AU2008291094A1

AU2008291094A1 - Method and device for converting thermal energy into mechanical energy

Info

Publication number: AU2008291094A1
Application number: AU2008291094A
Authority: AU
Inventors: Thomas Hauer; Jorg Lengert; Markus Neefischer; Reinhold Striegel
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2007-08-31
Filing date: 2008-08-21
Publication date: 2009-03-05
Also published as: RU2479727C2; WO2009027302A2; RU2010112413A; US20110000205A1; CN101842558A; WO2009027302A3; KR20100074166A; EP2188500A2

Description

PCT/EP2008/060921 - 1 2007P17898WOUS Description Method and apparatus for conversion of thermal energy to mechanical energy The invention relates to a method and apparatus for conversion of thermal energy to mechanical energy, according to the preamble of patent claim 1 and of patent claim 10, respectively; a method such as this and an apparatus such as this are known, for example, from WO 2005/100755 Al. In recent years, widely differing technologies have been developed for low-temperature heat sources with temperatures up to a maximum of 4000C, 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. 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 PCT/EP2008/060921 - la 2007P17898WOUS 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 PCT/EP2008/060921 - 2 2007P17898WOUS energy. The expanded agent is then condensed in a condenser with the aid of a coolant. Even more components may be connected in the circuit in order to improve the efficiency. For example - as disclosed in WO 2005/100755 Al - 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. 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. 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 PCT/EP2008/060921 - 3 2007P17898WOUS phase leads to "demixing" of the substance mixture and to the formation of an inhomogeneous, partially demixed two-phase flow in the line connection. 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. The object of the present invention is therefore to develop a method according to the precharacterizing clause of patent claim 1 and an apparatus according to the precharacterizing clause of patent claim 10 so as to make it possible to avoid such efficiency losses. The object with regard to the method is achieved by the method as claimed in patent claim 1. Advantageous refinements of the method are the subject matter of patent claims 2 to 9. The object with regard to the apparatus is achieved by an apparatus as claimed in patent claim 10. Advantageous refinements of the apparatus are the subject matter of patent claims 11 to 18.

PCT/EP2008/060921 - 3a 2007P17898WOUS The method according to the invention for conversion of thermal energy to mechanical energy using an agent which comprises a substance mixture having at least two substances which have PCT/EP2008/060921 - 4 2007P17898WOUS 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. 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. 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. 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. 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.

PCT/EP2008/060921 - 5 2007P17898WOUS The mixing process itself can likewise be carried out immediately before the condenser, or else directly in the condenser. 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. 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. 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. A geothermal fluid, industrial waste heat or waste heat from an internal combustion engine is preferably used as the external heat source. In this case, particularly high efficiencies can be achieved if a mixture of ammonia and water is used as the agent.

PCT/EP2008/060921 - 6 2007P17898WOUS The apparatus according to the invention 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. 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. 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. 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. The at least one nozzle may itself likewise be arranged immediately before or else in the condenser in the flow direction of the agent. According to one particularly advantageous refinement, the agent can be carried in a closed circuit in the apparatus, which closed circuit has at least the following components PCT/EP2008/060921 - 6a 2007P17898WOUS after the condenser in the flow direction of the agent: PCT/EP2008/060921 - 7 2007P17898WOUS - 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. The invention as well as further advantageous refinements of the invention according to the features of the dependent claims will be explained in more detail in the following text with reference to exemplary embodiments in the figures, in which: Figure 1 shows a circuit according to one particularly advantageous refinement of the invention, Figure 2 shows one example of demixing of a two-substance mixture in a line connection, PCT/EP2008/060921 - 8 2007P17898WOUS Figure 3 shows a mixing device with spraying in jointly for a plurality of condensers, Figure 4 shows a mixing device with spraying directly into the condensers, and Figure 5 shows a mixing device with separate spraying in for each individual condenser. An apparatus 1 as shown in Figure 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 8 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. 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. 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 PCT/EP2008/060921 - 9 2007P17898WOUS the turbine 6 and the mixing device 7, for combination of the separated liquid phase and the expanded vapor phase. 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. 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. 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. 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 Figure 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 PCT/EP2008/060921 - 9a 2007P17898WOUS 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 PCT/EP2008/060921 - 10 2007P17898WOUS 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. 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. 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. 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. The separator 20 therefore separates the vapor phase which is rich in ammonia, from the liquid phase, with little ammonia. The liquid phase, with little ammonia is passed to the nozzle 21 via the bypass line 14. In this case, the pump 22 PCT/EP2008/060921 - 10a 2007P17898WOUS 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 PCT/EP2008/060921 - 11 2007P17898WOUS 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. When there are a plurality of condensers 8 connected in parallel in the flow direction of the agent - as illustrated in. Figure 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. 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 Figure 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. However, as illustrated in Figure 5, the nozzles 21 can also be arranged immediately before the respective condensers 8, that PCT/EP2008/060921 - 11a 2007P17898WOUS 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.

