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Analysis of a Two-Body Floating Wave Energy Converter With Particular Focus on the Effects of Power Take-Off and Mooring Systems on Energy Capture
Abstract and Figures
The present paper summarizes analyses of a two-body floating wave energy converter (WEC) including the mooring system. An axi-symmetric Wavebob type WEC is chosen as the object of investigation here. However, the PTO system is modeled in a simplified manner as ideal linear damping and spring terms that couples the body 1 and the body 2 motions. The analysis is done using SIMO, a time domain simulation tool which accommodates simulation of multi-body systems with hydrodynamic interactions. In SIMO, docking cone features have been introduced between the two bodies to let them move as per actual operation and fenders are applied to represent end stops. Six alternative mooring configurations are applied to investigate the effect of mooring on power capture. In this paper, the software HydroD using WAMIT for hydrodynamic is used to determine hydrodynamic loads. The analysis is carried out for several regular and irregular wave conditions as representative of operational conditions. Simulations are performed with the purpose to study the effects of power take off (PTO) system, end stops setting and several mooring configurations on power captured by the WEC.
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... In recent decades, a significant portion of wave energy research has focused on the deployment of WABs in regions with extreme wave climates [19,20]. Such wave characteristics can yield higher power densities. ...
... To validate the correct set-up of the hydrodynamic model in ANSYS ® AQWA, the results obtained by Muliawan et al. in [20] were used as a reference for comparison. Figs 8 and 9 show the RAO results obtained from [20] and the present hydrodynamic model, respectively. ...
... To validate the correct set-up of the hydrodynamic model in ANSYS ® AQWA, the results obtained by Muliawan et al. in [20] were used as a reference for comparison. Figs 8 and 9 show the RAO results obtained from [20] and the present hydrodynamic model, respectively. The heave RAO value shows that the Wave Period (s) Fig. 9: RAO results from ANSYS ® AQWA maximum RAO values for both sway and roll in the ANSYS ® AQWA simulation occurred at a wave period of 18 s, which is the same as the results in the reference. ...
Recently, most wave energy research has targeted sea areas with high power density under extreme wave conditions for deployment. This trend often leads to higher energy costs for wave power plants. However, many marine areas with low power density, characterized by wavelet conditions and less destructive forces than extreme wave conditions, remain underexplored. Therefore, this study aimed to propose a wave-activated body model for sea regions with wavelet conditions. The wave-activated body design process encompassed site selection, parameter determination, geometry design, comparison and performance evaluation using the ANSYS® AQWA model. The results indicated that the proposed device achieved the desired heave motion, with an amplitude range of 1.2 to –2.5 m, validating its potential for deployment in marine regions with wavelet conditions. Notably, while the proposed design is optimized for wavelet conditions, it was found to have potential limitations in extreme wave environments. This observation emphasizes the challenge of formulating a generalized design suitable for both conditions. Consequently, it is pivotal for wave-activated body designs to be customized based on the specific ocean conditions they target, underscoring the need for specialized designs of wave energy converters that consider the unique wave characteristics of their deployment sites.
... They are proportional to the velocity and displacement of the relative heave directly. For detailed information about modeling, please refer to (Muliawan et al., 2011) [45]. ...
... They are proportional to the velocity and displacement of the relative heave directly. For detailed information about modeling, please refer to (Muliawan et al., 2011) [45]. ...
To reduce the cost of offshore wind and wave power, an innovative combined wind–wave energy generation system constituting of a 15 MW semi-submersible floating offshore wind turbine (FOWT) and four torus-type wave energy converters (WECs) is proposed. A wholly coupled numerical model of aero-hydro-elastic-servo-mooring was built to evaluate the mooring line and motion dynamics of this novel combined system. Additionally, a practical mooring optimization framework is proposed with the Latin Hypercube sampling method, Kriging model, and the combined optimization techniques of the Genetic Algorithm and Gradient Algorithm. The optimization results demonstrate that the optimized mooring scheme satisfies all the strict constraints, validating the effectiveness of the optimization method. Moreover, the hydrodynamic characteristics of the combined system and the effects of the WECs on the mooring system under both rated and extreme conditions are discussed, including changes in time-series mooring tension, power spectral density, and statistical characteristics. The research findings provide a reference for the further development and optimization of this novel combined system, contributing to the efficient utilization of offshore renewable energy.
... Two research groups must be noted. First, the consistent numerical work done by Muliawan et al. (2011Muliawan et al. ( , 2012Muliawan et al. ( , 2013aMuliawan et al. ( , 2013b, that introduced the STC concept and performed several dynamic analyses. The investigation was later updated by Ren et al. (2015), and followed by an in-depth experimental investigation (Wan et al., , 2016a(Wan et al., , 2016b. ...
