Electrostatically enhanced mechanical vapour compression

Abstract

A mechanical vapour compression system for water purification comprising: a low-pressure evaporator chamber for receiving impure liquid input water and configured to evaporate the input water under low-pressure conditions to form water vapour; a high-pressure condenser chamber for receiving the water vapour and configured to condense the water vapour under high-pressure conditions to form liquid water; a compressor arranged between the low-pressure evaporator chamber and the high-pressure condenser chamber and configured to generate the low-pressure and high-pressure conditions; and one or more charged particle generators configured to generate charged particles that initiate condensation of the water vapour to form liquid droplets.

Description

[0001] ELECTROSTATICALLY ENHANCED MECHANICAL VAPOUR COMPRESSION

[0002] TECHNICAL FIELD

[0003] The present disclosure relates mechanical vapour compression systems and methods for water purification.

[0004] BACKGROUND ART

[0005] Mechanical vapour compression (MVC) is a mature industrial technology. MVC involves using a compressor to draw a vacuum on a solution in an evaporator chamber. This causes the solution to boil and form a vapour. The vapour travels through the compressor that compresses the vapour into a condenser chamber, causing the vapour to condense into a pure, liquid solvent. The latent heat of vaporisation is recycled from the solvent as it condenses to the evaporating solution via heat exchange surfaces. This results in a substantial reduction in energy consumption when compared to boiling via an eternal heat source.

[0006] Due to its robust and contaminant agnostic nature, MVC has applications in a wide variety of industries, wherever there is high water usage; food & beverage, pulp & paper, mining & metals, oil & gas, chemical production, pharmaceuticals, textile manufacturing, and agriculture. In practice, MVC can be applied as desalination for saline or brackish water, waste effluent volume reduction, process chemical recovery & concentration, and aqueous product recovery / crystallization.

[0007] Despite its attractive qualities, MVC is often eschewed in favour of membrane-based technologies like ultrafiltration, nanofiltration, and reverse osmosis, due to the comparatively high energy consumption of the compression process. Membrane technologies come with their own issues: low flow rates, high chemical usage, sensitivity to contaminants, high labour and maintenance costs, and sensitivity to fouling.

[0008] It is an objection of the present disclosure to at least partially address some of these problems.

[0009] SUMMARY OF THE INVENTION

[0010] According to a first aspect of the disclosure there is provided a mechanical vapour compression system for water purification comprising: a low-pressure evaporator chamber for receiving impure liquid input water and configured to evaporate the input water under low-pressure conditions to form water vapour; a high-pressure condenser chamber for receiving the water vapour and configured to condense the water vapour under high- pressure conditions to form liquid water; a compressor arranged between the low-pressure evaporator chamber and the high-pressure condenser chamber and configured to generate the low-pressure and high-pressure conditions; and one or more charged particle generators configured to generate charged particles that initiate condensation of the water vapour to form liquid droplets.

[0011] Optionally, a condenser- side charged particle generator is provided within the high- pressure condenser chamber. Optionally, the condenser-side charged particle generator is arranged adjacent to an outlet of the compressor. Optionally, the condenser-side charged particle generator is configured to impart charge to liquid droplets from the compressor.

[0012] Optionally, an evaporator-side charged particle generator is provided within the low- pressure evaporator chamber. Optionally, the evaporator- side charged particle generator is arranged no more than 50 mm from a surface of the input water. Optionally, the one or more charged particle generators comprises a high-voltage electrode configured to produce a corona discharge.

[0013] Optionally, the one or more charged particle generators are configured to generate charged particles of a single polarity.

[0014] Optionally, the mechanical vapour compression system further comprises: a heat exchanger arranged between the low-pressure evaporator chamber and the high-pressure condenser chamber and configured to thermally couple the low-pressure evaporator chamber and the high-pressure condenser chamber to transfer latent heat of condensation from the high-pressure condenser chamber to the low-pressure evaporator chamber.

[0015] Optionally, the heat exchanger comprises an electrically conductive heat exchange surface within the condenser chamber to attract charged liquid droplets. Optionally, the heat exchange surface is electrically grounded. Optionally, the heat exchanger comprises a plate-type heat exchange surface. Optionally, the heat exchanger is configured as a falling film evaporator.

[0016] Optionally, the pressure in the low-pressure condenser chamber is below atmospheric pressure.

[0017] Optionally, the pressure in the low-pressure condenser chamber is configured to boil the input water without external heating.

