• Energy Research

Abstract A 30 kW fixed-guide-vanes biradial turbine-generator set was designed by Instituto Superior Tecnico (IST) and manufactured by Kymaner, within the framework of the H2020 European project OPERA, for open sea testing at the Mutriku breakwater oscillating water column (OWC) power plant and at the IDOM MARMOK-A-5 OWC spar-buoy, deployed at BiMEP, Basque Country, Spain. The paper reports the design and construction of the prototype, and its dry testing at IST, in a variable-flow test rig, simulating the oscillating flow induced by irregular waves. Results show that the turbine exhibits a peak efficiency of 70% under steady-state conditions and an average efficiency of 56% for the control laws tested.

AbstractThe quest for conquering the ocean and understanding its behaviour has been a challenge with increasing needs for innovation and technology investments in many areas of strategic value for the promotion, growth and competitiveness of the marine economy worldwide. Current oceanographic buoy systems are limited to low power levels and intermittency of data acquisition and transmission, among other aspects that need to be overcome to comply with new and more demanding applications. The development of marine activities requires more powerful and reliable data-acquisition systems to guarantee their future sustainability. This work presents a new systematic methodology for optimum design of wave energy converters. The methodology was applied to design two self-powered sensor buoys for long term monitoring based on the oscillating-water-column principle. The optimisation focussed on buoy hydrodynamic shape, sizing and selection of the turbine and the generator, as well as the control law of the generator electromagnetic torque. The performance was assessed through the use of the power matrix and a set of performance indicators. These performance indicators were defined to allow a simple comparison between different wave energy concepts. The results confirm the applicability of the designed buoys for a next generation of oceanographic monitoring systems.

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 both ends 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 non-uniform). Numerical results in regular and irregular waves are presented for a cylindrical buoy with a conical bottom, including the optimization of the acceleration tube geometry and PTO damping coefficient for several wave periods.

Abstract The Wells turbine is the most frequently used or proposed self-rectifying air turbine for oscillating-water-column wave-energy converter application, largely because of its conceptual and mechanical simplicity. Biplane Wells turbines allow a higher total blade solidity to be attained than monoplane turbines do, but this results in larger aerodynamic losses associated with the swirl kinetic energy loss at turbine exit. This may be overcome by the presence of a row of guide vanes between the two rotor planes, a solution that had been proposed and investigated theoretically or by numerical modelling. Results of turbine overall performance and flow details are reported from laboratory tests of a biplane Wells turbine without guide vanes and with specially designed guide vanes. The presence of the guide vanes was found to increase the peak efficiency by seven percentual points, while reducing (for fixed rotational speed) the damping provided by the turbine. Measured losses in the guide vane row were much smaller than in the rotors. Experimental results are compared with previously published numerical results. A stochastic theoretical transform was applied to obtain averaged results for the turbine performance subject to the irregular bidirectional air flow induced by real sea waves.

Abstract The quest for exploiting the ocean resources and understanding its behaviour has been a challenge with increasing needs for innovation and technology. Model testing is an essential step in offshore renewable energy technology development. It involves challenges that require experience and guidance. Costly mistakes might arise with the subsequent waste of time and resources. This paper presents the model design and testing processes as part of wave energy projects and the results of experimental testing of two types of oscillating-water-column (OWC) wave energy converters (WEC). The model design aims at the creation of a reduced-scale model to simulate the physical phenomena found in full-scale devices. It is a process that requires several skills and an adequate compromise among all variables. This design involves several approaches as different physical phenomena do not follow the same similarity conditions, requiring adjustments in scale, materials, and other relevant properties. Besides, the model testing process comprises the necessary planning and actions to execute the tests and post-processing of data. This process is addressed here through model design and testing of two WECs: the coaxial-duct and the spar-buoy OWCs. The configurations have been designed and studied for large-scale energy production and small-scale power in oceanographic applications. Although the devices are both OWCs, the designs exhibit significant differences. The development process of the models and results are presented for the two OWC devices. Free-decay tests, hydrodynamic performance and mooring tension results are presented and discussed. These may serve as guidelines and numerical modelling validation.

The paper concerns the hydrodynamic analysis and optimization of an OWC spar buoy, possibly the simplest concept for a floating oscillating water column (OWC) wave energy converter. It is an axisymmetric device consisting basically of a submerged vertical tail-tube-fixed to an axisymmetric floater that oscillates essentially in heave. The air flow displaced by the water motion inside the tube drives a self-rectifying air turbine. The possible advantages of using a tube of non-uniform inner cross section are investigated theoretically and numerically, especially as a way of reducing the draught of the device without significantly impairing its power performance. The unsteady water flow in the tube is modelled as one-dimensional. The frequency-dependent hydrodynamic coefficients of the tube-floater pair were computed with a boundary-element code. A linear air turbine is assumed. The hydrodynamics of the wave energy absorption is analysed in the frequency domain, including the effect of air compressibility in the chamber; special attention is devoted to optimization. Numerical results are presented for device's performance in regular and irregular waves, including especially optimization of the tube geometry and of the turbine characteristic. Practical implications of these results are discussed.

Abstract The paper presents a fast design method for the inlet guide vanes of low-cost mini hydraulic bulb turbines. The guide vanes are positioned between two conical surfaces with a common vertex and have constant thickness distribution, except close to the leading and the trailing edges. The conical-walled inlet guide vane row is designed using a quasi-three-dimensional calculation method, by prescribing the angular-momentum distribution along the span at the outlet section of the guide vanes. The meridional through-flow is computed by a streamline curvature method and the blade-to-blade flow by a singularity surface method. The stagger angle and the vane camber are computed to fulfil the required design circulation and zero-incidence flow at the leading edge. The final vane shape is a single-curvature surface with straight leading and trailing edges. To validate the design method, a conical-walled inlet guide vane row nozzle-model with six fixed vanes was designed, manufactured and tested in an airflow rig. Traversing measurements along the circumferential and radial directions were made with a five-hole probe. The experimental results are compared with the prescribed design conditions and with numerical results from the three-dimensional inviscid and viscous flow computed with the FLUENT code.

The Mutriku breakwater wave power plant is located in the Bay of Biscay, in Basque Country, Spain. The plant is based on the oscillating water column (OWC) principle and comprises 16 air chambers, each of them equipped with a Wells turbine coupled to an electrical generator with a rated power of 18.5 kW. The IDMEC/IST Wave Energy Group is developing a novel self-rectifying biradial turbine that aims to overcome several limitations of the Wells turbine, namely the sharp drop in efficiency above a critical flow rate. The new turbine is symmetrical with respect to a mid-plane perpendicular to the axis of rotation. The rotor is surrounded by a pair of radial-flow guide vane rows. Each guide vane row is connected to the rotor by an axisymmetric duct whose walls are flat discs. In the framework of the “OPERA” European H2020 Project, the new biradial turbine will be tested at Mutriku and later will be installed and tested on a floating OWC wave energy converter — the OCEANTEC Marmok-5’s — to be deployed at BiMEP demonstration site in September of 2017. The aim of the present paper is to perform critical comparisons of the performance of the new biradial and the Wells turbine that is presently installed at Mutriku. This is based on results from a time-domain numerical model. For the purpose, a new hydrodynamic frequency domain model of the power plant was developed using the well know WAMIT software package. This was used to build a time-domain model based on the Cummins approach.