Multi–chamber oscillating water column wave energy converters and air turbines: A review

The oscillating water column (OWC) is a more common type of wave energy converter (WEC) that has been the subject of the study and development for several decades. Multi–chamber oscillating water column (MC–OWC) devices or arrays have the advantage of being more efficient in energy extraction compared to a single chamber system, particularly in more chaotic sea states. A variety of single and array OWC devices have been proposed and studied on a small–scale, whereas few large–scale devices have been tested under ocean wave conditions. This paper provides a concise review of the current state of MC–OWC device development in laboratory conditions. The review highlights explicitly the main stages of MC–OWC device development for one ongoing study as an example. This review was based on the available information in the literature from 2003 to 2012, in addition, further work is presented as part of the current study at the University of Technology Sydney. This study is from 2015 to 2018. The discussion shows the challenges that a device needs to overcome to be more competitive with other WECs in the global of wave energy converter area.     

Simulation of electricity supply of an Atlantic island by offshore wind turbines and wave energy converters associated with a medium scale local energy storage

The problem of sizing an electricity storage for a 5000 inhabitants island supplied by both marine renewables (offshore wind and waves) and the mainland grid is addressed by a case study based on a full year resource and consumption data. Generators, transmission lines and battery storage are accounted for through basic simplified models while the focus is put on electricity import/export budget. Self-sufficiency does not seem a reasonable goal to pursue, but partial autonomy provided by renewable sources and a medium size storage would probably be profitable to the island community.     

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.     

A time-domain numerical simulator for oscillating water column wave power plants

This paper presents a time-domain numerical simulator of oscillating water column (OWC) wave power plants. The whole problem (the simulation of the movements of the inner free surface due to the excitation of the outer sea-waves) is split into two subproblems: an outer one, dealing with the incident, diffracted and radiated waves, solved once for all, and an inner one, concerning the inner water volume behavior. The kernel of this simulator is a hybrid numerical method [Josset C, Clément AH, Duclos G. A hybrid method for time-domain simulation of oscillating water column wave power plants. Ocean Engineering, to appear.] which allows for the coupling of these two problems. After the validation of each part of this simulator, it was applied to the European wave energy power plant seated on Pico Island, Azores, first to estimate the annual performance of the plant, then to simulate different alternative configurations, in a way to improve the productivity.     

A review of impulse turbines for wave energy conversion

Oscillating Water Column based wave energy plants convert wave energy into low pressure pnuematic power in the form of bi-directional air flows. Air turbines which are capable of rotating uni-directionally in bi-directional air flow, otherwise also known as self-rectifying turbines, are used to extract mechanical shaft power which is further converted into electrical power by a generator. This paper reviews the state of the art in self-rectifying impulse air turbines. New results on optimum parameters for the fixed-guide-vane impulse turbine are also presented. Starting characteristics and conversion efficiencies of two types of impulse turbines are compared with the well known Wells turbine.     

A linearized model for estimating the performance of submerged resonant wave energy converters

A realistic performance analysis of oscillating water column wave energy converters (WECs) addresses to a set of non-linear differential equations that need to be integrated in time, by using a stochastic approach, under the hypothesis of random wind-generated sea waves, for all the sea states which characterize the location of the system. Non-linearities of the differential equations have several origins:

• minor and major losses of the unsteady flow of water and air;

• compressibility of air and heat exchange with the walls of the air chamber;

• non-linear characteristics of the turbine.

Under the hypothesis of random sea waves with Gaussian distribution, the authors propose an original methodology for linearizing the differential equations that describe the flow motion inside a wholly submerged WEC. Under such hypothesis, the linearized model can be used for predicting the power output by means of the calculations in the frequency domain and for control design. The developed methodology has been applied to the estimation of the performance of the new “resonant sea wave energy converters”, called REWEC, patented by Boccotti in 1998, and consisting of several caissons, characterized by a structure similar to the caissons of the traditional breakwaters and placed on the seabed, close one to each other, to form a submerged breakwater. Each caisson is connected to a vertical duct wholly beneath the sea level, where a hydraulic Wells turbine is placed.The matching between turbine and resonance characteristic of the system is carefully analysed in order to maximize the energy conversion efficiency.Some results, given for a small installation in the Mediterranean sea, confirm that the REWEC system is able to absorb a large share of the incident wave energy due to a very simple regulation system which permits the tuning on sea states with different significant heights.     

