Field of the invention

The present invention relates to refrigeration systems, in particular C0 ₂ refrigeration systems.

Background

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or an form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Vapour compression refrigeration systems typically include a refrigerant continuously cycled between the primary components of an evaporator, compressor, condenser and expansion valve. At the evaporator, the refrigerant collects heat from the air to be cooled. This heat exchange causes the temperature of the refrigerant in the evaporator to increase and vaporise. The refrigerant is then compressed by a compressor which increases the refrigerants pressure and temperature, and the resultant hot gaseous refrigerant is then directed to a condenser where the heat can be removed by cooling the vapour. As the refrigerant is cooled heat is given up heat in the condenser via indirect heat transfer to a secondary fluid such as air, water or glycol. The gaseous refrigerant thereby liquefies (condenses) and is returned, via an expansion valve, to the evaporator so the process can be continuous.

The type of refrigerant selected depends on the application and temperature of refrigeration required. Some examples include Carbon Dioxide, Ammonia and Freon (CFC's and HCFC's). In the past Freon was widely used, however due to its harmful effects on the environment, use of Freon in refrigeration systems is now prohibited in some countries. The current trend is towards more environmentally friendly refrigerants such as C.0 ₂ . C0 ₂ refrigeration systems are more efficient than conventional Freon refrigeration systems when the ambient temperature is low. However, as the ambient temperature rises, a significant decrease in performance is observed.

Warm ambient temperatures have an adverse effect on both the operation and efficiency of a C0 ₂ vapour compression refrigeration system. Once the ambient temperature exceeds approximately 18 degrees Celsius, the performance of the refrigeration compressor is reduced in terms of work done and of power consumed, and the C0 ₂ system becomes less efficient that a conventional Freon (CFC or HCFC) system.

Any further increase in ambient temperature has a further negative effect on the systems performance typically increasing power consumption and decreasing the refrigerant systems cooling capacity. The loss of performance becomes more and more serious as the ambient temperature continues to rise resulting in a significant increase in the required compressor displacement and energy consumption required to achieve a given task.

Carbon dioxide has a low critical temperature (the temperature above which the refrigerant will not liquefy) of 31 deg C. Some carbon dioxide systems are able to operate both above and below the critical temperature and these systems are called trans-critical systems.

Accordingly a condenser for use in a carbon dioxide system can operate in two ways:

1. If it is operating in the sub critical area (below 31 °C) it will be a refrigerant

condenser.

2. If it is operating in the super critical mode (above 31 °C) it will operate as a gas cooler, and will not liquefy the refrigerant passing through it.

Generally when a refrigeration system is operating above the critical temperature, it is operating in the super critical mode. This type of operation is less energy efficient than sub critical operation and should be avoided when seeking low operating power consumption.

The present invention seeks to provide a refrigeration system that will prevent the loss of performance at warmer ambient temperatures whilst still permitting the system to take advantage of energy saving opportunities when lower ambient temperatures are available.

Summary of the Invention

In one broad form the present invention provides a carbon dioxide (C0 ₂ ) based refrigeration system including: a primary refrigeration circuit cycling C0 ₂ , the primary refrigeration circuit including a main condenser and an auxiliary heat exchanger, the auxiliary heat exchanger for reducing the temperature of C0 ₂ flowing out of the main condenser, wherein the auxiliary heat exchanger is independently operable with respect to the primary refrigeration circuit such that when the temperature of C0 ₂ flowing out of the main condenser is at or above a threshold temperature, the auxiliary heat exchanger operates to reduce the temperature of the C0 ₂ .

In one form, the C0 ₂ based refrigeration system further includes a sensor for detecting the temperature of C0 ₂ flowing out of the main condenser, and a control unit connected to the sensor, the control unit configured to operate the auxiliary heat exchanger when the temperature of C0 ₂ flowing out of the main condenser is at or above the threshold temperature.

In another form, the control unit is configured to stop operation of the auxiliary heat exchanger when the temperature of C0 ₂ flowing out of the main condenser below the threshold temperature. In a further form, the auxiliary heat exchanger is connected downstream, in series with, the main condenser. In one form, the auxiliary heat exchanger is connected in parallel with the main condenser.

In another form, a C0 ₂ based refrigeration system the auxiliary heat exchanger includes: an auxiliary condenser which is part of the primary refrigeration circuit such that it receives the C0 ₂ flowing through the circuit; and an evaporator which is part of a secondary refrigeration circuit cycling a second refrigerant, the evaporator indirectly cooling the C0 ₂ flowing through the auxiliary condenser.

