Abstract:
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.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is a U.S. National Phase application under 35 U.S.C §371 of International Application No. PCT/GB09/002,360, filed Oct. 2, 2009, entitled PROCESS AND APPARATUS FOR COOLING, which claims priority to Great Britain patent application number 0818162.0, filed Oct. 3, 2008. 
     
    
     BACKGROUND 
       [0002]    The invention relates to processes and apparatus for using the cooling duty provided by an absorption refrigeration plant. 
         [0003]    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. 
         [0004]    Refrigerant fluid evaporates at low pressure and temperature in the evaporator  3 , and in doing so provides a useful cooling duty UC. 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 by heat supply HS 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  where it is cooled by cooling water CW ready to absorb more refrigerant vapour. 
         [0005]    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. 
         [0006]    The absorption process illustrated in  FIG. 1  may be modified to improve performance, for example by the use of multi-staging. 
         [0007]    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) (or other lithium halides) solution in water/Refrigerant: Water (H 2 O), and   2) Carrier: Water/Refrigerant: Ammonia (NH 3 )—often called “Aqua-Ammonia”,
 
but other combinations are possible.
       
 
         [0010]    The above-mentioned Aqua-Ammonia combination can achieve cooling temperatures down to −35° C. or lower but the equipment is complex and is generally only economic on large industrial installations for 1 MW of cooling or above. 
         [0011]    The most commonly available combination of carrier fluid and refrigeration fluid is LiBr/H 2 O. 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 LiBr/H 2 O 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. 
         [0012]    The present inventors have identified that it would be desirable to utilise absorption refrigeration in retail situations such as supermarkets. Supermarkets have various cooling needs, including both display and storage refrigeration and freezing as well as air conditioning applications. Further, supermarkets often have their own on-site power generation, and it would be efficient to utilise the waste heat from the power generation to drive absorption refrigeration. 
         [0013]    However, supermarkets typically operate two cooling circuits, for use in different operations: one cooling circuit operating at approximately −5° C. to −10° C., and another operating at approximately −25° C. to −35° C. Therefore, using conventional absorption refrigeration techniques, it would be necessary to resort to expensive systems such as the above-mentioned Aqua-Ammonia carrier/refrigerant combination, in order to achieve the necessary cooling temperatures. This makes the integration of absorption cooling a less attractive prospect. 
         [0014]    U.S. Pat. No. 4,745,768 discloses a refrigeration system, which utilises absorption refrigeration, but which reaches temperatures below that obtained by the absorption unit. The cooling duty from the absorption unit is used to condense a refrigerant in a compressive refrigeration circuit, before that refrigerant is subsequently expanded (thereby achieving a lower temperature than the absorption unit refrigerant). However, the present inventors have identified that the system of U.S. Pat. No. 4,745,768 suffers from being inefficient. This is because the cooling duty of the absorption unit is transferred to the compressive refrigeration circuit via an intermediate water circuit. 
         [0015]    The use of the intermediate water circuit in U.S. Pat. No. 4,745,768 introduces losses in efficiency compared to a direct transfer of heat from the refrigerant in the compressive circuit to the refrigerant in the absorption unit. However, it has conventionally been accepted that this loss of efficiency is acceptable, because the engineering challenges of providing the direct heat exchange at a condenser between the of compressive and absorption circuits are too great. This is because the high pressure of the compressive circuit and the vacuum of the absorption circuit result in large pressure differentials across the heat exchange equipment, causing a high risk of leakage. 
         [0016]    U.S. Pat. No. 4,873,839 (which refers to U.S. Pat. No. 4,745,768) illustrates this prejudice. Compared to U.S. Pat. No. 4,745,768, direct heat transfer between a compressive and an absorption refrigeration circuit is disclosed. However, the heat transfer occurs at a sub-cooler rather than a condenser, thereby avoiding, rather than overcoming, the difficulties mentioned above. Further, the system uses an ammonia refrigerant. As the system is designed for large tonnage refrigeration, the expense of an ammonia absorption system is bearable, and allows for a sub-zero cooling of the compressive refrigerant by the absorption refrigerant. For supermarket applications, the expense of an ammonia absorption unit is not economic for the amount of refrigeration required. There is also a risk with the system of U.S. Pat. No. 4,873,839 that the low temperatures of the absorption refrigerant could lead to over-cooling of the compressive refrigerant, leading to a loss of pressure in the compressive circuit. 
