Abstract:
A method for automatically controlling a steam content of a working medium heated in an evaporator of a system for carrying out a thermodynamic cycle with the following steps: carrying out a phase separation for the working medium downstream from the evaporator, wherein liquid components are separated from vapor components of the working medium; conducting the separated liquid components to a reservoir; determining a level in the reservoir; and varying a control variable for automatically controlling the steam content as a function of the level.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority of DE 10 2014 206 012.5, filed Mar. 31, 2014, the priority of this application is hereby claimed and this application is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention pertains to a method for automatically controlling a vapor content of a working medium heated in a evaporator of a system for carrying out a thermodynamic cycle, to a control unit for a corresponding system, to a system for a thermodynamic cycle, and to an arrangement with an internal combustion engine and a system. 
         [0003]    Thermodynamic cycles of the type in question here are known. A working medium is conveyed through a circuit by a conveying device, wherein the medium is vaporized in a evaporator and sent to an expansion device, wherein it performs mechanical work. The expanded working medium is cooled in a condenser and thus condensed, after which it is sent back to the evaporator by the conveying device. A typical example of a thermodynamic cycle of this type is the Clausius-Rankine cycle. Very similar to it is the organic Rankine cycle (ORC), which, because of the lower temperature level in the evaporator thanks to the use of an organic working medium, is especially adapted to the use of waste heat in stationary applications such as geothermal power plants or to the use of the waste heat of an internal combustion engine. Whereas, in classical steam-powered machines and/or steam power plants, the steam is usually superheated after the vaporization of the working medium, so that dry steam can be expanded in the expansion device, it has been found, especially in conjunction with the use of waste heat and with the ORC process, that it can be advantageous to operate in the wet steam region. To ensure that the process can be carried out in stable fashion, however, it is necessary to work with a defined steam content, which means in particular that this content must be automatically controlled. The working medium is typically vaporized at a constant pressure and a constant temperature. There is thus no clear correlation between these variables on the one hand and the steam content on the other. It is therefore impossible to determine the quality of the steam directly by means of the rudimentary measuring techniques which would normally be present in any case. It is possible to make use of capacitive moisture measurements to determine the steam content, but this approach is complicated and expensive. 
       SUMMARY OF THE INVENTION 
       [0004]    The invention is based on the goal of creating a method for automatically controlling a steam content, namely, a method which is easy to carry out and at the same time allows stable and accurate control. The invention is also based on the goal of creating a control unit for carrying out a process of this type, a system for a thermodynamic cycle which can be automatically controlled by means of such a method, and an arrangement consisting of an internal combustion engine and a system of this type. 
         [0005]    The goal is achieved in a method in which a phase separation is carried out for the working medium downstream from the evaporator. Liquid components are therefore separated from the vapor components of the working medium. The separated liquid components of the working medium are conducted to a reservoir, and the level reached in the reservoir is determined. As a function of this level, a control variable is adjusted to control the steam content. This means that the level of the separated liquid working medium is used as the measurement value for the automatic control process, which ultimately represents the indirect control of the wet steam content. Because the amount of liquid separated during the phase separation step will be larger or smaller as a function of the steam content of the wet steam generated in the evaporator, it is possible, by determining the level in the reservoir, to infer the steam content. This content can thus be determined very easily and without complicated and expensive measuring devices. The automatic control method makes it possible to achieve robust control behavior especially in the face of changes in the boundary conditions and/or disturbances, wherein the reaction of the thermodynamic state of the process always occurs reliably within the wet steam region. This applies above all to load changes in the system, which will therefore never lead outside the wet steam region, which means that the process as a whole remains stable. This also makes partial load control easier to implement, and the behavior of the system when the load increases quickly is improved. This is especially important in conjunction with the use of waste heat of an internal combustion engine, where the operating state of the system changes as a function of the operating state of the engine, because the exhaust gas temperature and thus the amount of heat available depend on the operating state of the internal combustion engine. Depending on the manner in which the internal combustion engine is being used, load changes occur more or less frequently and at more or less regular intervals. Against this background, stable automatic control of the steam content makes it possible for the first time to operate the process in the wet steam region, which in turn makes it possible to increase the power yield of the system, especially when this is configured as an ORC system. 
