GB2464121A

GB2464121A - Processes and Apparatus for Cooling

Info

Publication number: GB2464121A
Application number: GB0818162A
Authority: GB
Inventors: Tony Coleman; Ciaran Wallace
Original assignee: Energyexcel LLP
Current assignee: Energyexcel LLP
Priority date: 2008-10-03
Filing date: 2008-10-03
Publication date: 2010-04-07
Anticipated expiration: 2028-10-03
Also published as: US20110173998A1; GB2464121B; GB0818162D0; EP2342510A1; WO2010038037A1

Abstract

A of hybrid refrigeration systems. In one embodiment a low pressure booster circuit (10) is linked to an absorption plant (20) to provide cooling at lower temperatures that can be achieved by the absorption plant (20) alone. The combined systems are efficient compared to vapour compression systems, especially when "waste" heat from other processes is used to drive the absorption part (20) of the circuit. The absorption plantn (20) can be provided with heat either by direct firing of a fuel, by waste heat from a combined heat and power (CHP) prime mover (such as a gas engine (30) or gas turbine for example), or by any suitable source of waste heat from another process.

Description

Processes and Apparatus for Cooling The invention relates to processes and apparatus for using the cooling duty provided by an absorption refrigeration plant.

Absorption refrigeration is a process for producing cooling using heat as the main driving energy supply instead of electricity, which is needed to power the compressor of a vapour compression refrigeration plant. Absorption refrigeration plants work with a combination of a refrigerant fluid and a carrier liquid. An absorption refrigeration circuit 20 is shown in Fig. 1, but other circuits can be configured with additional equipment to provide improved performance features.

Heat rejection to ambient is illustrated in Fig. 1 and throughout the application by cooling water, but could be via other devices such as air blast coolers, evaporative draught condensers or other.

Refrigerant fluid evaporates at low pressure and temperature in the evaporator 3, and in doing so provides a useful cooling duty. The refrigerant vapour is passed into the absorber 2 where it is absorbed into a carrier liquid, also at low pressure. The heat of solution so evolved is removed with cooling water. The carrier liquid with the refrigerant in solution is then pumped by pump 22 up to high pressure and delivered to the generator 23. In the generator 23 the solution is heated and the refrigerant vapour is re-released and flows, still at high pressure, to the condenser 24. In the condenser 24 the refrigerant vapour condenses, so releasing heat to the cooling water.

The condensed refrigerant liquid is then expanded back to low pressure through an expansion valve 26 causing some flash evaporation and self cooling and supplied back to the evaporator 3. Likewise, the spent carrier liquid from the generator 23 is expanded back to the absorber 21 ready to absorb more refrigerant vapour.

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In essence, the carrier liquid circuit of an absorption refrigeration circuit 20 can be considered as being used in place of the compressor in a vapour compression refrigeration plant.

The absorption process illustrated in Fig. 1 may be modified to improve performance, for example by the use of multi-staging.

There are many combinations of carrier fluid and refrigeration fluid that can be used for absorption refrigeration but the two most common are: 1) Carrier: Lithium Bromide (LiBr) solution in water / Refrigerant: Water (H20), and 2) Carrier: Water / Refrigerant: Ammonia (NH3) -often called "Aqua-Ammonia", but other combinations are possible.

The above-mentioned Aqua-Ammonia combination can achieve cooling temperatures down to -3 5°C or lower but the equipment is complex arid is generally only economic on large industrial installations for 1 MW of cooling or above.

The most commonly available combination of carrier fluid and refrigeration fluid is LiBrIH2O. There are thousands of plants of this nature in service across the world, mostly used for some form of air conditioning application. There are several large manufacturers of this type of equipment and the competition has led to equipment costs being optimised to a highly economic level. The LiBrTH2O combination does, however, suffer from a limitation in the temperature at which it can provide cooling. The water refrigerant freezes at 0°C so, after allowance for a temperature differential to provide heat flow, the minimum cooling temperature that such plants can achieve in practice is in the range +3 to +6°C.

