Abstract:
A new thermodynamic cycle is disclosed for converting energy from a low temperature stream from an external source into useable energy using a working fluid comprising of a mixture of a low boiling component and a high boiling component. The cycle is designed to improve the efficiency of the energy extraction process by mixing into an intermediate liquid stream an enriched liquid stream from which the energy from the external source stream is extracted in a vaporization step and converted to energy in an expansion step. The new thermodynamic process and the system for accomplishing it are especially well-suited for streams from low-temperature geothermal sources.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process and system to convert thermal energy from low temperature sources, especially from low temperature geothermal fluids, into mechanical and/or electrical energy. 
     More particularly, the present invention relates to a process and system to convert thermal energy from moderately low temperature sources, especially from geothermal fluids, into mechanical and electrical energy, where a working fluid comprises a mixture of at least two components, with the preferred working fluid comprising a water-ammonia mixture. The present invention also relates to a novel thermodynamic cycle or process and a system to implement it. 
     2. Description of the Related Art 
     Prior art methods and systems for converting heat into useful energy are well documented in the art. In fact, many such methods and systems have been invented and patented by the inventor. These prior art systems include U.S. Pat. Nos.: 4,346,561, 4,489,563, 4,548,043, 4,586,340, 4,604,867, 4,674,285, 4,732,005, 4,763,480, 4,899,545, 4,982,568, 5,029,444, 5,095,708, 5,440,882, 5,450,821, 5,572,871, 5,588,298, 5,603,218, 5,649,426, 5,822,990, 5,950,433 and 5,953,918; Foreign References: JP 94815 B2 and Journal References: NEDO Brochure, “ECO-Energy City Project”, 1994 and NEDO Report published 1996, pp. 4-6, 4-7, 4-43, 4-63, 4-53, incorporated herein by reference. 
     Although all of these prior art systems and methods relate to the conversion of thermal energy into other more useful forms of energy from moderately low temperature sources, all suffer from certain inefficiencies. Thus, there is a need in the art for an improved system and method for converting thermal energy from moderately low temperature sources to more useful forms of energy, especially for converting geothermal energy from moderately low temperature geothermal streams into more useful forms of energy. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for implementing a thermodynamic cycle comprising the steps of expanding a gaseous working stream, transforming its energy into usable form and producing a spent stream. After expansion and work extraction, the spent stream is mixed with at least one lean stream to form a lean spent stream. The lean spent stream is then used to heat a liquid first working stream to form a heated first working stream and a pre-condensed stream which is then condensed to form a liquid stream. The liquid stream is then mixed with an enriched stream to formn the liquid first working stream. A portion of this stream is then depressurized to an intermediate pressure and separated into an enriched vapor stream and the lean stream; while a second portion of the liquid first working stream is heated to form the gaseous working stream. 