         [0006]    The possibility of operating the system, i.e., the cycle, in the wet steam region offers additional advantages with respect to long-term service life and/or component stress: Larger wetted surfaces in the evaporator prevent inhomogeneous temperature distributions in the area of the evaporator wall, which considerably reduces the thermal load on the material of the evaporator. At the same time, the wetting prevents oil from being coked in the evaporator. In addition, the fluid-dynamic stability of the process is increased: As a result of operation in the wet steam region, the phase discontinuity in the evaporator occurs at a later point, which means a smaller gas volume and a slower flow rate in the evaporator. Associated with this is a reduced pressure loss, as a result of which ultimately the tendency to develop fluid-dynamic instabilities, especially a tendency to develop the Ledinegg instability, is decreased. Finally, the amount of circulating lubricant provided to lubricate the expansion device can be reduced, because the circulating lubricant has less of a tendency to form deposits in dead volumes and on the walls of the system. The lubricant is washed away by the liquid components of the working medium and transported further along the circuit. The liquid components therefore have an advantageous washing effect, so that ultimately a greater amount of lubricant can be circulated permanently through the system while at the same time the total amount of lubricant can be reduced. 
         [0007]    The advantageous effects of automatic wet steam control with respect to the service life of the system components are important criteria in particular for so-called off-road applications, that is, couplings of the system with an internal combustion engine for use of its waste heat in areas separate from conventional highway traffic such as in stationary systems or in special types of vehicles, so that the acceptance of this type of waste heat use is considerably increased by the method proposed here. 
         [0008]    The phase separation for the working medium is preferably carried out directly downstream from the evaporator, especially upstream of the expansion device. Thus it is possible to control the fresh steam content at the evaporator outlet very precisely, wherein at the same time saturated steam as free as possible of liquid components is sent to the expansion device. 
         [0009]    An embodiment of the method is especially preferred in which an ORC process is carried out. In this type of process, the advantages of the method are realized in a special way, wherein the process at the same time is especially adapted to the use of waste heat, especially to the use of the waste heat of an internal combustion engine. In a preferred embodiment of the method, ethanol is used as the working medium. Other organic working media are also possible, of course. 
         [0010]    It is possible for the level in the reservoir to be determined intermittently, that is, at certain times. Under certain conditions this can be sufficient for the stable automatic control of the steam content. As an alternative, it is preferable, however, for the level in the reservoir to be determined, especially monitored, at all times. In this way, the automatic control of the steam gas can be carried out with particular precision. Intermittent determination, however, leads to a less complicated method and therefore offers certain cost advantages. 
         [0011]    A preferred embodiment of the method is characterized in that a mass flow of the working medium in the system is varied as a function of the level. The mass flow of the working medium in the system is, to this extent, used as a control variable. This makes it possible to regulate the steam content very efficiently and precisely. It has been found that the pressure at the evaporator outlet is essentially dependent on the mass flow on the one hand and on the rotational speed of the expansion device on the other. When the amount of heat entering the system increases because, for example, an internal combustion engine coupled to the system comes under load and its exhaust gas temperature increases, there is a tendency for the fresh steam to become a superheated, wherein the temperature at the evaporator outlet increases at the same time. The pressure and/or the volume flow rate in front of the expansion device also increases. Simultaneously, the level in the reservoir either falls, increases with at least a reduced separation rate, or remains constant. The level falls especially when working medium is being withdrawn continuously from the reservoir. In any case, the level changes, or the change in level is different, which means that the change in the steam content can be determined. The mass flow in the system is then increased so that the increased amount of heat can be absorbed while keeping the steam content as constant as possible. If, conversely, the amount heat being supplied to the evaporator falls, because, for example, the load on the internal combustion engine decreases significantly, thus leading to a drop in the exhaust gas temperature, the steam content falls simultaneously. As a result, more liquid is separated, and the level in the reservoir rises. In this case, the mass flow in the system is reduced and thus adapted to the smaller amount of available heat. 
