Abstract:
In a method and device ( 1 ) for converting thermal energy of a low temperature heat source ( 20 ) into mechanical energy in a closed circuit, a liquid working agent is heated by transmitting heat from the low temperature source ( 20 ) and partially evaporating it in an expansion device ( 3 ). Erosion to the condenser ( 8 ) for condensing the partially evaporated working agent can be prevented by separating the liquid phase from the evaporator phase in the partially evaporated working agent that is directly in front of the condenser ( 8 ), and only the evaporator phase is transferred to the condenser ( 8 ) for condensing and subsequently, the condensed evaporator phase and the liquid phase are merged.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/EP2007/062147 filed Nov. 9, 2007, which designates the United States of America, and claims priority to German Application No. 10 2007 041 457.0 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 heat energy from a low-temperature source to mechanical energy. 
       BACKGROUND 
       [0003]    A method such as this and an apparatus such as this are known, for example, from U.S. Pat. No. 7,093,503 B1. 
         [0004]    In order to use the heat energy from low-temperature heat sources, for example geothermal sources, gas, vapor or liquid waste-heat sources or solar energy, it is already known for an agent in a circuit not to be vaporized by the heat source, but only to be heated. As a result of the lack of vaporization, the heat energy which is normally required to vaporize the agent can be used, for example, to heat the considerably greater mass flow of the agent. This makes it possible to achieve considerable efficiency advantages over circuits in which the agent is vaporized, for low-temperature sources in the temperature range below 400° C. 
         [0005]    In the case of a circuit which is known from U.S. Pat. No. 7,093,503 B1, in a first step, a liquid agent is raised to an increased pressure by a pump. In a second step, the increased-pressure, liquid agent is heated in a heat exchanger by heat transfer from a low-temperature source. In a third step, the heated, liquid agent is expanded in a two-phase turbine, with an expanded, partially vaporized agent with a liquid phase and a vapor phase being produced by partial vaporization of the agent, and with heat energy in the agent being converted to mechanical energy. 
         [0006]    The two-phase turbine for this purpose has nozzles directly adjacent to its inlet, in which the agent is expanded by increasing its volume from a relatively high inlet pressure to a lower outlet pressure, as a result of which the agent is partially vaporized. The water-steam jet which is created in this way is passed to turbine blades of the turbine, by means of which the kinetic energy of the water-steam jet is converted to mechanical energy of a rotor shaft. The rotor shaft is in turn connected to a generator, via which the mechanical energy of the rotor shaft is converted to electrical energy. 
         [0007]    The two-phase agent leaving the turbine is then supplied to a condenser. In a fourth step, the vapor phase of the expanded, partially vaporized agent is then condensed in the condenser, thus producing the initially mentioned liquid agent. This is supplied to the pump that has already been mentioned, thus closing the circuit. The T-s-diagram illustrated in  FIG. 2  shows the circulating process which takes place in this case. In this case, SL denotes the boiling line, TL the dew line and K the critical point of the agent. The agent is heated along the boiling line SL from the point A to the point B in the vicinity of the critical point K, is expanded, being partially vaporized, from point B to point C, and is condensed from point C to point A. 
         [0008]    It is furthermore known from WO 2005/031123 A1 for a two-phase mixture leaving a two-phase turbine to be supplied to a separator in order to separate the vapor phase from the liquid phase. The vapor phase is then expanded further in a steam turbine in order to produce additional mechanical energy. The expanded steam leaving the steam turbine is supplied to a condenser in which it is condensed, is then raised to an increased pressure by means of a pump, and is then combined with the liquid phase, which has been separated in the separator, of the two-phase mixture. The agent flow created in this way is then pumped into a heat exchanger with the aid of a further pump, by being heated by heat transfer from a low-temperature source. The condenser is therefore supplied only with the exhaust steam from the steam turbine, but not with the two-phase mixture of the two-phase turbine. Although this circuit is distinguished by very high efficiency, it is, however, also distinguished by being considerably more complex and by involving considerably greater investment costs. 
         [0009]    In the case of a circuit which is known from EP 0 485 596 A1, only one heated liquid, that is to say not vaporized, agent is likewise supplied to an expansion device, in which it is partially vaporized. The water-steam mixture leaving the expansion device is then supplied to a separator, which is used only to measure the liquid components in the steam. 
         [0010]    If the two-phase mixture leaving the turbine is supplied to the condenser in the initially mentioned circuit, then the liquid components can lead to erosion of the condenser, thus shortening the life of the condenser. 
