The present invention relates to a desalination system. Salinisation of soil and groundwater is a widespread problem affecting significant areas of land in all inhabited continents. For inland areas having access to saline groundwater but to no other water resource, desalination can provide clean water for drinking and sanitation. Examples of countries where groundwater desalination systems are in use include the United States, Morocco, Egypt, Jordan, the United Arab Emirates, India and Australia. As with seawater, the desalination of groundwater requires energy. The need to satisfy the growing demand for desalinated water while reducing the environmental impact associated with the energy usage makes it important to the improve energy efficiency of the process and to make use of renewable energy such as solar energy where possible.

Of the many solar-driven desalination technologies that have been studied or implemented, with some exceptions most fall into one of two categories: (i) thermal distillation processes and (ii) pressure driven membrane processes ie. reverse osmosis (RO). In the first category, single effect solar stills can provide of the order of litres of water per square metre of captured sunlight per day while multiple effects solar stills may provide tens of litres per square metre.

Reverse osmosis is generally considered the more energy efficient method of desalination. It requires as input mechanical work as opposed to heat. Most of the solar-RO plants that have been built to date use photovoltaics (PV) to capture the sun's energy. For example, systems producing about 0.5 m /day of freshwater from groundwater at a salinity of 3000 ppm, using a 0.26 kWp PV generators occupying an area of 2 m 2 have been used. This corresponds to 250 litres/m 2 per day and is therefore an order of magnitude above that readily achievable with solar stills. At this small scale, PV is a reliable technology that is simple to implement. However, it does not benefit from significant economies or efficiencies of scale and this may explain why the largest system built produced only 76 m per day. For comparison, there exist several desalination plants powered by conventional fuels with capacities exceeding 100000 m ³ per day.

Instead of PV, solar thermal power-plant may be used to generate the work needed to drive the RO process; however, so far very few plant have been constructed. Notably, the plant constructed at El Hamrawin in Egypt in 1981 reportedly produced 130 litres per m of solar collector, from feedwater of salinity 3500 ppm. The thermodynamic cycle used was the Rankine cycle operating a steam turbine while the working fluid used was Freon-11 which is an ozone depleting substance now banned under the Montreal Protocol.

Most thermal solar power plants in use today are based on the steam Rankine cycle used in conjunction with a steam turbine and are currently used to generate electricity for feeding to the grid. The possibility of coupling to desalination equipment has been studied and could be implemented in the future. However, steam power plant (in direct contrast to PV) are most efficient and economical at large scales, typically >10 MW electrical output, which if used for desalination by RO would result in water outputs of 10 000 m day or more. Despite the continued growth of interest in solar- powered RO, there is still a lack of proven and viable solutions at the intermediate scale of roughly 50 to 5 000 m /day. For groundwater desalination in particular, such scales are needed since groundwater sources are by nature distributed, such that individual wells and aquifers may have limited capacity to supply water to the desalination system. Groundwater is typically less saline than seawater. On account of this, the energy requirement for desalination by RO is lower. On the other hand, desalination of groundwater usually requires higher recovery ratios. This is to avoid wasting the precious groundwater resources and to minimise the volume of concentrated brine. High recovery ratios tend to require higher energy inputs thus partially offsetting the energy saving from the low feed salinity. This trade off is illustrated by the following standard equation for the thermodynamic minimum energy E required to recover a volume V of freshwater from saline feedwater having an osmotic pressure of P _osm , at a recovery ratio r.

The conventional use of a Rankine-RO system relies on a turbine to expand the working fluid and drive the high pressure pump supplying saline water to the RO membrane. One reason for the unattractiveness of the steam Rankine cycle at the smaller power outputs is the inefficiency of small steam turbines, which is a result of blade friction loss and leakage loss. An alternative is to use a piston to expand the steam. Once this approach is adopted, it is logical to consider omitting the step of converting the linear motion of the steam power piston to rotary motion. Instead, a so- called steam pump may be used, in which the steam power piston is coupled directly to the piston of a reciprocating pump. However, a difficulty with steam pumps is that the force provided by the power piston varies as the piston moves over its cycle. To overcome this problem, US6470683 (Childs et al) describes a hydraulic unit to add to or subtract from the power output and thus compensate for cyclic variations. However, such a system is complicated in as much as it requires the additional hydraulic drive unit.