Accordingly, it is an aim of the present invention to provide improved cooling processes and apparatus by at least partiy deciding problems of the prior art.

According to a first aspect of the invention, there is provided a system comprising: a compressor arranged to increase the pressure, and thereby increase the temperature, of a vapour of a first refrigerant; a heat exchanger arranged to remove heat from the first refrigerant, after the compressor has increased the pressure, and to deliver a liquid of the first refrigerant, an absorption refrigeration circuit for cooling a second refrigerant, and arranged such that the cooled second refrigerant provides the heat removal duty for the heat exchanger arranged to remove heat from the first refrigerant; an expansion valve arranged to reduce the pressure of the first refrigerant after heat has been removed from the first refrigerant, and to thereby cause flash evaporation of some of the first refrigerant and to reduce the temperature of the first refrigerant to be lower than that of the cooled second refrigerant.

The system of this aspect allows for heat removal by use of an absorption refrigeration circuit to be utilised at lower temperatures than would otherwise be possible. A commercially available absorption refrigeration system may not provide heat removal at a low enough temperature for a particular service. However, directly modifying an existing system to use a different refrigerant! carrier fluid combination is difficult and potentially expensive. According to this aspect, the lower temperatures can be reached without altering the absorption refrigeration system itself, and therefore maintaining the benefits associated with using an optimised system.

In preferred embodiments, the heat exchanger is arranged to cool the first refrigerant by heat exchange with the cooled second refrigerant, or the system further comprises a heat exchanger arranged to cool a third refrigerant by heat exchange with the a second refrigerant; and the heat exchanger arranged to remove heat from the first refrigerant is arranged to remove heat from the first refrigerant by heat exchange with the cooled third refrigerant, such that the cooled second refrigerant provides the heat removal duty for the heat exchanger indirectly. Direct heat exchange between the first and second refrigerant is energetically efficient, however it may be desirable to use the cooling from the absorption refrigeration for various different cooling duties, which may use different refrigerants or be used only intermittently. The use of an intermediate third refrigerant is one way to provide the necessary cooling in such circumstances.

The heat exchanger arranged to remove heat from the first refrigerant may be arranged to condense the first refrigerant. In this embodiment, the first refrigerant circuit forms a booster circuit, using the cooling duty from the absorption refrigeration circuit to condense the first refrigerant, and providing liquid first refrigerant to perform a cooling duty.

The system may further comprise a condenser arranged to condense the vapour of the first refrigerant produced by the compressor, wherein the heat exchanger arranged to remove heat from the first refrigerant is arranged to sub-cool the first refrigerant. In this embodiment the first refrigerant is already condensed before it exchanges heat with the absorption refrigeration circuit. However, the sub-cooling further enhances the cooling duty that the first refrigerant can perform.

In one embodiment, the system further comprises: an evaporator arranged to evaporate the liquid of the first refrigerant received from the expansion valve, thereby performing a cooling duty and producing a vapour of the first refrigerant, wherein the vapour of the first refrigerant from the evaporator is supplied to the compressor.

According to this embodiment, the cooled first refrigerant is used for a cooling duty in an evaporator, where it is boiled to a vapour that is returned to be cooled by the absorption refrigeration plant via the compressor.

Preferably, the second refrigerant is water. LiBrTH2O absorption refrigeration units are common and well optimised, but the water refrigerant freezes at 0°C, so has limited application. Using the water to cool another refrigerant in accordance with the invention allows LiBrIH2O absorption refrigeration units to be used in situations where they previously would not have been practical.

In a preferred embodiment the absorption refrigeration system is arranged to use heat from a combined heat and power plant to drive the absorption refrigeration cycle. The combination of CHP and absorption refrigeration allows for the use of heat that would otherwise be wasted. Therefore, this combination is economically and environmentally advantageous, as well as thermally efficient. The combination also allows for the load balancing between heating and refrigeration duties, which may vary from season to season, and therefore ensures that the heat produced during power production can always be put to use.

