Oscillating Water Column Plant Research Articles

Multi-stage control design for oscillating water column-based ocean wave energy conversion system

Despite the enormous global potential of ocean wave energy, it has yet to achieve a level of maturity and economic competitiveness that would result in a substantial impact. Challenges include direct integration into weak or isolated microgrids, a high proportion of uncertain marine environments, nonlinear dynamics, oscillating water column (OWC) device limitations, slower response times, unplanned power outages, power fluctuations, high capital and operational costs in ocean wave energy conversion (OWEC) systems. To this end, a new independent multi-stage design approach is proposed for the performance enhancement of an OWC-based OWEC system. Firstly, an airflow and rotational speed optimal control stage enhances power capture in the Wells turbine-based OWC plant. Secondly, compared to conventional control, the proposed permanent magnet synchronous generator control incorporates an adaptive nonlinear back-stepping control algorithm based on Lyapunov stability theory. Thirdly, introducing reconfigurable control into the conventional six-leg power converter ensures the uninterrupted operation of an OWEC system. Lastly, a model-predictive control-based energy management system is integrated with a bidirectional DC-DC converter that delivers steady power from the grid-connected OWC OWEC system. Hence, MATLAB simulations ensure the overall performance enhancement and feasibility of the OWEC system application and verify that the proposed multi-stage solution is efficient, robust, and reliable.

Hydrodynamic and energy-harvesting performance of an isolated oscillating water column device: An experimental study

Most previous experimental studies on the hydrodynamic and aerodynamic performances of an oscillating water column (OWC) wave energy converter were conducted in a wave flume. In this study, an experimental study on an isolated OWC plant is carried out in a wave tank. The interactions between the OWC system and incident regular and irregular waves are analyzed by measuring the surrounding wave fields using a wave gauge array. The wave distribution patterns and reflection ratios of the OWC model are presented. In addition, the free-surface elevation, air pressure varying amplitude, turbine torque output, and relative capture width ratios and efficiencies in regular and irregular wave scenarios are measured and analyzed. The operating performances of the OWC model are analyzed and compared to the results derived from the flume tests. An overall performance reduction of over 20% can be observed in the wave tank tests. In addition, the representative results of the overall capture width ratio of the OWC model derived from irregular wave tests are corrected from the scaling effects caused by the similarity contradiction.

Long term wave load trends against offshore monopile structures: A case study in the Bay of Biscay

This study examines the trend in wave parameters over available annual records in the Bay of Biscay, where EWTEC2023 is being hosted. There have been various renewable offshore energy projects in the 21st century in this region,such as the OWC (Oscillating Water Column) plant in Mutriku [1], or BiMEP (Biscay Marine Energy Platform)[2],the infrastructure for testing ocean devices in Armintza, where better characterization of the wave resource could helpto provide design information to developing wave energy technology. Other studies [3] have shown that wave heighthas increased in recent decades, as well as the frequency at which extreme waves occur, which affects the fatigueexperienced by deployed devices. The aim is to study the effect of the maximal wave height, its frequency, and loadson monopile cylindrical structures, which are basic component structures for wave energy converters and offshorewind turbines. The effect of this evolution, against these generic structures in offshore wind and wave energy devices,is studied using the 20th century reanalysis ERA20 (1900-2010) of ECMWF (European Centre for Medium-RangeWeather Forecasts) [3]. A significant increase in two forces, drag forces and inertial forces, of up to 15%, is computedover 110 years at a gridpoint near the Western Basque coast, which constitutes a strong positive slope that can be alsoassociated with climate change, and has strong implications for the design of new marine renewable energy technology.

A nonlinear generalized predictive control scheme for the oscillating water column plants

Under unknown waves, the efficiency of the wave energy conversion systems can be improved by applying a novel control algorithm that is able to anticipate the behavior of the plant. In this paper, a generalized predictive control scheme for wave power generation systems that includes a Wells turbine and an induction generator is proposed. The main idea is to maintain the flow coefficient at its optimal point for generated power maximization and, at the same time, improve the efficiency of the Wells turbine and avoid the stalling phenomenon by means of regulating the turbine rotational speed. Moreover, by using the capabilities of the predictive controller, the limitations of the stator current and the effect of the mechanical dead time of the system are integrated into the regulator design process, as well as the flux weakening technique. Simulation and experimental results, through a test bench that has been designed and constructed, show the validity and capability of the proposed scheme in both the rated and flux weakening regimes.