     The present invention provides a method for implementing a thermodynamic cycle comprising the steps of expanding a gaseous second working stream, transforming its energy into usable form and producing a low pressure spent stream. After expansion, the spent stream is mixed with a first lean stream forming a lean spent stream. Heat is then transferred from this stream to a first working solution to form a heated first working solution. The cooled lean spent stream is then mixed with a second lean stream to form a pre-condensed stream, which is then condensed to form a liquid stream. The liquid stream is then mixed with a first enriched vapor stream to form the first working solution. A first portion of the heated first working stream is separated into a second enriched vapor stream and the second lean stream. A second portion of the heated first working stream is then heated with an external heat source fluid stream to form a partially vaporized first working stream. The partially vaporized first working stream is then separated into a fourth enriched stream and a third lean stream. A first portion of the third lean stream is then separated into the first lean stream and a third enriched stream and the third enriched stream is mixed with the second enriched stream to form the first enriched stream. A second portion of the third lean stream is mixed with the fourth enriched stream to form the second working stream, which is then fully vaporized to from the gaseous second working stream. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention can be better understood with reference to the following detailed description together with the appended illustrative drawings in which like elements are numbered the same: 
     FIGS. 1A&amp;B depict a diagram of a preferred embodiment of a system of this invention for converting heat from a geothermal source to a useful form of energy; 
     FIG. 2 depicts a diagram of another preferred embodiment of a system of this invention for converting heat from a geothermal source to a useful form of energy; 
     FIG. 3 depicts a diagram of another preferred embodiment of a system of this invention for converting heat from a geothermal source to a useful form of energy and 
     FIG. 4 depicts a diagram of another preferred embodiment of a system of this invention for converting heat from a geothermal source to a useful form of energy. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventors have found that a system utilizing a novel thermodynamical cycle (process) can be designed to increase the work output derived from low temperature heat sources. The system and the process or method use a working fluid comprising a mixture of at least two components. The preferred working fluid for the systems and processes of this invention is a water-ammonia mixture, though other mixtures, such as mixtures of hydrocarbons and/or Freons can be used with practically the same results. The systems and methods of this invention are more efficient for converting heat from relatively low temperature geothermal source into a more useful form of energy. The system uses a multi-component basic working fluid to extract energy from one or more (at least one) geothermal source streams in one or more (at least one) heat exchangers or heat exchanges zones. The heat exchanged basic working fluid then transfers its gained thermal energy to one or more (at least one) turbines and the turbines convert the gained thermal energy into mechanical energy and/or electrical energy. The system also includes pumps to increase the pressure of the basic working fluid at certain points in the system and one or more (at least one) heat Exchangers which bring the basic working fluid in heat exchange relationships with one or more (at least one) cool streams. One novel feature of the systems and methods of this invention, and one of the features that increases the efficiency of the systems, is the result of absorbing a vapor stream into the condensed liquid working solution stream prior to fully pressurization via pumping. The vapor stream changes the composition of the solution prior to heating and vaporization by the geothermal stream. 
     The basic working fluid used in the systems of this inventions preferably is a multi-component fluid that comprises a lower boiling point fluid—the low-boiling component—and a higher boiling point fluid—the high-boiling component. Preferred working fluids include an ammonia-water mixture, a mixture of two or more hydrocarbons, a mixture of two or more freon, a mixture of hydrocarbons and freon, or the like. In general, the fluid can comprise mixtures of any number of compounds with favorable thermodynamic characteristics and solubility. In a particularly preferred embodiment, the fluid comprises a mixture of water and ammonia. 
     Referring now to FIG. 1A, a flow diagram, generally  100 , is shown that illustrates a preferred embodiment a system and method of energy conversion of this invention and will be described in terms of its components and its operation. 
     A fully condensed basic solution of working fluid with parameters as at a point  2  enters into a pump P 1 , where it is pumped to a chosen, elevated pressure, (hereafter referred to as the “intermediate pressure”), and obtains parameters as at a point  3 . The basic working solution at the point  2  is in a state of a saturated liquid, and as a result of increasing pressure in the process  2 - 3  obtains a state of sub-cooled liquid. The stream of sub-cooled liquid, having parameters as at the point  3 , is mixed with a stream of vapor having parameters as at a point  64  (see below). This vapor, with parameters as at the point  64 , has a significantly higher concentration of the low boiling component, (e.g., in case of water-ammonia basic working solution, the solution would have a higher concentration of ammonia), than the liquid with parameters as at a point  3 . As a result of this mixing, the liquid fully absorbs the vapor, and obtains parameters as at a point  11 . The composition of the solution having parameters as at the point  11  corresponds to a state of saturated liquid, but the composition of the solution is such that a concentration of the low boiling component in the solution at the point  11  is higher than a concentration of the low boiling component in the solution at the points  2  and  3 . The solution having that composition at the point  11  will hereafter be referred to as a first working solution. 