In a preferred embodiment a pump is arranged to increase the pressure of the first refrigerant after heat has been removed from it. This allows the first refrigerant to be used in extended refrigeration systems which involve other first refrigerant, compressor and condenser combinations in which the delivery pressure from the compressor(s) is higher than that of the compressor of the invention. The pumped refrigerant is subsequently expanded to provide a cooling duty.

According to another aspect of the invention, there is provided a method of using the cooling duty provided by an absorption refrigeration system, comprising: increasing the pressure of a vapour of a first refrigerant, and thereby increasing the temperature of the vapour of the first refrigerant; removing heat from the first refrigerant after the pressure has been increased, and thereby delivering a liquid of said first refrigerant; providing the heat removal duty for the step of removing heat from the first refrigerant via a second refrigerant cooled by the absorption refrigeration system; and reducing the pressure of the first refrigerant after heat has been removed from it, and thereby causing flash evaporation of some of the first refrigerant and reducing the temperature of the first refrigerant to be lower than that of the cooled second refrigerant.

The invention relates to the creation of hybrid refrigeration systems. In one embodiment a low pressure booster circuit is linked to an absorption plant to provide cooling at lower temperatures that can be achieved by the absorption plant alone. The combined systems are efficient compared to vapour compression systems, especially when "waste" heat from other processes is used to drive the absorption part of the circuit. The absorption plant can be provided with heat either by direct firing of a fuel, by waste heat from a combined heat and power (CHP) prime mover (such as a gas engine or gas turbine for example), or by any suitable source of waste heat from another process.

The present invention is described further below with reference to exemplary embodiments and the accompanying drawings, in which: Fig. 1 shows an absorption refrigeration circuit of the prior art; Fig. 2 shows a configuration of a new hybrid circuit according to a first embodiment of the invention; Fig. 3 shows a configuration of a new hybrid circuit of the first embodiment of the invention as connected to a gas engine CHP plant; Fig. 4 shows a second embodiment of the invention configured for sub-cooling of liquid refrigerant from plant main condensers; Fig. 5 shows the second embodiment of the invention as configured for the sub-cooling of liquid refrigerant from condensers of multiple main refrigeration circuits; Fig. 6 shows a third embodiment of the invention as configured for addition to an existing extended system; and Fig. 7 shows a fourth embodiment of the invention configured to including an interposing fluid to distribute cooling from an absorption refrigeration plant.

First Embodiment Fig. 2 shows a first embodiment of the invention. A low pressure booster circuit 10 is connected to an absorption plant 20. Here a LiBr/H20 absorption plant is selected for illustration. However, the principles of the invention here described below can be applied to an absorption plant using any combination of refrigerant and carrier fluids. The refrigerant in the low pressure booster circuit 10 in the example illustrated here is R404a, but could be any refrigerant that evaporates and condenses at suitable pressures to meet the temperature duty cycle.

The choice of refrigerant used will be determined by the temperature at which refrigeration needs to be provided and the temperature of which heat is to be removed from the refrigerant. For instance, refrigerant R134a is often used in the range -15°C to 40°C where its corresponding saturation pressure is from around 1 6OkPa to just over 1 000kPa. However, in the range -25°C to +40°C a common refrigerant is ammonia (where safety concerns allow) and its corresponding saturation pressure is in the range l5OkPa to 1555kPa. For lower temperatures or where ammonia is not appropriate refrigerant R404a is often used. In the range -3 7°C to +40°C refrigerant R404a has corresponding saturation pressures of I 5OkPa to 1 85OkPa. There are many other refrigerants that can be chosen each suitable for different duties, giving different efficiencies and different environmental impacts.

The choice of refrigerants is also affected by the temperature-pressure relationships of the saturation pressures for particular refrigerants. In general it is preferable to operate a refrigeration system so that the evaporator does not have to operate below atmospheric pressure and the condenser does not have to operate above 4000 kPa. This ensures that equipment costs are minimised, as process units do not need to be constructed to withstand relative vacuums or very large pressures.