This paper presents an overview of the recent developments in hybrid wind-wave energy. With the focus on floating concepts, the possible configurations introduced in the literature are categorized and depicted, and the main conclusions obtained from the references are summarized. Moreover, offshore wind and wave resources are discussed in terms of complementarity and supplementarity, offering a new perspective to developing hybrid wind-wave energy systems that look for synergies not limited to maximizing power output. Then, the feasibility of the concepts under development is discussed in detail, with focus on technical feasibility, dynamic feasibility and limitations of the methods employed. The hybrid configurations that surpassed the experimental validation phase are highlighted, and the experimental results are summarized. By compiling more than 40 floating wind turbine concepts, new relations are drawn between power, wind turbine dimensions, platforms' draft and displacement, which are further related to the payload allowance of the units to accommodate wave devices and onboard power takeoff systems. Bearing in mind that it is a challenge to model the exact dynamics of hybrid floating wind-wave platforms, this paper elucidates the current research gaps, limitations and future trends in the field. Lastly, based on the overview and topics discussed, several major conclusions are drawn concerning hybrid synergies, dynamics and hydrodynamics of hybrid platforms, feasibility of concepts, among other regards.
... In these three systems, the floating or fixed foundation of the platform has a single cylindrical column, which means they can be appropriately called single coaxial-cylinder hybrid systems. The design was originally inspired by a coaxial-cylinder WEC Wavebob ( Jaya Muliawan et al., 2013). Therefore, the annular WECs used in these hybrid systems were directly borrowed from the configuration of the outer cylinder of the Wavebob. ...
Installing annular wave-energy converters (WECs) on the columns of floating wind platforms in the form of a coaxi-al-cylinder provides a convenient means of integration. Extant coaxial-cylinder-type wind-wave hybrid systems are mostly based on single-column platforms such as spars ('single coaxial-cylinder hybrid system' hereafter). Systems based on multiple-column platforms such as semi-submersible platforms ('multiple coaxial-cylinder hybrid systems' hereafter) are rarely seen or studied, despite their superiority in wave-power absorption due to the use of multiple WECs as well as in dynamic stability. This paper proposes a novel WindFloat platform-annular WECs hybrid system, based on our study investigating its dynamic and power features, and optimizing the geometry and power takeoff of the WECs. Our results show that the dynamic and power features of a multiple coaxial-cylinder hybrid system are different from those of a single coaxial-cylinder hybrid system; thus the same optimization parameters cannot be directly applied. Flatter annular WECs absorb slightly more power in a wider wave-period range, but their geometry is confined by limitations in installation and structural strength. The overall effect of an oblique incident wave is greater intensity in the motions of the hybrid system in yaw and the direction perpendicular to propagation, although the difference is small and may be negligible.
... The geometry and the PTO of the annular WEC are, therefore, deterministic to the energy conversion performance of the hybrid system. In most previous studies, the configuration of the annular WEC was directly brought from the Wavebob device [31], and the PTO damping was assigned randomly chosen, unoptimized values [19]. Till recently, a study by the authors systematically optimized the dimensions and PTO damping of the annular WEC in a hybrid system based on the OC3 Hywind wind platform [32]. ...
Over the last two decades, a large body of academic scholarship has been generated on wave and tidal energy related topics. It is therefore important to assess and analyse the research direction and development through horizon scanning processes. To synthesise such large-scale literature, this review adopts a bibliometric method and scrutinises over 8000 wave/tidal energy related documents published during 2003–2021. Overall, 98 countries contributed to the literature, with the top ten mainly developed countries plus China produced nearly two-thirds of the research. A thorough analysis on documents marked the emergence of four broad research themes (dominated by wave energy subjects): (A) resource assessment, site selection, and environmental impacts/benefits; (B) wave energy converters, hybrid systems, and hydrodynamic performance; (C) vibration energy harvesting and piezoelectric nanogenerators; and (D) flow dynamics, tidal turbines, and turbine design. Further, nineteen research sub-clusters, corresponding to broader themes, were identified, highlighting the trending research topics. An interesting observation was a recent shift in research focus from solely evaluating energy resources and ideal sites to integrating wave/tidal energy schemes into wider coastal/estuarine management plans by developing multicriteria decision-making frameworks and promoting novel designs and cost-sharing practices. The method and results presented may provide insights into the evolution of wave/tidal energy science and its multiple research topics, thus helping to inform future management decisions.
A hybrid system of a spar-type floating offshore wind turbine and a heaving annular wave energy converter (WEC) provides a promising solution for collocated ocean renewable energy exploitation. The performance of the hybrid system depends on the dimensions of the WEC. Here an optimization method is proposed to determine the outer radius and the draft of the WEC under the wave condition in a randomly chosen operational site. First, three candidate models are selected based on three operational conditions of energy harvest: (1) The natural frequency of the system is matched with the peak wave frequency in the target site (referred to as synchronized mode), where the wind turbine and the WEC nearly heave together in a near-resonance condition, (2) The natural frequency of the WEC is matched with the peak wave frequency (ring mode), (3) The maximum wave power is harnessed under the peak wave frequency (target mode). Then the candidate modes are evaluated to obtain an optimum. Results show that the extracted wave power under the above operational conditions has an upper bound that can hardly be surpassed by enlarging the dimensions of the WEC only. The optimal annual wave energy production is achieved in the synchronized mode because of the superior performance of WEC over a wide bandwidth of effective energy conversion.
