Patent Application
20220032233
The present invention relates a sea water desalination system employing ocean wave-driven pumps to pressurize a flow of sea water delivered to a plurality of reverse osmosis (RO) membranes and a hydraulic motor-generator set supplying power for RO system peripheral devices.
Over 1 billion people suffer from the effects of water scarcity. Desalinating sea water is an excellent potential solution but traditional reverse osmosis (RO) desalination systems require a connection to an electrical grid or dedicated diesel engine-driven power generator. The disclosed wave-driven desalination system can be deployed quickly, operate completely “off-grid” and supply large quantities of fresh water at a competitive cost. Wave energy is well suited for driving a RO plant as both the energy and raw material (sea water) are co-located.
Present day RO plants employ an electric motor-driven pump to pressurize a portion of the total feed water flow of sea water supplied to the RO membranes which deliver two process streams, namely, 1) fresh water (a/k/a “permeate”) and 2) brine (a/k/a “concentrate”). Typically, a RO plant operates at a controlled “recovery ratio” wherein the permeate flow may be approximately 40% of the total feed water input flow and the rejected concentrate approximately 60%. The feed water input pressure may be approximately 850 psi and the concentrate marginally less-e.g., only 5 psi. The considerable hydraulic power of the concentrate flow is employed by an energy recovery unit (ERU)to pressurize the remaining feed water flow required to maintain the process. The most efficient ERU methods directly transfer concentrate pressure to incoming feed water. Small losses in the process are compensated by an electric motor-driven pump which may be integral to the ERU.
With the benefit of concentrate energy recovery, the electrical energy required to run the feed water pressurization pump, and other supporting devices and controls, may be approximately in the range of 2 to 4 kWh per cubic meter of permeate produced per day (m3/d) according to the scale of the plant. Where grid power is available at a modest cost—e.g., $0.08/kWh—the energy cost of the most efficient RO plants operating at 2 kWh/ m3/d would be an acceptable $0.16/m3/d. However, in remote coastal and island regions, smaller scale plants operating at 4 kWh/m3/d and employing electrical power supplied by a standalone diesel-generator set at a cost of $0.30/kWh would incur a relatively high water production energy cost of $1.20/m3/d. Amortized capital expense (CAPEX) and recurring operating expense (OPEX) might add an additional $1.00/m3/d or more to the associated cost. This high cost water and the negative environmental impact of diesel engine-powered generators motivate the concept of employing captured ocean wave energy in order to drive the RO process.
No ocean wave-driven RO plants—experimental or commercial—are known to be operational today. As reported in Matt Folley, Baltasar Peñate Suarezb, Trevor Whittaker, An autonomous wave-powered desalination system, Science Direct, 2008 (Reference [1]), the first demonstration of a wave-driven RO device was the Delbuoy which produced 2 m3/d circa 1980. DelBuoy employed a linear reciprocating pump driven by wave motion to pressurize feed water delivered to a RO membrane. A proposed contemporary embodiment of the DelBuoy concept disclosed in Reference [1] and in Matt Folley, Trevor Whittaker—The cost of water from an autonomous wave-powered desalination plant, Renewable Energy 34 (2009) 75-81 (Reference [2]), similarly uses wave-driven linear pumps to pressurize the RO plant feed water supply.
As reported in References [1] and [3], an attempt was made circa 2004 to employ electricity generated by an experimental oscillating water column (OWC) wave energy plant in Kerela India to power a RO plant. The energy demand, kWh/ m3/d, of the Kerela electrically powered RO plant, or other wave-to-electric powered system, would be significantly higher than the method of “direct pressurization” demonstrated by DelBuoy, or the contemporary embodiment disclosed in References [1] and [2], since additional power conversion steps and associated losses are incurred:
Wave-driven device mechanical or hydraulic power conversion to electricity;
Electrical power conversion to feed water pump motor shaft power;
Motor shaft power conversion to pump-pressurized RO feed water hydraulic power.
As a consequence, a wave-driven RO system employing direct pressurization of feed water will be less costly to build and operate.
Conventional RO plants supplied with electric power at stable voltage and frequency readily deliver feed water to the RO membranes at constant pressure and hence constant flow. While there is speculation that a fluctuation of pressure and flow will shorten membrane service life, little, if any, research in this regard has been reported. The relatively contemporary prior art wave-driven RO system with direct pressurization of the feed water supply disclosed by References [1] and [2] does not appear to have positive means of controlling this pressure. A pressure exchanger-intensifier device, pressure relief valve and large volume accumulator are provided to suppress and limit pressure fluctuations, but it is not obvious that mean pressure can be regulated to a preferred set point value to minimize any concerns about membrane life degradation.
