Abstract:
A method and apparatus for implementing a thermodynamic cycle that includes: (a) expanding a gaseous working stream, transforming its energy into usable form and producing a spent working stream; (b) heating a multicomponent oncoming liquid working stream by partially condensing the spent working stream; and (c) evaporating the heated working stream to form the gaseous working stream using heat produced by a combination of cooling geothermal liquid and condensing geothermal steam.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for transforming thermal energy from a geothermal heat source consisting of a mixture of geothermal liquid and geothermal steam (&#34;geofluid&#34; ) into electric power. This invention further relates to utilizing the energy potential of both geothermal liquid and geothermal steam in one integrated system. 
     Geothermal heat sources can generally be divided into two groups. In the first group are &#34;liquid-dominated&#34; heat sources that produce mostly hot geothermal liquid (brine). In the second group are &#34;steam-dominated&#34; heat sources that produce mostly geothermal steam with some geothermal liquid. 
     Methods for converting the thermal energy released by geothermal heat sources into electric power present an important and growing area of energy generation. Geothermal power plants generally belong to one of two categories, namely, steam plants and binary plants. 
     In steam plants, the geothermal source is utilized directly to produce steam (e.g., by throttling and flashing geothermal liquid). That steam is then expanded in a turbine, producing power. In binary plants, heat extracted from the geothermal liquid is used to evaporate a working fluid that circulates within the power cycle. The working fluid is then expanded in a turbine, producing power. 
     Steam plants are generally used for steam-dominated geothermal heat sources, while binary plants are generally used for liquid-dominated geothermal heat sources. U.S. Pat. No. 4,982,568 describes a method and apparatus for transforming thermal energy from geothermal liquid into electrical power in a binary plant. This method increases efficiency by using a thermodynamic cycle with a multi-component working fluid and internal recuperation. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the invention features a method of implementing a thermodynamic cycle that includes the steps of: 
     expanding a gaseous working stream, transforming its energy into usable form and producing a spent working stream; 
     heating a multicomponent oncoming liquid working stream by partially condensing the spent working stream; and 
     evaporating the heated working stream to form the gaseous working stream using heat produced by a combination of cooling geothermal liquid and condensing geothermal steam. 
     In preferred embodiments, the liquid working stream is superheated following evaporation using heat produced by cooling geothermal liquid to form the gaseous working stream. The multicomponent oncoming liquid working stream is preferably preheated by partially condensing the spent working stream, after which it is divided into first and second substreams. The first substream is then partially evaporated using heat produced by partially condensing the spent working stream, while the second substream is partially evaporated using heat produced by cooling geothermal liquid. The partially evaporated first and second substreams are then combined and evaporated to form the gaseous working stream using heat produced by a combination of cooling geothermal liquid and condensing geothermal steam. The difference between the boiling temperature of the second substream and the temperature of the geothermal liquid preferably is greater than the difference between the boiling temperature of the first substream and the temperature of the condensed spent working stream. 
     The geothermal steam is expanded, transforming its energy into usable form and producing a spent geothermal stream. The spent geothermal stream is then condensed to heat and partially evaporate the liquid working stream, after which it is combined with the geothermal liquid and used for further evaporation of the liquid working stream. Where the geothermal steam content of the geofluid is relatively high, it is preferable to perform multiple expansions of the geothermal steam. Thus, in one preferred embodiment, the spent geothermal stream produced by a first expansion of geothermal steam is divided into first and second geothermal substreams. The first geothermal substream is condensed to heat and partially evaporate the liquid working stream, and then combined with the geothermal liquid. The second geothermal substream is expanded, transforming its energy into usable form and producing a spent geothermal substream, which is then condensed to heat and partially evaporate the liquid working stream. The spent geothermal substream is then combined with the geothermal liquid. 
     In a second aspect, the invention features apparatus for implementing a thermodynamic cycle that includes: 
     means for expanding a gaseous working stream, transferring its energy into usable form and producing a spent stream; 
     a heat exchanger for partially condensing the spent stream and for transferring heat from the spent stream to an oncoming multicomponent liquid working stream; 
     a separator for separating geofluid into geothermal liquid and geothermal steam; and 
     a multiplicity of heat exchangers for cooling geothermal liquid and condensing geothermal steam, and for transferring heat from the geothermal liquid and geothermal steam to evaporate the liquid working stream and form the gaseous working stream. 