Ocean Wave Energy Control Using Aquila Optimization Technique

This paper presents ocean wave energy control using the Aquila optimization (AO) technique. An oscillating water column (OWC)-type wave energy converter has been considered that is fitted with a Wells turbine and doubly fed induction generator (DFIG). To achieve maximum power point tracking (MPPT), the rotor speed of the DFIG must be controlled as per the MPPT law. The MPPT law is designed in such a way that the Wells turbine flow coefficient remains within the threshold limit. It avoids the turbine from stalling which generates the maximum power. The MPPT law provides the reference rotor speed which is followed by the actual rotor speed. For this, a backstepping controller (BSC)-based rotational speed control strategy has been designed using the Lyapunov stability theory. The BSC has unknown control parameters which should be selected such that tracking errors are minimum. Hence, the objective of this work is to find the unknown control parameters using an optimization approach. The optimization approach of selecting BSC control parameters for an OWC plant has not been explored yet. To achieve this, an integral square error (ISE)-type fitness function has been defined and minimized using the AO technique. The results achieved using the AO technique have been compared with particle swarm optimization (PSO) and a genetic algorithm (GA), validating its superior performance. The rotor speed error maximum peak overshoot is least for AO-BSC as compared to PSO-BSC and GA-BSC. The fitness function value for AO comes out to be least among all the optimization methods applied. However, all tested methods provide satisfactory results in terms of turbine flow coefficient, rotor speed and output power. The approach paves the way for future research on ocean wave energy control.

Oscillating water column plant

This article is describing the way of construction and operation of an oscillating water column system in order to recover as much as possible from the waves energy. The oscillating water column plant is used for the production of electrical energy by tidal currents, and it is currently the most widespread and economical method for the conversion of wave motion. The environmental impact of these infrastructures remains very low: no emissions of gas or any waste during their operation. In addition, the swell is a formidable source of energy.

Electric propulsion of ships and its advantages for anchor lifting towing vessels

This article is describing the way of construction and operation of an oscillating water column system in order to recover as much as possible from the waves energy. The oscillating water column plant is used for the production of electrical energy by tidal currents, and it is currently the most widespread and economical method for the conversion of wave motion. The environmental impact of these infrastructures remains very low: no emissions of gas or any waste during their operation. In addition, the swell is a formidable source of energy.

Mathematical modeling of breakwater-integrated oscillating water column wave energy converter devices under irregular incident waves

In the present study, the performance of breakwater-integrated OWC (oscillating water column) wave energy converter devices is analyzed under the action of unidirectional regular and irregular incident waves. Two different types of OWC devices: (i) LIMPET device (sloping-face OWC device) and (ii) quarter-circle-shaped front wall OWC device are considered for the present study. Detailed derivations of the parameters associated with the performance of the OWC devices under the action of irregular waves are provided. To analyze the efficiency of the OWC devices in real sea conditions, the Bretschneider Spectrum is taken as the incident wave spectrum along with nine sea states represent the local wave climate at the OWC plant site Pico, Portugal. The annual-averaged plant efficiencies of the two aforementioned OWC devices are analyzed as a function of chamber length, submergence depth, turbine rotor diameter, and rotational speed of the Wells turbine. It is observed that the annual-averaged plant efficiency can be enhanced significantly with appropriate combinations of chamber length, submergence depth, and turbine characteristics.

Wave-flume tests of a model-scaled OWC chamber-turbine system under irregular wave conditions

The rapid development of the oscillating water column (OWC) wave energy converter proposes a demand to predict its overall performance accurately. Traditional experimental and numerical studies only employed the orifice or porous media as the power take-off substitutes, causing an inaccurate evaluation of the OWC performance. In this study, an impulse turbine model was used directly in a generic OWC model for the secondary-stage energy conversion. The wave-flume tests were conducted under irregular wave conditions and the rotation speeds of the turbine were fixed. Typical time histories of some performance parameters were presented. The primary-stage and secondary-stage efficiencies were analyzed under various test conditions. It was found that the distribution characteristics of the overall efficiency of the chamber-turbine system is significantly determined by the secondary-stage efficiency. The different distribution characteristics of the efficiencies for the regular and irregular wave conditions indicated that the irregular wave tests are preferred for the shape parameter and operating mode design of the OWC plant.

Experimental Assessment of a Fixed On-Shore Oscillating Water Column Device: Case Study on Oman Sea

Ocean wave is one of the renewable energy resources that these days various devices are used to extract its energy. Oscillating Water Column (OWC) installed on the shore is one of the wave energy absorption systems which has received attention due to its simple structure. Investigation of the pneumatic power is of great importance in such systems as the conversion of wave energy to pneumatic energy is the first step in OWCs’ energy conversion cycle. This study aims to assess the power available in a fixed onshore OWC plant according to the wave characteristics of the Oman Sea on the shores of Chabahar. For this purpose, a small-scale model of OWC was tested in the wave tank, and its pneumatic power was evaluated. In these experiments, the effects of various parameters including incident wave height and frequency, the front wall draft and the orifice diameter on the pneumatic power were investigated. The results show that increasing the wave height generally increases the pneumatic power; however, variations of power with the draft depends on the incident wave frequencies. The maximum achieved capture performance and maximum generated pneumatic power of the model are 18% and 0.7 watts, respectively.

