EP2040946A1

EP2040946A1 - Air conditioning system operating on a supercritical cycle for use in motor vehicles

Info

Publication number: EP2040946A1
Application number: EP06779016A
Authority: EP
Inventors: Thomas Justin
Original assignee: Renault Trucks SAS
Current assignee: Renault Trucks SAS
Priority date: 2006-07-04
Filing date: 2006-07-04
Publication date: 2009-04-01
Also published as: WO2008003841A1; US20090199582A1

Abstract

The invention relates to an air conditioning system operating on a supercritical cycle for a vehicle, comprising a circuit through which a refrigerant flows, which circuit comprises an evaporator (22) where the refrigerant collects heat from the ventilation air to be conditioned, a compressor (23), a cooler (24) where the refrigerant releases heat into the outside air, and an expander (25), characterized in that it comprises means (48) for humidifying the outside air that contacts the cooler (24) and in that it has no internal heat exchanger.

Description

AIR CONDITIONING SYSTEM OPERATING ACCORDING TO A

SUPERCRITICAL CYCLE FOR USE ON VEHICLES

AUTOMOBILES

Technical area

The invention relates to the field of air conditioning systems used on motor vehicles. It relates more particularly to particular arrangements of the air conditioning system to optimize performance, while simplifying the refrigerant circuit, especially when it operates with a supercritical cycle.

Previous techniques

In general, and as illustrated in FIG. 1, an air-conditioning unit 1 comprises four main elements, namely an evaporator 2, a compressor 3, a gas-cooler exchanger or condenser 4, and an expander 5 connected by a circuit traveled by a refrigerant.

The thermodynamic cycle of an air-conditioning unit operating according to a basic supercritical cycle, as shown in bold lines in FIG. 2, comprises a first phase corresponding to the transition between the points A1 and A2. During this first phase, the evaporator 2 captures the calories of the ventilation air circuit to be cooled 7, remaining at constant pressure, of the order of a few bars. The refrigerant which has thus captured these calories is then compressed in a compressor 3, during the phase corresponding to the transition between points A2 and A3. The compressed fluid then enters the cooler or condenser 4. This cooler is a heat exchanger, at which the refrigerant transfers part of its heat to the outside environment 8, according to the phase substantially corresponding to the transition between the points A3 and A4 . The fluid remains at a high pressure, greater than ten bars. Then, the coolant sees its pressure drop by its passage in the expander 5, corresponding to the transition phase between the points A4 and Al. The fluid then enters the evaporator 2. Among the criteria governing the choice of a refrigerant are in particular regulatory issues with respect to the environment. These criteria led in particular to consider the replacement of chlorofluorocarbon refrigerants by fluids such as carbon dioxide for example, which also allows air conditioning systems to operate in a supercritical cycle.

In supercritical air conditioning systems, the temperature and pressure may be beyond the critical point, meaning that the transition from gaseous to liquid state is not physically definable. Among the consequences, the organ by which the calories captured are transmitted to the external environment is not the seat of a phenomenon of condensation, but simply a cooling of the refrigerant. There is therefore no condensation in the gas cooling exchanger. Thus, the term "condenser", used for the definition of a conventional air conditioning system, is not physically correct in a system operating in a supercritical cycle. Therefore, in the rest of the description, this organ will be called "cooler". Although the use of systems operating on supercritical cycles is appropriate for environmental reasons, it does have certain design constraints. Indeed, the pressures in the refrigerant circuit are frequently greater than 100 bar, for temperatures of the order of 15O ⁰ C or more. The various components of the refrigerant circuit, and in particular the pipes and fittings, must therefore be designed and dimensioned accordingly.

