Research On The Power Capture Ability Of The Two-raft-type Wave Energy Converter And Its Hydraulic Power Take-off System

The growing scarcity of traditional energy is driving the implementation of renewable energy on an increasingly large scale.In addition,using the traditional energy to produce electricity would introduce green house effect,smog and other environmental pollution issues.Utilizing wave energy as the driving force of power take-off(PTO)system to output electricity gives a new way to reduce consumption of traditional energy.Among the wide variety of wave energy conversion devices proposed so far,raft-type wave energy converters(WECs)are proven to have a good survivability in the aggressive sea climate and a relatively high wave energy conversion efficiency due to its floaters aligned to the wave propagation direction and its PTO system reacts against its own body rather than against a separate external reaction frame,which make its operating mode is to “ride” the incident wave.Therefore,wave energy conversion and utilization based on raft-type WECs is fundamentally attractive.In this paper,analytical derivation,physical model validation and numerical simulation method are used to study the power capture ability of a two-raft-type WEC and the performance of its hydraulic PTO system.Firstly,to solve the problem that the previous time-domain analysis model based on Cummins equation with a convolution term is not suited for numerical simulation and controller design,and the problem that identification times is too much when using state-space model to replace the convolution term,a state-space time domain model with few identification times is proposed for the study of the hydraulic power take-off(PTO)system and the control strategies.The details of how to reformulate the convolution term into a state-space model and the realization of such a state-space model are reported.Based on the proposed model,the power capture ability of the two-raft-type WEC with typical PTO systems is investigated.Result show that the peak power captured by the coulomb PTO system is larger than than captured by the linear one in most regular wave periods.The previous research on the raft-type WECs treat the hydraulic PTO system as a linear PTO system and do not consider the dynamics of the hydraulic circuit and components.This paper proposes a combined hydrodynamic and hydraulic PTO system model with the consideration of the dynamics of the hydraulic PTO system to investigate the performance of the two-raft-type WEC and the effect of hydraulic PTO parameters on the power capture ability.The findings show that an approximately square wave type hydraulic PTO force is produced by the hydraulic PTO system,which causes the performance of the two-raft-type WEC not to be sinusoidal and the energy capturing manner different from that of the device using a linear PTO system.Results also show that the peak capture width ratio of the device under each optimal hydraulic PTO system parameters present a little correlation to the wave height,but mainly depends on the wave period.In order to study the effect of undesirable physical effects of the hydraulic PTO system on the power capture ability and conversion ability of the hydraulic PTO systm,a hydraulic PTO system model with the account of the undesirable physical effects such as friction,leakage,pressure drops and characteristics of hydraulic fluid is presented.Simulation results show that the undesirable physical effects have a little effect on the power capture ability,while make a large difference to conversion ability.Results also show that the power loss increases as the wave amplitude increases or the ratio of raft length to wavelength approaches to the optimal value,and the main loss comes from the hydraulic motor.In order to improve the capture ability of the device,the paper also pays attention to the latching control.To address the problem that the determination of latching duration is difficult especially when the device is a multi-DOF system and consider its hydraulic PTO system,the optimal control method is adopted to decide the control law between latching and unlatching,which avoids computing the latching duration directly.The numerical control algorithm is presented and simulations are performed to evaluate how large the benefit that latching control can introduce.Results also show that latching control can improve the power capture of device in moderate and large wave period while has negative effect in small wave period.To solve the negative effect introduced by latching control,an individual control that four switch valves are employed such that each chamber of the hydraulic cylinders can be independently connected to the high-pressure(HP)or low-pressure(LP)gas accumulators by switching the working position of each switching valveis proposed.The switching sequence of each valve is determined based on optimal control theory with the aim to maximize the power capture.Results show that the individual control has a good complementation with the latching control.At last,a scaled hardware-in-loop test rig is establised to validate the above simulation results.Results show that the above simulation results are correct and the control strategies can improve the power capture ability of the device.