Wave Power Conversion Research Articles

Numerical investigation on the hydrodynamic and conversion performance of a dual cylindrical OWC integrated into a caisson-type breakwater

The hydrodynamic performance of an integrated oscillating water column (OWC) combined with a cylindrical caisson type breakwater was investigated. A three-dimensional numerical wave tank was established to simulate the overall energy conversion performance and hydrodynamics of the integrated system with the software Star-CCM+. The developed Computational Fluid Dynamics (CFD) model was initially verified with the published physical results and numerical results derived from an analytical solution. The effects of the structural geometry and the pneumatic damping of the PTO system on the wave absorption and power conversion efficiency of the proposed device were investigated using this developed model. Based on this proposed numerical model, the velocity of the air flow, the corresponding air pressure characteristics in the air turbine nozzle and chamber zone were discussed. The results indicate that this proposed half-open land based OWC is adapted to absorb shorter nearshore waves, characterized by dimensionless wave number kd approaching 1.49. Furthermore, a larger opening inlet zone for this proposed OWC would shift the resonant corresponding kd to higher wave frequencies, while lowering the opening inlet could help to increase the power extraction efficiency for long waves. The peak hydrodynamic efficiency is observed at a ratio of OWC inlet width to diameter B/D of 0.97, which is approximately six times larger than that at B/D = 0.5. The inclusion of an impulse turbine yields a more uniform pressure distribution within the central chamber and effectively mitigates wave reflection for smaller incident waves, with maximum efficiency achieved using 37 blades. The property of the efficiency mitigation with insufficient certain intake depth of the wave chamber should be avoided for system design.

Wave power conversion systems for electrical energy production

Due to economic social cohesion, the European Union is promoting to improve the production of electrical energy from renewable energy sources. Sea waves have associated a form of renewable energy which can be captured by using a hydro mechanical device that in turn drives an electrical generator to produce electrical energy. After a brief description of wave formation and quantifying the power across each meter of wave front associated to the wave, the paper describes several devices used presently to extract mechanical energy from the waves and their advantages and disadvantages are presented as conclusions. In particular, the modern Pelamis system is described in some detail. Wave energy market is also discussed.

Performance of Resonant Chambers in Oscillating Water Column Devices

The utilization of marine power potential in Colombia holds a great promise, and the oscillating water column (OWC) is one option that deserves to be explored. The wave energy conversion process using an OWC typically involves two stages: the conversion of wave power to pneumatic power in an air chamber, and the conversion of pneumatic power to electricity using a selfrectifying air turbine coupled to an electric generator. In order to improve the efficiency of the primary stage, this study is aiming at determining the hydrodynamic performance of an OWC air chamber using a numerical model based on the Reynolds Averaged Navier-Stokes equations and the Volume of Fluid approach for free surface simulation. The chamber geometry was based on a U-shaped OWC. The maximum efficiency and the mean velocity of the chamber free surface were found to be 66.8% and 0.17 m/s, respectively. The numerical results for the wave height obtained through simulation were validated by comparing them to analytical expressions reported in the existing literature, and a significant level of agreement was observed. Optimizing the chamber shape parameters for the specific wave characteristics is concluded to be crucial to enhance the operating efficiency of the OWC.

Performance Validation of Resonant Wave Power Converter with Variable Moment of Inertia

This study focuses on the performance validation of a wave energy converter (WEC) equipped with resonant frequency adjustment capability. Achieving resonance with the oscillation period of waves is crucial for maximizing the efficiency of wave energy conversion systems. For this purpose, we introduce a variable moment of inertia (VMI) device that allows for tuning of the resonant frequency by adjusting the position of the mass. For validation of the proposed approach, performance verification experiments were conducted using a vibration platform. Through free vibration experiments, we identified the system’s resonant frequency and characteristics. In our forced vibration tests considering the wave frequency band of the coastal area, harmonic vibration experiments were performed over a range of 0.4–0.8 Hz. In these experiments, we considered three separate cases by adjusting the distance from the reel’s center to the mass. As a result, we found that the resonant frequency of the WEC can be adjusted from 0.65 Hz when the mass is furthest away to 0.55 Hz when it is closer. This research confirms the benefits obtained by utilizing VMI to adjust the resonant frequency of WECs without changing the mass or springs. This offers a promising opportunity to enhance the energy conversion efficiency of resonant-type WECs utilizing wave energy.

