Abstract:
A new thermodynamic cycle is disclosed for converting energy from a moderate temperature stream, external source into useable energy using a working fluid comprising of a mixture of a low boiling component and a higher boiling component and including a higher pressure circuit and a lower pressure circuit. The cycle is designed to improve the efficiency of the energy extraction process by recirculating a portion of a liquid stream prior to further cooling. The new thermodynamic process and system for accomplishing the improved efficiency is especially well-suited for streams from moderate-temperature geothermal sources.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a thermodynamic cycle and an apparatus for implementing the thermodynamic cycle for converting a portion of thermal energy associated with superheated stream of a multi-component fluid in a high efficient manner. 
   More particularly, the present invention relates to a thermodynamic cycle and an apparatus for implementing the thermodynamic cycle for converting a portion of thermal energy associated with superheated stream of a multi-component fluid in a high efficient manner, where the cycle utilizes four different compositions of the multi-component fluid and heats, vaporizes three of the compositional streams and superheats one of the compositional streams to form the superheated stream from which useable energy is produced. The cycle is designed to use with moderate temperature heat source stream. 
   2. Description of the Related Art 
   In U.S. Pat. No. 6,769,256, issued Aug. 31, 2004, a system is disclosed which utilizes heat from moderate and low temperature heat sources. This system is presented in three variants ranging from a highest efficiency and highest complexity variant, to a moderate variant, and finally to a lowest efficiency and lowest complexity variant. A detailed calculation of this system demonstrates than when the initial temperature of the heat source exceeds 325–330° F., the high complexity and moderate variants of the system (in which the working fluid is not fully vaporized, and the remaining liquid is recycled) degenerate and are thus in effect converted into the lowest complexity, lowest efficiency variant (in which all working fluid is vaporized). 
   Although prior systems for improving energy extraction from moderate temperature geothermal or other heat sources have been disclosed, there is still a need in the art for an improved and simplified system for energy extraction from moderate temperature sources. 
   SUMMARY OF THE INVENTION 
   The present invention provides an energy extraction apparatus comprising eight heat exchangers, at least three mixers, at least three splitters, two pumps, a separator and a turbine, where the heat exchangers are designed to produce a fully condensed basic working fluid stream and a superheated working fluid stream utilizing an external coolant stream, an external heat source stream and two working fluid streams. 
   The present invention also provides a method for energy extraction including the steps of 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a flow diagram of a preferred embodiment of a power cycle and system for utilizing moderate temperature heat sources of this invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The inventors have found that an improved power cycle and system for utilizing moderate temperature heat sources can be designed. The system has been developed for the purpose of producing useful power from heat sources, such as geothermal fluids, waste heat sources and other similar sources, with a moderate initial temperature, i.e., a temperature between about 325° F. and about 500° F. The inventor has found that the system of this invention has higher efficiency than the systems described in the prior art with heat sources whose initial temperatures are greater than or equal to 325° F. 
   The proposed system uses, as a working fluid, a multi-component mixture of at least two components with different normal boiling temperatures. In the preferred embodiment of the system, this mixture consists of water and ammonia, but other working fluids, such as a mixture of hydrocarbons, freons or other substances can be used as well. 
   Referring now to  FIG. 1 , the power cycle and system, generally  1 , is shown. A fully condensed working fluid stream  3  having a high concentration of a low boiling component of a multi-component fluid, hereafter referred to as a basic solution, and having parameters as at a point  1  enters into a pump, P 1 . The stream  102  is pressurized to a desired higher pressure and becomes a higher pressure stream  5  having parameters as at a point  2 . The stream  104  having the parameters as at the point  2  then passes through a recuperative pre-heater or a second heat exchanger HE 2 , where the stream  104  is heated in counterflow by a returning stream  7  having parameters as at a point  26  of condensing basic solution in a first heat exchange process  26 - 27  or  2 - 3  described below. The first heat exchange process  26 - 27  produces a pre-heated stream  9  having parameters as at a point  3  and a condensed stream  11  having parameters as at a point  27 . The parameters of the pre-heated stream  108  correspond to a state of saturated or slightly subcooled liquid. 