     In preferred embodiments, the apparatus includes a heat exchanger for cooling geothermal liquid and transferring heat from the geothermal liquid to superheat the liquid working stream and form the gaseous working stream. The apparatus also preferably includes a stream separator for dividing the heated liquid working stream into first and second substreams; a heat exchanger for partially condensing the spent working stream and transferring heat from the spent working stream to partially evaporate the first substream; a heat exchanger for cooling the geothermal liquid and transferring heat from the cooled geothermal liquid to partially evaporate the second substream; and a stream mixer for combining the partially evaporated first and second substreams. 
     The apparatus further preferably includes means for expanding geothermal steam, transforming its energy into usable form and producing a spent geothermal stream; a heat exchanger for condensing the spent geothermal stream and transferring heat from the spent geothermal stream to partially evaporate the liquid working stream; and a stream mixer for combining the spent geothermal stream with the geothermal liquid. To accommodate geofluids with relatively high geothermal steam content, the apparatus further includes a stream separator for dividing the spent geothermal stream produced in the first expansion into first and second geothermal streams; a heat exchanger for condensing the first geothermal substream and transferring heat from the first geothermal substream to partially evaporate the liquid working stream; a stream mixer for combining the first geothermal substream with the geothermal liquid; means for expanding the second geothermal substream, transforming its energy into usable form and producing a spent geothermal substream; a heat exchanger for condensing the spent geothermal substream and transferring heat from the spent geothermal substream to partially evaporate the liquid working stream; and a stream mixer for combining the spent geothermal substream with the geothermal liquid. 
     The invention provides an integrated system that utilizes the energy potentials of both geothermal steam and geothermal liquid (brine). The system can handle practically all geothermal resources in almost any proportion between steam and liquid. Geofluids from different wells having different temperatures and different proportions of steam and liquid may be used as well. Higher outputs and efficiencies are achieved relative to systems in which geothermal liquid and geothermal steam are utilized separately. In addition, the efficiency and output are higher relative to steam power systems that are currently used for utilization of such geothermal resources. 
     Because the heat source for the thermodynamic cycle involves a combination of cooling geothermal liquid and condensing geothermal steam, only a one stage expansion of the working fluid is necessary (as opposed to two stages of expansion with intermediate reheat). Moreover, by splitting the liquid working fluid into two substreams, one of which is partially evaporated by heat transferred from cooling geothermal liquid and the other of which is partially evaporated by heat transferred from partially condensing spent working fluid, geofluid having a high degree of mineralization (which can be cooled only to relatively high temperatures) can be used as well. 
     Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of one embodiment of the method and apparatus of the present invention. 
     FIG. 2 is a schematic representation of a second embodiment of the method and apparatus of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The schematic shown in FIG. 1 shows an embodiment of preferred apparatus that may be used in the above-described cycle. Specifically, FIG. 1 shows a system 100 that includes a gravity separator 101, a preheater in the form of a heat exchanger 109, a superheater in the form of a heat exchanger 104, and a boiler in the form of heat exchangers 103, 106, 107, and 108. In addition, the system 100 includes turbines 102 and 114, pumps 105 and 111, and condenser 110. Further, the system 100 includes stream separator 112 and stream mixer 113. 
     The condenser 110 may be any type of known heat rejection device. For example, the condenser 110 may take the form of a heat exchanger, such as a water cooled system, or another type of condensing device. 
     As shown in FIG. 1, geofluid consisting of geothermal liquid (brine) and geothermal steam leaving the geothermal well is sent into gravity separator 101, where geothermal liquid and geothermal steam are separated. Steam leaves separator 101 with parameters as at point 41, and liquid leaves separator 101, with parameters as at point 51. Thereafter, the steam is sent into steam turbine 102 where it expands, producing power which is converted into electric power, and leaves turbine 102 with parameters as at point 43. The steam is then sent into heat exchanger 103 where it condenses, releasing its heat of condensation and being completely condensed. The condensate leaves heat exchanger 103 with parameters as at point 44. Heat from the condensation of the steam is transferred in heat exchanger 103 to the working fluid of the power cycle. 
