Development of a Desalination System Driven by

Development of a Desalination System Driven by Low Energy Ocean Surface Waves Author(s): Manuel Gerardo Verduzco-Zapata, Aramis Olivos-Ortiz, Marco Liñán-Cabello, Christian Ortega-Ortiz, Marco Galicia-Pérez, Chris Matthews, Omar Cervantes-Rosas Source: Journal of Coastal Research, 85(sp1):1321-1325. Published By: Coastal Education and Research Foundation https://doi.org/10.2112/SI85-265.1 URL: http://www.bioone.org/doi/full/10.2112/SI85-265.1

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Journal of Coastal Research

SI

85

1321-1325

Coconut Creek, Florida

2018

Development of a Desalination System Driven by Low Energy Ocean Surface Waves Manuel Gerardo Verduzco-Zapata†*, Aramis Olivos-Ortiz††, Marco Liñán-Cabello†, Christian OrtegaOrtiz†, Marco Galicia-Pérez††, Chris Matthews‡, and Omar Cervantes-Rosas†. †

Facultad de Ciencias Marinas Universidad de Colima Manzanillo, Colima, México.

††

Centro Universitario de Investigaciones Oceanológicas Universidad de Colima Manzanillo, Colima, México. 19.5 Colonia El Naranjo. C.P. 28868, Manzanillo, Colima, México.

‡ SAROS Department EcoH2O Innovation, Charlotte, United States of America.

www.cerf-jcr.org

ABSTRACT

Verduzco-Zapata, M.G.; Olivos-Ortiz, A.; .; Liñán-Cabello, M.; Ortega-Ortiz, C.; Galicia-Pérez, M.; Matthews, C., and Cervantes-Rosas, O., 2018. Development of a Desalination System Driven by Low Energy Ocean Surface In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1321-1325. Coconut Creek (Florida), ISSN 0749-0208. www.JCRonline.org

As an effort to mitigate the water scarcity and to achieve a water security status in remote coastal communities with few or no hydraulic systems, or in places with compromised services due to natural disasters, a new semiportable wave driven desalination device is being developed and tested using a state of the art numerical model. In this early stage of development, the main challenge is to further optimize mechanisms that allow to adjust to tidal and wave variations and to efficiently resist the stresses exerted by cycle motions. The solution is partially handled through the use of a taut-line mooring mechanism which allows the device to work at large range of wave conditions. This prototype consists of two subsystems: a point absorber WEC conformed by a single buoy, and a standard reverse osmosis (RO) system. When interacting with the incoming waves, the mooring line is pulled and pressurizes seawater enough to drive it through several filters including the RO membranes. The FLOW-3D numerical model was used to test the efficiency of the WEC subsystem under several sea states associated with low energy wave conditions. The displacements and motions of the WEC as well as the forces in its anchor line were calculated. The results suggest that it provides the necessary force to pressurize the seawater for the desalination process. Further testing is needed to improve the reliability and survivability of the system which in turn will help to scale the prototype in order to obtain greater quantities of freshwater and thus be competitive with other technologies. ADDITIONAL INDEX WORDS: Water scarcity; wave-powered desalination system; FLOW-3D model.

INTRODUCTION Water scarcity is one of the biggest challenges around the world. It is originated mainly by the growing overpopulation and hence the overexploitation of the natural resource. Moreover, due to industrial and housing activities, there is an increasing pollution rate of the available water reserves which, coupled with extreme poverty and lack of hygiene, could affect the most vulnerable population due to the proliferation of infectious diseases. In coastal regions, nowadays is more often to turn to the construction and start-up of desalination plants to get from the sea a source of fresh water. There are several technologies for desalination, being the reverse osmosis (RO) one of the most preferred methods due to the continuously price dropping of the membranes employed in the process (Karagiannis and Soldatos, 2008). However, these plants requires a large amount of energy which frequently comes from the combustion of fossil fuels. As an alternative, renewable energies as wind, solar and ocean ____________________ DOI: 10.2112/SI85-265.1 received 30 November 2017; accepted in revision 10 February 2018. *Corresponding author: [email protected] © Coastal Education and Research Foundation, Inc. 2018

