Autonomous Wave Powered Desalination

Abstract

A wave powered water desalinating device may receive untreated salt water, and produce desalinated fresh water. The device consists of a pressure chamber, with a piston coupled with a pitching-type wave energy converter and configured to move along the major axis of the compression chamber; an inlet one-way valve configured to permit flow into the compression chamber from the exterior; a spring in fluid communication with the piston configured to absorb and control the cyclic pressure of the system; and a reverse osmosis membrane in the interior of the compression chamber such that motion of the piston in the direction of the distal end of the chamber exerts contents of the interior of the chamber against the reverse osmosis membrane producing fresh water.

Claims

1. A desalinating device, comprising: a compression chamber having an interior and being configured to receive water, the compression chamber having an inlet, a proximal end, a distal end, and a major axis extending longitudinally from the proximal end to the distal end; an inlet one-way valve sealably engaged with the inlet, the inlet one-way valve being configured to permit flow into the compression chamber from exterior to the compression chamber; a piston operatively coupled to a wing, the piston being sealably disposed within the compression chamber, and, the piston being configured to move along the major axis of the compression chamber in the direction of the distal end of the chamber in response to the movement of the wing; a reverse osmosis membrane being in fluid communication with the interior of the compression chamber such that motion of the piston in the direction of the distal end of the compression chamber exerts contents of the interior of the compression chamber against the reverse osmosis membrane; and a spring member in fluid communication with the interior of the compression chamber, the spring member being configured to compress when the piston moves in the direction of the distal end of the compression chamber.

2. The water desalinating device of claim 1, further comprising a check valve disposed within the compression chamber, the check valve being disposed between a first portion of the compression chamber and a second portion of the compression chamber, the check valve being disposed between the piston and the spring member, and the check valve being configured to permit flow in the direction of the spring member and resist flow away from the spring member.

3. The water desalinating device of claim 2, wherein the spring member, when decompressing and when the check valve is closed, is configured to exert contents of the compression chamber against the reverse osmosis membrane.

4. The water desalination device of any of claims 1-3, wherein the inlet one-way valve is configured to allow the water to pass in one direction through the valve into the compression chamber when the pressure inside the compression chamber is lower than the pressure outside the compression chamber, and to prevent the water from passing in the opposite direction through the valve out of the compression chamber.

5. The water desalination device of any of claims 1-4, wherein the spring member has a spring stiffness in the range from 10 N/mm to 500 N/mm, or a non-linear spring stiffness.

6. The water desalination device of any of claims 1-5, further comprising a controller positioned between the wing and the piston, the controller having an adjustable gear set configured to regulate the velocity and displacement of piston movement.

7. The water desalination device of any of claims 1-6, wherein the wing includes a lighter-than-water buoy portion attached to the wing.

8. The water desalination device of any of claims 1-7, wherein the driver includes an air receiving portion that is configured to receive air caused by motion of the water source and to use the received air to propel the piston.

9. The water desalination device of any of claims 1-8, wherein the spring member is positioned between the proximal end and the distal end of the compression chamber.

10. The water desalination device of any of claims 1-9, wherein the reverse osmosis membrane is positioned between the proximal end and the distal end of the compression chamber.

11. The water desalination device of any of claims 1-10, further comprising a plurality of wings and compression chambers, wherein the plurality of wings and compression chambers are in fluid communication with the same reverse osmosis membrane.

12. The water desalination device of any of claims 1-11, wherein the spring member includes a plurality of springs, some of which may be positioned in series relative to one another and some may be positioned in parallel relative to one another.

13. The water desalination device of any of claims 1-12 wherein the spring member includes a spring actuator configured to engage and disengage the spring member in response to a command.

14. The water desalination device of claim 13, wherein the command comes from a sensor connected to the device through wires or wirelessly.

15. The water desalination device of any claims 1-14, where a damping member is positioned in parallel, in series with, or instead of any of the spring members.

16. The water desalination device of any of claims 1-15, further comprising a plurality of reverse osmosis membranes, wherein the plurality of reverse osmosis membranes is in fluid communication with the same compression chamber, some of which may be isolated by an actuator.

