WO2006087549A2

WO2006087549A2 - Heat engines and compressors

Info

Publication number: WO2006087549A2
Application number: PCT/GB2006/000538
Authority: WO
Inventors: Abdulsalam Al-Mayahi
Original assignee: Abdulsalam Al-Mayahi
Priority date: 2005-02-16
Filing date: 2006-02-16
Publication date: 2006-08-24
Also published as: WO2006087549A3

Abstract

A heat engine operating on a vapour compression cycle employing as working fluid one of absolute alcohol, at least 25% ethanol in water, methanol and at least 25% methanol in water and/or ethanol.

Description

HEAT ENGINES AND COMPRESSORS

Field of the Invention

This invention relates, in alternative aspects thereof to heat engines, including, in particular (though not exclusively) heat pumps, and also to compressors.

We describe herein not only novel heat engines, but also novel compressors useful in such heat engines. The said compressors may also be used for other purposes, and a number of such uses are described.

Background to the Invention

Heat pumps are reverse heat engines (see Fig. 1 of the accompanying drawings) in which Work Wj_n is applied to take in Heat Qj_n from a source at temperature T_00M to transfer Heat Q_out to a heat sink at a higher temperature T_ho_t- The reverse heat engine of Fig. 1 may be used to heat the sink in which case its Coefficient of Performance (COP) as a heat pump will be given by the formula COPh_p- Q_0Ut/ Wi_n. Alternatively, the reverse heat engine of Fig. 1 may be used as a refrigerator to abstract heat from the refrigerator source and transfer it, for example to atmosphere, at the sink. Its Co-efficient of Performance as a refrigerator will be given by the formula COP_ref = Qi_n / Wj_n. Moreover, in this system Q_out - Qm ^ Wj_n. Apart from the relevant Co-efficient of Performance, heat pumps used for heating and refrigerators used for cooling are essentially the same apparatus, and the term "heat pump" is used hereinbelow as the generic term for such apparatus.

Although heat pumps are based on a variety of heat engine cycles, the most common such cycle is the vapour compression cycle, as employed in refrigerators. A simplified refrigeration heat pump cycle is illustrated in Fig. 2 of the accompanying drawings.

A working fluid circulates around a closed cycle. The working fluid as gas is compressed in compressor 1 where Work Wj_n is applied. The compressed gas at a higher pressure and hence higher temperature passes to a condenser 2, where the compressed vapour is condensed into a saturated or sub-cooled liquid, releasing Heat Q_0Ut- The working fluid then passes through a throttling or expansion valve 3 which expands the fluid, so reducing its pressure and its temperature. As a result, the working fluid may absorb Heat Qj_n at an evaporator 4 where the fluid returns to its vapour state before passing again to the compressor 1. The effect of this cycle is to continuously absorb Heat at the evaporator, to continuously give off Heat at the condenser and to apply Work to the cycle at the compressor.

Absorption heat pumps work on different principles, being thermally rather than mechanically driven. Absorption heat pumps used for space heating are often gas-fired, while high-pressure steam or waste water has been used in industry to drive such heat pump installations. Absorption systems utilize the ability of liquids or salts to absorb the vapour of the working fluid. The most common working pairs are: water (working fluid) and lithium bromide (absorbent); and ammonia (working fluid) and water (absorbent).

Traditionally, the most common working fluids for closed cycle compression heat pumps have been CFCs (chlorofluorocarbons) and related chemicals, usually identified in the refrigeration industry as CFC-12, CFC-114, R500, R502 and

HCFC-22.

