R.P.F. Gomes

Air Turbine and Primary Converter Matching in Spar-Buoy Oscillating Water Column Wave Energy Device

The oscillating water column (OWC) equipped with an air turbine is possibly the most reliable type of wave energy converter. The OWC spar-buoy is a simple concept for a floating OWC. It is an axisymmetric device (and so insensitive to wave direction) consisting basically of a (relatively long) submerged vertical tail tube open at both ends and fixed to a floater that moves essentially in heave. The air flow displaced by the motion of the OWC inner free-surface, relative to the buoy, drives an air turbine. The choice of air turbine type and size, the regulation of the turbine rotational speed and the rated power of the electrical equipment strongly affect the power performance of the device and also the equipment’s capital cost. Here, numerical procedures and results are presented for the power output from turbines of different sizes equipping a given OWC spar-buoy in a given offshore wave climate, the rotational speed being optimized for each of the sea states that, together with their frequency of occurrence, characterize the wave climate. The new biradial self-rectifying air turbine was chosen as appropriate to the relatively large amplitude of the pressure oscillations in the OWC air chamber. Since the turbine is strongly non-linear and a fully-nonlinear model of air compressibility was adopted, a time domain analysis was required. The boundary-element numerical code WAMIT was used to obtain the hydrodynamic coefficients of the buoy and OWC, whereas the non-dimensional performance curves of the turbine were obtained from model testing.© 2013 ASME

Latching Control of a Floating Oscillating Water Column Wave Energy Converter in Irregular Waves

The paper concerns the phase control by latching of a floating oscillating-water-column (OWC) wave energy converter of spar-buoy type in irregular random waves. The device is equipped with a two-position fast-acting valve in series with the turbine. The instantaneous rotational speed of the turbine is controlled through the power electronics according to a power law relating the electromagnetic torque on the generator rotor to the rotational speed, an algorithm whose adequacy had been numerically tested in earlier papers. Two alternative strategies (1 and 2) for the latching/unlatching timings are investigated. Strategy 1 is based on the knowledge of the zero-crossings of the excitation force on the floater-tube set. This is difficult to implement in practice, since the excitation force can neither be measured directly nor predicted. Strategy 2 uses as input easily measurable physical variables: air pressure in the chamber and turbine rotational speed. Both strategies are investigated by numerical simulation based on a time-domain analysis of a spar-buoy OWC equipped with a self-rectifying radial-flow air turbine of biradial type. Air compressibility in the chamber plays an important role and was modelled as isentropic in a fully non-linear way. Numerical results show that significant gains up to about 28% are achievable through strategy 1, as compared with no phase control. Strategy 2, while being much easier to implement in practice, was found to yield more modest gains (up to about 15%).Copyright

Latching Control of an OWC Spar-Buoy Wave Energy Converter in Regular Waves

The present paper concerns an OWC spar-buoy, possibly the simplest concept for a floating oscillating-water-column (OWC) wave energy converter. It is an axisymmetric device (and so insensitive to wave direction) consisting basically of a (relatively long) submerged vertical tail tube open at both ends, fixed to a floater that moves essentially in heave. The length of the tube determines the resonance frequency of the inner water column. The oscillating motion of the internal free surface relative to the buoy, produced by the incident waves, makes the air flow through a turbine that drives an electrical generator. It is well known that the frequency response of point absorbers like the spar buoy is relatively narrow, which implies that their performance in irregular waves is relatively poor. Phase control has been proposed to improve this situation. The present paper presents a theoretical investigation of phase control by latching of an OWC spar-buoy in which the compressibility of air in the chamber plays an important role (the latching is performed by fast closing and opening an air valve in series with the turbine). In particular such compressibility may remove the constraint of latching threshold having to coincide with an instant of zero relative velocity between the two bodies (in the case under consideration, between the floater and the OWC). The modelling is performed in the time domain for a given device geometry, and includes the numerical optimization of the air turbine rotational speed, chamber volume and latching parameters. Results are obtained for regular waves.Copyright

Performance Assessment of a Floating Coaxial Ducted OWC Wave Energy Converter for Oceanographic Purposes

This paper presents a numerical study on a floating coaxial ducted OWC wave energy converter equipped with a biradial air turbine to meet the requirements of an oceanographic sensor-buoy. The study used representative sea states of the Monterey Bay, California, USA. The geometry of the coaxial ducted OWC was hydrodynamically optimized using a frequency domain approach considering a linear air turbine. Afterwards, a time domain analysis was carried out for the system equipped with a biradial turbine. The turbine rotor diameter and the optimum generator’s control curves were determined, based on results for representative sea states. Results show that mean power output fulfills the requirement for oceanographic applications (300–500W) using a turbine rotor diameter of 0.25 m. Furthermore, the system’s performance is strongly influenced by the inertia of the turbine and the generator rated power. These results confirmed the suitability of using the coaxial ducted OWC as a self-sustainable oceanographic sensor-buoy.Copyright

Results From Numerical Simulation and Field Tests of an Oceanographic Buoy Powered by Sea Waves

The paper presents a performance analysis of a wave-energy converter designed for oceanographic applications to generate 300–500 W of electrical power on average. The prototype consists of a small draft cylindrical buoy connected to a submerged anti-heave plate by a hydraulic power takeoff system and cable. Engineering details and design of the system are based on the results of a detailed time-domain hydrodynamic analysis of the system, discussed here. The performance of the prototype is assessed with respect to the submerged plate size/added-mass and the effectiveness of latching schemes. Additionally, results for the power extraction are statistically compared with measurements performed during field tests in Monterey Bay, California. During these tests performance measurements were logged by the buoy, and wave data was recorded by a measurement buoy positioned 1 km from the deployment site. This allows the comparison of actual performance to model predictions run for the sea state present during testing. In general, the results of the numerical model match fairly well with the data acquired in field testing. The proposed causal latching control schemes have been shown to be ineffective for this type of wave-energy converter.Copyright