Patent Publication Number: US-2012039701-A1

Title: Closed Cycle Brayton Cycle System and Method

Description:
BACKGROUND 
     1. Technical Field 
     Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for increasing an efficiency of a closed cycle Brayton cycle system. 
     2. Discussion of the Background 
     During the past years, the use of gas turbines for providing thrust for most aircrafts, generating electricity, etc. has become popular. Gas turbines operate on a Brayton cycle and have a working fluid (usually air). The gas turbines may use an open or a closed Brayton cycle.  FIG. 1  illustrates a system  10  that operates based on the open Brayton cycle. Fresh air is provided at a compressor  12  in step  1 . After being compressed, the air is provided in step  2  to an internal combustor  14 . At the same time, fuel  16  is injected and ignited into the combustion chamber  14  for heating the compressed air. After being heated, the high temperature, high pressure gases from the combustion chamber are provided in step  3  to a turbine  18 . The exhaust gases rotate a shaft  20  of the turbine  18  for producing rotational energy  22 . The exhaust gasses, having now a lower temperature (500° C.) and pressure are discharged in step  4  into the atmosphere. 
     In terms of the thermodynamic processes taking place in this open cycle, it is noted that an isentropic compression  24  takes place in the compressor  12 , a constant pressure heat addition  26  takes place in the combustion chamber  14 , an isentropic expansion  28  takes place in the turbine  18  and a constant pressure heat rejection  30  takes place when the exhaust gases are released into the environment. Those skilled in the art would appreciate that thermodynamic processes  24 ,  26 ,  28  and  30  are ideal processes, i.e., the air in the various elements of the system  10  do not experience exactly these transformations but transformations that are substantially closed to the ideal transformations. However, for the purpose of characterizing a real life system, it is accepted in the art to use ideal transformations that only approximate the real transformations. 
     Closed Brayton cycle systems have been developed to address some concerns related to the open Brayton cycle. A closed Brayton cycle system  40  is shown in  FIG. 2 . This system includes the same compressor  12  and turbine  18  but the combustion chamber  14  is replaced by a first heat exchanger  42  and the medium used through the system  40  is recirculated via a second heat exchanger  44 , i.e., not released into atmosphere. In this way, no part of the medium is released into the atmosphere. However, for this kind of system, a heat source need to be provided such that heat is transferred via the first heat exchanger  42  to the medium and this heat source may be nuclear, geothermal, solar, conventional, electric, etc. 
     However, the existing closed Brayton cycle systems are not very efficient. Accordingly, it would be desirable to provide systems and methods that increase the efficiency of the closed Brayton cycle systems. 
     SUMMARY 
     According to one exemplary embodiment, there is a Brayton cycle unit. The unit includes a multistage compressor configured to compress a flowing medium; a first heat exchanger fluidly connected to the multistage compressor and configured to transfer heat from a working medium passing the first heat exchanger to the compressed flowing medium; an expander fluidly connected to the first heat exchanger and configured to expand the heated compressed flowing medium for producing a rotation of a shaft of the expander; and a second heat exchanger fluidly connected between the expander and the compressor and configured to remove heat from the expanded flowing medium. A path of the flowing medium through the multistage compressor, the first heat exchanger, the expander and the second heat exchanger is closed. There is at least one inter-cooler mechanism between first and second stages of the multistage compressor configured to cool the flowing medium between the first and second stages to a predetermined temperature. 
     According to still another exemplary embodiment, there is a Brayton cycle system. The system includes a closed cycle Brayton unit; an external source circuit configured to provide heat to the closed cycle Brayton unit; and a cooling circuit configured to remove heat from the closed cycle Brayton unit. The closed cycle Brayton unit includes a multistage compressor configured to cool a flowing medium between the stages to a predetermined temperature. 
