A TRANSCRITICAL COOLING SYSTEM WITH IMPROVED COOLING CAPACITY

The present invention relates to a transcritical refrigeration system, and in particular the present invention relates to a transcritical refrigeration system intended for a supermarket.

Transcritical refrigeration systems with CO ₂ as a refrigerant are well known in the art. The critical temperature of CO ₂ is 31.0 ⁰ C and the critical pressure is 73.8 bar. At higher temperatures and pressures no clear distinction can be drawn between liquid and vapour, and CO ₂ is said to be in the so-called super-critical fluid region.

In a conventional refrigeration system, heat release from the refrigerant is based on condensation of the refrigerant. Considering the temperature difference needed in a heat exchanger, i.e. app. 10 ⁰ C, the "real life" upper limit for heat release based on condensation of CO ₂ will be around 20 ⁰ C ambient temperature. Below this temperature, the CO ₂ stays below the critical point and the refrigeration system operates in subcritical cycles.

For refrigeration systems used in supermarkets, the ambient temperature will exceed 20 ⁰ C during the summer in a large part of the world. At these temperatures, cooling of the CO ₂ is a single-phase cooling, namely a gas cooling. CO ₂ is above the critical point at the high- pressure side of the system, and the refrigeration system operates in transcritical cycles.

The efficiency and the cooling capacity of the refrigeration system are lower in transcritical operation than in subcritical operation.

It is an important disadvantage of known CO ₂ refrigeration systems that they have a lowered performance at elevated ambient temperatures above app. 20 ⁰ C, i.e. when a high performance is actually desired.

It is an object of the present invention to provide a transcritical refrigeration system with improved performance during transcritical operation.

According to the present invention the above-mentioned and other objects are fulfilled by provision of a transcritical refrigeration system comprising a flow circuit for recirculation of a refrigerant, the flow circuit comprising a compressor for generation of a refrigerant flow from a low-pressure side to a high-pressure side of the compressor and, in the order defined by the flow direction, connected in series with a gas cooler for cooling of the refrigerant towards the ambient temperature, a first pressure reducing device, such as a reduction valve, separating the low-pressure side and the high pressure side of the compressor, and a first evaporator for evaporation of the refrigerant, e.g. in a cooling furniture.

In order to improve the performance of the transcritical refrigeration system during transcritical operation, a heat exchanger with a refrigerant flow channel is connected in the flow circuit between the gas cooler and the pressure-reducing device. The heat exchanger

also has a thermal medium flow channel for a thermal medium flow. The thermal medium flow channel is connected in series with a storage tank for storage of the thermal medium, e.g. water, the storage tank further comprising a second evaporator connected with the flow circuit in parallel with the first evaporator for cooling of the thermal medium. It is an important advantage of the present invention, that the cooling capacity of the system that is available at low ambient temperatures, i.e. during subcritical operation of the system, may be stored in the storage tank for later use, such as for use during transcritical operation of the system, when the system without the stored cooling capability would operate with a lowered performance. This makes it possible to lower the performance requirements of the compressor in the system, e.g. by 10 - 20 %, and this lowers the manufacturing cost.

Preferably, the thermal medium is water, and preferably the water is cooled to ice in the storage tank during subcritical operation of the system.

Alternatively, the thermal storage tank may also comprise a thermal medium flow path connected in series with the thermal medium flow path of the heat exchanger so that the thermal medium flows into thermal contact with a second thermal medium, e.g. water, stored in the thermal storage tank and cooled by the second evaporator during subcritical operation of the system. The second thermal medium may undergo a phase transition, e.g. water to ice, during cooling.

A phase transition, e.g. from water to ice, makes it possible to store a large cooling capacity in the storage tank.

Preferably, the refrigerant is CO ₂ due to its low global warming potential (GWP = 1), availability, and reasonable cost.

For improved performance of the system, a second pressure reducing device may be connected in series with and between the gas cooler and the heat exchanger for adjustment of a desired pressure in the gas cooler. As will be further explained below with reference to the drawing, an optimum gas cooler pressure exists during transcritical operation of the system. The second pressure reducing device, such as an expansion valve, may be controlled so that the gas cooler pressure attains the optimum value or approximately the optimum value. The heat exchanger also causes a pressure reduction from the input to the output of the heat exchanger as a result of the cooling of the refrigerant. Thus, the heat exchanger may also function as the second pressure reducing device and the flow of thermal medium through the heat exchanger may be controlled so that the gas cooler pressure attains the optimum value or approximately the optimum value.