In one form, the threshold temperature is such that the C0 ₂ flowing through the primary refrigeration circuit is prevented from becoming a super critical fluid.

In one form, the threshold temperature is about 31° C.

In another form, the threshold temperature is about 25° C.

In one form, the primary refrigeration circuit includes a plurality of auxiliary heat exchangers.

In another form, the refrigeration system includes a plurality of primary refrigeration circuits, and a plurality of auxiliary heat exchangers, any one or a combination of the plurality of auxiliary heat exchangers operating to reduce the temperature of C0 ₂ within any one of the plurality of primary refrigeration circuits.

In a further broad form, the present invention provides a C0 ₂ based refrigeration system including:

a primary refrigeration circuit cycling C0 ₂ , the primary refrigeration circuit including a main condenser; and

a cooling assembly independently operable with respect to the primary refrigeration circuit,

wherein the cooling assembly is operable to reduce the temperature of the main condenser.

In one form, the cooling assembly operates to reduce the temperature of the main condenser when the temperature of C0 ₂ flowing out of the main condenser reaches a threshold temperature.

In a further form, the refrigeration system further includes a sensor for detecting the temperature of C0 ₂ flowing out of the main condenser, and a control unit connected to the sensor, the control unit configured to operate the cooling assembly when the temperature of C0 ₂ flowing out of the main condenser is at or above the threshold temperature.

Brief Description of the Drawings

Examples of the present invention will now be described with reference to the accompanying drawings, in which:

'

Figure 1 illustrates a refrigeration system according to one example of the invention having "parallel" configuration.

Figure 2 illustrates a refrigeration system according to one example of the invention having "series" configuration.

Figure 3 illustrates a refrigeration system with two auxiliary heat exchangers.

Figure 4 illustrates a larger refrigeration system according to one example of the invention having multiple primary refrigeration loops and multiple auxiliary heat exchangers. Figure 5 illustrates a larger refrigeration system according to one example of the invention having multiple primary refrigeration loops and multiple auxiliary heat exchangers.

Detailed Description

The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive.

In the context of this specification, the word "comprising" means "including principally but not necessarily solely" or "having" or "including", and not "consisting only of. Variations of the word "comprising", such as "comprise" and "comprises" have correspondingly varied meanings.

Embodiments of the present invention provide a carbon dioxide (C0 ₂ ) based refrigeration system including a primary refrigeration circuit cycling C0 ₂ , the primary refrigeration circuit including a main condenser and an auxiliary heat exchanger, the auxiliary heat exchanger for reducing the temperature of C0 ₂ flowing out of the main condenser. The auxiliary heat exchanger is independently operable with respect to the primary refrigeration circuit such that when the temperature of C0 ₂ flowing out of the main condenser is at or above a threshold temperature, the auxiliary heat exchanger operates to reduce the temperature of the C0 ₂ .

Figure 1 illustrates a refrigeration system according to one embodiment of the invention. The refrigeration system (1) includes a primary refrigeration circuit (2) cycling C0 ₂ . The primary refrigeration circuit (2) includes a compressor, main condenser (4), auxiliary condenser (5), expansion valve (6) and evaporator (7). A secondary refrigeration circuit (8) can operate to maintain the auxiliary condenser (5) at a temperature lower than the main condenser (4). Accordingly this auxiliary heat exchange operates to reduce the temperature of the C0 ₂ flowing out of the main condenser if necessary. For example, if the temperature of C0 ₂ leaving the main condenser is too high (above about 31°C), the C0 ₂ can become super critical, lessening efficiency of the compressor and overall circuit. In practice one may wish to keep the C0 ₂ temperature below about 25°C so as to avoid fluctuations in C0 ₂ density.

The secondary refrigeration circuit (8) is independently operable to the primary refrigeration circuit and cycles a second refrigerant (such as, for example, Ammonia or R134A). Although not all shown in the figures, the secondary refrigeration circuit would also typically also include a compressor, condenser, expansion valve and evaporator (9). It is the evaporator (9) of the secondary refrigeration circuit (8) that indirectly cools the auxiliary condenser (5) of the first refrigeration circuit (2) such that it is reduced . to a temperature lower than the main condenser (5). This allows the auxiliary condenser to further cool C0 ₂ leaving the main condenser.