Of these, all but HCFC-22 are CFCs. These chemicals are harmful to the global environment having both a high Ozone Depletion Potential and a high Global Warming Potential. Since January 1996, the use of CFCs has been phased out, and these chemicals are now prohibited for use as refrigerants. HCFCs (hydrochlorofluorocarbons), of which the most commonly used has been HCFC-22, which has been used in virtually all refrigerators since 1996, may be less harmful than CFCs, but they still have significant Ozone Depletion and Global Warming Potentials. Since January 2004, the use of HCFCs in the European Union is required to be reduced, and they are required to be phased out by January 2015. As a consequence, the industry is turning to HFCs (hydrofluorocarbons) such as R- 134a, R-152a, R-32, R-125 and R-507, which may not contribute to ozone depletion, as they have no chlorine content, but still possess significant Global Warming Potential. However, special attention must be given to the use of lubricants, as mineral oils are non-miscible with these refrigerants, so that any mineral oil residues must be completely removed, which is a serious problem in retrofitting, only special expensive ester-based lubricant oils recommended by the manufacturers being useable.

These problems have held back the development and application of heat pumps on a wider scale, notwithstanding the apparent economic attractiveness of heat pumps, whether used for refrigeration or for heating, that they commonly have Co-efficients of Performance that are in theory substantially greater than unity (in other words: when used for heating the systems produce more Heat then the energy put in as applied Work, and, when used for cooling/refrigeration, the Heat abstracted by cooling is again greater than the energy supplied as Work put into the system).

A possible alternative to the otherwise almost ubiquitous use of fossil fuels or electricity derived from fossil fuels or nuclear energy for space heating and for cooling might be heat pumps based upon alternative "natural" working fluids but, as explained below, all such fluids used to date suffer from significant disadvantages.

So-called "natural" working fluids are substances that naturally exist in the biosphere and so do not represent a significant Global hazard. They will generally have essentially negligible Global environmental drawbacks (zero or near zero Ozone Depletion or Global Warming Potentials). Examples of natural working fluids that have been employed in heat pump cycles include ammonia (once widely used as a working fluid for refrigeration), hydrocarbons, carbon dioxide and water.

Water appears at first sight to be an excellent choice of working fluid for high-temperature industrial heat pumps due to its generally favourable thermodynamic properties, particularly its high latent heat of evaporation, and the fact that it is neither flammable nor toxic. Water has mainly been employed as a working fluid in open and semi-open mechanical vapour recompression systems. The major disadvantage of water, however, is its low volumetric heat capacity. This requires large (and hence expensive) compressors, especially when it is operating at low temperatures. A further disadvantage is that water freezes which means that the evaporator in a closed cycle cannot be exposed to temperature below O⁰C₅ which rules water out for use in refrigerators.

The problem with hydrocarbons is their high flammability, requiring precautions to be taken in enclosure of the heat pump, the adoption of fail-safe ventilation systems, the addition of tracer gas to the working fluid and/or use of gas detectors.

Carbon dioxide is a potentially attractive working fluid being non-toxic, non- flammable and compatible with most normal lubricants and construction materials. Its volumetric refrigeration capacity is high. However, the theoretical Co-efficient of Performance of a conventional heat pump cycle employing carbon dioxide as the working fluid is rather poor. As a result, the use of carbon dioxide has largely been restricted to use as a secondary refrigerant in cascade systems.

The problem with ammonia is its relative toxicity and the fact that many construction materials are incompatible with its use. In high-temperature industrial heat pumps ammonia is not suitable because efficient high-pressure compressors able to operate at the pressure required when using ammonia as the working fluid are simply not available.

When a heat engine is employed for heating it is generally referred to as a heat pump. When it is used for cooling, it is generally referred to as a refrigerator. The term "heat engine" is used hereinbelow to refer to all such arrangements, whether the primary emphasis of the apparatus is to heat, or to cool.

Summary of the Invention

The present invention, in some of its principal aspects, seeks to overcome the problems inherent in previously proposed heat engine systems by the adoption of one of a number of novel working fluids. In accordance with a first aspect of the present invention, there is provided: a heat engine operating on a vapour compression cycle and employing a working fluid consisting of one of absolute alcohol and at least 25% ethanol in water.

Preferably the percentage of ethanol will be 50% or more. More preferably, the working fluid is the azeotrope of ethanol and water.