In Figure 2, the refrigerant (e.g. R404a) boils in an evaporator 1, and in so doing performs a useful cooling duty at, for example, -25°C. The refrigerant vapour produced in the low pressure booster circuit evaporator 1 is pressure boosted for example, from 240 kPa to 800 kPa by a low power booster compressor 2. Heat is then removed by the intermediate pressure refrigerant vapour direct de-superheating and condensation on the other side of the evaporator 3 of the absorption plant 20, at a temperature preferably between +3°C and +15°C and more preferably in the region of 5°C. However, the temperature will depend on the detailed selection of working fluids. After condensation, condensed intermediate pressure liquid refrigerant is then expanded back to low pressure by an expansion valve 4 and the now cold low pressure vapour and liquid passes to the evaporator 1 to provide another cooling duty by re-evaporation and continuation of the cycle. Preferably, but not essentially, the pressure difference across the expansion valve 4 is nearly constant as the absorption refrigeration system 20 provides the interface to ambient conditions that otherwise causes variation in pressure difference and consequent difficulties in vapour compression cooling systems.

The cooling duty at the evaporator I may be provided, for example, to cool a heat transfer fluid that is being circulated around a factory or process, or the R404a refrigerant could alternatively be piped to a remote evaporator such as a display cabinet or cold room in a supermarket.

The creation of a direct heat exchange link between the low pressure booster system 10 and the absorption system 20 produces valuable cooling at lower temperatures than can otherwise be achieved. This feature increases substantially the applications where absorption refrigeration can be applied, and in turn increases enormously the applications where CHP with absorption refrigeration (so called "tn-generation") can be applied.

Fig. 3 below shows how the embodiment of Fig. 2 may be linked to a CHP engine to create an enhanced tn-generation process. A gas engine 30 is illustrated which is consuming gas and providing both electrical power and hot water from its jacket. The heat in the engine exhaust is used to drive the absorption system 20, which is taking heat from the booster circuit 10 and discharging it to atmosphere (or providing it for warm air heating of a building or other heating duty). The low pressure booster 10 provides low temperature cooling to a process of choice.

In a system where the products are power, hot water, warm air heating and refrigeration the system might be called "Quadra Generation".

Second Embodiment In certain applications, such as a frozen food factory or a process that is heavily cooling dominated, the cooling capacity required to cool the condenser(s) may exceed the capacity of the optimum selection of absorption refrigeration. In particular, this may be the case, for example, where the heat to drive the absorption plant is provided by a CHP unit. In this case, benefit can be achieved by sub-cooling the refrigerant liquid from the condenser 6 of the existing refrigeration plant, as shown in Fig. 4, instead of installing a booster compressor 2 and associated equipment as described in the section above. Sub-cooling of liquid refrigerant achieves increased cooling capacity even at low temperatures at the cost of only very little extra power input.

In such instances, a preferred arrangement is to directly exchange heat between the liquid refrigerant (R404a, for example) from the main condenser(s) 6 being supplied to the liquid expansion valve(s) 4 and the evaporator 3 of the absorption plant 20.

In the arrangement of Fig. 4, liquid from the condensers 6 of a main vapour compression refrigeration plant is diverted to the evaporator 3 of the absorption plant

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20. The evaporator 3 removes heat from the liquid and thereby cools it below its saturation temperature (that is, it is sub-cooled). This results in additional cooling capacity and improved coefficient of performance. The resulting sub-cooled liquid passes to the expansion valve(s) 4 of the plant. Due to the lower temperature of the refrigerant, in comparison to the non-subcooled embodiments previously discussed, the expansion produces a smaller amount of flash gas. The expanded liquid and flash gas is passed to the evaporator 1 of the vapour compression refrigeration plant where the liquid is boiled so producing useful cooling. The resulting refrigerant vapour is compressed at the compressors 5 and so delivered back to the condensers 6. The heat from the useful cooling duty performed by the vapour compression refrigeration circuit evaporator is rejected to cooling water via the vapour compression refrigeration circuit condenser 6. The sub-cooling heat, removed by the absorption refrigeration plant 20, is ultimately released by the condenser 24 of the absorption refrigeration plant 20. Other plant and device piping and heat exchanger configurations are possible that achieve the same process steps.