A control unit for the refrigeration system is able to turn the secondary refrigeration circuit (8) on and off as required. For example, when the main condenser is air cooled, the primary refrigeration circuit can operate either on its own, when low temperature ambient conditions are available, or in conjunction with the secondary refrigeration circuit, when surrounding ambient temperatures are not sufficiently cool.

In some embodiments the refrigeration system may further include a sensor connected to the control unit for detecting the temperature and/or pressure of C0 ₂ leaving the main condenser, or the surrounding ambient temperature. The control unit may be configured to operate the cooling assembly at or above a threshold C0 ₂ temperature or pressure, or at or above a threshold ambient temperature. The control unit may also be configured to stop operation of the secondary refrigeration circuit below the threshold C0 ₂ temperature or pressure, or below ambient temperature.

In any case, the independently operable secondary refrigeration circuit (8) operates to allow the primary refrigeration circuit to operate at its most energy efficient condition when ambient temperatures are warm, by keeping the C0 ₂ below its super critical point. Once C0 ₂ reaches the super critical phase, the efficiency of the compressor is greatly reduced. When ambient temperatures are cool enough, the secondary refrigeration circuit need not be operated as the primary circuit can take advantage of all the available ambient cooling (low ambient temperatures at night or during winter months for example).

Within the primary refrigeration circuit (2) of the refrigeration system in figure 1 the auxiliary condenser (5) is arranged in parallel to the main condenser (4). The first refrigerant is thereby cooled alternatively at the main condenser (4) or by heat exchange between the auxiliary condenser (5) and the evaporator (9) of the secondary refrigeration circuit (8). In this arrangement no control valves are required to control the direction of refrigerant flow between the two condenser options. Instead the cooling will take place in which ever condenser is in operation. C0 ₂ would be naturally drawn to the cooler condenser as a cooler condenser converts more gas to liquid and thus lowers the pressure within the condenser, ultimately drawing in more C0 ₂ .

For example, the air cooled main condenser (4) with fans running will draw cooling air through it and perform refrigerant cooling. Should the surrounding ambient air temperature rise to a level whereby cooling provided by the main condenser is insufficient, the auxiliary condenser (5) can take over cooling. Additionally, the auxiliary condenser (5) will take over refrigerant cooling if the airs on the primary condenser (4) are stopped and the secondary refrigeration circuit (8) is activated. As the auxiliary condenser (5) is cooled by the secondary refrigeration circuit (8) rather than the ambient environment, it can achieve sufficient cooling even when ambient temperatures are too high for the main condenser to provide efficient operation. The parallel arrangement is best suited to systems that have height restrictions where the primary condenser and the auxiliary condenser must be mounted at the same level.

Figure 2 illustrates a different configuration wherein the main condenser (4) of the first refrigeration circuit is connected in series with the auxiliary condenser (5) such that both the condensers are able to cool and condense the low stage refrigerant. This "in series" configuration cools the refrigerant (e.g., carbon dioxide) in the main condenser (4) prior to entering the auxiliary condenser (5). This arrangement can provide two-stage cooling and therefore may also have power absorbed by both main (4) and auxiliary condensers (5). Energy usage may need to be managed more closely to make the system run at the optimum power consumption possible. A C0 ₂ refrigeration system in accordance with the "in series" embodiment of figure 2 series may operate as follows in under hot, cool and cold conditions:

1) In hotter weather a C0 ₂ refrigerant may de-super heat in the main condenser to a point about 2 deg C above ambient temperature, reducing the load on the auxiliary condenser, which would thereafter condense the C0 ₂ and sub cool it to about 15 deg C.

2) In cooler weather where the C0 ₂ can condense in the main condenser (3) the auxiliary condenser can act as a sub-cooler, so as to improve the efficiency of the C0 ₂ system.

3) In cold weather where the main condenser is able to cool the C0 ₂ to 15 deg or below the auxiliary condenser remains in the circuit, but does not provide any further cooling,, and does not absorb any power. In larger systems the refrigeration system may include a primary refrigeration loop with a plurality of auxiliary heat exchangers.

It will be appreciated that the auxiliary heat exchanger down stream from the main condenser does not need to be facilitated by a secondary refrigeration circuit. For example other cooling means to cool the C0 ₂ leaving the main condenser can be provided, such as, for example, a natural source such as river water or other water source.

