WO2014128543A1

WO2014128543A1 - Desalination apparatus using nanofluid as heat carrier from solar collector

Info

Publication number: WO2014128543A1
Application number: PCT/IB2014/000131
Authority: WO
Inventors: Donald Earl SPENCE; Cody Jason SPENCE
Original assignee: Spence Donald Earl; Spence Cody Jason
Priority date: 2013-02-22
Filing date: 2014-02-10
Publication date: 2014-08-28
Also published as: GB2511075A; GB201303142D0

Abstract

Desalination apparatus (10) is provided for removing impurities from an impure liquid, and comprises a vacuum chamber (14) including a vaporization compartment (70) and a condensation compartment (72), and a solar heater (12) for raising the temperature of the impure liquid. The condensation compartment (72) includes a cooling element (78) for causing condensation of vapour and the solar heater (12) includes a solar-energy collector (26) for absorbing solar radiation and a heat exchanger (28) for transferring heat to the impure liquid. The solar-energy collector (26) and the heat exchanger (28) are fluidly interconnected in a heating circuit containing a heating fluid which includes a suspension of nanoparticles in a liquid. A system and method are also provided.

Description

DESALINATION APPARATUS USING NANOFLUID AS HEAT CARRIER FROM SOLAR COLLECTOR

The present invention relates to desalination apparatus, and more particularly but not necessarily exclusively to desalination apparatus for removing salt from sea water or brackish water. Desalination provides a fresh water supply in areas where fresh water is otherwise unavailable, but where a plentiful supply of salt or brackish water may be found. For example, desalination is used in areas of the world with low rainfall, and onboard ocean-going ships which are at sea for long periods.

Desalination is typically an energy-intensive means of producing fresh water. The most common desalination technique is reverse osmosis, which involves the use of pumps to force water through a membrane. The pumps are typically powered by electricity, which is primarily derived from burning fossil fuels.

An alternative desalination technique works by distillation. That is, salt water is made to evaporate, and the vapour is then condensed. The condensate is desalinated water. Distillation is generally more energy intensive than reverse osmosis. However, distillation is able to make use of waste heat at low temperatures which cannot be used to generate electricity efficiently. In particular, solar energy may be used to heat saline water for distillation.

Solar water heating is advantageous due to the low environmental impact of the energy source. However, to heat water to a useful temperature at a reasonable rate requires very large solar panels. A reduction in panel size results in either a lower temperature of water, requiring more energy input from polluting sources, or a lower rate of flow through the panel, reducing the output of a solar desalination plant.

It is an object of this invention to provide solar distillation, and more particularly desalination, apparatus which reduces the above mentioned problems.

According to a first aspect of the present invention, there is provided desalination apparatus for removing impurities from an impure liquid comprising: a vacuum chamber including a vaporization compartment and a condensation compartment having a cooling element for condensing vapour; a solar-energy collector for absorbing solar radiation; and a heat exchanger for transferring heat energy absorbed by the solar- energy collector to the impure liquid, so as to raise a temperature of the impure liquid, the solar-energy collector and the heat exchanger being fluidly interconnected in a heating circuit having a heating fluid which is or includes a heat energy absorbent nanofluid.

The impure liquid may be salt water or brackish water, with the product of the desalination apparatus preferably being fresh water.

In use, the impure liquid is heated by the solar heater, typically to 38°C or to around 38°C. The vaporization compartment of the vacuum chamber is at low pressure, typically around 137mbar. At this temperature and pressure, water boils.

The vacuum chamber may be maintained at a low pressure by means of a vacuum pump, which is typically powered by electricity. It is therefore advantageous to heat the liquid to as high a temperature as is possible, in order to increase the vapour pressure of the liquid, consequently reducing the energy consumed by the vacuum pump.