Claims

1. 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 (8), in which it is condensed, characterized in 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 (8).

2. The method as claimed in claim 1, characterized in that, for mixing in the two-phase flow, the liquid phase is 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.

3. The method as claimed in claim 2, characterized in that, before being sprayed in, the pressure of the separated liquid phase is increased to a value which is higher than the pressure of the vapor phase.

4. The method as claimed in claim 2 or 3, characterized in that, the separation of the liquid phase from the vapor phase is carried out immediately before the condenser (8). PCT/EP2008/060921 - 12a 2007P17898WOUS

5. The method as claimed in one of the preceding claims, characterized in that, the mixing process is carried out immediately before or in the condenser (8). PCT/EP2008/060921 - 13 2007P17898WOUS

6. The method as claimed in one of the preceding claims, characterized in that the agent passes through at least the following method steps in a closed circuit (2) after the condensation: - increasing the pressure of the agent, - producing a vapor phase of the agent by heat transfer from an external heat source (5), and - expanding the vapor phase and converting its thermal energy to mechanical energy.

7. The method as claimed in claim 6, characterized in that, before the expansion of the vapor phase of the agent, a liquid phase of the agent is separated from the vapor phase, and the vapor phase is supplied again after it has been expanded.

8. The method as claimed in claim 6 or 7, characterized in that, a geothermal fluid, industrial waste heat or waste heat from an internal combustion engine is used as the external heat source (5).

9. The method as claimed in one of the preceding claims, characterized in that, a mixture of ammonia and water is used as the agent.

10. An apparatus (1) 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 (8) 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 (8), characterized by a mixing device (7) for mixing the liquid phase of the two-phase flow with the vapor phase of the two-phase PCT/EP2008/060921 - 14 2007P17898WOUS flow before or during the condensation of the agent in the condenser (8)

11. The apparatus (1) as claimed in claim 10, characterized in that, the mixing device (7) has a separator (20) for separation of the liquid phase from the vapor phase, and has at least one nozzle (21) for spraying the separated liquid phase into the vapor phase.

12. The apparatus (1) as claimed in claim 11, characterized in that, the mixing device (7) has a pump (22) , 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.

13. The apparatus (1) as claimed in claim 11 or 12, characterized in that, the separator (20) is arranged immediately before the condenser (8) in the flow direction of the agent.

14. The apparatus (1) as claimed in one of claims 11 to 13, characterized in that, the at least one nozzle (21) is arranged immediately before or in the condenser (8) in the flow direction of the agent.

15. The apparatus (1) as claimed in one of claims 10 to 14, characterized in that, the agent can be carried in a closed circuit (2) in the apparatus (1), which closed circuit (2) has at least the following components after the condenser (8) in the flow direction of the agent: - a pump (3) for increasing the pressure of the agent - a heat exchanger (4) for producing a vapor phase of the PCT/EP2008/060921 - 14a 2007P17898WOUS agent by heat transfer from an external heat source (5), and - an expansion device (6), in particular a turbine, for expansion of the vapor phase and conversion of its thermal energy to mechanical energy. PCT/EP2008/060921 - 15 2007P17898WOUS

16. The apparatus (1) as claimed in claim 15, characterized in that, the circuit (2) additionally comprises a separator (15), which is arranged between the heat exchanger (4) and the expansion device (6), for separation of a liquid phase of the agent from a vapor phase, and a combination means (16), which is arranged between the expansion device (6) and the mixing device (7), for combination of the separated liquid phase and the expanded vapor phase.

17. The apparatus (1) as claimed in claim 15 or 16, characterized in that, the external heat source (5) is a geothermal flow, industrial waste heat or waste heat from an internal combustion engine.

18. The apparatus (1) as claimed in one of claims 10 to 17, characterized in that, the agent is a mixture of ammonia and water.