Abstract:
The present invention relates to a device for controlling the working fluid with low freezing point circulating in a closed circuit ( 10 ) operating according to a Rankine cycle, said circuit comprising a compression pump ( 12 ) for the fluid in liquid form, a heat exchanger ( 22 ) swept by a hot source ( 28 ) for evaporation of said fluid, expansion means ( 30 ) for expanding the fluid in vapour form and a cooling exchanger ( 40 ) swept by a cold source (F) for condensation of the working fluid. 
     According to the invention, the device comprises a fluid collection tank ( 52 ) for draining said circuit.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a device for controlling a working fluid with low freezing point, in particular water, contained in a closed circuit operating according to a Rankine cycle, and to a method using same. 
         [0002]    It notably aims to associate this device with an internal-combustion engine, in particular for motor vehicles. 
         [0003]    As it is widely known, a Rankine cycle is a closed-circuit thermodynamic cycle whose specific feature is to involve a (liquid/vapour) phase change of a working fluid. 
         [0004]    This cycle is generally broken down into a stage wherein the working fluid used, water here, in liquid form, is compressed in an isentropic manner, followed by a stage where this compressed water is heated and vaporized on contact with a source of heat, this water vapour is then expanded, in another stage, in an isentropic manner in an expansion machine, then, in a last stage, this expanded vapour is cooled and condensed on contact with a cold source. 
         [0005]    To carry out these various stages, the circuit comprises a positive-displacement pump (or compressor) for compressing the water in liquid form, a heat exchanger (or evaporator) that is swept by a hot fluid for at least partial vaporization of the compressed water, an expansion machine for expanding the vapour, such as a turbine that converts the energy of this vapour into another energy such as a mechanical or electrical energy, and another heat exchanger (or condenser) by means of which the heat contained in the vapour is yielded to a cold source, generally outside air that sweeps this condenser so as to convert this vapour into water in liquid form. 
       BACKGROUND OF THE INVENTION 
       [0006]    It is also well known, notably through document FR-2,884,555, to use the calorific energy conveyed by the exhaust gas of internal-combustion engines, in particular those used for motor vehicles, as the hot source providing heating and vaporization of the fluid flowing through the evaporator. 
         [0007]    This allows to improve the energy efficiency of this engine by recovering a large part of the energy lost at the exhaust in order to convert it to an energy that can be used for the motor vehicle through the Rankine cycle circuit. 
         [0008]    The selection of this working fluid, which undergoes a succession of liquid/vapour phase transformations, is therefore determining. 
         [0009]    In fact, the saturation curve of this fluid has to be optimized according to the temperature of the hot source and of the cold source. 
         [0010]    Using an aqueous working fluid in a Rankine cycle circuit therefore affords the advantage of having characteristics allowing to obtain a maximum saturation curve while having the advantage of not being dangerous. 
         [0011]    However, water has the specific feature of having a freezing point at low temperatures (around 0° C.) and antifreeze additives such as glycol are usually added thereto in order to lower this freezing point to acceptable temperature levels, of the order of −15° C. to −30° C. 
         [0012]    Adding such additives has the drawback of changing the characteristics of water, in particular its vaporization characteristics, and the hot source from the exhaust gas may be insufficient to perform this vaporization in a satisfactory manner. 
         [0013]    Furthermore, in the course of time, this additive-containing water undergoes unpredictable aging as the liquid/vapour phase changes take place. This unpredictable aging can lead to incomplete phase changes for this water, which generates a Rankine cycle circuit dysfunction. 
         [0014]    The present invention aims to overcome the aforementioned drawbacks by means of a device and of a method that limit or even prevent freezing of the working fluid without causing changes in the liquid/vapour phase transformation characteristics. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention therefore relates to a device for controlling the working fluid with low freezing point circulating in a closed circuit operating according to a Rankine cycle, said circuit comprising a compression pump for the fluid in liquid form, a heat exchanger swept by a hot source for evaporation of said fluid, expansion means for expanding the fluid in vapour form and a cooling exchanger swept by a cold source for condensation of the working fluid, characterized in that it comprises a fluid collection tank for draining said circuit. 
         [0016]    The tank can be an insulated tank, an expansible tank, a tank whose capacity is larger than the volume of the fluid contained in the circuit. 
         [0017]    The tank can comprise a system for heating the fluid contained therein. 
