Abstract:
A turbine-generator device for use in electricity generation using heat from industrial processes, renewable energy sources and other sources. The generator may be cooled by introducing into the gap between the rotor and stator liquid that is vaporized or atomized prior to introduction, which liquid is condensed from gases exhausted from the turbine. The turbine has a universal design and so may be relatively easily modified for use in connection with generators having a rated power output in the range of 50 KW to 5 MW. Such modifications are achieved, in part, through use of a modular turbine cartridge built up of discrete rotor and stator plates sized for the desired application with turbine brush seals chosen to accommodate radial rotor movements from the supported generator. The cartridge may be installed and removed from the turbine relatively easily for maintenance or rebuilding. The rotor housing is designed to be relatively easily machined to dimensions that meet desired operating parameters.

Description:
RELATED APPLICATION DATA 
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 14/797,639, filed Jul. 13, 2015, entitled “ORC TURBINE AND GENERATOR, AND METHOD OF MAKING A TURBINE,” which application is a divisional application of U.S. patent application Ser. No. 13/937,978, filed Jul. 9, 2013, entitled “Overhung Turbine and Generator System With Turbine Cartridge,” now patented, which application claims the benefit of priority of U.S. Provisional Patent Application No. 61/699,649, filed Sep. 11, 2012, entitled “Axial Overhung Turbine and Generator System For Use In An Organic Rankine Cycle.” All of these applications are incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to the field of turbine generator power systems for industrial waste heat recovery and other applications. In particular, the present invention is directed to an overhung turbine coupled to a direct-drive, electrical power generator. 
       BACKGROUND 
       [0003]    Concerns about climate change and rising energy costs, and the desire to minimize expenses in various industrial operations, together lead to an increased focus on capturing waste heat developed in such operations. Organic Rankine Cycle (“ORC”) turbine generator electrical power systems have been used in industrial waste heat recovery. Unfortunately, known systems for capturing waste heat and converting it to electricity are often too large for the space available in certain industrial operations, are less efficient than desired, require more heat to operate efficiently than is available, are too expensive to manufacture for certain applications, or require more maintenance than is desired. In other applications, such as geothermal energy recovery and certain ocean thermal energy projects, abundant heat is available and an efficient ORC system is a satisfactory means for conversion of such heat to electricity. Even in such other applications, however, known ORC systems tend to be too expensive for some such applications, are less efficient than desired and/or require more maintenance than is desired. 
       SUMMARY OF THE INVENTION 
       [0004]    In one implementation, the present disclosure is directed to a system for conversion of heat energy into electricity. The system includes an electric generator having a proximal end, a distal end, a generator rotor and a generator stator, said generator rotor being disposed for rotational movement within said stator about a rotational axis; and a turbine having at least one turbine stator and at least one turbine rotor supported for rotational movement relative to said at least one turbine stator about said rotational axis, said at least one turbine rotor being coupled with said generator rotor so as to rotationally drive said generator rotor, wherein said turbine includes a housing having a cavity, said at least one turbine stator includes a plurality of stator plates, and said at least one turbine rotor includes a plurality of turbine rotor plates, further wherein said plurality of stator plates and said plurality of turbine rotor plates together form a cartridge that is sized and configured to be releasably mounted in said cavity in said housing. 
         [0005]    In another implementation, the present disclosure is directed to a turbine cartridge designed to be releasably mounted in a cavity of turbine housing. The cartridge includes a plurality of rotor plates, each having a centerline, a first rotor plate contact surface, and a second rotor plate contact surface contacting said first contact surface; a plurality of stator plates, each having a centerline, a first stator plate contact surface, and a second stator plate contact surface contacting said first stator plate contact surface; and wherein said plurality of rotor plates are positioned in alternating relationship with corresponding respective ones of said plurality of stator plates so as to define a multi-stage rotor assembly with an upstream direction, and further wherein the size and configuration of said plurality of rotor plates and plurality of stator plates is selected so that said cartridge may be releasably mounted in a cavity of a turbine housing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    For the purpose of illustrating the invention, the drawings show aspects of one or more embodiments of the invention. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
           [0007]      FIG. 1  is a schematic depiction of an ORC turbine-generator system; 
           [0008]      FIG. 2  is a schematic depiction of the turbine and generator of the system shown in  FIG. 1 , with interior details of the generator being schematically illustrated; 
           [0009]      FIG. 3  is similar to  FIG. 1 , except that an alternative embodiment of the ORC turbine-generator system is depicted; 