DelBuoy and the relatively contemporary prior art wave-driven RO system with direct pressurization of the feed water supply disclosed by References [1] and [2] employ wave-driven linear pumps. As these are submerged in the ocean they are subject to development of corrosion and bio-fouling. In particular, the exposed portion of the piston rod is at risk as corrosion and formation of bio-fouling films may wear the pump seals as the rod translates to and fro. A scraper may be provided to remove bio-fouling in advance of the seal and the rod may be formed of an anti-corrosive steel alloy. Nonetheless, over a long period of time, it is to be appreciated residual bio-films and corrosion may shorten the service life of the pump seals.
Another shortcoming of the linear pump configuration, as disclosed in both References [1] and [2], is that the piston rod and seals bear a bending load which may impair their operational life. Moreover, the pump and its piping connections must articulate with that of the wave-driven flap. Hence, there is potential for fatigue failure of pump mechanical connections to the flap and stationary chassis and hydraulic connections to piping.
A further shortcoming of the linear pump configuration, as disclosed in both of References [1] and [2], is that there is no provision for controlling the pump reaction torque presented to the wave-driven flap to maximize wave power capture efficiency. Developed flap mechanical power depends on pump reaction torque load and akin to the operation of electrical sources maximum conversion of incident wave power is converted to flap mechanical power at a particular flap torsional load (a/k/a “flap damping torque”). It is to be appreciated that either too little or too much load will result in sub-optimal power conversion. Pump reaction torque is determined by the RO feed water pressure which may not provide the optimum flap load and, moreover, as explained above, the prior art disclosed by References [1] and [2] provides no means for controlling feed water pressure to a preferred set point value.
Wherefore, it is an object of the present invention to overcome the above-mentioned shortcomings and drawbacks associated with the prior art.
The disclosed system, according to the invention, employs an oscillating wave surge converter (OWSC) which comprises an ocean bottom-mounted hinged flap that preferably drives a pair of rotary hydraulic pumps which deliver a pressurized flow of filtered sea water to both (1) on-shore RO membranes and (2) a hydraulic motor-generator set supplying electrical “house power” for peripheral pumps and controls. The generator and hydraulic motor operate at variable speed enabled by a power electronic interface with an energy storage unit (ESU). Variable speed operation of the hydraulic motor enables control of the mean feed water pressure, supplied to the RO membranes, to be at a preferred set point value. Moreover, the disclosed invention provides switch-mode hydraulic means to modulate the effective displacement of the rotary (or possibly linear) hydraulic pumps so that pump reaction torque (Nm), determined by the product of displacement (m3/rad) and pressure (N/m2), may be adjusted independently of RO membrane feed water pressure so that flap load can be set to a value for optimum wave power capture.
A block diagram of the complete RO desalination system and house power generation system, identifying salient components and their interconnection, is depicted by
A first object of the present invention is to provide means for controlling a mean RO plant feed water pressure to a preferred value as available wave power fluctuates. This is accomplished by adjusting the speed of the hydraulic motor driving the house power electric generator by controlling the generator electric load. Control of hydraulic motor speed determines the rate at which sea water is extracted from a high-pressure accumulator and by this means its pressure can be maintained at a set point value. The fluctuation of generator power accompanying speed adjustment is compensated by the provision of an electrical energy storage unit (ESU) which comprises of a battery, a flywheel, a super-capacitor bank or combination thereof to assure continuous delivery of power to peripheral pumps and controls.
A second object of the present invention is to provide switch-mode means, similar to methods employed in the field of power electronics, to control flap pump reaction torque independently of the RO membrane feed water pressure so flap torsional load can be set to a value for optimum wave power capture while simultaneously achieving a preferred feed water pressure. Such means comprise a valve shunting the pump ports and a method for pulse width modulation (PWM) of the valve state for controlling the effective displacement (m3/rad) of the pump. With a PWM duty cycle of 0, the valve is closed, and no pump fluid is bypassed in which case the effective displacement equals the maximum displacement of the pump, as determined by its interior dimensions and any internal leakage. With a duty cycle of 1, the valve is continuously open, and the effective displacement is zero. For duty cycles between 0 and 1, intermediate values of effective displacement may be readily obtained. By this mechanism, the pump effective displacement and consequent flap loading can be slowly varied over minutes or hours, according to average sea conditions (a/k/a “Coulomb damping control”) or potentially in real-time, e.g., with flap load proportional to flap angular velocity (a/k/a “linear or viscous damping control”) to enhance wave power capture.
