Inertial Sea Wave Energy Converter Research Articles

Dynamic analysis and performance assessment of the Inertial Sea Wave Energy Converter (ISWEC) device via harmonic balance

Given the particular energy-maximising performance objective, wave energy converter (WEC) systems are prone to exhibit highly nonlinear behaviour. We present, in this paper, a detailed dynamic analysis and control synthesis for the Inertial Sea Wave Energy Converter (ISWEC) system, deriving and considering a comprehensive associated nonlinear model. In particular, we adopt a harmonic balance (HB) method to achieve this objective, producing the so-called amplitude-frequency curves (AFC) for the corresponding ISWEC nonlinear model, derived via a Lagrangian approach. We demonstrate that the system can present a variety of different behaviours which are completely neglected by its linear model counterpart. Leveraging both the efficiency, and convenient representation of the HB method, we synthesise so-called ‘passive’ (i.e. proportional) energy-maximising controllers using a variety of input conditions. We provide a comparison of the obtained control parameters with those arising from standard linear modelling, showing a consistent improvement in performance by effectively considering the relevant nonlinear ISWEC dynamics.

Satellite Multi/Hyper Spectral HR Sensors for Mapping the Posidonia oceanica in South Mediterranean Islands

The Mediterranean basin is a hot spot of climate change where the Posidonia oceanica (L.) Delile (PO) and other seagrasses are under stress due to its effect on marine coastal habitats and the rising influence of anthropogenic activities (i.e., tourism, fishery). The PO and seabed ecosystems, in the coastal environments of Pantelleria and Lampedusa, suffer additional growing impacts from tourism in synergy with specific stress factors due to increasing vessel traffic for supplying potable water and fossil fuels for electrical power generation. Earth Observation (EO) data, provided by high resolution (HR) multi/hyperspectral operative satellite sensors of the last generation (i.e., Sentinel 2 MSI and PRISMA) have been successfully tested, using innovative calibration and sea truth collecting methods, for monitoring and mapping of PO meadows under stress, in the coastal waters of these islands, located in the Sicily Channel, to better support the sustainable management of these vulnerable ecosystems. The area of interest in Pantelleria was where the first prototype of the Italian Inertial Sea Wave Energy Converter (ISWEC) for renewable energy production was installed in 2015, and sea truth campaigns on the PO meadows were conducted. The PO of Lampedusa coastal areas, impacted by ship traffic linked to the previous factors and tropicalization effects of Italy’s southernmost climate change transitional zone, was mapped through a multi/hyper spectral EO-based approach, using training/testing data provided by side scan sonar data, previously acquired. Some advanced machine learning algorithms (MLA) were successfully evaluated with different supervised regression/classification models to map seabed and PO meadow classes and related Leaf Area Index (LAI) distributions in the areas of interest, using multi/hyperspectral data atmospherically corrected via different advanced approaches.

The inertial sea wave energy converter (ISWEC) technology: Device-physics, multiphase modeling and simulations

In this paper we investigate the dynamics of the inertial sea wave energy converter (ISWEC) device using fully-resolved computational fluid dynamics (CFD) simulations. Originally prototyped by the Polytechnic University of Turin, the device consists of a floating, boat-shaped hull that is slack-moored to the sea bed. Internally, a gyroscopic power take-off (PTO) unit converts the wave-induced pitch motion of the hull into electrical energy. The CFD model is based on the incompressible Navier–Stokes equations and utilizes the fictitious domain Brinkman penalization (FD/BP) technique to couple the device physics and water wave dynamics. A numerical wave tank is used to generate both regular waves based on fifth-order Stokes theory and irregular waves based on the JONSWAP spectrum to emulate realistic sea operating conditions. A Froude scaling analysis is performed to enable two- and three-dimensional simulations for a scaled-down (1:20) ISWEC model. It is demonstrated that the scaled-down 2D model is sufficient to accurately simulate the hull’s pitching motion and to predict the power generation capability of the converter. A systematic parameter study of the ISWEC is conducted, and its optimal performance in terms of power generation is determined based on the hull and gyroscope control parameters. It is demonstrated that the device achieves peak performance when the gyroscope specifications are chosen based on the reactive control theory. It is shown that a proportional control of the PTO control torque is required to generate continuous gyroscopic precession effects, without which the device generates no power. In an inertial reference frame, it is demonstrated that the yaw and pitch torques acting on the hull are of the same order of magnitude, informing future design investigations of the ISWEC technology. Further, an energy transfer pathway from the water waves to the hull, the hull to the gyroscope, and the gyroscope to the PTO unit is analytically described and numerically verified. Additional parametric analysis demonstrates that a hull length to wavelength ratio between one-half and one-third yields high conversion efficiency (ratio of power absorbed by the PTO unit to wave power per unit crest width). Finally, device protection during inclement weather conditions is emulated by gradually reducing the gyroscope flywheel speed to zero, and the resulting dynamics are investigated.