     The stream of first working solution, with parameters as at the point  11 , enters a pump P 2 , where it is pumped to an elevated pressure, hereafter referred to as a high pressure, and obtains parameters as at the point  12 . Thereafter, the stream of the first working solution passes through a heat exchanger HE 1 , where it is heated, and obtains parameters as at a point  13 . In a preferred embodiment of this system, the stream, with parameters as at the point  13 , corresponds to a state of saturated or slightly sub-cooled liquid. Thereafter, the stream, with parameters as at the point  13 , is divided into two sub-streams, with parameters as at points  14  and  16 , respectively. 
     The sub-stream, with parameters as at the point  16 , passes through a throttle valve TV 1 , where its pressure is reduced to the intermediate pressure (see above) and obtains parameters as at a point  17 . As a result of the throttling in the process  16 - 17 , the stream, with parameters as at the point  17 , corresponds to a state of a two-phase fluid, i.e., a mixture of saturated liquid and saturated vapor. The stream, with parameters as at the point  17 , is then sent into a separator S 1 , where liquid is separated from vapor. The vapor, leaving the separator S 1 , with parameters as at a point  62 , is then mixed with another stream of vapor having parameters as at a point  63 , thus creating a stream of vapor having parameters as at the point  64 . This stream of vapor, with parameters as at the point  64 , is then mixed with liquid stream, with parameters as at the point  3 , creating a stream, with parameters as at the point  11  (see above). 
     The sub-stream of first working solution, with parameters as at the point  14 , passes through a heat exchanger HE 2 , where it is heated and partially vaporized, leaving the heat exchanger HE 2  as a stream, with parameters as at a the point  15 , corresponding to a state of a two-phase fluid. The stream of first working solution, with parameters as at the point  15 , then enters into a separator S 2 , where liquid is separated from vapor. A liquid stream leaving the separator S 2  has parameters as at a point  21 ; while a vapor stream leaving separator S 2  has parameters as at a point  61 . 
     The stream of liquid, with parameters as at the point  21 , is then divided into two sub-streams having parameters as at points  22  and  23 , respectively. The sub-stream of liquid, with parameters as at the point  22 , passes through a throttle value TV 2 , where its pressure is reduced to the intermediate pressure, and as a result the stream obtains parameters as at a point  24 , corresponding to a state of a two-phase fluid. The stream, with parameters as at the point  24 , is then sent into a separator S 3 , where it is separated into a stream of saturated vapor having parameters as at the point  63 , and a stream of saturated liquid having parameters as at a point  31 . The stream of vapor, with parameters as at the point  63 , is mixed with the stream of vapor, with parameters as at the point  62 , and forms the stream of vapor, with parameters as at the point  64  (see above). 
     The sub-stream of liquid, with parameters as at the point  23 , is mixed with the stream of vapor, with parameters as at the point  61 , forming a new stream having parameters as at a point  71 . The new stream, with parameters as at the point  71 , is referred to as a second working solution. 
     The stream of second working solution, with parameters as at the point  71 , is sent through a heat exchanger HE 3 , where it is heated and fully vaporized, so that the stream has parameters as at a point  72 . A composition of the stream of the second working solution, in the process  71 - 72  is chosen such that stream having the parameters at the point  72  corresponds to stream having a state of saturated or superheated vapor. The stream of second working solution, with parameters as at the point  72 , passes through a turbine T 1 , where it is expanded, producing useful work, and leaves turbine T 1  as a spent stream having parameters as at a point  73 . 
     The stream of liquid, with parameters as at the point  31 , leaving separator S 3  (see above) passes through a throttle value TV 3 , where its pressure is reduced to a pressure equal to a pressure of the stream at the point  73 , and the stream obtains parameters as at a point  32 . Then the streams with parameters as at the points  73  and  32  are combined, forming a stream of condensing solution having parameters as at a point  81 . The stream, with parameters as at the point  81 , passes through the heat exchanger HE 1  in counter-flow to the entering stream, with parameters as at the point  12 , where the stream, with parameters as at the point  81 , is partially condensed, releasing heat, and forming a stream with parameters as at a point  82 . The heat released in a process  81 - 82  is utilized to provide heat to the process  12 - 13  (see above). 
