Paulo Alexandre Justino

OWC wave energy devices with air flow control

A theoretical model is developed to simulate the energy conversion, from wave to turbine shaft, of an oscillating-water-column (OWC) plant equipped with a Wells air-turbine and with a valve (in series or in parallel with the turbine) for air-flow control. Numerical simulations show that the use of a control valve, by preventing or reducing the aerodynamic stall losses at the turbine rotor blades, may provide a way of substantially increasing the amount of energy produced by the plant, particularly at the higher incident wave power levels. From the hydrodynamic point of view, a by-pass valve or a throttle valve should be used depending on whether the wave energy absorbing system is over-damped or under-damped by the turbine.

Hydrodynamics of Multiple Floating Point-Absorber Wave Energy Systems With Inter-Body and Bottom Slack-Mooring Connections

Point absorbers constitute an important class of offshore wave energy converters. If employed in the extensive exploitation of the offshore wave energy resource, they should be deployed in arrays, the distance between elements in the array being possibly tens of meters. In such cases, it may be more convenient that the array is spread moored to the sea bottom through only some of its elements (possibly located in the periphery), while the other array elements are prevented from drifting and colliding with each other by connections to adjacent elements. This kind of mooring arrangement is addressed in the paper in a simplified way by considering an array of two buoys. Two opposed slack-mooring lines connect the floater pair to the bottom, while a third line, from whose mid-point hangs a weight, connects the two buoys pulling them towards each other. The centres of the buoys and the mooring lines are in a vertical plane parallel to the incoming wave direction, so that body and mooring motions are two-dimensional. The whole system — buoys, moorings and power take-off systems (PTOs) — is assumed linear, so that a frequency domain analysis may be employed. In the numerical simulations, two identical hemispherical buoys oscillate in heave and surge, acted upon by the waves, the mooring system and their PTOs. The PTO consists of a linear damper whose force is proportional to the heave velocity. Results from numerical simulations, with regular and irregular waves, are presented for the motions and power absorption of the converters, for different mooring and PTO parameters. Comparisons with the simpler cases of one single buoy in the moored and unmoored situations show significant differences.Copyright

Nonlinear Dynamics of a Floating Wave Energy Converter Reacting Against the Sea Bottom Through a Tight Mooring Cable

Tightly moored single-body floating devices are an important class of offshore wave energy converters. Examples are the devices under development at the University of Uppsala, Sweden, and Oregon State University, USA, prototypes of which were recently tested off the western coast of Sweden and off the Oregon coast, respectively. These devices are equipped with a linear electrical generator. The mooring system consists of a cable that is kept tight by a spring or equivalent device. This cable also prevents the buoy from drifting away by providing a horizontal restoring force. The horizontal and (to a lesser extent) the vertical restoring force are nonlinear functions of the vertical and horizontal displacements of the buoy, which makes the system a nonlinear one (even if the spring and damper are linear), whose modelling requires a time-domain analysis. Such an analysis is presented, preceded by a simpler frequency-domain approach. Numerical results (motions and absorbed power) are shown for a system consisting of a hemispherical buoy in regular and irregular waves, a tight mooring cable and a power take-off system consisting of a linear spring and a linear damper.Copyright

Optimization of Mooring Configuration Parameters of Floating Wave Energy Converters

Floating point absorbers devices are a large class of wave energy converters for deployment offshore, typically in water depths between 40 and 100m. As floating oil and gas platforms, the devices are subject to drift forces due to waves, currents and wind, and therefore have to be kept in place by a proper mooring system. Although similarities can be found between the energy converting systems and floating platforms, the mooring design requirements will have some important differences between them, one of them associated to the fact that, in the case of a wave energy converter, the mooring connections may significantly modify its energy absorption properties by interacting with its oscillations. It is therefore important to examine what might be the more suitable mooring design for wave energy devices, according to the converters specifications. When defining a mooring system for a device, several initial parameters have to be established, such as cable material and thickness, distance to the mooring point on the bottom, and which can influence the device performance in terms of motion, power output and survivability. Different parameters, for which acceptable intervals can be established, will represent different power absorptions, displacements from equilibrium position, load demands on the moorings and of course also different costs. The work presented here analyzes what might be, for wave energy converter floating point absorber, the optimal mooring configuration parameters, respecting certain pre-established acceptable intervals and using a time-domain model that takes into account the non-linearities introduced by the mooring system. Numerical results for the mooring forces demands and also motions and absorbed power, are presented for two different mooring configurations for a system consisting of a hemispherical buoy in regular waves and assuming a liner PTO.Copyright

Frequency and Time-Domain Models for a Two-Body Wave Power Device That Extracts Energy From Sea Waves by Relative Pitch Motion

In this paper analytical and numerical tools are used to describe the behaviour of a new wave power device. This device extracts energy from sea waves by relative pitch motion between their two independent parts. Frequency-domain analysis is carried out assuming that its hydrodynamic performance and its power take-off equipment behaviour may be considered linear. For regular waves optimal mechanical damping coefficients are computed assuming restrictions to hold for relative pitch motion between the two device parts. Useful power, capture length as well as rotations and displacements are computed. Time-domain analysis is necessary if one needs to compute the device’s performance under more realistic operational conditions. In order to get device’s variables trajectories for irregular sea conditions, a time-domain approach is adopted since the hydraulic power take-off equipment exhibits a non-linear behaviour. Results are obtained for irregular waves, i.e. for different sea states, and thus trajectories for several device variables are presented. Power available to the hydraulic machine is computed and presented.© 2008 ASME

Modelling of the IPS buoy wave energy converter including the effect of non-uniform tube cross-section

An important class of floating wave energy converters (that includes the IPS buoy, the Wavebob and the PowerBuoy) comprehends devices in which the energy is converted from the relative (essentially heaving) motion between two bodies oscillating differently. The paper considers the case of the IPS buoy, consisting of a floater rigidly connected to a fully submerged vertical (acceleration) tube open at both ends. The tube contains a piston whose motion relative to the floater-tube system (motion originated by wave action on the floater and by the inertia of the water enclosed in the tube) drives a power take-off mechanism (PTO) (assumed to be a linear damper). To solve the problem of the end-stops, the central part of the tube, along which the piston slides, bells out at either end to limit the stroke of the piston. The use of a hydraulic turbine inside the tube is examined as an alternative to the piston. A frequency domain analysis of the device in regular waves is developed, combined with a one-dimensional unsteady flow model inside the tube (whose cross-section is in general nonuniform). Numerical results are presented for a cylindrical buoy in regular waves, including the optimization of the acceleration tube geometry and PTO damping coefficient for several wave periods.

Hydrodynamic Interactions for Small Arrays of Wave Energy Devices

The main object of this work is to develop accurate solutions for hydrodynamics coefficients of arrays of simple-geometry wave-energy devices. Use is made of a computationally very efficient method, to model the wave energy absorption by a rigid sphere moving in heave and sway in water of constant (finite) depth, that employs a series of pulsating multipoles located at the centre of the sphere; a monopole or source can be added to simulate the effect of pulsating volume. In the present work, this is extended to model the interaction between spheres in a finite array, by locating series of multipoles at the centre of each sphere and computing the multipoles intensities from the boundary conditions at the spheres’ surfaces in order to find the matrices of damping and added-mass coefficients in several oscillating modes. Numerical results are obtained for small arrays. This may be used to validate results from boundary-element methods.Copyright