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Response and conversion efficiency of two degrees of freedom wave energy device

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Abstract

This paper concerns the response and efficiency of a two-body wave power conversion device. The physical problem of the device is simplified as a forced vibration system with viscous damping in two degrees of freedom. Based on the linear wave theory, wave excitation forces, added masses and damping are derived by use of an eigenfunction expansion matching method. The expressions of the response and efficiency are deduced from the motion equations of the device, and the optimal principle is presented as well. Numerical results indicate that damping optimal curve has a wave peak that is independent of the spring, and the value of the peak and the corresponding frequency are only related to the calculation conditions. Optimal curve presents two peaks when the spring exists, and the corresponding frequency width decreases with the increase of elastic coefficient. The damping is relative small at low frequency peak, while the response is relative high. The damping is relative high at high frequency optimization peak, while the response is relative small and almost smaller than the amplitude of the incident wave. With the decrease of the external damping coefficient, the relative motion amplitude and the efficiency increase, while the width of crest decreases.

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... He found that the submerged mass appears to perform better overall. Bijun Wu et al. [26] also studied the response and conversion efficiency of a two-body wave energy converter under regular wave excitation and they found that the performance of a two-body wave energy converter highly depends on the system parameters, like the physical properties of the buoy and incident wave frequency. The results obtained by the previous literature helped people to understand that it is possible to design a submerged body to achieve the theoretical maximum absorption power under regular wave excitation by neglecting the viscous damping. ...
... As a result, the maximum absorption power of the suboptimal design is achieved, as one can find in Figure 6(b). The corresponding optimal mass can be obtained by take the derivative of Eq. (18) with 2 + 22 and set it to zero, = 0 (26) where, ( ) is expressed in Eq. (18); For a given wave condition and buoy information, one can solve the optimal mass from Eq. (26). One should notice that optimal mass > critical mass when 2 > 0 and = when 2 = 0. ...
... As a result, the maximum absorption power of the suboptimal design is achieved, as one can find in Figure 6(b). The corresponding optimal mass can be obtained by take the derivative of Eq. (18) with 2 + 22 and set it to zero, = 0 (26) where, ( ) is expressed in Eq. (18); For a given wave condition and buoy information, one can solve the optimal mass from Eq. (26). One should notice that optimal mass > critical mass when 2 > 0 and = when 2 = 0. ...
Article
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An axisymmetric two-body wave energy converter is studied here. Previous results show that the cross couple terms of the added mass and damping can be neglected when the distance between the two buoys is far enough. A linear dynamic model of this two-body system is established and analyzed based on linear wave theory. The optimal power takeoff stiffness and damping were obtained. The numerical results show that large bottom body will result in a large output power. However, it is also constrained by the power tak eoff stiffness. The viscous damping will always have bad influence on energy harvesting. The performance of a two-body point in irregular wave was also investigated.
... Though the WAB wave energy converters have been investigated for a while, the optimization of the PTO for the WAB wave energy converters for maximizing capture power has not been well carried out, especially for those with multimotion modes, in which the experience and trial-and-error method are used [12]- [16]. In this research, maximization of wave energy production is targeted for how to optimize the PTO so to extract maximal power from regular or irregular waves. ...
... For a further simplification of analysis in this research, only the relative heave motions are included for capture power, though more motion modes are possible, but it will be the topic of the future research work. In calm water, the structures and are right angled, so the length of the PTO unit is calculated as (16) Due to the difference between and (the heave motions of the two barges), the PTO frame can take a shape in Fig. 3. If the change is small, that is, the angle is small, so (17) where is the horizontal length of the connection between two barges. ...