In general, the efficiency of an air conditioning unit is evaluated by measuring a coefficient of performance. This coefficient of performance is equal to the ratio of the power taken from the flow of ventilation air to be cooled, divided by the power consumed by the compressor. Thus, to obtain a satisfactory coefficient of performance with the use of a system operating in a supercritical cycle, it is necessary to use a complementary device, called "internal exchanger" as described for example in the document EP This internal heat exchanger 9, as illustrated in FIG. 1, makes it possible to cool the high-pressure refrigerant at the outlet of the cooler 8, by transferring part of its heat to the low-pressure refrigerant exiting from it. the evaporator 2. The corresponding thermodynamic cycle is illustrated in dotted lines in FIG. 2. The passage in the internal exchanger of the low-pressure refrigerant leaving the evaporator 2 corresponds to the transition from the point A2 to the point A2 ' . During this phase, a quantity of heat is received from the high-pressure fluid leaving the cooler 8. The compression takes place between the points A2 'and A3', and continues with the cooling within the cooler 8 between the points A3 'and A4. A complementary temperature drop is obtained by crossing the internal exchanger 9, corresponding to the transition between the points A4 and A4 '. The crossing of the expander 5 causes a drop in pressure, between the points A4 'and Al'. Then, the fluid is heated inside the evaporator 2 between the points A1 'and A2. Overall, the coefficient of performance is improved thanks to this internal exchanger 9, since it is equal to the ratio of the differences in enthalpy between the phases Al '-A2 and A2'-A3'.

It is understood that the use of this internal exchanger, if it is necessary to obtain a satisfactory coefficient of performance, however, causes a complexity of the refrigerant circuit. This is all the more penalizing as it leads to high temperature and pressure levels. Indeed, thanks to the temperature drop of the fluid upstream of the expander 5, the heat exchange in the evaporator 2 is more efficient because the coolant is at a lower temperature. However, this improvement of the exchanges in the evaporator results in an increase in the temperature of the fluid to be compressed at the level of the internal exchanger 9. The compressed fluid is therefore found in the cooler 4 at a very high temperature level and at high pressure. This therefore generates constraints in terms of sealing and holding materials. The use of complementary components also causes an increase in the weight and the cost price of the global circuit.

SUMMARY OF THE INVENTION One of the objectives of the invention is to allow the operation of the air conditioning circuit with a higher coefficient of performance than those measured on existing systems. Another object of the invention is to simplify the refrigerant circuit by reducing the weight, the volume and consequently the cost of the air conditioning circuit. Another objective is to reduce the number of areas exposed to the risk of leakage and forcing a delicate sizing. Another object of the invention is to allow operations at lower pressure levels with regard to the refrigerant fluid, so as to reduce the internal pressure losses, and allow the use of less expensive materials for the realization of the different organs of the circuit.

Thus, the air conditioning system according to the invention is characterized in that it comprises means for humidifying the outside air used for cooling the cooler, and in that it is free of internal exchanger.

In other words, the invention consists in improving the cooling of the coolant at the cooler, causing a phenomenon of evaporation of a certain amount of water introduced into the outside air coming into contact with the cooler, which captures the calories dissipated by the cooler. In a particular embodiment, this water can be projected in the form of microdroplets at the cooler. In this way, the heat captured by these microdroplets can vaporize a certain amount, within the limit of the saturation of air vapor in contact with the cooler. Thus, the coolant temperature at the outlet of the cooler is lowered, with the consequence that it is possible to dispense with the internal heat exchanger, used in previous systems to deliver to the pressure reducer a coolant at a sufficiently low temperature. This lowering of the temperature is obtained without the inconvenience observed in the art previous, which was an increase in the temperature and pressure of the fluid before entering the cooler.

In other words, the use of a phenomenon of evaporation at the cooler level makes it possible to improve the performance of this cooler, and consequently, the coefficient of performance of the whole system. The improvement in the coefficient of performance is all the more appreciable because it is combined with a reduction in the complexity of the refrigerant circuit, and more specifically by the elimination of the internal exchanger required in the systems of the art. prior. This advantage is further accentuated by the fact that the maximum pressure and / or temperature are reduced. Indeed, the compression phase is from the dew point and not from a higher temperature point as after passing through the internal exchanger of the prior art, which is favorable in terms of sizing the installation air conditioning.

The water spray can be projected directly on the chiller, or even in a flow of air to ventilate the chiller. The ventilation can be forced, or even result from a natural convection, depending on the desired performance and the powers involved. It is also possible to use porous moist materials, known by the general name of "wetted media ", Which can be passed through the ventilation air so as to load moisture which is then vaporized in contact with the cooler.