         [0012]    Within the scope of the method, preferably the steam content at the evaporator outlet is regulated. In particular, this corresponds to a regulation of the wet steam at the evaporator outlet. The steam content is preferably regulated automatically to match a constant, previously determined value. What this regulation preferably does, therefore, is to maintain a constant steam content. 
         [0013]    Another embodiment of the method is characterized in that a change in the level in the reservoir is determined, wherein the mass flow is varied as a function of the change in level. This takes into account the knowledge that the absolute level of the reservoir, in and of itself, typically says little about the steam content. In contrast, an increase in the level in the reservoir indicates an increase in the separation of liquid components and thus a decrease in the steam content, whereas a decrease in the level—as working medium is being withdrawn continuously from the reservoir—or a reduction in the separation rate and possibly even a constant level indicates an increase in the steam content. To this extent, a method is also preferred in which the rate at which the level changes is determined and evaluated with respect to the steam content or a change in the steam content. On this basis, it is possible to achieve an especially precise automatic control of the steam content. 
         [0014]    Another embodiment of the method is characterized in that the mass flow is varied by variation of an output of a conveying device, wherein the conveying device is used to convey the working medium in the system. In this way, the mass flow can be varied directly, very precisely, and easily. A pump is preferably used as the conveying device, especially a feed pump. To vary the mass flow, preferably the rotational speed of the pump, especially of the feed pump, is varied. A further embodiment of the method is characterized in that a pressure and/or a temperature of the working medium is determined downstream from the evaporator and used as input for the automatic control of the steam content. According to one embodiment of the method, therefore, it is provided that the pressure of the working medium is determined downstream from the evaporator, preferably directly downstream from the evaporator, in particular at the evaporator outlet, and used as input for the automatic control. Alternatively or in addition, it is provided that a temperature of the working medium downstream from the evaporator, especially directly downstream from the evaporator, preferably at the evaporator outlet, is determined and used as input for the automatic control. By determining or acquiring at least one of these measurement variables, the thermodynamic state of the working medium downstream from the evaporator and upstream of the expansion device, especially at the evaporator outlet, can be determined. 
         [0015]    Under certain circumstances, this allows, in and of itself, conclusions to be drawn concerning the steam content. In particular, it is thus possible under certain conditions to determine a superheating of the working medium at the evaporator outlet. In any case, the accuracy of the automatic control can be increased by combining the determination of the level, especially the determination of a change in the level, with the evaluation of the pressure and/or the temperature of the working medium downstream from the evaporator. 
         [0016]    Alternatively or in addition, it is possible to determine the pressure and/or the temperature of the working medium upstream of a separation device, which is provided to separate liquid components of the working medium from the vapor components. The pressure and/or the temperature is preferably determined directly in front of the separation device. Alternatively or in addition, it is possible for the pressure and/or the temperature of the working medium to be determined upstream of an expansion device of the system, especially directly in front of the expansion device. 
         [0017]    Another embodiment of the method is characterized in that the automatic control of the steam content is calibrated by operating the system on or beyond the saturated steam curve of the medium and by intentional deviation from the saturated steam curve into the wet steam region. The phrase “beyond the saturated steam curve” means that the working medium is superheated, so that dry fresh steam is produced. The reservoir is preferably completely emptied prior to the calibration, so that no liquid is present in the reservoir. For the calibration, preferably pure working medium without any lubricant components in the circuit of the system is used. The system is then operated initially on or beyond the saturated steam curve, wherein no liquid components of the working medium separate in the reservoir. By intentional deviation of the operating state of the system from the saturated steam curve into the wet steam region, it is then possible to determine how the level in the reservoir changes when the steam content changes in a defined manner. The data on the level and/or on the change in level in the reservoir as a function of the absolute and/or changing steam content thus acquired are preferably used as input for drawing up a characteristic diagram, which is then used later for automatic control during operation of the system. 