A surface-piercing truncated cylindrical meta-structure operating as a wave energy converter

In this paper, we study the interaction of water waves with a surface-piercing truncated cylindrical meta-structure consisting of two overlapping arrays of closely spaced vertical thin plates. The fluid resonance promoted in the narrow vertical channels formed by the cylindrical meta-structure is exploited by a novel design concept of the wave power converter by covering the surface of the cylinder with an array of small cuboid buoys which float in the gaps between the intersecting plate arrays. Each buoy is attached to its own spring and power takeoff damping mechanism, and the vertical displacement of individual buoys is replaced by a continuous two-dimensional function of position which follows from homogenization of the plate/fluid structure of the cylinder. Effective medium equations and boundary conditions are derived under both full depth-dependent theory and shallow-water theory, allowing semi-analytical methods to be developed to investigate the wave scattering and wave energy absorption properties of this cylindrical meta-structure. Results illustrate that the internal resonance of the cylindrical meta-structure can lead to significant wave power capture across a broad range of frequencies.

Evaluation of Geometrical Influence on the Hydrodynamic Characteristics and Power Absorption of Vertical Axisymmetric Wave Energy Converters in Irregular Waves

Abstract To obtain the mechanical energy of waves from arbitrary directions, the vibration absorbers of wave energy converters (WEC) are usually vertically axisymmetric. In such case, the wave-body interaction hydrodynamics is an essential research topic to obtain high-efficiency wave energy. In this paper, a semi-analytical method of decomposing the complex axisymmetric boundary into several ring-shaped stepped surfaces based upon the boundary approximation method (BAM) is introduced and examined. The hydrodynamic loads and parameters, such as the wave excitation forces, added mass and radiation damping of the vertical axisymmetric oscillating buoys, can then be achieved by using the new boundary discretisation method. The calculations of the wave forces and hydrodynamic coefficients show good convergence with the number of discretisation increases. Comparison between the constringent results and the results of the conventional method also verifies the feasibility of the method. Then, simulations and comparisons of the hydrodynamic forces, motions and wave power conversions of the buoys with series draught and displacement ratios in regular and irregular waves are conducted. The calculation results show that the geometrical shape has a great effect on the hydrodynamic and wave power conversion performance of the absorber. In regular waves, though the concave buoy has the lowest wave conversion efficiency, it has the largest frequency bandwidth for a given draught ratio, while in irregular waves, for a given draught ratio, the truncated cylindrical buoy has the best wave power conversion, and for a given displacement of the buoy, the concave buoy shows the best wave power conversion ability.

Study on an Oscillating Water Column Wave Power Converter Installed in an Offshore Jacket Foundation for Wind-Turbine System Part II: Experimental Test on the Converting Efficiency

This study is an experimental study corresponding to an analytical study presented previously, where a scaled-down model was built and tested in a water tank by following the size and shape of the structure applied in the analytical study. In this study, a wave energy converter of an oscillating water column (OWC) system is integrated with the infrastructure of a jacket-type offshore platform applied to an offshore wind turbine system. The purpose is to conduct a combination system through the simultaneous utilization of both wind power and wave power so that sustainable energy can be maximized. During the analytical study’s analysis, the airflow response and the converting efficiency of wave energy from an OWC system integrated with an offshore template structural system were evaluated. By following the analytical study’s analysis, the performance of all the systems is tested, including the airflow velocity, pneumatic power, and the converting efficiency of the power from waves. The experimental data are analyzed and discussed in terms of the variations of the OWC system’s geometrical parameters. The parameters under consideration include the exhale orifice-area of airflow, gate-openings of inflow water, and the submerged chamber depth. It is found from the experimental results that, through the comparison between the experimental data and the analytical results, the results of the analytical study’s analysis are countable, and an open sea OWC system can be successfully applied to the template structure of offshore wind power infrastructure as a secondary generating system for the multi-purpose utilization of the structure.

A Review of Power Electronics for Nearshore Wave Energy Converter Applications

In compliance with the green energy policy, mitigation from high fossil fuel dependency is becoming a new objective for most countries, including Malaysia. Wave energy is among extensively explored renewable energy relatively clean, sustainable, and inexhaustible resources. To this day, neither definite wave energy technology nor widely available commercial wave farm supplying the grid has existed. Therefore, wave energy harvesting is the most compelling solution, especially in regions where the possibility of grid connection is low in the nearest future. Like other renewable energy, the voltage amplitude and frequency generated from waves are unstable and may vary continuously. This uncertainty has created energy transfer challenges since the grid requires a stable and uninterrupted energy supply. Therefore, power electronics devices are employed to modulate the controller circuits’ pulse width. Further understanding of the relationship between wave energy conversion technology and its power conversion, particularly for nearshore applications, is summarized. This work also discussed selected wave energy conversion research and power conversion system implemented and studied in Malaysia. Finally, this review can provide extensive overview and broad understanding into power conversion system for Wave Energy Conversion, especially for nearshore applications.

