This application claims priority of Canadian Patent Application No. 2,818,357 filed June 10, 2013, the contents of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0001] The present invention generally relates to hydraulic gas compressors. In particular, the invention relates to uses and systems incorporating the same.

BACKGROUND OF THE INVENTION

[0002] An Hydraulic Air Compressor (HAC) is a large scale installation, typically formed in rock tunnels, that constitutes a method of harnessing hydropower, a renewable source of energy, towards the production of compressed air. The technology was first established in 1890 in Ontario by Charles Taylor. Eighteen examples of the technology have reported to have been constructed, in 9 different countries, on three different continents, mostly for mining applications. The largest of these was at Ragged Chutes, on the Montreal River, 20km south of Cobalt in Ontario. Other than a pneumatic, and subsequently, an hydraulic power assembly to move the intake head vertically up or down in response to natural watercourse head and discharge variations, these systems have no moving parts and hence have high reliability; the system at Cobalt operated more-or-less continuously for 70 years, operations only being interrupted twice for maintenance to the intake head.

[0003] Compressed air generated by the HACs was then transported through a distribution network of pipes to supply a variety of different applications requiring compressed air. With electricity becoming a more marketable form of energy than compressed air around when HACs were developing and the niche demands for compressed air that they serviced falling, almost all HACs have since been decommissioned. However new niche demands have since arisen and as such, there is a need to resurrect the use of HACs for applications where cost effective energy solutions are required. SUMMARY OF THE INVENTION

[0004] According to an aspect of the present invention, there is provided use of an hydraulic gas compressor for cooling an underground mine. The compressed gas produced by the hydraulic gas compressor being mixed with the airstream of an gas intake ventilation shaft of an underground mine to lower the temperature of the airstream.

[0005] According to a second aspect of the present invention there is provided a method for cooling an underground mine. The method involves supplying compressed gas from an hydraulic gas compressor to an gas intake airstream of a ventilation shaft of an underground mine to lower the temperature of the airstream.

[0006] According to a third aspect of the present invention there is provided a system for cooling an underground mine. The system includes: a ventilation shaft for delivering an airstream to an underground mine; and a hydraulic gas compressor for supplying compressed gas to the ventilation airstream. In the system, expanding the compressed gas and mixing it with the airstream decreases the overall temperature of the airstream.

[0007] In one embodiment, the hydraulic gas compressor comprises a down-comer shaft, a gas-liquid separator in communication with an outlet of the down-comer shaft and an inlet of an outlet shaft that transports compressed gas to the air intake ventilation shaft.

[0008] In a second embodiment, the compressed gas is transported through a network of conduit prior to entering the air intake ventilation shaft.

[0009] In a third embodiment, the compressed gas enters the air intake ventilation shaft through a nozzle. In some situations, the nozzle resembles a venturi jet pump.

[0010] In a fourth embodiment, the diameter of the air intake ventilation shaft is reduced in a collar section with a gradual angling of the air intake ventilation shaft walls towards the collar section and a more gradual angling of the walls away from the collar section at the point where the compressed air is introduced into the airstream of the ventilation shaft. [001 1] According to a fourth aspect of the present invention, there is provided a system for cooling a mine deep underground. The system includes: an hydraulic gas compressor; a gas inlet for injecting gas or atmospheric air into water prior to or once the water enters the down- comer shaft; a first gas-liquid separator at the outlet of the down-comer shaft for exhausting a first compressed gas into an gas intake ventilation shaft of a mine; a riser shaft for transporting water from the first gas-liquid separator to a second gas-liquid separator. The formerly dissolved gases are exhausted at the second gas-liquid separator into the gas intake ventilation shaft of the mine.

[0012] In one embodiment, the first gas-liquid separator is a high pressure separator and/or the second gas-liquid separator is a low pressure separator. The first and second gas- liquid separator being centrifugal separators or separation galleries. The centrifugal separator is a cyclone, hydrocyclone, cyclonic chamber or funnel.

