Abstract:
When converting thermal into mechanical energy by a working medium containing a mixture of at least two materials having different boiling and condensation points, which is fed to a condenser, and is condensed therein, the condenser condensation pressure may increase and the efficiency for generating the mechanical energy thus decreases because the mixture of materials is separated into a liquid phase and a vapor phase upstream of the condenser. To prevent this, the liquid phase of the working medium is mixed with the vapor phase of the working medium before or while the working medium is condensed, thus once again creating a homogeneous mixture of materials which condenses at a lower pressure than the separated working medium, thereby preventing loss of efficiency. This can be applied to the use of thermal energy from low-temperature sources such as geothermal fluids, industrial waste heat, or waste heat from internal combustion engines.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/EP2008/060921 filed Aug. 21, 2008, which designates the United States of America, and claims priority to German Application No. 10 2007 041 458.9 filed Aug. 31, 2007, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to a method and apparatus for conversion of thermal energy to mechanical energy. 
       BACKGROUND 
       [0003]    A method such as this and an apparatus such as this are known, for example, from WO 2005/100755 A1. 
         [0004]    In recent years, widely differing technologies have been developed for low-temperature heat sources with temperatures up to a maximum of 400° C., for example geothermal fluids or industrial waste heat, which allow the heat from these sources to be converted to mechanical and/or electrical energy with high efficiency. In addition to the Rankine process which uses an organic agent (Organic Rankine Cycle, ORC), the so-called Kalina cycle process, in particular, is distinguished by considerably higher efficiencies than the classical Rankine process. Various circuits for widely differing applications have already been developed on the basis of the Kalina cycle process. Instead of using water, these circuits use a two-substance mixture (for example of ammonia and water) as the agent, with the different boiling and condensation temperatures of the two substances and the non-isothermal boiling and condensation process of the mixture resulting from this being exploited in order to increase the efficiency of the circuit in comparison to a Rankine circuit. 
         [0005]    A Kalina circuit such as this normally comprises at least one pump for increasing the pressure of the agent, a heat exchanger for producing a vapor phase of the agent by heat transfer from an external heat source, for example a geothermal liquid or industrial waste heat, and an expansion device, preferably a turbine, for expansion of the vapor phase and conversion of its thermal energy to mechanical energy. The expanded agent is then condensed in a condenser with the aid of a coolant. 
         [0006]    Even more components may be connected in the circuit in order to improve the efficiency. For example—as disclosed in WO 2005/100755 A1—a separator can be arranged in the circuit between the heat exchanger and the expansion device, by means of which any liquid phase of the agent which is still present in the event of any partial vaporization of the agent in the heat exchanger can be separated from the vapor phase before being supplied to the expansion device. The separated liquid phase can then be combined with the expanded vapor phase by means of a mixing device which is arranged in the circuit between the expansion device and the condenser. Further heat exchangers can be provided in order to transfer heat from the expanded agent to the agent before it is supplied to the heat exchanger. 
         [0007]    A Kalina circuit with an ammonia-water mixture as the agent and which is known from EP 0756069 B1 additionally has a distillation unit, which is arranged in the circuit between the condenser and the pump, for separation of a weak ammonia liquid from the agent flow. This weak ammonia liquid is supplied to the agent that has been expanded in the turbine, before this agent is supplied to the condenser. 
         [0008]    As a result of partial condensation of the agent, the agent may contain a continuously increasing proportion of the liquid phase in a line connection between the expansion device and the condenser. In addition, feeding a liquid phase of the agent, which for example has been separated before the expansion device, into the expanded vapor phase leads to an increase in the proportion of the liquid phase in the agent before it is supplied to the condenser. The increasing proportion of the liquid phase leads to “demixing” of the substance mixture and to the formation of an inhomogeneous, partially demixed two-phase flow in the line connection. 