17. The water desalination device of any of claims 1-16, the water desalination device being configured to, during operation, permit pressure fluctuation on the reverse osmosis membrane of up to 60%, up to 50%, up to 40% up to 30% or up to 20%.

18. The water desalination device of any of claims 1-17, further comprising a spring chamber having an interior that is in fluid communication with the interior of the compression chamber, wherein at least a portion of the spring member is disposed within the spring chamber.

19. The water desalination device of any of claims 1-18 wherein the piston includes a set of a first piston and a second piston, each of the first and second pistons being operatively connected to the wing such that movement of the wing in a first direction causes the first piston to move along the major axis toward the reverse osmosis membrane, and where movement of the wing in a second direction opposite the first direction causes the second piston to move along the major axis toward the reverse osmosis membrane.

20. The water desalination device of any of claims 1-19, wherein the driving force is any form of Wave Energy Convertor, or any other form of mechanical driver.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] For a more complete understanding of the present invention, the objects and advantages thereof, reference is now made to the ensuing descriptions taken in connection with the accompanying drawings briefly described as follows:

[0039] FIG. 1 illustrates a basic variant of an Oscillating Surge Wave Energy Convertor;

[0040] FIG. 2 illustrates a WEC based desalination system through use of an electric generator, battery and pump;

[0041] FIG. 3 illustrates the pressures on a WEC due to wave passage and a simplified depiction of a reverse osmosis desalination membrane;

[0042] FIG. 4 illustrates a simplified single piston desalination system;

[0043] FIG. 5 illustrates a simplified dual piston desalination system;

[0044] FIG. 6 illustrates a single WEC piston and single spring driven piston desalination system;

[0045] FIG. 7 illustrates a physical embodiment of FIG. 4, with the cross shaped discharge nozzle used as a proxy for a reverse osmosis membrane;

[0046] FIG. 8 illustrates a computer rendering and a physical embodiment of FIG. 6, with the cross shaped discharge nozzle used as a proxy for a reverse osmosis membrane;

[0047] FIG. 9 illustrates a physical data from the second embodiment depicted in FIG. 8, with piston load force (N) on the y-axis and time (s) on the x-axis, with each plot depicting a different stiffness spring;

[0048] FIG. 10 illustrates a simplified rendering of the third physical embodiment;

[0049] FIG. 11 illustrates the fourth physical embodiment of the invention in operation on the test stand, producing desalinated water;

[0050] FIG. 12 illustrates physical data produced from the fourth physical embodiment showing pressure experienced by the reverse osmosis membrane while desalinated water is produced;

[0051] FIG. 13 illustrates physical test data from the fourth physical embodiment of the invention, with pressure experienced by reverse osmosis membrane modulated, while pressure experienced by the piston drops below 0 psi gauge before returning to high values;

[0052] FIG. 14 illustrates a summary of physical data produced from the fourth physical embodiment depicting several results with proving the efficacy of the invention;

[0053] FIG. 15 illustrates the use of a gearbox to disassociate the displacement of the piston from that of the WEC;

[0054] FIG. 16 illustrates a potential use of buoy based WEC to drive the piston;

[0055] FIG. 17 illustrates a plurality of buoys being used to increase the energy directed to the piston;

[0056] FIG. 18 illustrates a piston driven by air pressure variations inside of an enclosed chamber;

[0057] FIG. 19 illustrates a desalination system according to an embodiment of the invention, similar to the fourth physical embodiment depicted in FIG. 11;

[0058] FIG. 20 illustrates a desalination system pressurized by two separate pistons, each driven by a discrete WEC;

[0059] FIG. 21 illustrates the use of a plurality of springs used in series to provide a customized pressure modulation profile;

[0060] FIG. 22 illustrates the use of a plurality of springs used in parallel to provide a customized pressure modulation profile;

[0061] FIG. 23 illustrates a detail of an embodiment of the invention using springs both in series and in parallel to provide further customization of the modulation profile;