         [0018]    The device can comprise at least one line connecting the circuit to the tank. 
         [0019]    The device can comprise a line for draining off the fluid from the circuit into the tank and a line for filling the circuit with the fluid from this tank. 
         [0020]    Preferably, the line can comprise a valve. 
         [0021]    At least one of the lines can comprise a fluid circulation pump. 
         [0022]    At least one of the lines can be connected to a point of a circulation line between the compression pump and the heat exchanger for evaporation of said fluid. 
         [0023]    The circulation line can be provided with a valve located between the point and the heat exchanger for evaporation of said fluid. 
         [0024]    Preferably, the working fluid can be water without an antifreeze additive. 
         [0025]    The hot source can come from the exhaust gas of an internal-combustion engine. 
         [0026]    The invention also relates to a method of controlling a working fluid with low freezing point circulating in a closed circuit operating according to a Rankine cycle, said circuit comprising a compression pump for the fluid in liquid form, a heat exchanger swept by a hot source for evaporation of said fluid, expansion means for expanding the fluid in vapour form and a cooling exchanger swept by a cold source 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. 
         [0027]    The method can consist in transferring the fluid to the tank, while the circuit is turned off, when the ambient temperature is below the freezing temperature of the fluid. 
         [0028]    The method can consist in transferring the fluid contained in the tank to the circuit when the circuit is turned on. 
         [0029]    The method can consist in circulating the fluid in a line connecting the circuit to the tank under the action of the compression pump. 
         [0030]    The method can consist in circulating the fluid in a line connecting the circuit to the tank under the action of a circulation pump carried by said line. 
         [0031]    The method can consist in transferring through gravity the fluid contained in the tank into the circuit when the circuit is turned on. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0032]    Other features and advantages of the invention will be clear from reading the description hereafter, given by way of non limitative example, with reference to the accompanying figures wherein: 
           [0033]      FIG. 1  shows a device for controlling a closed circuit operating according to a Rankine cycle, and 
           [0034]      FIG. 2  illustrates a variant of the device of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    In  FIG. 1 , Rankine cycle closed circuit  10  comprises a circulation and compression pump  12  (or compressor) for a working fluid, with an inlet  14  for the working fluid in liquid form and an outlet  16  for this working fluid, also in liquid form, but compressed at high pressure. This compressor is advantageously driven in rotation by an electric motor (not shown). 
         [0036]    This circuit also comprises a heat exchanger  18 , referred to as evaporator, traversed by the compressed working fluid between an inlet  20  for this liquid fluid and an outlet  22  through which the working fluid flows out of this evaporator in form of compressed vapour. This evaporator is swept by a hot source  24  coming from the exhaust gas circulating in exhaust line  26  of an internal-combustion engine  28 , and more particularly an engine for motor vehicles. 
         [0037]    This circuit also comprises an expansion machine  30 , referred to as expander, receiving through its inlet  32  the working fluid in form of high-pressure compressed vapour, this fluid flowing out through outlet  34  of the expander in form of low-pressure expanded vapour. 
         [0038]    Advantageously, this expander can come in form of an expansion turbine whose rotor is driven in rotation by the working fluid in vapour form while driving a connecting shaft (not shown). Preferably, this shaft allows to transmit the energy recovered to any transformer device such as, for example, an electric generator. 
         [0039]    The circuit also comprises a cooling exchanger  36 , or condenser, with an inlet  38  for the expanded low pressure vapour and an outlet  40  for the working fluid converted to liquid form after passing through this condenser. The condenser is swept by a cold source, generally a cold fluid (arrow F), with air at ambient temperature, in order to cool the expanded vapour so that it condenses and is converted to liquid. 
         [0040]    Fluid circulation lines  42 ,  44 ,  46  and  48  allow to connect successively the various elements of this circuit so that the fluid circulates in the direction shown by arrows C. More precisely, line  42  connects the compressor outlet to the evaporator inlet, line  44  connects the outlet of this evaporator to the expander inlet, line  46  connects the expander outlet to inlet  42  of the condenser, and line  48  connects the condenser outlet to the compressor inlet. 
         [0041]    In the description below, water is mentioned as the working fluid with low freezing point (around 0° C.) circulating in this circuit. This water has the specific feature of comprising no additive and, more particularly, no additive preventing freezing thereof. Any other (liquid/vapour) phase change fluid without antifreeze additive, that can freeze at low temperature (around 0° C.), can be used as the working fluid, such as organic fluids for example. 