In certain applications refrigeration circuits are kept purposely separate to provide extra supply security to essential loads or to loads on different temperature circuits. A supermarket will sometimes have, for example, two refrigeration circuits operating at -6°C and two circuits operating at -24°C. In this instance there would be four separate sub-coolers directly exchanging heat with the evaporator of the absorption plant and providing liquid refrigerant sub-cooling to four separate refrigeration circuits. Other temperature range combinations could be present for alternate applications with varying numbers of circuits.

As a variant upon the sub-cooling system of the second embodiment, an arrangement for multiple circuit sub-cooling using a single absorption refrigeration circuit 20 is shown in Fig. 5, where the position numbers are analogous to those in Fig. 4. Other plant and device piping and heat exchanger configurations are possible that achieve the same process steps.

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Third Embodiment In a third embodiment, the system is arranged to be part of a larger extended refrigeration system that may, for example, already be existing and includes multiple compressors and multiple user end loads. This is shown in Fig. 6. In this example application, which could represent a supermarket, there is a common low pressure suction header 8 drawing refrigerant vapours from cooling operations where liquid refrigerant has been expanded and then evaporated to provide the cooling. These operations may be embodied by, for example, freezer cabinets or other user end cooling need, and are preferably at any temperature below +3°C. There are, in this example, several compressors 5 fitted that draw off the common suction header 8.

However, any number of compressors 5, including a single compressor 5, may be used. The compressors 5 in turn deliver to one or more condensers 6 which in turn drain to a receiver 9 that supplies liquid refrigerant to the common liquid main 7 that then supplies the user end loads, such as the freezer cabinets.

In this embodiment, the hybrid system is arranged with a low pressure booster compressor 2 which delivers refrigerant to the direct heat exchanger condenser that is also the evaporator 3 of the absorption refrigeration plant 20, both as previously illustrated in Fig. 2. In this embodiment, the liquid refrigerant that is condensed needs to be delivered into the main liquid header 7 but the header is at a higher pressure in order to supply the required end loads (for example 1500 kPa instead of the delivery of 800 kPa from the low pressure booster).

In this embodiment a liquid refrigerant pump 11 is fitted that pumps the liquid refrigerant up to the pressure of the main condensers and can therefore deliver the liquid from the low pressure booster hybrid condenser 3 into the main receiver 9 for mixing with the other refrigerant and supply to the common liquid feed header 7. In order to achieve stable control of the pump 11 delivery, a small receiver 12 may be fitted that acts as an accumulator and provides the pump II with a regular feed. Other plant and device piping and heat exchanger configurations are possible that achieve the same process steps.

Fourth Embodiment In a fourth embodiment, as shown in Fig. 7, the heat removal and cooling effect developed by the evaporator 3 of the absorption refrigeration plant 20 is transferred to other use(s) by means of an interposing fluid (such as chilled water) rather than by direct exchange to the refrigerant used in the end loads (as in the previous embodiments arranged for peak energy efficiency). The use of an interposing fluid does however have the advantage of being able to distribute cooling to meet varying load swings between different systems as may happen from time to time, such as in the example application of a supermarket.

In this example configuration as illustrated in Fig. 7, an interposing fluid transfers cooling from the evaporator 3a of the absorption refrigeration plant 20 to the condenser 3b of the low pressure booster hybrid. The booster circuit 10 may or may not be part of an extended system which would require it to be fitted with a liquid refrigerant pump 11 as described in more detail in the third embodiment and in Fig. 6.

The interposing fluid may also be distributed to provide liquid sub-cooling to refrigeration compressor plants as described in the second embodiment and in Figs. 4 and 5.

The interposing fluid in this embodiment may also be used to cool the condensers 6 of the main refrigeration plant. In this example of the use of the new hybrid system, an interposing fluid is used to reduce the condensing pressure of the main condensers 6 by supplying them at times of low demand elsewhere with a cooling fluid at a lower temperature than ambient. This produces energy savings when compared with normal refrigeration plant operation condensing against ambient temperatures yet allows operation to return to the higher, less efficient, condensing conditions if the absorption refrigeration plant is un-available or over-stretched. Fig.

