EP2360355B1 - Apparatus for controlling a working fluid with a low freezing point flowing through a closed cycle operating according to a Rankine cycle and method using such an apparatus - Google Patents

Description

The present invention relates to a device for controlling a low-freezing working fluid, in particular water, contained in a closed circuit operating according to a Rankine cycle and to a method using such a device.

It aims in particular the association of this device with an internal combustion engine, in particular for a motor vehicle.

As is known, the Rankine cycle is a closed circuit thermodynamic cycle having the particular feature of using a phase change (liquid / vapor) of a working fluid.

This cycle is generally broken down into a step during which the working fluid used, here water in liquid form, is compressed isentropically, followed by a step where the compressed water is heated and vaporized in contact with a source of heat, this water vapor is then relaxed, in another step, isentropically in an expansion machine, then, in a final step, this relaxed vapor is cooled and condensed in contact with a cold source .

To perform these various steps, the circuit comprises a positive displacement pump (or compressor) for compressing the water in liquid form, a heat exchanger (or evaporator) which is swept by a hot fluid to achieve the at least partial vaporization of the compressed water, an expansion machine for relaxing the steam, such as a turbine, which converts the energy of this steam into another energy, such as mechanical or electrical energy, and another heat exchanger (or condenser) with which the heat contained in the steam is yielded to a cold source, generally outside air which sweeps this condenser, to transform this vapor into water in liquid form.

He is also known, in particular by the document FR 2 884 555 , to use the heat energy conveyed by the exhaust gas of an internal combustion engine, in particular that used for motor vehicles, as a hot source for heating and vaporization of the fluid passing through the evaporator.

This makes it possible to improve the energy efficiency of this engine by recovering a large part of the energy lost in the exhaust to transform it into an energy that can be used for the motor vehicle through the Rankine cycle circuit.

The choice of this working fluid, which undergoes a succession of liquid / vapor phase transformation, is therefore critical.

Indeed, the saturation curve of this fluid must be optimized according to the temperature of the hot source and the cold source.

In this respect, the use of an aqueous working fluid in a Rankine cycle circuit has the advantage of having characteristics that make it possible to obtain a maximum saturation curve while having the advantage of being non-dangerous. .

However, water has the specificity of having a freezing point at low temperatures (around 0 ° C) and it is usual to add antifreeze additives, such as glycol, to lower this freezing point to acceptable temperature levels, of the order of -15 to -30 ° C.

The addition of such additives has the disadvantage of changing the characteristics of the water, in particular its vaporization characteristics, and the hot source from the exhaust gas may be insufficient to satisfactorily achieve this vaporization.

In addition, over time, this additive water undergoes unpredictable aging as the liquid / vapor phase changes. This unpredictable aging can lead to incomplete phase changes of this water, which generates a malfunction of the Rankine cycle circuit.

It is also known by the patent application DE 102 28 868 a circuit in which the working fluid reservoir is placed between the condenser and the pump. He is also known, by the intermediate patent application EP-2357349 a circuit in which the working fluid reservoir is placed in the same module with the coolant expansion vessel of an engine. This provision has the disadvantage, during low ambient temperatures, to cause the freezing of a very large amount of fluid in the tank, which is therefore difficult to thaw in order to achieve the operation of the circuit.

The present invention proposes to overcome the above disadvantages by means of a device and a process which limit or even prevent the freezing of the working fluid without this resulting in a modification of its liquid / vapor phase transformation characteristics.

For this purpose, the invention relates to a closed loop operating on a Rankine cycle comprising a device for controlling the low-freezing working fluid circulating in this circuit, said circuit comprising a fluid compression pump in liquid form, a heat exchanger swept by a hot source for evaporation of said fluid, means for expanding the fluid in vapor form, and a cooling exchanger swept by a cold source for condensing the working fluid, characterized in that it comprises a fluid receiving tank for draining said circuit and at least one pipe carrying a valve for connecting the circuit to said tank and in that the tank comprises a heating system placed in said tank for heating the fluid contained therein.

The tank may be an insulated tank, an expandable tank, a tank comprising a larger capacity than the volume of the fluid contained in the circuit.