Shape optimisation of floating wave energy converters for a specified wave energy spectrum

Ocean wave energy is one of the world's most powerful forms of energy and the energy density in ocean waves is the highest among renewable energy sources. Wave energy converters are employed to harness this energy and convert it into usable electrical energy. However, in order to efficiently extract the energy, the wave energy converter must be optimised in the design stage. Therefore, in this paper, a methodology is presented which aims to optimise the structural geometric configuration of the device to maximise the average power extraction from its intended deployment site. Furthermore, a case study of the Atlantic marine energy test site, off the west coast of Ireland, is undertaken in order to demonstrate the methodology. Using the average annual wave energy spectrum at this site as the input, the optimum structural geometric configuration was established, along with an analysis of the optimum configuration for different radius devices. In addition, the optimum damping coefficient of the PTO mechanism is determined and the total mean absorbed power for the structure at the site over the entire scatter diagram of data is calculated.     

Wells turbine for wave energy conversion: a review

In the past 20 years, the use of wave energy systems has significantly increased, generally depending on the oscillating water column concept. Wells turbine is one of the most efficient oscillating water column technologies. This article provides an updated and a comprehensive account of the state‐of‐the‐art research on Wells turbine. Hence, it draws a roadmap for the contemporary challenges, which may hinder future reliance on such systems in the renewable energy sector. In particular, the article is concerned with the research directions and methodologies, which aim at enhancing the performance and efficiency of Wells turbine. The article also provides a thorough discussion of the use of CFD for performance modeling and design optimization of Wells turbine. It is found that a numerical model using the CFD code can be employed successfully to calculate the performance characteristics of W‐T as well as other experimental and analytical methods. The increase of research papers about CFD, especially in the last 5 years, indicates that there is a trend that considerably depends on the CFD method. Copyright © 2016 John Wiley & Sons, Ltd.     

Wave-power absorption from a finite array of oscillating wave surge converters

Semi-analytical and fully numerical modelling is developed in the framework of the inviscid potential flow theory to investigate the dynamics of a wave farm made by flap-type wave energy converters in the nearshore. The hydrodynamic parameters and the efficiency of the system in typical layouts are calculated with both models. Good agreement is shown between the two approaches. Parametric analysis undertaken with the semi-analytical model allows to identify a near-resonant phenomenon which is responsible for increasing the absorbed power by the single elements of the array. Such result could be used as a preliminary design criterion. The numerical model is then applied to analyse a configuration of practical engineering interest, i.e. an array of two staggered converters. The dynamics arising in this more complex system is explained, showing that non-symmetric layouts can be less effective.     

On the park effect in arrays of oscillating wave energy converters

This paper aims to provide guidelines for designing the layout of arrays of oscillating Wave Energy Converters (WECs) based on a review of the literature of wave interactions and park effect in WEC arrays that has been published over the past 30 years.First, the fundamentals of wave energy absorption by oscillating bodies are summarised, and the principal differences between the park effect in arrays of wave energy converters and wind turbines are highlighted. Then, the numerical approaches commonly used to deal with WEC arrays are outlined briefly and their limitations are discussed. It is argued that, at present, only Boundary Element Methods (BEM) are capable of the appropriate analysis. Finally, previous work on wave interactions and park effect in WEC arrays is reviewed. Similar trends are found in these studies, which allow conclusions to be drawn regarding the significance of the park effect as a function of the number of WECs in the array and their spacing. Based on these conclusions, the following tentative guidelines are proposed:For small arrays of conventional devices (fewer than 10 devices of typical dimension 10–20 m) with usual layouts (regular or shifted grids with separating distance of order 100–200 m), the park effect appears to be negligible. For larger arrays (more than 10 devices), a negative park effect seems to be increasingly important with increasing number of rows (the lines of WECs perpendicular to the incident wave direction). Therefore, the number of rows should remain as small as possible, with a separating distance as large as possible. For arrays of non-conventional WECs (WECs of typical dimensions much larger than 10–20 m), no information has been found. However, trends similar to the previous cases could be expected, provided that aspect ratios are maintained.     