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7 also shows a direct use of chilled water for an end use demand at the site that may be occasional or alternatively may provide a base load of cooling requirement.

The invention is illustrated using sample illustrations of configurations of apparatus that are picked as examples from a range of possible apparatus that all may achieve the same process and its attendant benefits. The invention may be realised through other detailed apparatus that differ from the configurations illustrated.

Claims

SCLAIMS1. A system comprising: a compressor arranged to increase the pressure, and thereby increase the temperature, of a vapour of a first refrigerant; a beat exchanger arranged to remove heat from the first refrigerant, after the compressor has increased the pressure, and to deliver a liquid of the first refrigerant; an absorption refrigeration circuit for cooling a second refrigerant, and arranged such that the cooled second refrigerant provides the heat removal duty for the heat exchanger arranged to remove heat from the first refrigerant; an expansion valve arranged to reduce the pressure of the first refrigerant after heat has been removed from the first refrigerant, and to thereby cause flash evaporation of some of the first refrigerant and to reduce the temperature of the first refrigerant to be lower than that of the cooled second refrigerant.
2. The system according claim 1, wherein: the heat exchanger is arranged to cool the first refrigerant by heat exchange with the cooled second refrigerant.
3. The system according claim 1, further comprising a heat exchanger arranged to cool a third refrigerant by heat exchange with the a second refrigerant; and the heat exchanger arranged to remove heat from the first refrigerant is arranged to remove heat from the first refrigerant by heat exchange with the cooled third refrigerant, such that the cooled second refrigerant provides the heat removal duty for the heat exchanger indirectly.
4. The system according to any previous claim, wherein the heat exchanger arranged to remove heat from the first refrigerant is arranged to condense the first refrigerant.
5. The system according to any previous claim, further comprising a condenser arranged to condense the vapour of the first refrigerant produced by the compressor, and wherein the heat exchanger arranged to remove heat from the first refrigerant is arranged to sub-cool the first refrigerant.
6. The system according any previous claim, further comprising: an evaporator arranged to evaporate the liquid of the first refrigerant received from the expansion valve, thereby performing a cooling duty and producing a vapour of the first refrigerant, and wherein the vapour of the first refrigerant from the evaporator is supplied to the compressor.
7. The system according to any one of the previous claims, wherein the second refrigerant is water.
8. The system according to any one of the previous claims, wherein the absorption refrigeration system is arranged to use heat from a combined heat and power plant to drive the absorption refrigeration cycle.
9. The system according to any one of the previous claims, further comprising a pump arranged to increase the pressure of the first refrigerant after heat has been removed from it.
10. A method of using the cooling duty provided by an absorption refrigeration system, comprising: increasing the pressure of a vapour of a first refrigerant, and thereby increasing the temperature of the vapour of the first refrigerant;Sremoving heat from the first refrigerant after the pressure has been increased, and thereby delivering a liquid of said first refrigerant; providing the heat removal duty for the step of removing heat from the first refrigerant via a second refrigerant cooled by the absorption refrigeration system; and reducing the pressure of the first refrigerant after heat has been removed from it, and thereby causing flash evaporation of some of the first refrigerant and reducing the temperature of the first refrigerant to be lower than that of the cooled second refrigerant.
11. The method according to claim 10, wherein the step of removing heat from the first refrigerant comprises cooling the first refrigerant by heat exchange with the second refrigerant.
12. The method according to claim 10, wherein the step of removing heat from the first refrigerant comprises: cooling a third refrigerant by heat exchange with the second refrigerant; and removing heat from the first refrigerant by heat exchange with the third refrigerant, such that the cooled second refrigerant provides the heat removal duty for removing heat from the first refrigerant indirectly.
13. The method according to any one of claims 10 to claim 12, wherein the step of removing heat from the first refrigerant further comprises condensing the first refrigerant.
14. The method according to any one of claims 10 to claim 13, further comprising: a step of condensing the first refrigerant after the pressure has been increased, and wherein the step of removing heat from the first refrigerant further comprises sub-cooling the first refrigerant after it has been condensed.