         [0009]    For example, if the agent comprises an ammonia-water mixture, then this results in an inhomogeneous, partially demixed, two-phase flow in the line connection, comprising a saturated vapor which is rich in ammonia and a condensate with little ammonia. In consequence, the condenser is partially flooded with condensate with little ammonia, and the ammonia vapor fills only the remaining residue of the heat exchanger. The flooded component reduces the effectiveness of the condenser. Furthermore, the condensation pressure of the vapor which is rich in ammonia and which (for example comprises 95% ammonia) is considerably higher than that of a homogeneous water-ammonia mixture. The higher the condensation pressure is in the condenser, the shallower, however, is the pressure gradient to be dissipated across the turbine. In consequence, the circuit generates less mechanical and/or electrical power, with a poorer efficiency. 
       SUMMARY 
       [0010]    According to various embodiments, a method can be developed so as to make it possible to avoid such efficiency losses. 
         [0011]    According to an embodiment, a method for conversion of thermal energy to mechanical energy using an agent which comprises a substance mixture having at least two substances which have different boiling and condensation temperatures, wherein the agent which is expanded in an expansion device is supplied as a two-phase flow with a liquid phase and a vapor phase to a condenser, in which it is condensed, may comprise the step of mixing the liquid phase with the vapor phase in the two-phase flow before or during the condensation of the agent in the condenser. 
         [0012]    According to a further embodiment, for mixing in the two-phase flow, the liquid phase can be separated from the vapor phase, and the separated liquid phase is then combined with the vapor phase again, wherein the separated liquid phase is preferably sprayed into the vapor phase for combination. According to a further embodiment, before being sprayed in, the pressure of the separated liquid phase can be increased to a value which is higher than the pressure of the vapor phase. According to a further embodiment, the separation of the liquid phase from the vapor phase can be carried out immediately before the condenser. According to a further embodiment, the mixing process can be carried out immediately before or in the condenser. According to a further embodiment, the agent may pass through at least the following method steps in a closed circuit after the condensation: —increasing the pressure of the agent, —producing a vapor phase of the agent by heat transfer from an external heat source, and—expanding the vapor phase and converting its thermal energy to mechanical energy. According to a further embodiment, before the expansion of the vapor phase of the agent, a liquid phase of the agent can be separated from the vapor phase, and the vapor phase can be supplied again after it has been expanded. According to a further embodiment, a geothermal fluid, industrial waste heat or waste heat from an internal combustion engine can be used as the external heat source. According to a further embodiment, a mixture of ammonia and water can be used as the agent. 
         [0013]    According to another embodiment, an apparatus for conversion of thermal energy to mechanical energy using an agent which comprises a substance mixture with at least two substances which have different boiling and condensation temperatures, having a condenser for condensation of the agent, wherein the agent, which is expanded in an expansion device, is in the form of a two-phase flow with a liquid phase and a vapor phase before it is supplied to the condenser, may comprise a mixing device for mixing the liquid phase of the two-phase flow with the vapor phase of the two-phase flow before or during the condensation of the agent in the condenser. 
         [0014]    According to a further embodiment, the mixing device may have a separator for separation of the liquid phase from the vapor phase, and has at least one nozzle for spraying the separated liquid phase into the vapor phase. According to a further embodiment, the mixing device may have a pump, by means of which the pressure of the separated liquid phase can be increased to a value which is higher than the pressure of the vapor phase. According to a further embodiment, the separator can be arranged immediately before the condenser in the flow direction of the agent. According to a further embodiment, the at least one nozzle can be arranged immediately before or in the condenser in the flow direction of the agent. According to a further embodiment, the agent can be carried in a closed circuit in the apparatus, which closed circuit has at least the following components after the condenser in the flow direction of the agent: —a pump for increasing the pressure of the agent; —a heat exchanger for producing a vapor phase of the agent by heat transfer from an external heat source, and—an expansion device, in particular a turbine, for expansion of the vapor phase and conversion of its thermal energy to mechanical energy. According to a further embodiment, the circuit additionally may comprise a separator, which is arranged between the heat exchanger and the expansion device, for separation of a liquid phase of the agent from a vapor phase, and a combination means, which is arranged between the expansion device and the mixing device, for combination of the separated liquid phase and the expanded vapor phase. According to a further embodiment, the external heat source can be a geothermal flow, industrial waste heat or waste heat from an internal combustion engine. According to a further embodiment, the agent can be a mixture of ammonia and water. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The invention as well as further refinements will be explained in more detail in the following text with reference to exemplary embodiments in the figures, in which: 
           [0016]      FIG. 1  shows a circuit according to one particularly embodiment, 
           [0017]      FIG. 2  shows one example of demixing of a two-substance mixture in a line connection, 
           [0018]      FIG. 3  shows a mixing device with spraying in jointly for a plurality of condensers, 
           [0019]      FIG. 4  shows a mixing device with spraying directly into the condensers, and 
           [0020]      FIG. 5  shows a mixing device with separate spraying in for each individual condenser. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The method according to various embodiments for conversion of thermal energy to mechanical energy using an agent which comprises a substance mixture having at least two substances which have different boiling and condensation temperatures, wherein the agent which is expanded in an expansion device is supplied as a two-phase flow with a liquid phase and a vapor phase to a condenser, in which it is condensed, provides that the liquid phase is mixed with the vapor phase in the two-phase flow before or during the condensation of the agent in the condenser. 