         [0018]    Alternatively it is also possible for the calibration to be carried out when the system is operating with a mixture of working medium and lubricant. It is possible to include the lubricant separating in the reservoir during the operation of the system in the data used to draw up the characteristic diagram. This can increase the accuracy of the calibration and ultimately also the accuracy of the automatic control. 
         [0019]    In another embodiment of the method a withdrawal of liquid from the reservoir is used as input for the automatic control of the steam content. Liquid is withdrawn from the reservoir, first, to prevent the reservoir from overflowing. Liquid is therefore sent back, preferably continuously, from the reservoir to the circuit of the working medium. Exact knowledge of the amount of liquid withdrawn makes possible the precise automatic control of the steam content as a function of the level or change in level. 
         [0020]    It has also been found that at least some of the lubricant used to lubricate the expansion device is typically conveyed around the circuit together with the working medium. The lubricant is not vaporized in the evaporator and is thus separated together with the liquid components of the lubricant as part of the phase separation process in the reservoir. These amounts of separated lubricant are preferably also used as input for the automatic control of the steam content. Lubricant or a mixture of working medium and lubricant is withdrawn from the reservoir—preferably by means of a metering pump; this is then sent to the expansion device by way of lubricant pathways separate from the circuit. The lubricant demand depends on the operating point of the expansion device, especially on its rotational speed, and thus also on the operating point of the system. This operating point-dependent withdrawal of liquid from the reservoir is preferably stored in a characteristic diagram and can thus be used for the automatic control of the steam content as a function of the level. Under steady-state operating conditions, i.e., a constant withdrawal of lubricant or mixture from the reservoir, it is possible to detect a change in the steam content quickly and easily by detecting a change in the level in the reservoir. 
         [0021]    The goal of the invention is also achieved in that a control unit for a system for a thermodynamic cycle is created. This is set up to carry out a method for automatically controlling a steam content of a working medium heated in a evaporator of the system, wherein the control unit is set up to determine a level in a reservoir, which is located downstream from a evaporator and is connected to a separation device for separating liquid components of the working medium, wherein the control unit is also set up to vary a control variable for automatically controlling the steam content as a function of the level. The control unit is preferably set up to carry out a method according to one of the previously described embodiments. Thus, in conjunction with the control unit, the advantages already explained in connection with the method are realized. 
         [0022]    The method can be permanently implemented in an electronic structure, i.e., in the hardware, of the control unit. Alternatively, it is preferred that a computer program product be loaded into the control unit, namely, a program which contains instructions on the basis of which the method is carried out when the computer program product is running on the control unit. 
         [0023]    The control unit is preferably set up to vary a mass flow of the working medium in the system as a function of the level, especially to vary an output of a conveying device in the system. 
         [0024]    The control unit is preferably set up to acquire a change in the level, wherein it is also set up to vary the mass flow as a function of the change in level. 
         [0025]    The control unit is also preferably set up to acquire a pressure and/or a temperature of the working medium downstream from the evaporator, in particular upstream of an expansion device, and especially preferably at the evaporator outlet. 
         [0026]    The control unit is set up to use at least one of these measurement values for the automatic control of the steam content. 
         [0027]    The control unit is also preferably set up to use the withdrawal of liquid from the reservoir as input for the automatic control of the steam content. 
         [0028]    The control unit comprises appropriate interfaces to appropriate sensors and actuators. 
         [0029]    The goal of the invention is also achieved in that a system for a thermodynamic cycle is created. This is characterized by a control unit according to one of the previously described exemplary embodiments. Thus, in conjunction with the system, the advantages previously explained in connection with the control unit and especially the advantages already explained in connection with the method are realized. 
         [0030]    The system preferably comprises a evaporator; a separating device, set up to separate liquid components of the working medium from the vapor components of the working medium; an expansion device; a condenser; and a conveying device for the conveying the working medium in the circuit—arranged in series in the flow direction of the working medium through the circuit of the system. The system is preferably set up to carry out an organic Rankine cycle (ORC), especially with ethanol as the working medium. This makes it possible to employ the system especially effectively as a means of using waste heat. 
