Overtopping Wave Energy Converter Research Articles

Optimal Design of the Overtopping Wave Energy Converter Based on Fluid–Structure Interaction Simulation

Optimal Design of the Overtopping Wave Energy Converter Based on Fluid–Structure Interaction Simulation

Study with Large Eddy Simulations of energy dissipation due to backwash flows in wave overtopping.

Renewable energy market growth is imperative to reach a sustainable society. This necessity demands improving existing methodologies to exploit green energy sources and explore new technologies. Wave energy is one of the sources with the larger offer in power available in the oceans to be harvested; however, the remoteness and extreme conditions of the best locations to harvest wave energy have made the development of wave energy converters expensive and less attractive than other green energy sources that operate on land. Overtopping wave energy converters (OWEC) operate on the shore, facilitating their construction and maintenance. They can be integrated into coastal defences, sharing the cost of construction and their environmental impact and creating new benefits from the coastal defence. The effectiveness of an OWEC relies strongly on their ability to capture wave-overtopping volumes. The geometry of the run-up ramp (slope and shape) plays an important role in the efficiency of an OWEC. An optimum geometry for each location and wave condition needs to be found to maximise the operation of any OWEC. The efficiency of these devices, quantified by the hydraulic efficiency, must be improved to make them an attractive solution for energy conversion. Three distinguishable processes occurred on the ramp of an OWEC; the wave collapse, run-up and run-down (backwash). The first two have attracted more attention from researchers, specially oriented to the design of coastal defences, but fewer studies have analysed the impact of the backwash on the energy losses during the collapse of the waves. When an incoming wave approaches the run-up ramp, it crashes against the backwash, composed of the water volume that didn't overtop the structure in the previous wave. These interactions increase turbulences and, therefore, energy dissipation, affecting the performance of an OWEC. This study analyses the hydrodynamics during the backwash flows' and incoming waves' interaction on sloped structures. A numerical code Hydro3D, based on Large Eddy Simulations, is used to conduct the present study. As a reference condition, a structure with a freeboard high enough to not allow wave overtopping is tested; in this case, all the water that run-up the slope runs down after the water tongue reaches its highest elevation. Structures with lower freeboards are tested to allow different wave overtopping levels and backwash flows. The impact of different backwash flow conditions on energy dissipation is then evaluated. The founding of this study allows us to verify if a reduction in the backwash flows can improve the efficiency of a OWEC.

Numerical Study on Overtopping Performance of Multi-stage Overtopping Wave Energy Converters

The characteristics of wave conditions with large tidal ranges and small wave heights in China's coastal waters limit the operation time of traditional single-stage overtopping devices, while the multi-stage overtopping wave energy converters can increase the overall operation time of the device by distributing the upper and lower reservoirs. To study the overtopping performance of the multi-stage overtopping wave energy converter under real sea conditions, a two-dimensional numerical model of the device is developed and verified by physical model tests. The effects of the lower reservoir opening width, slope angle combination, and slope inundation length on the overtopping performance are investigated under regular wave conditions. It is found that a smaller opening width and two 30° slope angles improve the overtopping performance of the device, while the slope inundation depth has less effect. Further, based on the full-scale prototype device, the database of overtopping in a complete tidal cycle (12h) were constructed under real sea conditions in different seasons to provide the basic data for the Wave-to-wire model of power generation simulation.

Technology, Geometry, Performance and Challenges in Wave Energy Converters

Recently, the worldwide energy scarcity raised the demand for integrated renewable energy in the modern grid. Considering the potential and ocean areas, wave energy-based power generation becomes an essential sustainable source for industry and academia. Notably, the wave energy converter (WEC) is the most common solution for extracting power from wave energy and convert into electricity. Wave electricity generation is the main method of using wave energy, and wave energy converters that are the core of wave energy technology have been widely and rapidly evolved. In this paper, the state-of-art WECs are reviewed with the classification based on different ways and different principle of operation of capturing energy of waves, including oscillating water column WEC (OWC-WEC), oscillating buoy WEC (OB-WEC) and overtopping WECs (O-WEC) are systematically reviewed in terms of operation principles, geometric structure and power take-off (PTO) strategies. Additionally, the energy conversion efficiency between different technologies is compared and summarized. The challenges and optimization aspects are delivered.

