Study On Hydrodynamic Characteristics Of The Raft-type Wave Powered Desalination Device

The growing scarcity of freshwater is driving the implementation of desalination on an increasingly large scale.Using wave power as the driving force of reverse osmosis(RO)system gives a new way to reduce desalination plants costs by bridging the fields of renewable energy and desalination.Among the wide variety of devices proposed thus far,raft-type wave energy converters(WECs)are proven to have a high wave energy conversion efficiency.Therefore,the wave powered RO system based on raft-type WECs is fundamentally attractive.In this paper,both analytical analysis and numerical method are used to study the hydrodynamic characteristics of a raft-type wave powered desalination device,in which the desalination system is assumed to be a Power Take-Off(PTO)system of raft-type WECs first and then to be a RO convection-diffusion model for a detail study on real desalination process.Firstly,an analytical model for the analysis of hydrodynamic characteristics of two-dimensional raft-type WECs is proposed based on linearized velocity potential theory.The unknown coefficients of velocity potentials are determined by utilizing eigen-function expansion matching method.The analytical model is utilized to examine the effect of PTO damping coefficient,raft draught,spacing between adjacent rafts,raft numbers,raft length and raft length ratio on power absorption efficiency and wave transmission coefficient.The results show that a raft-type WEC consisting of two rafts with differenct length is capable to capture more power from waves compared to that with two same rafts.The paper also presents a dynamic analysis of a raft-type WEC,which consists of two hinged cylindrical rafts of elliptical cross section,based on the three-dimensional wave radiation-diffraction theory and boundary element method(BEM).The effect of raft length,linear damping and spring coefficient in the PTO system,axis ratio and raft radius of gyration on wave energy capture factor has been investigated in frequency domain.While the effects of a nonlinear Coulomb damping,raft radius of gyration and latching control have been studied in time domain.The difference in the performance of a raft-type WEC obtained using a linear damping and a Coulomb damping is also illustrated.It is revealed that with the consideration of the mass non-uniform distribution along rafts,the phase lags between pitch excitation moments and pitch velocities can be reduced,leading to a much larger power capture factor.The maximum mean power that can be absorbed by the raft-type WEC cannot be obtained accurately or rapidly by using numerical model.In addition,in practice applications,most WECs have physical liminations placed upon their excursions due to restraints such as mooring lines or pump stroke.To evaluate the maximum mean power that can be captured by the raft-type WEC under motion constraints,two mathematical models are presented by directly calculating time-averaged power absorption of PTO system and considering power absorption as the difference between excitation and radiated power without consideration of PTO system,respectively: the former one is mainly used for two interconnected rafts with relative pitch motion constraint;the other one can be applied for multiple connected floating rafts under a weighted global constraint.Two novel raft-type WECs,one consisting of spring-damping-mass oscillator system inside the rafts,the other one with a water tank inside each raft,are proposed.Effects of damping of PTO system,mass,damping and stiffness of the oscillator system,the size and position of the tanks on dynamics of the two novel raft-type WECs mainly in terms of wave energy absorption are all investigated by using numerical models.For wave powered RO device,the pressure required for RO comes from the ocean waves,leading to a non-steady state process with the feed pressure and flow rates varying with time.Therefore,a transient model accounting for the time variation of desalinating process is required to simulate the behavior of wave powered desalination device.In the end of this paper,a numerical model based on BEM and finite volume discretization is proposed to solve for the hydrodynamics of raft-type WEC and the concentration variation in the RO polarization layer.The model considers the interaction between raft-type WEC,high pressure accumulator and RO module,and accounts for the time variation of permeate flux and concentration.Results indicate that there is a proper RO width and a proper wave period to maximize averaged permeate water flux and to minimize the averaged permeate concentration as well.