[0062] FIG. 24 illustrates the use of actuation devices to impede spring motion to control pressure profiles;

[0063] FIG. 25 illustrates an embodiment of the invention with a plurality of reverse osmosis desalination membranes;

[0064] FIG. 26 illustrates the use of a sensor for pressure, salinity, or other conditions controls the performance of the spring systems.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0065] Given that electrical systems do not provide an optimal utilization of the energy profile from WECs, this disclosure provides a novel system that embraces the periodic nature of a WEC, and mechanically couples it with a small reverse osmosis membrane to generate freshwater. To do this, one may integrate a piston or series of pistons, driven by the OSWEC, and a Reverse Osmosis pressure vessel.

[0066] Single vs. Multiple Piston Systems: The simplest mechanical systems use a piston to pressurize and drive water past an RO membrane. The chamber with the membrane may be sealed using a check valve while the piston retracts, as in FIG. 7. The high pressure generated in the chamber will cause a percentage of seawater to pass through the RO membrane; the problem is that the pressure within the chamber will immediately begin to decline due to mass flow out if the vessel volume remains constant.

[0067] Single Piston: Due to the incompressibility of water, the loss of even a miniscule amount of water will cause a massive drop in pressure. Thus, the first tenet requires that either additional seawater be constantly supplied, or the volume constantly change. This makes a single chamber, single piston system an unviable solution.

FIG. 4Hypothetical Single Piston System

[0068] Multiple Driving Piston: The use of a multi stroke system, with multiple WECs and therefore multiple pistons and check valves working in tandem may solve the issue (see FIG. 5). However, if one assumes the force curves are perfect sine waves, superimposing them will still give two points with substantially lower force. Another embodiment may be to integrate three or four WECs in harmony, like a 3 stroke or 4 stroke engine. The increased mechanical complexity, small power generated by each WEC and distance between each unit make this infeasible.

FIG. 5Hypothetical Dual-Piston System

[0069] Pressure Exchanger Intensifier: Another option is a Pressure-Exchanger Intensifier, or Clark Pump. This system uses a pair of pistons, driving in opposite directions, with a single rod connecting them. This enables a low pressure pump source (either a WEC, or conventional pump) to drive each piston chamber sequentially, causing modulated, single direction flow. Some systems are designed to utilize the latent energy of the brine outflow from the Reverse Osmosis membrane as a power source, known as Power Take Off (PTO).

[0070] Variable Volume Systems: If manipulating the timing of each piston is not sufficient to maintain a consistent, high pressure, one must use some form of energy storage. While liquids are not ideal gasses by any means, the physical basis of the ideal gas law (PV=mRT) can give insight. This suggests that a system can maintain a constant pressure with decreasing mass by either altering the volume or the temperature. Heating a flowing liquid underwater via a purely mechanical system is extremely challenging, and would cause substantial issues due to thermal expansion of components. Therefore most devices must vary the volume of the system.

[0071] Bladder Tanks: A bladder pressure tank contains pressurized air and water separated by a flexible membrane (bladder). These tanks are typically precharged with air at the factory. As water pressure changes, the volume of air in a bladder tank contracts and expands. One may consider the use of this technology as a means of energy storage to maintain or rectify pressure consistently. Typically underwater air bladders are rated for pressures of up to 50 psi. While an attractive option, this pressure would be insufficient to efficiently execute reverse osmosis desalination, a process that requires at least twice the capability of an air bladder. Flow rates are linked proportionally to the driving pressure meaning that, generally higher pressures yields greater outflow rates of potable water.

[0072] Spring-Based Energy Storage System: Another type of system is a spring-based energy storage system. On the forward stroke of a wave, the saltwater is compressed and pressurized as in any other case, but additionally, the spring (which may be attached to a sealed piston) compresses and stores energy. Then, on the backstroke of the wave, the spring slowly returns to its initial position, releasing the energy that it stored on the forward stroke. This accomplishes the change in volume that is necessary to keep the pressure high on the water inside the system, thereby avoiding the rapid pressure fluctuations created by ocean waves. Additionally, this type of system is resistant to wear over time and makes maintenance much simpler than a bladder tank would, for example. A schematic of such a system appears in FIG. 6.