     The stream of liquid, with parameters as at a point  41 , leaving the separator S 1 , passes through a throttle valve TV 4 , where its pressure is reduced to a pressure equal to the pressure of the stream, with parameters as at the point  82 , and the stream obtains parameters as at a point  42 . Thereafter, the streams, with parameters as at the points  42  and  82 , are combined, forming a stream of basic solution having parameters as at a point  1 . The stream, with parameters as at the point  1 , passes through a condenser, i.e., a heat exchanger HE 4 , where it is cooled and fully condensed, forming a stream having parameters as at the point  2 . The cooling and condensation of the stream, with parameters as at the point  1  to the stream, with parameters at as the point  2  in the process  1 - 2  is provided by a stream of ambient fluid (air or water) which enters the heat exchanger HE 4  with parameters as at a point  91  and exists the heat exchanger HE 4  with parameters as at a point  92 . 
     A stream of hot geothermal fluid, with initial parameters as at a point  51 , passes through a heat exchanger HE 3 , in counter-flow to the stream having parameters as at the point  71 , providing heat for the process  71 - 72 , and the geothermal stream, with parameter as at the point  51 , forms a geothermal stream having parameters as at a point  52 . Thereafter, the stream geothermal fluid, with parameters as at the point  52 , passes though the heat exchanger HE 2 , where it is further cooled, providing heat for the process  14 - 15 . The thermodynamic cycle involving the basic working solution is a closed cycle. 
     In a simplified preferred embodiments of the system and process of this invention, generally  150 , a separator S 3  and a throttle valve TV 3  can be excluded as shown in FIG.  1 B. In such a case, a pressure of the stream of liquid, with parameters as at the point  22 , is reduced in the throttle valve TV 2 , in one step to a stream having parameters at a point  24 , where a pressure of the stream is equal to a pressure of the turbine exhaust stream, with parameters as at the point  73 . Once the pressure of the stream, with parameters as at the point  22 , has been reduced, forming the stream, with parameters as at the point  24 , the stream, with parameters at the point  24 , is mixed with this turbine exhaust stream, with parameters as at the point  73 , forming a condensing stream, with parameters as at the point  81 . As a result, the stream of vapor with parameters as at the point  63  of the system  100  of FIG. 1A, does not exist, and the absence of the stream, with parameters as at the point  63  of the system  100 , reduces a rate of enrichment of the basic solution in the process of mixing the stream with parameters as at the point  63  of the system  100  with the stream having parameters as at the point  64 . Additionally, the basic solution will become slightly richer and therefore the pressure after the turbine must be slightly increased. As a result, such a simplified version wilt have slightly lower overall efficiency. 
     Referring now to FIG. 2, a further simplified preferred embodiment of this invention, generally  200 , is shown. The system  200  not only excludes the separator S 3  and the throttle valve TV 3  of the system  100 , the system  200  also excludes the heat exchanger HE 3 . Thus, the vapor stream, with parameters as at the point  72 , is forwarded directly to the turbine T 1 . In such a case, the separator S 2  is preferred a very high quality and very efficient separator or separating apparatus to prevent or minimize droplets of liquid in the stream, with parameters as at the point  72 , as it enters the turbine T 1 . 
