OPTIMIZATION AND CONTROL OF ARRAYS OF WAVE ENERGY CONVERTERS

Wave Energy Converter Array is a practical approach to harvest ocean wave energy. To leverage the potential of the WEC array in terms of energy extraction, it is essential to have a properly designed array configuration and control system. This thesis explores the optimal configuration of Wave Energy Converters (WECs) arrays and their optimal control. The optimization of the WEC array allows both dimensions of individual WECs as well as the array layout to varying. In the first optimization problem, cylindrical buoys are assumed in the array where their radii and drafts are optimization parameters. Genetic Algorithms are used for optimization. Three case studies are investigated of different array sizes: 3, 5, and 7 devices in the array. Two types of controls are assumed; the first is the standard impedance matching control while the second is a derivative control. The numerical test cases demonstrate that a higher q-factor is achieved when optimizing the buoys dimensions simultaneously with the array layout. In the conducted test cases, it is shown that optimizing the array layout can increase the q-factor on average by 39.21% when using optimal control, and increase it on average by a factor of 8.87% when using a derivative control. Arrays of wave energy converters (WECs) usually have large spacing between members of the array to avoid negative hydrodynamic interaction between members in the array. Errors in estimating the spacing between members may result in a significant degradation in the performance of the array in terms of the total harvested energy, due to destructive hydrodynamic interaction between members of the array. In this thesis, a hybrid design of wave energy converter arrays, that contains two types of WECs, the heaving buoys, and the floating flap-type devices, is investigated and compared against traditional WEC arrays of heaving buoys. The resulting q-factor is less sensitive to deviations in the spacing from the design layout. This hybrid array, hence, enables more WECs in the same ocean area. The two types of arrays are tested using 40 layouts that have different separation distances ranging from small to large. With the hybrid configuration, the array achieved a variance of the q-factor as low as 0.006. The traditional array has a variance of 0.024 which is four times larger. The optimization is conducted on the hybrid array with both layout and dimension as design variables. The optimal control algorithm for the WEC array is developed using the optimality condition. Devices in the array are assumed to be identical heaving buoys. The optimization objective is to maximize energy extraction at each time step. Both regular and irregular waves are used to excite the array. The unconstrained optimal control problem is solved with saturation on the control force. The solutions show that good wave estimations and sufficient accuracy of the radiation sub-system are the keys to the desired WEC array performance

A Multiline Anchor Concept for Floating Offshore Wind Turbines

Floating offshore wind turbines (FOWTs) hold great potential for the renewable energy industry, but capital costs remain high. In efforts to increase FOWT substructure efficiency and reduce costs, this thesis investigates a novel multiline anchor concept in which FOWTs share anchors instead of being moored separately. The goal of this thesis is to evaluate the force dynamics, design, and potential cost reduction of the system. Anchor forces are simulated using the NREL 5 MW reference turbine and OC4-DeepCwind semisubmersible platform, and multiline anchor force is computed as the vector sum of the contributing mooring line tensions. The use of a multiline anchor configuration in large scale farms would result in reductions in the number of anchors approaching 67% and 83% for 3-line and 6-line anchor systems, respectively. Maximum anchor force is up to 16% smaller in 3-line anchor systems and up to 20% larger in 6-line anchor systems, compared to conventional single-line anchor systems. Direction of the multiline anchor force generally aligns with the wind, wave, and current (WWC) direction, and direction reversal within a single force cycle occurs in extreme load cases. Spatial coherence of the wave fields is considered due to the interconnectedness of the system, revealing that spatial correlation of the waves decays to insignificant levels within several hundred meters. Given that FOWT spacing is greater than 500 m, it is asserted that the assumption of independent wave fields at different FOWT locations is sufficient for obtaining multiline anchor load characterizations. The site characteristics of the first and only floating offshore wind farm as of writing this thesis are used to evaluate the multiline anchor concept in the context of a real farm. Average anchor size is similar between the installed single-line and novel multiline anchor layouts, but the multiline system results in a 41% reduction in the total anchor weight required. A capital cost analysis of the mooring lines, anchors, anchor installation, and geotechnical site investigation is carried out over a range of water depths, turbine spacings, and farm sizes, resulting in cost reductions for the multiline anchor system of 8-16% for a 100-turbine wind farm

Renewable Energy in Marine Environment

The effects of human-caused global warming are obvious, requiring new strategies and approaches. The concept of business-as-usual is now no longer beneficial. Extraction of renewable energy in marine environments represents a viable solution and an important path for the future. These huge renewable energy resources in seas and oceans can be harvested, including wind, tide, and waves. Despite the initial difficulties related mostly to the elevated operational risks in the harsh marine environment, newly developed technologies are economically effective or promising. Simultaneously, many challenges remain to be faced. These are the main issues targeted by the present book, which is associated with the Special Issue of Energies Journal entitled “Renewable Energy in Marine Environment”. Papers on innovative technical developments, reviews, case studies, and analytics, as well as assessments, and papers from different disciplines that are relevant to the topic are included. From this perspective, we hope that the results presented are of interest to for scientists and those in related fields such as energy and marine environments, as well as for a wider audience

Assessment of the power output of a two-array clustered WEC farm using a BEM solver coupling and a wave-propagation model

One of the key challenges in designing a Wave Energy Converter (WEC) farm is geometrical layout, as WECs hydrodynamically interact with one another. WEC positioning impacts both the power output of a given wave-energy project and any potential effects on the surrounding areas. The WEC farm developer must seek to optimize WEC positioning to maximize power output while minimizing capital cost and any potential deleterious effects on the surrounding area. A number of recent studies have shown that a potential solution is placing WECs in dense arrays of several WECs with space between individual arrays for navigation. This innovative arrangement can also be used to reduce mooring and cabling costs. In this paper, we apply a novel one-way coupling method between the NEMOH BEM model and the MILDwave wave-propagation model to investigate the influence of WEC array separation distance on the power output and the surrounding wave field between two densely packed WEC arrays in a farm. An iterative method of applying the presented one-way coupling to interacting WEC arrays is used to compute the wave field in a complete WEC farm and to calculate its power output. The notion of WEC array ‘independence’ in a farm from a hydrodynamic point of view is discussed. The farm is modeled for regular and irregular waves for a number of wave periods, wave incidence angles, and various WEC array separation distances. We found strong dependency of the power output on the wave period and the wave incidence angle for regular waves at short WEC array⁻array separation distances. For irregular wave operational conditions, a large majority of WEC array configurations within a WEC farm were found to be hydrodynamically ‘independent’