FIG. 6Hypothetical Spring-Based Energy Storage System

[0073] Multiple Spring Systems: One may also consider the possibility of using several springs either in series or in parallel to achieve the perfectly ideal spring constant, or to create a non-linear pressure recovery curve. Tuning a system by installing multiple springs both in series and parallel would lead to far more efficient devices.

Exemplary Embodiments

[0074] The first iteration pressure vessel can be seen in FIG. 7. It consisted of a simple single spring system, similar to the initial components of FIG. 4. On the right of the image, a tapped hole and adaptor for a pressure transducer can be seen. The clear tube leads to a pressure reduction device, using a microfluidic tube to cause large pressure losses, before releasing the liquid to a bucket. The piston was of relatively standard design, with grooves for two O-rings included.

FIG. 7Photograph of First Embodiment

[0075] The second prototype was tested with an MTS (Tension-Compression system). The MTS drove a large flat plate downwards, at a chosen, fixed displacement rate and allowed for observation of the pressure fluctuations inside the pressure vessel. The MTS was not used as a tension system on the second prototype, as there was no water inlet yet installed. This meant there was no physical attachment between the MTS Crosshead and the pistonthey lay flat against each other in simple compression.

[0076] The tests performed on the second prototype allowed for understanding the effects of spring stiffness on the pressure decay inside the system. For a given displacement rate, diameter and outflow resistance, there must be an optimal stiffness spring. One may purchase a series of springs of different stiffnesses and test them in the prototype setup to observe empirically how the stiffness of a spring affected the performance of the system. Four springs were tested with stiffnesses of: 17 N/mm, 40 N/mm, 70 N/mm, and 150 N/mm.

FIG. 8Rendering and Photograph of Second Embodiment

[0077] Tests were performed with each of these springs. Without any spring, the pressure decay would look like a square wave with close to 90 angles between the vertical and horizontal sections (depicted by the uppermost line in FIG. 9). The weakest spring, (shown by the diagonal line in FIG. 9) was not always optimal, while the stiffest spring, generally follows the square wave, albeit with a slower ramp up and some limited energy storage at the end. A medium stiffness spring, has a more gradual, curved ramp in pressure, and does store a substantial portion of energy (defined as the area under a Force-Time plot, when displacement is fixed at a constant rate). This indicated that there were springs that were both too stiff and too pliant for the system, and one may optimize to find a middle ground. One may model these pressures and decays using mathematical software, e.g., MATLAB.

FIG. 9Pressure Decay Curves for Springs of Various Stiffnesses

[0078] Move to Modular System Design: The final embodiment was modular, due to a number of benefits. For example, if a single component of the system malfunctions, it can simply be removed and replaced. In addition, using commercially available NPT threaded connections, off the shelf valves could be easily integrated, tested and serviced.

[0079] FIG. 10 provides a further view of an illustrative system (the third embodiment). An exemplary system contained three main sections: an inlet chamber, a spring and reverse osmosis chamber, and a check valve separating the two. A system may be designed to allow the integration of WECs with RO desalination by utilizing a spring and a series of valves. These components are able to modulate pressure inside a chamber containing a reverse osmosis membrane such that the outflow of clean water remains nearly constant, and the damage to the membrane is minimized. A reverse osmosis system has been installed to produce desalinated water, while an inlet valve allows a brine mixture to refill the test system for continuous operation.

FIG. 10Rendering of an Illustrative System (Third Embodiment)

[0080] Piston: On the forward stroke of an ocean wave, a WEC drives the inlet piston forward. On the back stroke of the wave, as the WEC returns to its natural position due to buoyancy, that piston simply retracts out of the chamber. The pistons used were machined out of aluminum, and had industry standard nitrile o-rings installed inside of grooves to provide liquid sealing under pressure. Packing or mechanical seals could be used in future iterations, as well as other pressure isolation systems.