[0013] In a second embodiment, the diameter of the air intake ventilation shaft is reduced in a collar section with a gradual angling of the air intake ventilation shaft walls towards the collar section and a more gradual angling of the walls away from the collar section at the point where the compressed air is introduced into the airstream of the ventilation shaft.

[0014] In a third embodiment, the system further comprises a conduit from the second gas-liquid separator for recirculating the liquid to the down-comer shaft. In some systems, a pump is positioned in series with the conduit for recirculating the liquid to the down-comer shaft.

[0015] In a fourth embodiment, a cooling heat exchanger is placed in series with the conduit.

[0016] In a fifth embodiment, a co-solute is added to the liquid in the down-comer shaft.

The co-solute being, for example, a salt, such as sodium sulphate.

[0017] In a sixth embodiment, at least portions of the system are provided as insulated conduit.

[0018] In a seventh embodiment, the system further comprises: a second hydraulic gas compressor; a second air inlet connected to the second gas-liquid separator for introducing gas into liquid prior to or once the liquid enters a second down-comer shaft; a third gas-liquid separator at the outlet of the second down-comer shaft for exhausting a second compressed gas into an air intake ventilation shaft or drift of a mine; a second riser shaft for transporting liquid from the third gas-liquid separator to a fourth gas-liquid separator, wherein oxygen is exhausted at the fourth gas-liquid separator into the air intake ventilation shaft of the mine.

[0019] According to a fifth aspect of the present invention there is provided a method for separating chemical compounds from a gaseous mixture, such as an exhaust combustion gas from a plant. The method involves the steps of: injecting the gaseous mixture into a down-comer shaft of a hydraulic gas compressor to generate a two-phase mixture of gas and liquid; removing one species within the gaseous phase mixture of the two-phase mixture before the outlet of the down-comer shaft by dissolving it in the liquid; separating the gaseous phase from the liquid phase at the bottom of the downcomer shaft; isothermally depressurizing the separated liquid portion of the two-phase mixture to recover previously dissolved gaseous species thereform; and either exhausting the previously dissolved species or collecting them for economic purpose.

[0020] According to a sixth aspect of the present invention, there is provided a system for separating chemical compounds from a gaseous mixture, such as an exhaust combustion gas. The system includes: a hydraulic gas compressor comprising a down-comer shaft, a gas-liquid separator in communication with an outlet of the down-comer shaft and an inlet of an outlet shaft; a connection to bring the gaseous mixture to the hydraulic gas compressor; a primary compressed gas outlet connected to the gas-liquid separator to deliver high pressure, separated, compressed gas; and a secondary outlet positioned near or in conjunction with the outlet of the outlet shaft for exhausting or collecting isothermally decompressed gas from the mixture of liquid and formerly dissolved gas.

[0021] According to a seventh aspect of the present invention, there is provided a method for cooling a building. The method involving supplying compressor gas from a closed-loop hydraulic gas compressor to the atmospheric air of a building; and depressurizing the compressed gas allowing it to expand and cool the atmospheric air.

[0022] In one embodiment, a receiver vessel is positioned in series with the compressed gas outlet. [0023] In a second embodiment, a co-solute is added to the liquid in the down-comer shaft. The co-solute being, for example, a salt, such as sodium sulphate.

[0024] In a third embodiment, at least portions of the system are provided as insulated conduit.

[0025] In a fourth embodiment, the separated compressed gas comprises nitrogen gas.

[0026] In a fifth embodiment, the previously dissolved chemical compounds comprise carbon dioxide.

[0027] According to an eighth aspect of the present invention, there is provided a domestic gas conditioner system. The domestic gas conditioner system having: a gas-liquid separator for positioning in a borehole; a down-comer shaft connected to an inlet port on the gas- liquid separator; a delivery pipe connected to the gas-liquid separator for transporting compressed gas from the gas-liquid separator; a return pipe for returning liquid to the down- comer shaft; and an gas intake for introducing gas into liquid prior to or near when the liquid enters the down-comer shaft.