Ethanol is a "natural" working fluid. It occurs naturally in the biosphere. It is produced on a substantial scale, for human consumption, by fermentation and distillation. For industrial use it is also commonly produced by an addition reaction between ethene and steam. It is impossible, by fermentation and distillation alone to produce ethanol with a higher degree of purity than 95% alcohol. Instead, an azeotropic mixture of 95% ethanol and 5% water is obtained, the azeotrope having its own boiling point of 78.15⁰C, which is less than the boiling point of 78.3°C of pure ethanol (absolute alcohol). Fractional distillation does not occur when the azeotrope is heated because the vapour has the same composition as the liquid azeotrope. In effect, the 95% alcohol azeotrope behaves as if it were a pure compound. Most preferably, the working fluid for the heat pump is the 95% azeotrope rather than either absolute alcohol (100% ethanol) or an aqueous solution with less than 95% ethanol. Wherever "ethanol" is used in the specific description below with reference to Fig. 3 of the accompanying drawings, this is intended to mean absolute alcohol, the 95% azeotrope, or an aqueous solution with less than 95% ethanol, it being understood that - for cost reasons in the case of absolute alcohol, and to avoid separation of a less than 95% aqueous solution into the azeotrope and water - the fluid used in practice will almost always be the azeotrope.

Use of ethanol produces significant advantages over previously proposed working fluids. It has a freezing point of about -73⁰C and a boiling point (in the case of the azeotrope) of 78.15⁰C. It has a very high evaporation enthalpy (latent heat of vapourization) of 838kJ/kg which, though about one-third that of water, is substantially better than all other previously proposed working fluids other than water and ammonia. Ethanol also has a reasonable value of thermal conductivity in comparison with other substances that have been employed as working fluids in heat pumps. It is not toxic, but is flammable. Its detectable smell gives a warning of any leakage. Environmentally it has zero Ozone Depletion Potential. Cost-wise, ethanol is very cheap as the azeotrope, though rather more expensive if absolute alcohol is used, and readily available. It has less corrosion effect on most common processed materials that might be used in construction of a heat pump, than do other previously proposed working fluids.

The only perceived disadvantage in employing ethanol as a heat pump working fluid (even though it would still be substantially better than water in this regard) is that it has a relatively low volumetric heat capacity (kJ/m³) in comparison with other previously proposed working fluids. This means that the compressors employed in the heat pump cycle will need to be relatively large and relatively expensive. The present invention in its broadest context contemplates the use of any of the known types of compressor, namely reciprocating compressors, rotary screw compressors, rotary centrifugal compressors and jet gas compressors. These may be varied in any of a number of respects including the number of compression stages, the cooling method employed (whether air, water or oil) the drive method (whether motor, engine or steam driven) and whether lubrication is required or the compressor can operate oil-free. However, the present invention also contemplates the use of novel forms of compressor which enable the cost of the compression stage substantially to be reduced.

As explained in detail below with reference to the accompanying drawings, these systems rely upon an ammonia-water contactor system, which is believed novel per se.

Preferred compressors comprise: a housing defining a first chamber therein, and a second chamber therein divided from said first chamber by a displaceable wall so that at any time the pressure in the first chamber is substantially equal to the pressure in the second chamber, the chambers being otherwise substantially isolated from each other; first inlet means arranged to provide entry of a fluid into the first chamber; first outlet means arranged to provide exit from the first chamber of said fluid compressed by the compressor; a third chamber containing a solution of ammonia in water and defining a gas space containing ammonia gas above the level of solution in said second chamber; a heater for periodically heating said solution to drive ammonia into said gas space to create a body of ammonia under pressure; a cooler for periodically condensing ammonia from said second chamber to reduce the pressure therein; said gas space being connectable to said second chamber, whereby pressurised ammonia gas from said gas space is enabled to pressurise said second chamber, thereby to compress fluid in said first chamber via the displaceable wall; and said first inlet and said first outlet being provided with valves, enabling flow of fluid into said first chamber to be timed for periods when the pressure of ammonia in said second chamber is relatively low, and flow of fluid from said first chamber to be timed for periods when the pressure of ammonia in said second chamber is relatively high.