   The pre-heated stream  108  having the parameters as at the point  3  is then divided into two substreams  13  and  15  having parameters as at points  4  and  5 , respectively. The basic solution substream  112  having the parameters as at the point  4  passes through a fourth heat exchanger HE 4 , where it is heated and partially vaporized in counterflow with a fifth heat source fluid stream  17  having parameters as at a point  42  in a second heat exchange process  42 - 43  or  4 - 6  as described below. The second heat exchange process  42 - 43  produces a stream  19  having parameters as at a point  6  and a sixth cooled heat source stream  21  having parameters as at a point  43 . The basic solution substream  114  having the parameters as at the point  5  passes through a recuperative boiled-condenser or third heat exchanger HE 3 , where it is heated and partially vaporized in counterflow with a condensing working fluid stream  23  having parameters as at a point  20  in a third heat exchange process  20 - 21  or  5 - 7  as described below. The third heat exchange process  20 - 21  produces a stream  25  having obtains parameters as at a point  7  and a partially condensed working fluid stream  27  having parameters as at a point  21 . In the preferred embodiment of this system, the parameters of the streams  118  and  124  having the parameters as at the points  6  and  7 , respectively, are identical or close to identical, where close to identical means that the parameters of each of the stream  118  and  124  are with about 5% of each other. 
   Thereafter, the basic solution streams  118  and  124  having parameters as at the points  6  and  7 , respectively, are combined forming a stream  29  having parameters as at a point  8 . The parameters of the stream  128  are such that the stream  128  is generally in a state of a liquid-vapor mixture. The stream  128  having the parameters as at the point  8  is then sent through a seventh heat exchanger HE 7 , where it is further heated and vaporized in counterflow with a third cooled heat source fluid steam  31  having parameters as at a point  46  in a fourth heat exchange process  46 - 42  or  8 - 14  as described below. The fourth heat exchange process  46 - 42  produces a first mixed stream  33  having parameters as at a point  14  and a fifth cooled heat source stream  116  having parameters as at a point  42 . In the preferred embodiment of this system, the parameters of the basic working fluid stream  132  is such that the stream  132  is either in a state of saturated vapor, i.e., fully vaporized, or has some very small amount wetness generally less than about 5% wetness. 
   Thereafter, the stream  132  having the parameters as at the point  14  is combined with a liquid stream  37  having parameters as at a point  29 , forming a working solution stream  39  having parameters as at a point  10 . The stream  136  having the parameters as at the point  29  is referred to herein as a recirculating solution. The parameters of the stream  136  at the point  29  is such that the stream  136  is in a state of saturated or slightly subcooled liquid as described below. The working solution stream  138  having the parameters as at the point  10  then passes though a fifth heat exchanger HE 5 , where it is heated and vaporized in counterflow with a first cooled heat source fluid stream  41  having parameters as at a point  41  in a fifth heat exchange process  41 - 44  or  10 - 11  as described below. The fifth heat exchange process  41 - 44  produces a second mixed stream  43  having parameters as at a point  11  and a second cooled heat source stream  45  having the parameters as at a point  44 . 
   In the preferred embodiment of this system, the parameters of the stream  142  at the point  11  is such that the stream  142  is in a state of a saturated vapor. The stream  142  having the parameters as at the point  11  is sent into a sixth heat exchanger HE 6 , where it is superheated in counterflow with a heat source fluid stream  47  having parameters as at a point  40  in a sixth heat exchange process  40 - 41  or  11 - 17  as described below. The sixth heat exchange process  40 - 41  produces a fully vaporized and superheated stream  49  having obtains parameters as at a point  17  and the first cooled heat source stream  140  having the parameters as at the point  41 . The stream  148  having the parameters as at the point  17  then enters a turbine T 1 , where it is expanded, producing power, and the spent stream  122  having parameters as at a point  20 . 
   The spent stream  122  having the parameters as at the point  20  is then sent into the third heat exchanger HE 3 , where it is cooled and partially condensed, releasing heat for the third heat exchange process  20 - 21  as described above forming the stream  126  having the parameters as at the point  21 . The parameters of the stream  126  at the point  21  is in a state of a vapor-liquid mixture. The stream  126  with parameters as at point  21  then enters into a separator S 1 , where it is separated into a saturated vapor stream  51  having parameters as at a point  22 , and a saturated liquid stream  53  having parameters as at a point  23 . The concentration of a low boiling component in the vapor stream  150  having the parameters as at the point  22  must be higher or equal to the concentration of the low boiling component in the basic working solution as described above. 
   The liquid stream  152  having the parameters as at the point  23  is divided into two substreams  55  and  57  having parameters as at points  24  and  25 , respectively. The liquid stream  156  having the parameters as at the point  25  is then combined with the vapor stream  150  having the parameters as at the point  22 , forming a basic working solution stream  106  having the parameters as at the point  26 . The stream  106  of basic working solution having the parameters as at the point  26  then passes through the recuperative pre-heater or second heat exchanger HE 2 , where it is cooled and partially condensed, releasing heat for process  2 - 3  or  26 - 27  as described above becoming the stream  110  having parameters as at point  27 . 
