Optimizing ballast design of wave energy converters using evolutionary algorithms

Wave Energy Converters (WECs) promise to be a viable alternative to current electrical generation methods. However, these WECs must become more efficient before wide-scale industrial use can become cost-effective. The efficiency of a WEC is primarily dependent upon its geometry and ballast configuration which are both difficult to evaluate, due to slow computation time and high computation cost of current models. In this thesis, we use evolutionary algorithms to optimize the ballast geometry of a wave energy generator using a two step process. First, we generate a function approximator (neural network) to predict wave energy converter power output with respect to key geometric design variables. This is a critical step as the computation time of using a full model (e.g., AQWA) to predict energy output prohibits the use of an evolutionary algorithm for design optimization. The function approximator reduced the computation time by over 99% while having an average error of only 3.5%. The evolutionary algorithm optimized the weight distribution of a WEC, resulting in an 84% improvement in power output over a ballast-free WEC

Improving Computational Efficiency in WEC Design: Spectral-Domain Modelling in Techno-Economic Optimization

Wave energy converter (WEC) optimization often underlines incremental and iterative approaches that result in suboptimal solutions, since all the elements that concur with a techno-economical evaluation are optimized separately due to computation constraints. A design process should rely on precise WEC models to ensure high result accuracy while minimizing the computational demand. These conflicting objectives can be addressed with non-linear time-domain models, known to be numerically accurate, and frequency-domain models due to their high computational efficiency. This work pursues the development of an all-encompassing optimization tool for a gyroscopic-type WEC called ISWEC that applies a new modelling technique named spectral-domain technique as a substitution to the complex time-domain model previously employed. In particular, the spectral-domain technique provides accurate and fast performance predictions of the ISWEC system and offers the possibility to model a hydraulic power take-off, not representable in the frequency domain. The article illustrates techno-economic trends associated with an early-stage design of the ISWEC in high-energy sea-sites, where the low-speed and high-torque profiles call for the use of hydraulic transmissions as opposed to the old electro-mechanical transmissions. The design tool proposed could facilitate the development of WEC technologies via efficient and accurate power assessment and via the possibility of carrying out advanced techno-economic optimisation that goes beyond linear models

Design Parameter Analysis of Point Absorber WEC via an Evolutionary-Algorithm-Based Dimensioning Tool

Wave energy conversion has an essential difference from other renewable energies since the dependence between the devices design and the energy resource is stronger. Dimensioning is therefore considered a key stage when a design project of Wave Energy Converters (WEC) is undertaken. Location, WEC concept, Power Take-Off (PTO) type, control strategy and hydrodynamic resonance considerations are some of the critical aspects to take into account to achieve a good performance. The paper proposes an automatic dimensioning methodology to be accomplished at the initial design project stages and the following elements are described to carry out the study: an optimization design algorithm, its objective functions and restrictions, a PTO model, as well as a procedure to evaluate the WEC energy production. After that, a parametric analysis is included considering different combinations of the key parameters previously introduced. A variety of study cases are analysed from the point of view of energy production for different design-parameters and all of them are compared with a reference case. Finally, a discussion is presented based on the results obtained, and some recommendations to face the WEC design stage are given

Low-Cost Heaving Single-Buoy Wave-Energy Point Absorber Optimization for Sardinia West Coast

This work presents the Water Energy Point Absorber (WEPA), which is a heaving single-buoy point absorber optimized for a specific site off the west coast of Sardinia Island. The aim of the study is to present the optimization process undertaken to identify the best configuration in terms of performance and cost. The optimization is carried out thanks to a simulation tool developed in Matlab-Simulink environment and verified through to the commercial software Orcaflex. Simulations are performed in the time domain with the installation site’s waves as input. The hydrodynamics parameters are computed thanks to the commercial software Ansys Aqwa and given to the model as input. The yearly energy production is computed as output for each configuration. Several parametric analyses are performed to identify the optimal Power Take Off (PTO) and buoy size. Among the main findings, it shall be mentioned that the PTO-rated torque has a strong influence on the energy production, higher PTO-rated torque proved to have better performance. The optimal hull size is strictly related to the incoming waves, and for the given site the smaller hulls are performing better than larger ones. The hull height, hull mass and hull draft have little impact on productivity. Finally, a comprehensive techno–economic analysis is performed, showing that the best configuration can be identified only after a detailed feasibility study and rigorous cost analysis