[0018] Optionally, the pressure in the low-pressure condenser chamber is no more than 10 kPa.

[0019] Optionally, the input water is received by the low-pressure evaporator chamber as a continuous or intermittent feed. Optionally, the low-pressure condenser chamber comprises at least one concentrate outlet for removing high impurity concentrate. Optionally, the concentrate outlet comprises an evacuation pump configured to raise the pressure of the removed high impurity water to around atmospheric pressure or higher.

[0020] Optionally, a temperature within the high-pressure evaporator chamber is no more than 60 °C.

[0021] Optionally, the pressure ratio between the low-pressure evaporator chamber and the high- pressure condenser chamber is less than 1.2.

[0022] Optionally, the high-pressure evaporator chamber comprises at least one distillate outlet for removing purified water. Optionally, the distillate outlet comprises an evacuation pump configured to raise the pressure of the removed high purity water to around atmospheric pressure.

[0023] Optionally, the compressor comprises any one of: a centrifugal compressor, an axial compressor, a Roots-type lobe compressor, a twin-screw compressor, and a steam -jet ejector.

[0024] Optionally, the compressor is configured to receive a mixed flow of water vapour and charged liquid droplets from the condenser chamber at a relatively low pressure and eject the mixed flow to the condenser at a relatively high pressure. Optionally, the compressor is configured so as to prevent neutralisation of the charged liquid droplets passing through the compressor. Optionally, an internal surface of the compressor configured to contact the charged liquid droplets passing through the compressor is electrically isolated from ground potential. Optionally, the mechanical vapour compression system further comprises a solar thermal collector configured to heat a feed of the impure liquid input water received by the low- pressure evaporator.

[0025] Optionally, the compressor comprises a steam -jet ejector, and the mechanical vapour compression system further comprises a solar thermal collector configured to provide heat for generating motive steam for the steam -jet ejector.

[0026] Optionally, the mechanical vapour compression system further comprises a photovoltaic array configured to provide electrical power to the mechanical vapour compression system. Optionally, the photovoltaic array comprises a second heat exchanger configured to heat a feed of the impure liquid input water received by the low-pressure evaporator. Optionally, the second heat exchanger comprises a gravity-driven plate-type heat exchange surface.

[0027] Optionally, the mechanical vapour compression system further comprises a controller configured to control the pressure in the low-pressure evaporation chamber. Optionally, the controller is configured to adjust the pressure in the low-pressure evaporation chamber in response to a measured impurity concentration of a high impurity concentrate in the low-pressure evaporation chamber.

[0028] According to a second aspect of the disclosure there is provided a method of mechanical vapour compression for water purification comprising: evaporating impure liquid input water under low-pressure conditions to form water vapour in a low-pressure evaporator chamber; condensing the water vapour under high-pressure conditions to form liquid water in a high-pressure condenser chamber; wherein a compressor arranged between the low- pressure evaporator chamber and the high-pressure condenser chamber generates the low- pressure and high-pressure conditions; and condensation of the water vapour to form liquid droplets is initiated by generating charged particles using one or more charged particle generators.

[0029] According to a third aspect of the disclosure there is provided a process of producing purified water from impure input water, comprising: performing the method of the second aspect by inputting the impure input water to the low-pressure evaporator chamber as impure liquid input water and extracting purified water from the high-pressure condenser chamber.

[0030] According to a fourth aspect of the disclosure there is provided a process for producing a water-based product, comprising: performing the process of the third aspect to produce purified water and further processing the purified water to produce the water-based product. Optionally, wherein the further processing comprises the addition of minerals to the purified water.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Further features of the disclosure will be described below, by way of non-limiting examples and with reference to the accompanying drawings, in which:

[0033] Fig. 1 shows a comparative example mechanical vapour compression system.

[0034] Fig. 2 shows a first example electrostatic mechanical vapour compression system according to the disclosure;

[0035] Fig. 3 shows a second example electrostatic mechanical vapour compression system according to the disclosure;

[0036] Fig. 4 shows a third example electrostatic mechanical vapour compression system according to the disclosure;

[0037] Fig. 5 shows a fourth example electrostatic mechanical vapour compression system according to the disclosure; Fig. 6 shows a fifth example electrostatic mechanical vapour compression system according to the disclosure;

[0038] Fig. 7 shows a sixth example electrostatic mechanical vapour compression system according to the disclosure;

[0039] Fig. 8 shows a seventh example electrostatic mechanical vapour compression system according to the disclosure; and

[0040] Fig. 9 shows a further example electrostatic mechanical vapour compression system according to the disclosure.

