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In the struggle against increasing global warming, wave renewable energy could help progress to the increasingly necessary renewable-based energy-mix. Wave energy has many applications, one of which involves its capture and conversion into usable energy. To date, there has been no agreement in the scientific community with respect to a single wave energy converter design, and given the numerous concepts involved, many approaches have been developed to investigate the performance of these different devices. This work summarises these approaches in addition to completing and extending them regarding wave spectrum, wave direction, generator capacity limitations, and use of the Froude similarity law for wave energy converter scaling. Ultimately, this study aims to improve the selection process of the most appropriate converter configuration for a given resource (i.e. wave distribution climate). The main results showed that neglecting the wave direction can lead to a 10 % energy production overestimation when the method is applied to the Wavepiston device. Furthermore, for Weptos and Wavepiston wave energy converters, different standard wave spectra were found to estimate the energy production with less than 8 % difference, below the 16.5 % estimated in previous studies. Additionally, the Reverse Froude Law method is introduced to help assess the applicability of the Froude similarity law for converter configuration scaling. Finally, a threshold in the decrease of generator capacity is determined such that, despite limiting the device power production potential, annual energy production remained almost as high as without generator limitations for an approximately 40 % capacity factor increase.

The paper discusses results of regular wave tests conducted at the University of Naples ???Federico II??? with the purpose of investigating the qualitative features and the magnitude of the wave pressures acting onto the front face of a Seawave Slot-Cone Generator (SSG). Various wave conditions have been considered, ranging from standing to plunging. Calculation formulae for engineering applications are proposed and discussed.

The WEPTOS is a well-known wave energy converter (www.weptos.com), formed by several rotors, with a shape that draws upon the reputable Salters Duck geometry. The WEPTOS has a large efficiency under waves of one particular frequency, i.e. when resonance conditions occur. In order to extend the range of resonance conditions, the possible use of a negative stiffness in the rotor system is analysed. This note presents some considerations on the effectiveness of negative stiffness based on physical model experiments and on the numerical simulations used to interpret the test results. More specifically, the aim of the work is to evaluate how the WEPTOS performance is affected by the presence of a spring into the system that enhances the rotor oscillations applying a negative (un-stabilizing) torque proportional to the degree of rotation. Experiments were carried out in the deepwater wave basin of Aalborg University, on a 1:30 scale model of one rotor of the WEPTOS. Three different configurations were analysed, providing different values of the negative stiffness. A set of 16 regular and 5 irregular wave conditions were tested, with maximum heights up to approximately 6 m at prototype scale, periods ranging from 5 to 9 s. The incident wave characteristics, the device rotations and the power dissipated by a dummy power converter were accurately measured. The effect of the negative stiffness was partly hindered by the presence of friction, so that no conclusive evidence could be drawn. A simple numerical model, where the rotor was interpreted as an oscillating mass-damper- spring system, was set up and calibrated to the experimental results. Reasonably good agreement between predictions and measurements were found at model scale. The numerical simulations at prototype dimensions, where friction effects have a relatively lower importance, revealed that the springs responsible of the negative stiffness increase in fact the efficiency of the device.

The purpose of this paper is to examine the hydrodynamics around floating wave energy converters (f- WECs). In particular, the paper considers the case of the f-WEC of the Wave Activated Body type, named DEXA. Based on 3D wave experiments in the Laboratory of the Aalborg University (DK), the modified wave field around a wave energy farm (composed by three 1:60 scale models) and around a single device (1:30 scale model) is investigated. Specific results include wave reflection, wave transmission and wave disturbance around the device. The results are examined considering scale effects, influence of wave length and wave steepness.

This paper analyses the performance of a new Wave Energy Converter (WEC) of the Oscillating Water Column type (OWC), named WavePiston. This near-shore floating device is composed of plates (i.e. energy collectors) sliding around a cylinder, that is placed perpendicular to the shore. Tests in the wave basin at Aalborg University allowed to investigate power production in the North Sea typical wave climate, with varying design parameters such as plate dimensions and their mutual distance. The power produced per meter by each collector is about the 5% of the available wave power. Experimental results and survivability considerations suggest that the WavePiston would be particularly suited for installations also in milder seas. An example application is therefore presented in the Mediterranean Sea, off-shore the island of Sicily. In this case, each collector harvests the 10% of the available wave power. In order to allow the extension of the experimental dataset, an analytic model is also developed based on mass-spring-damper system concept. The model is calibrated and validated to represent device displacements and loads in order to obtain the power production based on device geometry and conditions of incident wave attack.