     The geothermal liquid, with parameters as at point 51, is cooled in heat exchanger 104, which it leaves with parameters as at point 52 and transfers heat to the working fluid of the power cycle. The temperature of the steam condensate at point 44 is substantially equal to the temperature of the geothermal liquid at point 52. The steam condensate with parameters as at point 44 is pumped by a pump 105 to a pressure equal to that of the geothermal liquid at point 52, obtaining parameters as at point 45. Thereafter, the steam condensate with parameters corresponding to point 45 is combined with the geothermal liquid with parameters corresponding to point 52, obtaining parameters as at point 53. 
     The combined liquid having parameters as at point 53 passes through heat exchanger 106 where it is further cooled, releasing heat which is transferred to the working fluid of the power cycle and obtaining parameters as at point 56. Finally, liquid with parameters as at point 56 passes through heat exchanger 107 where it is further cooled, releasing heat which is transferred to the working fluid of the power cycle and obtaining parameters as at point 57. Thereafter, geothermal liquid is removed from the system and reinjected into the geothermal strata. 
     From the above discussion, it can be seen that the thermodynamic power cycle according to the invention utilizes two sources of geothermal heat, i.e., heat released in the process of condensation of geothermal steam and heat released by the cooling of liquid and steam condensate (geothermal liquid). The power cycle operates as follows. 
     The fully condensed working fluid of the power cycle with parameters as at point 21 passes through a recuperative preheater 109 where it is preheated up to boiling temperature and exits preheater 109 with parameters as at point 60. Thereafter, the working fluid is divided into two substreams at stream separator 112 having parameters, correspondingly, as at points 61 and 62. The first substream with parameters as at point 61 passes through heat exchanger 107, where it is heated by a stream of liquid geofluid and partially evaporated. It leaves heat exchanger 107 with parameters as at point 63. 
     The second substream having parameters as at point 62 passes through heat exchanger 108 where it is also heated and partially evaporated. It leaves heat exchanger 108 with parameters as at point 64. Thereafter, both substreams are combined at stream mixer 113, obtaining parameters as at point 66. The combined substreams are then sent into heat exchanger 106 where further evaporation occurs using heat transferred from a stream of liquid geofluid. 
     The temperature difference between the boiling point of the working fluid having parameters at point 62 and the temperature of the condensing working fluid stream at point 38 is minimized. However, the temperature difference between the initial boiling temperature and final temperature of the geothermal liquid used for evaporation in heat exchanger 107 can significantly exceed the minimum temperature difference between points 62 and 38 in heat exchanger 108. Thus, it is possible to optimize temperature and corresponding pressure at point 60 even where the geothermal liquid can only be cooled to relatively high temperatures because of a high degree of mineralization. 
     The working fluid leaves heat exchanger 106 having parameters as at point 69 and enters heat exchanger 103, where evaporation is completed using heat produced by condensation of the geothermal steam. The working fluid leaves heat exchanger 103 with parameters as at point 68 and enters heat exchanger 104, where it is superheated by a stream of geothermal liquid. Thereafter, the working fluid, which leaves heat exchanger 104 with parameters as at point 30, enters turbine 114 where it is expanded, producing power. The expanded working fluid stream then leaves turbine 114 with parameters as at point 36. 
     The expanded working fluid at point 36 is usually in the form of a dry or a wet saturated vapor. It then passes through heat exchanger 108 where it is partially condensed. The heat released during condensation is utilized for an initial boiling of the liquid working fluid. Thereafter, the expanded working fluid leaves heat exchanger 108 with parameters as at point 38 and passes through heat exchanger 109, where it is further condensed. The heat of condensation is utilized to preheat oncoming working fluid. The partially condensed working fluid with parameters as at point 29 leaves heat exchanger 109 and enters heat exchanger 110, where it is fully condensed, obtaining parameters as at point 14. Condensation can be provided by cooling water, cooling air, or any other cooling medium. The condensed working fluid is then pumped to a higher pressure by pump 111, obtaining parameters as at point 21. The cycle is then repeated. 