The displaceable wall is preferably formed by an expandable sheet. The expandable sheet is preferably of balloon form, the second chamber being generally surrounded by the first chamber.

The novel compressors referred to above may be used in systems other than heat pumps. Notably, they may be used to provide an essentially silent air compressor, and in the spraying of liquids and/or solids.

Thus, in one preferred arrangement, said first inlet of said compressor is adapted to receive air from atmosphere, and said first outlet of said compressor is coupled to a high pressure storage tank for air compressed by said compressor. Preferably, the system comprises two or more compressors working in parallel, their respective first outlets being coupled to a single said storage tank for pressurised air.

In a second preferred arrangement, said first inlet of said compressor is adapted to receive a propellant gas (preferably atmospheric air); and said outlet of said compressor is coupled to a storage tank for a material to be sprayed selected from particulate materials and liquid materials, including solutions and dispersions, compressed propellant gas from said first outlet being received in said tank in a gas space above said material; said tank including an outlet tube having an opening in a lower region of said tank and being coupled to a spray exit outside said tank, whereby pressurised propellant gas is arranged to force said material under pressure into and through said outlet tube from said opening to said spray exit. In a second and alternative aspect of this invention, there is provided: a heat pump operating on a vapour compressor cycle and employing a working fluid consisting of methanol or of at least 25% methanol in water or ethanol.

Preferably the working fluid consists of methanol in one of its commercially available grades such as ultra high purity methanol which has a purity of around 99.98% methanol or the lesser metallurgical grade which has a purity of around 99.9% methanol.

Methanol is also a "natural" working fluid that occurs naturally in the biosphere. Methanol, like ethanol, has Zero Ozone Depletion Potential. It is also inexpensive to produce as wood alcohol. Like ethanol it has little corrosion effect on most common processed materials that might be used in construction of a heat pump.

In a preferred arrangement, a methanol working fluid heat pump is operated in a dual cycle together with a second heat pump using ethanol or water as its working fluid.

Thus, in a third alternative aspect of the present invention, there is provided: a heat pump system comprising a first heat pump operating on a vapour compression cycle and employing a first working fluid (preferably consisting of one of water, absolute alcohol and at least 25% ethanol in water); and a second heat pump operating on a vapour compression cycle and employing a working fluid different from the first (preferably consisting of one of methanol and at least 25% methanol in water and/or ethanol); the heat source for the first heat pump being the heat sink of the second heat pump.

Preferably, the first working fluid has a higher boiling point than the second

- as it will have in the case of the preferred fluids identified in parentheses. Brief Description of the Drawings

The invention is hereinafter more particularly described with reference to the accompanying drawings, in which:- Fig. 1 shows a schematic diagram for a reverse heat engine;

Fig. 2 shows a schematic compression heat pump cycle;

Fig. 3 schematically illustrates an embodiment of heat pump utilising ethanol as working fluid;

Fig. 4 is a schematic diagram illustrating application of an ammonia-water contact compressor to provide an essentially silent air compressor;

Fig. 5 is a schematic diagram illustrating application of an ammonia-water contact compressor for spraying; and

Fig. 6 is a schematic circuit diagram for a two-stage heat pump system.

