WO2015167333A1

WO2015167333A1 - Water desalination system

Info

Publication number: WO2015167333A1
Application number: PCT/NL2015/050290
Authority: WO
Inventors: Reinoud Willem FEENSTRA; Sid VOLLEBREGT
Original assignee: Elemental Water Makers B.V.
Priority date: 2014-04-30
Filing date: 2015-04-30
Publication date: 2015-11-05

Abstract

A renewable energy powered desalination system comprising a reverse osmosis membrane unit and an elevated water storage unit is described.

Description

WATER DESALINATION SYSTEM

Field of the Invention

The present invention relates to a water desalination system powered by renewable energy, and a process for the continuous desalination of water. Background of the Invention

According to the 2010 publications of the World Health Organization, 672 million people will still lack access to improved drinking water sources in 2015. The major areas suffering from water scarcity on a global scale would include Africa, the Middle East and South-East Asia. 84% of the world population without an improved drinking- water source lives in rural areas. In some of these places, desalination of water is the only option to provide access to safe drinking water.

Reverse Osmosis (RO) is the desalination technique which has the largest installed worldwide capacity, because of the low energy consumption for this process when compared to other desalination techniques. The energy consumption of reverse osmosis is directly related to the salt concentration.

The required pressure for the RO process is generated by an electric pump. Traditionally, the electricity is generated using fossil fuels, directly relating to carbon dioxide emission. Besides the environmental impact, this energy source is becoming scarcer and prices are rising. A logical step in the RO industry is the introduction to Renewable Energy Sources (RES), such as wind energy and solar power, which have already been in use for decades. This option is most suitable for remote communities who lack access to an electricity grid or fossil fuels, but also for resorts and industries, who require their own water supply and could benefit from a "green" image.

However, to enable maximum RO performance and water purifying capacity, the system has to run continuously. With fluctuating RES, this means the introduction of an energy buffer. Existing small scale RES-RO systems sometimes use batteries as the energy buffer. The disadvantages are the energy conversion losses, the high investments costs, the short lifetime, the required maintenance and the battery disposal. There exists a need for alternative forms of desalination system that are more energy efficient, have lower maintenance needs and are more environmentally friendly.

Summary of the Invention

According to a first aspect of the present invention there is provided a water desalination system, comprising:

a pump powered by renewable energy and in fluid communication with a water source to be desalinated;

a reverse osmosis membrane unit in fluid communication with the pump;

an energy recovery device disposed between and in fluid communication with the pump and the reverse osmosis membrane unit; and

a water storage unit in fluid communication with the pump and the reverse osmosis membrane unit, wherein the water storage unit is at a greater elevation than one or both of the energy recovery device and the reverse osmosis membrane unit.

According to a second aspect of the present invention there is provided a process for continuous desalination of water, comprising:

using renewable energy to power a pump to pump water to be desalinated to a water storage unit and/or to a reverse osmosis membrane unit at times of high renewable energy availability;

allowing water to flow from the water storage unit to the reverse osmosis membrane unit at times of low or zero energy availability; and

intensifying the feed pressure to the reverse osmosis membrane unit of the water to be desalinated using an energy recovery device.

Displacing water to an elevated storage provides the pressure required for continuous operation of the reverse osmosis membrane unit without the need of a battery-based energy buffer. When RES are available, water is pumped up towards the storage tank which is located at higher ground, continuously providing the required pressure for the process. The amount of water pumped up with RES available is sized to be able to cover the periods of low RES availability. The mechanical energy recovery device reuses the pressure available in the concentrate flow to intensify the feed pressure and significantly reduce the required elevation. The system works continuously, is stationary, autonomous and also self-regulating. As the reverse osmosis membranes are designed to operate on continuous and stationary conditions, this will result in longer product lifetime and makes use of the maximum water purification capacity. Also continuous and stationary operation prevents bio- fouling, which can damage the reverse osmosis membranes. Lastly, the autonomous function of the system allows installation in remote communities. No electricity grid or access to fossil fuels has to be provided, which makes the system suitable for many locations where conventional desalination plants are not.