According to another characteristic of the invention, the means for humidifying the air in contact with the cooler can be fed by the recovery of the condensed water at the evaporator. In this way, all or part of the water produced at the evaporator can be used and reused to cool the cooler according to the invention. In the case where the water production at the evaporator is sufficient compared to the consumption for cooling the cooler, the autonomy can be obtained. It is also possible to provide this spray from an independent and autonomous reserve. Brief description of the figures

The manner of carrying out the invention, as well as the advantages which result therefrom, will emerge clearly from the following description of the embodiment, in support of the appended figures, in which:

Figure 1 is a simplified diagram of an air conditioning system operating with a supercritical refrigerant fluid according to the state of the art.

FIG. 2 is an enthalpy / pressure diagram on which the steps of the thermodynamic cycle of systems of the prior art are simplified.

Figure 3 is a simplified diagram of the air conditioning system according to the invention.

Figure 4 is an enthalpy / pressure diagram on which are shown in a simplified manner the different steps of the thermodynamic cycle of the refrigerant.

Way of realizing the invention

The air-conditioning system illustrated in FIG. 3 conventionally comprises an evaporator 22 traversed by the stream of air 27 to be cooled. This evaporator 22 comprises an internal circuit connected to the refrigerant circuit 26. The outlet of the evaporator 22 is connected to a compressor 23 which compresses this refrigerant fluid. The cooling step takes place under temperature and pressure conditions beyond the critical point of the fluid, which justifies the term "supercritical". By way of example, the fluid used in a supercritical cycle may be carbon dioxide, the pressure and the temperature of the critical point of which are respectively 73 bars and 32 ^° C. After cooling in the cooler 24, the refrigerant is expanded at the expander 25 to then penetrate at reduced pressure into the evaporator 22.

According to the invention, the cooler 24 is associated with means for humidifying the outside air coming into contact with the cooler. In the illustrated embodiment, liquid water 32 is sprayed into the outside air, to capture a part of the heat dissipated by the cooler 24, in order to increase the heat exchange within the cooler 24. This spray can take place directly on the cooler 24 itself, or preferentially in the air flow 28 which will be brought by ventilation in contact with the cooler 24. With this evaporative cooling, the air conditioning system has a satisfactory coefficient of performance, without requiring the addition of an internal heat exchanger used on existing systems.

According to a characteristic of the invention, the water 32 which is used at the chiller 2 can be advantageously recovered at the level of the evaporator 22, at which a portion of the water contained in the flow of the condensate is condensed. ventilation air 27 to cool.

As illustrated in FIG. 3, this water 33 can be collected by flow in a manifold 34 and then conveyed via a pipe 35 suitable for a tank 36. The tank makes it possible to manage any discrepancies between the condensate flow rate and the flow rate necessary to humidification. This tank 36 can optionally be supplied by an external water supply via the opening 37, to start the operation of the device when the water production of the evaporator 22 has not been sufficient, or that the weather conditions do not do not allow it. This reservoir 36 may be equipped with a sensor 38 of the volume of water it contains, the information delivered by this sensor being conveyed to a suitable control and control unit 40, ensuring the management of the system. A drain mechanism 41 and an overflow outlet 42 may also be provided. The emptying 41 can also be obtained by opening a valve 43 controlled by the control unit already mentioned.

For the operation of the invention, a quantity of water can be taken from the lower part of the tank 36, so as to be conveyed near the cooler 24. To do this, a metering mechanism 41, including in particular a pump for example volumetric 45 controlled by the control unit 40, ensures the spraying characteristic of a quantity given, and at times chosen to optimize the operation of the air conditioning system. Filtration devices 47 may be provided upstream of the metering system, to prevent fouling of the pump 45, and downstream of the metering system 44, to prevent fouling of the spray members. In the case where the wetting is carried out by spraying, the bodies providing this spraying may in particular be constituted by nozzles 48 at high pressure, for which the diameter and the pressure of the water make it possible to determine the size of the droplets.

In a simplified manner, the thermodynamic cycle of the system according to the invention is illustrated in FIG. 4, in solid lines, in comparison with a system of the prior art including an internal exchanger shown in dashed lines.

Thus, the heat captured by the refrigerant at the evaporator 22 corresponds to the transition between points Bl and B 2 of the diagram, during which, at constant pressure, the enthalpy of the refrigerant increases. The variation of enthalpy during this phase corresponds to the energy captured by the system on the ventilation airflow 27 to be cooled. The transition between points

B2 and B3 correspond to the compression phase, in which the pressure of the coolant increases, typically from 30 to 40 bar to about 90 bar. The variation of enthalpy during this phase corresponds to the energy consumed by the compressor, to the efficiency of the latter.