Comparative assessment of control strategies for the biradial turbine in the Mutriku OWC plant

To be competitive against other renewable energy sources, energy converted from the ocean waves needs to reduce its associated levelised cost of energy. It has been proven that advanced control algorithms can increase power production and device reliability. They act throughout the power conversion chain, from the hydrodynamics of wave absorption to the power take-off to improve the energy yield. The present work highlights the development and test of several algorithms to control the biradial turbine which is to be installed in the Mutriku oscillating water column plant. A collection of adaptive and predictive controllers is explored and both turbine speed controllers and latching strategies are examined. A Wave-to-Wire model of one chamber of the plant is detailed and simulation results of six control laws are obtained. The controllers are then validated using an electrical test infrastructure to prepare the future deployment in the plant. Finally, the control strategies are assessed against criteria like energy production, power quality or reliability.

Power Takeoff Optimization to Maximize Wave Energy Conversions for Oscillating Water Column Devices

This paper presents an investigation on how to optimize the power takeoffs (PTOs) to maximize the mean wave energy conversion from seas for floating oscillating water column (OWC) devices. For this purpose, the linear air turbine PTOs are first analytically optimized to maximize the mean energy conversion for regular waves, based on which a simple and fast-turnaround assessment method is proposed to optimize the linear PTO damping coefficient for maximizing the mean energy conversion for the given sea states. Further on, the focus is on how we can reliably assess the maximized mean wave energy conversions if the OWCs are equipped with nonlinear air-turbine PTOs, as frequently seen in practical OWC plants. Conventionally, the time-consuming time-domain analysis is employed for searching the optimized nonlinear damping coefficients which may be both wave-height- and wave-period-dependent for the given sea state. This may not be suitable in the early design stages of the device when many different options may be compared and checked for optimizing the device itself. As such, a reliable fast-turnaround assessment is very desirable for the device performance and for the assessment of energy conversion for the sea states in the deployment site. From the examples in the study, it is shown that the OWC wave energy converters with the optimized linear PTOs have the same energy capture capacities as those with optimized nonlinear PTOs. As such, it can be suggested that the OWC wave energy capture capacity can be analyzed using the fast-turnaround frequency-domain analysis, regardless of whether the linear or nonlinear PTO is used in the wave energy converter.

Wave energy conversion of oscillating water column devices including air compressibility

This paper presents an investigation on air compressibility in the air chamber and its effects on the power conversion of oscillating water column (OWC) devices. As it is well known that for practical OWC plants, their air chambers may be large enough for accommodating significant air compressibility, the “spring effect,” an effect that is frequently and simply regarded to store and release energy during the reciprocating process of a wave cycle. Its insight effects on the device's performance and power conversion, however, have not been studied in detail. This research will investigate the phenomena with a special focus on the effects of air compressibility on wave energy conversion. Air compressibility itself is a complicated nonlinear process in nature, but it can be linearised for numerical simulations under certain assumptions for frequency domain analysis. In this research work, air compressibility in the OWC devices is first linearised and further coupled with the hydrodynamics of the OWC. It is able to show mathematically that in frequency-domain, air compressibility can increase the spring coefficients of both the water body motion and the device motion (if it is a floating device), and enhance the coupling effects between the water body and the structure. Corresponding to these changes, the OWC performance, the capture power, and the optimised Power Take-off (PTO) damping coefficient in the wave energy conversion can be all modified due to air compressibility. To validate the frequency-domain results and understand the problems better, the more accurate time-domain simulations with fewer assumptions have been used for comparison. It is shown that air compressibility may significantly change the dynamic responses and the capacity of converting wave energy of the OWC devices if the air chamber is very large.

Design of non-linear controller for ocean wave energy plant

This paper proposes nonlinear control strategies for oscillating water column (OWC) wave power-take-off (PTO) systems. The system consists of Wells turbine, doubly fed induction generator (DFIG) and irregular wave models. This study considers a separately excited DC motor for emulating the Wells turbine. Plant emulation needs advanced control algorithms so that it reflects the practical behavior of actual plant under varying characteristics of ocean waves. Therefore, three non-linear controllers (NLC-1, 2 and 3) for OWC plant emulation and DFIG control have been designed using the Lyapunov stability theorem. The Pierson-Moskowitz and JONSWAP irregular wave models which represent realistic sea wave conditions have been applied for validating the proposed controllers. The OWC plant parameters such as Wells turbine flow coefficient, turbine torque and electrical output power has been compared between uncontrolled and controlled cases. The proposed rotational speed controller NLC-2 provides enhanced mean output power against uncontrolled operation of plant. The enhancement in output power is obtained as 25.14% and 39.24% for JONSWAP and Pierson-Moskowitz wave spectrum.