surface waves may be employed to provide the require amount of energy necessary in the desalination process. In this regard, the use of ocean surface waves is tempting as the energy and the water sources are readibly available from the sea. The main purpose of this paper is to show the initial stage of development of a point absorber wave energy converter (WEC) which consists of a hydraulic Power Take-Off (PTO) and a floating device which uses the vertical motion of each wave to produce work. This WEC is intended to be deployed in near and/or off-shore conditions. In the following section a brief background is presented to give some context. BACKGROUND The efficiency of point absorbers to pressurize sea water depends mostly on the dimensions of the buoys, the mooring design and the sea state of the deployment site. The DELBUOY (Hicks et al., 1989) was one of the first that used the pressurized water to desalinate it using Reverse Osmosis (RO) membranes. The Mc-Cabe Wave Pump (Brooke, 2003) consisted on several pontoons that moved relative to each other and extracted the wave energy from the rotation about hinge points by linear hydraulic pumps. Another example is the CETO (Karimirad, 2014), which consists on a fully-submerged buoy that captures

1322 Verduzco-Zapata et al

_________________________________________________________________________________________________ the wave energy and transfers it to a pump which delivers highpressure water onshore for energy conversion. One of the most recent WEC is the HiWave buoy, developed by KIC InnoEnergy, which is a highly advanced device that implements a phase control which allows the natural frequency of the WEC to fall near the frequency of the incoming waves hence improving its efficiency (Budal et al., 1982; Clément and Babarit, 2012). Verduzco-Zapata et al. (2017) studied several aspect ratio point absorbers interacting with regular waves to evaluate the buoys hydrodynamics. The results showed that a buoy size of around 2 m of diameter and 0.35 m height is suitable to give the enough force required for pressurizing the sea water. Although there has been several researchs in this topic, this kind of technology is still under development and more research is needed to design efficient and cost-effective prototypes for sea water desalination. The aim of this paper is to describe the first stages of development of a point-absorber WEC coupled to a desalination system, to directly pump water using only the wave motion associated to low energy sea states (e.g. Mexican Pacific). The design is based on the results obtained by Verduzco-Zapata et al. (2017), with an improve of the numerical representation of the Power Take Off (PTO) system. In the next section a description of the WEC and the numerical tests setup are presented. The Results and Discussion sections, details and discusses the most relevant findings, respectively, while some concluding remarks are presented in the Conclusions section. METHODS In this section a brief description of the WEC and the numerical model setup are presented. WEC description The WEC consists of a small cilindrical buoy of 500 kgm-3 of density with a diameter of 2m and a height of 0.35m, tethered with a mooring line with 9m and 0.025m of length and diameter, respectively, with a linear density of 0.39 kg/m. This line is connected to the hydraulic PTO system, which has chambers were the sea water is pressurized with the rise of a piston due to the wave motion. This piston is then placed back to its rest position using a spring mechanism with a block length of 0.5 m, hence it cannot be compressed anyfurther when this limit is reached. The PTO is fixed at the bottom of the flume, but it can rotate in any direction. The RO system is comprised of standard available components, enhanced by an energy recovery system used to reduce the total amount of energy required in the process. The overall configuration of the WEC-RO system is presented in Figure 1, detached just after the surf zone. The force required to obtain the enough pressure for the RO process is estimated as 1.65 kN.

Figure 1. Diagram of the overall WEC-desalination system (courtesy of EcoH2OInnovation).