         [0022]    This makes it possible to avoid demixing of the two-substance mixture, allowing a homogeneous two-substance mixture to be produced again in the two-phase flow. If the coolant average temperature in the condenser remains constant, a homogeneous two-substance mixture actually condenses at a lower pressure. However a lower condensation pressure in the condenser results in an increase in the pressure gradient to be dissipated across the turbine, as a result of which more mechanical and/or electrical power can be produced, at a higher efficiency. 
         [0023]    The liquid phase can be mixed with the vapor phase very easily by separating the liquid phase from the vapor phase in the two-phase flow and then combining the separated liquid phase with the vapor phase again. The separated liquid phase is in this case preferably sprayed into the vapor phase. 
         [0024]    Particularly good mixing of the liquid and the vapor phases can in this case be achieved by increasing the pressure of the separated liquid phase to a value which is higher than the pressure of the vapor phase, in order to spray it in. The separated liquid phase is therefore supplied to the vapor phase at an increased pressure. 
         [0025]    In this case, separation of the liquid phase from the vapor phase is preferably carried out immediately before the condenser, in order to avoid the two-substance mixture demixing again on its way to the condenser. 
         [0026]    The mixing process itself can likewise be carried out immediately before the condenser, or else directly in the condenser. 
         [0027]    In this case, the agent advantageously passes through at least the following method steps in a closed circuit after the condensation:
       increasing the pressure of the agent,   producing a vapor phase of the agent by heat transfer from an external heat source, and   expanding the vapor phase and converting its thermal energy to mechanical energy.       
 
         [0031]    The agent can in this case be vaporized completely by the heat transfer (that is to say only a vapor phase exists), or can be only partially vaporized (that is to say a vapor phase and a liquid phase exist). In the case of only partial vaporization, before the expansion of the vapor phase, the liquid phase of the agent is advantageously separated from the vapor phase, and the vapor phase is supplied again after it has been expanded. The liquid phase therefore bypasses an expansion device for expansion of the vapor phase. 
         [0032]    After expansion, the agent can be supplied to the condenser directly or via one or more intermediate heat exchangers, which transfer the heat from the expanded vapor phase to the agent before its at least partial vaporization. 
         [0033]    A geothermal fluid, industrial waste heat or waste heat from an internal combustion engine is preferably used as the external heat source. 
         [0034]    In this case, particularly high efficiencies can be achieved if a mixture of ammonia and water is used as the agent. The apparatus according to various embodiments for conversion of thermal energy to mechanical energy using an agent which comprises a substance mixture with at least two substances which have different boiling and condensation temperatures, comprises a condenser for condensation of the agent, wherein the agent, which is expanded in an expansion device, is in the form of a two-phase flow with a liquid phase and a vapor phase before it is supplied to the condenser, and a mixing device for mixing the liquid phase of the two-phase flow with the vapor phase of the two-phase flow before or during the condensation of the agent in the condenser. 
         [0035]    The mixing device advantageously has a separator for separation of the liquid phase from the vapor phase, and advantageously has at least one nozzle for spraying the separated liquid phase into the vapor phase. 
