| Tidal current energy conversion systems (TCECS) are kind of innovative renewable energy systems which utilize the hydrokinetic energy for power generation. In this paper, the hydrodynamics of TCECS and some associated control issues in system's energy transfer process have been reviewed firstly. Then through theoretical analysis, mathematical modeling, numerical prediction, simulation tests, and the real prototype system's bench tests, workshop tests, and offshore validation tests, some key issues about TCECS have been studied, which include the design methods and the hydrodynamic performance of tidal current energy converters, the energy conversion efficiency of each part of TCECS, the maximum power tracking and variable-speed constant-frequency control of tidal system, the novel variable pitch device suitable for tidal current turbines and the associated power control, etc..The main contents of each chapter are as follows:In chapter 1, the features, distribution, and exploitation meanings of the tidal current energy are summarized firstly. Then the features and research status of the TCECS are reviewed by category, and the challenges of the tidal current energy conversion technologies are pointed out. The research development of the hydrodynamics, the maximum power tracking control, and the power stabilization control of the TCECS are summed up next. Finally, the supports, research significance, and main contents of this work are addressed.In chapter 2, the energy conversion principle of the horizontal axis TCECS are analyzed. First, based on the one-dimensional momentum theory and the blade element theory, the energy capture principle of horizontal axis tidal current turbines is analyzed from the point of view of energy change and the point of view of blade force. Then the calculation method of the load of the horizontal axis tidal current turbine is illustrated by the blade element momentum (BEM) theory introduced subsequently. In addition, the principle of the secondary energy conversion of the TCECS is presented from the aspects of the mechanical transmission and hydraulic transmission respectively. At the end, the control principles of the maximum power tracking operation and the power stabilization for TCECS are illuminated.In chapter 3, a horizontal axis tidal current energy converter is designed using the optimal blade design method for variable-speed operation based on BEM theory firstly. A hydrodynamic performance prediction numerical model including necessary modifications is then established, with which the turbine performance for various blade pitch angles is predicted. Subsequently, an electromechanical subsystem is designed and presented for a 25 kW TCECS prototype, and related numerical simulations, bench tests, and offshore tests are carried out in turn for the prototype. The results of these tests show that the power characteristic of the designed tidal current turbine, the energy conversion efficiency of the electromechanical subsystem, and the operation performance of the whole prototype system are basically satisfactory. This has illustrated that the design methods of the tidal current energy converter and the subsequent electromechanical subsystem are reasonable and valid, and the established hydrodynamic performance prediction model is correct and practical. Furthermore, the reliability and robustness of the whole tidal current energy conversion prototype are also validated.In chapter 4, the maximum power tracking control of the TCECS with mechanical transmission is studied firstly. A power control scheme based on the load regulation method is proposed. The system's mathematical model is established. Following that, simulation comparison and static power tracking tests are carried out to validate the proposed regulation method and implementation scheme. At the second part of this chapter, some exploratory research on the TCECS with hydraulic transmission is carried out. A scheme for a hydraulic TCECS with hydraulic pump, motor, and accumulator is proposed and then validated preliminary through the co-simulation with Simulink and Amesim, and the effects of the main hydraulic components are assessed as well. At the third part of this chapter, the variable-speed constant-frequency control of the hydraulic TCECS is studied. The mathematical modeling and theoretical analysis are performed for the whole system, and a regulation and control principle for the variable speed, constant frequency and pressure operation of the system is proposed, which is based on the adjustment of the pump displacement, the load size, and the motor displacement. Co-simulation is also carried out for validation. The results with comparison analysis have validated the correctness of the proposed control principle, and some corresponding system control strategies are discussed as well at the end.In chapter 5, a hydraulic TCECS prototype is designed and tested. The design of the hydraulic drive subsystem, mechanical subsystem, and electrical control subsystem of the prototype are described in turn. The energy conversion efficiency of the system is estimated. Then a running-in tests is performed for the prototype in a motor driving test bed, through which the operation performance characteristic of the prototype is obtained and the system's reliability and stability are checked. Thus, the feasibility of the application of the hydraulic transmission to the tidal current energy systems has been truly verified.In chapter 6, the variable pitch device suitable for TCECS and some related control issues are studied. Through the detail blade force analysis based on the theoretical mechanics and BEM theory, a pitching load calculation method is proposed firstly. Then according to the characteristics of the pitching motion of horizontal axis tidal current turbines, a novel variable pitch control system is designed, which consists of a pitching actuating mechanism with a fore hydraulic cylinder and a pinion-and-rack, and an associated electro-hydraulic proportional control subsystem. Subsequently, the mathematical modeling, simulation, and workshop tests of the pitch control system are performed. The Simulation and test results show that the designed pitching system has fast response and precise control of pitch angle, and also it is able to realize the bidirectional running of the TCECS with a large pitch adjustment range.In chapter 7, the major work of the study is summarized, and the conclusions and innovations are elaborated. Besides, the future work is also expected at the end.
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