Numerical Analysis of the Available Power in an Overtopping Wave Energy Converter Subjected to a Sea State of the Coastal Region of Tramandaí, Brazil

The present work proposes a numerical study of an overtopping wave energy converter. The goal of this study is to evaluate the theoretical power that can be converted by an overtopping device subjected to sea waves in the coastal region of Tramandaí, Brazil. For this, realistic irregular waves were generated using the WaveMIMO methodology, which allows numerical simulation of sea waves through the imposition of transient discrete data as prescribed velocity. For the numerical analysis, a two-dimensional computational model was employed using Fluent, where the device was inserted into a wave channel. The volume of the fluid multiphase model was used for the treatment of the air–water interaction. The results indicated that the free surface elevation obtained using the WaveMIMO methodology, which converts a realistic sea state into a free surface elevation series, was adequately represented. The evaluation of the theoretical power of the overtopping device during around 45 min indicated that 471.28 W was obtained. In addition, a monthly generation projection showed that this device would supply 100% of the electricity demand of a school in the city of Tramandaí. These results demonstrated that the conversion of sea wave energy into electrical energy can contribute to supplying electricity demand, especially for coastal cities.

Constructal Design of a Two Ramps Overtopping Wave Energy Converter Integrated into a Breakwater: Effect of the Vertical Distance between the Ramps over its Performance

A two-dimensional numerical study about the influence of a vertical distance between two ramps of an Overtopping Device Wave Energy Converter (OTD-WEC) integrated into a breakwater in the city of São José do Norte, Rio Grande do Sul, Brazil was analyzed. The main purpose was to evaluate the influence of the vertical distance between the two ramps (Hg) of OTD-WEC, on the average overtopping dimensionless flow () using the Constructal Design for the geometric evaluation defining: 1) degree of freedom, (Hg), and 2) constraints, horizontal distance between the ramps (Lg), ratio between the height and length of the ramps (H1/L1 and H2/L2), area as a function of the wave parameters (Awave), areas of the ramps (Ar,i), maximum ramp height (fixed as half of the significant wave height (HS/2) at the MWL) and area fractions of the ramps (φi). The equations of conservation of mass, momentum, and an equation for the transport of volumetric fraction were solved using the Finite Volume Method (FVM). The multiphase model Volume of Fluid (VOF) was applied for the air-water interaction. The results showed that, in general, lower values of the vertical distance between the ramps (Hg) led to higher values of the average overtopping dimensionless flow (). Moreover, the geometric evaluation of the degree of freedom Hg through the Constructal Design method proved to be an important tool because some configuration of the ramps of the overtopping device facilitated the flow of water to the reservoir of the device, and others made it difficult. The maximum value of the average overtopping dimensionless flow was max = 0.044, with a difference of 2.23% for the value obtained with empirical equation found in the literature, for the vertical distance equal to Hg = 0.10.

Advanced energy flexibility enhancement via the novel resources of wave energy converter reservoirs and electric storages for a hybrid wave-wind energy supported hotel energy system

Renewable resources are fluctuating and uncontrollable. The susceptible and intermittent generation not only causes the mismatch between the demand and supply but also brings grid pressure. Therefore, several measures were conducted on a coastal building energy system to enhance energy flexibility. From the aspects of operational cost, net present value, peak-shaving, valley-filling, and on-site electrical energy matching/fraction, a series of non-renewable and hybrid wave-wind-based renewable energy cases were studied to investigate the impact of the flexibility enhancement measures. Based on a specific electricity pricing scheme considering the monthly maximum power and the on-peak/off-peak energy demand, first, utilised the battery as a flexibility source to adjust the grid's interactions. It showed superior peak shaving capability during the on-peak period reaching 100%. Second, the battery control after the load which was partially covered by hybrid wind–wave energy reduced about 1% of operational cost. Generally, the battery control helped in various aspects whereas diminished the net present value due to high investment. Third, using the inherently integrated reservoir of the overtopping wave energy converter, the flexibility of the seawater discharging control like a reservoir-type power plant was investigated. The novel reservoir control presented outstanding comprehensive performance and feasibility. It enhanced the energy matching by 10% and reduced the operational cost to 83.99% compared with the case without employing control, bringing an extra 40% increment of relative net present value.

Numerical Analysis of an Overtopping Wave Energy Converter Subjected to the Incidence of Irregular and Regular Waves from Realistic Sea States

The present study aims to evaluate the difference in the fluid-dynamic behavior of an overtopping wave energy converter under the incidence of irregular waves based on a realistic sea state when compared to the incidence of regular waves, representative of this sea state. Thus, the sea data of three regions from the Rio Grande do Sul coast, Brazil, were considered. Fluent software was employed for the computational modeling, which is based on the finite volume method (FVM). The numerical generation of waves occurred through the imposition of the velocity boundary conditions using transient discrete values through the WaveMIMO methodology. The volume of fluid (VOF) multiphase model was applied to treat the water–air interaction. The results for the water amount accumulated in the device reservoir showed that the fluid-dynamic behavior of the overtopping converter has significant differences when comparing the two proposed approaches. Differences up to 240% were found for the water mass accumulated in the overtopping device reservoir, showing evidence that the results can be overestimated when the overtopping device is analyzed under the incidence of the representative regular waves. Furthermore, for all studied cases, it was possible to approximate the water volume accumulated over time in the overtopping reservoir through a first-degree polynomial function.