The circuit may comprise a pipe for emptying the circuit fluid in the tank and a pipe for filling the circuit with the fluid of this tank.

At least one of the conduits may comprise a fluid circulation pump.

At least one of the lines can be connected at a point of a circulation line between the compression pump and the heat exchanger for evaporation of said fluid.

The circulation line may carry a valve placed between the point and the heat exchanger for evaporation of said fluid.

Preferably, the working fluid may be water free of antifreeze additive.

The hot source can come from the exhaust gases of an internal combustion engine.

The invention also relates to a method for controlling a closed circuit operating on a Rankine cycle with a low-freezing working fluid, said circuit comprising a liquid-form fluid compression pump, a swept-in heat exchanger a hot source for evaporating said fluid, means for expanding the fluid in vapor form, and a cooling exchanger swept by a cold source for condensing the working fluid, characterized in that it consists, at the time of stopping the operation of the circuit, transferring at least a portion of the fluid contained in said circuit to a reservoir by at least one pipe carrying a valve and heating the fluid contained therein by a heating system placed in said reservoir when said fluid is likely to freeze.

The method may include transferring the fluid to the reservoir when the circuit operation is stopped when the ambient temperature is below the freezing temperature of the fluid.

The method can consist of transferring the fluid contained in the reservoir to the circuit during the operation of the circuit.

The method may include circulating the fluid in a conduit connecting the circuit to the reservoir under the action of the compression pump.

The method may comprise circulating the fluid in a pipe connecting the circuit to the tank under the action of a circulation pump carried by said pipe.

The method may consist in transferring by gravity the fluid contained in the reservoir to the circuit during the operation of the circuit.

• la figure 1 qui montre un dispositif de contrôle d'un circuit fermé fonctionnant selon un cycle de Rankine, et
• la figure 2 qui illustre une variante du dispositif de la figure 1.

The other features and advantages of the invention will appear on reading the description which follows, given by way of illustration and not limitation, and to which are appended:

• the figure 1 which shows a control device of a closed circuit operating on a Rankine cycle, and
• the figure 2 which illustrates a variant of the device of the figure 1 .

On the figure 1 , the Rankine cycle closed circuit 10 comprises a circulating and compression pump 12 (or compressor) of a working fluid with an inlet 14 of the working fluid in liquid form and an outlet 16 of this working fluid also under liquid form but compressed under high pressure. This compressor is advantageously rotated by an electric motor (not shown).

This circuit also comprises a heat exchanger 18, called evaporator, traversed by the compressed working fluid between an inlet 20 of the liquid fluid and an outlet 22 through which the working fluid emerges from this evaporator in the form of compressed steam. This evaporator is traversed by a hot source 24 from the exhaust gas flowing in the exhaust line 26 of an internal combustion engine 28 and more particularly of a motor vehicle engine.

This circuit also comprises an expansion machine 30, called expansion valve, receiving at its inlet 32 the working fluid in the form of vapor compressed at high pressure, this fluid emerging through the outlet 34 of the pressure regulator in the form of low-pressure expanded steam.

Advantageously, this expander may be in the form of an expansion turbine whose rotor is rotated by the working fluid in the form of steam by driving a connecting shaft (not shown). Preferably, this shaft makes it possible to transmit the recovered energy to any transformer device, such as for example an electric generator.

The circuit further comprises a cooling exchanger 36, or condenser, with an inlet 38 for the low-pressure vapor expanded and an outlet 40 for the working fluid converted into liquid form after passing through this condenser. This condenser is swept by a cold source, usually a cold fluid (Arrow F) with air at room temperature, so as to cool the expanded steam so that it condenses and turns into liquid.

Fluid circulation lines 42, 44, 46 and 48 make it possible to successively connect the various elements of this circuit so that the fluid circulates in the direction indicated by the arrows C. More specifically, the pipe 42 connects the outlet of the compressor to the fluid. At the inlet of the evaporator, line 44 connects the outlet of this evaporator to the inlet of the expander, line 46 establishes a connection between the outlet of the expander and the inlet 42 of the condenser and line 48 connects the outlet of the condenser. with the compressor inlet.

