System-level Co-design and Control of Large Capacity Wave Energy Converters with Multiple PTOs

Marine wave energy is still less mature than wind, with perceived higher levelized cost of energy (LCOE). Various commercial initiatives have unfortunately failed and there is no convergence of design concept for waves as there is for wind and marine turbines. This is due to various reasons, principally low equipment capacity factor, low conversion efficiency, uncertain survivability and poor power quality. Wave energy converters (WECs) consist of multiple energy conversion stages and components to capture wave energy and convert it to electricity. These components across the conversion stages have interactions and constraints. Optimal operation of each single component does not imply the optimality of the whole system. Most efforts have been made to improve the performance of particular components in each stage. This cannot guarantee low-risk robust optimality of the whole system due to failure to include the effects of the couplings of dynamics and constraints between: (i) the conversion stages in hardware design, (ii) control and (iii) the constraints made by operational requirements. These issues can be tackled by device design, controller design and the integrated design of both device and controller, i.e. co-design. For example, the maximisation of energy capture from waves can result in power spikes in generators and high voltage and current values in power electronic converters, which make the components out of their optimal operational range and even cause damages. Thus this is a muti-objective multi-variable optimal design and control problem in coupled multidisciplinary domains subject to mixed-constraints and dynamics across domains of hydrodynamic, electric generator, power electronics and super-capacitor for energy storage. In this project we develop a systematic control design framework based on wave-to-wire model describing the dynamics for whole energy capture and conversion process of the WEC system to achieve an optimal balance between electricity output maximisation and power smooth. By integrating the proposed W2W optimal control into device design, we can further achieve the system-level co-design of the WEC system to find the lowest LCOE by balancing with the hardware cost, especially the cost from the power-take-off (PTO). Furthermore, we incorporate deterministic sea wave prediction (DSWP) into our controller design to approximate the Falnes non-causal optimality. DSWP can also enable the shut-down mechanism to the control framework to enlarge the safety window for WEC operation and thus further improve the energy output and reliability.

We focus on the multi-float and multi-PTO large capacity WECs because the multiple PTOs can provide extra freedom to maximise the energy output and smooth the power flow, by coordinating the PTOs. This can bring much more challenges in control and co-design compared with the benchmark point absorbers which have been studied extensively as a benchmark problem. In particular, we employ a well-designed multi-float multi-PTO large capacity WEC, M4 as a case study, with the benefits of available tank-validated linear hydrodynamic designs. We then investigate the generality and transferability of the proposed control and co-design approaches using the WECs of our industrial partners.