Constructal Design of an Overtopping Wave Energy Converter Incorporated in a Breakwater

A numerical study was performed in the present work about an Overtopping Device Wave Energy Converter (OTD-WEC) with one and two ramps incorporated in a real breakwater. The Constructal Design method was applied to evaluate the effects on the average dimensionless overtopping flow of the degrees of freedom of the device with one and two ramps. In addition, a comparison was carried out among the different geometry configurations of the OTD-WEC to determine which one presents the best hydrodynamic performance. The work used the JONSWAP spectrum and the multiphase Volume of Fluid model. It also solved the conservation equations for mass, momentum, and an equation for the transport of the volume fraction using the Finite Volume Method. Results showed that a device with a two-ramps configuration presented an average dimensionless overtopping flow 6.48% higher than those obtained for the one ramp. Present results obtained using Constructal Design theoretical recommendations about the influence of a complex configuration with four degrees of freedom over the performance of an OTD-WEC integrated into the east breakwater of the city of São José do Norte, State of Rio Grande do Sul (RS), Brazil.

The Effect of Hydraulic Efficiency on the Design Variables of an Overtopping Wave Energy Converter

The Effect of Hydraulic Efficiency on the Design Variables of an Overtopping Wave Energy Converter

Three-dimensional effects on the performance of multi-level overtopping wave energy converter

The performance of a multi-level overtopping wave energy converter has been numerically investigated in a three-dimensional wave tank. The device has three extra slots and is positioned on a breakwater. The span length of the breakwater is the main parameter associated with the three-dimensional effects on the hydraulic efficiency of the wave energy device. It has been shown that the device with a finite span yields a lower captured crest energy due to waves falling-down the edges during the run-up process. The result also implies that as the span of the device is increased, the efficiency tends to increase further to resemble the two-dimensional device. The three-dimensional mechanism has a significant influence on the potential energy in the water being stored by the higher reservoirs, while that of lower ones have relatively smaller effects. In addition, a relation between hydraulic efficiencies of two and three-dimensional devices has been proposed.

Bibliometric Study Applied to the Overtopping Wave Energy Converter Device

The present study aims to analyze the state of the art of scientific studies about the Overtopping device used to convert sea wave energy into electrical energy, by means the Bibliometric methodology. The development of this study took place through the selection of articles from conference proceedings, as well as national and international journals. The Bibliometric methodology consists of a statistical tool that allows quantifying the measurement of production indexes. Using selected keywords, it was conducted a survey of studies in the online databases of Science Direct, SciELO and Google Scholar. The works found then went through a filtering process, in order to limit the Bibliometric study only to studies about Overtopping devices as sea Wave Energy Converter (WEC). Finally, the investigation of these selected articles was carried out under the optics of production and authorship study, content study and study of bibliographic references. Where it was identified growth in publications related to the topic, methodologies used and, among other indicators, the authors most cited in the analyzed articles. The predominant keywords used were “Wave Energy Converter” and “Overtopping”. It was noted that Brazilian universities are leaders in the productivity, presenting more than 36% of the scientific production regarding Overtopping WECs.

Numerical and experimental study on hydrodynamic performance of multi-level OWEC

The performance of a multi-level overtopping wave energy converter (OWEC) has been numerically and experimentally investigated in a two-dimensional wave tank in order to study the effects of opening width of additional reservoirs. The device is a fixed OWEC consisting of an inclined ramp together with several reservoirs at different levels. A particle-based numerical simulation utilizing the Lattice Boltzmann Method (LBM) is used to simulate the flow behavior around the OWEC. Additionally, an experimental model is also built and tested in a small wave flume in order to validate the numerical results. A comparison in energy captured performance between single-level and multi-level devices has been proposed using the hydraulic efficiency. The enhancement of power capture performance is accomplished by increasing an overtopping flow rate captured by the extra reservoirs. However, a noticeably large opening of the extra reservoirs can result in a reduction in the power efficiency. The overtopping flow behavior into the reservoirs is also presented and discussed. Moreover, the results of hydrodynamic performance are compared with a similar study, of which a similar tendency is achieved. Nevertheless, the LBM simulations consume less computational time in both pre-processing and calculating phases.