         [0042]    As illustrated in this figure, a working fluid control device  50  with means for storing the water contained in the circuit is associated with this circuit. 
         [0043]    These means comprise a closed storage tank  52  for storing the water collected after draining the circuit. This tank allows to keep this water in the liquid state even when the ambient temperature is at a level that can cause it to freeze or allows it to freeze without damage risks for the tank and/or the circuit. 
         [0044]    More precisely, the tank is an insulated tank  54  with a peripheral coating  56  that covers all or part of its walls  58  and insulates it thermally from the ambient air. 
         [0045]    Alternatively, the tank is an expansible tank  60  with at least part of its walls  62  elastically deformable under the effect of the volume increase of the frozen water. 
         [0046]    A tank of large volume can also be used. The configuration of this tank is such that it has an inner volume that is greater than the volume of the water contained in the circuit and leaves a gas overhead  64  between the water level and the upper wall of this tank. This gas overhead comprises a volume that is at least equal to the volume increase of the water after freezing. 
         [0047]    In all the aforementioned tank layouts, the tank can comprise a system  66  for heating the liquid contained in the tank. This system comprises, by way of example, an electric resistance heater  68  arranged within this tank and supplied by electric conductors  70 . 
         [0048]    Of course, any control means known to the person skilled in the art are connected to this heating system so as to control and/or actuate it with, for example, ambient temperature measurement by means of a temperature detector. 
         [0049]    This tank is connected to circulation line  42  by a drain line  72  starting in the upper part of this tank and ending at a connection point  74  with line  42 . This drain line is fitted with a two-position valve  76 , providing a fully open and a fully closed position, allowing the water circulation in this line to be controlled. A filling line  78  also connects the tank bottom to a junction point  82  with line  42 . This filling line also comprises a two-position valve  82 , providing a fully open and a fully closed position, and a circulation pump  84 , preferably electric, which allows the water circulation in this line to be controlled. Preferably, the drain and filling lines can be insulated so as to limit freezing of the water contained in these lines. 
         [0050]    Finally, line  42  is fitted with a control valve  86  arranged downstream from the two junction and connection points and upstream from inlet  20  of evaporator  18 . 
         [0051]    Of course, valves  76 ,  82  and  86  are controlled by any known means such as electric motors, under the control of a processing unit and more particularly of the calculator of the internal-combustion engine. 
         [0052]    Similarly, this processing unit controls the motors driving compressor  12  and pump  84 . 
         [0053]    During operation, the water only circulates in the circuit in a clockwise motion considering  FIG. 1  (arrows C). The drain  76  and filling 82 lines are therefore in closed position for lines  72  and  78 , whereas valve  86  is in open position for line  42 . Pump  84  is inactive and compressor  12  is driven in rotation by its electric motor. 
         [0054]    In this configuration, the water leaves compressor  12  in liquid form at a pressure of the order of 10 bars and a temperature close to 50° C. This compressed water circulates in line  42  and ends in evaporator  22 , control valve  86  being open; it cannot circulate in lines  72  and  78  that are closed by valves  76  and  82 . This compressed water flows through the evaporator so as to be converted to vapour under the effect of the heat sweeping this evaporator and coming from the exhaust gas of engine  28 . The water vapour flowing from the evaporator is carried by line  44  and flows through expander  30  while transmitting thereto the energy it contains. The expanded water vapour leaving this expander circulates in line  46  and flows through condenser  36  where it is converted to liquid water. This liquid water is then brought through line  48  to compressor  12  in order to be compressed. 
         [0055]    When the Rankine cycle circuit is turned off, the processing unit controls control valve  86  so as to prevent any circulation of the compressed water contained in line  42  towards the inlet of evaporator  18  while maintaining the closed position of filling valve  82  for filling line  78  and keeping pump  84  inactive. 
         [0056]    This unit also controls drain valve  76  so that it is in open position for drain line  72  in order to establish a communication between line  42  and tank  52  through connection point  74  and this drain line  72 . 
         [0057]    Driving of compressor  12  is maintained and the water that leaves compressor  12  is fed into filling line  72  through point  74  in order to be transferred to tank  52 , here at the top of the tank, as shown by arrows V in  FIG. 1 . 