           [0010]      FIG. 4 a    is a cross-sectional view of a multi-stage axial turbine embodiment of the turbine assembly depicted in  FIG. 1  and a partially broken-away view of the generator depicted in  FIG. 1  showing, schematically, bearings included in one embodiment of the generator, with the rotor and stator of the generator removed for clarity of illustration; 
           [0011]      FIG. 4 b    is similar to  FIG. 4 a   , except that a single-stage radial turbine embodiment of the turbine assembly depicted in  FIG. 1  is shown; 
           [0012]      FIG. 4 c    is similar to  FIG. 4 b   , except that a multi-stage radial turbine embodiment of the turbine assembly depicted in  FIG. 4 b    is shown; 
           [0013]      FIG. 4 d    is similar to  FIG. 4 c   , except that the rotors of the multi-stage radial turbine assembly depicted in  FIG. 4 c    are arranged in back-to-back configuration; 
           [0014]      FIG. 5  is cross-sectional view of one embodiment of a turbine cartridge usable in the turbine shown in  FIG. 4   a;    
           [0015]      FIG. 6  is an enlarged cross-sectional view of a portion of the turbine shown in  FIG. 4 a   , illustrating a portion of the hood backplate and the entire turbine cartridge; 
           [0016]      FIG. 7  is a perspective view showing the relative placement of two stator plates and one rotor plate with its stationary spacer plate used in a multi-stage embodiment of the turbine depicted in  FIG. 4   a;    
           [0017]      FIG. 8  is a perspective view of three rotor plates used in a multi-stage embodiment of the turbine depicted in  FIG. 4 a    showing the relative placement of the plates; 
           [0018]      FIG. 9  is a cross-sectional side view of a portion of the turbine shown in  FIG. 6  illustrating brush seals and other details of the turbine; and 
           [0019]      FIG. 10  is similar to  FIG. 9 , except that it depicts an alternative embodiment of the turbine. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The present disclosure is directed to a turbine powered electrical generator for use in an Organic Rankine Cycle (ORC), Kalina cycle, or other similar cycles, industrial operations that generates waste heat, or in connection with other heat sources, e.g., a solar system or an ocean thermal system. High-pressure hot gas from a boiler, which is heated by the heat source, enters the turbine housing and is expanded through the turbine to turn the rotor, which turns the generator shaft to generate electricity, as described more below. 
         [0021]    Referring to  FIG. 1 , turbine-generator assembly  20  is intended for use in an ORC system  22 . For convenience of discussion, system  22  is referred to and described as ORC system  22 . It is, however, to be appreciated that other thermodynamic processes, such as a Kalina cycle process and bottoming cycle processes, are also encompassed by the present invention. Turbine-generator assembly  20  includes a turbine  24  and a generator  26  connected to, and driven by, the turbine. Before discussing turbine-generator assembly  20  in more detail, discussion of ORC system  22  is provided. 
         [0022]    ORC system  22  includes a boiler  28  that is connected to a heat source  30 , such as waste heat from an industrial process. Boiler  28  provides high-pressure hot vapor via connection  32  to turbine  24 . As discussed more below, the hot vapor, aka, the working fluid, is expanded in turbine  24 , where its temperature drops, and is then exhausted from the turbine and delivered via fluid connection  34  to condenser  36 . In condenser  36 , the vapor cooled in turbine  24  is cooled further, typically to a liquid state, and then a first volume of such liquid is delivered via fluid connection  38  to pump  40 , where the liquid is returned via connection  42  to boiler  28 . This liquid is then reheated in boiler  28  by heat from heat source  30  through a heat exchanger or other structure (none shown) in the boiler and then, repeating the cycle, is returned as high-pressure hot vapor via fluid connection  32  to turbine  24 . 
         [0023]    Turning now to  FIGS. 1 and 2 , a second volume of the cooled liquid exiting condenser  36  is, in one embodiment, delivered by pump  50  via fluid connection  52  to vaporizer  54  and from the vaporizer to generator  26  via fluid connection  58 . Fluid from pump  50  is also delivered via fluid connection  56  to generator  26 , in particular cooling jacket  76 , discussed more below. In other embodiments, it may be desirable to omit pump  50  and instead deliver liquid that is output from pump  40  via fluid connection  57  to fluid connections  52  and  56 . Vaporizer  54  vaporizes at least some of the second volume of liquid from condenser  36  and delivers the cooling vapor via fluid connection  58  to generator  26 . As illustrated in  FIG. 2 , generator  26  includes a fluid gap  70 , a stator  72  and a generator rotor  74 , with the fluid gap (e.g., gas or atomized liquid) being positioned between the stator and rotor. Generator rotor  74  rotates relative to stator  72  about rotational axis  106 . 
         [0024]    The cooling vapor is introduced into gap  70 , and as the vapor passes through gap  70  it extracts heat from stator  72  and generator rotor  74 , which vapor is then exhausted via fluid connection  34 , along with the hot vapor exhausted from turbine  24 , for cooling by condenser  36 . Optionally, as illustrated in  FIGS. 1 and 3 , vapor exhausted from generator  26  may be delivered via fluid connection  37  directly to condenser  36  rather than being combined with vapor exhausted from turbine  24 . Turbine  24  has a through flow rate and, in one embodiment, the second volume of the vapor (working fluid) introduced into gap  70  travels through the gap with a flow rate that is no more than 50% of the through flow rate. Typically, although not necessarily, generator  26  is hermetically sealed to ensure working fluid present in gap  70  does not escape except via fluid connection  34 , or fluid connection  37 , if provided. 