       SUMMARY 
       [0011]    According to various embodiments, a method and an apparatus can be developed such that it is possible to reliably prevent erosion of the condenser, without significantly increasing the complexity of the circuit. 
         [0012]    According to an embodiment, a method for conversion of heat energy from a low-temperature heat source to mechanical energy in a closed circuit may comprise the following steps: 
         [0013]    step 1: increasing the pressure of a liquid agent, 
         [0014]    step 2: heating of the increased-pressure, liquid agent by transferring heat from the low-temperature heat source to the agent, without vaporizing the agent, 
         [0015]    step 3: expanding the heated, liquid agent, wherein an expanded, partially vaporized agent with a vapor phase and a liquid phase is produced by partial vaporization of the agent, and heat energy in the agent is converted to mechanical energy, 
         [0016]    step 4: condensing the vapor phase produced in step  3  in a condenser in order to produce the liquid agent from step  1 , wherein 
         [0017]    in the case of the expanded, partially vaporized agent produced in step  3 , the liquid phase is separated from the vapor phase immediately before the condenser 
         [0018]    only the vapor phase is supplied to the condenser, 
         [0019]    the condensed vapor phase and the liquid phase are combined after the condenser but before step  1 , in order to produce the liquid agent. 
         [0020]    According to a further embodiment, the pressure of the agent in the condenser can be set to an optimum between the droplets of the liquid phase in the vapor phase of the agent being as small as possible and the mechanical energy produced being as great as possible in step  3 . According to a further embodiment, the condensed vapor phase and the liquid phase can be combined in an agent reservoir. According to a further embodiment, the low-temperature source can be at a temperature of less than 400° C. 
         [0021]    According to another embodiments, an apparatus for conversion of heat energy from a low-temperature heat source to mechanical energy in a closed circuit, may comprise a pump for increasing the pressure of a liquid agent, a heat exchanger for heating the increased-pressure, liquid agent by transferring heat from the low-temperature heat source to the agent, without vaporizing the agent, an expansion device for expanding the heated, liquid agent, wherein an expanded, partially vaporized agent with a liquid phase and a vapor phase can be produced by partial vaporization of the agent in the expansion device, and heat energy in the agent can be converted to mechanical energy, a condenser for condensation of the vapor phase of the partially vaporized agent in order to produce the liquid agent, a separator for separation of the liquid phase from the vapor phase of the expanded, partially vaporized agent, wherein the separator is arranged immediately before the condenser in the flow direction of the agent, and is connected to the condenser in order to supply the vapor phase to the condenser, and a combination means for combining the liquid phase and the condensed vapor phase of the partially vaporized agent, wherein the combination means is arranged before the pump in the flow direction of the agent and is connected to the separator in order to supply the liquid phase, and to the condenser in order to supply the condensed vapor phase, to the combination means. 
         [0022]    According to a further embodiment, the pressure of the agent in the condenser can be set to an optimum between the droplets of the liquid phase in the vapor phase of the agent being as small as possible and the mechanical energy produced being as great as possible in the expansion device. According to a further embodiment, the combination means can be in the form of an agent reservoir. According to a further embodiment, a nozzle and a turbine can be arranged successively in the flow direction of the agent in the expansion device. According to a further embodiment, the nozzle and the turbine may form a single physical unit. According to a further embodiment, the low-temperature source can be at a temperature of less than 400° C. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    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: 
           [0024]      FIG. 1  shows a simplified, schematic illustration of a circuit for an apparatus according to an embodiment, and 
           [0025]      FIG. 2  shows a T-s-diagram of a circuit known from the prior art with an agent being heated (without vaporization) by a low-temperature source. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The method according to various embodiments provides that in the case of the expanded, partially vaporized agent, the liquid phase is separated from the vapor phase immediately before the condenser. Only the vapor phase is supplied to the condenser for condensation. The condensed vapor (that is to say then liquid) phase and the separated liquid phase are combined after the condenser but before step  1 , that is to say the increase in the pressure of the liquid agent, in order to produce the liquid agent. The liquid phase therefore bypasses the condenser, thus making it possible to prevent erosion of the condenser. All that is required for this purpose is a separator for separation of the liquid phase from the vapor phase, a bypass line for the liquid phase line to bypass the condenser and a combination means for combining the (separated) liquid and condensed vapor (that is to say then liquid) phase. The complexity of the circuit is therefore increased only insignificantly. 
