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

2010 ◽  
Author(s):  
Pedro C. Vicente ◽  
Anto´nio F. O. Falca˜o ◽  
Paulo A. P. Justino
Keyword(s):  
Wave Energy ◽  
Time Domain Analysis ◽  
Irregular Waves ◽  
Western Coast ◽  
State University ◽  
Restoring Force ◽  
Wave Energy Converters ◽  
Sea Bottom ◽  
Mooring Cable ◽  
Offshore Wave

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.

scholarly journals OPTIMIZATION OF THE WAVECAT WAVE ENERGY CONVERTER

2012 ◽  
Vol 1 (33) ◽  
pp. 5 ◽  
Author(s):  
Hernan Fernandez ◽  
Gregorio Iglesias ◽  
Rodrigo Carballo ◽  
Alberte Castro ◽  
Marcos Sánchez ◽  
...  
Keyword(s):  
Physical Model ◽  
Wave Energy ◽  
Model Tests ◽  
Irregular Waves ◽  
Intermediate Water ◽  
Plan View ◽  
Santiago De Compostela ◽  
Wave Energy Converters ◽  
Physical Model Tests ◽  
The Difference

The development of efficient, reliable Wave Energy Converters (WECs) is a prerequisite for wave energy to become a commercially viable energy source. Intensive research is currently under way on a number of WECs, among which WaveCat©—a new WEC recently patented by the University of Santiago de Compostela. In this sense, this paper describes the WaveCat concept and its ongoing development and optimization. WaveCat is a floating WEC intended for operation in intermediate water depths (50–100 m). Like a catamaran, it consists of two hulls—from which it derives its name. The difference with a conventional catamaran is that the hulls are not parallel but convergent; they are joined at the stern, forming a wedge in plan view. Physical model tests of a 1:30 model were conducted in a wave tank using both regular and irregular waves. In addition to the waves and overtopping rates, the model displacements were monitored using a non-intrusive system. The results of the physical model tests will be used to validate the 3D numerical model, which in turn will be used to optimize the design of WaveCat for best performance under a given set of wave conditions.

A Comparison of Selected Strategies for Adaptive Control of Wave Energy Converters

2011 ◽  
Vol 133 (3) ◽  
Author(s):  
Jørgen Hals ◽  
Johannes Falnes ◽  
Torgeir Moan
Keyword(s):  
Wave Energy ◽  
Reactive Power ◽  
Average Power ◽  
Volume Ratio ◽  
External Input ◽  
Irregular Waves ◽  
Optimal Velocity ◽  
Wave Energy Converters ◽  
Absorbed Power ◽  
Energy Converters

Wave-energy converters of the point-absorbing type (i.e., having small extension compared with the wavelength) are promising for achieving cost reductions and design improvements because of a high power-to-volume ratio and better possibilities for mass production of components and devices as compared with larger converter units. However, their frequency response tends to be narrow banded, which means that the performance in real seas (irregular waves) will be poor unless their motion is actively controlled. Only then the invested equipment can be fully exploited, bringing down the overall energy cost. In this work various control methods for point-absorbing devices are reviewed, and a representative selection of methods is investigated by numerical simulation in irregular waves, based on an idealized example of a heaving semisubmerged sphere. Methods include velocity-proportional control, approximate complex conjugated control, approximate optimal velocity tracking, phase control by latching and clutching, and model-predictive control, all assuming a wave pressure measurement as the only external input to the controller. The methods are applied for a single-degree-of-freedom heaving buoy. Suggestions are given on how to implement the controllers, including how to tune control parameters and handle amplitude constraints. Based on simulation results, comparisons are made on absorbed power, reactive power flow, peak-to-average power ratios, and implementation complexity. Identified strengths and weaknesses of each method are highlighted and explored. It is found that overall improvements in average absorbed power of about 100–330% are achieved for the investigated controllers as compared with a control strategy with velocity-proportional machinery force. One interesting finding is the low peak-to-average ratios resulting from clutching control for wave periods about 1.5 times the resonance period and above.

Coupled Multi-Phase CFD and Transient Mooring Analysis of the Floating Wave Energy Converter OWEL

2013 ◽  
Author(s):  
Rachel Nicholls-Lee ◽  
Adam Walker ◽  
Simon Hindley ◽  
Richard Argall
Keyword(s):  
Wave Energy ◽  
Dynamic Environment ◽  
Cfd Analysis ◽  
Heavy Duty ◽  
Operational Parameters ◽  
Energy Converter ◽  
Aggressive Environment ◽  
Wave Energy Converters ◽  
Offshore Wave ◽  
Multi Phase

Floating wave energy converters are surface based thus facilitating installation and maintenance. They tend to be moored offshore and consequently have less of an impact than other devices both visually and audibly. Mooring these devices is a challenging task, as not only are they subject to drift forces due to the aggressive environment, but they are also designed to operate at their resonant frequency in order to obtain as much power as possible. Such operational parameters require heavy duty mooring systems, capable of coping with the dynamic environment. These moorings will, in turn, affect the performance of the device by restraining the motions and thus modifying the energy absorption characteristics. In this paper a free floating representation of the Offshore Wave Energy Ltd. device (OWEL) has been modeled in RANS CFD in order to obtain initial mooring loads. Subsequently, a preliminary mooring arrangement for OWEL was developed, and using these loads it was modeled using OrcaFlex. The dynamic, non-linear loads were then coupled to a fully transient, multiphase CFD analysis of the device in order to obtain performance characteristics for further detailed design. The numerical results have been compared to results obtained through physical model scale tests of the device and show a good degree of correlation.