     Referring to FIG. 3, another preferred embodiment of the system and process of this invention, generally  300 , is shown, which has enhanced efficiency through the addition of a fifth heat exchanger. When liquid streams, having parameters as at points  17  and  22 , respectively, are throttled in the throttling valves TV 1  and TV 2 , the quantities of vapor produced in these processes will increase as the pressure after the throttle valves is decreased. Therefore, flow rates of the streams having parameters as at the point  62  and  63  will be increase, which in turn increases a flow rate of the stream have parameters as at the point  64 . But this will in turn require lowering a pressure of the liquid stream having parameters as at the point  3  leaving the pump P 1 , and, therefore, reduce an ability of the stream having parameters as at the point  3  to absorb the vapor stream having the parameters as at the point  64 . When the liquid stream having parameters as at the point  3  and the vapor stream having parameters as at the point  64  are mixed, it may be necessary to install an additional condenser or heat exchanger HE 5  into which the stream having parameters as at the point  11  is sent. As a result, the fully condensed stream having parameters as at a point  18  is produced. Thereafter, the stream having parameters as at the point  18  is sent into the pump P 2 . In this preferred embodiment, the streams of liquid having the parameters as at the point  32  and  42  become leaner (i.e., contain a smaller concentration of the low boiling component, e.g., a smaller concentration of ammonia in a water-ammonia mixture), and a composition of the streams having parameters as at the points  1 ,  2  and  73  also correspondingly become leaner, which results in a lowering of a pressure of the streams having parameters  1 ,  2  and  73  increasing the work output of the turbine T 1 . 
     The introduction of the additional condenser or heat exchanger HE 5  does not increase the total quantity of heat which is rejected to the ambient surroundings. To the contrary, the amount of heat rejected to the ambient is decreased as a result of the increased output of the turbine T 1 . In general, the embodiment  300  of FIG. 3 is more efficient than the embodiment  100  of FIG.  1 . 
     The embodiment  300  of FIG. 3 provides for a significantly higher degree of enrichment of the basic working solution in the process of mixing it with a stream of vapor having parameters as at the point  64 . This, in turn, allows for a significant simplification of this embodiment. The first working solution may be enriched to such an extend that it can be used as a second working solution, thus excluding the need for two separate working solutions. Such a simplified version of this embodiment, generally  400 , is shown in FIG.  4 . The system  400  differs from the system  300  of FIG. 3 as set forth below. 
     The working solution form in the condenser or heat exchanger HE 5 , after being heated by a stream of turbine exhaust in the heat exchanger HE 1 , is divided into two sub-streams having parameters as at the point  14  and  16 , respectively. Thereafter, the sub-stream having parameters as at the point  14  is sent into the heat exchanger HE 2 , where it is vaporized in counter-flow relationship to the geothermal stream having parameters as at the point  51 , forming a stream having parameters as at the point  15 . A composition and pressure of the working solution must be chosen such that the stream having parameters as at the point  15  corresponds to a stream having a state of saturated or superheated vapor. Thereafter, the stream of working solution having parameters as at the point  15  passes through the turbine T 1 , where it expands, producing useful work. The stream exits the turbine T 1  having parameters as at the point  73  is sent them through the heat exchanger HE 1 , where it is partially condensed, providing heat for heating the stream having parameters as at the point  12  in the heating process  12 - 13 . After leaving the heat exchanger HE 1 , the stream of working solution having the parameters as at the point  73  forms a stream having parameters as at the point  82 . The stream having the parameters as at the point  82  is then combined with the lean stream having parameters as at the point  42  as previously described, forming a stream of basic working solution having the parameters as at the point  1 . In all other particulars, the embodiment  400  of FIG. 4 operates in the same manner as the embodiment  300  of FIG.  3 . 
     As one can see, the variant of the proposed system presented in FIG. 4 is significantly simpler than the variant presented in FIG.  3 . As compared to the system  300  presented in FIG. 3, the system  400  presented in FIG. 4 includes four heat exchangers instead of five heat exchangers, two throttled valves instead of four throttled valves and one separator instead of three separators. However, such a simplification reduces the flexibility and to some degree the efficiency of the system  400  of FIG. 4 compared to the system  300  of FIG.  3 . 
     The choice amongst the four presented preferred embodiment of this invention depends upon the initial and final temperature of the utilized geothermal fluid stream or other heat carrying fluid stream, upon the ambient temperature, and upon economics conditions in which the system has to operate. One of ordinary skill in the art can choose the particular embodiment of this invention that best suits the conditions and constraints of the environment in which the system is to be installed and operated. 
