Numerical and Experimental Investigation of a Novel Wave Energy Converter

2012 ◽  
Author(s):  
Øyvind Ygre Rogne ◽  
Svein Ersdal ◽  
Torgeir Moan
Keyword(s):  
Wave Energy ◽  
Lagrange Equation ◽  
Model Tests ◽  
Wave Energy Converter ◽  
Domain Model ◽  
Hydrodynamic Theory ◽  
Energy Converter ◽  
Mean Power ◽  
Large Power ◽  
Domain Identification

This paper deals with the numerical and experimental work carried out as part of the development of a new wave energy converter device suggested by Aker Solutions. The device consists of one catenary moored platform hinged to several smaller buoys. Energy is extracted through the relative rotation in the hinges. Model tests have been carried out in a towing tank with the aim to verify the use of linear hydrodynamic theory as a mean to predict the annual mean power production. The mathematical model of the device relies on linear hydrodynamic theory, and the commercial BEM code WAMIT is used to obtain the hydrodynamic coefficients. The equation of motion is formulated using generalized coordinates and Lagrange equation. The non-linear characteristics of the pneumatic cylinders, used to represent the power take-off forces in the model tests, necessitate the use of a time domain model. The convolution integral representing the radiation forces is replaced by an approximate state-space model, which is obtained using an frequency domain identification method. The results show that the simulation model overpredicts the mean absorbed power by approximately 20% in sea states that result in moderate hinge response, whereas in one particular sea state that produce large hinge response, and consequently large power absorption, the overprediction is as large as 49%. This trend is expected, since large hinge response imply that the linear assumption is violated.

scholarly journals Sensitivity of a Wave Energy Converter Dynamics Model to Nonlinear Hydrostatic Models

2015 ◽  
Author(s):  
Ryan G. Coe ◽  
Diana L. Bull
Keyword(s):  
Linear Model ◽  
Wave Energy ◽  
Three Dimensional ◽  
Wave Energy Converter ◽  
Domain Model ◽  
Interpolation Model ◽  
Surface Integral ◽  
Energy Converter ◽  
Motion Constraints ◽  
Large Improvement

A three dimensional time-domain model, based on Cummins equation, has been developed for an axisymmetric point absorbing wave energy converter (WEC) with an irregular cross section. This model incorporates a number of nonlinearities to accurately account for the dynamics of the device: hydrostatic restoring, motion constraints, saturation of the power-take-off force, and kinematic nonlinearities. Here, an interpolation model of the hydrostatic restoring reaction is developed and compared with a surface integral based method. The effects of these nonlinear hydrostatic models on device dynamics are explored by comparing predictions against those of a linear model. For the studied WEC, the interpolation model offers a large improvement over a linear model and is roughly two orders-of-magnitude less computationally expensive than the surface integral based method.

Optimization of Mooring Configuration Parameters of Floating Wave Energy Converters

2011 ◽  
Author(s):  
Pedro C. Vicente ◽  
Anto´nio F. O. Falca˜o ◽  
Paulo A. P. Justino
Keyword(s):  
Wave Energy ◽  
Oil And Gas ◽  
Wave Energy Converter ◽  
Domain Model ◽  
Floating Point ◽  
Mooring System ◽  
Energy Converter ◽  
Energy Devices ◽  
Wave Energy Converters ◽  
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.

scholarly journals Influence of a Taut Cable on the Performance of a Point-Absorber Wave Energy Converter

2018 ◽  
Author(s):  
R. Wang ◽  
Y. Wei ◽  
M. van Rooij ◽  
B. Jayawardhana ◽  
A. I. Vakis
Keyword(s):  
Frequency Domain ◽  
Wave Energy ◽  
Alternative Energy ◽  
Frequency Domain Analysis ◽  
Wave Energy Converter ◽  
Domain Model ◽  
Energy Harvest ◽  
Energy Converter ◽  
Point Absorber ◽  
Improved Model

In recent years, wave energy converters (WECs) have received considerable attention as an efficient way to harvest alternative energy sources. Within this class of systems, point-absorbers are popular and have become one of the most widely used renewable energy harvest designs all over the world, at least in the preliminary R&D stage, with many relevant research works having been published as well. However, unlike the single buoy and PTO systems which already have a comprehensive research basis, the connection cable has received little attention. The traditional taut cable analysis approach, initiated from the needs of the oil&gas industry, has been applied for WEC investigations. However, this approach utilizes an essential assumption that the oscillating term (PTO force) is much smaller than the static term of the cable force (pre-tension) and could be neglected, which may not be proper for WEC applications. In this work, a conventional frequency domain model is utilized to test and verify the validity of the previously mentioned assumption by presenting the ratio between two force terms. Then the ratio could be applied in combination with sea state contours to reveal the critical state of the cable. Then, a fully nonlinear time domain method of a numerical solution of the point-absorber wave energy converter is presented. According to the critical states obtained from the frequency domain analysis, an improved model of a slack cable is proposed. Its influence on the energy extraction performance is investigated using the open source code — WEC-Sim. This work provides insight into simulating a proper model of the cable and how the design of the cable influences the WEC performance.