[0041] DETAILED DESCRIPTION

[0042] The mechanical vapour compression (MVC) system according to the disclosure enhances the energy efficiency of the MVC process to be competitive with membrane -based technologies. This results in water treatment technology that has advantages of both membrane and phase change technologies and mitigates the downsides.

[0043] Fig. 1 shows a comparative example of a mechanical vapour compression apparatus 1. As shown, the apparatus comprises an evaporator chamber 2, which receives water through water inlet 3. A compressor 4 draws a vacuum in evaporator chamber 2 causing the vapour to move into a condenser chamber 5 (flow of vapour is shown by the arrows at the compressor 4). As the vapour compresses, it returns to its liquid form and can be removed as product water at outlet 7. The evaporator chamber 2 and the condenser chamber 5 are most often contained within the same vessel, as shown. Evaporation and condensation are illustrated by arrows at the water surface, the water being illustrated by dashed lines within the lower volume.

[0044] The two chambers are physically separated but thermally connected through a heat exchanger 6 comprising heat exchange surfaces (flow of heat is shown by the arrow at the heat exchanger 6). While the surface is labelled on the condenser side, the evaporator side has the opposite surface. Concentrate in the evaporator chamber 2 is removable through port 8. Compressors can take a wide variety of forms, but many systems use what is known as a roots blower, a high- volume positive displacement pump. Alternatively, some systems, often referred to as thermal vapour compression, may use a venturi pump, most often using steam as the motive fluid.

[0045] The primary energy consumer of an MVC system is the compressor, and the energy consumed by the compressor is directly proportional to the heat of the vapour being compressed, as well as the pressure ratio between condenser and evaporator. The majority of industrial MVC systems operate at high temperatures of over 60 C, and high-pressure ratios of over 1.2. These values are well in excess of the thermodynamic minimum values to drive the evaporation-condensation process but are necessary to maintain a high level of heat transfer from condenser to evaporator. As shown the compressor may be powered by an external power source 9, e.g. an electrical grid.

[0046] The examples according to the disclosure may operate much closer to the thermodynamic minimums. Some examples may boil the solution at ambient temperatures by drawing a significant vacuum in the range of ~5 kPa internal operating pressure and running the compressor at compression ratios around 1.15, or lower. Under conventional MVC operation this would result in very poor heat transfer and necessitate prohibitively large heat transfer surfaces, but the embodiments here enhance the process using electrostatic condensation enhancement.

[0047] Electrostatic condensation enhancement is a phenomenon by which free vapour molecules are attracted to charged particles via dielectrophoretic forces. Due to the polar nature of water molecules, they are affected by electric fields. In a uniform electric field, water molecules will tend to orient themselves aligned with the electric field but will experience no net force.

[0048] In a non-uniform electric field, such as one produced by a charged particle, the electric field will attract one pole of a water molecule with greater force than it repels the opposite pole, resulting in a net force towards the centre of charge. When a particle, in this case a nanometre scale water droplet, is sufficiently charged, the dielectrophoretic force will be greater than the vapour pressure of the water. This results in water vapour condensing under conditions in which it would otherwise evaporate.

[0049] According to literature, these charged droplets possess 2-3 times the enthalpy of an uncharged droplet, which equates to tens of degrees Celsius higher temperature. After growing sufficiently in size, these droplets would be electrostatically attracted to grounded heat exchange surfaces covered in liquid water films, and once incorporated into the bulk liquid, the droplets would be prevented from re-evaporating. The process of electrostatically condensing water vapour into higher temperature droplets circumvents the thermal energy barrier in conventional MVC processes, in which the vapour must release the enthalpy of vaporisation prior to taking a liquid state. This allows the proposed process to operate at low temperatures and pressure ratios, and still maintain high levels of heat transfer from condenser to evaporator.

[0050] The charged particles in the water droplets would be either dissolved impurities in the water, such as salts, or water molecules themselves split into either H+ or OH-. The high voltage DC charge injection more or less ensures that only one polarity ends up in the system (either + or -), which is then neutralized on the grounded heat exchanger. This neutralization would generate a very small amount of gas, and in the case of salts being used as charge carriers, they would remain in the produced water. However, the ratio of charged particles to water molecules is very small, less than 1 charged particle per 1 million water molecules; which is still considered highly pure.