   The stream  110  of basic working solution with parameters as at point  27  is then sent through a condenser or first heat exchanger HE 1 , where it is cooled and fully condensed, in counterflow with a stream  59  of coolant (air or water) stream having parameters as at a point  50  in a seventh heat exchange process  50 - 51  or  27 - 1 . The seventh heat exchange process  50 - 51  produces a spent coolant stream  61  having parameters as at a point  51  and the stream  102  having parameters as at the point  1  as described above. 
   The stream  154  of liquid with the parameters as at the point  24  as described above enters into a second pump P 2 , where its pressure is increased to form a higher pressure stream  63  having parameters as at a point  9 . The parameters of the stream  162  are such that the stream  162  correspond to a state of subcooled liquid. The stream  162  having the parameters as at point  9  then passes through an eighth heat exchanger HE 8 , where it is heated in counterflow with a fourth cooled heat source fluid stream  65  having parameters as at a point  47  in an eighth heat exchange process  47 - 48  or  9 - 29  described below. The eighth heat exchange process  47 - 48  produces a seventh cooled heat source stream  67  having parameters as at a point  48  and the stream  136  having the parameters as at the point  29 . The parameters of the stream  136  are such that the stream  136  corresponds to a state of saturated or slightly subcooled liquid. Thereafter, the stream  136  having the parameters as at the point  29  is combined with the stream  132  having the parameters as at the point  14 , forming the stream  138  having the parameters as at the point  10  as described above. 
   The heat source fluid stream  146  having the initial parameters as at the point  40 , passes through the sixth heat exchanger HE 6 , where it is cooled, providing heat for process  11 - 17  as described above forming the first cooled heat source stream  140  having the parameters as at the point  41 . Thereafter, the first cooled heat source stream  140  having the parameters as at the point  41  passes through the fifth heat exchanger HE 5 , where it is cooled, providing the fifth heat exchange process  10 - 11  as described above forming the stream  144  having the parameters as at the point  44 . Thereafter, the stream  144  of heat source fluid having the parameters as at the point  44  is divided into two substreams  130  and  164  having the parameters as at the points  46  and  47 , respectively. 
   The stream  130  having the parameters as at the point  46  passes through the seventh heat exchanger HE 7 , where it is cooled, providing heat for the fourth heat exchange process  8 - 14  as described above to form the fifth cooled heat source stream  116  having the parameters as at the point  42 . The stream  116  of heat source fluid having the parameters as at the point  42  then passes through the fourth heat exchanger HE 4 , where it is further cooled, providing heat for the second heat exchange process  4 - 6  as described above to form the sixth cooled heat source stream  120  having the parameters as at the point  43 . 
   The stream  164  of heat source fluid having the parameters as at the point  47  passes through the eighth heat exchanger HE 8 , where it is cooled, providing heat for the eighth heat exchange process  9 - 29  as described above to form the seventh cooled heat source stream  166  having the parameters as at the point  48 . Thereafter, the sixth cooled heat source streams  120  and the seventh cooled heat source  166  of heat source fluid having the parameters as at the points  43  and  48  are combined, forming a spent heat source stream  69  having parameters as at a point  49  which is sent out of the system. 
   The cycle is closed. 
   The complete vaporization of the basic solution and the preheating of the recirculating solution prior to the combination of the basic solution with the recirculating solution reduces the irreversibility in the process of mixing of these two streams and therefore increases the efficiency of the overall process. Moreover, this approach increases the heat load in the process cooling the heat source fluid from point  44  down. This, in turn, requires an increase of a flow rate of the heat source fluid per unit of a flow rate of the basic solution. As a result, a flow rate of the recirculating solution can also be increased leading to an increase of a flow rate of the working solution passing through the turbine, and thus an increase in a power output. At the same time, a flow rate of the basic solution passing through the final condenser or first heat exchanger HE 1  of the seventh heat exchange process  27 - 1 , remains unchanged, and a quantity of heat rejected in the first heat exchanger HE 1  also remains unchanged. As a result, the overall efficiency of the system is increased. 
   A summary of a performance of the system of this invention is presented in Table 1 and the parameters of all key points described above are tabulated in Table 2. 
   Comparing these results with the results of the system presented in the prior art shows that the system of this invention within a temperatures range between about 325° F., and about 500° F. has a net thermal efficiency that is from 7% to 10% higher than the efficiency of the system presented in the prior art. 
   