Article
This paper presents a fundamental investigation on optimizing the power takeoff (PTO) for maximizing wave energy conversion of the wave-activated bodies (WABs) wave energy converters. In this research, two relative heave motions are taken for capture power, and a linear PTO is considered in the primary analysis. For such a linear dynamic system, the frequency-domain analysis can be carried out, and an analytical formula can be derived for the optimized frequency-dependent PTO damping coefficient, which can be used to determine an optimized PTO damping for maximizing wave energy conversion in regular waves. However, when an optimized PTO is required for maximizing energy conversion from ocean waves, the PTO damping optimization may be very different and much less certain, because it may be based on one of many characteristic periods of the given sea state and the dependency may change from different sea states. For this reason, the optimized PTO damping coefficient for a given sea state must be studied carefully. Another important aspect of the research is to examine whether an optimized nonlinear PTO can take more energy out from waves than that of an optimized linear PTO. For this purpose, the maximized capture powers by the optimized linear and nonlinear PTOs are compared using the time-domain analysis. It has been shown from the examples that the maximized capture power by a nonlinear PTO system may exceed that by the linear PTO, but only marginally (less than 1%). Hence, it can be generally concluded that the maximized power using a linear PTO system can be a very good indicator for the device in extracting the maximal energy from waves regardless of the linear or nonlinear PTO in actual use. This conclusion may help simplify the analysis of the wave energy converters in terms of the energy production as well as the device optimization for improving energy conversion capacity.
... He found that the submerged mass appears to perform better overall. Bijun Wu et al. [26] also studied the response and conversion efficiency of a twobody wave energy converter under regular wave excitation and they found that the performance of a two-body wave energy converter highly depends on the system parameters, like the physical properties of the buoy and incident wave frequency. These results obtained by the previous literature helped people to understand that it is possible to design a submerged body to achieve the theoretical maximum absorption power under regular wave excitation by neglecting the viscous damping. ...
... (18) with m 2 þA 22 and set it to zero,d ðP ave Þ subopt dðm 2 þ A 22 Þ ¼ 0(26)where: (P ave ) subopt is expressed in Eq.(18); For a given wave condition and buoy information, one can solve the optimal mass ratio m o from Eq.(26). One should notice that m o >m c when z 2 >0 and m o ¼m c when z 2 ¼0 based onFig. ...
... Yu and Li [22] analyzed the power of a two body WEC with variable power take-off damping ratio using both a computational fluid dynamics approach and an experimental one and demonstrated the nonlinear hydrodynamic effects on the performance of the system. Wu et al. [23] studied the response of a two-body WEC under the variation of the design parameters and also derived an analytical solution for the non-dimensional added mass, damping coefficient, and wave excitation force for both the floater and the submerged body, and concluded that the generated power is highly dependent on the design parameters, especially those that affect the resonant frequency. Beatty et al. [19] conducted a numerical and experimental comparison between two different commercial two-body WECs. ...
... It is also noted that Figs. 13-15 have the same trends as the figures of the non-dimensional analytical solutions proposed in the literature [23]. ...
Article
In this paper, the effects of increasing the number of degrees of freedom through adding submerged bodies to a point absorber wave energy converter (WEC) were studied in an attempt to capture more power at a lower resonant frequency while keeping the same total mass and volume of the system, which is similar to those of the piezoelectric vibration energy harvesters. Novel multiple degrees of freedom wave energy harvesters were proposed, starting from a typical two-body point absorber and increasing the number of degrees of freedom through adding submerged bodies from 2 degrees of freedom (DOF) to 5 DOF while keeping the same total mass and volume of the system. Both spherical and cylindrical submerged bodies were studied. The dynamic model of a multiple degree of freedom wave energy converter system was formulated with linearized viscous damping drag force effect. The hydrodynamic properties were simulated using the commercial software ANSYS AQWA. It was found that increasing the number of degrees of freedom through adding spherical submerged bodies increases the average captured power by 26% for the WECs going from 2 DOF to 4 DOF and decreases the resonant frequency by 19% going from 2 DOF to 5 DOF. These results are important for point absorbers, as they translate to capturing power more efficiently at a lower resonant frequency, which is closer to the realistic ocean wave excitation frequencies. As for the wave energy converters with cylindrical submerged bodies, even though the resonant frequency reduces with the increase of the number of degrees of freedom, the power massively decreases as well because of the dominant effect of the viscous damping drag force on the system’s dynamics.
... Previous experimental and numerical studies of SRPAs focus predominantly on single device designs (French, 1985;Korde, 1999Korde, , 2003Weinstein et al., 2003;Hals et al., 2007;Beatty et al., 2009;Elwood et al., 2010;Candido and Justino, 2011;Henriques et al., 2012;de Andres et al., 2013;Wu et al., 2014). From these studies, it is difficult to draw equitable comparisons among different SRPA designs due to differences in mooring and PTO control strategies and insufficient characterization of experimental and numerical uncertainties. ...