         [0036]    If the mixing device has a pump, by means of which the pressure of the separated liquid phase can be increased to a value which is higher than the pressure of the vapor phase, particularly good mixing of the two phases can be achieved when it is sprayed in. 
         [0037]    If the separator is arranged immediately before the condenser in the flow direction of the agent, it is possible to avoid the two-substance mixture demixing again on its way to the condenser. 
         [0038]    The at least one nozzle may itself likewise be arranged immediately before or else in the condenser in the flow direction of the agent. 
         [0039]    According to one embodiment, the agent can be carried in a closed circuit in the apparatus, which closed circuit has at least the following components after the condenser in the flow direction of the agent:
       a pump for increasing the pressure of the agent   a heat exchanger for producing a vapor phase of the agent by heat transfer from an external heat source, and   an expansion device, in particular a turbine, for expansion of the vapor phase and conversion of its thermal energy to mechanical energy.
 
In this case, the agent may be completely vaporized by the heat transfer (that is to say only a vapor phase exists) or only partially vaporized (that is to say a vapor phase and a liquid phase exist). In the case of only partial vaporization, the circuit advantageously also comprises a separator, which is arranged between the heat exchanger and the expansion device, for separation of a liquid phase from the vapor phase, and a combination means, which is arranged between the expansion device and the mixing device, for combination of the separated liquid phase and the expanded vapor phase. In this case, the liquid phase can in this way bypass the expansion device. The heat source is preferably a geothermal fluid, industrial waste heat or waste heat from an internal combustion engine. The agent is advantageously a mixture of ammonia and water.
       
 
         [0043]    An apparatus  1  as shown in  FIG. 1  for conversion of thermal energy to mechanical energy comprises a circuit  2  in which a pump  3  for increasing the pressure of the agent, a heat exchanger  4  for producing a vapor phase of the agent by heat transfer from an external heat source  5 , a turbine  6  for expansion of the vapor phase of the agent and conversion of its thermal energy to mechanical energy, a mixing device  7  for mixing a liquid and a vapor phase of the agent and a condenser for complete condensation of the agent with the aid of a coolant  9  are arranged successively as major components in the flow direction of an agent. By way of example, the external heat source  5  is a geothermal fluid, industrial waste heat or waste heat from an internal combustion engine. By way of example, the turbine  6  drives a generator, which is not illustrated but converts the mechanical energy to electrical energy. 
         [0044]    The agent comprises a substance mixture having at least two substances which have different boiling and condensation temperatures. The following text is based on the assumption that a mixture of ammonia and water is used as the agent. 
         [0045]    As further components, the circuit  2  comprises a separator  15 , which is arranged between the heat exchanger  4  and the turbine  6 , for separation of a liquid phase of the agent from the vapor phase, and a combination means  16 , which is arranged between the turbine  6  and the mixing device  7 , for combination of the separated liquid phase and the expanded vapor phase. 
         [0046]    During operation of the circuit  2 , the agent is exclusively in the form of a liquid after the condenser  8 . The liquid agent is raised to a higher pressure by means of the pump  3  and is then at least partially vaporized in the heat exchanger  4 , that is to say the agent exists in a vapor phase and possibly a liquid phase with little ammonium after the heat exchanger. The liquid phase which may possibly still be present is separated from the vapor phase in the separator  15 . 
         [0047]    The vapor phase is expanded in the turbine  6 , and its thermal energy is converted to mechanical energy. The mechanical energy can then be used further, for example for electricity generation. 
         [0048]    The vapor phase, which has now been expanded, is combined again with the liquid phase, which was possibly previously separated, in the combination means  16 . 