         [0025]    Referring now to  FIGS. 1-4 , in one embodiment generator  26  is surrounded by a cooling jacket  76  ( FIGS. 2 and 4 ) for cooling the generator. Cooling liquid pumped by pump  50  to generator  26  via fluid connection  56  is delivered to cooling jacket  76  via inlets  77  ( FIG. 4 ). As the cooling liquid circulates through cooling jacket  76 , it extracts heat from stator  72  and other components of generator  26 . After completing its passage through cooling jacket  76 , the cooling liquid, now somewhat hotter, is removed from generator  26  via fluid connection  78 , after exiting fluid outlet  79  in the cooling jacket, and returned to condenser  36 . 
         [0026]    Turning next to  FIGS. 2 and 3 , in another embodiment of ORC system  22 , atomized cooling liquid, rather than vaporized liquid, is provided to gap  70  in generator  26 . Except as specifically discussed below, the embodiment of ORC system  22  illustrated in  FIG. 3  is essentially identical to the embodiment of the system shown in  FIG. 1 , and so description of identical elements is not provided in the interest of brevity. Unlike the embodiment of ORC system  22  illustrated in  FIG. 1 , no vaporizer is provided in the embodiment illustrated in  FIG. 3 . Instead a portion of the cooling liquid delivered via fluid connection  56  to generator  26  is provided by fluid connection  80  to atomizer  82  positioned proximate to the generator. Atomizer  82  atomizes the cooling liquid, which is then delivered to gap  70  in generator  26 , where the relatively cool atomized liquid extracts heat from stator  72  and generator rotor  74  as it travels through the gap, including through the latent heat of vaporization with respect to portions of the atomized liquid that are vaporized by the heat in the stator and rotor. The atomized liquid is then extracted from generator  26  via fluid connection  34  along with the working fluid exhausted from turbine  24 . In  FIGS. 2 and 3 , atomizer  82  is depicted in dotted view to indicate that it is an optional element used in connection with one embodiment of the invention. As discussed above, in one embodiment, the second volume of the atomized liquid (working fluid) introduced into gap  70  travels through the gap with a flow rate that is no more than 50% of the through flow rate of turbine  24 . 
         [0027]    In some applications, it may be desirable to provide just cooling of stator  72  via cooling jacket  76 , and not provide vapor or atomized liquid to gap  70 . In other applications, the reverse may be desired. 
         [0028]    Various high molecular weight organic fluids, alone or in combination, may be used as the working fluid in system  20 . These fluids include refrigerants such as, for example, R125, R134a, R152a, R245fa, and R236fa. In other applications fluids other than high molecular weight organic fluids may be used, e.g., water and ammonia. 
         [0029]    System  22  also includes a power electronics package  86  connected to generator  26 . Package  86  converts the variable frequency output power from generator  86  to a frequency and voltage suitable for connection to the grid  87 , e.g. 50 Hz and 400 V, 60 Hz and 480 V or other similar values. 
         [0030]    Discussing generator  26  in more detail, in one embodiment the generator is a direct-drive, permanent magnetic, generator. Such a construction is advantageous because it avoids the need for a gearbox, which in turn results in a smaller and lighter system  20 . Various aspects of the invention described herein may, of course, be effectively implemented using a generator having a gearbox mechanically coupled between turbine rotor  104  of turbine  24  and generator rotor  74  of generator  26 , and a suitable wound rotor that does not include permanent magnets, e.g., a doubly wound, induction-fed rotor. In addition, in certain applications direct-drive synchronous generators may be used as generator  26 . The rated power output of generator  26  will vary as a function of the intended application. In one embodiment, generator  26  has a rated power output of 5 MW. In another embodiment, generator  26  has a rated power output of 50 KW, and in yet other embodiments, generator  26  has a rated power output somewhere in between these values, e.g., 200 KW, 475 KW, 600 KW, or 1 MW. Rated power outputs for generator  26  other than those listed in the examples above are encompassed by the present invention. 
         [0031]    To permit high-speed (e.g., on the order of 20,000-25,000 rpm) operation, and to minimize maintenance, it may be desirable in some embodiments of generator  26  to support generator rotor  74  for rotational movement using magnetic radial bearings  88  (see  FIG. 4 ). In one embodiment, magnetic radial bearing  88   a  is positioned adjacent an end of generator rotor  74  proximate turbine  24  and magnet radial bearing  88   b  is positioned adjacent an opposite end of the rotor. As discussed more below, this placement of bearings  88  enables in large part the overhung construction of turbine  24 . Similarly, axial movement of generator rotor  74  may be controlled through the use of magnetic axial thrust bearing  89 . Magnetic radial bearings  88  and magnetic axial thrust bearing  89  are controlled by a controller  90  that adjusts power delivered to the bearings as a function of changes in radial and axial position of generator rotor  74 , as detected by sensors (not shown) coupled to the controller, all as well known to those of ordinary skill in the art. 
         [0032]    In another embodiment of the invention, fluid-film bearings may be used in place of magnetic radial bearings  88  and thrust bearing  89 . For purposes of illustration, the schematic depiction of magnetic bearings  88  and  89  in  FIG. 4  should be deemed to include, in the alternative, fluid-film bearings. As is known, fluid-film bearings support the total rotor load on a thin film of fluid, i.e., gas or liquid. 