[0028] According to a ninth aspect of the present invention, there is provided a vapour compression refrigerator. The vapour compression refrigerator having: a gas-liquid separator; a down-comer shaft connected to an inlet port on the gas-liquid separator; a delivery pipe connected to the gas-liquid separator for transporting compressed gas from the gas-liquid separator to a condensing heat exchanger, an expansion device and an evaporating heat exchanger; a return pipe for returning liquid to the down-comer shaft; and an gas intake for introducing gas from the evaporating heat exchanger into liquid prior to or near when the liquid enters the down-comer shaft.

[0029] In one embodiment, the gas is a refrigerant, such as R22 or Rl 34a.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] These and other features, aspects and advantages of the present invention will become better understood with regard to the following description and accompanying drawings wherein: [0031] FIG. 1 is a schematic diagram of a hydraulic gas compressor;

[0032] FIG. 2 is a schematic diagram of a hydraulic gas compressor according to an embodiment of the present invention;

[0033] FIG. 3 is a schematic diagram of a hydraulic gas compressor according to an embodiment of the present invention;

[0034] FIG. 4 is a schematic diagram of a hydraulic gas compressor according to an embodiment of the present invention;

[0035] FIGs. 5a-f are schematic diagrams of hydraulic gas compressors according to an embodiment of the present invention;

[0036] FIGs. 6a-c are schematic diagrams of hydraulic gas compressors according to an embodiment of the present invention; and

[0037] FIG. 7 is a schematic diagram of minimum work vapour compression refrigerator according to an embodiment of the present invention.

DESCRIPTION OF THE INVENTION

The following description is of an illustrative embodiment by way of example only and without limitation to the combination of features necessary for carrying the invention into effect.

[0038] The present invention relates to hydraulic gas compressors (HGCs), such as those developed by Charles Taylor in the late 1800's. As shown in FIG. 1, an HGC 1 includes a down- comer shaft 2, having a water inlet 3 and a water outlet 4. The water inlet 3 being in fluid communication with a natural or man-made source of moving water, such as a river or the like. At or near the water inlet 3 of the down-comer shaft 2 is positioned a gas intake 5. The gas intake 5 introduces, by means of varying mechanisms, air or gas into the stream of water flowing down the down-comer shaft 2. The down-comer shaft 2 terminates in a chamber 6 buried below the surface of the earth. The length of the riser shaft 8 can vary depending on the amount of gas compression desired. The deeper into the earth that the chamber 6 is positioned, thus extending the length of the riser shaft 2, the greater the compression of the gas. Depths of 100m or more produce sufficient compression to allow for the compressed gas to be used in industrial applications.

[0039] In operation, the chamber 6 houses a combination of compressed gas and liquid, mostly in the form of water. The compressed gas can be exhausted through a compressed gas outlet 7, which is interconnected with a network that is capable of transporting the compressed gas to one or more endpoints, which will be discussed in further detail below. An riser shaft 8 having an inlet 9 connected to the chamber 6 and an outlet 10 in fluid communication with a surface body of water, transports the water from the chamber 6 to the surface water body. This surface water body can be directly or indirectly connected to the same source of water that feeds the down-comer shaft 2 or can be a separate watercourse altogether. In some cases, the outlet shaft 8 may be directly or indirectly connected to a pump at the surface water body and returned to the primary water source that feeds the down-comer shaft 2. If the outlet shaft 8 is directly connected to the pump, then a cooling heat exchanger may be added in series with the conduit to transfer any heat accumulated in the water.