Structural Design and Motion Response Analysis of AutoNaut Wave Glider

Click here and insert your abstract text.Wave glider is a surface unmanned detection platform and wave propulsion is realized by using wave power conversion mechanism. Its excellent endurance performance and pollution-free have been widely concerned and recognized. However, due to the complex dual-body structure of the wave glider, it has certain requirements for operating water depth and it also has some disadvantages such as weak maneuverability and harsh deployment and recovery, which restricts the development of its full sea state coverage. To overcome the above disadvantages, it is very necessary to develop a new type of wave glider (AutoNaut Architecture Wave Glider, ANWG) driven by heave and pitch. Firstly, the propulsion mechanism of the ANWG is analyzed, its overall functional requirements and design objectives are determined, and the overall structural design of the ANWG prototype is completed, including float structure design and wave power conversion mechanism design. Secondly, the motion response of the float is analyzed and the influence of hydrofoil on the motion response of the float is studied. Finally, by analyzing the relationship between ANWG resonance frequency and motion response, the most suitable sea wave frequency for ANWG navigation is obtained.

Spatial variation in the periods of ion and neutral waves in a solar magnetic arcade

Context.We present new insight into the propagation of ion magnetoacoustic and neutral acoustic waves in a magnetic arcade in the lower solar atmosphere.Aims.By means of numerical simulations, we (a) study two-fluid waves propagating in a magnetic arcade embedded in the partially ionised, lower solar atmosphere and (b) investigate the effect of the background magnetic field configuration on the observed wave-periods.Methods.We considered a 2D approximation of the gravitationally stratified and partially ionised lower solar atmosphere consisting of ion plus electron and neutral fluids that are coupled by ion-neutral collisions. In this model, the convection below the photosphere causes the excitation of ion magnetoacoustic-gravity and neutral acoustic-gravity waves.Results.We find that in the solar photosphere, where ions and neutrals are strongly coupled by collisions, ion magnetoacoustic-gravity and neutral acoustic-gravity waves have periods ranging from 250 s to 350 s. In the chromosphere, where the collisional coupling is weak, the wave characteristics strongly depend on the magnetic field configuration. Above the footpoints of the considered arcade, the plasma is dominated by a vertical magnetic field along which ion magnetoacoustic-gravity waves propagate. These waves exhibit a broad range of periods, and the most prominent periods are 180 s, 220 s, and 300 s. Above the main loop of the solar arcade, where mostly horizontal magnetic field lines guide ion magnetoacoustic-gravity waves, the main spectral power reduces to the period of about 180 s, and no longer wave-periods exist.Conclusions.In photospheric regions, ongoing solar granulation excites a broad spectrum of wave-periods that undergoes complex interactions: mode-coupling, refractions through the inhomogeneous atmosphere, real physical absorption, and conversion of wave power. We found that, in addition, the magnetic arcade configuration with a partially ionised plasma drastically changes the image of wave-periods observed in the upper layers of the chromosphere and corona. Our results agree with recent observational data.

Optimization of Low Head Axial-Flow Turbines for an Overtopping BReakwater for Energy Conversion: A Case Study

Overtopping-type wave power conversion devices represent one of the most promising technology to combine reliability and competitively priced electricity supplies from waves. While satisfactory hydraulic and structural performance have been achieved, the selection of the hydraulic turbines and their regulation is a complex process due to the very low head and a variable flow rate in the overtopping breakwater set-ups. Based on the experience acquired on the first Overtopping BReakwater for Energy Conversion (OBREC) prototype, operating since 2016, an activity has been carried out to select the most appropriate turbine dimension and control strategy for such applications. An example of this multivariable approach is provided and illustrated through a case study in the San Antonio Port, along the central coast of Chile. In this site the deployment of a breakwater equipped with OBREC modules is specifically investigated. Axial-flow turbines of different runner diameter are compared, proposing the optimal ramp height and turbine control strategy for maximizing system energy production. The energy production ranges from 20.5 MWh/y for the smallest runner diameter to a maximum of 34.8 MWh/y for the largest runner diameter.

