Patent Publication Number: US-2020300174-A1

Title: Power train architectures with low-loss lubricant bearings and low-density materials

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part application of, and claims priority to, U.S. patent application Ser. No. 14/460,410, entitled “POWER TRAIN ARCHITECTURES WITH LOW-LOSS LUBRICANT BEARINGS AND LOW-DENSITY MATERIALS,” filed on Aug. 15, 2014 and currently pending. The present application is related to the following commonly-assigned patent applications: U.S. patent application Ser. No. 14/460,560, entitled “MULTI-STAGE AXIAL COMPRESSOR ARRANGEMENT”; U.S. patent application Ser. No. 14/460,576, entitled “POWER TRAIN ARCHITECTURES WITH MONO-TYPE LOW-LOSS BEARINGS AND LOW-DENSITY MATERIALS”; U.S. patent application Ser. No. 14/460,595, entitled “POWER TRAIN ARCHITECTURES WITH HYBRID-TYPE LOW-LOSS BEARINGS AND LOW-DENSITY MATERIALS”; U.S. patent application Ser. No. 14/460,606, entitled “MECHANICAL DRIVE ARCHITECTURES WITH MONO-TYPE LOW-LOSS BEARINGS AND LOW-DENSITY MATERIALS”; U.S. patent application Ser. No. 14/460,620, entitled “MECHANICAL DRIVE ARCHITECTURES WITH HYBRID-TYPE LOW-LOSS BEARINGS AND LOW-DENSITY MATERIALS”; and U.S. patent application Ser. No. 14/460,418, entitled “MECHANICAL DRIVE ARCHITECTURES WITH LOW-LOSS LUBRICANT BEARINGS AND LOW-DENSITY MATERIALS.” Each patent application identified above is incorporated by reference herein for all purposes. 
    