[0051] Example systems according to the present disclosure may have the same basic arrangement as the comparative example MVC system shown in Fig. 1. In particular, example systems according to the disclosure comprise an evaporator chamber, a condenser chamber and a compressor arranged between the evaporator chamber and the condenser chamber. However, example systems according to the disclosure additionally comprise one or more charged particle generators configured to generate charged particles that initiate condensation of the water vapour to form liquid droplets.

[0052] Fig. 2 shows a first example system 10 according to the disclosure. As shown, the system 10 comprises a low-pressure evaporator chamber 2 for receiving impure liquid input water and a high-pressure condenser chamber 5 for receiving the water vapour. As shown, a compressor 4 is arranged between the low-pressure evaporator chamber and the high- pressure condenser chamber and configured to generate the low-pressure and high-pressure conditions. The low-pressure evaporator chamber 2 is configured to evaporate the input water under low-pressure conditions to form water vapour. The high-pressure condenser chamber 5 is configured to condense the water vapour under high-pressure conditions to form liquid water.

[0053] As shown, examples according to the invention may further comprise at least one water inlet 3 through which the impure liquid input water is received by the low-pressure evaporator chamber 2. The input water may be received by the low-pressure evaporator chamber as a continuous or intermittent feed.

[0054] As shown, examples according to the invention may further comprise at least one concentrate outlet 8 for removing high impurity concentrate from the low-pressure evaporator chamber 2 and at least one distillate outlet for removing purified water from the high-pressure condenser chamber 5. As shown, the outlets may comprise a respective evacuation pump 15, 16. The evacuation pumps 15, 16 may be high pressure, low flow rate pumps. The pumps 15, 16 may be configured to raise the pressure of the removed water to around atmospheric pressure.

[0055] As shown in Fig. 2, the system may comprise an evaporator- side charged particle generator 11 within the evaporator chamber 2. As in the present example, the charged particle generator 11 may be a high voltage electrode configured to generate a corona discharge which generates charged particles of a single polarity. As described above, the generated charged particles initiate condensation of the water vapour to form liquid droplets (the droplets being illustrated by dots emanating from the side charged particle generator 11). A mixture of water vapour and charged liquid droplets may pass through the condenser 4 from the evaporator chamber 2 to the condenser chamber 5.

[0056] This evaporation process is enhanced by the injection of charged particles into the water vapour, which rapidly form nanoscopic droplets of liquid water, converting large volumes of water vapour into much smaller volumes of liquid water, further lowering the pressure experienced by the input water. Further, compressing a mixture of water vapour and liquid droplets results in a mass flow rate that is substantially higher than the mass flow rate of compressing water vapour alone. Due to the incompressibility of the droplets, the work input required to run the compressor is not increased. Therefore, process efficiency can be enhanced by the evaporator-side charged particle generator 11.

[0057] Further improvement may be achieved by the evaporator-side charged particle generator 11 being arranged no more than 50 mm from a surface of the input water within the evaporator chamber 2. Ions close to the water surface generate a vapour pressure deficit, driving the water to evaporate. As shown in Fig. 2, the system may comprise a condenser- side charged particle generator 12 within the condenser chamber 5. As in the present example, the charged particle generator 12 may be a high voltage electrode configured to generate a corona discharge which generates charged particles of a single polarity. As described above, the generated charged particles initiate condensation of the water vapour passing through the compressor 4 to form liquid droplets (the droplets being illustrated by dots emanating from the side charged particle generator 12). Further, the condenser-side charged particle generator 12 may impart charge to liquid droplets from the compressor, which may have been neutralised when passing through the compressor 4.

[0058] In alternative examples of the disclosure, only the evaporator-side charged particle generator 11 may be provided. Alternatively, only the condenser- side charged particle generator 12 may be provided. Such an example is shown in Fig. 9 by example system 30.

[0059] As shown, the charged particle generators 11, 12 may be connected to a high voltage power supply 14. The power supply 14 may convert a lower input voltage from a power source to high voltage, for example.

[0060] Within the condenser chamber 5, water vapour generated within the evaporator chamber 2 condenses to liquid water, enhanced by the generated charged particles, and is collected with the condenser chamber 5, e.g. under gravity, to form a volume of distillate (purified water). It should be noted that the purity level of the distillate may vary, but is of higher purity than the input water. Although in some example systems, a purity of substantially 100% may be achieved.