       SUMMARY 
       [0017]    In particular, in the case of the lithium halide/water absorption units the low pressures required in the absorption unit, have been considered incompatible with a direct heat transfer to a compressive refrigeration circuit. However, the present inventors have realised that these engineering constraints no longer exist. Further it is preferable to connect directly to the condenser to enable the exploitation of the absorption refrigeration (which may be powered by waste heat) without the need to introduce further cooling operations into the compressive refrigeration cycle. That is, including a sub-cooler requires the provision of another unit operation which incurs further expense when fitting an absorption cooling unit to an existing refrigeration cycle. Also, using a sub-cooler to try and realise energy efficiency gains risks loss of pressure in the compressive circuit, and makes less efficient use of the refrigeration available than a condenser would. 
         [0018]    Accordingly, it is an aim of the present invention to provide improved cooling processes and apparatus by at least partly deciding problems of the prior art. 
         [0019]    According to a first aspect of the invention, there is provided 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 to condense the first refrigerant, after the compressor has increased the pressure, and to deliver a liquid of the first refrigerant; an absorption refrigeration circuit configured to circulate a second refrigerant in an absorption refrigeration cycle, whereby the second refrigerant is cooled, and arranged such that the cooled second refrigerant provides the heat removal duty for the heat exchanger by direct heat exchange with the first refrigerant in the heat exchanger; and a first 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 below 0° C. 
         [0020]    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. The heat exchanger is arranged to cool the first refrigerant by direct heat exchange with the cooled second refrigerant, because heat exchange between the first and second refrigerant is energetically efficient. The second refrigerant is the refrigerant used in the absorption cycle of the absorption refrigeration circuit. That is, the second refrigerant is the refrigerant cooled during the absorptive refrigeration process. For example, where the absorption refrigeration is based on a water refrigerant and an LiBr carrier, the second refrigerant is the water. 
         [0021]    The heat exchanger arranged to remove heat from the first refrigerant condenses the first refrigerant. As a result, 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. 
         [0022]    In one embodiment, the system further comprises: an evaporator arranged to evaporate the liquid of the first refrigerant received from the first 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. 
         [0023]    Preferably, the second refrigerant is water. LiBr/H 2 O 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 LiBr/H 2 O absorption refrigeration units to be used in situations where they previously would not have been practical. 
         [0024]    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. 
         [0025]    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. 
         [0026]    In a preferred embodiment the first refrigerant is carbon dioxide. Carbon dioxide is viewed as an environmentally benign refrigerant, but suffers from the need for high pressures in conventional compressive refrigeration systems. The invention overcomes this need. 
         [0027]    In a preferred embodiment, the system may further comprise two circuits, each connected to receive first refrigerant from the heat exchanger arranged to cool the first refrigerant and to return the first refrigerant to the compressor arranged to increase the pressure of the first refrigerant; wherein the first expansion valve is located in a first of the two circuits, and a second expansion valve is provided a second of the two circuits. The first expansion valve may be arranged to cause flash evaporation of some of the first refrigerant and to reduce the temperature of the first refrigerant to a temperature in the range of from 0 to −10° C., and preferably from −4 to −6° C. The second expansion valve may be arranged to cause flash evaporation of some of the first refrigerant and to reduce the temperature of the first refrigerant to a temperature in the range of from −20 to −35° C., and preferably from −25 to −30° C. These embodiments are particularly relevant to supermarket applications. 
         [0028]    According to another aspect of the invention, there is provided a method of using the cooling duty provided by an absorption refrigeration system, in a 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; circulating a second refrigerant in an absorption refrigeration cycle, thereby cooling the second refrigerant; providing the heat removal duty for the step of removing heat from the first refrigerant via direct heat exchange with the second refrigerant in a heat exchanger; 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 providing cooling, via the first refrigerant, at below 0° C. 
         [0029]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The present invention is described further below with reference to exemplary embodiments and the accompanying drawings, in which: 
           [0031]      FIG. 1  shows an absorption refrigeration circuit of the prior art; 
           [0032]      FIG. 2  shows a configuration of a new hybrid circuit according to a first embodiment of the invention; 
           [0033]      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; 
           [0034]      FIG. 4  shows a second embodiment of the invention as configured for addition to an existing extended system; and 
           [0035]      FIG. 5  shows a third embodiment of the invention configured to operate in a “cascade” refrigeration system. 
