| In order to improve the efficiency and survivability of the wave energy converter(WEC),a new type of pendulum WEC is designed in this paper,and it generates electricity through pitch movement driven by waves.A new mechanism for adjusting the resonance between the WEC and waves is proposed,that is,changing the height of the counterweight block inside the pendulum to adjust the inherent period of the device to be consistent with the wave period.Consequently,efficient energy capture in a wide range of sea conditions can be achieved.The WEC can be installed with the fixed offshore wind turbine to jointly generate power,or installed in the fixed offshore working platform to persistently supply electricity.The key mechanical and electrical equipment is sheltered inside the hull to avoid contact with seawater,which has strong adaptability to the sea states and high survivability.This paper devises the overall structure of the resonant pendulum WEC and establishes the basic model of the energy capture device and the PTO(Power Take-off)system.Further,the three fundamental problems,which are hydrodynamic analysis,stable operation and efficient power generation,are focused.The nonlinear characteristics and modeling of hydrodynamic force,the hydraulic energy conversion and control on stability,and the optimal wave energy conversion control considering efficient power generation and safety constraints are studied.This paper can provide theoretical guidance for the engineering application of resonant pendulum WEC in the future and promote the commercial development of wave power generation.The specific research contents are as follows:1)In order to comprehensively investigate the conversion process from wave energy to electric energy and provide a model basis for subsequent research,firstly,the shape of pendulum is proposed,which can increase wave excitation and reduce radiation energy dissipation.The hydrodynamic models in frequency domain and time domain of the pendulum are established based on the Boundary Element Method(BEM).The analytic expressions of the optimal phase condition,the optimal amplitude condition and the natural frequencies are derived from the frequency domain model,which are used to guide the maximization extraction of the wave energy.The time domain model takes into account the different types of irregular wave and the "memory effect" of the radiation torque,and the Realization method is adopted to convert the "memory effect" to state space model which is convenient for the subsequent control model.Next,the structures of mechanical PTO and hydraulic PTO are respectively designed,and the nonlinear models of hydraulic cylinder,check valve,accumulator,hydraulic motor and permanent magnet synchronous generator are established.Then,the complete waveelectricity nonlinear state space model of structural transmission and energy conversion is constructed.2)For further research on nonlinear effect of the actual flow for the resonance pendulum WEC and improve the accuracy and speed of the nonlinear hydrodynamic calculation,the modified potential flow model with nonlinear hydrodynamics is put forward.The main framework is to describe the linear-characteristics-dominating hydrodynamics based on BEM,and then introduce correction terms of the nonlinear-characteristics-dominating hydrodynamics,and the model coefficients are identified by fully nonlinear numerical simulation.The static Froude-Krylov(FK)moment and fluid viscosity moment are identified as the main nonlinear hydrodynamics factors of the resonant pendulum WEC.For the former,a model considering the nonlinear variations of gravity and buoyancy arm lengths is established.For the latter,based on the Morison viscous model,an improved form considering the additional viscous added moment of inertia is proposed in order to increase the accuracy of calculating the motion period.Subsequently,the CFD-based numerical wave tank is established to simulate the real fluid characteristics,and the free-decay tests are carried out.Through the comparison between different models,the nonlinear effects of the static FK moment and fluid viscosity torque are analyzed.The distribution of the fluid viscosity moment coefficients are identified in the wide range of operating conditions.Furthermore,the influences of the nonlinear hydrodynamics,in particularly the uncertainty of the fluid viscous moment coefficients,on the evaluation of wave energy extraction are explored.Finally,a prototype model is built to verify the rationality of the nonlinear hydrodynamic model.3)To improve the smoothness of the traditional hydraulic PTO in the irregular waves and guarantee the quality of output power of WEC,a novel scheme of stable operation is put forward and a power regulation module is proposed.The power regulation module can maintain the flow rate of the throttle valve when the inlet pressure or outlet pressure changes through the feedback mechanism of pressure compensation valve.As a consequence,the fluctuation of generator speed and output power is attenuated.The structural dynamics model of the power regulation module is derived,and then the above module is introduced into the traditional hydraulic PTO and the complete wave-electricity state space model is established.Then,the effect of the power regulation module is verified under different significant wave heights,peak periods,wave spectrum types and resistance load.Next,the influences of key parameters,including the stiffness and preset elastic force of the pressure compensation valve’s spring,the initial opening of the pressure compensation valve’s spool and the opening of the throttle valve on the control performance are investigated.On this basis,taking the opening of the throttle valve which is easy to operate as the regulating object,the real-time power control is implemented by utilizing the anti-windup PID controller.Therefore,the balance between the supply and demand of power can be achieved when the resistance load and the power demand change.Furthermore,a hydraulic PTO experimental platform is built,and the system responses when the hydraulic cylinder pistons move and the flow rate-pressure difference characteristics of the power regulation module are tested,validating the correctness of the numerical model.4)In order to enhance the power output of the resonant pendulum WEC in actual sea conditions and reduce the adjustment burden of the PTO system,and meanwhile ensure the system operates within a safe range,the optimal wave energy conversion control is proposed.It includes adjusting the counterweight block in the energy capture device to achieve resonance,and utilizing model predictive control(MPC)in PTO system to calculate the optimal PTO torque numerically and handling state constraints.The augmented state space model and the future multi-step output prediction model of the device are derived.Aiming at maximizing the generation power and guaranteeing the safe operation of the device in the prediction horizon,the compound objective function and constraint matrix are constructed,and the Hildreth algorithm with superior computational stability is employed to conduct the numerical optimization.Then,under the regular wave conditions,the optimal wave energy conversion control is compared with the traditional control of maximum power extraction(including the complex-conjugate control and damping control).The resonance condition,average output power,the reactive power,energy flow,PTO torque amplitude are investigated with emphasis,and the advantages of optimal wave energy conversion control are demonstrated,and the correctness of the model is verified.Furthermore,the optimal wave energy conversion control is applied to a variety of irregular sea conditions to improve the output power,and its effectiveness is verified by comparing with the effect of the optimal fixed PTO damping control.Finally,the effects of the optimal wave energy conversion control to constrain the PTO torque,the change rate of PTO torque and the pendulum angle are discussed.By additionally adding the pendulum-angle penalty term into traditional objective function,the stability of optimization calculation in extreme wave conditions is improved. |
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