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

海洋波浪能仍然不如风能成熟，能源成本(LCOE)被认为是更高的。不幸的是，各种商业举措都失败了，波浪的设计理念没有像风力涡轮机和船用涡轮机那样趋同。究其原因是多方面的，主要是设备容量系数低、转换效率低、生存能力不确定、电能质量差。波能转换器(WEC)由多个能量转换级和部件组成，用于捕获波能并将其转换为电能。这些跨转换阶段的组件具有交互作用和约束。每一个组件的最佳运行并不意味着整个系统的最优。在每个阶段都作出了最大努力来改进特定部件的性能。这不能保证整个系统的低风险稳健最优性，因为未能包括动态和约束之间的耦合的影响：(I)硬件设计中的转换阶段，(Ii)控制和(Iii)由操作要求造成的约束。这些问题可以通过设备设计、控制器设计以及设备和控制器的集成设计来解决，即协同设计。例如，最大限度地从波浪中捕获能量会导致发电机的功率尖峰和电力电子变流器的高电压和高电流值，使组件超出其最佳运行范围，甚至造成损坏。因此，这是一个跨水力、发电机、电力电子和超级储能电容器等多学科领域的混合约束和动态的多学科耦合领域的多目标多变量优化设计与控制问题。在这个项目中，我们开发了一个基于波线模型的系统控制设计框架，描述了WEC系统整个能量捕获和转换过程的动力学，以实现电力输出最大化和功率平滑之间的最佳平衡。通过将所提出的W2W最优控制集成到器件设计中，可以进一步实现WEC系统的系统级协同设计，通过权衡硬件成本，特别是功率输出(PTO)成本，找到最低LCOE。此外，我们将确定性海浪预报(DSWP)融入到控制器设计中，以逼近Falnes非因果最优性。DSWP还可以使控制框架中的关闭机制能够扩大WEC运行的安全窗口，从而进一步提高WEC的能量输出和可靠性。由于多个PTO可以提供额外的自由度，通过协调PTO来最大化能量输出和平滑功率流，因此本文重点研究多浮子和多PTO大容量WEC。与作为基准问题被广泛研究的基准点吸振器相比，这在控制和协同设计方面带来了更多的挑战。特别是，我们采用了精心设计的多浮子多PTO大容量WEC，M4作为案例研究，具有可用水箱验证的线性流体动力设计的好处。然后，我们使用我们的工业合作伙伴的WEC来调查所建议的控制和共同设计方法的通用性和可转移性。

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.