     
       
             
           
             
             
             
             
             
           
             
             
           
             
             
             
           
             
           
             
             
           
         
             
               TABLE 1 
             
             
                 
             
           
           
             
               Plant Performance Summary 
             
             
                 
             
           
        
         
             
               Heat in 
               30,470.49 
               kW 
               538.65 
               Btu/lb 
             
             
               Heat rejected 
               24,800.44 
               kW 
               438.41 
               Btu/lb 
             
             
               Turbine enthalpy Drops 
               5,803.26 
               kW 
               102.59 
               Btu/lb 
             
             
               Gross Generator Power 
               5,533.70 
               kW 
               97.82 
               Btu/lb 
             
             
               Process Pumps (−2.35) 
               −144.79 
               kW 
               −2.56 
               Btu/lb 
             
             
               Cycle Output 
               5,388.91 
               kW 
               95.26 
               Btu/lb 
             
             
               Other Pumps and Fans (−2.25) 
               −136.61 
               kW 
               −2.41 
               Btu/lb 
             
             
               Net Output 
               5,252.30 
               kW 
               92.85 
               Btu/lb 
             
             
               Gross Generator Power 
               5,533.70 
               kW 
               97.82 
               Btu/lb 
             
             
               Cycle Output 
               5,388.91 
               kW 
               95.26 
               Btu/lb 
             
             
               Net Output 
               5,252.30 
               kW 
               92.85 
               Btu/lb 
             
           
        
         
             
               Net thermal efficiency 
               17.24% 
             
             
               Second Law Limit 
               29.50% 
             
             
               Second Law Efficiency 
               58.43% 
             
           
        
         
             
               Specific Brine Consumption 
               95.20 
               lb/kW-hr 
             
             
               Specific Power Output 
               10.50 
               W-hr/lb 
             
             
                 
             
           
        
         
             
               Overall Heat Balance Btu/lb 
             
             
                 
             
           
        
         
             
               Heat In: 
               Source + pumps = 538.65 + 2.35 = 541.00 
             
             
               Heat Out: 
               Turbines + condenser = 102.59 + 438.41 = 541.00 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
             
             
             
             
             
             
             
           
             
           
             
             
             
             
             
             
             
             
             
           
             
           
             
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Parameters of Key Points 
             
             
                 
             
           
           
             
               Working Fluid 
             
           
        
         
             
                 
               X 
               T 
               P 
               H 
               S 
               Ex 
               G rel 
               Ph. 
               Wetness 
             
             
               Pt. 
               lb/lb 
               ° F. 
               psia 
               Btu/lb 
               Btu/lb-R 
               Btu/lb 
               G/G = 1 
               lb/lb 
               or T ° F. 
             
             
                 
             
             
               1 
               0.9000 
               69.81 
               115.587 
               8.7511 
               0.0717 
               53.6564 
               1.00000 
               Mix 
               1 
             
             
               2 
               0.9000 
               71.09 
               474.724 
               10.8310 
               0.0725 
               55.3018 
               1.00000 
               Liq 
               −95.67° F. 
             
             
               3 
               0.9000 
               165.00 
               464.724 
               121.8394 
               0.2649 
               67.9204 
               1.00000 
               Mix 
               1 
             
             
               4 
               0.9000 
               165.00 
               464.724 
               121.8394 
               0.2649 
               67.9204 
               0.39329 
               Mix 
               1 
             
             
               5 
               0.9000 
               165.00 
               464.724 
               121.8394 
               0.2649 
               67.9204 
               0.60671 
               Mix 
               1 
             
             
               6 
               0.9000 
               227.47 
               462.724 
               533.3776 
               0.9076 
               150.7830 
               0.39329 
               Mix 
               0.1799 
             
             
               7 
               0.9000 
               227.47 
               462.724 
               533.3776 
               0.9076 
               150.7830 
               0.60671 
               Mix 
               0.1799 
             
             
               8 
               0.9000 
               227.47 
               462.724 
               533.3778 
               0.9076 
               150.7830 
               1.00000 
               Mix 
               0.1799 
             
             
               9 
               0.3811 
               170.79 
               464.724 
               48.6950 
               0.2189 
               15.9998 
               0.17026 
               Liq 
               −114.35° F. 
             
             
               10 
               0.8245 
               284.57 
               462.224 
               606.6533 
               1.0093 
               171.7561 
               1.17026 
               Mix 
               0.1686 
             
             
               11 
               0.8245 
               322.52 
               460.724 
               757.8078 
               1.2073 
               221.6375 
               1.17026 
               Vap 
               −0.1° F. 
             