FLOW-3D governing equations The FLOW-3D model (F3D) version 11.2.2.01 is a RANS model that solves the continuity equation (Eq. 1) and the NavierStokes equations (Eqs. 2-4) by finite difference schemes. It uses a staggered grid with non-uniform rectangular cells sizes in a Cartesian coordinate system. The free surface is solved with the Volume of Fluid technique (VOF) (Hirt and Nichols, 1981) (Eq. 5), which gives the evolution of the water surface through the domain: u Ax x

+

v Ay y

+

w Az z

= Sp

1  u u u  1  p  + vAy + wAz −  uAx =  + Gx + fx − bx + Su   x  V  x y z  F v 1  v v v  1  p  + + vAy + wAz −  uAx =  + Gy + fy − by + Sv  t V  x y z   y  F w 1  w w w  1  p  + + vAy + wAz −  uAx =  + Gz + fz − bz + Sw   z  t V  x y z  F  F 1  + S (FAx u ) + y FAy v + z (FAzw ) = F t V  x  F

u t

+

(

)

(1 ) ( 2) (3) ( 4) (5)

where u is the velocity vector; A and VF are the fractional area and volume open to flow, respectively; F is the water filled volume within a cell;  is the water density; p is the pressure; b and G are the flow losses in porous media and the gravity, respectively; f is a term for the viscous accelerations and Sp and S are energy inputs functions active only at the inlet boundary and/or near obstacles. The effects of the turbulence was neglected as only the overall responds of the WEC is of interest to evaluate its capability to achieve the required force for pressurizing the sea water. In order to define the WEC geometry, the Fractional Area/Volume Obstacle Representation (also known as FAVOR method) (Hirt and Sicilian, 1985) was used to partial or totally block specific cells. This method coupled with the General Moving Objects (GMO) model (Wei, 2005) allows to calculate the coupled wave-WEC movement in its six degrees of freedom considering the effects of hydraulic and gravitational forces. At each time step, the locations and orientations of the WEC are tracked and the area and volume fractions are updated by resolving the equations of rigid body motion. Mooring systems can be represented using springs, ropes and mooring lines elements. The main assumptions of the springs and ropes are that they are weightless and are always straight in shape when they are in tension, which is presumed to be constant along the element. On the other hand, the mooring line model (MLM) (Wei, 2015) uses a finite segment approach

Journal of Coastal Research, Special Issue No. 85, 2018

1323 Development of a Desalination System Driven by Low Energy Ocean Surface Waves

_________________________________________________________________________________________________ (Figure 2a) to calculate its dynamics, which may be influenced by the surrounding fluid, gravity forces and tethered moving objects (see Eq. 6): mp

dv p dt

= G + B +T + D

(6 )

; mp and v̅ p are the mass of the segment and the mass point ̅ B, ̅ T ̅ and D ̅ represent the velocity, respectively; and G, gravitational, buoyancy, tension and drag forces (in normal and tangential directions of the segment), respectively (Figure 2b). For initial condition, the mooring line is assumed to be in static equilibrium. At each time step Eq. 6 is solved explicity for each mass point velocitites using a reduced time step to integrate the equation to ensure numerical stability. Location of mp is calculated by integrating v̅ p over the reduced time steps. The instantaneous line shape is determined when the locations of all mass points at the end of the integration are updated. The dynamic coupling between the mooring lines and moving objects like buoys, is achieved by data exchange between the GMO and MLM solutions. The GMO gives the coordinates of the mooring line ends, and in return the MLM exerts tension forces at those points affecting the movement of the thethered object.

a)

b)