Brief Description of the Figure Figure 1 shows one possible configuration of a water desalination system powered by solar energy.

Detailed Description of the Invention

A first aspect of the present invention provides a water desalination system powered by renewable energy. The water desalination system generally comprises a pump, the pump being powered by renewable energy, a reverse osmosis membrane unit, an energy recovery device and a water storage unit.

Renewable energy source

The water desalination system may be powered by any renewable energy source. For example the water desalination system may be powered by solar energy, wind energy, wave power or a hybrid system involving any combination of these. In the example in which the water desalination system is powered by solar energy, the system may further comprise a solar panel, for example a photovoltaic device, for converting solar energy into electrical energy. In one example in which the water desalination system is powered by wind energy, the system may further comprise a wind turbine, for converting wind energy into electrical energy. Alternatively, in the example in which the water desalination system is powered by wave power, the system may further comprise a wave power device, for example a point absorber buoy, an oscillating water column or an oscillating wave surge controller, to convert wave power into electrical energy. Alternatively, in one example in which the water desalination system is powered by wind energy, the system may comprise a windpump, i.e. a windmill which converts wind power directly into mechanical energy for pumping water. Alternatively, in one example in which the water desalination system is powered by wave power, the system may comprise a wavepump, i.e. a pump which uses the oscillating movement from the waves to directly pump water from the ocean. The water desalination process carried out in the water desalination system requires no energy input beyond that provided by the renewable energy source, as is explained below. The water desalination process is operated as a continuous process, despite the fluctuations in the availability of the renewable energy source. That is, even at times of low to zero renewable energy availability (e.g. at nighttime in the example of solar energy), the water desalination process is still operational, due to the presence of the elevated water storage unit.

Unless otherwise stated, the term high energy availability and the term low energy availability in relation to harnessing solar power as the renewable energy source are to be understood as meaning times of daylight, with periods of low energy availability specifically referring to periods after dawn or before dusk. Furthermore, and unless otherwise stated, the term zero energy availability in relation to harnessing solar power as the renewable energy source is to be understood as meaning times of zero solar irradiance, i.e. nighttime.

Unless otherwise stated, the term low energy availability in relation to harnessing wind power as the renewable energy source is to be understood as meaning periods when wind velocities are less than 10 m/s. Furthermore, and unless otherwise stated, the term high energy availability in relation to harnessing wind power as the renewable energy source is to be understood as meaning periods when wind velocities are from 10 m/s to 25 m/s. Although wind speeds in excess of 25 m/s are possible, wind turbines usually stop producing energy at these speeds for safety reasons. Furthermore, and unless otherwise stated, the term zero energy availability in relation to harnessing wind power as the renewable energy source is to be understood as meaning times of zero wind, i.e. still air.

Unless otherwise stated, the term low energy availability in relation to harnessing wave power as the renewable energy source is to be understood as meaning periods when wave power is less than 15 kW/m. Furthermore, and unless otherwise stated, the term high energy availability in relation to harnessing wave power as the renewable energy source is to be understood as meaning periods when wave power is at least 15 kW/m. Furthermore, and unless otherwise stated, the term zero energy availability in relation to harnessing wave power as the renewable energy source is to be understood as meaning times when no waves are present, i.e. flat water.

Pump

The pump may be any pump which can be used to displace water to be desalinated from a water source into the water desalination system and to the water storage unit and the reverse osmosis membrane unit. The pump may be a mechanical pump or an electrical pump. For example, the pump may be an electrical pump powered by electrical energy converted from a renewable energy source such as solar power or wind power. Alternatively, the pump may be a mechanical pump in the form of a windpump in which the wind vanes of the windpump directly convert the wind power into mechanical motion. Alternatively, the pump may be a mechanical pump in the form of a wavepump which uses the oscillating movement from the waves to directly pump water from the ocean. An example of a wavepump is the SEADOG® pump

In the example in which the pump is an electrical pump and is powered by electrical energy converted from a renewable energy source, the pump may be associated with a power controller or inverter to convert the direct current generated from the renewable energy into alternating current to power the pump.