15. The method according to any one of claims 10 to claim 14, further comprising: evaporating the first refrigerant after the step of reducing the pressure, to perform a cooling duty and produce a vapour of the first refrigerant.
16. The method according to any one of claims 10 to claim 25, wherein the second refrigerant is water.
17. The method according to any one of claims 10 to claim 16, wherein the absorption refrigeration system is arranged to use heat from a combined heat and power plant to drive the absorption refrigeration cycle.
18. The method according to any one of claims 10 to claim 17, further comprising: pumping the first refrigerant to increase its pressure after heat has been removed from it.
19. A system substantially as hereinbefore described with reference to or as illustrated in the accompanying drawings.Amendments to the claims have been filed as follows 1. A refrigeration system comprising: a compressor arranged to increase the pressure, and thereby increase the temperature, of a vapour of a first refrigerant; a heat exchanger arranged to remove heat from the first refrigerant, after the compressor has increased the pressure, and to thereby condense the first refrigerant and deliver a liquid of the first refrigerant; an absorption refrigeration circuit configured to circulate a second refrigerant in an absorption refrigeration cycle, and arranged such that the cooled second refrigerant provides the heat removal duty for the heat exchanger arranged to remove heat from the first refrigerant, the heat exchanger being arranged to cool and condense the first refrigerant by direct heat exchange with the cooled second refrigerant; and an expansion valve arranged to reduce the pressure of the first refrigerant after :::: heat has been removed from the first refrigerant, and to thereby cause flash evaporation of some of the first refrigerant and to reduce the temperature of the first refrigerant to be lower than that of the cooled second refrigerant; * wherein the refrigeration system is configured to provide cooling, via the first *S4**S * * 20 refrigerant, at below +3 degrees Celsius. * S. * -.2. The system according any previous claim, further comprising: an evaporator arranged to evaporate the liquid of the first refrigerant received from the expansion valve, thereby performing a cooling duty and producing a vapour of the first refrigerant, and wherein the vapour of the first refrigerant from the evaporator is supplied to the compressor.3. The system according to any one of the previous claims, wherein the absorption refrigeration system is arranged to use heat from a combined heat and power plant to drive the absorption refrigeration cycle.4. The system according to any one of the previous claims, further comprising a pump arranged to increase the pressure of the first refrigerant after heat has been removed from it.5. A method of using the cooling duty provided by an absorption refrigeration system, comprising: increasing the pressure of a vapour of a first refrigerant, and thereby increasing the temperature of the vapour of the first refrigerant; removing heat from the first refrigerant after the pressure has been increased, to condense the first refrigerant and thereby delivering a liquid of said first refrigerant, wherein the removing of heat from the first refrigerant is achieved via direct heat exchange with a water refrigerant that is cooled in an absorption refrigeration cycle of the : * 15 absorption refrigeration system; reducing the pressure of the first refrigerant after heat has been removed from it, and thereby causing flash evaporation of some of the first refrigerant and reducing the temperature of the first refrigerant to be lower than that of the cooled second refrigerant; and : * 20 providing cooling, via the first refrigerant, at below +3 degrees Celsius.* * 6. The method according to claims 5, further comprising: a step of condensing the first refrigerant after the pressure has been increased, and wherein the step of removing heat from the first refrigerant further comprises sub-cooling the first refrigerant after it has been condensed.7. The method according to claim 5 or 6, further comprising: evaporating the first refrigerant after the step of reducing the pressure, to perform a cooling duty and produce a vapour of the first refrigerant.8. The method according to any one of claims 5 to 7, wherein the absorption refrigeration system is arranged to use heat from a combined heat and power plant to drive the absorption refrigeration cycle.9. The method according to any one of claims 5 to 8, further comprising: pumping the first refrigerant to increase its pressure after heat has been removed from it.10. A system substantially as hereinbefore described with reference to or as illustrated in the accompanying drawings. S... * *1 *. * * . S...4 S*S** * SS * S * *. * . S S...SS SS S 55