             
               14 
               0.9000 
               284.57 
               462.224 
               679.1791 
               1.1111 
               192.5432 
               1.00000 
               Mix 
               0.0271 
             
             
               17 
               0.8245 
               361.00 
               460.224 
               784.8355 
               1.2411 
               231.3555 
               1.17026 
               Vap 
               38.6° F. 
             
             
               20 
               0.8245 
               232.47 
               121.587 
               697.1728 
               1.2635 
               132.2385 
               1.17026 
               Mix 
               0.0442 
             
             
               21 
               0.8245 
               170.00 
               119.587 
               483.8153 
               0.9440 
               82.2867 
               1.17026 
               Mix 
               0.2499 
             
             
               22 
               0.9722 
               170.00 
               119.587 
               629.3327 
               1.1858 
               104.8123 
               0.87779 
               Mix 
               0 
             
             
               23 
               0.3811 
               170.00 
               119.587 
               47.0820 
               0.2183 
               14.6817 
               0.29247 
               Mix 
               1 
             
             
               24 
               0.3811 
               170.00 
               119.587 
               47.0820 
               0.2183 
               14.6817 
               0.17026 
               Mix 
               1 
             
             
               25 
               0.3811 
               170.00 
               119.587 
               47.0820 
               0.2183 
               14.6817 
               0.12221 
               Mix 
               1 
             
             
               26 
               0.9000 
               170.00 
               119.587 
               558.1742 
               1.0676 
               93.7972 
               1.00000 
               Mix 
               0.1222 
             
             
               27 
               0.9000 
               112.84 
               117.587 
               447.1658 
               0.8843 
               76.5215 
               1.00000 
               Mix 
               0.2273 
             
             
               29 
               0.3811 
               284.57 
               462.224 
               180.6858 
               0.4111 
               49.6667 
               0.17026 
               Mix 
               1 
             
             
                 
             
           
        
         
             
               Heat Source 
             
           
        
         
             
                 
               X 
               T 
               P 
               H 
               S 
               Ex 
               G rel 
               Ph. 
             
             
               Pt. 
               lb/lb 
               ° F. 
               psia 
               Btu/lb 
               Btu/lb-R 
               Btu/lb 
               G/G = 1 
               lb/lb 
             
             
                 
             
             
               40 
               BRINE 
               370.00 
               14.693 
               352.5340 
               0.5047 
               94.4232 
               2.58868 
               Liq 
             
             
               41 
               BRINE 
               358.29 
               14.693 
               340.3156 
               0.4899 
               89.7893 
               2.58868 
               Liq 
             
             
               42 
               BRINE 
               234.59 
               14.693 
               211.2994 
               0.3189 
               48.2247 
               2.40263 
               Liq 
             
             
               43 
               BRINE 
               170.00 
               14.693 
               143.9340 
               0.2171 
               32.9407 
               2.40263 
               Liq 
             
             
               44 
               BRINE 
               292.77 
               14.693 
               271.9834 
               0.4028 
               65.9851 
               2.58868 
               Liq 
             
             
               46 
               BRINE 
               292.77 
               14.693 
               271.9834 
               0.4028 
               65.9851 
               2.40263 
               Liq 
             
             
               47 
               BRINE 
               292.77 
               14.693 
               271.9834 
               0.4028 
               65.9851 
               0.18605 
               Liq 
             
             
               48 
               BRINE 
               176.96 
               14.693 
               151.1910 
               0.2285 
               34.3364 
               0.18605 
               Liq 
             
             
               49 
               BRINE 
               170.50 
               14.693 
               144.4556 
               0.2179 
               33.0388 
               2.58868 
               Liq 
             
             
                 
             
           
        
         
             
               Coolant 
             
           
        
         
             
                 
               X 
               T 
               P 
               H 
               S 
               Ex 
               G rel 
               Ph. 
               T 
             
             
               Pt. 
               lb/lb 
               ° F. 
               psia 
               Btu/lb 
               Btu/lb-R 
               Btu/lb 
               G/G = 1 
               lb/lb 
               ° F. 
             
             
                 
             
             
               50 
               water 
               51.70 
               54.693 
               19.9395 
               0.0394 
               0.1617 
               15.6119 
               Liq 
               −235 
             
             
               51 
               water 
               51.81 
               64.693 
               20.0833 
               0.0397 
               0.1914 
               15.6119 
               Liq 
               −245.84 
             
             
               52 
               water 
               79.92 
               54.693 
               48.1655 
               0.0932 
               0.9127 
               15.6119 
               Liq 
               −206.78 
             
             
                 
             
           
        
       
     
   
   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.