Seawater lubricated polymer journal bearings for use in wave energy converters

This study investigates the wear characteristics of polymeric journal bearings while immersed in seawater, and their applicability towards wave energy conversion. A block on ring wear machine was used to test four commercially available bearing materials under unidirectional and oscillatory sliding conditions at low pressure. It was observed that wear generally increases with counterface roughness; however, major deviations to this trend exist, depending on the bearing’s composition. Stable wear rates were shown to vary widely depending on velocity profile, though a general trend could not be established for the sample group as a whole. It was shown that polymer wear rates cannot be attributed to any one parameter, and that detailed testing at several characteristic pressures and velocities is needed to determine a material’s applicability towards wave energy converters operating in real seas.     

A review on front end conversion in ocean wave energy converters

Harvesting the energy from ocean waves is one of the greatest attractions for energy engineers and scientists. Till date, plenty of methods have been adopted to harvest the energy from the ocean waves. However, due to technological and economical complexity, it is intricate to involve the majority of these energy harvesters in the real ocean environment. Effective utilization and sustainability of any wave energy harvester depend upon its adaptability in the irregular seasonal waves, situation capability in maximum energy extraction and finally fulfilling the economic barriers. In this paper, the front end energy conversions are reviewed in detail which is positioned in the first stage of the wave energy converter among other stages such as power take off (PTO) and electrical energy conversion. If the recent development of these front end energy conversion is well known then developing wave energy converter with economic and commercial viability is possible. The aim of this review is to provide information on front end energy conversion of a point absorber and emphasize the strategies and calamity to be considered in designing such kinds of devices to improve the energy harvesting competence. This will be useful to the engineers for speeding up the development of a matured point absorbing type wave energy converter.     

Performance of OWC wave energy converters: influence of turbine damping and tidal variability

The performance of oscillating water column (OWC) systems depends on a number of factors in a complex manner. The objective of this work is to analyse the influence of the wave conditions, the damping caused by the turbine and the tidal level on the efficiency of the conversion from wave to pneumatic energy that occurs in the OWC chamber. To achieve this, a comprehensive experimental campaign is carried out, involving in total 387 tests of a model OWC under varying wave conditions (both with regular and irregular waves), damping coefficients and tidal levels. It is found that the damping exerted by the turbine is the factor that most affects the chamber efficiency—even more than the wave conditions. It follows that a proper selection of the turbine is crucial not only to the performance of the turbine itself but also to that of the chamber, which reflects the importance of the turbine–chamber coupling in OWC systems. The next factor in order of importance is the wave period. Finally, we find that the influence of the tidal level, which is examined in this work for the first time, is significant under certain conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

A modified Wells turbine for wave energy conversion

The method of wave energy conversion utilises an oscillating water column (OWC). The OWC converts wave energy into low-pressure pneumatic energy in the form of bi-directional airflow. Wells turbine with its zero blade pitch setting has been used to convert this pneumatic power into uni-directional mechanical shaft power. Measurements in OWC based wave energy plants in India and Japan show that the airflow velocity is not equal in both directions. The velocity is more when the airflows out to the atmosphere (exhalation) than in the reverse direction. It may be advantageous to set the rotor blade pitch asymmetrically at a positive pitch so as to achieve a higher mean efficiency in a wave cycle. Towards this objective, performance characteristics of a turbine with different blade setting angles in steady flow were found by experimentation. Quasi-steady analysis was then used to predict the mean efficiency for a certain variation of air velocity with time. This variation with time was taken as pseudo-sinusoidal wherein the positive part of the cycle was taken as a half sine-wave whose amplitude is greater than that of the negative half sine-wave. Such a variation is representative of what happens in reality. For exhalation velocity amplitude to inhalation velocity ratios 0.8 and 0.6, a rotor blade setting angle of 2° was found to be optimum.     

A wave energy resource assessment case study: Review,analysis and lessons learnt

A case study of the development of an overall resource assessment for the Wave Hub site in the southwest UK is presented. Wave Hub is one of the earliest large-scale wave farms planned. Several resource assessment studies have been performed for the site, but the published results are high-level and predicted power availability varies significantly. This paper provides a detailed overview and re-analysis of the multiple datasets used in the original studies, which consisted of a combination of physical measurement and numerical modelling. The quality of the datasets is assessed, and reasons for the discrepancies between predicted resource levels investigated. Results from a SWAN model for the region illustrate significant levels of spatial variability in the resource due to the complexity of the local bathymetry, and examination of long-term global model datasets shows notable inter-annual variability. It is thus concluded that a resource assessment methodology utilising datasets from multiple locations and of short duration significantly reduces the accuracy of the predicted levels of resource. From these results, key learnings for future developments are discussed.     