In the remainder of the description, mention is made of water as a low-freezing working fluid (at around 0 ° C.) circulating in this circuit. This water has the particularity of not having any additive and more particularly no additive avoiding its frost. As working fluid, any other phase change fluid (liquid / vapor) without antifreeze additive, can freeze at low temperature (around 0 ° C), can be used, such as organic fluids.

As illustrated in this figure, a device for controlling the working fluid 50 with means for storing the water contained in the circuit is associated with this circuit.

These means comprise a closed storage tank 52 of the water collected after emptying the circuit. This reservoir makes it possible to keep this water in a liquid state even when the ambient temperature is at a level that can lead to its freezing or to freeze it without risk of damaging the reservoir and / or the circuit.

More specifically, the tank is a heat insulated tank 54 with a peripheral coating 56 which covers all or part of its walls 58 by thermally insulating it from the ambient air.

Alternatively, the reservoir is an expandable reservoir 60 with at least a portion of its walls 62 which is elastically deformable under the effect of the increase in volume of the frozen water.

It can also be expected to use a large volume tank. This reservoir has a configuration such that it comprises an internal volume that is greater than the volume of the water contained in the circuit leaving a gaseous sky 64 between the water level and the upper wall of the reservoir. This gaseous sky comprises a volume at least equal to the increase in volume of the water after freezing.

In all the reservoir arrangements mentioned above, the reservoir comprises a heating system 66 of liquid contained in the reservoir. This system comprises, by way of example, an electrical heating resistor 68 placed inside this tank and supplied with current by electrical conductors 70.

Of course, all control means within the reach of those skilled in the art are connected to this heating system to regulate and / or activate it with for example a measurement of the ambient temperature by means of a temperature sensor.

This tank is connected to the circulation pipe 42 by a drain line 72 from the upper part of this tank and arriving at a connection point 74 with the pipe 42. This drain pipe carries a valve 76 with two positions, full opening and full closure, to control the flow of water in this pipe. A filling line 78 also connects the bottom of the tank to a junction point 80 with the line 42. This filling line also comprises a two-position valve, full opening and full closure, and a circulation pump 84, preferably electric, which manages the flow of water in this pipe. Preferably, the emptying and filling pipes may be insulated so as to limit the freezing of the water contained in these pipes.

Finally, the pipe 42 carries a control valve 86 placed downstream of the two junction and connection points and upstream of the inlet 20 of the evaporator 18.

Of course, the valves 76, 82 and 86 are controlled by any known means, such as electric motors, under the control of a computing unit and more particularly of the calculator of the internal combustion engine.

Likewise, this computing unit controls the drive motors of compressor 12 and pump 84.

In operation, the water circulates only in the circuit in a clockwise direction considering the figure 1 (arrows C). For this, the drain valves 76 and filling 82 are in a closed position for the pipes 72 and 78 while the valve 86 is in an open position for the pipe 42. The pump 84 is inactive and the compressor 12 is rotated by its electric motor.

In this configuration, the water leaves the compressor 12 in liquid form with a pressure of the order of 10 bars and a temperature of 50 °. This compressed water circulates in the pipe 42 to reach the evaporator 22 through the opening of the control valve 86 and can not flow in the pipes 72 and 78 closed by the valves 76 and 82. This compressed water passes through the evaporator so as to become vapor under the effect of the heat sweeping this evaporator and from the exhaust gas of the engine 28. The water vapor leaving the evaporator is conveyed by the pipe 44 to pass through the evaporator. expander 30 by transmitting the energy it contains. The water vapor released from this regulator flows in line 46 to pass through the condenser 36 in which it is converted into a liquid water. This liquid water is then fed via line 48 to compressor 12 to be compressed.

When stopping the operation of the Rankine cycle circuit, the computing unit controls the control valve 86 so that it prevents any flow of the compressed water contained in the pipe 42 to the inlet of the evaporator 18 while maintaining the closed position of the filling valve 82 for the filling line 78 as well as the inaction of the pump 84.