ISWEC Devices on a Wave Farm Handled by a Multi-Agent System

This paper aims to contribute to the improvement of the energy extracted from Wave Energy Converters, arrayed in a wave farm, through the implementation of several actions, namely wave extrapolation using a dynamic neural network, wave type identification resorting to a machine learning system, and coordination of the interoperability, communication, and control of all the devices belonging to the wave farm via a Multi-Agent System. The Multi-Agent System was initially developed and subsequently tested through simulations. Simulations used the Inertial Sea Wave Energy Converter, which extracts power from sea waves, using gyroscopic motion, to generate electricity. This device can be originally controlled by proportional-derivative controllers with parameters that change with the sea state. With this Multi-Agent System, ocean waves and corresponding sea states are identified using a machine learning wave classifier method. Information is passed on so that all devices can use optimal control parameters. Therefore, energy extraction by the wave farm is enhanced due to the combination of the Multi-Agent System with automatic learning methods, while its architecture is resilient to communication problems between devices. Simulation results for a case study showed an increase in the average power absorbed, proving the effectiveness of combining a Multi-Agent System with automatic learning methods. Conclusions demonstrated that the proposed system architecture is a viable alternative to the original wave forecasting model. Additionally, this Multi-Agent System can be adapted to wave farms with other Wave Energy Converters and other types of control.

Experimental Investigation of the Mooring System of a Wave Energy Converter in Operating and Extreme Wave Conditions

A proper design of the mooring systems for Wave Energy Converters (WECs) requires an accurate investigation of both operating and extreme wave conditions. A careful analysis of these systems is required to design a mooring configuration that ensures station keeping, reliability, maintainability, and low costs, without affecting the WEC dynamics. In this context, an experimental campaign on a 1:20 scaled prototype of the ISWEC (Inertial Sea Wave Energy Converter), focusing on the influence of the mooring layout on loads in extreme wave conditions, is presented and discussed. Two mooring configurations composed of multiple slack catenaries with sub-surface buoys, with or without clump-weights, have been designed and investigated experimentally. Tests in regular, irregular, and extreme waves for a moored model of the ISWEC device have been performed at the University of Naples Federico II. The aim is to identify a mooring solution that could guarantee both correct operation of the device and load carrying in extreme sea conditions. Pitch motion and loads in the rotational joint have been considered as indicators of the device hydrodynamic behavior and mooring configuration impact on the WEC.

Optimizing energy production of an Inertial Sea Wave Energy Converter via Model Predictive Control

We introduce an original Model Predictive Control (MPC) approach, aimed at optimizing the energy production of an Inertial Sea Wave Energy Converter (ISWEC). The aim of the proposed method is to handle the performance tradeoff among different control objectives such as energy production, control effort and speed limitation through a suitable MPC formulation. Mechanical and electrical physical constraints are taken into account to ensure proper working conditions to the ISWEC system. We employ a linear model of the ISWEC device to cast the underlying MPC optimization problem as a quadratic program, to allow a fast online implementation of the controller by means of an efficient solver. We introduce extensive simulation tests performed on a detailed nonlinear ISWEC model to show the effectiveness of the presented approach.

