Research On Control Parameter Identification Method And Control Strategy Of Resonant Wave Energy Converter

The exploitation of renewable energy is a major strategy and concerted action to accelerate energy structure transformation and cope with climate change globally.Ocean contains abundant renewable energy,and wave energy,as a high-quality ocean energy resource,has high energy density,large reserves,and is inexhaustible.And many countries are devoted to the exploitation of wave energy.However,wave energy technology has not yet converged and is not mature enough for large-scale commercial application,and the low wave energy conversion efficiency is the key factor hinders the development of wave energy technology.Resonance can significantly improve the capture efficiency of a wave energy converter(WEC).However,ocean waves are mostly irregular and time-varying,and there is a strong fluid-structure interaction during the running of the WEC,which makes it difficult to control a WEC to resonate with waves,thus resulting in inefficient and intermittent power generation of the WEC.With this background,the team of Prof.Yuanqi Cai at Wuhan University proposed an active resonance C-type buoyant flap wave energy converter(abbreviate to C-WEC).C-WEC has the ability to perceive wave information and can actively control its natural period to resonate with incident waves based on the perceived wave information,thus significantly increasing the wave energy conversion efficiency and providing a technical support for the large-scale exploitation of wave energy.In this thesis,the hydrodynamic performance of C-WEC,the identification methods of the parameters to be used to control WEC system to resonate such as radiation hydrodynamic parameters and wave force,control strategy of WECs in irregular waves are studied by numerical simulation and wave flume tests.And the researches and results are as follows:(1)The dynamic equation of C-WEC is established.On the basis of linear assumptions and potential flow theory,the dynamical control equation of C-WEC is obtained by bringing the parameters describing the radiation hydrodynamic characteristics of the WEC into the dynamic equation.And C-WEC’s natural period control model is derived and numerically verified,which provides a theoretical basis for resonance control.(2)Based on the commercial software package FLUENT and user-defined functions(UDF)and the momentum source term making and eliminating wave method and C-WEC dynamic equation,a CFD numerical simulation platform of C-WEC is established,which provides a reliable and powerful research tool for studying C-WEC.(3)The method to identify wave forces acting on C-WEC is proposed,and online identifying wave force can be realized.Based on the online wave force,the in-situ wave information can be perceived by C-WEC,which provides information to judge a wave energy system to be resonance and to control C-WEC to resonate.(4)Based on the relationship between wave force and WEC’s velocity,the wave energy conversion mechanism is studied,and the effects of resonance,nonlinearity and damping on wave energy conversion are revealed,providing a theoretical basis for the design and control of WECs.The research shows that resonance ensures that the wave force does as much positive work as possible on the WEC and increase the input energy;nonlinearity will cause WEC’s velocity to be detuned with the wave force,thus reducing the wave energy input;reducing the damping of the WEC system can increase the work stroke of the wave force and increase the wave energy input.It is also shown that the wave energy conversion laws in regular and irregular waves are the same,and the laws obtained in regular waves are also applicable to in irregular waves.(5)The criterion of online judging C-WEC to be resonance and the strategy of online controlling C-WEC’s natural period are proposed,which provides the theoretical basis for controlling a WEC system to resonate.(6)The method of in-situ testing the WEC’s radiation hydrodynamic parameters is proposed.By this method,the analytical expression of WEC’s radiation hydrodynamic parameters for frequency can be obtained according to one free decay response.So this method can be used to testing WEC’s radiation hydrodynamic parameters in real seas easily to obtain accurate control parameters of controlling the WEC to resonate.(7)The control strategies of C-WEC in irregular waves are proposed.The resonance judgment criterion,the natural period control strategy,and the optimal PTO damping prediction method and PTO control strategy in irregular waves are proposed,which provide the theoretical basis to control C-WEC to resonate in real sea.(8)The wave flume experiments of C-WEC are designed and the experimental studies are carried out.The wave flume experiments confirmed that the wave energy system consisting of the C-WEC prototype is a weakly nonlinear and close to linear,thus proving the correctness of the premise assumptions of the derivation of the C-WEC dynamical control equation.And the flume experiments also verified the proposed hydrodynamic parameters testing method,natural period control model,wave force identification method,resonance judgment criterion and control method.The hydrodynamic performance of the C-WEC prototype is tested,and it is confirmed that the prototype can be controlled to resonate with the target waves and keep efficient wave energy conversion in any regular and irregular wave,thus the intermittency of wave power generation can be greatly reduced.The wave flume experiments show that for the waves with height of 10 cm and periods of 1.62s～ 3.20 s,the prototype’s capture width ratio is 54%～86% in regular waves and 35% ～ 45%in irregular waves.(9)The numerical simulation studies of C-WEC are carried out.The numerical simulation tests confirmed that the nonlinearity of C-WEC system is weak and the system is close to linearity;verified the radiation hydrodynamic parameters testing method,natural period control model,wave force identification method,resonance judgment criterion and control method;validated the control strategy of C-WEC in irregular waves,and also confirmed that the resonance of C-WEC is controllable and C-WEC can achieve efficient wave energy conversion in any wave condition.