         [0031]    A reservoir is preferably connected to the separating device, so that the liquid components separated in the separating device can be conducted to it. The system preferably comprises a level sensor, by means of which the level or the change in level in the reservoir can be detected. The control unit of the system is preferably functionally connected to the level sensor for determining, preferably for monitoring, the level in the reservoir. The control unit is also preferably functionally connected to the conveying device for varying its output and thus in particular for varying a mass flow in the system. 
         [0032]    The system also preferably comprises a pressure sensor and/or a temperature sensor downstream from the evaporator, especially at an evaporator outlet. The control unit is preferably functionally connected to at least one of these sensors and is set up to determine a thermodynamic state of the working medium downstream from the evaporator, especially at the evaporator outlet. The control unit is also preferably set up to use at least one of these measurement values for the automatic control of the steam content. 
         [0033]    Another exemplary embodiment of the system is characterized in that the separating device is configured as a cyclone separator. Baffles cause the working medium to flow in circles in the cyclone separator, as a result of which liquid components strike the baffles and run down them. The cyclone separator makes highly efficient phase separation possible while simultaneously having an extremely low pressure loss. 
         [0034]    Another exemplary embodiment of the system is characterized in that the expansion device is configured as a helical screw expander. A helical screw expander has been found to be especially favorable in terms of power yield especially in the case of an ORC process. This is especially true for ORC systems which operate without superheating, i.e., which operate in the wet steam region. A helical screw expander is a displacement machine free of dead spaces, the working chambers of which are formed by the spaces between the teeth of two helical gear wheels, also called rotors. The teeth of one rotor, which preferably extend in spiral fashion over the rotor&#39;s circumferential surface, which is elongated in the axial direction, engage in the tooth spaces of the other rotor. When the two rotors are in relative rotation, working chambers of variable volume are thus formed, in which the working medium expands as it passes through the helical screw expander from the inlet to the outlet. 
         [0035]    Alternatively, it is also possible for the expansion device to be configured as a continuous-flow machine, especially as a turbine, as a displacement machine of some other type, or as a volumetric expansion device, in particular as a reciprocating piston machine, a scroll expander, a rotary vane machine, or a Roots expander. 
         [0036]    Finally, an exemplary embodiment of the system is characterized in that it is set up to use the waste heat of an internal combustion engine. This makes it possible to employ the system advantageously for the mobile or stationary use of waste heat and to increase the efficiency of the internal combustion engine. 
         [0037]    The goal of the invention, finally, is also achieved by an arrangement that comprises an internal combustion engine and a system according to one of the previously described exemplary embodiments. The system is functionally connected to the internal combustion engine for the use of its waste heat. It is possible for the exhaust gas of the internal combustion engine to be conducted to the system so that use can be made of the waste contained in that gas. Alternatively or in addition, coolant of the internal combustion engine can be conducted to the system so that use can be made of the waste heat contained in the coolant. In this way, the overall efficiency of the internal combustion engine can be increased, and beneficial use can be made of its waste heat. It is possible for the mechanical work performed in the system to be returned directly to a crankshaft of the internal combustion engine to support the work of the engine. Alternatively or in addition, the mechanical work can be used elsewhere in directly mechanical fashion. It is also possible for a generator to be functionally connected to the expansion device, so that the mechanical work is converted to electrical energy. This can be sent, by means of an electric motor, for example, back to the crankshaft of the internal combustion engine to support it. Alternatively or in addition, the electrical energy thus generated can be used elsewhere such as in an on-board power supply system, or it can be fed into a power grid. 