[0040] It should be noted that the hydraulic gas compressors described herein are not just used to compress air and that other gases can be compressed by such hydraulic gas compressors. For the purposes of the present discussion, "air" and "gas" are used interchangeably herein to describe the same element. For example, but not limited to, methane (natural gas) could be used in the hydraulic gas compressor of the present invention. Moreover, in closed loop applications, the gas could be in the form of refrigerants, such as, but not limited to, R22 or R134a. Similarly, in preceding and following descriptions, reference has been or will be made to the use of water as the liquid that passes through the system. In further embodiments of the invention, the use of water could be replaced by another liquid, particularly when the liquid is returned to the intake of the down-comer shaft by means of a pump. For gas separation embodiments of the invention, alternative liquids could be selected based on the differential pressure solubility in the selected liquid of the gaseous species in the gaseous mixture to be separated. Water may be the most frequently selected solvent due to its availability and low cost relative to other solvents, however, both "water" and "liquid" are used interchangeably herein to describe the same element. [0041] In one embodiment, the compressed gas exhausted by the HGC 1 could be used to reduce the temperature of air flowing to a mine (FIG. 4). In this case, the compressed gas outlet directly or indirectly, depending on whether the compressed gas is delivered to the mine through a network, terminates at a mine ventilation shaft or drift 30, or is temporarily stored in a receiver, mixes with the airstream traveling down the ventilation shaft or drift 30 to the mine 31. In one example, using the compressed gas from HGC 1 in an ideal device that could expand the gas isentropically would produce a 3.8 kg/s stream of -126.1°C compressed gas with a cooling power of (419.14-271.94) kJ/kg x 3.8 kg/s = 560 kW _th , deliverable to the bulk mine ventilation air through the direct contact of mixing, (see FIG. 3) This is sufficient cooling power to reduce a shaft bottom ventilation inflow of 800 m ³ /s (1,695,120 cfm) by 0.58°C. In another example, where deeper mining is being carried out, it is possible that greater depths i.e. approximately 600 m or more in depth, 11.2 kg/s air at 56 bar gauge could be produced by such a system, which, if expanded isentropically could cool the same amount of ventilation air by 2.4°C.

[0042] As well as cooling the air, compressed gas introduced into the ventilation air from the HGC 1 can pass through a nozzle to a mine airway shaped similarly to 135 in Figure 4, such that this embodiment could act as an integrated mine air cooler and mine air booster fan.

[0043] In another embodiment, the concept of the HGC is provided as a closed loop

HGC 50. In this case, the down-comer shaft 102 is not in fluid communication with a natural water body. Instead, water is recycled and propelled into the down-comer shaft 102 by a pump 1 10. Prior to or at the same time as the water enters the down-comer shaft 102, ambient air is injected into the stream of water by gas inlet 112. Optionally, between the pump 1 10 and the inlet of the down-comer shaft 102 the conduit carrying the water can be narrowed and the walls of the conduit properly angled to the narrowed portion to produce an arrangement similar to a venturi injector. At the narrow portion of the venturi injector, ambient air is drawn into the system through the gas inlet 112.

[0044] The mixture of gas and water travels down the down-comer shaft to a gas-liquid separator system, or cyclone 122. Similar to the gaseous mixture separation system described above, as the air/water mixture travels down the down-comer shaft 102, 0 ₂ in the air will be dissolved in the water and the N ₂ will be compressed and released in the form of gas at the compressed gas outlet 123 attached to the gas-liquid separator system 122.

[0045] The N ₂ gas exhausted from the high pressure gas-liquid separator system 122 can be transferred to air intake ventilation shaft of the mine. A receiver vessel 60 may be placed in series with the compressed gas outlet 123 in order to store the compressed gas produced at the gas-liquid separator system 122. Regulators and/or valves 61 can be placed along the length of the compressed gas outlet 123 to control flow rate into the receiver vessel 60 and/or air intake ventilation shaft of the mine. In order to improve the overall cooling efficiency of the system, the air intake ventilation shaft 30 may be configured to resemble a venturi jet pump 135 prior to the atmospheric air from the surface being drawn into the mine workings 31. In this case, the gas compressed outlet 123 terminates at or near the entrance of the venturi jet pump 135 allowing for the atmospheric air to be enriched with compressed N ₂ .

[0046] In the embodiment where the air intake ventilation shaft or drift 30 is configured to resemble a venturi jet pump 135, the diameter of the air intake ventilation shaft 30 is reduced in a collar section 90, with a gradual angling of the air intake ventilation shaft walls towards the collar section 90 and a more gradual angling of the walls away from the collar section 90. This arrangement allows for cooler air, having a consistency similar to atmospheric air, to be drawn into the mine workings 31 and up the upcast exhaust shaft 158 by main mine fan 170.

