Plate Shape Effect on the Performance of the Vertical Axis Auto Rotation Current Turbine (VAACT)

The present paper explores experimentally the performance of two types of hinged plates which rotate about vertical axis when submitted to uniform current. A flat plate configuration and also a flapped plate (say, S shape) configuration have been investigated. The Vertical axis Auto rotation Current Turbine (VAACT) is one degree of freedom system (free to rotate in yaw direction). It is shown that a high efficiency for S shape type can be obtained of the order of 30 percent while flat blade type reaches approximately to 7 percent. Upper limit of tip speed ratio for flat blade type has been expanded approximately 0.9 whereas S shape approaches 1.3.

Dynamic Mudline Damping for Offshore Wind Turbine Monopiles

Fatigue is often a design driver for large (e.g. 5–10 MW) offshore wind turbines (OWTs), necessitating a thorough examination of damping sources: aerodynamic, hydrodynamic, structural, and soil. Of these sources, soil damping has been least considered by researchers with respect to OWTs. Aeroelastic programs, such as the National Renewable Energy Laboratory (NREL) code FAST, are typically used for time history analysis of aerodynamic and hydrodynamic loads experienced by OWTs. To take into account foundation flexibility while minimizing computational expense, reduced-order foundation models such as the mudline stiffness matrix are often used. Mudline stiffness and damping matrices are derived here for the NREL 5MW reference turbine. By recompiling FAST with mudline stiffness and damping matrices, the contribution of soil damping to OWT dynamic behavior is then quantified by comparing time history analysis results including and excluding soil damping.

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%).

Numerical Simulation and Optimization of a Wave Energy Converter for the Izu Islands Sea in Japan

The purpose of this study is the numerical simulation and control optimization of a wave energy converter to estimate the power at a test site in the Izu Islands. In Japan, ocean energy is once again being seriously considered; however, since there are many inherent problems due to severe conditions such as the strong swells and large waves, estimations are important when designing such devices. The numerical simulation method in this study combines the wave interaction analysis software WAMIT and an in-house time-domain simulation code using the Newmark-β method, and introduces approximate complex-conjugate control into the code. The optimized parameters were assessed for a regular sine wave and an irregular wave with a typical wave spectrum. With the optimized parameters, average and maximum output power were estimated for the observed wave data at the test site. The results show a more than 100 kW average power output and a several times larger maximum power output.

Numerical Assessment on Primary Wave Energy Conversion of Oscillating Water Columns

Oscillating water column (OWC) wave energy converters (WECs) are probably the simplest and most promising wave energy converters due to their good feasibility, reliability and survivability in practical wave energy conversions and also regarded as the most studied and developed when compared to other types of the wave energy converters. This research aims to develop a reliable numerical tool to assess the performance of the OWC wave energy converters, particularly in the primary wave energy conversion. In the numerical assessment tool, the hydrodynamics of the device and thermodynamics of the air chamber can be studied separately. However, for the complete dynamic system when a power takeoff (PTO) system is applied, these two dynamic systems are fully coupled in time-domain, in which the PTO can have a simple mathematical expression as the relation between the pressure difference across the PTO (the chamber pressure) and its flowrate through the PTO. And the application of a simple PTO pressure-flowrate relation very much simplifies the complicated aerodynamics and thermodynamics in the air turbine system so the whole dynamic system can be simplified. The methodology has been applied to a generic OWC device and the simulation results have been compared to the experimental data. It is shown that the developed numerical method is reliable in and capable of assessing the primary wave energy conversion of oscillating water columns.

Nonlinear Motion Analysis of the Wavestar Wave Energy Converter With a Focus on the Structural Responses

The scope of this paper is to connect a nonlinear WEC numerical model with a structural response model. The numerical WEC model takes into account the nonlinear hydrostatic restoring moment of the Wavestar float. A parameterized structural model of the Wavestar arm is developed in ANSYS APDL. Based on the assumption that the structural displacements remain small, linear first order theory is used to calculate the structural response. The section moments and forces are thus superimposed according to the superposition law. The effect of the nonlinear hydrostatic restoring moment on the structural response is investigated. Moreover, an analysis is carried out which shows that reactive control, applied as a closed loop control, increases the section moments and shear forces.

Mooring Analysis of a NOMAD Buoy Through Field Testing and Numerical Simulation

This study presents a mooring analysis of the Ocean Sentinel buoy, which is a mobile test platform for Wave Energy Converters (WECs). The Ocean Sentinel is owned and operated by the Northwest National Marine Renewable Energy Center (NNMREC) at Oregon State University (OSU). The study involved a field observation as well as numerical modeling. The Ocean Sentinel was deployed from 7/29/2013 – 10/04/2013 at the NNMREC North Energy Test Site, which is located between 2–3 nautical miles (3.7–5.6 km) offshore of Yaquina Head, north of Newport, OR. It was configured in a three-point mooring with load cells on each mooring line. Prior to deployment, the numerical model was used for design and testing of the Ocean Sentinel mooring system. After deployment, recorded environmental conditions were coupled with the model to simulate deployed conditions, and model predictions of tension in the mooring lines were compared with actual results.

Offshore Deployment of Point Absorbing Wave Energy Converters With a Direct Driven Linear Generator Power Take-Off at the Lysekil Test Site

Within the year 2013, four linear generators with point absorber buoy systems were deployed in the Lysekil test site. Until now, deployments of these point absorbing wave energy converters have been expensive, time consuming, complicated and raised safety issues. In the present paper, we focus on the analysis and optimization of the offshore deployment process of wave energy converters with a linear generator power take-off which has been constructed by Uppsala University. To address the crucial issues regarding the deployment difficulties, case study of previous offshore deployments at the Lysekil test site are presented regarding such parameters as safety, cost and time efficiency. It was discovered that the deployment process can be improved significantly, mainly by using new technologies, e.g., new specialized deployment vessels, underwater robots for inspections and for connecting cables and an automatized pressurizing process. Addressing the main deployment difficulties and constrains leads us to discovery of methods that makes offshore deployments more cost-efficient and faster, in a safety context.

On the Tidal Stream Resource of Two Headland Sites in the English Channel: Portland Bill and Isle of Wight

There are various candidate sites for tidal stream energy extraction in the English Channel. In this paper we examine the tidal stream resource at Portland Bill and the south coast of the Isle of Wight. A depth-averaged numerical model is developed and compared to field measurements. The presence of rows of tidal turbines is simulated using a line-discontinuity to represent the head loss across the turbines. The head loss is given by linear momentum actuator disc theory. At each site the length of the turbine rows, the local blockage ratio, and the location of the turbines are varied. For Portland Bill the presence of an array with multiple rows of turbines is also considered. We find that it is likely that (based purely on the hydrodynamics) power could viably be extracted at each site, with the mean power produced by each site being in the order of 10s MW.

Optimising Reactive Control in Non-Ideal Efficiency Wave Energy Converters

When analytically optimising the control strategy in wave energy converters which use a point absorber, the efficiency aspect is generally neglected. The results presented in this paper provide an analytical expression for the mean harvested electrical power in non-ideal efficiency situations. These have been derived under the assumptions of monochromatic incoming waves and linear system behaviour. This allows to establish the power factor of a system with non-ideal efficiency. The locus of the optimal reactive control parameters is then studied and an alternative method of representation is developed to model the optimal control parameters. Ultimately we present a simple method of choosing optimal control parameters for any combination of efficiency and wave frequency.