Numerical experiments setup The numerical wave flume (Figure 3) had a constant width and height (16 and 13 meters, respectively) and it was divided in three mesh blocks. The Block I was 155 m long, and it had a uniform mesh with a cell size of 0.5 m with 257,920 (310 x 32 x 26) cells. Block II was 145 m long, and it was used as a damping zone, with a cell size of 1 m with 30,160 (145 x 16 x 13) cells. Block III was a nested grid needed to have enough resolution to resolve the movement of the floating WEC (Figure 4). It was 16 m long, with a width and height of 8 and 12 m, respectively, with a cell size of 0.06 m with 7,102,200 (267 x 133 x 200) cells. The anchor and PTO system were represented by a mooring line (with a spring coefficient of 2.16x106 Nm-1) and a massless spring element (with a spring coefficient of 4,400 Nm-1 and a block length of 0.5 m), respectively, joined by a six DOF connection element of radius 0.25 m and a density equal to the density of the surrounding fluid (1000kgm-3). The initial still water elevation was set at z=0. Second order Stokes waves, associated with low energy sea states, were applied at the inlet of Block I (with a wave height of 0.70, 1.06 and 1.41 m and a period of 10 seconds, for Tests 1, 2 and 3, respectively), while at the outlet boundary of block II a Sommerfeld radiation condition plus a numerical sponge with a length of 145 m were defined to treat the open boundary. At the flume bottom and its sides, as well as at solid boundaries, noslip and impenetrability conditions were applied. Along the channel four wave sensors were placed to detect the water surface elevation, and one more attached to the WEC to measure its displacement and velocity approximately every 0.01 s. Sensor 1 and 2 were located at 25 and 15 m upstream of the WEC, the third was located at the initial position of the WEC while the fourth was 15 m downstream from the buoy. Sensor 5 was attached to the buoy’s mass centre GM. The time step (Δt) was automatically set so the Courant stability condition was always achieved during the experiment. The tests were carried on a server with 1 processor Intel Xeon E5-2690 v4 @2.6 GHz with 14 cores, 64 GB of RAM and 1 TB of solid hard drive.

Figure 3. Numerical wave tank conformed by three mesh blocks. Block II is used as a dissipation zone. Figure 2. Description of the mooring line method: (a) discrete representation of the segments; (b) forces applied in each mass point (adapted from Wei, 2015, pp. 2-3).


1324 Verduzco-Zapata et al

_________________________________________________________________________________________________

Figure 4. Block III with a grid resolution of 0.06 m needed for resolving the movement of the floating WEC and the small connection element between the anchor line and the PTO.

RESULTS In order to properly compare the responses of the buoy, its vertical displacements were normalized with the incoming wave amplitudes (0.35, 0.53 and 0.705 m for Tests 1, 2 and 3, respectively). The normalized displacement (*) is shown in Figure 5. It can be seen that the vertical response of the buoy to the waves is similar in all tests. The total displacement of the buoy pulled the anchor line and in turn it forced the PTO to rotate and/or to compress or elongate. These changes in the PTO length produced tension and compression forces (Figure 6). When these forces exceed the estimated required force of 1.65 kN, then they pressurize the sea water inside the chambers of the PTO.

Figure 5. Vertical displacement of the buoy normalized with the wave amplitude: 0.35, 0.53 and 0.705 m for Tests 1, 2 and 3, respectively.

Figure 6. Tension and compression forces calculated on the PTO.

DISCUSSION As expected, the vertical movement and therefore the total displacement of the buoy increased when the wave height increased. As a result, the highest force achieved in the PTO was found in Test 3. Although in all three cases the required force of 1.65 kN was exceeded, in Test 1 the WEC exhibited a low performance as the measured force barely went above the required limit. These results suggest that the configuration of the WEC prototype is satisfactory for pressurizing the sea water for desalination purposes when the wave height is at least 1 m. From Figure 6 it can be seen that only the tension force can be used for this purpose, as the compression force is too small for achieving the required pressure, then only a simple effect PTO most be employed. As a double effect PTO is a desirable characteristic of the WEC for improving its pumping capability, the design could be modified for allowing the PTO to compress even further to achieve greater pressures. A future work most include different anchor line and PTO lengths to give the system time to react downwards after every wave crest, and then evaluate if the compression force is enough for a double effect pump. It is also important to have physical measurements for a) validation of the simulations, and b) evaluation of the contribution of the damping forces in the PTO performance. These forces may be an important factor depending on the friction of the piston with the inner walls of the PTO, and in the effect of the water inside the chambers, therefore they must be included in the numerical model in order to met the conditions encountered in a real scenario. Finally, it would be worth exploring the inclusion of a latching control (Budal et al., 1982; Falnes, 1997) on the PTO for maximizing the buoy movement, in order to be able to use the device in low energy wave conditions with wave heights under 1m.


