Abstract：With low loss and flexible control, a direct-drive wave energy converter (WEC) based on oscillating body structure has become a research hotspot in wave energy. Through the mooring system, two-body direct-drive WEC, independent of off-shore fixed platforms, is a promising solution to wave energy utilization. However, the two-body WEC utilizes relative motion between upper and lower float to generate electricity, which makes modelling and optimal energy extraction control more difficult. A two-body WEC can be regarded as two power sources with different voltages and resistance paralleled to output electricity through the equivalent method. Furthermore, this two-paralleled power source can be equivalent to a single source model, similar to a single-body WEC. Therefore, modelling and energy extraction characteristics of two-body WEC are simplified. The stiffness of mooring has a great effect on maximum wave energy extraction. The results show that the mooring should be as stiff as possible to avoid energy extraction loss. In this case, the two-body WEC can be equivalent to a single-body WEC: the upper float harnesses wave energy, and the lower float along with the mooring system acts as a fixed base. Moreover, the design of a two-body WEC with a pretension mooring system is put forward. Model predictive control (MPC) is a state-of-the-art method for optimal energy extraction in wave energy. Based on the equivalent model, the MPC problem for maximizing wave energy extraction considering generator force and float displacement constraints is formulated. For wave excitation force prediction, the Kalman filter is used to achieve excitation force identification, and the autoregressive model is used for excitation force prediction. Considering the computation complexity of the MPC solution, a fast solution algorithm is put forward, and the previous step is set as the initial iteration of the current step. Thus, only one iteration is required. Compared with the conventional MPC that requires 3~5 interactions, the proposed fast solution algorithm can save up to 80% computing time. A two-body direct-drive WEC prototype with an upper float of 1.5 m diameter is designed for experimental test in the wave tank lab with a dimension of 100 m×18 m×5 m. This lab can generate waves with periods from 2~3 m and 0.25 m maximum wave height. Different power control methods include resistive control, damping control, latching control, conventional MPC, and proposed MPC. The results show that MPC can extract 70% more energy than traditional methods. The proposed MPC with the fast solution algorithm has the same energy extraction efficiency, and only 50% CPU operation time is required. The following conclusions can be drawn. (1) Pretension mooring has no adverse effect on energy extraction, and two-body WEC can be equivalent to one-body WEC through the equivalent circuit method, which will simplify control modelling significantly. (2) Compared with conventional methods, MPC has more energy extraction efficiency by injecting reactive power to increase the oscillation amplitude while limiting it within preset constraints. (3) The experimental results indicate that the proposed MPC is stable, considering modelling errors in the experiment. Besides, excitation force prediction is achieved by software algorithms, and the proposed MPC method only requires 50% computation time. The proposed method is promising for practical implementation without additional sensors and a high-performance control system.
黄宣睿, 林泽川, 肖曦. 双浮体直驱波浪发电装置建模分析与基于模型预测控制的能量提取算法研究[J]. 电工技术学报, 2024, 39(2): 445-454.
Huang Xuanrui, Lin Zechuan, Xiao Xi. Modelling and Analysis of the Two-Body Direct-Drive Wave Energy Converter and Optimal Energy Extraction Method Based on Model Predictive Control. Transactions of China Electrotechnical Society, 2024, 39(2): 445-454.
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2024, Vol. 39 