         [0033]    Optionally, in addition to magnetic bearings  88  and  89 , rolling element radial bearings  92 , e.g., radial bearings  92   a  and  92   b,  may be provided at opposite ends of rotor shaft  93  of generator rotor  74  surrounding the rotor shaft, typically adjacent magnetic bearings  88   a  and  88   b,  respectively. Rolling element radial bearings  92  support generator rotor  74  and its shaft  93  in substantially coaxial relation to rotational axis  106  when magnetic bearings  88  and  89  are not energized. More particularly, rolling element radial bearings  92  provide a rest point for generator rotor  74  when magnetic bearings  88  are not activated and provide a safe landing for the generator rotor in the event of a sudden electronic or power failure. It may be desirable in some cases to size rolling element radial bearings  92  to support generator rotor  74  with a relatively loose fit so that during operation when magnetic bearings  88  and  89  are energized, the rotor has limited, if any contact, with rolling element radial bearings  92 , even during times of maximum radial deflections of generator rotor  74  due to perturbations in the operation of magnetic bearings  88 . When fluid-film bearings are used in place of magnetic radial bearings  88 , rolling element radial bearings  92  are typically not required, although in some applications it may be desirable to include such radial bearings. 
         [0034]    In one embodiment, rolling element radial bearings  92  are sized to permit rotor shaft  93  to deviate radially from perfect coaxial alignment with rotational axis  106  an amount that is 1.01 to 5 times as great as the maximum radial deviation of shaft  93  from rotational axis  106  that may occur when magnetic radial bearings  88  are fully activated, including during times of major radial deflection that may occur due to perturbations of the magnetic radial bearings, e.g., from a fluid dynamic instability or a failed control system or a power failure (without backup). In another embodiment, this deviation permitted by radial bearings  92  is about 2 to 3 times as great as the radial deviation of shaft  93  from rotational axis  106  that occurs when magnetic bearings  88  are activated, again including during major perturbations that occur over time. Rolling element radial bearings  92  are often referred to as “bumper bearings” or “backup bearings” in the art. 
         [0035]    While beneficial for the reasons discussed above, rolling element radial bearings  92  also present a challenge because the radial clearance of such bearings is much higher than the desired clearances for the conventional seals (not shown in detail) of turbine  24 . Typical rolling element radial bearings  92  have a radial clearance on the order of 0.005 to 0.015 inch. By contrast, desired radial clearances for the seals of turbine  24  are typically on the order of 0.000-0.001 inch. As generator  26  is assembled, shipped and stored, or during a loss of levitation of generator rotor  74  during operation due to failure of magnetic bearings  88 , the generator rotor will drop to rolling element radial bearings  92 . A consequence of such “play” in generator rotor  74  is that portion of shaft  93  proximate rolling element radial bearings  92 , along with seals in turbine  24 , can be damaged over time. Indeed, in certain applications, as few as 1-10 “bumper” events can cause sufficient damage to components of turbine-generator assembly  20  that disassembly and repair/replacement of such components is required. 
         [0036]    A solution to this problem is to add a radial brush seal  94  ( FIG. 4 ) adjacent one or more of magnetic bearings  88  and/or rolling element radial bearings  92 , or to substitute a brush seal for the rolling element radial bearings (i.e., the bumper bearings). As used in such context, brush seal  94  is designed to withstand substantial radial forces before deforming. Such deformation is temporary, with brush seal  94  being constructed so that it springs back quickly to its prior configuration. In other words, brush seal  94  is self-healing. The stiffness of each brush seal  94  is selected based upon the weight of generator rotor  74  and turbine rotor  104  (discussed below) coupled with the generator rotor, and the extent of radial movement of the rotors  74  and  104  that is permissible given the overall design and operating parameters, respectively, of generator  26  and turbine  24 . In one embodiment, the stiffness of brush seals  94  is selected so that the extent of radial deviation of generator rotor  74  from co-axial alignment with rotational axis  106  that occurs when the rotor is supported by just the brush seals is 1 to 5 times greater than the extent of maximum radial deviation of generator rotor  74  from co-axial alignment with rotational axis  106  that occurs when magnetic bearings  88  are fully activated and supporting generator rotor  74  for rotational movement through the course of normal operation. In another embodiment, such extent of radial deviation is 1.2 to 4 times greater than the extent of radial deviation of generator rotor  74  from co-axial alignment with rotational axis  106  that occurs when magnetic bearings  88  are fully activated and supporting generator rotor  74  for rotational movement through the course of normal operation. In another implementation, generator rotor  74  is free to move a first radial distance out of co-axial alignment with rotational axis  106  when magnetic bearings  88  are not activated and the generator rotor does not move radially more than a second radial distance out of co-axial alignment with rotation axis when supported by brush seals  94 . In this implementation, the second radial distance is no more than 0.8 times the first radial distance, and in some implementations ranges from 0.2 to 0.6 times the first radial distance. 