Thus an object of the present invention is to provide a desalination unit that is simpler in operation and/or cheaper to manufacture/operate and/or that can be used in remote areas that do not have the benefit of an electricity grid system.

Thus, according to the present invention there is provided a desalination system including a power piston slidably mounted in a power cylinder,

a water pump for supplying pressurised water to a reverse osmosis unit, the water pump including a pump piston slideably mounted in a pump cylinder thereby defining a pump volume, and

a coupling mechanism coupling the power piston to the pump piston, the coupling mechanism providing a relatively low mechanical advantage when the pump volume is relatively large and a relatively high mechanical advantage when the pump volume is relatively small. Advantageously by varying the mechanical advantage as described, a higher pressure is available when some of the water has been desalinated and the pressure required to desalinate the remaining water is higher. In one embodiment the water pump is driven by solar energy. Since the amount of power available from solar energy is limited, by changing the mechanical advantage as described, the system enables a higher production rate of desalinated water when using solar energy. According to another aspect of the present invention there is provided a system for separating a solvent from a solution the system including a power piston slideably mounted in a power cylinder,

a pump for supplying a pressurised solution to a semi-permeable membrane the semi-permeable membrane allowing the passage of the solvent, the pump including a pump piston slideably mounted in a pump cylinder thereby defining a pump volume, and a coupling mechanism coupling the power piston to the pump piston, the coupling mechanism providing a relatively low mechanical advantage when the pump volume is relatively large and a relatively high mechanical advantage when the pump volume is relatively small. In one embodiment the solvent may be a polar solvent. The polar solvent may be water or alcohol. The solute in the solution may be salt. The salt may be sodium chloride or the salt may be potassium chloride.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic view of a desalination system according to the present invention,

Figure 2 is an enlarged view of part of figure 1,

Figure 3 is a plan view of part of figure 1,

Figure 4 is a second embodiment of a desalination system according to the present invention, and

Figure 5 is a third embodiment of a desalination system according to the present invention. With reference to figures 1 to 3 there is shown a desalination system 10 including expansion units (also known as power units) 12, 14 and 16, a water pump 18 and a crank mechanism 20.

Consideration of figures 2 and 3 show the expansion cylinders, water pump and crank mechanism in more detail.

The power unit 12 includes an expansion cylinder 30 (defining an expansion cylinder axis 31) within which is slideably mounted a power piston 32. The power piston is connected to a piston rod 34 which is linearly slideable in bearings 36 and 37 mounted on the chassis 11 of the desalination system 10. The cylinder 30 is mounted on bulkhead 22. Bearing 36 forms a seal and hence bulkhead 22, piston 32 and part of cylinder 30 define an under piston volume 24.

The water pump 18 includes a pump cylinder 40 (defining a pump cylinder axis 41) within which is slideably mounted a pump piston 42. Connected to pump piston 42 is a piston rod 44 which is slideably mounted linearly in bearings 46 and 47 of the chassis 11.

The piston rod 34 is connected to the piston rod 44 by the crank mechanism 20. In this case the crank mechanism 20 includes a linkage 38, a linkage 48 and a crank 50. Crank 50 is rotatably mounted (about crank axis 51) via a bearings 52 and 53 on the chassis 11. The crank 50 has a first arm 54, one end of which is pivotally mounted via a pivot 55 to one end of the linkage 38. The crank includes a second arm 56 which is pivotally connected to linkage 48 at one end via pivot 57.