A method to include the influence of mooring cables in the frequency domain analysis of wave energy converters is presented. In brief the method consists of:(i)A non-linear time domain solution of the mooring line in isolation and at an appropriate equilibrium condition. This is done by enforcing a sinusoidal displacement at the mooring attachment point in each translational degree of freedom. This is repeated at a number of frequencies.(ii)The amplitude and phase of the resulting force is recorded, allowing the equivalent linear resistive and reactive contribution of the mooring line to be estimated separately. Using results at a number of frequencies, frequency dependent impedance properties of the mooring cable can be estimated.(iii)Considering the attachment point and orientation of the mooring cables in a suitable equilibrium condition of the device, the contribution of each mooring cable is resolved to the global co-ordinates of the device and added to the frequency domain equation of motion.The method here is applied to a generic wave energy device based on a truncated vertical cylinder of 100 tonne displacement. The results for the unmoored device are compared to the same device with moorings of varying configuration. The results indicate that moorings may have a significant impact on the performance of devices of this scale, both beneficial and detrimental. The introduction of mooring terms can upset device symmetry and introduce significant cross coupling in the overall mechanical impedance of the device. Arrangements where this can benefit as well as detriment performance are studied.
To be effective, wave energy converter devices (WECs) must be installed in unsheltered locations with high wave energy density. Such regimes are more likely to be at offshore locations and much current interest is focused on offshore floating devices. Since the wave energy industry has little practical long-term experience with deployed WEC systems there is no historical experience upon which to base the formulation of appropriate station-keeping arrangements. The development of generic station-keeping arrangements that will allow reliable, yet cost-effective, mooring systems presents a challenging task. This paper raises issues pertinent to the station-keeping of floating WECs and discusses a preliminary design procedure to identify suitable mooring arrangements.
Sea waves are a promising potential source of renewable energy, but the technology has not yet settled down to one or two basic forms as has happened with most major inventions. New ideas continue to arise, and no general agreement about how to proceed seems to be emerging. Moreover, although practicable wave energy converters (WECs) have been developed, they are not as economical as might have been hoped.
This paper explores a particular aspect of the cost problem. Broadly speaking, the costs of WECs are high because they deal with large forces moving slowly. It is reasonable to expect that where a working surface can be identified, the bigger its area is relative to the overall surface area and the faster the surface moves, the more economical it is likely to be. This suggests two criteria for an economical WEC: a large working area relative to its size and a high ratio of the speed of that surface to the particle speed of the wave. The second criterion indicates a resonant system, and this paper is confined to WECs that have quasi-resonant working surfaces (QR WECs). The quasi- is because the mechanics involved is not quite that of classical resonance.
Promising types are listed in four groups according to the source of reaction, because this is generally the most difficult function to provide in QR WECs. Other types of WEC are left out, most often because they do not meet the criteria of a strongly coupled working surface and resonance, and usually require large amounts of material. While size is not uniquely related to cost, WECs using much material are likely to be expensive.
The aim of this study is to estimate the mean annual power absorption of a selection of eight Wave Energy Converters (WECs) with different working principles. Based on these estimates a set of power performance measures that can be related to costs are derived. These are the absorbed energy per characteristic mass [kWh/kg], per characteristic surface area [MWh/m(2)], and per root mean square of Power Take Off (PTO) force [kWh/N]. The methodology relies on numerical modelling. For each device, a numerical Wave-to-Wire (W2W) model is built based on the equations of motion. Physical effects are modelled according to the state-of-the-art within hydrodynamic modelling practise. Then, the W2W models are used to calculate the power matrices of each device and the mean annual power absorption at five different representative wave sites along the European Coast, at which the mean level of wave power resource ranges between 15 and 88 kW per metre of wave front. Uncertainties are discussed and estimated for each device. Computed power matrices and results for the mean annual power absorption are assembled in a summary sheet per device. Comparisons of the selected devices show that, despite very different working principles and dimensions, power performance measures vary much less than the mean annual power absorption. With the chosen units, these measures are all shown to be of the order of 1.
A system for absorbing and utilizing the energy carried by ocean waves is discussed. The 'point absorber' considered is a system in which the horizontal extent is much smaller than one wavelength. The point absorber is optimized for efficient energy conversion. The resonant characteristic frequency of the system is at all times tuned to the characteristic frequency of the wave.
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