[0061] As shown in Fig. 2, the example system may further comprise a heat exchanger 6. As shown, the heat exchanger 6 is arranged between the evaporator chamber 2 and the high- pressure condenser chamber 5 configured to thermally couple the evaporator chamber 2 and the condenser chamber 5. A first heat exchange surface of the heat exchanger 6 may be arranged within the evaporator chamber 2 and a second, opposite, heat exchange surface may be arranged within the condenser chamber. The heat exchanger thus provides a physical barrier between the evaporator chamber 2 and the high-pressure condenser chamber 5. The heat exchanger is configured to transfer latent heat of condensation from the condenser chamber 5 to the evaporator chamber 2. This improves process efficiency.

[0062] As shown in Fig. 2, the heat exchange surface arranged within the condenser chamber 5 may be electrically conductive and electrically grounded. Thus, the heat exchange surface is configured to attract and neutralise charged liquid droplets. This improves the process efficiency.

[0063] As shown, the heat exchanger 4 may comprise a plate-type heat exchange surface. For example, the heat exchange surface may be substantially flat. Condensed droplets may be configured to fall under gravity, e.g. as a film, down the heat exchange surface into the distillate volume. Thus, the heat exchanger 4 may be gravity driven. The heat exchanger 4 may be a falling film evaporator, for example.

[0064] The concentrate left in the evaporator chamber 2 and the distillate collected in the condenser chamber 5 are then evacuated from their respective chambers. However, this step may be part of a continuous cycle of water purification and / or may be performed periodically.

[0065] The compressor 4 is configured to generate the low-pressure and high-pressure conditions respectively. The absolute pressures in the evaporator chamber and condenser chamber may not be very different in absolute terms, provided that the pressure in the evaporator chamber is lower than the pressure in the condenser chamber. The compression ratio may be greater than 1, but less than 1.2, preferably less than 1.15. more preferably less than 1.10. In a specific example, the compression ratio may be around 1.15. The absolute pressure in the low-pressure evaporator chamber may be less than 10 kPa, preferably less than 7 kPa, more preferably less than 5 kPa.

[0066] The compressor 4 may be any one of a centrifugal compressor, an axial compressor, a Roots-type lobe compressor, a twin-screw compressor and a steam -jet ejector.

[0067] Although not shown, the example system may comprise a controller configured to control the pressure in the low-pressure evaporation chamber, e.g. by controlling the compressor 4. The controller may be configured to adjust the pressure in the evaporation chamber 2 in response to a measured impurity concentration of the concentrate, e.g. in order to achieve a desired recovery ratio. The recovery ratio is defined as the amount of purified water output divided by the amount of input water.

[0068] In the example system shown in Fig. 2, the system may be powered by an external electrical power source 9 such as an electrical grid. Sub-systems requiring electrical power may include the compressor 4, the charged particle generators 11, 12 and the pumps 15, 16.

[0069] Fig. 3 shows a second example system 17 according to disclosure. As shown, the second example system 17 is the same as the first example system 10, but additionally comprises a solar thermal collector 18 configured to use solar energy to heat the feed 3 of the input water. This may further improve the energy efficiency of the system.

[0070] Fig. 4 shows a third example system 19 according to the disclosure. As shown, the third example system 19 is the same as the first example system 10, but additionally comprises a solar thermal collector 20 configured to use solar energy to power the compressor 4. In this example, the compressor 4 comprises a steam -jet ejector, and the solar thermal collector is configured to provide heat for generating motive steam for the steam -jet ejector. This may further improve the energy efficiency of the system. Although not shown the compressor 4 may still draw some power from the external power source 9, e.g. grid.

[0071] In a variation, the additional features of the second and third example systems may be combined in a single system. Two different solar thermal collectors may be used or one single solar thermal collector for both purposes.

[0072] Fig. 5 shows a fourth example system 21 according to the disclosure. As shown, the fourth example system 21 is the same as the first example system 10, but additionally comprises a photovoltaic array as an electrical power source. This may be instead of or in addition to an external power source 9, e.g. grid.

[0073] Fig. 6 shows a fifth example system 23 according to the disclosure. As shown, the fifth example system 21 is the same as the third example system 19, but additionally comprises a photovoltaic array as an electrical power source as in the fourth example system. This may be instead of or in addition to an external power source 9, e.g. grid.