End 38A of linkage 38 is pivotally connected to piston rod 34 and end 48A of linkage 48 is pivotally connected to piston rod 44. The crank 50 includes a crank shaft 58. The power units 14 and 16 includes respective expansion cylinders and power pistons. Associated with power unit 14 is piston rod 134, bearing 136, bearing 137, linkage 138 and first arm 154. Associated with power unit 16 is piston rod 234, bearing 236, bearing 237, linkage 238 and first arm 254.

First arms 54, 154 and 254 and second arm 56 are all rotatably fast, i.e. rotatably fixed to the crank shaft 58.

In summary, and as best understood from figure 1, sunlight generates high pressure steam which is used in the power units 12, 14 and 16 to reciprocally move the associated power pistons. The power pistons are connected to associated piston rods which in turn are connected via associated linkages to first arms 54, 154 and 254. Thus, reciprocal motion of the pistons causes reciprocal rotation of the crank shaft 58. This in turn causes reciprocal motion of the second arm 56 which in turn causes reciprocal motion of the pump piston 42 (via reciprocal motion of the linkage 48 and piston rod 44). Movement of the pump piston 42 to the right when viewing the figures causes saline water to be pressure fed to the reverse osmosis (RO) unit 60 causing the saline ground water to be separated into fresh water and concentrate. Movement of the pump piston 42 to the left when viewing the figures causes the pump cylinder to be refilled with saline ground water. In particular the arrangement of the power units 12, 14 and 16 coupled to the water pump 18 via the crank mechanism 22 provides for a particularly beneficial varying mechanical advantage which will be described further below.

In more detail, a feed pump 70 pumps a working fluid 71, in this case water, into receiver tubes 72 of a solar steam generator 73. Sunlight 74 is reflected from mirrors 75 onto the receiver tube 72 thereby heating the water within to generate steam which is held in a steam reservoir 76 at pressure XI. As required, the steam from steam reservoir 76 is fed into power unit 12 to generate work and is then exhausted from power unit 12 and fed into steam reservoirs 77 at pressure X2 (lower than pressure XI). As required steam from steam reservoir 77 is fed into power unit 14 to do work and then exhausted therefrom into steam reservoir 78 at pressure X3, lower than pressure X2. The steam is then fed into power unit 16 as required and then exhausted therefrom into condenser 79 where the steam condenses into water. The water is then pumped via feed pump 70 into receiver tube 72 to continue the cycle.

Saline ground water is pumped from well 80 by feed pump 81 through the condenser where it used to help condense the working fluid.

The pipe circuitry is best seen in figure 1 and includes pipes PI, P2, P3, P4, P5, P6 and P7. Pipe PI includes a valve V3, pipe P5 includes valve V5, pipe P6 includes valve V4 and pipe P3 includes recirculation pump 82.

Operation of the desalination unit is as follows. For the purposes of explanation, the initial starting conditions are with steam reservoirs 76, 77 and 78 containing steam at pressures XI, X2 and X3 respectively. The power piston 32 is positioned at its leftmost position when viewing figure 2, i.e. the volume (also known as power volume) defined by the power piston and the power cylinder is at a minimum. Due to the crank mechanism, the pump piston 42 is also at its left-most position and the pump volume, i.e. the volume defined by the pump piston and pump cylinder is at a maximum. The pump is therefore full of saline ground water. Valve V3 is closed, valve V5 is closed and valve V4 is open. Recirculation pump 82 is in operation.

Valves VI, VI' and VI" are opened thereby admitting steam into the power units 12, 14 and 16 respectively. The steam causes the pistons of the power units 12, 14 and 16 to move to the right causing the crank shaft 58 to rotate clockwise when viewing figure 2 which in turn moves the pump piston 42 to the right. Because valves V3 and V5 are closed, the pressure in pipes P2, P3, P4 and P6 increases, in particular to above the threshold pressure level at which the RO unit 60 operates. Under these circumstances fresh water will flow through pipe P7 to be collected.