This unit also controls the drain valve 76 so that it is in the open position of the drain line 72 so as to establish a communication between the pipe 42 and the tank 52 through the connection point 74 and this pipe. emptying 72.

The drive of the compressor 12 is maintained and the water leaving the compressor 12 is introduced into the filling line 72 through the point 74 to be transferred into the tank, here in the top of the tank, according to the arrows V of the figure 1 .

Although extended, it is within the abilities of those skilled in the art to calculate the moment when the compressor drive is stopped to proceed with the complete emptying of the circuit water and its storage in the tank, or, at least so that only a small volume of water remains in the circuit which, if it were to freeze, would not damage the elements of the circuit.

Similarly, the person skilled in the art will place the connection points 74 and the stitching points 80 and the control valve 86 closer to the outlet 16 of the compressor 12 and limit the extent of the lines 72 and 78. This makes it possible to limit areas where residual water can freeze.

The water stored in the tank and which is initially at the outlet temperature of the compressor (of the order of 50 ° C), is then protected from the risks of frost by the heat insulation 56 of the insulated tank 54 or can freeze, either in deforming the walls of the deformable reservoir 60, either by occupying the volume of the gaseous sky 64 of the large volume tank without compromising the integrity of this reservoir.

Of course, it is envisaged to put into operation the heating system 66 when its control means will detect a temperature of the ambient air capable of generating the freezing of this water. In the case of a freeze of water in the tank, the heating system 66 is actuated by the computer so as to thaw the water to start the circuit 10.

When restarting the Rankine cycle circuit, the control valve 86 is in the open position of the circulation line 42, the valve 76 is placed in the closed position of the filling line 72 and the valve 82 is placed in an open position of the filling line 78.

The compressor 12 and the pump 84 are actuated with the result of introducing into the pipe 42, through the junction point 80, the water contained in the tank. This water is withdrawn from the tank under the action of the pump to circulate in the filling line 78 and then flows in the pipe 42 according to the arrows R of the figure 1 . This water introduced into the pipe 42 is then circulated in the circuit 10 under the effect of the compressor 12 undergoing the various phase changes, as mentioned above.

The skilled person will set the operating time of the pump 84 to determine its stop after the reintroduction of all the water from the tank in the circuit 10. It may alternatively place a detection means in the tank, such as a float , which will control the interruption of the drive pump 84 when the float will detect no presence of water in the tank.

As part of the figure 1 , it may be provided to remove the drain pipe 72 and its valve 76 and use only the pipe 78 with its valve 82 and pump 84 as a drain pipe and filling with the feature that the pump 84 is a bidirectional pump.

In this case, when stopping the operation of the circuit, the valve 86 is placed in a closed position of the pipe 42 and the valve 82 is in the open position of the pipe 42. The compressor 12 and the pump 84 are actuated in the same direction of rotation to introduce the circuit water in the pipe 78 and in the bottom of the tank 52 according to the arrows V '.

When restarting this circuit, the valve 82 remains in the open position of the pipe 78 and the valve 86 switches to a fully open position of the circulation pipe 42.

The compressor is operated in the same direction as for the emptying and the pump is controlled in a direction opposite to that of the emptying so as to extract the water contained in the tank to circulate in the pipe 78 according to the arrows R, as previously mentioned.

The variant of the figure 2 differs from the example of the figure 1 by specific positioning of the tank 52 and by the removal of the circulation pump on the filling line 78.

As visible on this figure 2 , the tank is positioned relative to the circuit 10 in such a way that the connection point 88 of the filling line 78 with the tank, placed here in the bottom of this tank, is situated above the junction point 80 of this line with the circulation line 42.

For this variant, the operation of the circuit is similar to that of the figure 1 with the closures of the valves 76 and 82, the opening of the valve 86 and a circulation of water according to the arrows C under the action of the compressor 12.

The step of emptying the water in the tank 52 to stop the operation of the circuit is also identical to that of the figure 1 with the closures of the valves 82 and 86, the opening of the valve 76 and an actuation of the compressor 12 to obtain a flow of water according to the arrows V.

