WO2007022778A1 - A transcritical cooling system with improved cooling capacity - Google Patents

A transcritical cooling system with improved cooling capacity Download PDF

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Publication number
WO2007022778A1
WO2007022778A1 PCT/DK2006/000459 DK2006000459W WO2007022778A1 WO 2007022778 A1 WO2007022778 A1 WO 2007022778A1 DK 2006000459 W DK2006000459 W DK 2006000459W WO 2007022778 A1 WO2007022778 A1 WO 2007022778A1
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WO
WIPO (PCT)
Prior art keywords
refrigerant
refrigeration system
transcritical
thermal medium
cooling
Prior art date
Application number
PCT/DK2006/000459
Other languages
French (fr)
Inventor
Finn Guldager Christensen
Original Assignee
Knudsen Køling A/S
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Knudsen Køling A/S filed Critical Knudsen Køling A/S
Priority to EP06761869A priority Critical patent/EP1920203A1/en
Publication of WO2007022778A1 publication Critical patent/WO2007022778A1/en
Priority to NO20081411A priority patent/NO20081411L/en

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B40/00—Subcoolers, desuperheaters or superheaters
    • F25B40/04—Desuperheaters
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B40/00—Subcoolers, desuperheaters or superheaters
    • F25B40/02—Subcoolers
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B41/00—Fluid-circulation arrangements
    • F25B41/30—Expansion means; Dispositions thereof
    • F25B41/39—Dispositions with two or more expansion means arranged in series, i.e. multi-stage expansion, on a refrigerant line leading to the same evaporator
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B5/00—Compression machines, plants or systems, with several evaporator circuits, e.g. for varying refrigerating capacity
    • F25B5/02—Compression machines, plants or systems, with several evaporator circuits, e.g. for varying refrigerating capacity arranged in parallel
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B9/00—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point
    • F25B9/002—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point characterised by the refrigerant
    • F25B9/008—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point characterised by the refrigerant the refrigerant being carbon dioxide
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25D—REFRIGERATORS; COLD ROOMS; ICE-BOXES; COOLING OR FREEZING APPARATUS NOT OTHERWISE PROVIDED FOR
    • F25D16/00—Devices using a combination of a cooling mode associated with refrigerating machinery with a cooling mode not associated with refrigerating machinery
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B2309/00—Gas cycle refrigeration machines
    • F25B2309/06—Compression machines, plants or systems characterised by the refrigerant being carbon dioxide
    • F25B2309/061—Compression machines, plants or systems characterised by the refrigerant being carbon dioxide with cycle highest pressure above the supercritical pressure
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B2400/00—General features or devices for refrigeration machines, plants or systems, combined heating and refrigeration systems or heat-pump systems, i.e. not limited to a particular subgroup of F25B
    • F25B2400/22—Refrigeration systems for supermarkets
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B2400/00—General features or devices for refrigeration machines, plants or systems, combined heating and refrigeration systems or heat-pump systems, i.e. not limited to a particular subgroup of F25B
    • F25B2400/24—Storage receiver heat
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D20/00—Heat storage plants or apparatus in general; Regenerative heat-exchange apparatus not covered by groups F28D17/00 or F28D19/00
    • F28D20/02—Heat storage plants or apparatus in general; Regenerative heat-exchange apparatus not covered by groups F28D17/00 or F28D19/00 using latent heat
    • F28D20/021—Heat storage plants or apparatus in general; Regenerative heat-exchange apparatus not covered by groups F28D17/00 or F28D19/00 using latent heat the latent heat storage material and the heat-exchanging means being enclosed in one container

Definitions

  • 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.
  • heat release from the refrigerant is based on condensation of the refrigerant.
  • 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.
  • 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.
  • 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.
  • 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.
  • the thermal medium is water, and preferably the water is cooled to ice in the storage tank during subcritical operation of the system.
  • 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.
  • GWP global warming potential
  • 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.
  • 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.
  • 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.
  • 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.
  • a method of operating a transcritical refrigeration system for improved cooling capacity 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.
  • a method of operating a transcritical refrigeration system for supplemental heating 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.
  • 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
  • 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 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.
  • the COP decreases for increased gas cooler pressure.
  • the valve 20 is adjusted in such a way that the gas cooler pressure attains, at least approximately, this optimum pressure value.
  • 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.
  • 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.
  • the evaporator 42 operates in parallel with the evaporator 30 so that the water in the storage tank 20 is cooled to ice.
  • 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.
  • 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.
  • 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.

Abstract

The invention relates to a transcritical refrigeration system with improved performance during transcritical operation. To improve the performance during transcritical operation, a heat exchanger with a refrigerant flow channel is connected in a flow circuit between a gas cooler and a pressure-reducing device. The heat exchanger also has a thermal medium flow channel which is connected in series with a storage tank, the storage tank further comprising a second evaporator connected with the flow circuit in parallel with a first evaporator for cooling of the thermal medium whereby the cooling capacity of the system that is available at low ambient temperatures, i.e. during subcortical 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 .

Description

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 CO2 as a refrigerant are well known in the art. The critical temperature of CO2 is 31.0 0C and the critical pressure is 73.8 bar. At higher temperatures and pressures no clear distinction can be drawn between liquid and vapour, and CO2 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 0C, the "real life" upper limit for heat release based on condensation of CO2 will be around 20 0C ambient temperature. Below this temperature, the CO2 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 0C during the summer in a large part of the world. At these temperatures, cooling of the CO2 is a single-phase cooling, namely a gas cooling. CO2 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 CO2 refrigeration systems that they have a lowered performance at elevated ambient temperatures above app. 20 0C, 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 CO2 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 CO2 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 CO2 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 CO2 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 CO2 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 CO2 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 CO2 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 CO2 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 CO2 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.