[0047] Water exiting the high pressure gas-liquid separator system 122 has 0 _2, and to a much lesser extent N ₂ , dissolved therein. As this water travels up a riser shaft 140, at least a portion of the 0 ₂ and N ₂ dissolved in the water is isothermally depressurized, so that when the gas and water mixture is delivered to a second low-pressure gas-liquid separator 150, the 0 ₂ and N ₂ are exhausted through an exhaust port 151, which can, in certain applications, terminate at a position along the air intake ventilation shaft 30. The second or low pressure gas-liquid separator 150 can be designed similar to the high pressure gas-liquid separator 122 or can have a different structure depending upon the installation and application. In any case, the second gas-liquid separator will also be able to separate gas from liquid using forced centrifugal separation. Since the gas traveling through exhaust port 151, contains mostly 0 ₂ and to a much lesser degree N ₂ , this gas can be added to the atmospheric air being drawn into air intake ventilation shaft 30 to enrich the 0 ₂ concentration thereof. This allows for the air eventually reaching the mine workings 31 to have a consistency, in terms of the percentages of 0 ₂ and N ₂ contained therein, that is more similar to atmospheric air.

[0048] Water exiting the second gas-liquid separator 150 enters back into the system via pump 1 10.

[0049] The use of an HAC, as described above, in the cooling of mine, deep or otherwise, offers significant energy savings over the current use of conventional compressors and/or powerful fan units.

[0050] In another embodiment, the gaseous mixture passing through gas intake 5 comes from an exhaust outlet 20 from a plant 21 (FIG. 2). In most cases, the plant 21 will be a fossil fuel powered plant, so the combustion gases will predominantly comprise C0 ₂ , water vapour, and N ₂ , with much smaller concentrations of undesirable species such as NO _x , S0 ₂ , and possibly unburnt hydrocarbons or 0 ₂ , if the plant operated with significant excess air. For the purposes of the present illustrative discussion, it is assumed that the combustion gas comprises only C0 ₂ , H ₂ 0 and N _2.

[0051] When the combustion gas bubbles come into contact with the water in the down- comer shaft 2, the water vapour will condense into the water readily (if the water has not already become condensate prior to being passed to the HAC as part of a heat recovery scheme). This will leave a stream gas bubbles with a composition of C0 ₂ and N ₂ .

[0052] Henry's Law (see for example, the useful compilation of Henry's Law constants in Sander, 1999, http://www.henrys-law.org or Battino et al, J. Phys. Chem. Ref Data 13(2):563- 600, 1984, both of which are incorporated herein by reference) governing the pressure solubility of gases can be described: where ?, is the partial pressure of the gas species in the gas phase, K _t is Henry's constant for species / and x, is the maximum mol fraction (concentration) of the species in the solvent (water), known as the solubility. Henry's constant for N ₂ is 155.88 MPa/(mol/dm ³ ) and for C0 ₂ is 2937 MPa/(mol/dm ³ ). It is thus evident that C0 ₂ has pressure solubility in water at least an order of magnitude higher than N ₂ and will thus dissolve completely first in the water as the pressure increases. In addition, a small amount of N ₂ will be dissolved in the water. A detailed analysis of the pressure solubility of gases is presented in Millar D, "A review of the case for modern-day adoption of hydraulic air compressors" Applied Thermal Engineering 69: 55-77, 2014, the complete contents of which is incorporated herein by reference.