A third object of the present invention is to replace the prior art linear pump with an oscillating rotary pump in which all critical working surfaces are fully enclosed and isolated from the surrounding sea water so that the modes of failure, due to bio-fouling and corrosion, are avoided or minimized at the very least. The bearings of a rotary pump may also serve as hinges for the flap and thereby reduce the associated capital expense. Moreover, neither the pump nor its hydraulic connections articulate with flap oscillation thus avoiding the potential for fatigue failures.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate various embodiments of the invention and together with the general description of the invention given above and the detailed description of the drawings given below, serve to explain the principles of the invention. The invention will now be described, by way of example, with reference to the accompanying drawings in which
It should be understood that the drawings are not necessarily to scale and that the disclosed embodiments are sometimes illustrated diagrammatical and in partial views. In certain instances, details which are not necessary for an understanding of this disclosure or which render other details difficult to perceive may have been omitted. It should also be understood that this disclosure is not limited to the particular embodiments illustrated herein.
The present invention will be understood by reference to the following detailed description, which should be read in conjunction with the appended drawings. It is to be appreciated that the following detailed description of various embodiments is by way of example only and is not meant to limit, in any way, the scope of the present invention.
Turning now to the system block diagram of
Referring to
The angular velocity amplitude of flap 1 fluctuates, as the flap oscillates back and forth, between zero and a maximum value. As a consequence, the rectified output of the flap-driven pumps pulsates at twice wave frequency—e.g., with a period of 5 seconds for waves of 10 second period. Moreover, due to variability of wave height and period the amplitude of these pulses will be modulated by these more slowly fluctuating wave characteristics. An off-shore high pressure accumulator 21 and typically larger volume on-shore accumulator 11, as depicted in
As can be seen in
A second portion of the output flow from the boost pump 15, i.e., the portion of boost pump 15 output not flowing to the flap pump suction line 16, is delivered to the RO energy recovery unit (ERU) 13 which transfers much of the pressure in the RO membrane reject concentrate (a/k/a “brine”) to this component of boost pump feed water output. This ERU pressurized supply of feed water 30 joins the pressurized feed water flow supplied to the RO membranes 12 by the flap pumps 2 and flow rectifiers 17 depicted in
Since the pressure drop from RO membrane feed water input to reject concentrate output is very small—perhaps only 1 percent or less of the feed water pressure—the concentrate hydraulic power is very high and absent the ERU would be wasted and the flow supplied by the flap pumps 2 would have to be much greater to meet a desired RO flow of the permeate 28. As a simplified example, assume zero pressure drop across the RO membrane from the feed water input to concentrate output as well as a 100% efficient ERU. Then, if the desired permeate flow were to be 90 gpm (˜500 m3/d) and the RO process operated at a 50% recovery ratio, the flap pumps 2 would need deliver only 90 gpm since the remaining 50% of the required feed water flow (90 gpm) would be supplied by the ERU.
In practice, a recovery ratio somewhat less than 50% might be desirable to minimize membrane maintenance. Also ERU efficiency would be less than 100% and its loss would be compensated by house power supplied to its internal pump. As recovery ratio declines, more feed water must be supplied to the ERU by the boost pump 15 which increases its size and cost as well as its house power demand. Moreover, the consequent additional feed water hydraulic power demand of the house power generation system would exact a penalty on permeate production capacity. These and other design tradeoffs can be evaluated with a comprehensive computer model of the complete plant depicted by
The performance of a desalination plant of the present invention is rated at a stated permeate productivity Qp (m3/d) at rated sea state conditions—“significant wave height” Hs (m) and “Peak Power Period” Tp (s). Hs is defined as the mean trough to crest height of the highest third of the waves and Tp is the inverse of the frequency at which the value of the wave power frequency spectrum is a maximum. For example, an illustrative system may deliver a permeate flow Qp of 500 m3/d for Hs=2.5 m and Tp=12 s. For these rated conditions, there will be a particular RO plant feed water pressure which achieves Qp at a preferred recovery ratio RR.
Wave power incident on the flaps 1 and developed flap mechanical and flap pump 2 feed water power is proportional to Hs2 and Tp.
For below-rated sea conditions—in particular Hs<Hs rated—the preferred RO feed water pressure to maintain best permeate flow, at a desired recovery ratio, will be less than the rated pressure, as illustrated by trend line 9 in
In similar fashion to the preceding example, if RO feed water pressure drops below a set point value, the generator 20 current and reaction torque presented to the hydraulic motor 19 can be momentarily increased to slow down the hydraulic motor 19 and the rate at which fluid is extracted from the HPA 11 to raise RO feed water pressure.