[0074] Fig. 7 shows a sixth example system 26 according to the disclosure. As shown, the sixth example system 24 is the same as the first example system 10, but power is provided by a hybrid photovoltaic solar thermal system 27. Fig. 8 shows a variation forming a seventh example system 28 according to the disclosure. As shown, the seventh example system 28 is the same as the sixth example system 28, but power may additionally be provided by an external power source 9, e.g. grid, as well as a hybrid photovoltaic solar thermal system 29. In a further variation of the above examples, a photovoltaic array may comprise a second heat exchanger configured to heat a feed of the impure liquid input water received by the low-pressure evaporator.

[0075] The MVC systems according to the disclosure may operate at a significant vacuum, e.g. in the range of about 5% of atmospheric pressure. Typical MVC systems operate near atmospheric pressure.

[0076] The examples above comprise a high-voltage power supply, that charges a supply of particles to produce charged particles. As described above a corona discharge may generate free ions that would adsorb water molecules and bombard existing droplets to maintain a high level of charge. In other examples, an electrospray apparatus generates charged nanometre to micron scale droplets that would adsorb additional water molecules.

[0077] These examples would involve the application of high voltage. Corona discharge may take the form of high voltage sharp needles or fine wires over which the vapour would flow. The electrospray apparatus may take the form of an array of high voltage, small diameter nozzles, in the range of less than 1 mm, injecting charged droplets into the vapour stream.

[0078] The input water may be saline or brackish water, for example, or any other suitable aqueous solution for purification. The purified distillate may be potable water. In some examples, the purified distillate may undergo further processing, e.g. remineralisation. Remineralisation may add minerals to the purified water, e.g. minerals lost during purification. These minerals may include calcium and magnesium, for example.

[0079] All features disclosed in the specification, including the claims, abstract, and drawings, and all the steps in any method or process disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. Each feature disclosed in the specification, including the claims, abstract, and drawings, can be replaced by alternative features serving the same, equivalent, or similar purpose, unless expressly stated otherwise. It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the embodiments.

Claims

CLAIMS1. A mechanical vapour compression system for water purification comprising: a low-pressure evaporator chamber for receiving impure liquid input water and configured to evaporate the input water under low-pressure conditions to form water vapour; a high-pressure condenser chamber for receiving the water vapour and configured to condense the water vapour under high-pressure conditions to form liquid water; a compressor arranged between the low-pressure evaporator chamber and the high- pressure condenser chamber and configured to generate the low-pressure and high-pressure conditions; and one or more charged particle generators configured to generate charged particles that initiate condensation of the water vapour to form liquid droplets.

2. The mechanical vapour compression system of claim 1, wherein a condenser- side charged particle generator is provided within the high-pressure condenser chamber.

3. The mechanical vapour compression system of claim 2, wherein the condenser- side charged particle generator is arranged adjacent to an outlet of the compressor.

4. The mechanical vapour compression system of claim 2 or 3, wherein the condenser- side charged particle generator is configured to impart charge to liquid droplets from the compressor.

5. The mechanical vapour compression system of any preceding claim, wherein an evaporator- side charged particle generator is provided within the low-pressure evaporator chamber.

6. The mechanical vapour compression system of claim 4, wherein the evaporatorside charged particle generator is arranged no more than 50 mm from a surface of the input water.

7. The mechanical vapour compression system of any preceding claim, wherein the one or more charged particle generators comprises a high-voltage electrode configured to produce a corona discharge.

8. The mechanical vapour compression system of any preceding claim, wherein the one or more charged particle generators are configured to generate charged particles of a single polarity.

9. The mechanical vapour compression system of any preceding claim, further comprising: a heat exchanger arranged between the low-pressure evaporator chamber and the high-pressure condenser chamber and configured to thermally couple the low-pressure evaporator chamber and the high-pressure condenser chamber to transfer latent heat of condensation from the high-pressure condenser chamber to the low-pressure evaporator chamber.

10. The mechanical vapour compression system of claim 9, wherein the heat exchanger comprises an electrically conductive heat exchange surface within the condenser chamber to attract charged liquid droplets.

11. The mechanical vapour compression system of claim 10, wherein the heat exchange surface is electrically grounded.

12. The mechanical vapour compression system of any one of claim 9 to 11, wherein the heat exchanger comprises a plate-type heat exchange surface.

13. The mechanical vapour compression system of any one of claim 9 to 12, wherein the heat exchanger is configured as a falling film evaporator.