... Harvesting energy from ocean in electricity power generation has been gaining vast interests from the researchers in the development of renewable energy and multiple types of ocean energy conversion methods, such as tidal [1][2][3][4][5][6], overtopping [7][8][9], oscillating water column [10][11][12][13], ocean currents [14-Penerbit Akademia Baru 16], ocean thermal energy conversion [17,18], salinity gradient power [19], and heaving point absorber [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] have been investigated. A point absorber is an ocean energy conversion device for harvesting wave energy and converting it into electricity through its power take-off system. ...
Article
Full-text available
Wave energy conversion by using point absorber has recently gained intensive research in renewable energy. However, a majority of research works only focused on the regions with high wave heights, which may not be readily achievable in Malaysian seas condition. As the technology of point absorber facing the concern on less-applicability in low wave height conditions in Malaysia, a numerical modeling to understand the maximum potential power output to be generated by point absorber is now in demand to predict the power capture ability of point absorber in Malaysian waters. In order to complete this research gap, this paper is aiming to determine the sensitivity of different configurations of power takeoff system in point absorber and to numerically analyze the potential maximum power output to be generated by the point absorber in Malaysian water, under regular wave motion. The significance of this study leads to a better understanding of the envelope of power output generated by point absorber in Malaysian seas. The methodology is conducted with theoretical modeling of point absorber, developing a numerical model of power takeoff system to identify the maximum magnetic flux density of different stator-translator configuration, and simulating the power output of point absorber in time-domain under regular wave condition based on Malaysia seas data. The results show that power output of point absorber can be increased by a double-sided stator. The envelope of maximum power output to be generated has been identified. This research provides a further understanding of the development of point absorber technologies in Malaysian seas condition.
... Harvesting energy from ocean in electricity power generation has been gaining vast interests from the researchers in the development of renewable energy and multiple types of ocean energy conversion methods, such as tidal [1][2][3][4][5][6], overtopping [7][8][9], oscillating water column [10][11][12][13], ocean currents [14-Penerbit Akademia Baru 16], ocean thermal energy conversion [17,18], salinity gradient power [19], and heaving point absorber [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] have been investigated. A point absorber is an ocean energy conversion device for harvesting wave energy and converting it into electricity through its power take-off system. ...
Article
Wave energy conversion by using point absorber has recently gained intensive research in renewable energy. However, a majority of research works only focused on the regions with high wave heights, which may not be readily achievable in Malaysian seas condition. As the technology of point absorber facing the concern on less-applicability in low wave height conditions in Malaysia, a numerical modeling to understand the maximum potential power output to be generated by point absorber is now in demand to predict the power capture ability of point absorber in Malaysian waters. In order to complete this research gap, this paper is aiming to determine the sensitivity of different configurations of power takeoff system in point absorber and to numerically analyze the potential maximum power output to be generated by the point absorber in Malaysian water, under regular wave motion. The significance of this study leads to a better understanding of the envelope of power output generated by point absorber in Malaysian seas. The methodology is conducted with theoretical modeling of point absorber, developing a numerical model of power takeoff system to identify the maximum magnetic flux density of different stator-translator configuration, and simulating the power output of point absorber in time-domain under regular wave condition based on Malaysia seas data. The results show that power output of point absorber can be increased by a double-sided stator. The envelope of maximum power output to be generated has been identified. This research provides a further understanding of the development of point absorber technologies in Malaysian seas condition.
... Harvesting energy from ocean in electricity power generation has been gaining vast interests from the researchers in the development of renewable energy and multiple types of ocean energy conversion methods, such as tidal [1][2][3][4][5][6], overtopping [7][8][9], oscillating water column [10][11][12][13], ocean currents [14-Penerbit Akademia Baru 16], ocean thermal energy conversion [17,18], salinity gradient power [19], and heaving point absorber [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] have been investigated. A point absorber is an ocean energy conversion device for harvesting wave energy and converting it into electricity through its power take-off system. ...