     According to yet another exemplary embodiment, there is a method for rotating a shaft of an expander that is part of a closed cycle Brayton system. The method includes compressing a flowing medium with a multistage compressor; cooling the flowing medium with at least one inter-cooler mechanism between first and second stages of the multistage compressor to a predetermined temperature; circulating the compressed flowing medium to a first heat exchanger fluidly connected to the multistage compressor; transferring heat from a working medium passing the first heat exchanger to the compressed flowing medium; circulating the heated flowing medium to an expander that is fluidly connected to the first heat exchanger; expanding the heated flowing medium in the expander for rotating a shaft of the expander; circulating the expanded flowing medium to a second heat exchanger that is fluidly connected between the expander ( 64 ) and the multistage compressor; removing heat from the expanded flowing medium in the second heat exchanger; and circulating the cooled flowing medium back to the multistage compressor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings: 
         FIG. 1  is a schematic diagram of an open cycle Brayton cycle unit; 
         FIG. 2  is a schematic diagram of a closed cycle Brayton cycle unit; 
         FIG. 3  is a schematic diagram of a closed cycle Brayton cycle system according to an exemplary embodiment; 
         FIG. 4  is a schematic diagram of a multistage compressor with a bull gear; 
         FIG. 5  is a schematic diagram of a closed cycle Brayton cycle system according to an exemplary embodiment; 
         FIG. 6  illustrates a pressure versus enthalpy phase space for a flowing medium through the closed cycle Brayton cycle system according to an exemplary embodiment; and 
         FIG. 7  is a flow chart illustrating a method for generating energy according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a system having an integrally geared compressor (technology to be discussed later) and a multiple stage radial or axial expander. However, the embodiments to be discussed next are not limited to these systems, but may be applied to other systems that use multistage compressors and expanders in a closed cycle. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
     According to an exemplary embodiment illustrated in  FIG. 3 , a novel closed Brayton cycle system  60  may include a compressor  62  fluidly connected to an expander  64 . The compressor  62  may be a multistage compressor and the expander  64  may be a multistage expander. In one application, the compressor  62  has four stages and uses SRL technology (to be discussed next) and the expander  64  is a two-stage radial expander. However, other types of compressors and expanders may be used. 
     SRL or integrally geared compressors (produced by Nuovo Pignone S.p.A., Florence, Italy) are used in several petrochemical applications, either for low-flow/high pressure, or high-flow/low pressure conditions. This type of compressor, which is illustrated in  FIG. 4 , has a bull gear  66  and from one to four high speed pinions  68 . One or two impellers  70  can be mounted on each pinion-shaft as show in  FIG. 4 . Inter-cooling mechanisms  72  may be provided between the stages for cooling in a desired fashion the compressed medium when passing from one stage of the compressor to another stage of the compressor. 
     Returning to  FIG. 3 , a flowing medium that is compressed by the compressor  62  follows the following closed path inside system  60 . From the compressor  62 , the flowing medium enters a recuperator  74 , a first heat exchanger  76 , the expander  64 , again the recuperator  74 , a second heat exchanger  78  and goes back to the compressor  62 . Thus, the flowing medium is confined to this closed path and does not contaminate or interacts with the ambient or other fluids. The flowing medium may be CO 2  or another fluid having a high molecular density. In one exemplary embodiment, system  60  is designed such that the CO 2  remains in a gas phase irrespective of the location in the system, i.e., no phase change takes place inside system  60 . The expander  64  may be connected to a power generation unit  63  for producing electricity. The expander  64 , compressor  62  and power generation unit  63  may share a same shaft  65 , as shown in  FIG. 3 . As would be recognized by those skilled in the art, the expander  64  may be connected to other devices (e.g., a pump) for providing the necessary energy to activate them. 