On wave energy focusing and conversion in open water

This paper investigates wave energy conversion in open water where the goal is to utilize the wave-field focusing effect of a stationary disc submerged a short depth beneath the water surface. Dynamic interaction of the disc with additional coupled, submerged inertias is used to minimize its oscillation. The method used to enable and extend this favorable dynamic coupling is discussed here. An oscillating water column in a submerged duct attached under a small circular opening in the disc and driven by the wave-field over the disc is used for wave energy conversion. Non-real-time reactive control of the water column response to enhance energy absorption is examined. Added mass, radiation damping, and exciting force values for the submerged disc are computed, and the focusing effect of a submerged stationary disc is confirmed with numerical calculations of surface elevation over the disc. Calculations of hydrodynamic performance suggest that energy absorption from the oscillating water column is significantly greater under control holding the disc stationary, and can be improved further by applying reactive loads tuned to the optimal susceptance and conductance associated with the oscillating water column. Although the control forces involved in holding the disc stationary may be large at lower wave numbers, the maximum deflection amplitudes of the compensation system are found to be within reasonable limits.     

Dynamic modelling and simulation of a new air conditioning prototype by solar energy

This work presents a new design of an air conditioning prototype by solar energy developed at the Laboratory of Electromechanical Systems of the National Engineering School of Sfax, Tunisia. The new conception permits to produce heat or cold by using solar energy without polluting the environment. The installation, composed of four compartments, consists of three functioning modes according to the season of the year and according to the climatic conditions.A numerical model is developed to study the behaviour of the unit. This model uses real meteorological data to predict the performance of a thermal solar driven system. The dynamic modelling and simulation of only two modes of functioning (winter mode and summer mode without pre-cooling of air) are presented in this paper. This theoretical model is expected to help in predicting the behaviour of the installation in various climatic conditions. Besides, it would enhance the performance of such installation.     

Analysis of the SEA-OWC-Clam wave energy device part B: Structural integrity analysis

In this paper the dynamic loads determined for the SEA-OWC-Clam wave energy device, treated as a floating offshore structure with six degrees-of-freedom with partial internal sealed-off channels, are applied to assess its structural integrity. This task necessitates the matching of the boundary element determined dynamic pressures to the corresponding Finite Element Analysis (FEA) to determine the effective von Mises stress levels. A decomposition of the total load to permit attribution to different aspects of radiation and diffraction loading is presented. Possible modification of structural details is identified by undertaking a more local stiffened plate analysis to improve structural details.     

Speed control of oil-hydraulic power take-off system for oscillating body type wave energy converters

The variable displacement oil-hydraulic pumps for the Power Take-Off (PTO) of wave energy converters must work above 80% of maximum displacement in order to have an overall efficiency of approximately 94.5%. This is achieved by controlling their rotational speed when the oil-hydraulic power fluctuates in time. Three speed control strategies have been presented, the first fixing the maximum possible speed in each sea state, the second by slowly varying the pump speed between speed peak values and average ones, and the third by working with highly variable speed reference values. The worst pump efficiency is achieved with the first strategy while the best one with the third strategy. However, the first has less impact than the third one in the pump lifecycle. On the other hand, the second strategy is used to make a trade-off between pump efficiency and lifecycle. However, this paper presents a fourth speed control strategy, which is a hybrid of the second and third strategies. So, the objectives of this paper were to know if these strategies are implementable in a test rig and also on a new PTO concept and determining what modifications should be introduced in these PTO strategies and hardware. This paper also contributes with the application of new methodologies in this field of research for the modelling of pump efficiency and pressure control, such as Neuro-Fuzzy modelling and Fuzzy Logic control systems.     

Assessment of wave energy potential and its harvesting approach along the Indian coast

The present scenario of energy market is highly volatile due to large oscillation in the fossil fuel prices. During these periods, the high energy demand for the industries is being partially met through non-conventional energy sources such as wind and solar power. The large untapped energy potential in the Ocean is yet to be exploited due to many technological constraints. The recent decades have shown positive developments worldwide towards the ocean wave energy converters. In the present study, an improved wave energy potential estimate has been made. Based on various parameters such as physical site characteristics, environmental conditions and socio-economic regional state, the selection criteria have been suggested. This would form the basis for energy device selection for the decision makers.