         [0049]    Because of partial condensation of the expanded vapor phase and possibly liquid phase supplied via the combination means  16  the proportion of liquid in the ammonium-water mixture will increase in the line connection  10  between the turbine  6  and the condenser  8 , with demixing taking place into saturated vapor  11  which is rich in ammonia, and condensate  12  with little ammonia (see  FIG. 2 ). The condenser  8  would therefore be supplied with an inhomogeneous, partially demixed agent flow. This would result in the condenser  8  being partially flooded with the condensate  12  with little ammonia, with the saturated vapor  11  which is rich in ammonia filling the rest of the condenser. The flooded component would decrease the effectiveness of the condenser and would therefore increase the condensation pressure, since the condensation pressure of the saturated vapor which is rich in ammonia (approximately 95% ammonia) is considerably higher than that of a homogeneous water-ammonia mixture. As the condensation pressure rises in the condenser, however, the pressure gradient to be dissipated across the turbine decreases, and therefore the mechanical and/or electrical power which can be produced also decreases. 
         [0050]    In order to avoid such efficiency losses, the circuit  2  has a mixing device  7 . The mixing device  7  comprises a separator  20  for separation of the liquid phase with little ammonia from the vapor phase which is rich in ammonia, and a nozzle  21  for spraying the separated liquid phase into the vapor phase, wherein the separator  20  and the nozzle  21  are arranged successively in the connecting line  10 , between the turbine  6  and the condenser  8  and after the combination means  16 , in the flow direction of the agent. The liquid phase which is separated in the separator  20  is supplied via a bypass line  14  to the nozzle  21 . A pump  22  and a control valve  23  are connected in the bypass line  14 . 
         [0051]    The pump  22  makes it possible to increase the pressure on the separated liquid phase which carried in the bypass line  14  to a value which is higher than the pressure of the vapor phase after the separator  20 . The amount of liquid phase supply to the nozzle  21  can be controlled by means of the control valve  23 . 
         [0052]    The separator  20  is arranged immediately before the condenser  8  in the flow direction of the agent, in order to avoid demixing of the agent again on the rest of its way to the condenser  8 . The nozzle  21  can be arranged immediately before or in the condenser  8 , in the flow direction of the agent. 
         [0053]    The separator  20  therefore separates the vapor phase which is rich in ammonia, from the liquid phase, with little ammonia. 
         [0054]    The liquid phase, with little ammonia is passed to the nozzle  21  via the bypass line  14 . In this case, the pump  22  increases the pressure of the liquid phase with little ammonia to a value which is higher than the pressure of the vapor phase which is rich in ammonia. The liquid phase with little ammonia is thus sprayed at an increased pressure into the vapor phase, which is rich in ammonia in the nozzle  21 . This once again results in a homogeneous ammonia-water mixture being able to be produced and being able to be supplied to the condenser  8 , which mixture actually condenses at a lower pressure than the vapor phase, which is rich in ammonia, assuming that the cooling temperature in the condenser remains constant. However, with a lower condensation pressure in the condenser, the pressure gradient to be dissipated across the turbine rises, and the circuit can therefore produce more electrical power, at a higher efficiency. 
         [0055]    When there are a plurality of condensers  8  connected in parallel in the flow direction of the agent—as illustrated in FIG.  3 —a mixing device  7  can be provided with a single separator  20  and a single nozzle  21  for all the condensers  8 . The separator  20  and the nozzle  21  are then preferably arranged immediately before the condensers  8 . The liquid phase is therefore sprayed jointly into the vapor phase for all the condensers  8 . 
         [0056]    Alternatively, when there are a plurality of condensers  8  which are connected in parallel in the flow direction of the agent, it is also possible to provide a mixing device  7  with a single separator  20  and in each case one or more nozzles  21  for each of the condensers  8 . In the exemplary embodiment shown in  FIG. 4 , the separator  20  is arranged immediately in front of the condensers  8 , and the nozzles  21  are arranged in the condensers  8 . The liquid phase is therefore sprayed directly into the condensers  8 . In this case, the supply of the liquid phase to the nozzles  21  can be controlled by means of a joint control valve  23 . 
         [0057]    However, as illustrated in  FIG. 5 , the nozzles  21  can also be arranged immediately before the respective condensers  8 , that is to say the spraying-in process is carried out separately for each individual condenser  8 . In this case, supply of the liquid phase to each of the nozzles  21  can be controlled by means of a separate control valve  23  for each of the condensers  8 .