     Two more circuits are shown in  FIG. 3  and discussed next. The first heat exchanger  76  is configured to transfer heat from an external source circuit  80  to the flowing medium of the Brayton closed circuit  60 . In one exemplary embodiment, the external source circuit  80  may include a compressor that absorbs ambient air at inlet  82   a  and output compressed air at outlet  82   b . Prior to providing this compressed air to a combustion chamber  84 , the compressed air is flown through a heat exchanger  86  for heating the compressed air. The heat source is the exhaust gasses from the combustion chamber  84 . It is noted that path  86   a  of the compressed air from compressor  82  and path  86   b  of the combustion chamber  84  do not intersect but are distributed so that heat is exchanged between the two. 
     The heated and compressed air is then provided to the combustion chamber  84  where fuel is inserted at inlet  84   a . The compressed air is heated by the burning of the fuel. The hot mixture of air, fuel, and exhaust gas may enter an expander  87  for generating energy or may be supplied directly to the first heat exchanger  76  for transferring heat to the flowing medium in the closed Brayton cycle system  60 . After removing part of the heat of the gas exhaust, the gas exhaust may enter the heat exchanger  86  to heat the compressed air from the compressor  82  prior to being disposed in the atmosphere at outlet  86   c . A temperature of the exhaust gas may be around 150° C. 
     The second heat exchanger  78  is configured to transfer heat from the flowing medium in the closed Brayton cycle system  60  to a cooling circuit  90 . The cooling circuit  90  may include a working medium (e.g., water) that is circulated through the second heat exchanger  78  for removing heat from the flowing medium coming from the expander  64 . The heat is provided to a sink  92 , for example, a water tower. 
     Recuperator  74  includes at least two separate paths  74   a  and  74   b  that accommodate the flowing medium coming from the compressor  62  and the expander  64 , respectively. Recuperator  74  is configured to remove heat from the flowing medium coming from the expander and to provide that heat to the flowing medium coming from the compressor  62 . 
     A more specific example in terms of pressures and temperatures of the closed Brayton cycle system  60  is illustrated in  FIG. 5 . This figure shows that compressor  62  increases the pressure of the flowing medium from around 1 bar to around 35 bar and the first heat exchanger  76  increases the temperature of the flowing medium to around 1200° C. prior to reaching the expander  64 . This system shows a higher efficiency than the existing systems as the flowing medium is cooled between the stages of the compressor  62  and heat extraction in the external source circuit  80  is up to 150° C. In one application, the efficiency of the system shown in  FIG. 5  reached 49%. 
     In an exemplary embodiment,  FIG. 6  shows a P-H diagram (P indicates the pressure and H indicates the enthalpy of the flowing medium at a certain point) for the flowing medium (CO 2 ) of the closed Brayton cycle system  60 . As previously discussed, those skilled in the art would appreciate that the thermodynamic transformations shown in  FIG. 6  are ideal and are meant to approximate the real transformations that take place in the real system  60 . However, these ideal transformations are a good indicator of the characteristics of the real system. 
     Various points are shown in  FIG. 6  and they correspond to physical locations in the closed Brayton cycle system  60  as will be described next. Consider that the CO 2  enters the compressor  62  at  100  at a certain temperature (close to 15° C.) and pressure (1 bar). The CO 2  is compressed during a first stage (assume that the compressor has four stages) from 1 bar to around 4 bars so that the CO 2  reaches point  102 . At this point, the temperature of the compressed CO 2  may reach a value around 70 to 100° C. The compression of the CO 2  between points  100  and  102  is isentropic. Once the CO 2  exits the first stage and prior to entering the second stage, the compressed CO 2  is cooled in step  105  to a predetermined temperature, e.g., around 25° C. It is noted that the first cooling step  105  takes place between points  102  and  104  at a substantially constant pressure. Next, the CO 2  enters the second stage of the compressor where its pressure further increases to, for example, around 8 bar when reaching point  106 . The temperature of the CO 2  also increases to around 70 to 100° C. at point  106 . From here, the CO 2  undergoes a second cooling step  107  that takes place between points  106  and  108 . The temperature of the CO 2  is reduced again to around the predetermined temperature. The process describing the compression of the CO 2  in the compressor  62  may be described in the phase space defined by pressure versus enthalpy as having a see-saw shape. 