Numerical and Experimental Identification of the Aerodynamic Power Losses of the ISWEC

The wave energy sector is experiencing lively years of conceptual innovation and technological advances. Among the great variety of candidates, only a few are going to be able to reach maturity and, eventually, industrial feasibility and competitiveness. The essential requisite for success is the continuous innovation in response to the incremental experience gained during the design and prototyping stages. In particular, the ability to generate detailed mathematical models, representative of every phenomenon involved in the system, is crucial for informing the design and control stages, allowing to maximize productivity while minimizing costs, and inspiring technological breakthrough and innovation. This papers considers the case of the ISWEC (Inertial Sea Wave Energy Converter), where a technological leap is tightly linked with the modelling of aerodynamic losses around its spinning flywheel, the core of the energy conversion chain. Two mathematical models of increasing complexity are considered, one semi-empiric and one based on computational fluid dynamics, which are successfully validated against experimental data. Such models are used to quantify the benefits of a technological innovation consisting of enclosing the flywheel in a sealed container, allowing pressure regulation to reduce aerodynamic friction. Compared to the free configuration, power losses with the enclosed configuration are about half already at atmospheric pressure, and about one third at half the atmospheric pressure.

State of the Art and Perspectives of Wave Energy in the Mediterranean Sea: Backstage of ISWEC

According to the European Commission, sea waves have a great potential as renewable energy source. Despite wave energy technology is a field in continuous development, it is not yet competitive with the other renewables, due to the small quantities of devices sold, most of them being prototypal solutions at level. So far, various Wave Energy Converter concepts have been developed and some of them tested in full scale. The most recurrent test environment is the North Atlantic Ocean, which possesses high energy potential. The Mediterranean Sea on the other hand is less energetic, but also possesses less dangerous extreme conditions. It represents a favourable starting point to develop technologies that later will be scaled up to more powerful sites. This article illustrates the wave energy potential of the Mediterranean and analyses the wave energy converters engineered according to sea states characteristic of the Mediterranean Sea. Focus is brought to the Inertial Sea Wave Energy Converter (ISWEC) technology, which is one of the few Mediterranean concept to have reached Technology Readiness Level (TRL) 7. The article will document the deployment and the following open sea test campaign of a full scale prototype off the shore of Pantelleria Island, Italy.

Mathematical Modeling and Scaling of the Friction Losses of a Mechanical Gyroscope

Friction is a complicated phenomenon that plays a central role in a wide variety of physical systems. An accurate modeling of the friction forces is required in the model-based design approach, especially when the efficiency optimization and system controllability are the core of the design. In this work, a gyroscopic unit is considered as case study: the flywheel rotation is affected by different friction sources that needs to be compensated by the flywheel motor. An accurate modeling of the dissipations can be useful for the system efficiency optimization. According to the inertial sea wave energy converter (ISWEC) gyroscope layout, friction forces are modeled and their dependency with respect to the various physical quantities involved is examined. The mathematical model of friction forces is validated against the experimental data acquired during the laboratory testing of the ISWEC gyroscope. Moreover, in the wave energy field, it is common to work with scale prototypes during the full-scale device development. For this reason, the scale effect on dissipations has been correlated based on the Froude scaling law, which is commonly used for wave energy converter scaling. Moreover, a mixed Froude–Reynolds scaling law is taken into account, in order to maintain the scale of the fluid-dynamic losses due to flywheel rotation. The analytical study is accompanied by a series of simulations based on the properties of the ISWEC full-scale gyroscope.

On-board sea state estimation method validation based on measured floater motion

This paper presents a method to estimate the sea state PSD (Power Spectral Density) of the current wave climate, by using the measured floater motion and the body hydrodynamic response. The knowledge of PSD and the sea state synthetic parameters derivable from the PSD, such as the Significant Wave Height and the Energy Period, is fundamental for the navigation and operation in naval field and also for the control strategies of the Wave Energy Converters (WEC). The ISWEC (Inertial Sea Wave Energy Converter) is used as case study for the validation of the sea state estimation method. ISWEC is a floating device using the inertial effects of a gyroscopic system to convert a floater motion into electric energy. Sea state parameters are used in the control of the device to tune gyroscope speed and the generator torque law to achieve maximum power absorption. The heave measurements are used to estimate the PSD of the incoming wave and it is compared with the wave PSD measured by a wave measurement system. The method is studied and validated for three different sea state cases. At this stage the method presents satisfying results, with an accuracy under the 10% of the estimated parameters. Such accuracy is comparable with the short term (1-3h) wave forecast produced by ECMWF.