Suspensions of nanoparticles in a liquid, or nanofluids, absorb solar radiation with very high efficiency. By using a nanofluid as the heating fluid in the solar heater, the impure liquid is raised to a greater temperature than with the use of conventional heating fluids. This reduces the amount of non-solar energy required to operate the apparatus. The heat exchanger may include a heated liquid outlet fluidly connected with a heated liquid inlet to the vacuum chamber, for transferring heated liquid from the heat exchanger into the vacuum chamber. In other words, the liquid may be heated in the heat exchanger before being vaporized in the vacuum chamber. The heated liquid will flash vaporize as it passes into the low-pressure vacuum chamber. The heat exchanger may be provided integrally with the solar-energy collector, or may be a separate unit.

Alternatively, the heat exchanger may be provided inside the vacuum chamber. In other words, the liquid may pass into the vacuum chamber before being heated. The solar-energy collector may be in the form of a panel, and may include a base, at least one side, a heat absorption surface and at least one baffle disposed within the panel substantially parallel with the heat absorption surface. The baffle may include an aperture near to a side of the panel, creating a tortuous flow path for the heating fluid from a side of the panel, across substantially the entire area of the collector beneath the baffle, and across substantially the entire area of the collector above the baffle.

Alternatively, the baffle may be substantially perpendicular to the heat absorption surface, creating a tortuous flow path for the heating fluid, the heating fluid passing into the solar-energy collector to one side of the baffle, through the aperture, to the other side of the baffle, and then out of the solar-energy collector.

Preferably, the aperture may be in the form of a slot.

The solar-energy collector may be rectangular, circular, or any other shape. Where the solar-energy collector is rectangular, a baffle parallel with the heat absorption surface may adjoin three of the four sides of the panel, the space between the baffle and the fourth side of the solar-energy collector defining a slot. Liquid may therefore flow across a length of the solar-energy collector below the baffle, then up through the slot and back across the same length of the solar-energy collector, above the baffle. Alternatively, the liquid may flow above the baffle before flowing below the baffle.

Where the solar-energy collector is circular, the slot may be in the shape of a segment or a crescent. Again, the aperture may be provided between the baffle and the side of the solar-energy collector.

Providing a baffle parallel with the heat absorption surface increases the efficiency of the solar-energy collector, because it allows solar radiation which is not absorbed by the heating fluid running across the top of the baffle to be absorbed by heating fluid underneath the baffle. Where fluid runs underneath the baffle and then above the baffle, the effect is that the fluid is pre-heated underneath the baffle before the main heating stage above the baffle. This increases the temperature of the heating fluid after it has passed through the panel, whilst maintaining a high flow rate through the panel. Providing baffles to separate layers of heating fluid ensures that substantially all of the available solar radiation is absorbed, and is preferable to simply increasing the thickness of the layer of heating fluid passing through the solar-energy collector. With a thick layer of heating fluid in the panel, warmed heating fluid would convect to the top of the layer, cooler fluid at the bottom of the layer not being exposed to the sun and therefore remaining cool. Using baffles to separate the flow ensures that the entire volume of liquid flowing out of the solar-energy collector is heated to substantially the same temperature.

The heat absorption surface may be, for example, a glass panel. It may alternatively be a surface of the heating fluid which is directly exposed to the sun, although some form of barrier is preferred as contamination of the heating fluid is thus avoided. A plurality of baffles may be provided, the baffles being disposed substantially parallel with each other and adjacent baffles having apertures near to opposing sides of the absorption panel. Where the solar-energy collector is circular or elliptic and therefore has only one side, apertures near to opposing sides should be taken to mean apertures which oppose each other across the circular solar-energy collector. Providing multiple baffles further reduces the above described problem of uneven heating, whilst ensuring that all of the solar radiation available is utilised to the maximum possible effect.

The baffles may be made from glass. Glass allows solar radiation to pass through for absorption by the heating fluid, but prevents heat from leaving the panel by convection. The vaporization compartment and the condensation compartment of the vacuum chamber may be divided by a wall, the height of the wall being less than the height of the vacuum chamber. Vapour may therefore pass over the top of the wall, from the vaporization compartment to the condensation compartment.