Figure 3 illustrates an embodiment wherein a secondary refrigeration circuit provides two evaporators (36a, 36b) connected in parallel, each evaporator operating to cool a corresponding auxiliary condenser (41a, 41b) of a primary C0 ₂ refrigeration loop (40a, 40b). This configuration shows how a secondary refrigeration circuit may cool more than one primary refrigeration circuit. The secondary refrigeration circuit of figure 3 includes an accumulator (31), compressor (30), oil separator (32), condenser (33), receiver (34) and two evaporators (36a, 36b) connected in parallel. The two evaporators (36a, 36b) are arranged to interact and cool the auxiliary condensers (41a, 41b) of the two primary loops (40a, 40 b).

Alternatively, the multiple evaporator configuration of figure 3 may be used to cool a single large primary refrigeration circuit. Multiple evaporators of the secondary circuit could operate to cool one or more condensers (arranged in series or otherwise) of a large primary refrigeration circuit. Such a configuration would allow the heat transfer load at each heat exchange point to thereby be divided among the evaporators of the secondary circuit.

Some larger refrigeration systems may include multiple primary C0 ₂ refrigeration circuits, the respective auxiliary condensers of each primary circuit being cooled by any one or a combination of secondary refrigeration circuits or otherwise.

For example, figure 4 provides a refrigeration system including two primary C0 ₂ refrigeration circuits (50,50a) each having respective main condensers (51, 51a). Each primary refrigeration circuit has 3 auxiliary heat exchangers (52, 53, 54) (52a, 53a, 54a). The auxiliary heat exchangers are connected in parallel with each other and with the main condenser.

Figure 5 shows a similar arrangement with two primary C0 ₂ refrigeration loops (60,60a) each having respective main condensers (61,61a). Each loop has 3 auxiliary heat exchangers (62, 63, 64), (62a, 63a, 64a). However, figure 5 has the auxiliary heat exchangers of the primary circuits in parallel with one another, but in series with respective main condensers.

The larger system configurations such as those of figures 3, 4 and 5 can provide additional protection against individual refrigeration circuit failure. The main and auxiliary condensers do not necessarily need to be air cooled. The cooling fluid may be anything that is cooler than the condensing temperature. For example the main condenser could be cooled by air, water, brine or any other fluid. The same applies to the auxiliary condenser and/or second refrigerant.

This allows usage of cheap cooling fluids when they are available, and other cooling fluids to take over if the primary fluid is no longer able to provide sufficient cooling. A further embodiment does not utilise an auxiliary heat exchanger in combination with a main condenser, but rather a cooling assembly operates to cool the main condenser itself. For example, the system may have chilled water as the main condenser cooling fluid, which requires operation of a cooling assembly (chiller). In winter the chiller may be shut down and the colder surrounding ambient air may be used to cool the main condenser. Conversely, when the air is too warm to provide adequate cooling to the main condenser, the cooling assembly/chiller may be switched on such that chilled water is used for cooling.

Operating the chiller constantly, in cold weather, would not be energy efficient as one would need to artificially raise the operating temperature of the chiller to keep it running.

As previously described, the present invention allows the auxiliary heat exchanger to be switched on and off on demand. The auxiliary heat exchanger can provide a cooling fluid that is the below the temperature of the main condenser in the primary refrigeration circuit. For example, in colder ambient temperatures the auxiliary heat exchanger may be switched off such that the main condenser is cooled by surrounding air.

In one particular example, a C0 ₂ refrigeration system may be installed in the vicinity of one or more separate air conditioning systems (for example such systems may provide cooling via ammonia water chillers). Such an arrangement is typical in a supermarket environment wherein refrigeration and air conditioning can be required simultaneously. During cooler winter periods, the air conditioning systems would typically be off, and the cooler ambient temperatures would allow the C0 ₂ system to effectively provide refrigeration. During warmer periods, the air conditioning systems would typically be running. Accordingly, if the main condenser of the refrigeration system is not sufficiently cooling the C0 ₂ , any excess cooling capacity could be fed off the running air conditioning systems to provide further cooling to the C0 ₂ . This would prevent C0 ₂ becoming super critical and thus avoid compressor inefficiency in the C0 ₂ system.

It is therefore apparent how having an independently operable auxiliary heat exchanger can reduce energy wastage of the overall system. Optional embodiments of the present invention may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

Although a preferred embodiment has been described in detail, it should be understood that various changes, substitutions, and alterations can be made by one of ordinary skill in the art without departing from the scope of the present invention.

It will be appreciated that various forms of the invention may be used individually or in combination.