         [0037]    Referring now to  FIGS. 2 and 4-10 , turbine  24  will be described in more detail. In the embodiment illustrated in  FIG. 4 a   , turbine  24  is an overhung axial turbine and includes a housing  98  having an axial inlet  100  and a radial outlet  102 . Turbine  24 , in one embodiment, is a multi-stage turbine, with the embodiment shown in  FIG. 4 a    having three stages. In other embodiments discussed more below, turbine  24  may be a single-stage overhung radial turbine as show in  FIG. 4 b   , and a multi-stage overhung radial turbine as shown in  FIG. 4 c   . Consistent with this overhung configuration, no radial bearings are included in turbine  24 ,  324 ,  424  for radially supporting the rotor in the turbine for rotational movement, As discussed above, turbine  24  is constructed so that the working fluid is expanded as it is transported through the turbine, with the result that the cold end of the turbine, i.e., the end proximate radial outlet  102 , is positioned adjacent generator  26 . This arrangement reduces heat transfer from turbine  24  to generator  26 . 
         [0038]    Turbine  24  includes a turbine rotor  104  that rotates about rotational axis  106  and a stator  108  that is fixed with respect to housing  98 . As discussed more below, in one example of turbine  24  featuring a modular design, turbine rotor  104  includes a plurality of individual bladed plates  110  and stator  108  includes a plurality of individual plates  112  positioned in alternating, inter-digitated relationship with the rotor plates, as best seen in  FIGS. 5, 6 and 9 . Rotor plates  110  and stator plates  112  are positioned within housing  98  in the cavity  114  formed at the region between inlet  100  and outlet  102 . As best illustrated in  FIGS. 9 and 10 , radially innermost portions of stator plates  112  are spaced from portions of turbine rotor  104  positioned between rotor plates  110  so as to form a gap  115  sealed by seals  116  provided on such radially innermost portion of the stator plates. In the portion of turbine  24  illustrated in  FIG. 5 , a plurality of stator spacer segments  117 , one corresponding to each rotor plate  110 , is provided in alternating, inter-digitated relationship with radially outer portions of stator plates  112 . Each spacer segment  117  is positioned radially outwardly of a corresponding respective rotor plate  110 . In the alternative embodiments of turbine  24  illustrated in  FIGS. 9 and 10 , spacer segments  117  are formed as an integral portion of stator plates  112  (spacer segments are not separately labeled in  FIGS. 9 and 10 ). In any event, in each of these embodiments, each spacer segment  117  is sized with respect to its corresponding respective rotor plate  110  so that a gap  118  is provided between a radially outermost portion of the rotor plate and the radially innermost portion of the spacer segment. Seals  119  (see  FIG. 9 ) may be provided in gap  118  in certain embodiments of turbine  24 . 
         [0039]    As best illustrated in  FIGS. 9 and 10 , each rotor plate  110  includes a first contact surface  130  and a second contact surface  132  that contacts the first contact surface. Similarly, each stator plate  112  includes a first contact surface  134  and a second contact surface  136  that contacts the first contact surface. Contact surfaces  130 ,  132 ,  134  and  136  are substantially flat and substantially parallel. Further, they are arranged to be substantially perpendicular to rotational axis  106 . In one embodiment, contact surfaces  130 ,  132 ,  134  and  136  are flat in the range 0.00005″ to 0.020″, and in certain embodiments in the range 0.0005″ to 0.005″, as measured with respect to a root mean square version of such surfaces. Further, in one embodiment contact surfaces  130  and  132  of rotor plates  110 , and contact surfaces  134  and  136  of stator plates  112 , deviate from perfectly parallel by an amount in the range 0.0001″ to 0.015″, and in certain embodiments in the range 0.0005″ to 0.005″. Spacer segments  117 , when provided, preferably have contact surfaces that are similarly flat and parallel to contact surfaces  130 ,  132 ,  134 , and  136 , as discussed above. 
         [0040]    Referring now to  FIGS. 7 and 8 , in certain implementations of turbine  24 , it may be desirable to circumferentially clock one rotor plate  110  with respect to an adjacent rotor plate, e.g., clocking rotor plate  110   a  with respect to plate  110   b.  Similarly, it may be desirable to circumferentially clock one stator plate  112  with respect to an adjacent stator plate, e.g., clocking stator plate  112   a  with respect to plate  112   c . Desired performance specifications for turbine  24  will influence the extent of clocking provided, as those skilled in the art will appreciate. When pairs of rotor plates  110  being clocked both have an equal number of vanes  140 , in one embodiment a first rotor plate  110 , e.g., plate  110   a,  is clocked with respect to a second adjacent rotor plate, e.g., plate  110   b,  zero to one vane pitch, i.e., (0)S to (1)S. Similarly, when pairs of stator plates  112  being clocked both have an equal number of vanes  142 , in one embodiment a first stator plate  112 , e.g., plate  112   a  is clocked with respect to an adjacent stator plate, e.g., plate  112   c , zero to one vane pitch, i.e., (0)S to (1)S. When pairs of rotor plates  110  being clocked both have an unequal number of vanes  140 , in one embodiment a first rotor plate  110 , e.g., plate  110   a,  is clocked with respect to a second adjacent rotor plate, e.g., plate  110   b,  somewhere in the range of 0 to 360 degrees. Similarly, when pairs of stator plates  112  being clocked both have an unequal number of vanes  142 , in one embodiment a first stator plate  112 , e.g., plate  112   a  is clocked with respect to an adjacent stator plate, e.g., plate  112   c , somewhere in the range of 0 to 360 degrees. Known turbine flow analytical and experimental methods are used to guide selection of the optimal amount of clocking in this range of 0 to 360 degrees. 