Whilst fresh water is being generated, recirculation pump 82 circulates the saline ground water from the RO unit through the water pump 18 and back to the RO unit. This reduces concentration polarisation near the semi-permeable membrane of the RO unit. When the pump piston 42 reaches its rightmost position, valve V4 is closed, valve V5 is opened and valve V3 is opened. Feed pump 81 is operated and this flushes the concentrate from pipes P3, P4, P5 and from the RO unit. Once flushing has occurred, valve V5 is closed, valve V4 is opened (valve V3 remains open). Valves VI, VI' and VI" are all closed, valves V2, V2' and V2" are all opened and the pressure on either side of the power piston is equalised by venting the power volume to the under piston volume by a vent arrangement (not shown). The pressure generated by the pump 81 then forces the piston pump 42 to the left which in turn causes the crank mechanism 20 to, in turn, move the power pistons to the left.

Valve V3 is then closed, valves V2, V2' and V2" are closed, valves VI, VI' and VI" are opened and the under piston volume is isolated from the power volume so as to continue the cycle.

As mentioned above, the crank mechanism 20 provides a particularly beneficial system of varying the mechanical advantage between the power units 12, 14 and 16 and the water pump 18. This is best understood by considering a second embodiment of the invention as shown in figure 4.

Figure 4 shows a desalination system 310 with components that fulfil the same function as those in desalination system 10 labelled 300 greater.

Valves VI, V2, V3, V4 and V5 (as shown in Figure 4) fulfil the same function as their equivalently labelled valves (as shown in figure 1). In this case, the crank mechanism 320 includes a crank 350 having a single arm 390. Linkage 338 directly connects the piston 332 with an end of arm 390 (i.e. the piston rod 34 as shown in figure 1 has been dispensed with). Linkage 348 directly connects the pump piston 342 with an end of the arm 390, (i.e. piston rod 44 as shown in figure 1 has been dispensed with). In effect, linkage 338 and linkage 348 are connecting rods connecting their associated pistons to a common point on the crank 350. With the crank at angle θ _1; the power volume is at a minimum and the pump volume is at a maximum. With the crank at angle θ ₂ , the power volume is at a maximum and the pump volume is at a minimum. The steam is supplied from a high pressure steam reservoir to the power unit 312 and expands against the power piston. This piston drives the pump piston which is used to pressurise saline water against the semi-permeable membrane of the RO unit 360 that allows freshwater to pass while retaining the salt. As the steam expands, its pressure will decrease, typically according to the well- known polytropic expression:

— constant^

where n is a constant typically having a value of 1.135 for wet steam and 1.3 for superheated steam. As water is expelled from the saline solution, the concentration of salt will increase and thus will the osmotic pressure according to the Van't Hoff relation:

Py¥ _v - cvttalu-tti _t

Consequently, as the power piston 330 moves to the right and as pump piston 342 moves downwards, the force F _x available from the power piston decreases while the force F _y needed to move the pump piston increases. The crank mechanism provides an increasing mechanical advantage as the pistons move. From the equilibrium of moments on the crank, it is evident that the mechanical advantage of the system when the crank is at an angle Θ to the horizontal is given by:

^ = tan #

Therefore the mechanical advantage increases as the crank rotates clockwise and Θ increases. For example, Θ = 10° gives a mechanical advantage of 0.176 increasing 32- fold to 5.67 at Θ = 80°. Returning to desalination system 10, it is evidence that the mechanical advantage of the system on the crank at an angle Θ is given by F _y /F _x = tan Θ. Therefore the mechanical advantage increases as the crank rotates clockwise and Θ increases. For example, Θ = 10° gives a mechanical advantage of 0.176 increasing 32 fold to 5.67 at 9 = 80°.

Variations of the invention will readily be apparent to those skilled in the art. Thus, any number (including just 1) power units could be used to power the desalination system of figures 1 and 4. The desalination system 10 could use connecting rods to connect the power pistons directly to the crank arms. The desalination system 310 could use a combination of piston rods and linkages (as per desalination system 10) to connect the power piston to the crank or to connect the pump piston to the crank. The linkages 38 and 48 could be connected to a common point on the crank mechanism 50 (in a manner similar to the desalination system 310). The power piston and pump piston of desalination system 310 could be connected to separate arms of the crank mechanism (in a manner similar to desalination system 10).