Description of Preferred Embodiments

Referring first to Fig. 3 which shows a schematic ethanol based heat pump, compressed ethanol vapour issuing from compressor 5 passes to condenser 6 in which heat exchange takes place between the ethanol and a secondary fluid, suitably water or brine passing through the condenser/heat exchanger from an inlet 7 to an outlet 8, abstracting heat from the compressed ethanol vapour which condenses as a result, the secondary fluid issuing from outlet 8 at a significantly higher temperature than at inlet 7. The condensed ethanol passes to a flash evaporator 9, which serves both as the expansion valve and evaporator that would be present in a standard compression heat pump cycle. Liquid ethanol issues from nozzles 10 within a tank 11 in the vapour space 12 above liquid ethanol 13. Liquid from the liquid space 13 is recirculated via pump 14 to re-join the incoming liquid ethanol from the condenser. Vapour space 12 is maintained at a low pressure. The expansion and evaporation within tank 11 causes cooling. A low grade heat source (for example, ambient air, ground water, a river), dependent on the size, location and intended use (heating, cooling, domestic refrigeration etc.) for which the heat pump is intended, or a secondary fluid circulating in heat exchange with the low grade source (for example, water passing in a closed cycle through the subsoil), is circulated through piping etc. 16 in heat exchange with the liquid ethanol 13 which absorbs heat as a consequence. Vapour from the flash evaporator 9 passes to compressor 5. The compressor 5 is suitably heated to avoid condensation within the compressor and again may be provided in any of the well-known forms, including reciprocating compressors, rotary screw compressors, rotary centrifugal compressors and jet gas compressors. Since these conventional forms of apparatus are well known and commercially available as items of commerce, further description here is deemed unnecessary. However, in preferred arrangements, as described in more detail below, the compressor is suitably an ammonia-water contact system.

An exactly similar system may be operated with methanol as the working fluid.

Fig. 4 schematically illustrates what is in effect an essentially silent air compressor, in which two ammonia- water contact compressors 17 (which, apart from valves and the expandable wall, have no mechanical moving parts) operate in parallel to supply compressed air to a storage tank 62. Chambers 18 of compressors 17 in this embodiment have respective inlets 63 coupled via non-return valves 64 to receive air from atmosphere. After compression therein by ammonia within chambers 19, air from chambers 18 passes via respective non-return valves 65 at respective outlets to storage tank 62. In this case, each compressor 17 has an associated combined ammonia source and ammonia scrubber unit 66. When pressurised ammonia gas is required in a particular chamber 19 to compress air in the corresponding chamber 19, aqueous ammonium hydroxide solution 67 within unit 66 is heated via heat exchanger 68 to drive off ammonia under pressure. To return compressor 17 to its original state with its chamber 19 deflated, the now depleted solution/water 67 within scrubber unit 66 is cooled by heat exchanger 68, thereby allowing the depleted solution/water to absorb ammonia gas. It will be appreciated that there may be a bank of such compressors.

It will be appreciated that, rather than compressing a gas (here: air), an essentially exactly similar system may be employed, with appropriate timing of the valves, to pump an essentially incompressible liquid, such as water from a source thereof. Ammonia-water contact compressors may also be used to provide propellant gas to apparatus for spray of particulate solids or of non-aqueous liquid, as explained with reference to Fig. 5.

A compressor 17 essentially similar to the compressors of Fig. 4 using a similar combined ammonia source and ammonia scrubber 66 is coupled to receive air from atmosphere via non-return valve 64 and to pass compressed air via nonreturn valve 65 into a pressure tank 69. Although air is most suitable, being freely available, other gases may be used. The propellant gas (air) pressurises space 70 within tank 69 above a material 71 to be sprayed. An outlet tube 72 from pressure tank 69 controlled by a valve 73 has an opening 74 in a lower part of tank 69. Supply of compressed propellant gas/air into tank 69 and opening of outlet valve 73 allows a supply of material 71, which may be a particulate solid or a liquid, including solutions and dispersions, to be forced into opening 74, up tube 72 and, via valve 73, suitably to a spray head or spray nozzle, not illustrated.

The arrangement illustrated in Fig. 6 is a two-stage heat pump system involving a first heat pump cycle 201 using a first working fluid and a second heat pump cycle 202 using a second and different working fluid. Each of these cycles is substantially as illustrated in Fig. 3, but with the recirculation pump being omitted for clarity in the schematic circuit diagram of the present Fig 6.

Thus, the first cycle includes a compressor 203, a condenser/heat exchanger 204 and a flash evaporator 205. Similarly, the second heat pump cycle 202 comprises a second flash evaporator 207 coupled to a compressor 209.