     The CO 2  is further compressed between points  108  and  110  to a pressure around 17 bar and between points  112  and  114  to a final pressure of around 34 bar. Between the third stage and the fourth stage, the CO 2  is again cooled between points  110  and  112  during step  111  in order to bring the temperature of the CO 2  to the predetermined temperature. It is noted that the predetermined temperature may depend on the medium used, the final pressure of the medium, and other parameters of the system. 
     Once at point  114 , the compressed CO 2  leaves the compressor  62  (in  FIG. 3 ) and enters the recuperator  74 . The thermodynamic process  115  taking place inside the recuperator  74  is bounded by points  114  and  116  and the CO 2  increases its temperature while maintaining a substantially constant pressure during this process. Further heat is added to the CO 2  along path  117  bounded by points  116  and  118 , which correspond to the flowing medium being heated inside the first heat exchanger  76 . It is noted that the heat provided by the first heat exchanger  76  is produced in the combustion chamber  84 . 
     The CO 2  enters then expander  64  and the corresponding thermodynamic process is bounded by points  118  and  120  and this expansion is an isentropic expansion. It is noted that the temperature of the flowing medium at point  118  may be around 1200° C. while the temperature at point  120  may be around 600° C. To further reduce the temperature of the flowing medium at point  120  and to further extract energy, the flowing medium enters recuperator  74 , which corresponds to a recuperation process  121  that is bounded by points  120  and  122 . This process  121  takes place at substantially constant pressure (close to atmospheric pressure). The temperature drop for this process is around 500° C. However, other values may be implemented. The CO 2  is further cooled from point  122  to point  100  (cooling step  123 ) by circulating it through the second heat exchanger  78 . During this cooling process, the temperature and pressure of the CO 2  may reach the ambient temperature and pressure and a new cycle may be started by sending the CO 2  back to the compressor. 
     The inter-cooling steps  105 ,  107  and  111  (more or less of these steps may be implemented) help to improve the efficiency of the whole cycle. Other features of the novel embodiments, e.g., running a closed Brayton cycle system, using the CO 2  as the flowing medium and having the CO 2  in a gaseous phase through the system also help to improve the efficiency of the whole cycle. According to an exemplary embodiment, all these four features may be combined. However, not all features are required for achieving an improved efficiency Brayton cycles system. 
     If the system shown in  FIG. 3  is used with an external combustion chamber  84  (e.g., a furnace), an advantage of this setup is the freedom to use low grade fuels for combustion. Also, the system shown in  FIG. 3  is a green product as the exhaust gases from the combustion chamber  84  are discharged into the atmosphere at a lower temperature (around 150° C.) than the existing systems (around 500° C.). This feature is achieved due in part to the heat exchanger  86 . 
     Next, a method for producing energy based on a closed cycle CO 2  Brayton system is discussed with regard to  FIG. 7 . The method includes a step  700  of compressing a flowing medium with a multistage compressor; a step  702  of cooling the flowing medium with at least one inter-cooler mechanism between first and second stages of the multistage compressor to a predetermined temperature; a step  704  of circulating the compressed flowing medium to a first heat exchanger fluidly connected to the multistage compressor; a step  706  of transferring heat from a working medium passing the first heat exchanger to the compressed flowing medium; a step  708  of circulating the heated flowing medium to an expander that is fluidly connected to the first heat exchanger; a step  710  of expanding the heated flowing medium in the expander for rotating a shaft of the expander; a step  712  of circulating the expanded flowing medium to a second heat exchanger that is fluidly connected between the expander and the multistage compressor; a step  714  of removing heat from the expanded flowing medium in the second heat exchanger; and a step  716  of circulating the cooled flowing medium back to the multistage compressor. 
     The disclosed exemplary embodiments provide a system and a method for increasing an efficiency of a closed cycle Brayton system. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. 
     Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. 
     This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.