         [0038]    The internal combustion engine of the arrangement is preferably configured as a reciprocating piston engine. In a preferred exemplary embodiment, the internal combustion engine serves in particular to drive heavy land vehicles such as mining vehicles and trains or water craft, wherein the internal combustion engine is used in a locomotive or motor coach or in a ship. The use of the internal combustion engine to drive a vehicle serving defensive purposes such as a tank is also possible. In another exemplary embodiment of the internal combustion engine, it is stationary and used for stationary power generation to generate emergency power or to cover continuous-load or peak-load demands, wherein the internal combustion engine in this case preferably drives a generator. The stationary use of the internal combustion engine to drive auxiliary units such as fire-fighting pumps on offshore drilling rigs is also possible. An application of the internal combustion engine in the area of the recovery of fossil materials and especially fossil fuels such as oil and/or gas is also possible. The internal combustion engine can also be used in industry or in the construction field for the production of construction vehicles such as cranes and bulldozers. The internal combustion engine is preferably configured as a diesel engine; as a gasoline engine; or as a gas engine for operation with natural gas, biogas, customized gas, or some other suitable gas. Especially when the internal combustion engine is configured as a gas engine, it is suitable for use in block-type thermal power stations for stationary power generation. 
         [0039]    An especially preferred exemplary embodiment of the arrangement is provided for marine applications, especially for use on board a ferry, wherein the internal combustion engine is provided preferably to drive the ship. Electrical energy for an on-board power system of the ship can be recovered by means of the system. 
         [0040]    The description of the method on the one hand and of the control unit, the system, and the arrangement on the other hand are to be understood as complementary to each other. Features of the control unit, of the system, and/or of the arrangement which have been explained explicitly or implicitly in conjunction with the method are preferably steps, individually or in combination, of a preferred embodiment of the control unit, of the system, and/or of the arrangement. Method steps which have been explained explicitly or implicitly in conjunction with the control unit, the system, and/or the arrangement, are preferably steps, individually or in combination, of a preferred embodiment of the method. The method is preferably characterized by at least one method step which is required by at least one feature of the control unit, of the system, and/or of the arrangement. The control unit, the system, and/or the arrangement are characterized preferably by at least one feature which is required by at least one method step of the method. 
         [0041]    The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the drawings and descriptive matter in which there are illustrated and described preferred embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0042]    In the Drawing: 
           [0043]      FIG. 1  shows a schematic diagram of an exemplary embodiment of an arrangement with an internal combustion engine and a system for carrying out a thermodynamic cycle; and 
           [0044]      FIG. 2  shows a schematic diagram of an embodiment of the method in the form of an automatic control circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0045]      FIG. 1  shows a schematic diagram of an exemplary embodiment of an arrangement  1 , which comprises an internal combustion engine  3  and a system  5  for a thermodynamic cycle. The system  5  is set up here to carry out an organic Rankine cycle and to use waste heat of the internal combustion engine  3 . For this purpose, waste heat of the internal combustion engine  3  can be conducted to a evaporator  7  of the system  5 , especially the waste heat contained in the exhaust gas and/or in a coolant of the internal combustion engine  3 . 
         [0046]    The system  5  comprises a circuit  9  for a working medium, preferably ethanol, wherein a separation device  11 , an expansion device  13 , a condenser  15 , and a conveying device  17  for conveying the working medium through the circuit are provided around the circuit  9  downstream, in the flow direction of the working medium, from the evaporator  7 . The separation device  11  is preferably configured as a cyclone separator. The expansion device  13  is preferably configured as a helical screw expander. The conveying device  17  is preferably configured as a feed pump with variable speed. In any case, the conveying device  17  comprises variable output, wherein the mass flow of the working medium in the system  5  is adjustable by varying the output of the conveying device  17 . 
         [0047]    The system  5  is set up to operate the thermodynamic cycle, especially the ORC process, in the wet steam region, so that, downstream from the evaporator  7 , in particular at the evaporator outlet, wet steam containing both liquid and vapor components of the working medium is present. To guarantee a stable cycle, the system  5  comprises automatic control for the steam content of the working medium downstream from the evaporator  7  and upstream of the expansion device  13 , especially in the area of the evaporator outlet. In the separation device  11 , liquid components of the working medium are separated from the vapor components, wherein the separated liquid components are conducted to a reservoir  19 . Lubricant, which is provided to lubricate the expansion device  13 , is also preferably separated in the separation device  11 . At least some of this lubricant is conveyed together with the working medium around the circuit  9 , but it is not vaporized in the evaporator  7 . It is therefore in liquid form downstream from the evaporator and therefore is also separated in the separation device  11 . It then arrives in the reservoir  19  along with the liquid components of the lubricant. 