The pump may be a submersible pump and be submerged into the water source. Alternatively the pump may be located above the surface of the water source. It will be appreciated by those skilled in the art that the pump can be selected depending on the required power output of the pump in order to drive the water desalination system. In one embodiment, the pump is in fluid communication with an elevated water storage unit. The water desalination system may comprise a one-way non-return valve located downstream from the pump, to prevent undesired flow of water from the elevated water storage unit to the pump. Reverse osmosis membrane unit

The reverse osmosis membrane unit may be any device known in the art which uses a semi-permeable membrane and an applied pressure to overcome osmotic pressure and thus desalinate water. Only a part of the saline feed water pumped into the membrane assembly passes through the membrane with the salt removed. The remaining "concentrate" flow passes along the saline side of the membrane to flush away the concentrated salt solution. The percentage of desalinated water produced versus the saline water feed flow is known as the "recovery ratio". This varies with the salinity of the feed water and the system design parameters. The concentrate flow is at typically only up to 3 bar / 45 psi less than the feed pressure, and thus still carries much of the high pressure pump input energy.

The required inlet pressure for the reverse osmosis membrane unit is generated at least in part by the pump, although the high pressure concentrate from the reverse osmosis membrane unit can be utilised to drive the energy recovery device, as described below to reduce the energy requirements of the pump. Furthermore, the inlet pressure of the reverse osmosis membrane unit may be provided by the elevated water storage unit, also as described below.

Energy recovery device The energy recovery device may be any device which harnesses the high pressure of the RO membrane unit concentrate flow to intensify the feed pressure into the RO membrane unit of the water to be desalinated. In one example, the energy recovery device is a mechanical device. In one example the energy recovery device requires no auxiliary power. The energy recovery device may be termed an energy intensifier, and can be used to increase the feed pressure to the reverse osmosis membrane unit from 10 bar to a required pressure of, for example, 40 to 60 bar for operation of the reverse osmosis membrane unit in the case of seawater desalination, thus reducing the overall energy and elevation difference requirement for the system. In one example, the energy recovery device is or comprises a piston-based energy recovery device. Alternatively, the energy recovery device may be or comprise a turbine-based energy recovery device. Alternatively, the energy recovery device may be or comprise an axial piston energy recovery device. Exemplary energy recovery devices are available from Danfoss or Flowserve.

Water storage unit

The water desalination system comprises a water storage unit which is at a greater elevation than one or both of the energy recovery device and the reverse osmosis membrane unit. The water storage unit can provide the water pressure required for the desalination process by the force of gravity. It may function also as the energy buffer when there are no renewable energy sources available to enable continuous operation of the desalination part.

The difference in elevation between the water storage unit and the pump, the reverse osmosis membrane unit and the energy recovery device is defined by the salinity of the water to be desalinated, and thus the inlet pressure for the reverse osmosis membrane unit. That is, the water storage unit may be elevated at a sufficient height that the hydrostatic pressure is sufficient to drive the reverse osmosis membrane unit.

In one example, the water storage unit is spaced less than 250 m above one or more of the pump, the reverse osmosis membrane unit and the energy recovery device. Alternatively, the water storage unit is spaced less than 120 m above one or more of the pump, the reverse osmosis membrane unit and the energy recovery device. Alternatively, the water storage unit is spaced less than 50 m above one or more of the pump, the reverse osmosis membrane unit and the energy recovery device. The degree of spacing of the water storage unit above one or more of the pump, the reverse osmosis membrane unit and the energy recovery device is dependent in part on the source of the water to be desalinated, or the level of salinity of the water to be desalinated. For example, desalination of brackish water, which has a lower salinity than sea water, may be achieved by the system described herein with the water storage unit being spaced less than 50 m above one or more of the pump, the reverse osmosis membrane unit and the energy recovery device. Alternatively, desalination of sea water may be achieved by the system described herein with the water storage unit being spaced less than 250 m above one or more of the pump, the reverse osmosis membrane unit and the energy recovery device. In the absence of the energy recovery device, the required elevation of the water storage unit above at least the reverse osmosis membrane unit will be much greater than if the energy recovery device is present. Thus, the energy recovery device allows for the water storage unit to be positioned closer to the rest of the desalination system.

