter systems and facilities that are not connected to the electricity grid with focus on small scale wave energy systems as potential source of energy. For the proper identification, qualification and quantification of wave energy resource potential, wave properties such as wave height and period need to be characterized. This is used to properly determine and predict the probability of the occurrence of these wave properties at particular locations, which enables the choice of product design, installation, operation and maintenance to effectively capture wave energy. Meanwhile, the present technologies available for wave energy converters can be limited by location (offshore, nearshore or shoreline). Therefore, the potential applications of small scale stand-alone wave energy converter are influenced by the demand, location of the need and the appropriate technology to meet the identified needs. The paper discusses the identification of wave energy resource potentials, the location and appropriate technology suitable for small scale wave energy converter. Two simplified wave energy converter designs are created and simulated under real wave condition in order to estimate the energy production of each design.

<p indent=0mm>Coral reefs are widely distributed in the South China Sea, and its topography is different from the gentle slope of the coastal continental shelf, which has a great impact on wave propagation and deformation. At the same time, reefs are located in a harsh marine environment, so these factors having a profound effect on the stability of the backfilled sand island and the effective operation of other ancillary facilities. Seawall is an important protective structure in the protection works of reefs, and the wave overtopping of seawall plays a decisive role in the engineering safety of reefs. At present, there are few studies on the wave overtopping of reefs around the world, and the existing formula for calculating wave overtopping discharges over seawall on gentle slope terrain is not applicable to reefs terrain, so detailed research on wave overtopping of a vertical seawall on coral reefs is needed. A series of experiments are carried out in the Large Wave Flume (LWF) of Tianjin Research Institute of Water Transport Engineering (TIWTE), which is <sc>456 m</sc> long, <sc>12 m</sc> deep and <sc>5 m</sc> wide. The maximum wave height generated by LWF is 3.5 m, and the wave period range is from 2 to <sc>10 s.</sc> The model layout includes 3 parts: Deep-water zone, steep slope zone and platform zone. The depth of the deep-water zone is <sc>5 m.</sc> The front slope is divided into 3 sections with slope 1:1, 1:4 and 1:15. The steep slope toe is <sc>330 m</sc> away from the wave generator. The strongly non-linear physical process of wave transmission and breaking can be reproduced by this physical model. Based on Froude similarity criterion and a 1:15 scale model, the propagation and deformation process of regular waves on reef-top with seawall is analyzed. The effects of wave height, wave period, water depth and the distance between seawall and reef-edge on the mean overtopping discharge are studied. The results show that the mean overtopping discharge increases significantly with increasing of wave height and period, while it decreases with decreasing of water depth. However, the mean overtopping discharge is not the monotone function with distance. Mostly the mean overtopping discharge decreases with increasing distance. When wave period is longer than <sc>13.56 s,</sc> the mean overtopping discharge decreases firstly, then increases a little, finally decreases with increasing distance. The dimensionless mean wave overtopping discharge increases as the wave steepness decreases for given wave height. The dimensionless mean wave overtopping discharge is found to be functions of relative freeboard height, the dimensionless mean wave overtopping discharge decreases as relative freeboard height increases for given wave conditions. On the basis of the experimental data, a formula considering the effects of wave height, wave period, water depth and the distance between seawall and reef-edge on the wave overtopping, is presented to estimate the mean wave overtopping discharge of a vertical seawall on coral reefs.

Short-term density forecasting of wave energy using ARMA-GARCH models and kernel density estimation

The activities in the Gulf of Mexico are thriving with a considerable number of oil and gas platforms operating in this area. Wave power can provide a substantial portion of clean power to substitute for the inefficient gas turbines used on the platforms. Wave power density (wave energy flux) is one of the ways for directly assessing the potential and available wave power. The task of a wave energy capture device is to effectively capture the wave energy flux. The average wave power density is generally between 20 kW/m and 65 kW/m in the Gulf of Mexico. During tropical storms or hurricanes, the theoretical wave power density could reach 1,600 kW/m. However, higher wave height and period could damage the energy capture devices. This project explores the correlation pattern between available wave power density hotspots and hurricane-induced wave distribution within 10 years (2010-2019) using a deep convolutional neural network. The correlation pattern was explored and validated by applying meteorological data in the Gulf of Mexico area from the NOAA WAVEWATCH III system. A wave power hotspot was defined as an event when wave power density over a certain threshold during a period of time in a predefined location. The distribution of wave power hotspots has the segmentation threshold on wave power depending on the wave energy converter, location, wave height and wave period. The meteorological data from WAVEWATCH III was visualized as images of wave power density in 3-hour time steps, which were used for wave power density hotspot identification, classification and localization through a deep convolutional neural network. With the distribution of wave power density hotspots along with the coordinates attached through matching the local wave and hurricane-induced wave conditions and the GIS mapping support, the project was able to reveal the impacts of hurricanes based on pattern recognition between the wave power hotspots distribution and hurricane-induced wave track distribution. Meanwhile, the comparison of the wave power density hotspot distribution with extreme weather conditions and under the regular circumstances was analyzed. The results could also provide feedback to and support wave energy harvesting or hurricane forecasting and harvesting.