Heat to the combined heat pump cycles is supplied from a low grade heat source, for example solar and/or geothermal and /or waste heat from some other process, typically at a temperature of 5-40⁰C, by heat exchange at 210 with liquid in the evaporator 207 of the second cycle. The nature of the low grade heat source will depend upon whether the combined cycle is being used for heating, in which case the source may be ambient air, ground water, sea water or a river or a secondary fluid circulating in heat exchange with that low grade source. Similarly, when the system is being used for refrigeration, the low grade source will be air or water being cooled or a secondary fluid circulating in heat exchange with the region being cooled or refrigerated. Typically, the output from cycle 202 at heat exchanger 211 may be 55-75⁰C.

The first cycle uses as its low grade heat source 211 the compressed working fluid in the second cycle so that in effect the heat source for the first cycle is the heat sink from the second cycle. This system is particularly useful where the working fluid in the second cycle has a lower boiling point than the working fluid in the first cycle. Thus the second cycle is preferably a methanol cycle, employing as working fluid either methanol or at least 25% methanol in water and/or ethanol; while the first cycle is preferably an ethanol cycle, employing as working fluid either absolute alcohol or at least 25% ethanol in water. Preferably, the working fluid in the ethanol cycle is the azeotrope of ethanol and water; while the preferred working fluid in the methanol cycle is a commercial grade of methanol.

A combined ethanol-methanol system has a number of advantages over a simple one-stage ethanol cycle. Methanol has a higher vapour pressure than ethanol and a higher latent heat of evaporation than ethanol but its boiling point is only 64°C which is lower than that of ethanol or of the ethanol/water azeotrope. Thus, the methanol cycle has a better heat transfer in terms of heat transfer per unit mass of working fluid than does the ethanol cycle but is limited by its lower boiling point. Linking the two cycles together as in the Fig. 6 arrangement enables the benefits of both working fluids to be obtained.

In an alternative arrangement, a methanol working fluid cycle may be used alone and can reach a temperature of 9O⁰C after compression to about 2 bars.

Claims

1. A heat engine operating on a vapour compressor cycle and employing a working fluid selected from the group consisting of methanol and at least 25% methanol in water or ethanol.

2. A heat engine according to Claim 1, wherein the working fluid consists of methanol in a commercially available grade selected from ultra high purity methanol and metallurgical grade methanol.

3. A heat engine according to Claim 1 or Claim 2, wherein the heat engine includes an evaporator for the working fluid heated by a low grade heat source selected from the group comprising solar energy, geothermal energy and waste heat from another process.

4. A heat engine according to Claim 1, wherein the said heat engine is coupled for operation in a dual cycle with a second heat engine using ethanol or water as its working fluid, the heat sink of the first mentioned heat engine serving as the heat source of the second heat engine.

5. A heat engine operating on a vapour compression cycle and employing a working fluid selected from the group consisting of absolute alcohol and at least 25% ethanol in water.

6. A heat engine according to Claim 4, wherein the percentage of ethanol in the working fluid is 50% or more.

7. A heat engine according to Claim 5, wherein the working fluid is the azeotrope of ethanol and water.

8. A heat engine system comprising a first heat engine operating on a vapour compression cycle and employing a first working fluid; and a second heat pump operating on a vapour compression cycle and employing a working fluid different from the first; the heat source for the first heat pump being the heat sink of the second heat pump.

9. A heat engine system according to Claim 8, wherein the first working fluid has a higher boiling point than the second.

10. A heat engine system according to Claim 8, wherein the first working fluid is selected from the group consisting of water, absolute alcohol and at least 25% ethanol in water, and the second working fluid is selected from the group comprising methanol and at least 25% methanol in a solvent selected from at least one of water and ethanol.