Study on an Oscillating Water Column Wave Power Converter Installed in an Offshore Jacket Foundation for Wind-Turbine System Part I: Open Sea Wave Energy Converting Efficiency

This study is focused on the wave energy converter of an oscillating water column (OWC) system that is integrated with a jacket type infrastructure applied for an offshore wind turbine system. In this way, electricity generation by both wind power and wave power can be conducted simultaneously to maximize the utilization of sustainable energy. A numerical analysis was performed in this research to model and simulate the airflow response and evaluate the converting efficiency of wave energy from an OWC system integrated with an offshore template structural system. The performance of the system including the generating airflow velocity, air-pressure in the chamber, generating power and then the converting efficiency of power from waves are all analyzed and discussed in terms of the variations of the OWC system’s geometrical parameters. The parameters under consideration include the exhale orifice-area of airflow, gate-openings of inflow water and the submerged chamber depth. It is found that from the analytical results the performance of the OWC wave energy converter is influenced by the dimensional parameters along with the design conditions of the local environment. After a careful design based on the in-situ conditions including water depth and wave parameters, an open OWC system can be successfully applied to the template structure of offshore wind power infrastructure as a secondary generating system for the multi-purpose utilization of the structure.

Structural Safety Analysis for an Oscillating Water Column Wave Power Conversion System Installed in Caisson Structure

In this study, an alternative way, a so called caisson based type of oscillating water column (OWC) wave energy converting system was proposed to capture and convert wave energy. Since the caisson structure is constructed to protect the coastal line or ports, it is important to know if a built-in associated OWC system will be a burden to affect the safety of the structure or it is safe enough to work appropriately. In this study, three steps of structural analysis were performed: firstly, the analysis for the structural safety of the whole caisson structure; secondly, performing the mechanic analysis for the chamber of the associated OWC system; and finally, performing the analysis for the wave induced air-pressure in the chamber under the design conditions of a local location during the wave-converting operation. For the structural safety analysis, a typical structural model associated with caisson breakwater was built and analyzed while the shape of the structure, material applied to the construction, and associated boundary conditions were all set-up according to the wave and structures. The motion and the strain distribution of the caisson structure subjected to designated waves of 50-year return period were evaluated and compared to the safety requirement by the code. For the analysis of the energy converting performance, a numerical method by using a theorem of unsteady Navier–Stokes equations in conservation form was used to analyze the proposed OWC model when the structure subjected to an incident wave of a 10-year return period.

Adaptive Sliding Mode Control for a Double Fed Induction Generator Used in an Oscillating Water Column System

Wave power conversion systems are nonlinear dynamical systems that must endure strong uncertainties. Efficiency is a key issue for these systems, and the application of robust control algorithms can improve it considerably. Wave power generation plants are typically built using variable speed generators, such as the doubly fed induction generator (DFIG). These generators, compared with fixed speed generators, are very versatile since the turbine speed may be adjusted to improve the efficiency of the whole system. Nevertheless, a suitable speed controller is required for these systems, which must be able to avoid the stalling phenomenon and track the optimal reference for the turbine. This paper proposes a sliding mode control scheme aimed at oscillating water column (OWC) generation plants using Wells turbines and DFIGs. The contributions of the paper are (1) an adaptive sliding mode control scheme that does not require calculating the bounds of the system uncertainties, (2) a Lyapunov analysis of stability for the control algorithm against system uncertainties and disturbances, and (3) a validation of the proposed control scheme through several simulation examples with the Matlab/Simulink suite. The performance results, obtained by means of simulations, for a wave power generation plant (1) evidence that this control scheme improves the power generation of the system and (2) prove that this control scheme is robust in the presence of disturbances.

Assessment of the power conversion of wave energy converters based on experimental tests

Power estimation is the most important task in the performance assessment of wave energy converters (WEC). Depending on the type of WEC, power take-off (PTO) system, and available instrumentation, different approaches are usually applied to assess power from experimental tests. Since neither for tests execution nor for the analyses exist standard methodologies, the estimated power can significantly differ among the approaches, thus, introducing inaccuracies in the assessment of the WEC’s performance. This study analyzed an extensive set of experimental results of a WEC and applied different power estimation approaches to compare and validate the obtained results. The strengths and weaknesses of those methodologies were analyzed and discussed. The major outcomes were: (a) PTO damping characterization based on bench tests can lead to unrealistic values of the WEC’s absorbed power; (b) the hybrid (experimental plus numerical) approach provide more reliable results than the traditional ones; (c) the kinetic energy harvester approach underestimates power but can be useful for preliminary qualitative performance assessments; (d) the new proposed gross efficiency index allows the definition of upper boundaries for wave power conversion as well as the identification of the wave conditions for which WEC performance can be improved.