As mentioned above, when the pump cylinder is refilled using feed pump 81, the power volume and under piston volume are equalised. In an alternative embodiment, the under piston volume could be pressurised (i.e. the power unit could have a double acting piston) so as to reduce the power volume and increase the pump volume, thereby causing the pump volume to fill with ground water. Under these circumstances it may be possible to operate the system without a feed pump. Expansion cylinder 330 defines an expansion cylinder axis 331. Pump cylinder 340 defines a pump cylinder axis 341. Consideration of figure 4 shows that expansion cylinder axis 331 is at 90° to pump cylinder axis 341, though in further embodiments these axes could be angled relative to each other at any angle greater than 0° and less than 180°. However, typically expansion cylinder axis may be angled between 45° and 135° relative to the pump cylinder axis.

It will also be appreciated from figure 4 that the crank axis 351 is positioned on the expansion cylinder axis and is also positioned on the pump cylinder axis. In further embodiments a crank axis need not be positioned on the expansion cylinder axis. In further embodiments the crank axis need not be positioned on the pump cylinder axis.

In the example of figure 4 above, the crank arm 390 reciprocates between a minimum Θ angle (Θ of 10° and a maximum Θ angle (θ ₂ ) of 80°. This gives a mean Θ angle of 45°, i.e. half of 90°, which is the angle at which the expansion cylinder axis 331 is angled relative to the pump cylinder axis 340. Because the linkage 338 is the same length as the linkage 348, then under these circumstances the total stroke of power piston 332 is the same as the total stroke of pump piston 342 (excepting of course that the mechanical advantage varies during the stroke). In further embodiments, by changing the mean Θ angle the total stroke of the power piston 332 may be greater or less than the total stroke of the pump piston 342.

Figure 5 shows a desalination system 410 with components that fulfil the same function as those as desalination system 10 labelled 400 greater. For ease of explanation, the input and output couplings of the power unit 412 and water pump 418 and their associated pipe work has not been shown, though one skilled in the art would appreciate that it is similar to that shown in respect of desalination system 10 or desalination system 310. A comparison of figures 2 and 5 show that in figure 5 the expansion cylinder axis 431 is positioned at 90 degrees to the pump cylinder axis 441, whereas in figure 2 these axis are in line. Crank axis 451 is not in line with the expansion cylinder axis 431, nor is it in line with the pump cylinder axis 441 (in figure 2 the crank axis 51 is in line with both the expansion cylinder axis 31 and the pump cylinder axis 41). As shown in figure 5, the distance Rl between the crank axis 451 and the pivot 455 is less than the distance between the crank axis 451 and the pivot 457. Consideration of figure 2 shows a distance between the crank axis 51 and pivot 55 is the same as the distance between crank axis 51 and pivot 57. Thus, in figure 5 as the crank 450 reciprocates, pivot 457 travels a greater distance than pivot 455. This difference in distances Rl and R2 inherently creates a mechanical advantage which means that pump cylinder 440 can have a longer stroke and narrower bore when compared with expansion cylinder 430 (assuming the crank 50 reciprocates about a mean Θ angle of 45°). Whilst primarily directed towards desalination, the invention is also applicable to separating a solvent from a solution using a semi-permeable membrane. For example any solution which can have its solvent separated from it by applying the solution to one side of the semi-permeable membrane under pressure and allowing the solvent to pass through the semi-permeable membrane and then collecting the solvent can be used in the present invention. The solvent may be a polar solvent or a non-polar solvent. Where the solvent is a polar solvent it may be water or alcohol. The solute in the solution may be a salt. Where the solute is a salt, salt may be sodium chloride or salt may be potassium chloride.