[0081] Inlet and Check Valve: During this retraction phase, the inlet valve opens and lets new seawater into the system. The check valve, in the center of the device, is open on the forward stroke of the wave. On the back stroke of the wave it closes, effectively isolating the inlet chamber from the reverse osmosis chamber. A ball valve was used for the inlet due to the small sizes available off the shelf, and a larger swing check valve was used to separate the chambers.

[0082] Spring: On the forward stroke of the wave, the spring compresses and is able to store energy. This energy storage phase is defined as Phase I of system operation. On the back stroke of the wave, when the check valve is closed, the spring is slowly returning to its extended position, thereby releasing the energy that it stored on the forward stroke of the wave. This is defined as Phase II of system operation. This slow release of energy keeps high pressure on the reverse osmosis membrane. The membrane therefore does not experience the rapid pressure fluctuations that would subject it to damage, and it is constantly producing fresh, drinkable water.

[0083] Check Valves: The invention seeks to separate the pressures on the reverse osmosis section of the device from the driven side, isolated by a check valve. One wishes to avoid the backflow of fresh water because it would both increase the rate of membrane degradation and reduce output. A number of different options can be usedboth ball and swing valves were utilized in the embodiment.

[0084] Fourth Embodiment: The fourth (and final physical) embodiment added an elbow joint to allow more efficient testing and higher flow rates of potable water. However, due constant fluid communication, orientation has no impact on system performance due to low fluid head differences in pressure. FIG. 16 shows the full system with both a brine source (red cooler, center background), potable water collection pot (clear beaker, left foreground) and brine disposal (black bucket, right). The MTS driver is shown in the left of the frame.

FIG. 11Photograph of Fourth Embodiment in Testing Setup

[0085] The MTS driver was modified to drive the piston both up and down to simulate the motion that it would undergo if it were operating in an ocean. The crosshead was run at various speeds for a set of predetermined time periods. These speeds and periods were meant to simulate common, real-life ocean wave conditions.

[0086] The periods of time for which the tests were run mimicked ocean wave periods in real life. Ocean wave periods vary from about 8 seconds long up to around 25 seconds long. For this reason, exemplary embodiments underwent testing at simulated wave periods of 6, 8, 10, 12, 14, and 20 seconds.

[0087] Crosshead speeds were chosen based on the limits of the experimental setup. If the crosshead ran too slowly, it was unable to close the isolation check valve in the center of the system. Alternatively, at very fast crosshead speeds, sometimes the pressure inside the system became high enough that water could leak behind the spring piston. This would cause the pressure inside the system to build at a rapid pace. Devices were tested at crosshead speeds between 1 and 7 mm/s, in 0.5 mm/s intervals.

[0088] For every combination of wave period and wave speed, the system underwent six simulated wave cycles. This accounted for a settling period, to allow the pressure profile to steady. An example of such a trend is shown in FIG. 12. This particular pressure data arose when the spring inside the system had a stiffness of 40 N/mm and the crosshead speed was 4 mm/s. This data was collected for approximately 200 different test regimes.

FIG. 12Data Collected From One Test Regime

[0089] From graphs like the one in FIG. 12, one is able to understand how pressure fluctuates inside the system. For example, the data shows a pressure fluctuation of 21.63 psi about an average pressure of 76.30 psi, meaning that the pressure was fluctuating by 28.3% under this particular set of conditions. However, to understand the significance of this, it is important to note the difference between the force/pressure readings at the crosshead (which are equivalent to those in the inlet of the system) versus those at the pressure chamber and reverse osmosis chamber. To do this, one may create plots such as the one shown in FIG. 13.

FIG. 13Graph Showing Modulation of Pressure

[0090] This figure shows, conceptually, the pressure profile that the system would see without the disclosed technology (the paler line that dips below the x axis) versus the pressure profile created by the disclosed technology (upper, darker line). This plot is proof that the system is working; it is alleviating the hypothetical rapidly fluctuating pressure profile to something much closer to constant pressure. Testing a range of both wave frequencies, piston displacements and spring stiffnesses creates a plot of the two key metrics: pressure fluctuation at the membrane surface and the volume of potable water produced (FIG. 14).