Article
Full-text available
Wave energy conversion by using point absorber has recently gained intensive research in renewable energy. However, a majority of research works only focused on the regions with high wave heights, which may not be readily achievable in Malaysian seas condition. As the technology of point absorber facing the concern on less-applicability in low wave height conditions in Malaysia, a numerical modeling to understand the maximum potential power output to be generated by point absorber is now in demand to predict the power capture ability of point absorber in Malaysian waters. In order to complete this research gap, this paper is aiming to determine the sensitivity of different configurations of power take-off system in point absorber and to numerically analyze the potential maximum power output to be generated by the point absorber in Malaysian water, under regular wave motion. The significance of this study leads to a better understanding of the envelope of power output generated by point absorber in Malaysian seas. The methodology is conducted with theoretical modeling of point absorber, developing a numerical model of power take-off system to identify the maximum magnetic flux density of different stator-translator configuration, and simulating the power output of point absorber in time-domain under regular wave condition based on Malaysia seas data. The results show that power output of point absorber can be increased by a double-sided stator. The envelope of maximum power output to be generated has been identified. This research provides a further understanding of the development of point absorber technologies in Malaysian seas condition.
... The vertically floating cylinder is a natural model for a floating wave energy converter, whose study goes back at least to Berggren & Johansson's 1992 examination of two floating, axisymmetric cylinders oscillating in heave. More recent studies employing floating cylinders as WECs have been made by Garnaud and Mei 2009, Child and Venugopal 2010, Borgarino et al 2012, Teillant et al 2012, Wu et al 2014, and many others. The ease of solving the Laplace equation in cylindrical coordinates allows for an analytical treatment, first explored in work by Black & Mei 1970, and has allowed for a wealth of insight into the hydrodynamics of such a device. ...
Article
Full-text available
Using the Israeli Mediterranean as an example, we address the impact of resource variability and device survivability on the design of floating-body wave-energy converters (WECs). Employing a simplified heaving cylinder as a prototypical WEC, several device sizes, corresponding to the most frequently encountered and most energetic sea states in the Israeli Mediterranean, are investigated. The mean annual energy production is calculated based on the scatter-diagram/power-matrix approach. Subsequently, a measure for significant device motions under irregular sea-states akin to the spectral significant wave-height is developed, and cutoffs to regular operation are explored from the perspective of these significant displacements. The impact of this WEC downtime is captured in a refinement of mean annual energy production, which consists of supplementing the scatter-diagram/power-matrix calculations by a Boolean displacement matrix. In the Israeli Mediterranean, where most of the annual incident wave power comes in infrequent winter storms, larger WECs outperform smaller WECs by a greater margin when downtime is taken into account. Analogous displacement cutoffs for refining calculations of mean annual energy production may inform WEC design for other sites.
... It was concluded that the added mass of the submerged body and the buoyancy of the floater have the greatest effect on the generated power. The response of a two-body WEC was studied under the variation of the design parameters with a focus on the hydrodynamic mathematical modeling [17]. It was concluded that the design parameters have a big impact on the generated power, especially those parameters related to the resonant frequency. ...
Article
This paper studies the multidisciplinary nature of two body wave energy converters by a parametric study based on the Taguchi method which helps to understand the effect of different dependent parameters on the wave energy conversion performance. Seven different parameters are analyzed and their effect on the maximum captured power, resonance frequency and bandwidth is studied. An interesting comparison between a cylindrical submerged body and a spherical one was made in terms of the system's viscous damping and hydrodynamics. The best system parameter combinations based on the maximum output power, best resonant frequency and frequency bandwidth were identified from the outcomes of the Taguchi method and optimized to capture the maximum power to operate in the specific (Australian) sea regions where the waves’ frequencies are relatively low. This paper should provide a guideline for designers to tune their parameters based on the desired performance and sea state.
... A minor parameter study was conducted where the effect of the oscillating devices' radii on the hydrodynamic properties was investigated. Wu, et al. [90] studied the response of a two-body WEC while changing the design parameters, but more importantly derived and validated an analytical solution for the non-dimensional added mass, damping coefficient, and wave excitation force for both the floater, submerged body, and the interactions between them. A parameter study was conducted, and it was deduced that the design parameters, especially the ones related to the resonant frequency can have a large effect on the captured power. ...