A nozzle may be provided for discharging impure liquid into the vacuum chamber, the nozzle being positioned to discharge in a downward direction at a point within the vaporization compartment below the level of the top of the dividing wall. Where the liquid is heated before it enters the vacuum chamber, it is flash vaporized on entry to the vacuum chamber. Solid particles of impurities— for example, salt— will therefore be discharged from the nozzle. By placing the nozzle so as to discharge in a downward direction below the level of the dividing wall, the possibility of impurities being introduced into the condensation compartment is eliminated.

The condensation compartment may be divided into a condensing unit and a condensate storage area by a gas-tight seal. For example, a trap may be provided to allow passage of condensate from the condensing unit to the condensate storage area, but to prevent vapour from entering the condensate storage area.

It is possible that small solid particles of impurity may be carried by the vapour. It is therefore advantageous to reduce as far as possible the surface area of condensate which is exposed directly to the vapour, to reduce re-contamination and improve the purity of the output from the apparatus.

A holding cistern may be provided for storing a supply of impure liquid. The impure liquid may flow from the cistern under gravity. The cistern therefore allows the desalination apparatus to operate for a period of time when an inlet pump, for example, pumping salt water from the sea, is not operating. This ensures that a purified liquid supply can be maintained in the event of pump failure. In addition, it allows the inlet pump to be operated only at certain times of day when cheap electricity is available, for example during off-peak hours or when the weather conditions are such that output from wind turbines and photovoltaic cells is high.

The holding cistern may include a pre-heater for raising the temperature of the impure liquid prior to it entering the solar heater. The pre-heater may itself be a solar heater, and may be a glass panel on top of the cistern for admitting solar radiation into the cistern. It is also envisaged that water in the cistern may be pre-heated by waste heat from, for example, air conditioning systems.

A bleed outlet may be provided between the heated liquid outlet of the solar heater and the heated liquid inlet of the vacuum chamber for diverting liquid out of the apparatus, bypassing the vacuum chamber. This allows inadequately heated water to be flushed from the system, for example in the early morning when the solar heater is not yet heating the water to a useful temperature. IB2014/000131

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A liquid waste outlet may be provided in the vaporization compartment of the vacuum chamber, for disposing of excess liquid.

Where the liquid is being heated before being passed into the vaporization compartment, there should not be much liquid present in the vaporization compartment when the system is operating normally. If, however, the vacuum chamber is not at a low enough pressure or the liquid is not at a high enough temperature, then liquid may build up in the vaporization compartment and will need to be removed. Even at the correct operating temperature and pressure, some liquid vapour may condense against the walls of the vaporization compartment. A level sensor may be provided which activates a waste disposal valve and/or pump in the event that the liquid level in the vacuum chamber rises above a certain point.

A purified liquid outlet may be provided in the vacuum chamber for discharging condensate from the condensate section. Where the condensation compartment is divided into a condensing area and a condensate storage area, the purified liquid outlet may be provided as an outlet from the condensate storage area.

The cooling element may include a cooling pipe containing a cooling fluid. The cooling fluid may be circulated through the cooling pipe for absorbing heat from vapour in the vacuum chamber, and may be circulated through a cooling reservoir for removing heat from the cooling fluid. The cooling fluid may be continually circulated between the cooling element and the cooling reservoir in a closed circuit. Alternatively, cooling fluid may be pumped from, for example, the sea, and heated coolant returned to the sea.

The vacuum chamber may be substantially cylindrical and the vaporization compartment may be substantially cylindrical, the vaporization compartment having a diameter less than the diameter of the vacuum chamber and being disposed within the vacuum chamber, the condensation compartment being provided between the outside of the cylindrical vaporization chamber and the outside of the cylindrical vacuum chamber. The vaporization compartment may be disposed substantially at the centre of the vacuum chamber, the condensation compartment being in the shape of a cylindrical shell, the two compartments thus being concentric with each other. Providing a vaporization compartment in the centre of the vacuum chamber and a condensation compartment around the periphery of the vacuum chamber is a convenient arrangement, as the condensation compartment completely surrounds the vaporization compartment. This helps to ensure that a large proportion of the vapour is condensed in the condensation compartment, the amount of vapour which loses heat and condenses in the vaporisation section being kept small. It is advantageous to the efficiency of the system to reduce the amount of condensation which occurs in the vaporisation section, since the heat in this liquid is wasted, especially where the liquid is heated prior to its entry into the vacuum chamber and there is no heating element in the vacuum chamber.