The energy buffer in the form of the elevated water storage unit enables the process to be stationary and run continuously, which increases the lifetime of the reverse osmosis membranes and ensures maximum desalinated water generation as the membrane's capacity is fully used.

System location

The water storage unit may be elevated using natural elevation, i.e. hills or cliffs, or using man-made structures such as water towers. In one example, the water desalination system is located at sea level and the water storage unit is located above sea level.

Pre-filtration

The water desalination system may comprise a pre-filtration system to remove any solid matter, suspended impurities and/or biological organisms from the water to be desalinated. Such systems are known in the art and are advantageously incorporated into water desalination systems to prolong the lifetime of reverse osmosis membrane units.

Post-treatment Following desalination in the reverse osmosis membrane unit, the treated water may be optionally post-treated in order to render it suitable for human consumption, for example by addition of minerals. Example

The present invention will now be described, without limitation, with reference to the following Example, in which a water desalination system powered by solar energy is described. Photovoltaic panels 1 are installed at a suitable location. When there is solar irradiance available, solar power is converted into electric power. This generated electric power is transferred by electrical connection 2 to controller 3, where it is converted to AC power and transferred by electrical wire 4 to water pump 5. Water pump 5 is fluidly connected to the source water 6, which can contain various levels of salinity. At times of high renewable energy availability, i.e. at times of solar radiance, the water pump 5 is powered by the electrical energy generated by the photovoltaic panels and so will displace water. Water pump 5 is in fluid communication with elevated water storage unit 10 through conduits 7 and 9 (via T-splitter 8), and with energy recovery device 14 through conduits 7, 11 and 13. One outlet of energy recovery device 14 is fluidly connected to reverse osmosis membrane unit 16 by conduit 15 as an inlet for water to be desalinated, with outlet 17 from reverse osmosis membrane unit 16 taking the high pressure concentrate stream to drive energy recovery device 14 and intensify the feed pressure to reverse osmosis membrane unit 16. At times of renewable energy availability, i.e. solar irradiance, water is pumped from source 6 by pump 5 and travels through conduit 7 towards T-splitter 8. A portion of the water is pumped towards the desalination part (consisting of an optional pre-filtration step 12, energy recovery device 14, and reverse osmosis membrane unit 16). The remaining part of the solar pumped water is displaced towards the elevated water storage unit 10 through conduit 9.

When there is no solar irradiance available, the pump is no longer powered, and so the direction of the flow of tubing 9 will change. The water is now allowed to flow down towards the desalination part. Water pump 5 preferably contains a non-return valve to prevent the water from flowing back into the source 6. The difference in height between the level of the desalination part and the elevated water storage 10 will mostly determine the pressure at the desalination part. The amount of water entering the desalination part is determined by the combination of the inlet pressure (i.e. the elevation difference between elevated water storage unit 10 and the desalination part), the reverse osmosis membrane unit 16 and energy recovery device 14. This fixed flow allows enough water to be stored to cover periods of less irradiance and during nighttime. The flow entering the desalination part first enters the pre-filtration step 12, which could consist of for example a 5 micron filter. The water then enters the energy recovery device 14, with the remaining pressure caused by the elevated water storage 10 minus friction losses and pressure loss over the pre-filtration step 12. The produced permeate flow, containing the water from which most salt is removed, is collected by tubing 19 and either stored in a water tank or led to the water distribution system. The concentrate discharge 18, which contains the remaining salt, is disposed of at a suitable location.