Article
Full-text available
Even though ocean waves around the world are known to contain high and dense amounts of energy, wave energy harvesters are still not as mature as other forms of renewable energy harvesting devices, especially when it comes to commercialization, mass production, and grid integration, but with the recent studies and optimizations, the point absorber wave energy harvester might be a potential candidate to stand out as the best solution to harvest energy from highly energetic locations around the world’s oceans. This paper presents an extensive literature review on point absorber wave energy harvesters and covers their recent theoretical and experimental development. The paper focuses on three main parts: One-body point absorbers, two-body point absorbers, and power take-offs. This review showcases the high amount of work being done to push point absorbers towards technological maturity to eventually kick off commercialization and mass production. It should also provide a good background on the recent status of point absorber development for researchers in the field.
... Recently, Reference [30] pointed out that the mass and viscous damping of the submerged body needs to be designed larger to achieve a better energy absorption of the two body heaving system. Reference [31] shows the system response and the energy absorption of a heaving two-cylinder WEC system. Reference [32] presents a latching phase control for the Interproject Service (IPS) buoy which is a two-body heaving system. ...
Article
Buoys carrying scientific equipment usually need continuous power supply for the operation of these equipments. These buoys can be equipped with actuators and controlled to harvest power from the heaving motion of the buoy. A two-body wave energy converter can be designed such that the buoy heaves to harvest energy while the second (lower) body carries the science equipments. This paper presents a control approach for this type of two-body wave energy converter. This control approach is a multi resonant control that attempts to maximize the harvested energy from the buoy (upper body). In this model, the actuator is attached to both bodies. The lower body however is required to have minimal heave motion. The proposed multi resonant control utilizes measurements of the buoy position. The frequencies of the measured buoy position are estimated, along with the motion amplitudes of these frequencies, and used for feedback control. Estimation is carried out using two approaches; the first uses a linear Kalman filter while the second uses an extended Kalman filter. A new method for handling the motion and actuation limitations, suitable for the multi resonant control, is proposed. Various numerical simulation results are presented in the paper. Simulation results show that the linear Kalman filter estimation approach is more robust and computationally efficient compared to the extended Kalman filter.
... The hydrodynamic properties of a cylinder in other situations, such as locating in extremely shallow water, floating in water of infinite depth, also have been extensively studied (Drobyshevski, 2004;Koh and Cho, 2011;Yu and Srivastava, 2012;Finnegan et al., 2013;Jiang et al., 2013). In addition, wave radiation by a two-body axisymmetric system has also drawn some researchers' attention (Eidsmoen, 1995;Mavrakos, 2004;Zheng et al., 2005;Wu et al., 2014). ...
... In Fig. 2(a), the trend of the capture width ratio of the coaxial-cylinder WEC is shown against the dimensionless angular frequency with the dimensionless generator damping . The hydrodynamic coefficients pto = 0.0068 of a coaxial-cylinder system have been carefully studied in Chau and Yeung [25] and Chau and Yeung [26] and calculated using matched eigenfunction expansion method [27][28] [29]. Note that in the heave motion for finite water depth h, when the incident wave frequency tends to zero, the dimensionless heave radiation damping can reach an exact value that is inverse proportional with h [30]. ...
Article
To achieve a wider frequency range where the device has a larger capture width ratio, the performance of a heaving coaxial-cylinder wave energy converter is optimized through actively controlled generator damping and stiffness using a linear frequency domain model. The generator power take-off system is modeled as a damping-spring system, and the numerical model is validated against published results. The coupled dynamics of a two-body model is analyzed to search for the optimal generator damping and stiffness leading to maximal capture width ratio. The optimization process, which can be decoupled into two independent steps, leads to an improved performance of the device, with increased frequency bandwidth and better capture width ratio. The effects of water depth, mooring stiffness, and the dimensions of the WEC on the capture width ratio are also studied, and parameter values are identified which correspond to optimal performance of the device.
... The conversion efficiency and response of a two-body WEC in regular waves were studied by Wu et al. (2014). They found that the performance of a two-body WEC was highly dependent on system parameters, such as buoy physical properties and incident wave frequency. ...