scholarly journals Round Robin Testing: Exploring Experimental Uncertainties through a Multifacility Comparison of a Hinged Raft Wave Energy Converter

2021 ◽  
Vol 9 (9) ◽  
pp. 946
Author(s):  
Thomas Davey ◽  
Javier Sarmiento ◽  
Jérémy Ohana ◽  
Florent Thiebaut ◽  
Sylvain Haquin ◽  
...  
Keyword(s):  
Wave Energy ◽  
Physical Modelling ◽  
Round Robin ◽  
Wave Energy Converter ◽  
Energy Converter ◽  
Mean Power ◽  
Wave Heights ◽  
Significant Wave Heights ◽  
Similar Good ◽  
Six Degree Of Freedom

The EU H2020 MaRINET2 project has a goal to improve the quality, robustness and accuracy of physical modelling and associated testing practices for the offshore renewable energy sector. To support this aim, a round robin scale physical modelling test programme was conducted to deploy a common wave energy converter at four wave basins operated by MaRINET2 partners. Test campaigns were conducted at each facility to a common specification and test matrix, providing the unique opportunity for intercomparison between facilities and working practices. A nonproprietary hinged raft, with a nominal scale of 1:25, was tested under a set of 12 irregular sea states. This allowed for an assessment of power output, hinge angles, mooring loads, and six-degree-of-freedom motions. The key outcome to be concluded from the results is that the facilities performed consistently, with the majority of variation linked to differences in sea state calibration. A variation of 5–10 % in mean power was typical and was consistent with the variability observed in the measured significant wave heights. The tank depth (which varied from 2–5 m) showed remarkably little influence on the results, although it is noted that these tests used an aerial mooring system with the geometry unaffected by the tank depth. Similar good agreement was seen in the heave, surge, pitch and hinge angle responses. In order to maintain and improve the consistency across laboratories, we make recommendations on characterising and calibrating the tank environment and stress the importance of the device–facility physical interface (the aerial mooring in this case).

Optimum Power Capture of a New Wave Energy Converter in Irregular Waves

2009 ◽  
Author(s):  
George A. Aggidis ◽  
Mohammad T. Rahmati ◽  
Robert V. Chaplin ◽  
Andrew P. McCabe ◽  
Majid A. Bhinder ◽  
...  
Keyword(s):  
Control System ◽  
Wave Energy ◽  
Wave Spectrum ◽  
Wave Energy Converter ◽  
Irregular Waves ◽  
Energy Converter ◽  
Mean Power ◽  
Peak Period ◽  
Point Absorber ◽  
Power Capture

This paper presents the optimum power capture of a new point-absorber wave energy converter, in irregular waves. A stepwise control system for the wave energy converter (WEC) is developed. The control system is used to efficiently extract power from irregular waves where amplitudes vary from wave to wave. The Bretschneider spectrum is used in the experiment and the device is ‘tuned’ to the peak period of the sea state. It is shown that this WEC has a reasonable capture width in irregular waves. However, the optimum mean power depends on the wave spectrum, the shape of the collector body, its freeboard and the device pivot depth.

Power Absorption Modeling and Optimization of a Point Absorbing Wave Energy Converter Using Numerical Method

2014 ◽  
Vol 136 (2) ◽  
Author(s):  
Jeremiah Pastor ◽  
Yucheng Liu
Keyword(s):  
Wave Energy ◽  
Wave Energy Converter ◽  
Domain Model ◽  
Boundary Element Methods ◽  
Damping Coefficient ◽  
Energy Converter ◽  
Incoming Wave ◽  
Power Absorption ◽  
Point Absorber ◽  
Spring Force

This paper presents, assesses, and optimizes a point absorber wave energy converter (WEC) through numerical modeling, simulation, and analysis. Wave energy conversion is a technology uniquely suited for assisting in power generation in the offshore oil and gas platforms. A linear frequency domain model is created to predict the behavior of the heaving point absorber WEC system. The hydrodynamic parameters are obtained with AQWA, a software package based on boundary element methods. A linear external damping coefficient is applied to enable power absorption and an external spring force is introduced to tune the point absorber to the incoming wave conditions. The external damping coefficient and external spring forces are the control parameters, which need to be optimized to maximize the power absorption. Two buoy shapes are tested and a variety of diameters and drafts are compared. Optimal shape, draft, and diameter of the model are then determined to maximize its power absorption capacity.