Where the condensation compartment surrounds the vaporization compartment as described above, the cooling pipe may be in a loop, multiple concentric loops, or a spiral around the outside of the cylindrical vacuum chamber, at or adjacent to the top of the condensation compartment.

An auger may conveniently be provided for removing solid waste from the vaporization compartment of the vacuum chamber. The auger is preferably provided on or adjacent to the base of the vaporization compartment of the vacuum chamber. According to a second aspect of the invention, there is provided an impure water desalination system comprising desalination apparatus in accordance with the first aspect of the invention, and a source of salt or brackish water.

According to a third aspect of the invention, there is provided a method of desalinating seawater using desalination apparatus in accordance with the first aspect of the invention, the method comprising the steps of: exposing a heat-absorbent nanofluid to solar radiation; pre-heating salt water or brackish water by transferring heat energy of the heat-absorbent nanofluid thereto; reducing the pressure of the pre-heated salt water or brackish water, so that it vaporises wherein the salt separates; and condensing the remaining vapour to form fresh water condensate. According to a fourth aspect of the invention, there is provided desalination apparatus for removing impurities from an impure liquid, the apparatus comprising a solar heater for pre-heating an impure liquid to be distilled, a vacuum chamber for receiving the preheated impure liquid, and pressure reducing means for reducing a pressure of the preheated impure liquid at the vacuum chamber, the solar heater including a solar-energy collector having a heat transfer liquid which comprises a suspension of nanoparticles for heat absorption, and a heat exchanger in liquid communication with the solar-energy collector for receiving the impure liquid for pre-heating, and the vacuum chamber including a vaporisation compartment for receiving the pressure reduced pre-heated impure liquid as a vapour, and a condensation compartment which is spaced from the vaporisation compartment for condensing the vapour.

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example only to the accompanying drawings, in which:

Figure 1 shows a schematic of a desalination apparatus according to the invention; and

Figure 2 shows an enlarged schematic side view of a vacuum chamber forming part of the desalination apparatus of Figure 1.

Referring firstly to Figure 1, a desalination apparatus is shown generally at 10. This embodiment is designed for removing salt from sea water on an industrial scale, although it will be appreciated that the invention may be used for removing other impurities from other liquids. The desalination apparatus comprises a solar heater outlined by box 12, a vacuum chamber 14, a storage cistern 16 and a cooling reservoir 18.

In use, seawater is pumped from the sea 100 into a storage cistern 16 by a first seawater pump 50. The first seawater pump 50 does not need to operate continuously, as long as storage cistern 16 is kept topped up with seawater. The seawater pump 50 may therefore be activated when energy is available from intermittent renewable sources, such as wind turbines or photovoltaic cells, or at times of the day when electricity is cheaper. Seawater is pumped from the storage cistern 16 into the solar heater 12 by a second seawater pump 52. When the cistern 16 is full enough to provide adequate pressure at the base of the cistern 16, the water will be able to flow under gravity and the second pump 52 may be bypassed, reducing the energy required to operate the system. After the water is heated by the solar heater 12, it flows into the vacuum chamber 1 via pressure-reducing valve 21. The vacuum chamber 14 is kept at a low pressure by means of a vacuum pump 54. In this embodiment, the pressure is typically at or around 137mbar, but may be increased or reduced depending on the amount of solar energy available from the sun. The solar-heated water vaporises upon entry to the vacuum chamber 14, and is then condensed within the vacuum chamber 14. The condensate 94 is fresh water, which may be removed from the vacuum chamber 14 via a fresh water outlet 20. If necessary, the fresh water may be pumped by a fresh water pump 56.

A bleed valve 22 is preferably provided in the flow path between the solar heater 12 and the vacuum chamber 14. The bleed valve 22 may be opened to remove inadequately heated water from the system. The inadequately heated water is fed into a bleed tank 24, from where it may be reintroduced into the desalination apparatus 10 or simply dumped back into the sea 100.