ISWEC linear quadratic regulator oscillating control

Wave energy is one of the most promising technologies for the future of renewable energy. Its steady, even if not dashing, development, both in theoretical and applied solutions, will bring the field, in the mid-term, to the point of being a viable, economical and sustainable technology, competitive when compared to more mature ones. This paper deals with the ISWEC (Inertial Sea Wave Energy Converter) power take off (PTO) technology and control strategies. In particular it focuses on the control strategy used for harnessing energy during its first deployment, and presents the development of a new controller designed using optimal control theory. The approach to the problem of this work is not to seek for the theoretical optimal control strategy. In fact, during the test period of the ISWEC into the sea, limitations of a model-optimized control strategy came out, due to well known shortcomings of hydrodynamics modeling theory for wave energy devices, and the idea of an easy-to-tune in the field controller arose. This work points out in the direction of developing a practical and effective strategy for industrial application, with the main objective of increasing the power production while simplifying the control law by which the device harnesses energy.

ISWEC design tool

ISWEC (Inertial Sea Wave Energy Converter) is a gyroscopic effect based wave power extraction device. The aim of this paper is to present a tool developed for the design process of this WEC. The main purpose of the software is to allow to the system engineer to compute the annual electrical productivity using a simplified model, taking into account several considerations such as system geometrical and electrical configurations, the hull properties or the installation site. To obtain this result, an automatic optimization of control parameters is implemented. A cost function is minimized under various system constraints (e.g. PTO torque and power, flywheel speed) and the absorbed power of the system is evaluated. Output of the software is the device productivity, along with considerations about structural loads, PTO utilization, floater and mass properties. The software will help the system engineer to take preliminary decisions and to verify the adaptability of this WEC to the considered sea site. The methodology here proposed can be easily generalised to other applications, especially in wave energy field, where severe constraints and time consuming simulations are common, and the use of a tool like this can reduce develop time, giving different information from the very start of the project.

Productivity analysis of the full scale inertial sea wave energy converter prototype: A test case in Pantelleria Island

Wave power is one of the most rich and promising sources of renewable energy for the future. Approximately 2000 TWh/year can be produced through the exploitation of the wave energy potential. In the past four decades, hundreds of Wave Energy Converters have been proposed and studied, but so far a conclusive architecture to harvest wave power has not been identified. Many engineering problems are still to be solved; these include survivability, durability, and effective power capture in a variable wave climate. Reacting body devices use the inertia of a large mass to generate the reaction needed from the power take off (PTO). Heretically, in the case of a simple inertial mass, optimal control adjusts the dynamic parameters of the PTO, such as the spring constant and energy absorbing damping, to maximize energy absorption. The ISWEC (Inertial Sea Wave Energy Converter) uses a gyroscope to create an internal inertial reaction that is able to harvest wave power without exposing mechanical parts to the harsh oceanic environment. In the past few years, the ISWEC has been successfully tested using two scale models (scales 1:45 and 1:8) and several extensive laboratory experimental campaigns. In this paper, the first full scale ISWEC prototype is presented along with its control system and a refined control strategy. The goal of this paper is to identify an optimal control strategy in order to maximize wave power exploitation of the ISWEC. The control technique presented is numerically applied to the ISWEC full scale device with rated power of 60 kW. The control strategy is tested, and the expected production obtained, for the typical wave climate of Pantelleria Island, in the Mediterranean Sea where the first full scale ISWEC prototype was deployed in autumn 2015.