         [0058]    Of course, the person skilled in the art is able to calculate the time when driving of the compressor is stopped so as to completely drain off the water from the circuit and to store it in the tank, or at least so that only a minimum volume of water remains in the circuit which, if it should freeze, would not damage the elements of the circuit. 
         [0059]    Similarly, the person skilled in the art will position as close as possible to outlet  16  of compressor  12  the connection  74  and branch connection  80  lines, as well as control valve  86 , and limit the extent of lines  72  and  78 . This allows to limit the zones where the residual water can freeze. 
         [0060]    The water stored in the tank, and which is initially at the compressor outlet temperature (of the order of 50° C.), is then protected against freezing risks by insulation  56  of insulated tank  54 , or it can freeze, either by deforming the walls of deformable tank  60 , or by occupying the volume of gas overhead  64  of the large-volume tank without damaging the integrity of this tank. 
         [0061]    Of course, one may consider starting heating system  66  when its control means detect an ambient air temperature likely to generate freezing of this water. In the case of water freezing in the tank, heating system  66  is actuated by the calculator so as to thaw this water in order to turn on circuit  10 . 
         [0062]    When turning on the Rankine cycle circuit again, control valve  86  is in open position for circulation line  42 , valve  76  is in closed position for filling line  72  and valve  82  is in open position for filling line  78 . 
         [0063]    Compressor  12  and pump  84  are actuated, which results in feeding into line  42 , through junction point  80 , the water contained in the tank. This water is discharged from the tank under the action of the pump and circulates in filling line  78 , then in line  42  as shown by arrows R in  FIG. 1 . This water fed into line  42  is then circulated in circuit  10  under the effect of compressor  12  and undergoes various phase changes, as mentioned above. 
         [0064]    The person skilled in the art will parametrize the operating time of pump  84  so as to determine when to stop it after feeding again all of the water from the tank into circuit  10 . Alternatively, a detection means such as a float can be placed in the tank and control the interruption of pump  84  when this float detects no presence of water in the tank. 
         [0065]    Within the scope of  FIG. 1 , it can be considered removing drain line  72  and its valve  76 , and using only line  78  with its valve  82  and its pump  84  as the drain and filling line, the particular feature of pump  84  being that it is a bidirectional pump. 
         [0066]    In this case, when the circuit is turned off, valve  86  is in closed position for line  42  and valve  82  is in open position for line  42 . Compressor  12  and pump  84  are actuated in the same direction of rotation so as to feed the water from the circuit into line  78 , then to the bottom of tank  52  as shown by arrows V′. 
         [0067]    When turning this circuit on again, valve  82  remains in open position for line  78  and valve  86  switches to a fully open position of circulation line  42 . 
         [0068]    The compressor is actuated in the same direction as for draining and the pump is controlled in the opposite direction to draining so as to extract the water contained in the tank and circulate it in line  78  as shown by arrows R, as mentioned above. 
         [0069]    The variant of  FIG. 2  differs from the example of  FIG. 1  by a specific position of tank  52  and the removal of the circulation pump on filling line  78 . 
         [0070]    As can be seen in  FIG. 2 , the tank is positioned with respect to circuit  10  in such a way that connection point  88  between filling line  78  and the tank, arranged here in the bottom of this tank, is located above junction point  80  between this line and circulation line  42 . 
         [0071]    For this variant, the operation of the circuit is the same as in  FIG. 1 , with closing of valves  76  and  82 , opening of valve  86  and water circulation according to arrows C under the action of compressor  12 . 
         [0072]    The stage of draining the water from the circuit into tank  52  so as to turn off the circuit is also identical to  FIG. 1 , with closing of valves  82  and  86 , opening of valve  76  and actuation of compressor  12  in order to circulate the water as shown by arrows V. 
         [0073]    For the circuit filling stage, valve  76  is in closed position for line  72 , valves  82 ,  86  are in open position for lines  78  and  42 , and compressor  12  is actuated. 
         [0074]    Due to gravity, the water contained in the tank flows through connection point  88  and circulates in filling line  78 , then in circulation line  42  as shown by arrows R. 
         [0075]    Of course, without departing from the scope of the invention, it is possible to drain the circuit, after turning it off, only if the ambient temperature is likely to cause freezing of the water contained in the circuit, notably when it is below its freezing temperature. 
         [0076]    A dedicated temperature detector or the detector associated with heating system  66  can be used for this purpose.