14. The mechanical vapour compression system of any preceding claim, wherein the pressure in the low-pressure condenser chamber is below atmospheric pressure.

15. The mechanical vapour compression system of any preceding claim, wherein the pressure in the low-pressure condenser chamber is configured to boil the input water without external heating.

16. The mechanical vapour compression system of any preceding claim, wherein the pressure in the low-pressure condenser chamber is no more than 10 kPa.

17. The mechanical vapour compression system of any preceding claim, wherein the input water is received by the low-pressure evaporator chamber as a continuous or intermittent feed.

18. The mechanical vapour compression system of any preceding claim, wherein the low-pressure condenser chamber comprises at least one concentrate outlet for removing high impurity concentrate.

19. The mechanical vapour compression system of claim 18, wherein the concentrate outlet comprises an evacuation pump configured to raise the pressure of the removed high impurity water to around atmospheric pressure.

20. The mechanical vapour compression system of any preceding claim, wherein a temperature within the high-pressure evaporator chamber is no more than 60 °C.

21. The mechanical vapour compression system of any preceding claim, wherein the pressure ratio between the low-pressure evaporator chamber and the high-pressure condenser chamber is less than 1.2.

22. The mechanical vapour compression system of any preceding claim, wherein the high-pressure evaporator chamber comprises at least one distillate outlet for removing purified water.

23. The mechanical vapour compression system of claim 18, wherein the distillate outlet comprises an evacuation pump configured to raise the pressure of the removed high purity water to around atmospheric pressure.

24. The mechanical vapour compression system of any preceding claim, wherein the compressor comprises any one of: a centrifugal compressor, an axial compressor, a Roots-type lobe compressor, a twin-screw compressor, and a steam -jet ejector.

25. The mechanical vapour compression system of any preceding claim, wherein the compressor is configured to receive a mixed flow of water vapour and charged liquid droplets from the condenser chamber at a relatively low pressure and eject the mixed flow to the condenser at a relatively high pressure.

26. The mechanical vapour compression system of claim 25, wherein the compressor is configured so as to prevent neutralisation of the charged liquid droplets passing through the compressor.

27. The mechanical vapour compression system of claim 26, wherein an internal surface of the compressor configured to contact the charged liquid droplets passing through the compressor is electrically isolated from ground potential.2128. The mechanical vapour compression system of any preceding claim, further comprising a solar thermal collector configured to heat a feed of the impure liquid input water received by the low-pressure evaporator.

29. The mechanical vapour compression system of any preceding claim, wherein the compressor comprises a steam-jet ejector, and the mechanical vapour compression system further comprises a solar thermal collector configured to provide heat for generating motive steam for the steam -jet ejector.

30. The mechanical vapour compression system of any preceding claim, further comprising a photovoltaic array configured to provide electrical power to the mechanical vapour compression system.

31. The mechanical vapour compression system of claim 30, wherein the photovoltaic array comprises a second heat exchanger configured to heat a feed of the impure liquid input water received by the low-pressure evaporator.

32. The mechanical vapour compression system of claim 31, wherein the second heat exchanger comprises a gravity-driven plate-type heat exchange surface.

33. The mechanical vapour compression system of any preceding claim, comprising a controller configured to control the pressure in the low-pressure evaporation chamber.

34. The mechanical vapour compression system of any preceding claim, wherein the controller is configured to adjust the pressure in the low-pressure evaporation chamber in response to a measured impurity concentration of a high impurity concentrate in the low- pressure evaporation chamber.

35. A method of mechanical vapour compression for water purification comprising:22evaporating impure liquid input water under low-pressure conditions to form water vapour in a low-pressure evaporator chamber; condensing the water vapour under high-pressure conditions to form liquid water in a high-pressure condenser chamber; wherein a compressor arranged between the low-pressure evaporator chamber and the high-pressure condenser chamber generates the low-pressure and high-pressure conditions; and condensation of the water vapour to form liquid droplets is initiated by generating charged particles using one or more charged particle generators.

36. A process of producing purified water from impure input water, comprising: performing the method of claim 35 by inputting the impure input water to the low- pressure evaporator chamber as impure liquid input water and extracting purified water from the high-pressure condenser chamber.

37. A process for producing a water-based product, comprising: performing the process of claim 36 to produce purified water and further processing the purified water to produce the water-based product.

38. The process of claim 37, wherein the further processing comprises the addition of minerals to the purified water.23

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