Oscillating Water Column based wave energy plants built so far have experienced a low efficiency in the conversion of the bidirectional oscillating flow. A new concept is considered here, the LeanCon Wave Energy Converter (WEC), that unifies the flow direction by use of non-return valves, into a unidirectional flow, making the use of more efficient air turbines possible. Hereby, a more steady flow is also obtained.The general objective of this note is to examine, the power take off (PTO) efficiency under irregular wave conditions, for WECs with flow redirection. Final practical aim is to identify a method that allows the choice of the optimal power generation capacity for which the device should be designed, when subjected to any given wave climate.The analysis is based on the experimental results of existing tests carried out in the 3D deep water wave tank at Aalborg University, Denmark. First, the power measured at the modelled PTO is compared with the available incident wave power in order to examine the overall system response in a scaleindependent manner. Then, the power production density function is fitted to a simplified shape, whose parameters are related to the tested sea state conditions (wave period, wave height). Average performance and stochastic variability is thus obtained for any sea state and therefore also for the annual wave climate of interest.An example application of a LeanCon unit is carried out for a location off-shore Cagliari (Italy). Conclusions provide economic criteria for estimating the optimal value of power for which the PTO should be designed. Oscillating Water Column based wave energy plants built so far have experienced a low efficiency in the conversion of the bidirectional oscillating flow. A new concept is considered here, the LeanCon Wave Energy Converter (WEC), that unifies the flow direction by use of non-return valves, into a unidirectional flow, making the use of more efficient air turbines possible. Hereby, a more steady flow is also obtained.The general objective of this note is to examine, the power take off (PTO) efficiency under irregular wave conditions, for WECs with flow redirection. Final practical aim is to identify a method that allows the choice of the optimal power generation capacity for which the device should be designed, when subjected to any given wave climate.The analysis is based on the experimental results of existing tests carried out in the 3D deep water wave tank at Aalborg University, Denmark. First, the power measured at the modelled PTO is compared with the available incident wave power in order to examine the overall system response in a scaleindependent manner. Then, the power production density function is fitted to a simplified shape, whose parameters are related to the tested sea state conditions (wave period, wave height). Average performance and stochastic variability is thus obtained for any sea state and therefore also for the annual wave climate of interest.An example application of a LeanCon unit is carried out for a location off-shore Cagliari (Italy). Conclusions provide economic criteria for estimating the optimal value of power for which the PTO should be designed.

The paper presents the preliminary results of new physical tests carried out in the directional wave basin of Aalborg University (DK). The devices under exams are two floating 7 Degrees of Freedom Wave Activated Bodies moored with a spread system composed by 4 steel chains. The devices were subject to ordinary North Sea wave climate conditions and deployed in 1:60 scale. The main purpose of this paper is to analyse the performance of a Wave Energy Converter considering the interdependencies among energy production, loads on real moorings and device movements. The mooring effects on power production and on device movements are specifically investigated by varying the chain pre-tension level. Results suggest that the power production optimization is achieved with a slack mooring system providing a quasi-static response to the ordinary wave attacks.

The aim of this research is to develop a numerical modelling approach able to provide design criteria for the optimization of the layout of a farm of Wave Energy Converters (WECs). The numerical model MIKE 21BW is calibrated against the experimental results obtained in Aalborg wave tank for a farm of floating WECs. The WECs under exam belong to the Wave Activated Body type. The calibrated model allows to tests more configurations at lower cost than physical models. Specifically, different mutual distance among the devices and their mutual alignment are examined to maximise the density of the devices and the distribution of residual wave energy and therefore energy production. Simulated and measured wave heights recorded by the many wave gauges placed in the basin are compared, as well as wave transmission and reflection induced by the devices. A sensitivity analysis, i.e. the identification of the key model parameters to be tuned in order to optimise the comparison, is also carried out.