A cooling fluid is circulated between a condensing unit 88 within the vacuum chamber 14 and a cooling reservoir 18. The cooling fluid is circulated by a cooling pump 58. In the embodiment shown, the cooling fluid is circulated in a closed circuit between the cooling reservoir 18 and the vacuum chamber 14. The volume of the cooling reservoir 18 is approximately equal to the volume of fresh water to be produced daily by the desalination apparatus 10. The solar heater 12 includes a solar-energy collector provided as a solar panel 26, a heat exchanger 28, and a heating fluid pump 60. The solar panel 26, heat exchanger 28 and heating fluid pump 60 are connected in a closed heating circuit, and the heating fluid pump 60 circulates a heating fluid in that closed circuit. The heating fluid comprises a nanofluid, which is a suspension of nanoparticles in a liquid. The solar panel 26 is preferably cuboidal, having four sides, first and second sides 44, 46 being shown in Figure 1, a base 30 and a roof 32. Five glass baffles 34, 36, 38, 40, 42 are provided within the panel 26, parallel with the base 30 and the roof 32. Three of the glass baffles 34, 38, 42 abut three of the sides of the solar panel 26, being disconnected from the first side 44 of the panel 26. The other two glass baffles 36, 40 are disconnected from the second side 46 of the panel which opposes first side 44. In this way, a tortuous flow path is created through the panel 26 which passes over the base 30 of the panel 26 and over each baffle 34, 36, 38, 40, 42. The heating fluid circulating in the heating circuit therefore passes across the panel six times in its journey from an inlet 31 at or adjacent to the base 30 to an outlet 33 at or adjacent to the roof 32. The roof 32 of the panel 26 is preferably made from glass or other radiation- transmissible material.

Referring now to Figure 2, a detailed side view of the vacuum chamber 14 is shown. The vacuum chamber 14 in this embodiment is cylindrical, and is divided into a central vaporization compartment 70 and a condensation compartment 72 by cylindrical wall 76. Wall 76 has a height which is less than the height of the vacuum chamber 14, so that vapour may pass over the top of the wall 76, between the vaporization compartment 70 and the condensation compartment 72.

At the top of the condensation compartment 72, is provided a condensing unit 88. The condensing unit 88 comprises a cooling element 78 preferably formed as a coil of copper or aluminium pipe 79, and an upper shelf 80 supporting the cooling element 78, the upper shelf 80 extending from outer wall 81 of the vacuum chamber 14. The upper shelf 80 slopes downwardly from the outer wall 81 towards the vaporization compartment 70. The upper shelf 80 has an upper lip 82 which depends from a radially inner edge 83 of the shelf 80. A further lower shelf 84 extends from the wall 76, at a level just below the lowermost extent of the upper lip 82. The lower shelf 84 has a lower lip 86 extending upwardly from or adjacent to its radially outermost edge, almost meeting the upper shelf 80.

In use, salt water is introduced into the vaporization compartment 70 via nozzle 74, having passed through the pressure-reducing valve 21. As the salt water enters the vacuum chamber 14 via nozzle 74 and shown by arrows A, it flash vaporizes. Salt 92 falls and collects at the bottom of the vaporization compartment 70, and the vapour rises, as shown by arrows B. The vapour condenses when it reaches the vicinity of and contacts the cooling element 78, at which point it flows along the upper shelf 80 down the fall of the upper shelf 80, towards the centre of the vacuum chamber 14, until it runs off the edge and onto the further lower shelf 84. The condensate 94 accumulates in a trough 96 defined by wall 76, lower shelf 84 and lower lip 86 until it flows over an upper edge of the lower lip 86. The overflowing condensate 94 thus drains into a condensate storage area 90, which is radially outward of the vaporization compartment 70 and which extends around the outer side of the dividing wall 76. The upper and lower shelves 80, 84 and the upper and lower lips 82, 86 form a gas-tight seal between the condensate storage area 90 and the vaporization compartment 70 when the trough 96 is filled with condensate 94.

