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

High torque, low velocity pneumatic rotary servomotor

The work deals about the design of a pneumatic rotating servomotor intended for low speed, high torque operation. The servosystem is based on a pneumatic orbital motor working in the 0 – 80 rpm range with a maximum torque of 28 Nm, and hence it can be applied in heavy duty application. The paper analyses the kinematic layout of the motor, shows the results of characterisation tests and finally it discusses the layout and the components to implement closed loop velocity control. The effectiveness of control layout based on the use of pressure proportional and flow proportional valves is compared by experimental tests. The results show the capabilityof the servosystem to keep the set velocity and to react to external disturbance

Simulating water-entry/exit problems using Eulerian-Lagrangian and fully-Eulerian fictitious domain methods within the open-source IBAMR library

In this paper we employ two implementations of the fictitious domain (FD) method to simulate water-entry and water-exit problems and demonstrate their ability to simulate practical marine engineering problems. In FD methods, the fluid momentum equation is extended within the solid domain using an additional body force that constrains the structure velocity to be that of a rigid body. Using this formulation, a single set of equations is solved over the entire computational domain. The constraint force is calculated in two distinct ways: one using an Eulerian-Lagrangian framework of the immersed boundary (IB) method and another using a fully-Eulerian approach of the Brinkman penalization (BP) method. Both FSI strategies use the same multiphase flow algorithm that solves the discrete incompressible Navier-Stokes system in conservative form. A consistent transport scheme is employed to advect mass and momentum in the domain, which ensures numerical stability of high density ratio multiphase flows involved in practical marine engineering applications. Example cases of a free falling wedge (straight and inclined) and cylinder are simulated, and the numerical results are compared against benchmark cases in literature.Comment: The current paper builds on arXiv:1901.07892 and re-explains some parts of it for the reader's convenienc

Design Of a Spar Buoy for Offshore Wind Turbines

Offshore wind Energy demonstrated to be one of the most promising technologies for growing electric energy demand worldwide. The main objective of this research was to conceive a floating offshore structure for supporting wind turbines. The Mediterranean Sea is characterized by deep water, especially in the western area, from Italy towards Spain, and this makes floating support structures desirable, since in deep water they are cheaper than bottom fixed piles [1]. An aerodynamic, electric and mechanic model was developed and tested against experimental results of laboratory and wind tunnel tests on a small scale wind turbine, demonstrating its reliability, so it was used to determine actions on the floater and a first concept design was performed using commercial software for marine structures analysis. Further steps in this process will include integration of the turbine model with hydrodynamic calculations, as well flume, tank and open sea tests for the designed floate

Towards data-driven and data-based control of wave energy systems: Classification, overview, and critical assessment

Currently, a significant effort in the world research panorama is focused on finding efficient solutions to a carbon-free energy supply, wave energy being one of the most promising sources of untapped renewable energy. However, wave energy is not currently economic, though control technology has been shown to significantly increase the energy capture capabilities. Usually, the synthesis of a wave energy control strategy requires the adoption of control-oriented models, which are prone to error, particularly arising from unmodelled hydrodynamics, given the complexity of the hydrodynamic interactions between the device and the ocean. In this context, data-driven and data-based control strategies provide a potential solution to some of these issues, using real-time data to gather information about the system dynamics and performance. Thus motivated, this study provides a detailed analysis of different approaches to the exploitation of data in the design of control philosophies for wave energy systems, establishing clear definitions of data-driven and data-based control in this field, together with a classification highlighting the various roles of data in the control synthesis process. In particular, we investigate intrinsic opportunities and limitations behind the use of data in the process of control synthesis, providing a comprehensive review together with critical considerations aimed at directly contributing towards the development of efficient data-driven and data-based control systems for wave energy devices

Data-driven control of a Pendulum Wave Energy Converter: A Gaussian Process Regression approach

The energy coming from the motion of the waves of seas and oceans could be an important component in the solution of the energy problem related to the pursuit of alternatives to fossil fuels. However, wave energy is still technologically immature and it has not reached the economic feasibility required for economy of scale. One of the major technological challenges for the achievement of this goal is the development of control strategies capable of maximizing the extracted energy, adapting to the conditions of the seas and oceans that surround the Wave Energy Converter (WEC) devices. To perform this task, control systems often adopt explicitly control-oriented models, that are by nature affected by uncertainties. On the contrary, to address the problem a data-driven solution is proposed here. The presented strategy applies an optimization approach based on a Gaussian Process Regression (GPR) metamodel to learn the control strategy to be applied. In order to accelerate the learning process, we present a novel method that exploits in the initial phase a previous knowledge given by simulations with the system model and based on the co-kriging concept. To test this approach the Pendulum Wave Energy Converter has been adopted as a case study. To differentiate the previous knowledge and the real system behaviour, a simplified linear model is used to obtain the prior knowledge, while a complex nonlinear one acts as the environment in which simulate the behaviour of the real system. A month-long simulation is used to validate the effectiveness of the proposed strategy, showing the ability of adapting to a real system different from the simplified model on the basis only of data, and overcoming the model-based strategy in terms of performance

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

Mooring System Design and Analysis for a Floating Offshore Wind Turbine in Pantelleria

The mooring system plays a key role in a floating offshore wind turbine: it connects the floating structure to its anchor on the seabed and it is designed to prevent the platform from drifting under the action of wind, waves and currents. The layout of the mooring system is strictly connected to the installation site: in the first place it depends on the bathymetry and the type of seabed which conditions the type of anchor that can be used; secondly by the wind and waves loads in extreme sea states. To properly design the mooring system, three different configurations are proposed and discussed, respectively adapting catenary, taut leg and semi-taut methodologies for a floating offshore wind turbine located near the island of Pantelleria, in Sicily. For each configuration, the Hexafloat foundation, developed by Saipem, is considered. Important design constraints such as how large the nominal sizes are, how long the mooring lines are, how far the anchor points are located, are demonstrated in detail. The material used will range from steel chains and wires to polyester ropes, to grant economically viable solutions