Claims

1. 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 separating the low-pressure side and the high pressure side of the compressor, and a first evaporator for evaporation of the refrigerant, c h a r a c t e r i z e d in a heat exchanger with a refrigerant flow channel connected in the flow circuit between the gas cooler and the pressure-reducing device, and a thermal medium flow channel for a thermal medium flow and connected in series with a storage tank for storage of the thermal medium, 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.
2. A transcritical refrigeration system according to claim 1 , wherein the refrigerant is CO2.
3. A transcritical refrigeration system according to claim 1 or 2, wherein the thermal medium is water.
4. A transcritical refrigeration system according to claim 3, wherein part of the water is frozen to ice in the storage tank.
5. A transcritical refrigeration system according to any of the previous claims, further comprising a second pressure reducing device connected in series with and between the gas cooler and the heat exchanger for adjustment of a desired pressure in the gas cooler.
6. A transcritical refrigeration system according to claim 5, wherein the heat exchanger also constitutes the second pressure reducing device.
7. A transcritical refrigeration system according to any of the previous claims, further comprising a second heat exchanger with a refrigerant flow channel connected in the flow circuit between the compressor and the gas cooler, and 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.
8. A method of operating a transcritical refrigeration system for improved cooling capacity, 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.
9. A method of operating a transcritical refrigeration system for supplemental heating, 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.
PCT/DK2006/000459 2005-08-25 2006-08-24 A transcritical cooling system with improved cooling capacity WO2007022778A1 (en)

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NO20081411A NO20081411L (en) 2005-08-25 2008-03-18 Transcritical dress system with improved dress capacity

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WO2013050035A1 (en) 2011-10-07 2013-04-11 Danfoss A/S Method for controlling gas pressure in cooling plant
CN103175323A (en) * 2011-12-23 2013-06-26 东普雷股份有限公司 Refrigeration device using ternary pipe-type heat exchanger
US9194615B2 (en) 2013-04-05 2015-11-24 Marc-Andre Lesmerises CO2 cooling system and method for operating same
EP3112776A1 (en) * 2015-06-30 2017-01-04 Hiref S.p.A. Carbon dioxide compression refrigeration system
CN107933594A (en) * 2017-12-18 2018-04-20 中车大连机车研究所有限公司 One kind is based on CO2The trans critical cycle railway vehicle air conditioner system of refrigerant
EP3324134A1 (en) * 2016-11-08 2018-05-23 Heatcraft Refrigeration Products LLC Absorption subcooler for a refrigeration system
CN110186221A (en) * 2019-06-27 2019-08-30 中国科学院理化技术研究所 CO 2 trans-critical heat pump system
US10690389B2 (en) 2008-10-23 2020-06-23 Toromont Industries Ltd CO2 refrigeration system
IT201900010572A1 (en) * 2019-07-01 2021-01-01 Enex S R L IMPROVED REFRIGERATION SYSTEM
IT202000004375A1 (en) * 2020-03-02 2021-09-02 Claudio Marazzi REFRIGERATOR UNIT WITH CO2 IN SUBCRITICAL CONDITIONS.
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Cited By (14)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US8087256B2 (en) 2007-11-02 2012-01-03 Cryomechanics, LLC Cooling methods and systems using supercritical fluids
US10690389B2 (en) 2008-10-23 2020-06-23 Toromont Industries Ltd CO2 refrigeration system
WO2013050035A1 (en) 2011-10-07 2013-04-11 Danfoss A/S Method for controlling gas pressure in cooling plant
US9885509B2 (en) 2011-10-07 2018-02-06 Danfoss A/S Method for controlling gas pressure in cooling plant
CN103175323A (en) * 2011-12-23 2013-06-26 东普雷股份有限公司 Refrigeration device using ternary pipe-type heat exchanger
US9194615B2 (en) 2013-04-05 2015-11-24 Marc-Andre Lesmerises CO2 cooling system and method for operating same
US11656005B2 (en) 2015-04-29 2023-05-23 Gestion Marc-André Lesmerises Inc. CO2 cooling system and method for operating same
EP3112776A1 (en) * 2015-06-30 2017-01-04 Hiref S.p.A. Carbon dioxide compression refrigeration system
US10458685B2 (en) 2016-11-08 2019-10-29 Heatcraft Refrigeration Products Llc Absorption subcooler for a refrigeration system
EP3324134A1 (en) * 2016-11-08 2018-05-23 Heatcraft Refrigeration Products LLC Absorption subcooler for a refrigeration system
CN107933594A (en) * 2017-12-18 2018-04-20 中车大连机车研究所有限公司 One kind is based on CO2The trans critical cycle railway vehicle air conditioner system of refrigerant
CN110186221A (en) * 2019-06-27 2019-08-30 中国科学院理化技术研究所 CO 2 trans-critical heat pump system
IT201900010572A1 (en) * 2019-07-01 2021-01-01 Enex S R L IMPROVED REFRIGERATION SYSTEM
IT202000004375A1 (en) * 2020-03-02 2021-09-02 Claudio Marazzi REFRIGERATOR UNIT WITH CO2 IN SUBCRITICAL CONDITIONS.

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