Article
Full-text available
This work investigated the influence of two types of mooring systems on the hydrodynamic performance of a two-body floating wave energy converter (WEC). It also investigated the effects of the physical parameters of the mooring system on the amount of extractable power from incident waves in the frequency domain. The modeled converter comprised a floating body (a buoy), a submerged body with two mooring systems, and a coupling system for two bodies. The coupling system was a simplified power take-off system that was modeled by a linear spring-damper model. The tension leg mooring system could drastically affect the heave motion of the submerged body of the model and increase relative displacement between the two bodies. The effects of the stiffness parameter of the mooring system on power absorption exceeded those of the pretension tendon force.
... This is also a significant feature of the multi-degree-of-freedom motion of the multi-body system different from that of the single-body system. 20 In theory, under a certain wave parameter, the optimal PTO damping coefficient and optimal spring elasticity coefficient may be very large, so that the theoretical optimal dampers are almost impossible to apply in actual engineering. Therefore, in the design or test of this type of WEC, the PTO damping parameters should be set to the maximum values within a certain range according to the actual situation of the device and wave field. ...
Article
Full-text available
In the practical engineering applications of multi-body floating wave energy converter (WEC), the traditional geometric optimization is always expensive and time-consuming. This study aim to propose a more efficient geometry optimization strategy with a hinged double-body floating WEC as the study object. The influences of geometric parameters of the buoys on the pitching motion and energy conversion ability are analyzed by numerical simulation. Simulation results show that the resonance state of the pitching motion of the buoys mainly depends on their radius and draft rather than the length; But the length of the buoys, rather than the radius and draft, always has a significant effect on the pitching phase difference of the adjacent buoys. Based on the motion analysis and resonance response, an efficient multi-factor geometry optimization strategy is put forwarded. By the strategy, the sub-optimal and optimal geometrical parameters are solved out quickly at several typical wave conditions of China Seas. The results indicate that the optimal total length of WEC is approximately equal to the wave length. The optimal diameter of buoys is about 25% of the length of buoys. And the optimal draft should attain about 61% of the diameter.
... Previous experimental and numerical studies of SRPAs focus predominantly on single device designs [22,31,32,64,25,5,13,6,26,11,66]. From these studies, it is difficult to draw equitable comparisons among different SRPA designs due to differences in mooring and PTO control strategies and insufficient characterization of experimental and numerical uncertainties. ...
Thesis
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A comprehensive set of experimental and numerical comparisons of the performance of two self-reacting point absorber wave energy converter (WEC) designs is undertaken in typical operating conditions. The designs are either currently, or have recently been, under development for commercialization. The experiments consist of a series of 1:25 scale model tests to quantify hydrodynamic parameters, motion dynamics, and power conversion. Each WEC is given a uniquely optimized power take off damping level. For hydrodynamic parameter identification, an optimization based method to simultaneously extract Morison drag and Coulomb friction coefficients from decay tests of under-damped, floating bodies is developed. The physical model features a re-configurable reacting body shape, a feedback controlled power take-off, a heave motion constraint system, and a mooring apparatus. A theoretical upper bound on power conversion for single body WECs, called Budal's upper bound, is extended to two body WECs. The numerical analyses are done in three phases. In the first phase, the WECs are constrained to heave motion and subjected to monochromatic waves. Quantitative comparisons are made of the WEC designs in terms of heave motion dynamics and power conversion with reference to theoretical upper bounds. Design implications of a reactive power take-off control scheme and relative motion constraints on the wave energy converters are investigated using an experimentally validated, frequency domain, numerical dynamics model. In the second phase, the WECs are constrained to heave motion and subjected to panchromatic waves. A time domain numerical model, validated by the experimental results, is used to compare the WECs in terms of power matrices, capture width matrices, and mean annual energy production. Results indicate that the second WEC design can convert 30% more energy, on average, than the first design given the conditions at a representative location near the West coast of Vancouver Island, British Columbia, Canada. In the last phase, the WECs are held with three legged, horizontal, moorings and subjected to monochromatic waves. Numerical simulations using panelized body geometries for calculations of Froude-Krylov, Morison drag, and hydrostatic loads are developed in ProteusDS. The simulation results---mechanical power, mooring forces, and dynamic motions---are compared to model test results. The moored WEC designs exhibit power conversion consistent with heave motion constrained results in some wave conditions. However, large pitch and roll motions severely degrade the power conversion of each WEC at wave frequencies equal to twice the pitch natural frequency. Using simulations, vertical stabilizing strakes, attached to the reacting bodies of the WECs are shown to increase the average power conversion up to 190% compared to the average power conversion of the WECs without strakes.