[0053] A gas-liquid separation system 22 provided at the outlet 4 of the down-comer shaft 2 at the depth (pressure) at which the C0 ₂ becomes completely dissolved will cause the C0 ₂ to be separated from the input gas stream as it will leave by being dissolved in the water passing through the gas-liquid separation system 22. The gas-liquid separation system 22 can be, but is not limited to, a forced centrifugal separator, such as a cyclone, hydrocyclone, cyclonic chamber or funnel as shown in Figure 2 or a separation gallery 6 as shown in Figure 1. In the case of a forced centrifugal separator, the water and gas mixture that enters the separator is forced against the interior of the separator in a manner that generates a swirling or cyclonic movement of the mixture. The cyclonic movement of the pressurized gas and water results in most of the gas rising to the top of the separator and the water funnelling out of the separator, below. In the case where the input gaseous mixture contains N ₂ and C0 ₂ , and the gas-liquid separator is positioned at a depth (pressure) where C0 ₂ becomes completely dissolved in the water, then the gas exhausted from the gas-liquid separation system will be primarily pressurized N ₂ . In a system where a forced centrifugal separator 22 is not provided, the gas stream exiting the outlet 4 of the down-comer shaft 2, which contains high pressure nitrogen, N ₂ , can be vented through compressed gas outlet 23.

[0054] In order to ensure constant availability of pressurized gas from the compressed gas outlet 23, a receiver vessel 60 may be positioned in series along the compressed gas outlet 23 or the distribution network attached thereto.

[0055] As the water depressurises while it ascends, C0 ₂ becomes less soluble and will come out of solution (together with the minor amount of N ₂ that was dissolved as well). At the outlet 10 of the outlet shaft 8, the flow will be two phase and so the gas stream can be separated from the water with another gas-liquid separation system 25 having a secondary gas outlet 26 (as shown in Figure 2). The second gas-liquid separation system 25 can be of similar configuration to the first gas-liquid separator 22, or can have a different configuration. In this case, the gaseous phase of the gas and water mixture will be under less pressure than when the mixture passed through the first gas-liquid separator.

[0056] Gas dissolved in the water that is separated at depth provides a mechanism for compressed gas to escape the receiver plenum. The leakage has a direct bearing on the mechanical efficiency of the installation for air compression. For closed and open loop systems one means to mitigate the portion of the loss of efficiency that arises due to gas solubility is to consider the use of a co-solute. In general, the prior presence of a dissolved salt in water leads to reduced gas solubility; gas solubility reduces as the dissolved salt concentration increases. For example, sodium sulphate could be added to the circulating water of an open or closed loop HAC.

[0057] For closed loop HAC systems, a second means to mitigate efficiency loss due to solubility is to operate these systems at higher temperature than previously considered for run-of- river systems. In one embodiment, within a closed loop HAC, water circulating in insulated pipe work will gradually rise in temperature as a result of the heat transferred to it during the compression of the gas.

[0058] In another embodiment, the flow exiting the first HAC can be passed to a second, similar HAC system. This arrangement will be particularly advantageous when the purity of the C0 ₂ stream is low. As the solubility of gases in water depends on the gas species partial pressure, in the second HAC system, less of the N ₂ will dissolve as the pressure increases, than dissolved in the first HAC system at the same pressure. In the high pressure gas-liquid separator 22 at depth, less N ₂ will be carried, dissolved, in the liquid phase. In the overflow of the low pressure gas-liquid separator 25 at surface of the second HAC, the purity of the C0 ₂ will be higher.

[0059] When additional gas species are considered in the system, such as 0 ₂ , which may be present due to the combustion process taking place in excess air, whether or not these species predominantly arrive at the high pressure overflow 23 or the low pressure overflow 25 depends on their relative pressure solubility; 0 ₂ has Henry's constant value of 77.94 MPa/(mol/dm ³ ), about half that of N ₂ , meaning that it is about twice as soluble in water as N ₂ . The bulk of the 0 ₂ will be carried up the riser 8 dissolved in the water, but undissolved 0 ₂ will arrive at the overflow of the high pressure cyclone 22, reducing the purity of the predominantly N ₂ stream. To improve the nitrogen purity of this stream, it may be passed to another HGC, where the elevation of the high pressure separation cyclone 22 is located at a depth where the oxygen can be taken to have dissolved completely. The overflow of this cyclone will produce a high purity stream of compressed nitrogen gas. Thus it can be seen that when deployed as part of a combustion gas separation scheme, or carbon capture scheme, HGCs would be deployed in cascades.