For the step of filling the circuit, the valve 76 is in the closed position of the pipe 72, the valves 82, 86 are in the open position of the pipes 78 and 42 and the compressor 12 is actuated.

Due to the gravity, the water contained in the reservoir flows through the point of connection 88 and flows in the filling pipe 78 and then in the circulation pipe 42 according to the arrows R.

Of course, and without departing from the scope of the invention, it may be envisaged to proceed, after stopping the operation of the circuit, to its emptying only if the ambient temperature is likely to cause the freezing of the water contained in the circuit, especially when it is below its freezing temperature.

For this, it can be used a temperature sensor dedicated to this measurement or the sensor that is associated with the heating system 66.

Claims (16)

1. A closed circuit (10) operating on a Rankine cycle, said circuit comprising a device for controlling the low freezing point working fluid circulating in this circuit, said circuit comprising a compression pump (12) for the fluid in liquid form, a heat exchanger (18) swept by a hot source (24) for evaporation of said fluid, expansion means (30) for expanding the fluid in vapour form and a cooling exchanger (36) swept by a cold source (F) for condensation of the working fluid, comprising a fluid collection tank (52) for draining said circuit and at least one line (72, 78) carrying a valve (76, 82) for connecting circuit (10) to said tank and characterized in that tank (52) comprises a heating system (66) arranged in said tank for heating the fluid contained therein.
2. A circuit as claimed in claim 1, characterized in that the tank is a heat-insulated tank (54).
3. A circuit as claimed in claim 1, characterized in that the tank is an expansible tank (60).
4. A circuit as claimed in claim 1, characterized in that the capacity of the tank is larger than the volume of the fluid contained in the circuit.
5. A circuit as claimed in claim 1, characterized in that it comprises a line (72) for draining off the fluid from circuit (10) into the tank and a line (78) for filling circuit (10) with the fluid from said tank.
6. A circuit as claimed in any one of the previous claims, characterized in that at least one of lines (72,78) comprises a fluid circulation pump (84).
7. A circuit as claimed in any one of the previous claims, characterized in that at least one of lines (72, 78) is connected to a point (74, 80) of a circulation line (42) between compression pump (12) and heat exchanger (18) for evaporation of said fluid.
8. A device as claimed in claim 7, characterized in that circulation line (42) carries a valve (86) arranged between said point and heat exchanger (18) for evaporation of said fluid.
9. A device as claimed in claim 1, characterized in that the fluid is water without an antifreeze additive.
10. A device as claimed in claim 1, characterized in that hot source (24) comes from the exhaust gas of an internal-combustion engine (28).
11. A method of controlling a closed circuit (10) operating on a Rankine cycle with a low freezing point working fluid, said circuit comprising a compression pump (12) for the fluid in liquid form, a heat exchanger (18) swept by a hot source (24) for evaporation of said fluid, expansion means (30) for expanding the fluid in vapour form and a cooling exchanger (36) swept by a cold source (F) for condensation of the working fluid, characterized in that it consists, while the circuit is turned off, in transferring at least part of the fluid contained in said circuit into a tank (52) through at least one line (72, 78) carrying a valve (76, 82) and in heating the fluid contained therein by means of a heating system arranged in said tank when said fluid is likely to freeze.
12. A method as claimed in claim 11, characterized in that it consists in transferring the fluid to the tank, while the circuit is turned off, when the ambient temperature is lower than the freezing temperature of the fluid.
13. A method as claimed in claim 11 or 12, characterized in that it consists in transferring the fluid contained in the tank to circuit (10) when the circuit is turned on.
14. A method as claimed in any one of claims 11 to 13, characterized in that it consists in circulating the fluid in a line (72) connecting circuit (10) to tank (52) under the action of compression pump (12).
15. A method as claimed in any one of claims 11 to 13, characterized in that it consists in circulating the fluid in a line (78) connecting circuit (10) to tank (52) under the action of a circulation pump (84) carried by said line.
16. A method as claimed in any one of claims 11 to 14, characterized in that it consists in transferring through gravity the fluid contained in the tank into circuit (10) when the circuit is turned on.

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