     In prior art (see e.g., U.S. Pat. No. 5,029,444), the basic solution, after passing through the condenser, is pumped in one step to a high pressure, and is then sent into two heat exchangers, one of which is heated by turbine exhaust and another by liquid returning from a separator, which corresponds to liquid stream having parameters as at the point  22  of the systems of this invention. In these two heat exchangers, the basic solution is heated and then partially vaporized. But the quantity of heat required to raise the temperature by any given temperature difference in a process of vaporization is several times greater than the quantity of heat required to pre-heat a liquid by the same temperature difference. As a result, in these heat exchangers of the prior art, the heat from the returning stream of vapor and liquid is balanced only by the process of vaporization, and, therefore, is poorly utilized; i.e., excessive heat in the process of pre-heating is utilized only partially. 
     Moreover, if the initial temperature of the geothermal fluid is low, then a temperature of vapor exiting the turbine can be lower than an initial temperature of boiling of the basic solution. In this case, the pressure at which boiling occurs must be lowered, so as to provide for the initial boiling of the basic solution by heat exchange with the stream of turbine exhaust. Alternately, because a temperature of the vapor exiting the turbine must be higher than the initial temperature of boiling of the basic solution, a pressure of the vapor exiting the turbine has to be increased to provide, on one hand, a higher temperature of the vapor exiting the turbine, and on the other hand, a richer basic solution so that the initial temperature of boiling for the basic solution becomes lower. These results, when compared to the systems of this invention, in a lowering of the efficiency of the system in the prior art in cases where the initial temperature of the geothermal fluid or other heat source, is low. 
     In the prior art, in systems designed to utilize low-temperature heat sources (e.g., U.S. Pat. No. 5,953,918), the heat of condensation of the turbine exhaust stream is utilized only for preheating an upcoming high pressure stream of working solution. But for the same reason as described above, this heat is poorly utilized as well. 
     In contrast, in all of the embodiment of the system of this invention, the basic solution is enriched by absorbing a stream of vapor having parameters as at the point  64 , thus forming the first working solution. In the embodiments  300  and  400  of FIGS. 3 and 4, respectively, this absorption is enhanced by using an additional condenser or heat exchanger HE 5 . In the embodiments  100 ,  150  and  200  of FIG. 1A,  1 B and  2 , the turbine exhaust is mixed with liquid from the separator S 3 . In the embodiment  200  of FIG. 2, the turbine exhaust is mixed with liquid from the separator S 2 . In all fours embodiments, the heat released in the process of the condensation of the stream of turbine exhaust (whether not the stream is mixed with addition liquid)is used only for pre-heating of the first working solution up to the boiling temperature. Because the working solution is enriched by a low-boiling component in comparison to the basic working solution, it allows a higher boiling pressure of the first and, where applicable, of the second working solutions. All heat from the condensation of turbine exhaust is effectively used by being sent into the heat exchanger HE 1 , a stream of the first working solution with a weight flow rate significantly higher than the flow rate of the stream of this same solution which is sent into the boiler (Heat Exchanger HE 2 ). Excessive quantity of the first working solution is used to produce a stream of vapor with parameters as at the point  62 , which is then utilized to enrich the basic solution by adding this vapor stream to it, and rowing a richer stream of the first working solution. 
     To sum up, it is clear that the systems of this invention can provide for a higher pressure of vapor entering the turbine and a lower pressure of vapor exiting he turbine, thus providing a higher efficiency to the system as a whole. A preliminary assessment shows that the proposed system can, at the same border conditions, provide for an increase in power output of between 10% and 20%. It should be recognized that the working solution is in a closed thermodynamic cycle and the temperatures and pressures of the streams are self adjusting so that the system operates at maximum efficiency with little or no outside monitoring or control. 
     All references cited herein are incorporated by reference. While this invention has been described fully and completely, it should be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. Although the invention has been disclosed with reference to its preferred embodiments, from reading this description those of skill in the art may appreciate changes and modification that may be made which do not depart from the scope and spirit of the invention as described above and claimed hereafter.