Balancing Power Absorption and Structural Loading for an Asymmetric Heave Wave-Energy Converter in Regular Waves

2016 ◽  
Author(s):  
Nathan M. Tom ◽  
Farshad Madhi ◽  
Ronald W. Yeung
Keyword(s):  
Wave Energy ◽  
Wave Energy Converter ◽  
Degree Of Freedom ◽  
Hydrodynamic Theory ◽  
Cost Ratio ◽  
Perfect Absorber ◽  
Control Force ◽  
Energy Converter ◽  
Power Absorption ◽  
Restraining Force

The aim of this paper is to maximize the power-to-load ratio of the Berkeley Wedge: a one-degree-of-freedom, asymmetrical, energy-capturing, floating breakwater of high performance that is relatively free of viscosity effects. Linear hydrodynamic theory was used to calculate bounds on the expected time-averaged power (TAP) and corresponding surge restraining force, pitch restraining torque, and power take-off (PTO) control force when assuming that the heave motion of the wave energy converter remains sinusoidal. This particular device was documented to be an almost-perfect absorber if one-degree-of-freedom motion is maintained. The success of such or similar future wave energy converter technologies would require the development of control strategies that can adapt device performance to maximize energy generation in operational conditions while mitigating hydrodynamic loads in extreme waves to reduce the structural mass and overall cost. This paper formulates the optimal control problem to incorporate metrics that provide a measure of the surge restraining force, pitch restraining torque, and PTO control force. The optimizer must now handle an objective function with competing terms in an attempt to maximize power capture while minimizing structural and actuator loads. A penalty weight is placed on the surge restraining force, pitch restraining torque, and PTO actuation force, thereby allowing the control focus to be placed either on power absorption or load mitigation. Thus, in achieving these goals, a per-unit gain in TAP would not lead to a greater per-unit demand in structural strength, hence yielding a favorable benefit-to-cost ratio. Demonstrative results in the form of TAP, reactive TAP, and the amplitudes of the surge restraining force, pitch restraining torque, and PTO control force are shown for the Berkeley Wedge example.

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

2015 ◽  
Author(s):  
Juan C. C. Portillo ◽  
Joao C. C. Henriques ◽  
Luis M. C. Gato ◽  
Rui P. F. Gomes ◽  
Antonio F. O. Falcão
Keyword(s):  
Wave Energy ◽  
Numerical Study ◽  
Time Domain Analysis ◽  
Turbine Rotor ◽  
Wave Energy Converter ◽  
Energy Converter ◽  
Mean Power ◽  
Air Turbine ◽  
Mean Power Output ◽  
Frequency Domain Approach

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.

scholarly journals The performance of the Mocean M100 wave energy converter described through numerical and physical modelling

2020 ◽  
Vol 3 (1) ◽  
pp. 11-19
Author(s):  
J. Cameron McNatt ◽  
Christopher H. Retzler
Keyword(s):  
Frequency Domain ◽  
Wave Energy ◽  
Time Domain ◽  
Numerical Models ◽  
Average Power ◽  
Physical Modelling ◽  
Wave Energy Converter ◽  
Domain Model ◽  
Energy Converter ◽  
Cost Of Energy

Mocean Energy has designed a 100-kW hinged-raft wave energy converter (WEC), the M100, which has a novel geometry that reduces the cost of energy by improving the ratios of power per size and power per torque. The performance of the M100 is shown through the outputs of frequency-domain and time-domain numerical models, which are compared with those from 1/20th scale wave-tank testing. Results show that for the undamped, frequency-domain model, there are resonant peaks in the response at 6.6 and 9.6 s, corresponding to wavelengths that are 1.9 and 3.7 times longer than the machine. With the inclusion of power-take-off and viscous damping, the power response as a function of frequency shows a broad bandwidth and a hinge flex amplitude of 12-20 degrees per meter of wave amplitude. Comparison between the time-domain model and physical data in a variety of sea states, up to a significant wave height of 4.5 m, show agreements within 10% for average power absorption, which is notable because only simple, nonlinear, numerical models were used. The M100 geometry results in a broad-banded, large amplitude response due to its asymmetric shape, which induces coupling between modes of motion.

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