[0060] In the preceding paragraphs relevant to the embodiment of the invention that concerns the separation of gaseous mixtures, the use of a combustion gas mixture to illustrate the gas separation systems and methods, embodies specific methods and systems for effecting 'carbon capture' from new or existing fossil fuel burning plants using HGCs.

[0061] Regulators, valves, switches and the like can be positioned at various spots along the HGC and related systems to control flow of water, air and/or gases. These regulators, valves and switches can be controlled by a microprocessor and related circuitry.

[0062] The concept of the closed-loop HGC system described above can be used for a domestic air conditioning system, as shown in FIG. 5a. In this case, a borehole 200 is provided as the riser shaft. A gas-liquid separator 201, similar to the ones described above, is housed in the borehole 200, which is fed by a down-comer shaft 202. Compressed gas that is separated from the water in the gas-liquid separator 201 is exhausted from the gas-liquid separator 201 by compressed gas delivery pipe 203. Compressed gas from the delivery pipe 203 is fed to the domestic structure and depressurized causing expansion and cooling of the air. After the water exits the gas-liquid separator 201, it slowly (compared to the down-comer shaft) flows up and around the gas-liquid separator 201 and down-comer shaft 202 and delivery pipe 203 to eventually be pumped back into the down-comer shaft 202 by mechanical pump 204. Before the water re-enters the down-comer shaft 202, it passes through venturi injector 205, where gas is reintroduced into the system at gas inlet 206. Low-pressure gas accumulated in the borehole 200 can be exhausted by exhaust outlet 207. [0063] Systems comprising riser shafts 200, as shown in FIGs 5b-5f, can be used in situations where the horizontal space requirements of the systems described above may not be available. In the closed loop system shown in FIG. 5b, a second gas-liquid separator 208 exhausted by outlet 209 is provided at the top of the riser shaft 200 where the water exits the shaft 200. In this case, the exhaust outlet 207 is connected to the gas-liquid separator 208. Systems incorporating open-loop systems are shown in FIGs. 5c and 5d. In these cases, water is pumped from pump 204 through return 210 to the source of water 21 1 that feeds the down-comer shaft 202. Gas is injected into this system by gas inlet 206 that is positioned in the down-comer shaft 202. Systems where the water exiting the riser shaft 200 is not returned to the down-comer shaft 202 are shown in FIGs. 5e and 5f. In these arrangements, the water can be delivered to another watercourse or used for some other purpose.

[0064] In another embodiment, the system can include a separation gallery or chamber

320 in conjunction with riser shaft 300 (FIG. 6). In the various systems shown in FIG. 6, the down-comer shaft 302 empties into a separation gallery or chamber 320, where compressed gas is removed via delivery pipe 303. The water in the chamber is allowed to rise in riser shaft 300, where low-pressure gas is exhausted at exhaust outlet 307 (FIGs. 6a and 6b). Alternatively, the water is allowed to rise up the riser shaft and is introduced to a gas-liquid separator 308 which is connected to exhaust outlet 307 (FIG. 6c). The various reference numerals shown in FIG. 6 correspond to equivalent elements in FIG. 5.

[0065] In yet a further embodiment, the HGC described above is modified to act as a minimum work vapour compression refrigerator 400 (FIG. 7). The HGC loop shown in FIG. 7, is essentially the same loop as shown for deep mine cooling applications (see FIG. 4). In the minimum work vapour compression refrigerator shown in FIG. 7, the compressed gas that leaves the gas-liquid separator 422 is passed through what would be otherwise known as a conventional mechanical vapour compression refrigeration circuit, including condenser 453, evaporator 454 and expansion valve 455. The gas used in this system is typically a refrigerant, such as R22 or R134a. The various reference numerals shown in FIG. 7 correspond to equivalent elements in FIG. 4. [0066] The present invention has been described with regard to one or more embodiments. However, it will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined by the claims.