A Simple Model to Assess the Performance of an Overtopping Wave Energy Converter Embedded in a Port Breakwater

A numerical model for the optimization of the performance of an innovative overtopping breakwater for wave energy conversion is proposed. The model is based on the stochastic description of the overtopping phenomenon based on the results of extensive laboratory tests, and it is able to simulate the behavior of the device operating under any assigned sequence of sea states, thus allowing to easily obtain results that would otherwise require time consuming and costly physical model tests. The model is used here to identify the main geometrical parameters affecting the performance of the device and to optimize such parameters in order to maximize the average yearly output power. An application to a device embedded in the breakwater of Pantelleria Port (Sicily, Italy) is presented. The model is also proved to be useful to verify the possibility of further increasing the output power through the implementation of specific control strategies concerning the operation of the turbines. The work provides a better understanding of the influence that such a system could have on the energy system of small Mediterranean islands, for example in terms of contribution to the CO2 emission reduction.

A new average wave overtopping prediction formula with improved accuracy for smooth steep low-crested structures

Wave overtopping is a key process in coastal protection and its assessment defines the design of the sea defence structures. An existing knowledge gap in wave overtopping prediction is identified for steep low-crested structures, i.e., structures with steep slopes up to the limit case of vertical structures, with small relative freeboards down to the limit case of zero freeboards. This type of structure is increasingly relevant in a sea level rise context due to climate change. Additionally, steep low-crested structures are also of interest when used as overtopping wave energy converters. To cover the identified knowledge gap, more than 900 2D hydraulic model tests have been performed in the wave flume of the Department of Civil Engineering at Ghent University. Wave conditions and the overtopping performance have been measured. After analysing the average overtopping rates of the new tests, we found that there is a lack of accuracy of the recommended EurOtop 2018 manual overtopping prediction formulae for steep low-crested structures. Based on the new tests, a new average overtopping prediction formula for steep low-crested structures is obtained. This formula improves the prediction accuracy of the average overtopping rates for steep low-crested structures with respect to the recommended predictions in the EurOtop 2018 manual by reducing the RMSE by 35% for zero freeboards, by 16% for very small relative freeboards, by 31% for very steep slopes and by 24% for vertical structures. The accuracy of the EurOtop 2018 manual predictions for other structural types is maintained.

Parametric analysis of gravity Vortex turbines as a low cost renewable energy alternative from low head hydraulic resources

The long existent worldwide trend for large scale hydroelectric power plants, relying on dams are now under severe criticism for the large areas their reservoirs occupy, which are often fertile agricultural areas and sometimes flood cultural heritage sites. However, there are also environment-friendly alternatives for hydroelectric power production, which are capable to obtain energy from small scale streams with relatively low heads. Such smaller scale sources with low cost facilities can be used for electric production by alternative schemes that use small streams, irrigation canals and divertions from rivers, tidal pools, overtopping wave energy converters and urban wastewater. One of the recent types of such plants are the gravity vortex turbines that use the naturally occurring “sink vortex” draining such water. They are highly efficient and able to obtain energy from sources with flow rates as low as 1 m3/s and heads as low as 0.80 m. Such water sources are abundant in most of the rural areas and it is possible to obtain either an important part or the total need of the energy requirement of the nearby communities with such systems. Gravity vortex turbines have low costs due to their simple structure and are easy to maintain. They can also be implemented for overtopping wave energy and tidal energy systems, as well as recovery units of pumped energy storage schemes. The purpose of this paper is to propose relations for the design and parametric analysis to size the relevant parts of the plant- the pool and the turbine. Potential flow is assumed throughout the analysis. Attempts to obtain optimized relations between the relative sizes and rotational speeds for the pool, water source, turbine are made and inputs for preliminary design are obtained.