... Previous experimental and numerical studies of SRPAs focus predominantly on single device designs (French (1985); Korde (1999Korde ( , 2003; Weinstein et al. (2003); Hals et al. (2007); Beatty et al. (2009) ;Elwood et al. (2010); Candido and Justino (2011); Henriques et al. (2012); de Andres et al. (2013); Wu et al. (2014)). From these studies, it is difficult to draw equitable comparisons among different SRPA designs due to differences in mooring and PTO control strategies and insufficient characterization of experimental and numerical uncertainties. ...
Conference Paper
Full-text available
Two self reacting point absorber (SRPA) wave energy converter (WEC) design configurations are under analysis by the West Coast Wave Initiative (WCWI) at the University of Victoria. The analyses are motivated threefold. First, to provide benchmark comparisons of the two SRPA configurations given simple mooring systems in operating conditions. Second, to uncover dynamic and hydrodynamic aspects that hinder the performance of the SRPA designs, and third, to deliver recommendations for the design of a maximally effective SRPA. The first SRPA configuration features a streamlined reacting body, the second SRPA configuration features a damper plate reacting body, and both configurations feature the same float. The study involves both physical and numerical models. The physical model is a 1:25 scale WEC that features a feedback controlled power take off (PTO) simulator, a re-configurable external hull shape, and three-legged horizontal soft moorings. Uniquely optimized PTO damping levels for each WEC configuration and power production test were chosen by a numerical optimization. The numerical simulations, generated with ProteusDS, use panelized body geometries for instantaneous calculations of nonlinear Froude-Krylov, Morison drag, and hydrostatic loads. Radiation and diffraction forces, PTO forces, and mooring forces are all included in the simulation. The physical model test data—mechanical power, mooring forces, and dynamic motions in regular waves—are compared to simulation results. The wave conditions represent operational (nonextreme) conditions. Both WEC configurations exhibited high power conversion performance in some sea conditions. Large pitch and roll motion amplitudes observed at wave frequencies equal to twice the natural frequency of the WEC in pitch, severely degrade the power conversion performance. Using simulations, vertical stabilizing strakes, attached the reacting bodies of the WECs are shown to improve average power conversion by up to 190% compared to average power conversion of the WECs without strakes.
... They use this technology for power extraction which was used in oil and gas plants. Wu et al. [9] carried out detailed numerical simulation analysis and they found out that the net power extraction of a two-body heaving WEC system is heavily dependent on the physical system properties such as incident wave frequencies, buoy diameter, and shape, etc. ...
Article
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The paper aims to design and simulation of a wave energy harvesting system commonly known as point absorber for Ennore port located in the coastal area of Chennai, India. The geographical condition of India, which is surrounded by the three sides with seas and ocean, has enormous opportunity for power production through wave energy harvesting system. The wave energy converter device is a two-body floating system and its both parts are connected by power take-off unit which acts as spring mass damper system. In this paper, the hydrodynamic diffraction, stability analysis, frequency, and time response analysis is carried out on ansys-aqwa. The numerical results are compared with the results obtained from the similar experiments for validation of CFD solver. Effects of the properties featuring wave characteristics including wave height and wave period of Ennore port on the energy conversion, Froude-Krylov and diffraction force, response amplitude operator (RAO) are studied. Based on the study, float diameter, draft, geometry, and varying damping coefficient for power generation are optimized. Finally, the optimally designed point absorber is simulated as per Indian ocean energy harvesting standard and mass of the system, heave dimension, diffraction forces, and pressure variations are computed.
... Zheng et al [32], in a generalization of Berggren & Johansson to three modes of motion, considered the hydrodynamics of two unconnected, coaxial floating cylinders, again without considering power capture. The power capture for a heave-only two-cylinder WEC was recently obtained for attacking monochromatic incident waves by Wu et al [30], albeit with a rather terse discussion of their results. ...