While the systems and processes have been described with reference to certain examples, those skilled in the art will appreciate that various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the disclosure. It is intended, therefore, that the invention be limited by the scope of the following claims. The features of any dependent claim may be combined with the features of any of the other dependent claims or any and/or any of the independent claims.

Claims

1 . A water desalination system, comprising:

a reverse osmosis membrane unit in fluid communication with the pump;

2. The water desalination system of claim 1 , wherein the renewable energy is converted to electrical energy by one or more of a solar photovoltaic device, a wind turbine, or a wave power device, and wherein the electrical energy is used to power the pump.

3. The water desalination system of claim 1 , wherein the pump is part of a windpump or a wavepump.

4. The water desalination system of any one of claims 1 to 3, further comprising a non-return valve downstream of the pump.

5. The water desalination system of any one of the preceding claims, wherein the system further comprises a pre-filtration unit upstream of the reverse osmosis membrane unit and/or the energy recovery device.

6. The water desalination system of any one of the preceding claims, wherein the energy recovery device comprises a piston-based energy recovery device, a turbine- based energy recovery device or an axial piston energy recovery device.

7. The water desalination system of any one of the preceding claims, wherein the pump comprises a submersible pump or a surface pump.

8. The water desalination system of any one of the preceding claims, wherein the system is located at or above sea-level.

9. The water desalination system of any one of the preceding claims, wherein the water storage unit is spaced less than 250 m above one or more of the pump, the reverse osmosis membrane unit and the energy recovery device.

10. The water desalination system of any one of the preceding claims, wherein the water storage unit is spaced less than 120 m above one or more of the pump, the reverse osmosis membrane unit and the energy recovery device.

1 1 . The water desalination system of any one of the preceding claims, wherein the water storage unit is spaced less than 50 m above one or more of the pump, the reverse osmosis membrane unit and the energy recovery device.

12. The water desalination system of any one of the preceding claims, wherein the system is powered solely by renewable energy.

13. The water desalination system of any one of claims 1 to 1 1 , wherein a waste outlet of the reverse osmosis membrane unit is in fluid communication with an inlet of the energy recovery device.

14. A process for continuous desalination of water, comprising:

15. The process of claim 14, wherein the renewable energy is solar energy, wind energy or wave power.

16. The process of claim 14 or 15, wherein the water storage unit is at a greater elevation than one or both of the energy recovery device and the reverse osmosis membrane unit.

17. The process of any one of claims 14 to 16, wherein the water storage unit is spaced less than 250 m above one or both of the energy recovery device and the reverse osmosis membrane unit.

18. The process of any one of claims 14 to 17, wherein the water storage unit is spaced less than 120 m above one or both of the energy recovery device and the reverse osmosis membrane unit.

19. The process of any one of claims 14 to 18, wherein the water storage unit is spaced less than 50 m above one or more of the pump, the reverse osmosis membrane unit and the energy recovery device.

20. The process of claim 19, wherein the energy recovery device uses a waste concentrate stream from the reverse osmosis membrane unit to intensify the feed pressure to the reverse osmosis membrane unit of the water to be desalinated.

21 . The process of any one of claims 14 to 20, wherein the renewable energy is solar energy.

22. The process of claim 21 , wherein the times of low and high renewable energy availability correspond to hours of daylight.

23. The process of claim 21 , wherein the times of zero renewable energy availability correspond to hours of darkness.

24. The process of any one of claims 14 to 20, wherein the renewable energy comprises wind power.

25. The process of claim 24, wherein the times of high renewable energy availability correspond to wind velocities of from 10 m/s to 25 m/s.

26. The process of claim 24, wherein the times of low renewable energy availability correspond to wind velocities of less than 10 m/s.

27. The process of any one of claims 14 to 20, wherein the renewable energy comprises wave power.

28. The process of claim 27, wherein high renewable energy availability corresponds to wave power of at least 15 kW/m.

29. The process of claim 27, wherein low renewable energy availability corresponds to wave power of less than 15 kW/m.