         [0048]    The system  5  comprises a level sensor  21 , by means of which the level and in particular a change in the level in the reservoir  19  can be detected. To regulate the steam content of the working medium downstream from the evaporator  7 , the system  5  is set up to vary a control variable as a function of the level detected by the level sensor  21 , especially as a function of a change in level detected by the level sensor  21 . 
         [0049]    For this purpose, a control unit  23  is provided, which is functionally connected to the level sensor  21  to determine, especially to monitor, the level, especially a change in the level. The control unit  23  is set up to vary a mass flow of the working medium in the system  5  as a function of the level, especially of the change in the level. For this purpose, in the exemplary embodiment shown here, it is functionally connected to the conveying device  17  to vary its output and thus the mass flow of the working medium through the circuit  9  as a function of the signal acquired from the level sensor  21 . 
         [0050]    If, for example, more heat is being supplied from the internal combustion engine  3  to the evaporator  7 , the steam content at the evaporator outlet increases, so that less liquid working medium is separated in the separation device  11  and thus conducted to the reservoir  19 . The level therefore increases more slowly, no longer changes, or perhaps even falls. These data are acquired by the control unit  23  and evaluated quantitatively as an increase in the steam content. The conveying device  17  is actuated by the control unit  23  to increase its output, so that the mass flow in the circuit  9  increases. Thus the increased amount of heat supplied to the evaporator can be absorbed by the system  5  while the steam content remains at least approximately the same. 
         [0051]    If, conversely, the amount of heat supplied by the internal combustion engine  3  decreases, less vaporization will occur and thus the amount of liquid component of the working medium will increase, wherein more liquid is separated in the separation device  11 , which liquid is then conducted to the reservoir  19 . Thus the level in the reservoir  19  rises, which is again detected by the level sensor  21  and quantitatively evaluated by the control unit  23  as a decrease in the steam content. The control unit  23  then actuates the conveying device  17  in such a way that its output is reduced, so that the mass flow in the circuit  9  decreases. It is thus adapted to the smaller amount of available heat in the evaporator  7 , as a result of which the steam content again can be kept at least approximately constant. The control unit  23  is preferably set up automatically to keep the steam content at the evaporator outlet constant. The control unit  23  is also preferably configured to control the rotational speed of the conveying device  17 , configured as a feed pump. 
         [0052]    A sensor device  25  for detecting a pressure and/or a temperature of the working medium is preferably provided downstream from the evaporator  7 , especially at the evaporator outlet. The control unit  23  is preferably functionally connected to this sensor device  25  and is set up to determine a thermodynamic state of the working medium at the evaporator outlet on the basis of the at least one measurement value of the control unit  25 . This information is preferably used as input for the automatic control of the steam content, as a result of which the precision of the control process is increased. 
         [0053]    It has been found preferable to send the lubricant separated in the reservoir to the expansion device  13  to lubricate it by way of a lubricant route  27 , which is indicated only schematically. Alternatively or in addition, liquid present in the reservoir  19  is preferably returned to the circuit along a drain route  29 , preferably downstream from the expansion device and upstream of the condenser  15 . This option is preferably used especially to prevent the reservoir  19  from overflowing. In addition, it is possible in this way to ensure a high lubricant concentration—and thus a small amount of working medium—in the reservoir, which functions to this extent as a lubricant tank. 
         [0054]    In any case, it has been found preferable for liquid to be withdrawn from the reservoir  19  continuously and/or at regular intervals. In particular, the removal of lubricant to lubricate the expansion device  13  depends on the operating state of the system  5 , especially on the rotational speed of the expansion device. In the control unit  23 , preferably at least one characteristic diagram is stored, in which the operating point-dependent removal of liquid from the reservoir  19  is entered. Alternatively or in addition, it is possible for the system  5  to comprise a withdrawal sensor  31 , preferably in the form of a flow sensor, by means of which the withdrawal of liquid from the reservoir  19  can be detected directly. In this case, the control unit  23  is functionally connected to the withdrawal sensor  31  to detect the withdrawal of liquid from the reservoir  19 . In any case, the withdrawal of liquid from the reservoir is preferably used as input for the automatic control of the steam content, which increases its accuracy yet again. 