         [0027]    The size of the droplets in the liquid phase in the vapor phase of the agent after expansion is dependent on the pressure of the agent in the condenser. The higher the pressure of the agent is in the condenser, and thus at the outlet of the expansion device, the smaller the droplets. In turn, the smaller the droplets are, the less is the risk of erosion caused by the droplets. On the other hand, however, as the pressure of the agent in the condenser and at the outlet of the expansion device increases, the mechanical energy which can be produced by conversion of heat energy by the expansion device decreases. 
         [0028]    Preferably, therefore, the pressure of the agent during the condensation process is set to an optimum between the droplets of the liquid phase in the vapor phase of the agent being as small as possible and the mechanical energy produced being as great as possible in step  3 . The amount of mechanical energy produced is therefore deliberately reduced in order to prevent erosion of the condenser. Because of the enormous efficiency advantage resulting from heating rather than vaporization of the agent by the low-temperature heat source, however, considerable efficiency advantages can nevertheless still be achieved in comparison to conventional circuits in which the agent is vaporized by the low-temperature heat source. 
         [0029]    According to one embodiment of the method, the condensed vapor (that is to say then liquid) phase and the (separated) liquid phase are combined in an agent reservoir. Since a reservoir such as this is provided in any case in many circuits, there is no need for an additional component for combination of the two phases. 
         [0030]    In this case, particularly high efficiencies can be achieved if the low-temperature source is at a temperature of less than 400° C. 
         [0031]    The apparatus according to various embodiments has a separator for separation of the liquid phase from the vapor phase of the expanded, partially vaporized agent, wherein the separator is arranged immediately before the condenser in the flow direction of the agent. A combination means is used to combine the (separated) liquid phase and the condensed vapor (that is to say then liquid) phase of the expanded, partially vaporized agent, wherein the combination means is arranged before the pump in the flow direction of the agent. The separator is connected to the condenser in order to supply the vapor phase to the condenser. The combination means is connected to the separator in order to supply the (separated) liquid phase to the combination means, and is connected to the condenser in order to supply the condensed vapor (that is to say then liquid) phase to the combination means. The advantages that have been mentioned for the method according to various embodiments apply in a corresponding manner to the apparatus. 
         [0032]    The pressure of the agent in the condenser can preferably be set to an optimum between the droplets of the liquid phase in the vapor phase of the agent being as small as possible and the mechanical energy produced being as great as possible in the expansion device. 
         [0033]    According to one embodiment, the combination means is in the form of an agent reservoir. 
         [0034]    Advantageously, a nozzle and a turbine can be arranged successively in the flow direction of the agent in the expansion device in order to expand the heated agent. The agent can be expanded in the nozzle by increasing its volume from a higher inlet pressure to a lower outlet pressure, thus partially vaporizing the agent. The water-steam jet which is created in this way can then be passed to the turbine blades of the turbine, by means of which the kinetic energy of the water-steam jet is converted to mechanical energy of a rotor shaft. Instead of only a single nozzle, a plurality of nozzles can also be arranged at the turbine inlet, for example in an annular configuration, through which the agent can flow in parallel. 
         [0035]    In this case, the nozzle and the turbine may also form a single physical unit, that is to say the nozzles are arranged directly adjacent to the turbine inlet. 
         [0036]    An apparatus  1  according to various embodiments for conversion of the heat energy of a low-temperature heat source to mechanical energy comprises a thermodynamic circuit in which a heat exchanger  2 , an expansion device  3 , a separator  7 , a condenser  8 , an agent reservoir in the form of a condensate tank  9  and a pump  10  are arranged successively in the flow direction of an agent. 
         [0037]    The low-temperature heat source is a heat source at a temperature of less than 400° C. By way of example, heat sources such as these are geothermal sources (hot thermal water), industrial waste-heat sources (for example waste heat from plants used in the steel, glass or cement industries) and solar energy. 
         [0038]    By way of example, a coolant liquid of the R134 type may be used as an agent for temperatures of less than 300° C., and, for example, a cooling liquid of the R245 type may be used for temperatures of more than 300° C. The pump  10  is used to pump the liquid agent to an increased pressure. 
         [0039]    The heat exchanger  2  is used to heat the increased-pressure, liquid agent in the circuit by heat transfer from the low-temperature heat source  20  to the agent without vaporization of the agent, that is to say the agent is only heated and is not vaporized in the heat exchanger  2 . For this purpose, the low-temperature heat source  20 , for example hot geothermal water flows through the primary side of the heat exchanger, and the increased-pressure agent flows through its secondary side. A line  11  connects the secondary side of the heat exchanger  2  to the expansion device  3 . The agent is still liquid at the outlet on the secondary side of the heat exchanger  2 , when it enters the line  11 . 