Average sea state parameters Hs and Tp vary relatively slowly—perhaps only significantly over 15 minutes or longer. As the speed of motor 19 and generator 20 fluctuate to maintain a preferred RO feed water pressure, the power developed by the generator will also fluctuate. At the same time, the speed of boost pump 15 and energy recovery unit 13 will also be adjusted and hence their house power demands will also fluctuate. To assure that house power demands are satisfied under varying wave conditions, an energy storage unit (ESU) 25 is provided to buffer generator power output.
As depicted by trend line 10 in
The present invention provides control means for achieving optimal flap damping torque while simultaneously setting RO feed water pressure to its optimal value. This control is accomplished by provision of valves 22 which shunt the ports of flap pumps 2 depicted in
In an illustrative embodiment of the present invention, a shunt valve switching frequency of 1 to 5 Hz may be sufficient for the PWM control.
While shunt valves 22 may be also located across the pressure and suction lines, on the output side of the flow rectifier as depicted in
An illustrative embodiment of the present invention includes the interconnected elements depicted in
A second embodiment of the present invention only differs from the first embodiment with respect to location of shunt valves 22 connected across the output ports of their respective flow rectifiers 17, as depicted in
A third embodiment of the present invention differs from the first or second embodiments, described above, only with respect to the choice of flap pumps 2 for which cross-head driven linear pumps would be selected.
A fourth embodiment of the present invention differ from the first and second embodiment, described above, with respect to provisions made to support rotation of the flaps 1 for which purpose, as depicted in
In all embodiments, an illustrative system of the present invention would have a pair of wave-driven flaps approximately 8 m wide and 7 m tall deployed in a mean sea level depth of 7 m, each driving a pair of rotary pumps 2 or linear pumps 2′ designed to accommodate and pressurize sea water supplied by an on-shore filter 14, boost pump 15 and suction pipe line 16. Filtered sea water thus pressurized to a level in the range of approximately 500 to 1,000 psi (a/k/a “feed water”) would be delivered to membranes 12 of a reverse osmosis (RO) desalination plant and a hydraulic motor 19 driving an electric generator 20 which supplies electrical power to RO plant peripheral devices such as boost pump 15 and energy recovery unit (ERU) 13.
The flap-driven pumps deliver a flow of feed water to the RO membranes 12 nominally equal to the fresh water (a/k/a “permeate”) flow which constitutes a fraction (a/k/a “recovery ratio”) of the total required RO feed water required. The recovery ratio may have an illustrative value of 0.4 per unit (pu). The remaining fraction of the required RO feed water flow—e.g., 0.6 pu—is largely provided by the energy recovery unit which transfers the considerable pressure of the brine (a/k/a “concentrate”) rejected by the RO membranes 12 to the supply of filtered sea water provided by the boost pump.
The feed water pressure may be controlled to a preferred set point value by adjustment of the speed of the hydraulic motor 19 and generator 20 which determines the rate at which water is extracted from the on-shore accumulator 11. Generated electrical power is buffered by an electrical energy storage unit (ESU) 25 to assure that the electrical power demands of RO system peripheral devices can be met as available wave power fluctuates.
The reaction torque of the flap driven pumps 2 or 2′ may be adjusted independently of the feed water pressure by means of the pulse width modulated shunt valves 22 to adjust the effective displacement of the pumps in order to achieve flap damping torque enabling optimum wave power capture. Flap loading can be varied slowly over minutes or hours according to average sea conditions (a/k/a “Coulomb damping control”) or potentially in real-time—e.g., with flap load proportional to flap angular velocity (a/k/a “linear or viscous damping control”) to enhance wave power capture.
Nominal power flows in all embodiment for rated sea conditions are depicted in
Additionally, if sea conditions provide more than rated flap mechanical and feed water hydraulic power the PWM duty cycle of shunt valves 22 may be adjusted to reduce developed hydraulic power to its rated limit.
While various embodiments of the present invention have been described in detail, it is apparent that various modifications and alterations of those embodiments will occur to and be readily apparent to those skilled in the art. However, it is to be expressly understood that such modifications and alterations are within the scope and spirit of the present invention, as set forth in the appended claims. Further, the invention(s) described herein is capable of other embodiments and of being practiced or of being carried out in various other related ways. In addition, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items while only the terms “consisting of” and “consisting only of” are to be construed in a limitative sense.
The foregoing description of the embodiments of the present disclosure has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the present disclosure to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the present disclosure be limited not by this detailed description, but rather by the claims appended hereto.
A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the scope of the disclosure. Although operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results.
This invention was made with government support under Contract No. DE-SC0017699 awarded by the U.S. Department of Energy. The United States Government has certain rights in the invention(s).
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2019/052632 | 9/24/2019 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62735992 | Sep 2018 | US |