11. An essentially silent air-compressor, comprising an ammonia- water compressor and a high pressure storage tank; said ammonia-water compressor comprising: a housing defining: a first chamber therein, and a second chamber therein divided from said first chamber by a displaceable wall so that at any time the pressure in the first chamber is substantially equal to the pressure in the second chamber, the chambers being otherwise substantially isolated from each other; a first inlet arranged to provide entry of air into the first chamber; a first outlet arranged to provide exit from the first chamber of air compressed by the compressor; a third chamber containing a solution of ammonia in water and defining a gas space containing ammonia gas above the level of solution in said second chamber; a heater for periodically heating said solution to drive ammonia into said gas space to create a body of ammonia under pressure; a cooler for periodically condensing ammonia from said second chamber to reduce the pressure therein; said gas space being connectable to said second chamber, whereby pressurised ammonia gas from said gas space is enabled to pressurise said second chamber, thereby to compress air in said first chamber via the displaceable wall; the first inlet being adapted to receive air from atmosphere, and said first outlet of said compressor being coupled to said high pressure storage tank; and the high pressure storage tank including an outlet for compressed air therefrom; and said first inlet and said first outlet being provided with valves, enabling flow of air into said first chamber to be timed for periods when the pressure of ammonia in said second chamber is relatively low, and flow of pressurised air from said first chamber to be timed for periods when the pressure of ammonia in said second chamber is relatively high.

12. An essentially silent pump, comprising an ammonia-water contact compressor and a supply of water; said ammonia- water compressor comprising: a housing defining: a first chamber therein, and a second chamber therein divided from said first chamber by a displaceable wall so that at any time the pressure in the first chamber is substantially equal to the pressure in the second chamber, the chambers being otherwise substantially isolated from each other; a first inlet arranged to provide entry of water from said supply into the first chamber; a first outlet arranged to provide exit from the first chamber of water under pressure; a third chamber containing a solution of ammonia in water and defining a gas space containing ammonia gas above the level of solution in said second chamber; a heater for periodically heating said solution to drive ammonia into said gas space to create a body of ammonia under pressure; a cooler for periodically condensing ammonia from said second chamber to reduce the pressure therein; said gas space being connectable to said second chamber, whereby pressurised ammonia gas from said gas space is enabled to pressurise said second chamber, thereby to pump water present in said second chamber through said first outlet; and said first inlet and said first outlet being provided with valves, enabling flow of water into said first chamber to be timed for periods when the pressure of ammonia in said second chamber is relatively low, and flow of water from said first chamber to be timed for periods when the pressure in said second chamber is relatively high.

13. A spray propellant system, comprising an ammonia-water contact compressor, and a storage tank arranged to be charged with a material to be sprayed selected from particulate materials and liquid materials, including solutions and dispersions; said ammonia-water compressor comprising: a housing defining: a first chamber therein, and a second chamber therein divided from said first chamber by a displaceable wall so that at any time the pressure in the first chamber is substantially equal to the pressure in the second chamber, the chambers being otherwise substantially isolated from each other; a first inlet arranged to provide entry of a propellant gas into the first chamber; a first outlet arranged to provide exit from the first chamber of propellant gas compressed by the compressor; a third chamber containing a solution of ammonia in water and defining a gas space containing ammonia gas above the level of solution in said second chamber; a heater for periodically heating said solution to drive ammonia into said gas space to create a body of ammonia under pressure; a cooler for periodically condensing ammonia from said second chamber to reduce the pressure therein; said gas space being connectable to said second chamber, whereby pressurised ammonia gas from said gas space is enabled to pressurise said second chamber, thereby to compress propellant gas in said first chamber via the displaceable wall; the first inlet being adapted to receive propellant gas from a supply thereof (optionally air from atmosphere) when pressure in said second chamber is relatively low, and said first outlet of said compressor being coupled to said storage tank to provide compressed propellant gas from said first outlet when pressure in said second chamber is relatively high to a gas space defined above said material; and said tank including an outlet tube having an opening in a lower region of said tank and being coupled to a spray exit outside said tank, whereby pressurised propellant gas supplied by said compressor to said storage tank is arranged to force said material under pressure into and through said outlet tube from said opening to said spray exit.