       
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
       [0036]      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/H 2 O 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. 
         [0037]    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 160 kPa to just over 1000 kPa. 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 150 kPa to 1555 kPa. For lower temperatures or where ammonia is not appropriate refrigerant R404a is often used. In the range −37° C. to +40° C. refrigerant R404a has corresponding saturation pressures of 150 kPa to 1850 kPa. There are many other refrigerants that can be chosen each suitable for different duties, giving different efficiencies and different environmental impacts. 
         [0038]    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. 
         [0039]    In  FIG. 2 , the refrigerant (e.g. R404a) boils in an evaporator  1 , and in so doing performs a useful cooling duty UC 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. 
         [0040]    The cooling duty at the evaporator  1  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. 
         [0041]    Preferably, especially for supermarket applications, the invention provides cooling at below 0° C. The invention is particularly relevant to supermarket applications. Supermarket applications include the cooling of perishable and frozen goods, as well as, optionally, the provision of air conditioning. Typical refrigeration capacities of supermarket applications are in the range of approximately 15-300 kW of cooling duty, or approximately 5-85 tonnes of refrigerant. In supermarket applications, refrigeration is provided in both open and closed cabinets as well as cold storage rooms. 
         [0042]    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 “tri-generation”) can be applied. 
         [0043]      FIG. 3  below shows how the embodiment of  FIG. 2  may be linked to a CHP engine to create an enhanced tri-generation process. A gas engine  30  is illustrated which is consuming gas NG and providing both electrical power P and hot water HW from its jacket. The driving heat DH 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 as waste heat WH (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. 
         [0044]    In a system where the products are power, hot water, warm air heating and refrigeration the system might be called “Quadra Generation”. 
       Second Embodiment 
       [0045]    In a second 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. 4 . 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., and more preferably any temperature below 0° 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 L, such as the freezer cabinets. 
         [0046]    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). 
         [0047]    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  11  with a regular feed. Other plant and device piping and heat exchanger configurations are possible that achieve the same process steps. 
       Third Embodiment 
       [0048]    A third embodiment is shown in  FIG. 5 . In this embodiment the invention is incorporated as the topping cycle in a multi stage refrigeration plant, such as may be installed at a supermarket for cooling frozen foods and cooling fresh food display cases and other loads. As discussed previously, supermarkets often operate two refrigeration circuits at different temperatures, such as in the ranges of 0 to −10° C. and −20 to −35° C. The circuits may be arranged in a “cascade” configuration, in which the two circuits are connected so that coolant returned from the lower temperature circuit is discharged into the higher temperature circuit. In other embodiments, an interposing heat exchanger may be fitted between the lower temperature and higher temperature circuits. 
         [0049]    In certain applications the working fluid of the vapour compression system may be carbon dioxide (CO 2 ) which is preferred by many as an environmentally benign fluid. The drawback that a CO 2  refrigeration plant suffers from is that the pressures have to be very high, above the critical point of 31° C., 73 bar gauge in order to be able to reject heat from the condenser to ambient. 
         [0050]    The present invention allows for a CO 2  refrigeration plant to operate with reduced top end pressures, below 50 bar gauge, or in cases below 40 bar gauge. With reference to the diagram below, the system is fitted with a low pressure suction header  13 , that receives refrigerant gas from frozen food storage and other colder loads LL at typically around −30° C. Low pressure compressors  14  draw gas from the header and deliver it to the inter-stage header  15  that would typically operate at −5° C. saturation pressure equivalent. The gas produced by higher temperature loads LH such as from cooling of fresh perishable goods are introduced also into this header. It is noted that the temperatures shown in the drawing are examples only, and are not essential to the invention. 
         [0051]    High stage compressors  5  draws off this header and delivers to a delivery header  16  that typically runs in the range +6 to +16° C. (40 to 51 bar gauge), and normally at around +10° C. (44 bar gauge). The gas from the delivery header is drawn into the evaporator  3  of the absorption plant  20  where it is condensed. Liquid refrigerant runs into the high pressure liquid receiver  12  which accumulates liquid to feed to the loads along the liquid supply header  7 . 
         [0052]    The same system can be used with other refrigerants such as halogenated hydrocarbons, hydrocarbons and other mineral refrigerants such as ammonia. 
         [0053]    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.