1325 Development of a Desalination System Driven by Low Energy Ocean Surface Waves

_________________________________________________________________________________________________ CONCLUSIONS The performance of the WEC described in this document was studied under a train of regular waves. The results suggest that the small buoy provides enough force to the PTO device when the sea state corresponds to at least 1 m of wave height. For smaller waves it is necessary to modify the design. One option is to include technology involving latching controls, which would require to have, a priori, information of the incoming wave spectra. With the actual WEC configuration and within the range of these small wave heights, the PTO can only work as a simple effect pump. It is recommended to vary different anchor line and PTO lengths to give the system time to adjust after each wave and then to evaluate if the compression forces are enough for a double effect pump, which would positive impact on the efficiency of the WEC. It is important to recognize the need of more studies with a detailed configuration of the physics inside the PTO, as the damping forces due to friction and the water pressure itself may affect the estimated overall performance of the PTO system. More numerical tests are also needed with regular and irregular waves, in order to improve the design to obtain larger quantities of fresh water and thus be competitive with

other technologies, as well as to improve the reliability and survivability of the system and then decide the final design of the

WEC prior its contruction and testing in a real case scenario.

ACKNOWLEDGEMENT This work has been developed under the research program of the Wave Group at Colima University (geo.ucol.mx), supported by CONACYT (project PN-2015-01-674) and CEMIE-O. LITERATURE CITED Brooke, J., 2003. Wave energy conversion. Hungary. Elsevier Science, 262p. Budal, K., Falnes, J., Iversen, L., Lillebekken, P., Oltedal, G., Hals, T., Onshus, T., 1982. The Norwegian wavepowered buoy project. Proceedings of the the Second International Symposium on Wave Energy Utilization.

(Trondheim, Norway), pp. 323–344. Clément, A., Babarit, A., 2012. Discrete control of resonant wave energy devices. Philosopphical transactions or the Royal Society, 370(1959), 288–314. Falnes, J., 1997. Principles for capture of energy from ocean waves. Phase control and optimum oscillation. Department of Physics, NTNU, N-7034 Trondheim, Norway, pp. 1-8. Hicks, D., Mitcheson, G., Pleass, C., Salevan, J., 1989. Delbouy: ocean wave-powered seawater reverse osmosis desalination systems. Desalination, 73, 81–94. Hirt, C., Nichols, B., 1981. Volume of fluid (VOF) method for the dynamics of free boundaries. Journal of computational Physics, 39(1), 201–225. Hirt, C., Sicilian, J., 1985. A porosity technique for the definition of obstacles in rectangular cell meshes. Proceedings of Fourth International Conference on Numerical Ship Hydrodynamics. National Academy of Science. (Washington, DC, USA), pp. 441-450. Karagiannis, I., Soldatos, P., 2008. Water desalination cost literature: review and assessment. Desalination 223, 448– 456. Karimirad, M., 2014. Offshore energy structures: for wind power, wave energy and hybrid marine platforms. Switzerland. Springer, 288p. Verduzco-Zapata, M., Ocampo-Torres, F., Matthews, C., Olivos-Ortiz, A., Diego, E., 2017. Development of a Wave-Powered Desalination Device : Numerical Modelling. Proceedings of the 12th European Wave and Tidal Energy Conference. (Cork, Ireland), pp. 1–9. Wei, G., 2005. A fixed-mesh method for General Moving Objects in Fluid Flow. Modern Physics Letters, 19(28/29), 1719–1722. Wei, G., 2015. Development of the Compliant Mooring Line Model for FLOW-3D. Flow Science Report 08-15, 12p.