Advantages of an innovative vertical breakwater with an overtopping wave energy converter

This paper presents an innovative vertical breakwater cross-section integrating an overtopping wave energy converter, named OBREC-V, and the analysis of its hydraulic performance and stability response to hydraulic loading. The structure is composed of a vertically-faced caisson with a sloping ramp on the top, a reservoir and a set-back crown-wall. The analysis of the structure is carried out by performing numerical simulations based on the Volume-Averaged Reynolds Averaged Navier-Stokes (VARANS) equations. The numerical simulations are performed to compare the performance of a traditional and innovative vertical caissons under the action of irregular waves, in terms of wave reflection, overtopping and wave acting forces. Results show that the reflection coefficients are lower than those computed in front of the traditional breakwater, with a reduction of up to the 40%. New formulations are proposed to better estimate the reflection coefficient and the wave overtopping at the rear side of the structure taking into account the non-conventional geometry of the device. The analysis of the forces indicate that the non-conventional geometry of the innovative OBREC-caisson increases the overall stability of the structure. The values of the safety factor against sliding, Cs, on innovative caissons are similar or greater than those calculated on the traditional vertical structure for almost all the tests. The downward force on the ramp and reservoir and the time lag between the vertical and horizontal forces, lead to a significant reduction of the maximum destabilizing forces Fs in the innovative breakwater, whose values range between 60 and 80% of the ones computed on the traditional structure. The obtained results show the co-benefits, in terms of functionality and hydraulic stability, that an OBREC-V entails with respect to a traditional vertical breakwater.

Effect of wave steepness to relative wave run-up on OWEC breakwater

OWEC (Overtopping Wave Energy Converter) breakwater is a coastal structure model concept. The concept is to integrate the breakwater and the overtopping wave energy converter. This coastal protection model concept is equipped with a reservoir at the top of the structure to collect water that overflows from the top of the structure (overtopping). Many factors influence the amount of wave overtopping discharge; one of them is the run-up height. The height of wave run-up that occurs is influenced by many factors, one of which is the steepness of the incoming wave. The effect of the wave steepness to the relative wave run-up on OWEC breakwater is investigated experimentally. The study is carried out in wave flume with regular waves. The OWEC breakwater model is simulated with slope variations and freeboard variations. The results of the study show that small wave steepness value produces a large relative wave run-up value and vice versa. For a review of the structure slope, the results showed that the largest relative run-up value was obtained in the largest slope of the variation of the slope studied, namely 1: 2, at all different freeboard heights.

The effect of wave reflection coefficient to the breaker parameter on OWEC breakwater

OWEC breakwater is a coastal structure with an integrated concept of the breakwater and overtopping wave energy converter. This breakwater WEC concept (OWEC breakwater) model has a reservoir at the rear of the breakwater peak. This reservoir serves to catch and collect overtopping waves that overflow through the top of the breakwater. The amount of overtopping discharge is influenced by many factors, one of them is the characteristics of the incoming wave, including the characteristics of the breaking wave on the surface of the structure, named the breaker parameter. The effect of wave reflection coefficient to the breaker parameter on OWEC (overtopping wave energy converter) breakwater is investigated experimentally as a preliminary study. The study is carried out in wave flume with regular waves. The OWEC breakwater model is simulated with slope variations and freeboard variations. The result showed that most of the breaker parameter types obtained are plunging (0.4 < \U0001d709 < 2) and surging (\U0001d709 > 2), the result also showed that the higher the value of reflection coefficient, the higher the value of the breaker parameter.

Lifecycle Environmental Impact Assessment of an Overtopping Wave Energy Converter Embedded in Breakwater Systems

Overtopping breakwater systems are among the most promising technologies for exploiting wave energy to generate electricity. They consist in water reservoirs, embedded in piers, placed on top of ramps, higher than sea-level. Pushed by wave energy, seawater fills up the reservoirs and produces electricity by flowing back down through low head hydro turbines. Different overtopping breakwater systems have been tested worldwide in recent years. This study focuses on the Overtopping BReakwater for Energy Conversion (OBREC) system that has been implemented and tested in the harbor of Naples (Italy). The Life Cycle Assessment of a single replicable module of OBREC has been performed for analyzing potential environmental impacts, in terms of Greenhouse Gas Emissions, considering construction, installation, maintenance, and the operational phases. The Carbon Footprint (i.e. mass of CO2eq) to build wave energy converters integrated in breakwater systems has been estimated, more specifically the “environmental investment” (i.e. the share of Carbon Footprint due to the integration of wave energy converter) needed to generate renewable electricity has been assessed. The Carbon Intensity of Electricity (i.e. the ratio between the CO2eq emitted and the electricity produced) has been then assessed in order to demonstrate the profitability and the opportunity to foster innovation in the field of blue energy. Considering the impact for implementing an operational OBREC module (Carbon Footprint = 1.08 t CO2eq; Environmental Investment = 0.48 t CO2eq) and the electricity production (12.6 MWh/yr per module), environmental benefits (avoided emissions) would compensate environmental costs (i.e. Carbon Footprint; Environmental Investment) those provided within a range of 25 and 13 months respectively.