         [0041]    With continuing reference to  FIGS. 7 and 8 , in one embodiment adjacent stator plates  112  are clocked with respect to one another using an alignment system featuring a plurality of circumferentially spaced bores  160  positioned along a peripheral section  162  of a stator plate  112 , e.g., stator  112   c,  only five of which are illustrated in  FIG. 7  for convenience of illustration. In one implementation, adjacent bores  160  are circumferentially spaced one vane pitch S. The alignment system also includes a bore  164  in a peripheral section  166  of spacer segments  117 . Further, a blind bore  168  may be provided in a peripheral section  170  of a stator plate  112 , e.g., stator plate  112   a,  immediately adjacent the stator plate, e.g., stator plate  112   c,  in which bores  160  are provided (rotor plate  110   b  and spacer plate  117  are intervening, of course). In one embodiment, bores  160 ,  164  and  168  are spaced a substantially identical radial distance from rotational axis  106 , and have a substantially identical diameter. The alignment system further includes pin  172 , which is sized for receipt, typically using a mild friction fit, in a selected one of bores  160  and in bore  164 . When so positioned, pin  172  locks stator plate  112   c  in selected circumferential alignment with adjacent spacer segment  117 . The selected circumferential clocking between adjacent stator plates, e.g., plates  112   a  and  112   c,  is achieved by next locking spacer section  117  to stator plate  112   a  using pin  174  inserted in bores  164  and  168 . A similar system for clocking adjacent rotor plates  110  may also be employed, as discussed more below in connection with  FIGS. 9 and 10 . As discussed above, selection of one of the plurality of bores  160  that receives pin  172  is determined based on the extent of circumferential clocking desired between adjacent stator plates  112 . The present invention encompasses other approaches to circumferentially clocking adjacent rotor plates  110  and stator plates  112 , as those skilled in the art will appreciate. 
         [0042]    With particular reference to  FIG. 9 , rotor plates  110  and stator plates  112  are, in one implementation, spaced so that axial distance  178  between vanes  140  of a rotor plate  110  and vanes  142  of an adjacent stator plate  112  is in the range of two axial chords to ¼ of 1% of an axial chord, and in certain embodiments ⅓ to 1 chord, as measured with respect to the chord of the immediately upstream one of the rotor or stator plates. For example, vanes  140  of rotor plate  110  identified as R 3  in  FIG. 9  are axially spaced distance  178   a  from immediately adjacent vanes  142  of stator plate  112  identified as S 3  a chord distance C x ,S 3  that is in the range of two axial chords to ¼ of 1% of an axial chord, and in certain embodiments is spaced ⅓ to 1 chord. Additionally, the stage reaction for turbine  24  may be of any conventional level. When, however, axial thrust levels must be controlled to meet available thrust capability of generator  26 , then very low stage reaction may be desirable, with common values in one example ranging from −0.1 to 0.3 and often falling in the range of −0.05 to +0.15. When very low stage reaction cannot be achieved, for example with multi-stage radial inflow turbine  424  illustrated in  FIG. 4 c   , then the second stage may be reversed so that the two radial turbines work back-to-back, leaving the last stage discharge still facing the generator. 
         [0043]    Referring to  FIGS. 4-6, 9 and 10 , in connection with the assembly of this embodiment, rotor plates  110  and stator plates  112  are positioned in alternating, inter-digitated relationship. In one embodiment, rotor plates  110  include a plurality of bores  186  (see  FIG. 5 ) in radially inner portions of the plates, which bores are sized to receive a fastener, such as bolt stud  188 , which extends through the plates and is secured to stub shaft  189  via threaded bores  190  in the stub shaft. Generator rotor shaft  93  may include a threaded male end  192  that is received in a threaded bore  194  in stub shaft  189 . 