     The pressure at point 43 to which geothermal steam is expanded is chosen to achieve maximum total power output from both steam turbine 102 and working fluid turbine 114. The composition of the multicomponent working fluid (which includes a lower boiling point fluid and a higher boiling point fluid) is similarly chosen to maximize total power output. Specifically, the composition is chosen such that the temperature at which the expanded working fluid having parameters at point 36 condenses is higher than the temperature at which the same working fluid having parameters at point 60 boils. Examples of suitable multicomponent working fluids include an ammonia-water mixture, two or more hydrocarbons, two or more freons, mixtures of hydrocarbons and freons, or the like. In a particularly preferred embodiment, a mixture of water and ammonia is used. The multicomponent working stream preferably includes about 55% to about 95% of the low-boiling component. 
     Preferred parameters for the points corresponding to the points set forth in FIG. 1 are presented in Table I for a system having a water-ammonia working fluid stream. From the data it follows that the proposed system increases output in comparison with a traditional steam system by 1.55 times, and in comparison with a system that separately utilizes heat from brine and steam by 1.077 times. 
     
                                           TABLE I__________________________________________________________________________# P psiA X   T °F.          H BTU/lb                G/G30 Flow lb/hr                            Phase__________________________________________________________________________14  112.71 .7854      78.00          -12.37                1.0000                      2,682,656                            SatLiquid21  408.10 .7854      78.00          -11.12                1.0000                      2,682,656                            Liq 90°23  • Water      70.00          38.00 14.8173                      39,749,69424  • Water      94.70          62.70 14.8173                      39,749,69429  113.01 .7854     133.62          353.56                1.0000                      2,682,656                            Wet .403730  385.10 .7854     386.80          811.71                1.0000                      2,682,656                            Vap 67°36  113.61 .7854     240.46          724.15                1.0000                      2,682,656                            Wet .032138  113.31 .7854     170.00          450.61                1.0000                      2,682,656                            Wet .299840  113.61 .7854     244.90          755.37                1.0000                      2,682,656                            SatVapor41  224.94 Steam     391.80          1200.54                .1912   513,000                            SatVapor43   84.77 Steam     316.09          1132.63                .1912   513,000                            Vap 0°44   84.77 Steam     316.09          286.24                .1912   513,000                            SatLiquid45  224.94 Steam     316.09          286.42                .1912   513,000                            Vap 0°51  • Brine     391.80          305.83                1.4143                      3,794,00052  • Brine     316.09          241.48                1.4143                      3,794,00053  • Brine     316.09          241.48                1.6055                      4,307,00056  • Brine     240.46          177.19                1.6055                      4,307,00057  • Brine     170.00          117.30                1.6055                      4,307,00060  393.10 .7854     165.00          85.93 1.0000                      2,682,656                            SatLiquid61  391.10 .7854     235.46          455.64                .2601   697,740                            Wet .341262  391.10 .7854     235.46          455.64                .7399 1,984,916                            Wet .341266  391.10 .7854     235.46          455.64                1.0000                      2,682,656                            Wet .341269  389.10 .7854     269.56          558.84                1.0000                      2,682,656                            Wet .224870  387.10 .7854     311.08          720.70                1.0000                      2,682,656                            Wet .05__________________________________________________________________________ 
    
     Where the initial geofluid leaving the geothermal well contains a relatively large quantity of steam, it is preferable to expand and then condense the geothermal steam in two or more steps, rather than in one step as shown in FIG. 1. In such a case, heating and evaporation of the working fluid is performed alternately by cooling the geothermal liquid and condensing the geothermal steam. 
     In FIG. 2, a system which includes two stages of expansion of geothermal steam is presented. It differs from the system shown in FIG. 1 by the fact that after the first stage of expansion, part of the expanded steam with parameters as at 43 is sent into heat exchanger 103. A portion of partially expanded steam is further expanded in a second steam turbine 204 and then condensed in a second steam condenser shown as heat exchanger 203, from which it is pressurized via pump 201 and then recombined with geothermal liquid. Geothermal liquid is used to heat the working fluid of the power cycle between those two steam condensers in heat exchanger 204. 
     While the present invention has been described with respect to a number of preferred embodiments, those skilled in the art will appreciate a number of variations and modifications of those embodiments. For example, the number of heat exchangers may be increased or decreased. In addition, the geothermal steam may undergo more than two expansions depending on the steam content of the geofluid. Thus, it is intended that the appended claims cover all such variations and modifications as fall within the true spirit and scope of the present invention.