Preprint
We discuss the hydrodynamics of a wave energy converter consisting of two vertically floating, coaxial cylinders connected by dampers and allowed to heave, sway and roll. This design, viable in deep water and able to extract energy independent of the incident wave direction, is examined for monochromatic waves as well as broad-banded seas described by a Pierson Moskowitz spectrum. Several possible device sizes are considered, and their performance is investigated for a design spectrum, as well as for more severe sea states, with a view towards survivability of the converters. In terms of device motions and captured power, a quantitative assessment of converter design as it relates to survival and operation is provided. Most results are given in dimensionless form to allow for a wide range of applications.
Article
The dynamics of a novel wave energy converter based on a guided inclined point absorber are investigated. Thereby, it is studied through simulations and experiments whether different inclination angles of the guided point absorber lead to larger motion amplitudes and velocities in regular and irregular waves, from which energy can be harvested. For that, different simulations and experimental setups are analyzed in the presence of wave forcing. In the case of irregular waves a random non-white Gaussian stochastic process based on a sea spectrum is used. It is shown that the inclination angle has a significant influence on the energy harvesting output. Based on this insight, a simple control strategy is introduced in order to further increase the energy harvesting output.
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The interaction of water waves with a freely floating circular cylinder possessing a side-wall that is porous over a portion of its draft is investigated theoretically. The porous side-wall region is bounded top and bottom by impermeable end caps thereby resulting in an enclosed fluid region within the structure. The problem is formulated based on potential flow and linear wave theory and assuming small-amplitude structural oscillations. An eigenfunction expansion approach is then used to obtain semi-analytical expressions for the hydrodynamic excitation and reaction loads on the structure. Numerical results are presented which illustrate the effects of the various wave and structural parameters on these quantities. It is found that the permeability, size and location of the porous region may have a significant influence on the horizontal components of the hydrodynamic excitation and reaction loads, while its influence on the vertical components in most cases is relatively minor.
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Hydrodynamics coefficients for vertical circular cylinders at finite water depth are obtained and presented for different depth to radius and draft to radius ratios. A summary of equations for computer application is presented. Limiting values for heave added mass for zero frequency are discussed. A matching technique is used to satisfy the continuity of pressure and normal velocities.
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The Archimedes Wave Swing (AWS) is a system that converts ocean wave energy into electrical energy. A pilot plant of this system has been built. This paper describes the design of the permanent-magnet (PM) linear synchronous generator applied in the AWS. Based on a magnetic circuit model, it is concluded that saturation does not play an important role. The correlation between measured and calculated generator parameters and no-load voltage is reasonable, which indicates that the generator is adequate. Copyright © 2005 John Wiley & Sons, Ltd.
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This paper describes the research, design, construction and prototype testing process of a novel ocean energy direct drive permanent magnet linear generator buoy. The buoy employs the vertical component of the motion of ocean waves to power a linear generator. The generator consists of a permanent magnet field system (mounted on the central translator shaft) and an armature, in which the power is generated (mounted on the buoy). The translator shaft is anchored to the sea floor, and the buoy/floater moves armature coils relative to the permanent magnet translator to induce voltages. The electrical and mechanical structures of the buoy generator are provided, along with performance characteristics, including voltage, current and developed power.
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The progress in wave energy conversion in Europe during the past ten years is reviewed and current activities and initiatives in the wave energy sector at National and Union level are described. Other important activities worldwide are summarized. The technical and economical status in wave energy conversion is outlined and important wave energy developments are presented.
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Hydrodynamic coefficients of a wave energy device consisting of a buoy connected to a submerged plate, are presented. Both the buoy and the plate are idealised as vertical cylinders. For this two-body system the case with the buoy oscillating vertically and the case with the submerged plate oscillating vertically are treated. The coefficients are solved by the method of matched eigenfunction expansions. Numerical results showing the plate's influence on the hydrodynamic coefficients of the buoy and vice versa are presented.
Calculation of Hydrodynamic Forces on Floating Cylinder Wave Energy Conversion Devices and Energy Steady Control
  • B J Wu
Wu, B.J., 2005. Calculation of Hydrodynamic Forces on Floating Cylinder Wave Energy Conversion Devices and Energy Steady Control. Ph. D. Thesis. University of Science and Technology of China, Hefei.