         [0055]    It has also been found that, in the case of the exemplary embodiment of the system  5  illustrated here, the expansion device  13  is functionally connected to a generator  33 , so that the mechanical work performed by the working medium in the expansion device  13  can be converted into electrical energy by the generator  33 . 
         [0056]    It is especially preferable to provide the arrangement  1  for marine applications, especially for ferries. In this case, the internal combustion engine  3  preferably serves to drive the ship, especially the ferry. The electrical power generated by the generator  33  is especially preferably sent to, i.e., fed into, an on-board power system of the ship, especially of the ferry. Other applications of the use  1 , especially stationary applications or other mobile applications—as previously described in conjunction with the internal combustion engine—are also possible. 
         [0057]      FIG. 2  shows a schematic diagram of an embodiment of the method in the form of an automatic control circuit. The automatic control illustrated schematically here is preferably carried out in its entirety in the control unit  23 . A nominal value  35  for the steam content of the working medium downstream from the evaporator  7 , especially at the evaporator outlet, is sent to the automatic control circuit. In a comparison member  37 , this value is compared with an actual value  39  of the steam content, from which a control deviation  41  is obtained. This is sent to an automatic control device  43 , which calculates from it a control variable  45 , in particular in the form of an actuation signal for the conveying device  17 , to adjust its output. It is possible for the output of the conveying device  17  to be controlled on the basis of the control variable  45  alone. Alternatively, it is preferable, however, for the output of the conveying device  17 , especially the rotational speed of the feed pump, to be regulated to match the control variable  45  by means of a subordinate control process. This increases the accuracy of the method. 
         [0058]    The command variable  45  acts on a controlled system  47 , which comprises in particular the conveying device  17 , the evaporator  7 , the separation device  11 , and the reservoir  19 . On the basis of a change in the control variable  45 , the mass flow of the working medium in the circuit  9  is changed by variation of the output of the conveying device  17 , which influences the steam content of the working medium at the evaporator outlet and thus also the level in the reservoir  19 , which can be detected by the level sensor  21 . The controlled system  47  therefore results ultimately in a measurement value  49 , which represents the level or the change in the level, preferably detected by the level sensor  21 . The measurement value  49  is therefore in particular a measurement signal produced by the level sensor  21 . The measurement value  49  is sent to a calculation member  51 , which is set up to calculate the steam content of the working medium downstream from the evaporator  7 , in particular at the evaporator outlet, as a function of the measurement variable  49 . What therefore results finally from the calculation member  51  is the actual value  39  for the steam content, which is itself sent back to the comparison member  37 . 
         [0059]    Additional variables are preferably also input into the calculation member  51 . It is preferable for a pressure  53  of the working medium downstream from the evaporator  7 , especially at the evaporator outlet, to be entered. Alternatively or in addition, it is provided that a temperature  55  of the working medium downstream from the evaporator  7 , especially at the evaporator outlet, is entered. By means of at least one of these measurement variables, a thermodynamic state of the working medium at the designated point can be determined, as a result of which also—and possibly as a complement to the measurement variable  49 —the steam content of the working medium can also be inferred. This increases the accuracy of the automatic control. 
         [0060]    Alternatively or in addition, preferably a withdrawal  57  of liquid from the reservoir  19  is entered into the calculation member  41 , wherein the withdrawal  57  is read out from a characteristic diagram and/or measured by means of the withdrawal sensor  31 . This also increases the accuracy of the method. 
         [0061]    Overall, it has thus been found that, by means of the method, it is possible to regulate the steam content automatically in a very stable, accurate, and low-cost manner, which ultimately makes it possible to operate the system economically in the wet steam region with all its associated advantages. 
         [0062]    While specific embodiments of the invention have been shown and described in detail to illustrate the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.