The coefficient of performance of the system is therefore calculated by the ratio between the energy captured on the airflow, that is to say the difference in enthalpy between the points Bl and B2, relative to the energy consumed by compressor, ie the difference in enthalpy between points B2 and B3.

Therefore, in the particular example, corresponding to extreme ambient temperature conditions, of the external temperature type 40 ^° C., with a humidity level of the order of 50%, it can be seen that the coefficient of performance is order of 1.90. By comparison, a similar cycle, implemented on a system of the prior art, including an internal exchanger, has a coefficient of performance of the order of 1.5. This coefficient is calculated as taking into account the energy captured at the level of the evaporator 22, corresponding to the transition between the points

Al 'and B2, referred to the compression phase, illustrated between the points A2' and

A3. The increase in temperature between the outlet of the evaporator 22 and the inlet into the compressor 23, illustrated by the transition between the points B2 and A2 'corresponds to the heating occurring via the internal exchanger, such as illustrated in Figure 1.

As an example of an encrypted comparison, the flow temperature at the outlet of compressor 27 in the system according to the invention is of the order of 92 ^° C. (B3), compared with a temperature in the region of 160 ^° C. (A3 ') at the output of compressor 3 in the prior art. Similarly, in the prior art, a complementary lowering of temperature is carried out at the level of the internal exchanger 9, bringing the temperature of the order of 45 ^° C. at the outlet of the cooler 4 to a temperature of the order of 35 ⁰ C at the inlet of the expander 5, which corresponds to the transition illustrated between the points A4 and A4 '.

Conversely, in the device according to the invention, the same temperature of the order of 35 ⁰ C is obtained directly at the outlet of the cooler 24. In other words, the system according to the invention is advantageous in that it allows operation at a lower temperature, with a more favorable coefficient of performance ratio, and this combined with a simpler refrigerant circuit structure.

The comparison of the pressure levels achieved is also in favor of the invention, since at a better coefficient of performance (1.90 compared to 1.50), the maximum pressure reached in the circuit is of the order of 90 bars. , compared with the 120 bars observed in the prior art, in the presence of an internal exchanger. Such an improvement can be obtained when a quantity of water Sufficient is available, especially if the production of water recovered at the evaporator achieves autonomy. This of course depends on the climatic conditions, and in particular the humidity level and the ambient temperature. Thus, the maximum temperature and pressure values reached in the cooler can be optimized according to these climatic conditions.

Thus, with equal performances, it is possible to use compressors of smaller size or lower consumption. It is also possible to define cycles where the maximum pressure is lower, which is advantageous in terms of sizing the refrigerant circuit.

Claims

1 / air conditioning system operating in a supercritical cycle for a vehicle, comprising a circuit traversed by a refrigerant, said circuit comprising an evaporator (22) at which the coolant captures heat to the ventilation air at air conditioning, compressor (23), cooler

(24) at which the coolant transfers heat to the outside air, and an expander (25), characterized in that it comprises means (48) for humidifying the outside air coming into contact with the cooler ( 24), and in that it is free of internal heat exchanger.

2 / System according to claim 1, characterized in that the means for humidifying the outside air are supplied by the recovery of the condensed water at the evaporator (22).

3 / A system according to claim 1, characterized in that the means for humidifying the outside air provide a spray of water spray in the outside air.

4 / System according to claim 1, characterized in that the means for moistening the outside air ensure the passage of outside air through a humidified media.

5 / System according to claim 1, characterized in that the means for humidifying the outside air provide a spray of water spray on the cooler.

6 / air conditioning system according to claim 1, characterized in that it comprises means for ensuring ventilation of the cooler (24) by the humidified outside air. 11 air conditioning system according to claim 2, characterized in that it comprises a reservoir (36) supplied with condensed water at the evaporator (22).

8 / air conditioning system according to claim 1, characterized in that the refrigerant is carbon dioxide.

9 / air conditioning system according to claim 1, characterized in that it comprises a water metering system (44) humidifying the outside air.