Referring back to Figure 1, a waste outlet 48 is preferably provided for removing waste liquid from the vaporization compartment 70 of the vacuum chamber 14. In this case, a waste pump 62 is provided for pumping out the waste, and may be activated by a level sensor so that waste liquid is pumped only when it reaches a particular level in the vaporization compartment 70. The waste liquid is conveniently pumped back into the sea 100.

Beneficially, a salt auger 49 is provided at the base of the vaporization compartment 70 of the vacuum chamber 14, for removing waste salt from the vaporization compartment 70. However, other options for salt removal may be provided, for example, an access hatch for access by a labourer for manual removal, maintenance and cleaning.

The desalination apparatus may be used for providing a supply of fresh water on a large scale, where only salt water or brackish water is naturally available. The apparatus makes efficient use of solar energy to heat the water for desalination, reducing the amount of external energy which needs to be supplied to the system. The apparatus is also able to reduce its energy consumption at times of the day when electricity is expensive, making up the shortfall when electricity is cheap.

The embodiments described above are provided by way of example only, and various changes and modifications will be apparent to persons skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Claims

1. Desalination apparatus (10) for removing impurities from an impure liquid comprising: a vacuum chamber (14) including a vaporization compartment (70) and a condensation compartment (72) having a cooling element (78) for condensing vapour; a solar-energy collector (26) for absorbing solar radiation; and a heat exchanger (28) for transferring heat energy absorbed by the solar- energy collector (26) to the impure liquid, so as to raise a temperature of the impure liquid, the solar-energy collector (26) and the heat exchanger (28) being fluidly interconnected in a heating circuit having a heating fluid which is or includes a heat energy absorbent nanofluid.

2. Desalination apparatus (10) as claimed in claim 1, in which the heat exchanger (28) includes a heated liquid outlet in fluid communication with a heated liquid inlet of the vacuum chamber (14), for transferring heated liquid from the heat exchanger (28) into the vacuum chamber (14).

3. Desalination apparatus (10) as claimed in claim 1, in which the heat exchanger (28) heats impure liquid inside the vaporization compartment (70) of the vacuum chamber (14).

4. Desalination apparatus (10) as claimed in claim 1, in which the solar-energy collector (26) includes a base (30), at least one side (44, 46), a heat absorption surface and at least one baffle (34, 36, 38, 40, 42) disposed within the solar- energy collector (26).

5. Desalination apparatus (10) as claimed in claim 4, in which the or each baffle (34, 36, 38, 40, 42) is substantially parallel with the heat absorption surface.

6. Desalination apparatus (10) as claimed in claim 5, in which the or each baffle (34, 36, 38, 40, 42) includes an aperture near to a side of the solar-energy collector (26), creating a flow path for the heating fluid from a side (44, 46) of the collector (26), across substantially the entire area of the collector (26) beneath the or each baffle (34, 36, 38, 40, 42), and across substantially the entire area of the collector (26) above the or each baffle (34, 36, 38, 40, 42).

7. Desalination apparatus (10) as claimed in any of claims 4 to 6, in which a plurality of baffles (34, 36, 38, 40, 42) are included, the baffles (34, 36, 38, 40, 42) being disposed substantially parallel with each other and adjacent baffles (34, 36, 38, 40, 42) having apertures near to opposing sides (44, 46) of the solar- energy collector (26).

8. Desalination apparatus (10) as claimed in any of claims 4 to 7, in which the baffle or baffles (34, 36, 38, 40, 42) are made from glass.

9. Desalination apparatus (10) as claimed in any preceding claim, in which the vaporization compartment (70) and the condensation compartment (72) are divided by a wall (76), the wall (76) having an aperture near the top of the vacuum chamber (14).

10. Desalination apparatus (10) as claimed in claim 9, in which the vaporization compartment (70) and the condensation compartment (72) of the vacuum chamber (14) are divided by a wall (76), the height of the wall (76) being less than the height of the vacuum chamber (14).

11. Desalination apparatus (10) as claimed in claim 9 or claim 10, in which a nozzle (74) is provided for discharging impure liquid into the vacuum chamber (14), the nozzle (74) being positioned to discharge in a downward direction at a point within the vaporization compartment (70) below the level of the aperture in the dividing wall (76).