Performance assessment of a 2 DOF gyroscopic wave energy converter

Wave Power is one of the most investigated energy sources today. So far, several devices have been tested and built up to the pre-commercial stage. ISWEC (Inertial Sea Wave Energy Converter) has developed at the Politecnico di Torino, exploiting gyroscopes to extract wave energy. It allows power extraction without using any moving part immersed into water. The previous version of ISWEC presented 1 DOF (Degree Of Freedom), therefore requiring alignment of the device to the incoming wave; this paper describes a novel version of ISWEC, with 2 DOFs and consequently able to absorb power from every wave direction. The kinematics and the dynamics of the device are investigated, in order to compare the 1 DOF and the 2 DOF architectures from the point of view of the extracted power. The resulting simulations show that the 1 DOF prototype is more efficient when aligned with the incoming wave, while the behavior of the 2 DOF device is substantially independent of the wave direction. Such a difference of the performances is quantified and discussed along with considerations on the design and realization of the full-scale prototype.

Stochastic Control of Inertial Sea Wave Energy Converter

The ISWEC (inertial sea wave energy converter) is presented, its control problems are stated, and an optimal control strategy is introduced. As the aim of the device is energy conversion, the mean absorbed power by ISWEC is calculated for a plane 2D irregular sea state. The response of the WEC (wave energy converter) is driven by the sea-surface elevation, which is modeled by a stationary and homogeneous zero mean Gaussian stochastic process. System equations are linearized thus simplifying the numerical model of the device. The resulting response is obtained as the output of the coupled mechanic-hydrodynamic model of the device. A stochastic suboptimal controller, derived from optimal control theory, is defined and applied to ISWEC. Results of this approach have been compared with the ones obtained with a linear spring-damper controller, highlighting the capability to obtain a higher value of mean extracted power despite higher power peaks.

Hardware-In-the-Loop test rig for the ISWEC wave energy system

The Hardware-In-the-Loop (HIL) simulation is a powerful mean to reduce costs in the design and manufacturing process of an engineering system. HIL techniques allow to use real components inside a simulation of a mathematical model. In this work such techniques are used on the ISWEC wave energy system. ISWEC (Inertial Sea Wave Energy Converter) converts sea waves energy to electric energy by means of the gyroscopic effects produced by a spinning flywheel. The peculiarity of the system lays in the fact that all the moving parts needed to produce energy are sealed inside a hull and therefore protected from the aggressive ocean climate. During the research process on the ISWEC, the gyroscope and the electric generator have been manufactured and mounted on a test rig able to simulate the wave actions on the hull of ISWEC. Those real parts of the system have been replaced inside the full mathematical model of ISWEC. Such HIL system is validated against real wave tank tests carried out at the INSEAN in Rome. The HIL simulations proved to reproduce the real behavior in water waves of ISWEC with errors as small as the 10%.

Linear Tubular Permanent-Magnet Generators for the Inertial Sea Wave Energy Converter

Marine energy represents a very promising source of energy available worldwide. Although this energy is available in several forms, wave energy is the more widespread source characterized by high power density. In order to exploit energy from waves, numerous systems have been designed and commercialized. The Inertial Sea Wave Energy Converter is a device based on a floating body slack-moored to the seabed, using a gyroscope as a reference frame to produce electric power. This paper describes the design of the proposed converter equipped with two linear tubular permanent-magnet generators, exploiting the reciprocating motion between the gyroscopic system and the hull to generate power. The design of a two-phase linear tubular generator is analyzed. Experimental tests on a scale prototype have been performed.

ISWEC: A gyroscopic mechanism for wave power exploitation

Abstract In the past four decades, hundreds of Wave Energy Converters (WECs) have been proposed and studied, but so far a final architecture to harvest wave power has not been identified. Many engineering problems are still to be solved, like survivability, durability and effective power capture in a variable wave climate. ISWEC (Inertial Sea Wave Energy Converter) is a system using the gyroscopic reactions provided from a spinning flywheel to extract power. The flywheel works inside a sealed floating body in order to be protected from the outer environment and grant a reliable and durable operation. The article summarizes the design procedure of a 1:45 scaled ISWEC device with rated power 2.2 W and the tank tests performed with a simplified plain float to verify the actual prototype power capabilities. The article then focuses on the implementation of a non-linear coupled model (mechanics + hydrodynamics) to improve the float shape in order to maximize the power absorption. The final result is a float shape capable to absorb a power almost three times bigger (5.96 W) than the initial float shape.