A second heat exchanger with a refrigerant flow channel may be connected in the flow circuit between the compressor and the gas cooler and with a thermal medium flow channel connected with the storage tank for fluid flow of the thermal medium and heat exchange with the refrigerant at the high pressure side of the compressor for heating of the thermal medium and storage of the heated medium in the thermal storage tank.

Alternatively, the thermal storage tank may also comprise a thermal medium flow path connected in series with the thermal medium flow path of the second heat exchanger so that the thermal medium flows into thermal contact with a second thermal medium, e.g. water, stored in the thermal storage tank. Thus, according to the present invention, a method of operating a transcritical refrigeration system for improved cooling capacity is provided, comprising the steps of operating the refrigeration system in subcritical cycles when the ambient temperature is below a temperature that allows cooling of the refrigerant by condensation, during at least part of the time of subcritical operation, cooling a thermal medium contained in a storage tank, operating the refrigeration system in a transcritical cycle process when the ambient temperature is above the temperature that allows cooling of the refrigerant by condensation, and during at least part of the time of transcritical operation, utilizing the thermal medium in a heat exchanger for cooling of the refrigerant at the high-pressure side of the refrigeration system.

Further, a method of operating a transcritical refrigeration system for supplemental heating is provided, comprising the steps of operating the refrigeration system in subcritical cycles when the ambient temperature is below a temperature that allows cooling of the refrigerant by condensation, during at least part of the time of subcritical operation, heating a thermal medium contained in a storage tank with the refrigerant in a heat exchanger at the high- pressure side of the refrigeration system, and operating the refrigeration system in transcritical cycles when the ambient temperature is above the ambient temperature that allows cooling of the refrigerant by condensation.

Below the invention will be described in more detail with reference to the exemplary embodiments illustrated in the drawing, wherein

Fig. 1 is a blocked schematic of a first embodiment of a transcritical cooling system according to the present invention,

Fig. 2 is a plot of a subcritical cooling cycle,

Fig. 3 is a plot of a transcritical cooling cycle,

Fig. 4 is a plot illustrating control of gas cooler pressure, and

Fig. 5 is a blocked schematic of a second embodiment of a transcritical cooling system according to the present invention.

Fig. 1 is a blocked schematic of a first embodiment 10 of a transcritical cooling system according to the present invention. The system 10 comprises a refrigerant flow circuit for recirculation of CO ₂ refrigerant 12, the flow circuit comprising a compressor 14 for generation of a refrigerant flow in the direction of the arrow 16 from a low-pressure side to a high- pressure side of the compressor 14 and, in the order defined by the flow direction, connected in series with a gas cooler 18 for cooling of the refrigerant 12 towards the ambient temperature, a valve 20 for pressure reduction as will be further explained below, a heat exchanger 22 with a refrigerant flow channel 24 connected in the flow circuit between the gas cooler 18 and a receiver 26 for accommodation of CO ₂ refrigerant 12. The receiver 26 is connected to an expansion valve 28 that separates the low-pressure side and the high- pressure side of the compressor 14, and a first evaporator 30 for evaporation of the CO ₂ refrigerant. The refrigerant flow channel 24 of the heat exchanger 22 may alternatively be connected between the gas cooler 18 and the valve 20, or, between the receiver 26 and the expansion valve 28.

Fig. 2 illustrates subcritical operation of the system 10 in a conventional Log (p), h (enthalpy) diagram. The enthalpy H is defined by the equation: H = U + pV, where U is the internal energy, p is the pressure, and V is the volume of the system. Between point 1 and 2, the compressor 14 compresses the CO ₂ refrigerant, and subsequently heat is released from the refrigerant from point 2 to 3 below the critical point 32 by condensation of the refrigerant in the gas cooler 18 at a constant pressure. The expansion from point 3 to 4 takes place at constant specific enthalpy at passage of the expansion valve 28. The heat absorption takes place in the evaporator 30 in the cooling furniture of the system 10 from point 4 to 1 at constant pressure. The control valve 20 is fully open when the system 10 operates subcritically.

