António Sarmento

Wave generation by an oscillating surface-pressure and its application in wave-energy extraction

A two-dimensional analysis, based on linear surface-wave theory, is developed for an oscillating-water-column wave-energy device in water of arbitrary constant depth. The immersed part of the structure is assumed of shallow draught except for a submerged vertical reflecting wall. Both the cases of linear and nonlinear power take-off are considered. The results show that air compressibility can be important in practice, and its effects may in general be satisfactorily represented by linearization. The analysis indicates that using a turbine whose characteristic exhibits a phase difference between pressure and flow rate may be a method of strongly reducing the chamber length and turbine size with little change in the capability of energy extraction from regular waves. It was found in two examples of devices with strongly nonlinear power take-off that the maximum efficiency is only marginally inferior to what can be achieved in the linear case.

Turbine-controlled wave energy absorption by oscillating water column devices☆

Abstract The paper deals with phase control as a method of increasing the energy absorption by oscillating water column (OWC) devices, from regular as well as from irregular waves. The power take-off machine considered is a modified version of the self-rectifying axial-flow Wells air turbine, whose rotor blades are of variable setting angle; this allows the air pressure and flow rate to be controlled independently from each other. Results of numerical simulations are presented for three different control strategies applied to energy absorption from irregular waves by an OWC device of simple, two-dimensional geometry. Experimental data from a turbine model are used in the simulation.

Hydrodynamic coefficients of a submerged pulsating sphere in finite depth

Abstract By extending the work of Linton (Linton, C.M., 1991. Radiation and diffraction of waver waves by a submerged sphere in finite depth. Ocean Engineering 18 (1/2), 61–74), the problem of radiation of water waves by a submerged pulsating sphere in finite depth is formulated using the multipole method. As in Linton (1991), this leads to an infinite system of linear equations, which are easily solved numerically. Simple expressions are derived for the hydrodynamic characteristics of such a body. Results showing the effect of varying both the immersion depth and the water depth on the hydrodynamic coefficients of the pulsating sphere are given. The paper resumes the work presented in Lopes (Lopes, D.B.S., 1999. On the study of the Archimedes wave swing device for wave energy utilization (in Portuguese). MSc on the Management and Modelling of the Marine Environment, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa.).

Full-Scale WECs

In this chapter, some of the concepts that have reached the full-scale stage are de-scribed in detail. Other examples could be given, but the analysis was limited to four main technologies: OWC (Oscillating Water Column), AWS (Archimedes Wave Swing), Pelamis and Wave Dragon presented in 7.1 to 7.4. Each illustrates one particular power conversion mechanism that was addressed in Chapter 6, namely air turbines, direct drive linear generators and hydraulics. A subsection re-garding overtopping theory is also presented, as it was not addressed in the previ-ous chapter and is linked with one of the technologies (Wave Dragon). Section 7.5 gives an account of the operational experience gathered by the several technology developers. Such detail provides valuable lessons to those interested in the field and to a wide engineering audience. To conclude, section 7.6 provides a brief up-date on test centres, pilot zones, a review on the most relevant EU funded projects and a case study related to one of the technologies.

Improving R&D impact on wave energy commertial development

When analyzing the R&D activities in Europe, it is apparent that the agenda is not sufficiently driven by the main technological and economic issues that need to be solved if wave energy is expected to attain commercial success, namely energy costs, survivability and environmental impacts. Hence, the present paper aims to call for a collaborative project to assess from a techno-economical point of view the potential competitiveness and constraints of wave energy concepts, in order to narrow the amount of different concepts under development nowadays, so that funds can be focused more cost-effectivelly. In this sense, the paper presents a discussion regarding hydrodynamic modeling methodologies to assess the energy output and the survivability strategy of the most promising technologies proposed, in order to carry out a comparative study among them. Nevertheless, when performing a feasibility study, technical constraints should be considered to confirm if the device engineering, construction and operation and maintenance are viable. Accordingly, a technical feasibility discussion followed by an economic analysis, to evaluate if the technology is potentially competitive or not, is also presented.

Numerical and Experimental Modelling of WECs

The design of a wave energy converter relies heavily on results from numerical simulations and experiments with scale models. Such results allow not only fun-damental design changes but also the optimisation of selected configurations. For ongoing development, and particularly at an early stage, numerical models give the flexibility of assessing a large number of versions at a relatively low cost. Physical models are then tested in wave tanks to validate the numerical simula-tions and to investigate phenomena which are not evidenced by the computational packages. This chapter provides an overview on the numerical techniques that have been used to model the hydrodynamics of wave energy converters (WECs), details on wavemaker and wave tank design, guidelines on experimental tech-niques and finally a case study related to one of the most studied concepts which reached the full-scale prototype stage.