         [0040]    The expansion device  3  is used to expand the heated liquid agent, wherein an expanded, partially vaporized agent with a liquid and a vapor phase can be produced by partial vaporization of the heated liquid agent in the expansion device  3 , and heat energy in the heated liquid agent can be converted to mechanical energy. The expansion device  3  for this purpose comprises a nozzle  4  and a turbine  5 , which are arranged successively in the flow direction of the agent. The nozzle and the turbine may in this case form a single physical unit, that is to say the nozzle  4  is arranged immediately adjacent to the inlet of the turbine  5 . Instead of only a single nozzle  4 , it is also possible to arrange a plurality of nozzles  4  at the inlet of the turbine  5 , for example in an annular configuration, through which the agent can flow in parallel. 
         [0041]    On the outlet side, the turbine  5  is connected via a line  12  to the separator  7 . The separator  7  is used to separate the liquid phase from the vapor phase of the agent which has been partially vaporized in the expansion device  3 . The separator  7  is arranged immediately before the condenser  8  in the flow direction of the agent, is connected via a line  13  to the condenser  8  in order to supply the vapor phase to the condenser  8 , and is connected via a line  14  to the condensate tank  9  in order to supply the liquid phase to the condensate tank  9 . 
         [0042]    The condenser  8  is used to produce the liquid agent by condensation of the partially vaporized agent. 
         [0043]    The condensate tank  9  is used to combine the liquid phase and the condensed vapor (that is to say then liquid) phase of the partially vaporized agent. The condensate tank  9  is arranged after the condenser  8  and before the pump  10  in the flow direction of the agent, is connected via a line  14  to the separator  7  in order to supply the liquid phase, and via a line  15  to the condenser  8  in order to supply the condensed vapor phase to the condensate tank  9 . 
         [0044]    During operation of the apparatus  1 , in a first step, liquid agent from the condensate tank  9  is raised to an increased pressure by the pump  10 , and is pumped into the heat exchanger  2 . 
         [0045]    In a second step, the increased-pressure, liquid agent is heated, without being vaporized, in the heat exchanger  2  by transfer of heat to the agent from the low-temperature heat source  20  which flows through the primary side of the heat exchanger  2 . 
         [0046]    In a third step, the heated, liquid agent is expanded in the expansion device  3 , with the agent being partially vaporized and its heat energy being converted to mechanical energy. The expansion device  3  therefore produces an expanded, partially vaporized agent with a liquid phase and a vapor phase. For this purpose, the heated, liquid agent which is supplied to the nozzle  4  via the line  11  is expanded in the nozzle  4  and in the process is partially vaporized. The kinetic energy of the water-steam jet created in this way is converted in the turbine  5  into mechanical energy of a rotor shaft, and a generator  6  is thus driven, which in turn converts the mechanical energy to electrical energy. 
         [0047]    The expanded, partially vaporized agent which is produced in the third step and leaves the turbine  5  in the form of a two-phase mixture (steam/liquid) is supplied via a line  12  to the separator  7 , in that the vapor phase is separated from the liquid phase of the two-phase mixture. 
         [0048]    Only the vapor phase is supplied to the condenser  8  via the line  13 . In the condenser  8 , the vapor phase is condensed by cooling, for example by direct cooling, air cooling, hybrid cooling or water cooling, and the condensed vapor (that is to say then liquid) phase is supplied via the line  15  to the condensate tank  9 . 
         [0049]    The separated liquid phase, in contrast, bypasses the condenser  8  via the line  14  and only after this, but still before the pump  10  and therefore before the first step, is combined with the condensed vapor (that is to say then liquid) phase in the condensate tank  9 . 
         [0050]    Liquid agent from the condensate tank  9  is raised to an increased pressure with the aid of the pump  10  and is pumped into the heat exchanger  2 , thus closing the circuit. 
         [0051]    Erosion of the condenser  8  can be prevented by separation of the liquid phase from the vapor phase of the two-phase mixture leaving the turbine  5 , in the separator  7 , and by the liquid phase then being fed directly into the condensate tank  9 , bypassing the condenser  8 . 
         [0052]    The pressure of the agent in the condenser  8  is in this case set to an optimum between the droplets of the liquid phase in the vapor phase of the agent being as small as possible and the mechanical energy produced being as great as possible in the third step. This makes it possible to reduce the erosion of the condenser even further.