Optimization and Energy Maximizing Control Systems for Wave Energy Converters

The book, “Optimization and Energy Maximizing Control Systems for Wave Energy Converters”, presents eleven contributions on the latest scientific advancements of 2020-2021 in wave energy technology optimization and control, including holistic techno-economic optimization, inclusion of nonlinear effects, and real-time implementations of estimation and control algorithms

Investigation of new layout design concepts of an array-on-device WaveSub device

Wave Energy Converters (WECs) have not yet proven their competitiveness in the mainstream energy market. Research and development of this technology are necessary to find optimal solutions in terms both of energy produced and reduced cost. A WEC farm is expected to have reduced Levelized Cost of Energy (LCoE) compared to individual devices due to shared installation and grid connection costs. Studies show that energy yield of a WEC array is highly dependent on spacing and layout of the WECs. A method for selecting an optimal array layout is desirable.Here we show a comparison between 4 different design layouts of a WaveSub device with six floats. A six float configuration has been chosen because the LCoE reduces with increasing floats per device as shown in previous research. An optimal configuration in terms of LCoE and rated power is found for linear, rectangle, triangle and circular multi-float configurations. Parameters optimised are float spacing and Power Take Off (PTO) stiffness, damping and rated power. The optimisation algorithm uses a genetic algorithm combined with a Kriging surrogate model. Numerical simulations are solved in the time-domain in WEC-Sim while the hydrodynamic coefficients are calculated in Nemoh using a linear potential flow theory.For all geometric configurations, the smallest float spacing was the most promising because of the lower cost of the structure. In fact, the influence of the float spacing on the power produced by the device is shown to be less significant than the influence of float spacing on the capital cost. Overall, the circular configuration outperformed the other configurations. This study shows that layout configurations can be investigated with optimisation and this could be applied to other configurations and other WEC concepts in future

Hydrodynamic performance optimization of semi-submersible floaters for offshore wind turbines

Floating structures have become viable alternatives for supporting wind turbines as offshore wind projects move deeper into the water. The wind is prevalent in deep water (depths > 60 m) all around the world. Because of the amount of potential at these depths, wind turbines will require the design of a floating platform, as current wind turbines are usually fixed at the bottom and rely on ordinary concrete with a gravity base, which is not practical at these depths. Floating offshore wind offers a huge potential for green energy production offshore and the overall energy transition to zero carbon emission in general. With the development of even larger wind turbines in the range beyond 15 MW, the floating concepts become more attractive and competitive from a cost perspective. However, larger turbines and cost optimization also require a re-thinking of established solutions and concepts. New ideas and innovations are required to optimize floating offshore wind farms further. An approach for the optimization of semi-submersible floaters using different surrogate models has been developed in this thesis. A semi-submersible floater is selected and designed to support a 15-MW wind turbine in the North Sea. The optimization framework consists of automatic modeling and numerical simulations in open-source tools as well as obtaining the Pareto fronts using surrogate models and the Genetic Algorithm in CEASES software. A Python-SALOME-NEMOH interface is used to obtain the hydrodynamic properties for geometries defined by various variables. The geometries are subjected to three performance constraints: the static platform pitch, metacentric height, nacelle acceleration, and wind. Loads in operating and parked conditions are considered. Finally, the geometries are optimized using two objective functions related to material cost and nacelle acceleration, and the results are discussed. This work contributes to developing efficient design optimization methods for floating structures

Empowering wave energy with control technology: Possibilities and pitfalls

With an increasing focus on climate action and energy security, an appropriate mix of renewable energy technologies is imperative. Despite having considerable global potential, wave energy has still not reached a state of maturity or economic competitiveness to have made an impact. Challenges include the high capital and operational costs associated with deployment in the harsh ocean environment, so it is imperative that the full energy harnessing capacity of wave energy devices, and arrays of devices in farms, is realised. To this end, control technology has an important role to play in maximising power capture, while ensuring that physical system constraints are respected, and control actions do not adversely affect device lifetime. Within the gamut of control technology, a variety of tools can be brought to bear on the wave energy control problem, including various control strategies (optimal, robust, nonlinear, etc.), data-based model identification, estimation, and forecasting. However, the wave energy problem displays a number of unique features which challenge the traditional application of these techniques, while also presenting a number of control ‘paradoxes’. This review articulates the important control-related characteristics of the wave energy control problem, provides a survey of currently applied control and control-related techniques, and gives some perspectives on the outstanding challenges and future possibilities. The emerging area of control co-design, which is especially relevant to the relatively immature area of wave energy system design, is also covered