Up-Wave and Autoregressive Methods for Short-Term Wave Forecasting for an Oscillating Water Column

The real-time control of wave energy converters (WECs) requires the prediction of the wave elevation at the location of the device in order to maximize the power extracted from the waves. One possibility is to predict the future wave elevation by combining its past history with the spatial information coming from a sensor which measures the free surface elevation up-wave of the WEC. As an application example, this paper focuses on the prediction of the wave elevation inside the chamber of the oscillating water column (OWC) for the Pico OWC plant in the Azores, and two different sensors for the measurement of the free surface elevation up-wave of the OWC were tested. The study showed that the use of the additional information coming from the up-wave sensor does not significantly improve the linear prediction of the chamber wave elevation given by a forecasting model based only on the past values of the chamber wave elevation.

Wave energy resource assessment for a breakwater-integrated oscillating water column plant at Porto, Portugal

The objective of this paper is to present the wave energy resource assessment having in view the construction of an OWC (oscillating water column) to be integrated into a new breakwater at the mouth of the Douro River in Porto (northern Portugal). The paper starts with the evaluation of the offshore wave energy resource from measured data. This is followed by the wave transformation from offshore conditions to the plant location in about 11 m water-depth. Such transformation was carried out using an inverse-ray refraction model that is described in detail. As expected, the wave power level is lower, whereas the wave energy period is higher, at the plant coastal site as compared with deep water conditions. The average wave direction rotates from approximately NW offshore to nearly West at the plant location, and the directional spread becomes smaller, which are effects of refraction as the waves propagate in waters of decreasing depth.

Performance of an Ocean Energy Conversion System with DFIG Sensorless Control

The 2009/28/EC Directive requires Member States of the European Union to adopt a National Action Plan for Renewable Energy. In this context, the Basque Energy Board, EVE, is committed to research activities such as the Mutriku Oscillating Water Column plant, OWC. This is an experimental facility whose concept consists of a turbine located in a pneumatic energy collection chamber and a doubly fed induction generator that converts energy extracted by the turbine into a form that can be returned to the network. The turbo-generator control requires a precise knowledge of system parameters and of the rotor angular velocity in particular. Thus, to remove the rotor speed sensor implies a simplification of the hardware that is always convenient in rough working conditions. In this particular case, a Luenberger based observer is considered and the effectiveness of the proposed control is shown by numerical simulations. Comparing these results with those obtained using a traditional speed sensor, it is shown that the proposed solution provides better performance since it increases power extraction in the sense that it allows a more reliable and robust performance of the plant, which is even more relevant in a hostile environment as the ocean.

Sensitivity Analysis for the Encaged Turbine Concept in Oscillating Water Column Plants

Oscillating water column plants are one of the most popular wave energy device types. Prototype OWC units have been operating in various parts of the world since the mid-1980s and such developers have more field experience of this technology than any other relevant plant. The most common turbine used is the self-rectifying Well's turbine which has a rather low peak efficiency if compared to other designs but was preferred in terms of its simplicity and cycle performance. The present study exploits the merits of a new concept for the power extraction process, that of an encaged turbine for OWC plants, which allows conventional high-efficiency turbines to be employed in such plants. This is achieved by guiding the pressurized air into a sequence of three chambers, creating a unidirectional closed air circuit through the turbine. A theoretical model is deployed simulating the operation of the plant and a sensitivity analysis is carried out for the design and working parameters. Results indicate that the power extraction efficiency may exceed the 50% level in a real plant.

Sliding-Mode Control of Wave Power Generation Plants

The worldwide demand for energy requires alternatives to fossil fuels and nuclear fission, so renewable resources, particularly ocean energy, are called to play a relevant role in a near future. In particular, the oscillating water column (OWC) is one of the most promising devices to harness energy from the sea, as it is the case of the Nereida project plant, located in the Basque coast of Mutriku. This kind of devices consists of a particular type of turbine and a doubly fed induction generator. The turbogenerator module is usually controlled using a traditional proportional-integral (PI)-based vector control scheme, which requires an accurate knowledge of the system parameters and lacks of robustness, limiting, in some cases, the power extraction. To avoid these drawbacks, a novel sliding-mode-control-based vector control scheme for the OWC plant is presented in this paper. This variable-structure control is intrinsically robust under parameter uncertainties, which always appear in real systems, and presents a convenient disturbance rejection. The stability of the proposed controller is analyzed using the Lyapunov theory. The performance of the control scheme presented is proved by comparing it to the traditional PI-based vector control scheme in a series of representative maximum power generation case studies. Both numerical simulations and experimental results show that the proposed solution provides high-performance dynamic characteristics, improving the power extraction in spite of parameter uncertainties and system disturbances.