Wave Power as Solution for Off-Grid Water Desalination Systems: Resource Characterization for Kilifi-Kenya

Freshwater scarcity is one of humanity’s reoccurring problems that hamper socio-economic development in many regions across the globe. In coastal areas, seawater can be desalinated through reverse osmosis (RO) and transformed into freshwater for human use. Desalination requires large amounts of energy, mostly in the form of a reliable electricity supply, which in many cases is supplied by diesel generators. The objective of this work is to analyze the wave power resource availability in Kilifi-Kenya and evaluate the possible use of wave power converter (WEC) to power desalination plants. A particular focus is given use of WECs developed by Uppsala University (UU-WEC). The results here presented were achieved using reanalysis—wave data revealed that the local wave climate has an approximate annual mean of 7 kW/m and mode of 5 kW/m. Significant wave height and wave mean period are within 0.8–2 m and 7–8 s respectively, with a predominant wave mean direction from southeast. The seasonal cycle appeared to be the most relevant for energy conversion, having the highest difference of 6 kW/m, in which April is the lowest (3.8 kW/m) and August is the peak (10.5 kW/m). In such mild wave climates, the UU–WEC and similar devices can be suitable for ocean energy harvesting for water desalination systems. Technically, with a capacity factor of 30% and energy consumption of 3 kWh/m3, a coastal community of about five thousand inhabitants can be provided of freshwater by only ten WECs with installed capacity of 20 kW.

Spatial Distribution of the Baltic Sea Near-Shore Wave Power Potential along the Coast of Klaipėda, Lithuania

Wave power is an abundant source of energy that can be utilized to produce electricity. Therefore, assessments of wave power resources are being carried out worldwide. An overview of the recent assessments is presented in this paper, revealing the global distribution of these resources. Additionally, a study, which aims to assess the spatial distribution of the Baltic Sea near-shore wave power potential along the coast of Klaipėda (Lithuania), is introduced in this paper. The impacts of the wave propagation direction and decreasing depth on wave power resources were examined using the numerical wind-wave model MIKE 21 NSW. The wave height loss of the design waves propagating to shore was modelled, and the wave power fluxes in the studied depths were calculated using the JONSWAP wave spectrum modified for the Baltic Sea. The results revealed that all waves that propagate to the shore in the Baltic Sea near-shore area along the coast of Klaipėda from 30 m depth to 5 m depth lose at least 30% of their power. Still, most common waves in this area are low, and therefore, they start to lose their power while propagating to the shore at relatively low (10–14 m) depths. To turn this into an advantage the wave power converter would have to work efficiently under low power conditions.

Beam–Wave Interaction From FEL to CARM and Associated Scaling Laws

The development of a microwave tube providing high output power (~1 MW) at a high frequency (~250 GHz) with high efficiency of the beam–wave power conversion is a challenging task. A great deal of theoretical and experimental efforts is directed toward such a goal. A promising powerful source of microwave radiation is the cyclotron autoresonance maser (CARM) oscillator. In this paper, we revisit the well-known physical models in a way, which is suitable for their implementation in the numerical tools for computer-aided design and optimization of a CARM operating at high frequency. The analysis developed by us is an attempt directed toward the realization of an adequate design tool for the development of CARM devices.

ローリング運動を利用した直接波力発電方式(海洋開発と防災, 海洋工学)

ローリング運動を利用した直接波力発電方式(海洋開発と防災, 海洋工学)

MODEL EXPERIMENT AND FIELD TEST OF PW-OWC TYPE WAVE POWER EXTRACTING BREAKWATER

Many research studies on wave power conversion have been conducted in Japan over the past 30 years, but the outgrowth of these studies have yet to be put to practical use. In this study, medium-scale and large-scale hydraulic model experiments as well as prototype field test on a new oscillating water column with a projecting wall (PW-OWC) wave power extracting breakwater were carried out in order to investigate the efficiency of energy conversion. The primary energy conversion efficiency of the PW-OWC converter is about 10% larger than that of the standard OWC type used in the medium-scale experiments. The primary energy conversion efficiency of the large scale experiments is about 20-30-% smaller than that of the medium scale experiments. The secondary energy conversion efficiency ranges from 0.20 to 0.42 and becomes small when the wave period is short and the wave height is large.