         [0044]    Stator plates  112 , and spacer segments  117  if provided, may, for example, be secured together in alternating, inter-digitated relationship so as to form a unitary cartridge  198 . The latter may be releasably secured in cavity  114  ( FIG. 6 ) of housing  98  using known fasteners and other devices. In one embodiment, cartridge  198  may be secured in cavity  114  by lock ring  200 , which is engaged with a snap fit in a correspondingly sized recess  201  in in the cavity. With this construction, when lock ring  200  is installed, stator plates  112 , and segments  117  when provided, are driven against shoulder  202  formed in cavity  114  in housing  98 , thereby holding the plates and segments securely in place. In certain embodiments of turbine  24 , rotor plates  110  may be secured together with pins  203  (see  FIGS. 9 and 10 ) received in bores  204  (see  FIGS. 8, 9 and 10 ) to ensure no relative rotational movement occurs between rotor plates. Similarly, in other embodiments of turbine  24 , stator plates  112  and spacers  117  may be secured together with pins  172  (see  FIGS. 9 and 10 ), as discussed above, to ensure no relative rotational movement occurs. Pins  172  may also penetrate into floor  204  of housing  98  (such penetration not being shown) from the downstream-most spacer  117  or stator plate  112 , if desired to assure no relative motion. A nose cone  206  may be provided, with one embodiment being threadedly engaged with threaded bore  208  in the furthest upstream stator plate  112  (identified in  FIGS. 9 and 10  as S 1 ). Alternatively, machine screws may be used to fasten nose cone  206  to first stator plate  112 . With reference to  FIGS. 5, 6, 9 and 10 , in some implementations it may be desirable to rotationally align and secure together rotor plates  110 , stator plates  112 , and if provided, spacers  117 , using one or more pins  210  and/or one or more bolt studs  212  that extend through the rotor plates, stator plates and spacers. Pins  210  may be used for precision rotational alignment of rotor plates  110 , stator plates  112  and spacers  117 , and if received in these components with a sufficient force fit, may also hold these components together to form a unitary structure, namely unitary cartridge  198 . Bolt studs  212 , in addition to providing some measure of rotational alignment, also draw together the rotor plates, stator plates and spacers to form a unitary structure, namely unitary cartridge  198 . 
         [0045]    By providing separate rotor plates  110  and stator plates  112 , and by making such plates relatively flat as discussed above, these plates may be assembled as a cartridge  198  (see  FIG. 5 ) that may be positioned in and removed from cavity  114  in housing  98  as a unitary assembly. As discussed more below, the provision of cartridge  198  permits a universal turbine  24  to be readily adapted for its intended application and interchanged for maintenance or new loading requirements. 
         [0046]    In some applications, it will be desirable to more substantially isolate generator  26  from turbine  24 . To achieve this objective, as best illustrated in  FIG. 6 , it may be desirable to include a seal  220  surrounding stub shaft  189  of turbine  24  proximate the radially innermost portion of backplate  250 . Seal  220  may be implemented as a labyrinth seal, a brush seal, a close-tolerance ring seal or using other seals known in the art. 
         [0047]    The embodiment of turbine  24  shown in  FIGS. 4-6 , is designed to permit ready manufacture of versions of the turbine having differently sized rotors  104  and stators  108 . By providing a single housing  98  for turbine  24  while permitting construction of turbines with varying operating parameters using that single housing, the turbine can be manufactured on a cost-efficient basis to the specifications of a given application. This flexible design is achieved in part by designing and sizing housing  98  of turbine  24  so that the largest-diameter turbine rotor  104  contemplated for the turbine may be received within cavity  114  and through the use of the cartridge design discussed above. In particular, after the desired operating parameters of turbine  24  are determined for the application in which the turbine will be used, then the number and size of plates  110  used in turbine rotor  104 , and plates  112  and spacer segments  117  used in stator  108 , are determined. 
         [0048]    Consistent with the objective of providing a turbine  24  that can be readily modified to meeting desired operating parameters, housing  98  is designed to facilitate such modification. One aspect of such design of housing  98  involves providing floor  204  with a thickness sufficient to accommodate turbine rotor  104  and stator  106  having varying radial heights. Δr, as measured between said rotational axis and an outermost portion of said at least one turbine rotor, said axial turbine including a hood having a floor with a first thickness, wherein said first thickness is selected to permit said floor to be machined on the inside to a thickness sufficient to accommodate said at least one turbine rotor with a radial dimension that varies between Δr and 1.4Δr. Further, housing  98  is provided with a configuration that permits easy access to floor  204  by conventional machine tools, e.g., a 5-axis CNC milling machine or a CNC lathe, that can be used to machine the floor so as to create a cavity  114  sized to receive turbine rotor  104  and stator  106  with the desired radial heights. 
         [0049]    Another aspect of providing a modifiable housing  98  is to include a backplate  250  having a thickness that may be adjusted so as to selectively vary width l 4 , i.e., the distance l 4  between backplate  250  and housing wall  252 , and to selectively vary width l 1 , i.e., the exit width. In this regard, width l 4  may be varied so that it ranges from one half to four times the width of diffuser exit l 1 . Backplate  250  may be an integral portion of housing  98  in some embodiments and a separate element in others, as illustrated in  FIG. 4 . Backplate  250  preferably includes one or more ports  254  through which vapor in gap  70  may be exhausted and delivered to the exhaust flow path of turbine  24  and ultimately via fluid connection  34  to condenser  36 . If desired, flow splitter  256  may be provided immediately downstream of turbine rotor  104  and stator  106  as another way to tailor the performance of turbine  24 . As another optional feature, an extension plate  258  may be added to nose  260  of floor  204  of housing  98 , as best seen in  FIG. 6 . 