FIG. 14Water Produced Versus Pressure Fluctuation for All Tests

[0091] From this graph, one may impose several constraints regarding data points that are relevant. Firstly, any pressure fluctuation over the 30% mark may be potentially unsafe for reverse osmosis membranes. In addition, there is a minimum output flow rate that the system must produce in order to provide enough water for a family. This minimum amount of water is 1.25 L/hr. Finally, the ideal operation of the system is the point at which it produces the most water and experiences the minimum pressure fluctuation. Taking each of these points into consideration, one can identify the optimum operating point, shown on the following graph inside the oval. This means that the spring that works best for the exemplary system is that of stiffness 40 N/mm. Using this spring, and at particular operating conditions, the exemplary system was able to produce five liters of clean drinking water per hour while experiencing a pressure fluctuation as low as 25%.

[0092] Such a value is the solution for the testing of the fourth embodiment, but any other scenarios can also be solved for, based on the exact WEC specifications, RO membrane chosen and waves expected.

[0093] To predict such conditions, a number of modeling techniques can be used. For waves, there is a substantial body of literature on predicting and defining wave shapes and forces. Internal to the device, there are many different fluid dynamic theories that can be applied to modeling. Understanding the pressure decay inside the system when the check valve is closed and the RO membrane is in contact with the spring-piston system is possible with empirical data. To validate these results, one may pressurize the system to a variety of peak pressures and observe how the pressure decayed when the piston was held in a constant position. The four laws that were used for comparison were the Darcy-Weisbach equation, the HagenPoiseuille law, flow through an orifice, and fluid flow through porous media.

[0094] One can use comparisons between the pressure readings from the first, second and third embodiments with the above laws to develop a MATLAB simulation that models the system's dynamics.

Potential Variations of Desalination Device

FIGS. 15-26Hypothetical Implementation of Exemplary Systems on a WEC

[0095] FIGS. 15-26 disclose alternative embodiments of the desalination device. In some embodiments, the desalination device may include a gearbox. The gearbox can regulate the velocity, rate, or force of the piston, as well as other features of the device.

[0096] The desalination device may include a buoy configured to flow on water, the buoy being operatively connected to the piston. In some embodiments, a plurality of buoys or lighter-than-water floatation structures may be connected to the piston.

[0097] In some embodiments, the piston may be driven by air pressure. Air may enter an air receptacle having an interior, the interior being in fluid communication with the piston. Air may actuate the piston in one or more directions.

[0098] In some embodiments, the piston may be connected to a spring member, the spring member being configured to compress when contents of the compression chamber are moved toward the reverse osmosis membrane.

[0099] The desalination device may include a plurality of wings and a plurality of pistons configured to operate within one compression chamber and move contents of the compression chamber towards one reverse osmosis membrane.

[0100] In some embodiments, the desalination device may include a plurality of springs. The springs may be connected in series relative to one another. Alternatively, the springs may be connected in a parallel arrangement to one another. In a further embodiment, a desalination device may include springs that are connected in series and springs that are connected in parallel.

[0101] The desalination device may include a plurality of reverse osmosis membranes. The reverse osmosis membranes may have varying properties. In some embodiments, an actuator may isolate, open, or close one or more reverse osmosis membranes.

[0102] Spring members may include non-linear spring members configured to compress in a non-linear manner, such that force exerted on the piston by the spring member varies as a function of the state of the system.

[0103] In some embodiments, spring member stiffness may be controlled by an actuator connected to the desalination device. The actuator may isolate or disengage one or more springs. The actuator may operate in response to a command provided by a pressure sensor.

[0104] In further embodiments, the desalination device may include an electronic controller. The controller may receive data from a sensor attached to the device, either through a wire or wirelessly. The controller may be configured to provide an electrical current to a spring member such that the stiffness increases or decreases.