Fig. 3 illustrates transcritical operation of the system 10. The most important difference between the plot of Fig. 3 and the plot of Fig. 2 is that the CO ₂ refrigerant is above the critical point 32 at the high-pressure side of the compressor 14 and thus, heat is released from the refrigerant by CO ₂ gas cooling in the gas cooler 18. The coefficient of performance (COP) of the system 10 is less for transcritical cycles than for subcritical cycles due to the lacking phase transition, i.e. no condensation, during heat release.

The expansion from point 3 to 4 takes place in two steps, namely from point 3 to 5, and subsequently from point 5 to 4. The valve 20 reduces the pressure from point 3 to point 5 so

that CO ₂ in the liquid phase enters the heat exchanger 22 and is collected in the receiver 26. Further, the valve 20 is controlled in such a way that the pressure in the gas cooler 18 attains a value that gives the best possible COP. This is further illustrated in Fig. 4. In addition to the transcritical cooling cycle, Fig. 4 shows two isotherms 34, 36. It should be noted that a decrease of the gas cooler pressure at the point 3 moves the point 4 to the right by a large amount because of the low and almost horizontal slope of the isotherm 34 so that the available specific enthalpy for release in the evaporator decrease by a large amount. Since the specific enthalpy added by the compressor 14 decreases by a small amount, the resulting COP decreases by a large amount. Conversely, an increase of the gas cooler pressure at the point 3 moves the point 3 to the left by a small amount because of the steep slope of the isotherm 34 so that the available specific enthalpy for release in the evaporator increases by a small amount. Since the specific enthalpy added by the compressor 14 also increases by a small amount, the resulting COP hardly changes.

It should be noted that if the slope of the isotherm 34 is larger than the slope of the line between points 1 and 2, the COP decreases for increased gas cooler pressure. This illustrates that there is an optimum value for the gas cooler pressure that maximizes the COP, and preferably the valve 20 is adjusted in such a way that the gas cooler pressure attains, at least approximately, this optimum pressure value. Typically, the gas cooler pressure is app. 120 bar while the pressure at the low-pressure side of the compressor 14 is app. 40 bar.

A processor 100 is adapted to control the valve 20 based on the temperature and pressure on the low pressure side of the compressor 14 downstream the evaporator 30 and on the pressure at the output side of the gas cooler 18 in such a way that the gas cooler pressure attains, at least approximately, its optimum pressure value. For further improvement of the capacity of the system 10, the heat exchanger 22 also comprises a thermal medium flow channel 38 for a thermal medium flow and connected in series with a storage tank 40 for storage of water and ice. The storage tank 30 further comprises an evaporator 42 connected to an expansion valve 44 so that the evaporator 42 is connected in the flow circuit in parallel with the first evaporator 30. During subcritical operation, the evaporator 42 operates in parallel with the evaporator 30 so that the water in the storage tank 20 is cooled to ice. During transcritical operation the pump 46 is operated and the three-way valve 50 is opened to pump water at the freezing point through the thermal medium flow channel 38 for further cooling of the CO ₂ refrigerant whereby the capacity of the system 10 is increased during transcritical operation. The phase transition from water to ice, and vice versa, makes it possible to store a large cooling capacity for later use.

The embodiment of Fig. 5 corresponds to the embodiment of Fig. 1 with a further heat exchanger 50 inserted in the flow circuit between the compressor 14 and the gas cooler 18. The heat exchanger 50 has a refrigerant flow channel 52 connected in the flow circuit between the compressor 14 and the gas cooler 18. The heat exchanger 50 also comprises a thermal medium flow channel 54 for a thermal medium flow and connected in series with the storage tank 40 for storage of water and ice. Thus, in the winter time when the system 10 operates in subcritical cycles, the valves 48, 56, 58 are opened allowing the pump 46 to pump water from the storage tank 40 through the heat exchanger 50 for heating of the water, and back into the storage tank. The storage tank may further be connected to a heating system (not shown) that utilizes the heated water. During the summer time, the water is cooled to ice, when the system 10 operates in subcritical cycles and the ice water is used for cooling of the refrigerant in the heat exchanger 22 when the system 10 operates in transcritical cycles as described above with reference to Fig. 1.