         [0050]    Housing performance depends on several factors, but alignment of the entry flow at the housing inlet  100  and housing base dimensions are important as taught in the literature. A very good flow entry provides for diffuser exhaust flowing up the housing backplate  250 , as configured in  FIG. 4 . An essential design variable is to set L 4 =l 4 /l 1  to a value of 0.5 to 4, often in the range of 2 to 3, in order to have high performance (maintaining good diffuser Cp). This means that the diffuser exit width (l 1 ) and the hood floor width (l 4 ) must be controlled. The exit width l 1  also controls the performance of the diffuser as it controls the diffuser overall area ratio, which is a first order design parameter; hence a conflict can arise. If l 1  is increased for the diffuser, it will hurt the housing. This is controlled by starting with a generous housing design to cover a wide range of power levels (up to 5 MW for certain designs) and then adjusting operating parameters by modifying backplate  250  and the nose  260  of floor  204 . Another design variable is to introduce diffuser splitter  256  ( FIG. 6 ), which gives independent control on l 1 , thereby permitting a selected change in the diffuser exit value. Further performance tailoring can be achieved by selection of an extension plate  258  ( FIG. 6 ) of suitable height and thickness. 
         [0051]    Turbine  24  is depicted in  FIG. 4 a    as a multi-stage axial turbine  24 , but turbine-generator system  20  is not so limited. In this regard, and with reference to  FIG. 4 b   , in an alternative embodiment, turbine-generator system  20  may include a radial turbine  324  having a single stage. Like numbers are used in  FIGS. 4 a    and  FIG. 4 b    to identify like elements, and for brevity, a description of like elements is omitted in connection with the following description of radial turbine  324 . The latter includes a single rotor  104  and a single stator  108 . Like the axial turbine  24  depicted in  FIG. 4 a   , radial turbine  324  may be implemented as a unitary cartridge  198  that may be releasably secured to generator shaft  93  with a bolt stud  188 . Turbine  324  may include an inlet flange ring  333 , and an outer flow guide  334  attached to housing  98  with known fasteners. Nose cone  206  and stator  108  may be releasable secured to housing  98  with a known fastener, such as bolts  337 . 
         [0052]    Turning next to  FIG. 4 c   , in an alternate embodiment, turbine-generator system  20  may include a multi-stage radial turbine  424 . Like numbers are used in  FIGS. 4 a    and  FIG. 4 b    to identify like elements, and for brevity, a description of like elements is omitted in connection with the following description of radial turbine  424 . The latter includes two rotors  104  and two stators  108 . Radial turbine  424  may be implemented as a unitary cartridge  198  that may be releasably secured to generator shaft  93  with a bolt stud  188 . Turbine  424  may include an inlet flange ring  333 , and an outer flow guide  334  attached to housing  98  with known fasteners. Nose cone  206  and stators  108 , together with intermediate flow guide  441  positioned between the stators, may be releasable secured to housing  98  with a known fastener, such as bolts  337 . The two stators  108  of turbine  424  and intermediate flow guide  441  may be secured together with bolts  339  or other known fasteners so as to create unitary cartridge  198 . Intermediate flow guide  441  is functionally analogous to stator spacers  117  in the version of turbine  24  illustrated in  FIGS. 5 and 6 . 
         [0053]    Depending on the desired balancing of thrust in turbine-generator system  20 , it may be desirable to configure rotors  104  of a multi-stage radial turbine in a back-to-back arrangement, as illustrated in  FIG. 4 d    with respect to radial turbine  524 . In this regard, rotor  104   a  is positioned so it backs up to rotor  104   b,  with the rotors being coupled to rotate together. Stator  108  is positioned between rotors  104   a / 104   b,  and includes bearings  526  for rotatably supporting a portion of rotor  104   b  that extends through the stator. Turbine  524  further includes a front face plate  550  through which gas transfer tubes  552  extend, with the gas transfer tubes terminating at interior plenum  554 . Gas flow entering turbine  524  flows into tubes  552 , is delivered to interior plenum  554 , exits the plenum causing rotor  104   a  to rotate, flows over stator  108 , then drives rotor  104   b  and finally exits the turbine. 
         [0054]    Although not specifically illustrated, turbine-generator system  20  may also be implemented using a mixed-flow turbine. The latter is very similar in design to radial turbine generators  324  and  424 , and so is not separately illustrated. 
         [0055]    By placing rotor  104  in a reverse orientation so that the low-pressure, cooled working fluid is discharged from the last rotor stage of turbine  24  proximate generator  26 , heat transfer to the generator is minimized, thereby prolonging generator life. The low-pressure exhaust of turbine  24 , as a consequence of its reverse orientation, draws the second volume of working fluid out of gap  70  in generator  26  via ports  254  and into the discharge stream of turbine  24  while balancing thrust forces sufficiently so that the generator thrust bearing  89  can handle the remaining axial load of turbine  24 . Such a design is efficient, compact and thermally efficient. 
         [0056]    Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the spirit and scope of the present invention.