12. Desalination apparatus (10) as claimed in any of the preceding claims, in which the vaporization compartment (70) is separated from the condensate storage area (90) by a gas-tight seal.

13. Desalination apparatus (10) as claimed in any of the preceding claims, in which a holding cistern (16) is provided for storing a supply of impure liquid.

14. Desalination apparatus (10) as claimed in claim 13, in which the holding cistern (16) includes a pre-heater.

15. Desalination apparatus (10) as claimed in claim 14, in which the pre-heater is a solar heater (12).

16. Desalination apparatus ( 0) as claimed in claim 2, in which a bleed outlet (22) is provided between the heated liquid outlet of the heat exchanger (28) and the heated liquid inlet of the vacuum chamber (14) for diverting liquid out of the apparatus, bypassing the vacuum chamber (14).

17. Desalination apparatus (10) as claimed in any of the preceding claims, in which a liquid waste outlet (48) is provided in the vaporization compartment (70) of the vacuum chamber (14), for disposing of excess liquid.

18. Desalination apparatus (10) as claimed in any of the preceding claims, in which a purified liquid outlet (20) is provided in the vacuum chamber (14) for discharging condensate (94) from the condensation compartment (72).

19. Desalination apparatus (10) as claimed in any of the preceding claims, in which the cooling element (78) includes a cooling pipe (79) for containing a cooling fluid.

20. Desalination apparatus (10) as claimed in claim 17, in which circulation means (58) are provided for circulating the cooling fluid through the cooling pipe (79) for absorbing heat from vapour in the vacuum chamber (14), and for circulating the cooling fluid through a cooling reservoir (18) for removing heat from the cooling fluid.

21. Desalination apparatus (10) as claimed in any of the preceding claims, in which the vacuum chamber (14) is substantially cylindrical and the vaporization compartment (70) is substantially cylindrical, the vaporization compartment (70) having a diameter less than the diameter of the vacuum chamber (14) and being disposed within the vacuum chamber (14), the condensation compartment (72) being provided between the outside of the cylindrical vaporization chamber (70) and the outside of the cylindrical vacuum chamber (14).

22. Desalination apparatus (10) as claimed in claim 19, in which the vaporization compartment (70) is disposed substantially at the centre of the vacuum chamber

(14), the condensation compartment (72) being in the shape of a cylindrical shell.

23. Desalination apparatus (10) as claimed in any of the preceding claims, in which an auger (49) is provided for removing solid waste (92) from the vaporization compartment (70) of the vacuum chamber (14).

24. An impure water desalination system comprising desalination apparatus (10) as claimed in any one of the preceding claims, and a source of salt or brackish water (100).

25. A method of desalinating seawater utilising desalination apparatus (10) as claimed in any one of claims 1 to 23, the method comprising the steps of: a. exposing a heat-absorbent nanofluid to solar radiation; b. pre-heating salt water or brackish water by transferring heat energy of the heat-absorbent nanofluid thereto; c. reducing the pressure of the pre-heated salt water or brackish water, so that it vaporises wherein the salt (92) separates; and d. condensing the remaining vapour to form fresh water condensate (94).

26. Desalination apparatus (10) for removing impurities from an impure liquid, the apparatus comprising a solar heater (12) for pre-heating an impure liquid to be distilled, a vacuum chamber (14) for receiving the pre-heated impure liquid, and pressure reducing means (21) for reducing a pressure of the pre-heated impure liquid at the vacuum chamber (14), the solar heater (12) including a solar-energy collector (26) having a heat transfer liquid which comprises a suspension of nanoparticles for heat absorption, and a heat exchanger (28) in liquid communication with the solar-energy collector (26) for receiving the impure liquid for pre-heating, and the vacuum chamber (14) including a vaporisation compartment (70) for receiving the pressure reduced pre-heated impure liquid as a vapour, and a condensation compartment (72) which is spaced from the vaporisation compartment (70) for condensing the vapour.