Constrained Optimal Control of a Heaving Buoy Wave-Energy Converter

2010 ◽  
Vol 133 (1) ◽  
Author(s):  
Jørgen Hals ◽  
Johannes Falnes ◽  
Torgeir Moan
Keyword(s):  
Energy Absorption ◽  
Wave Energy ◽  
Optimal Operation ◽  
Wave Energy Converter ◽  
Irregular Waves ◽  
Energy Converter ◽  
Model Predictive Controller ◽  
Wave Energy Converters ◽  
Predictive Controller ◽  
Energy Converters

The question of optimal operation of wave-energy converters has been a key issue since modern research on the topic emerged in the early 1970s, and criteria for maximum wave-energy absorption soon emerged from frequency domain analysis. However, constraints on motions and forces give the need for time-domain modeling, where numerical optimization must be used to exploit the full absorption potential of an installed converter. A heaving, semisubmerged sphere is used to study optimal constrained motion of wave-energy converters. Based on a linear model of the wave-body interactions, a procedure for the optimization of the machinery force is developed and demonstrated. Moreover, a model-predictive controller is defined and tested for irregular sea. It repeatedly solves the optimization problem online in order to compute the optimal constrained machinery force on a receding horizon. The wave excitation force is predicted by use of an augmented Kalman filter based on a damped harmonic oscillator model of the wave process. It is shown how constraints influence the optimal motion of the heaving wave-energy converter, and also how close it is possible to approach previously published theoretical upper bounds. The model-predictive controller is found to perform close to optimum in irregular waves, depending on the quality of the wave force predictions. An absorbed power equal to or larger than 90% of the ideal constrained optimum is achieved for a chosen range of realistic sea states. Under certain circumstances, the optimal wave-energy absorption may be better in irregular waves than for a corresponding regular wave having the same energy period and wave-power level. An argument is presented to explain this observation.

Estimation of Technical Wave Energy Potential in Exclusive Economic Zone of India

2018 ◽  
Author(s):  
Garlapati Nagababu ◽  
Ravi Patel ◽  
Seemanth Moideenkunju ◽  
Abhinaya Srinivas Bhasuru ◽  
Surendra Singh Kachhwaha ◽  
...  
Keyword(s):  
Wave Energy ◽  
Coastal Region ◽  
Resource Assessment ◽  
Capacity Factor ◽  
Eastern Coast ◽  
Wave Power ◽  
Western Coast ◽  
Energy Potential ◽  
Wave Energy Converters ◽  
Energy Converters

Identification of the best location for wave farm installation, wave resource assessment needs to be carried out. In the present work, wave resource assessment along the Indian EEZ was carried out using the 17-year (2000 to 2016) output simulation of the third generation wave model WAVEWATCH-III (WWIII). Spatial distribution of significant wave height, mean wave energy period and annual mean of wave power is plotted. Further, the monthly and seasonal variation has been carried out to assess the effect on temporal variability at a specific location. The results show the annual mean wave power is in the range of 1–12 kW/m across the Indian EEZ. Further, it was observed that wave power along the western coast of India is more energetic than the eastern coast of India, with annual average wave power of 8–12 kW/m and 2–6 kW/m respectively. However, coastlines of Gujarat and Maharashtra experience the maximum seasonal and monthly variability across Indian EEZ, which is 2 and 3.5 respectively. By using different wave energy converters (WEC), the capacity factor and technical wave energy potential over the study area are estimated. Oceantec WEC shows maximum capacity factor (0.35) among the all selected wave energy converters. The results reveal that the electric wave power generation is 3 times more in the western coastal region as compared to the eastern coast of India.

The dynamics and power extraction of bottom-hinged plate wave energy converters in regular and irregular waves

2015 ◽  
Vol 96 ◽  
pp. 86-99 ◽  
Author(s):  
R.P.F. Gomes ◽  
M.F.P. Lopes ◽  
J.C.C. Henriques ◽  
L.M.C. Gato ◽  
A.F.O. Falcão
Keyword(s):  
Wave Energy ◽  
Irregular Waves ◽  
Plate Wave ◽  
Power Extraction ◽  
Wave Energy Converters ◽  
Hinged Plate ◽  
Energy Converters
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