    
     BACKGROUND 
     The present disclosure relates generally to power train architectures and, more particularly, to gas turbines, steam turbines, and generators used as part of a power train in a power-generating plant with low viscosity fluid bearings. In some embodiments, one or more rotating components in the power train may be made of low-density materials. 
     In one type of a power-generating plant, a gas turbine can be used in conjunction with a generator to generally form the plant&#39;s power train. In this plant, a compressor with rows of rotating blades and stationary vanes compresses air and directs it to a combustor that mixes the compressed air with fuel. In the combustor, the compressed air and fuel are burned to form combustion products (i.e., a hot air-fuel mixture), which are expanded through blades in a turbine. As a result, the blades spin or rotate about a shaft or rotor of the turbine. The spinning or rotating turbine rotor drives the generator, which converts the rotational energy into electricity. 
     Many gas turbine architectures deployed in such a power train of a power-generating plant use slide bearings in conjunction with a high viscosity lubricant (i.e., oil) to support the rotating components of the turbine, the compressor, and the generator. High viscosity oil bearings are relatively inexpensive to purchase, but have costs associated with their accompanying oil skids (i.e., for pumps, reservoirs, accumulators, etc.). In addition, high viscosity oil bearings have high maintenance interval costs and cause excessive viscous losses in the power train, which in turn can adversely affect overall output of a power-generating plant. 
     SUMMARY 
     In one aspect of the present disclosure, a power train architecture having a first gas turbine is disclosed. In this aspect, the first gas turbine comprises a compressor section, a turbine section, and a combustor section operatively coupled to the compressor section and the turbine section. A first rotor shaft extends through the compressor section and the turbine section of the first gas turbine. A first generator, coupled to the first rotor shaft, is driven by the turbine section of the first gas turbine. A plurality of bearings supports the first rotor shaft within the compressor section and the turbine section of the first gas turbine and the first generator, wherein at least one of the bearings is a low-loss lubricant bearing. The compressor section, the turbine section, and the generator include rotating components therein, at least one of the rotating components in one of the compressor section of the first gas turbine, the turbine section of the first gas turbine, and the first generator including a low-density material. 
     A second aspect of the present disclosure provides a power train architecture including: a first gas turbine comprising a compressor section, a turbine section, and a combustor section operatively coupled to the compressor section and the turbine section; a first rotor shaft extending through the compressor section and the turbine section of the first gas turbine; a first generator, coupled to the first rotor shaft and driven by the turbine section of the first gas turbine; and a first plurality of hydrodynamic bearings supporting the first rotor shaft within the compressor section and the turbine section of the first gas turbine and the first generator, where each of the first plurality of hydrodynamic bearings includes a low-loss lubricant, and where the low-loss lubricant is a mineral oil-based lubricant having a viscosity grade equal to or between VG8 and VG20, and a midpoint kinematic viscosity between about 8 centistokes and about 20 centistokes at 40° C., where VG represents the viscosity grade in centistokes at 40° C.; and where the compressor section, the turbine section, and the generator each includes a plurality of rotating components each include a plurality of rotating components, each one of the plurality of rotating components disposed in a section of one or more of the compressor section of the first gas turbine, the turbine section of the first gas turbine, and the first generator includes a low-density material, the low-density material having a density less than 0.2 lbm/in 3 , where each of the first plurality of hydrodynamic bearings including the low-loss lubricant supports a respective section in which a corresponding one of the plurality of rotating components including the low-density material is disposed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of the various embodiments will be apparent from the following more detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of these embodiments. 
         FIG. 1  is a schematic diagram of a simple cycle power train architecture including a front-end drive gas turbine, a generator, a bearing fluid skid, and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with the power train, according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of a simple cycle power train architecture including a rear-end drive gas turbine, a generator, a bearing fluid skid, and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with the power train, according to an embodiment of the present invention; 
         FIG. 3  is a schematic diagram of a simple cycle power train architecture including a front-end drive gas turbine having a reheat section, a generator, a bearing fluid skid, and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with the power train, according to an embodiment of the present invention; 
         FIG. 4  is a schematic diagram of a single-shaft steam turbine and generator (STAG) power train architecture including a front-end drive gas turbine, a multi-stage steam turbine, a generator, a heat exchanger, a bearing fluid skid, and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with the power train, according to an embodiment of the present invention; 
         FIG. 5  is a schematic diagram of an alternate architecture of  FIG. 4 , which illustrates a single-shaft steam turbine and generator (STAG) power train architecture including a front-end drive gas turbine, a generator, a clutch, a multi-stage steam turbine, a heat exchanger, a bearing fluid skid, and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with the power train, according to an embodiment of the present invention; 
         FIG. 6  is a schematic diagram of a single-shaft steam turbine and generator (STAG) power train architecture including a rear-end drive gas turbine, a generator, a multi-stage steam turbine, a heat exchanger, a bearing fluid skid, and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with the power train, according to an embodiment of the invention; 
         FIG. 7  is a schematic diagram of a single-shaft steam turbine and generator (STAG) power train architecture including a front-end drive gas turbine with a reheat section, a generator, a multi-stage steam turbine, a heat exchanger, a bearing fluid skid, and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with the power train, according to an embodiment of the invention; 
         FIG. 8  is a schematic diagram of a two-on-one (2:1) combined cycle power train architecture including two front-end drive gas turbines (each with its own generator, heat exchanger, and bearing fluid skid) and one multi-stage steam turbine with its own generator and bearing fluid skid, and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with any one or more of the power trains, according to an embodiment of the invention; 
         FIG. 9  is a schematic diagram of a two-on-one (2:1) combined cycle power train architecture including two rear-end drive gas turbines (each with its own generator, heat exchanger, and bearing fluid skid) and one multi-stage steam turbine with its own generator and bearing fluid skid, and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with any one or more of the power trains, according to an embodiment of the invention; 
         FIG. 10  is a schematic diagram of a three-on-one (3:1) combined cycle power train architecture including three rear-end drive gas turbines (each with its own generator, heat exchanger, and bearing fluid skid) and one multi-stage steam turbine with its own generator and bearing fluid skid, and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with any one or more of the power trains, according to an embodiment of the invention; 
         FIG. 11  is a schematic diagram of a multi-shaft, combined cycle power train architecture including a front-end drive gas turbine coupled on a first shaft to a first generator and having a first bearing fluid skid, and a multi-stage steam turbine coupled on a second shaft to a second generator and having a second bearing fluid skid, and further including a heat exchanger, at least one low-loss bearing with a low-loss lubricant, and at least one rotating component made of a low-density material in use with any one or more of the power trains, according to an embodiment of the invention; 
         FIG. 12  is a schematic diagram of a multi-shaft, combined cycle power train architecture including a rear-end drive gas turbine coupled on a first shaft to a first generator and having a first bearing fluid skid, and a multi-stage steam turbine coupled on a second shaft to a second generator and having a second bearing fluid skid, and further including a heat exchanger, at least one low-loss bearing with a low-loss lubricant, and at least one rotating component made of a low-density material in use with any one or more of the power trains, according to an embodiment of the invention; 
         FIG. 13  is a schematic diagram of a multi-shaft, combined cycle power train architecture including a front-end drive gas turbine with a reheat section coupled on a first shaft to a first generator and having a first bearing fluid skid, and a multi-stage steam turbine coupled on a second shaft to a second generator and having a second bearing fluid skid, and further including a heat exchanger, at least one low-loss bearing with a low-loss lubricant, and at least one rotating component made of a low-density material in use with any one or more of the power trains, according to an embodiment of the invention; 
         FIG. 14  is a schematic diagram of a gas turbine architecture including a rear-end drive power turbine and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with the power train, according to an embodiment of the present invention; 
         FIG. 15  is a schematic diagram of a multi-shaft gas turbine architecture including a rear-end drive power turbine and a reheat section and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with the power train, according to an embodiment of the present invention; 
         FIG. 16  is a schematic diagram of a single-shaft, front-end drive gas turbine architecture including a stub shaft and a speed-reduction mechanism to reduce the speed of forward stages of a compressor and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with the power train, according to an embodiment of the present invention; 
         FIG. 17  is a schematic diagram of a single-shaft, front-end drive gas turbine architecture with a reheat section, which includes a stub shaft and a speed-reducing mechanism to reduce the speed of the forward stages of a compressor and which further includes at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with the power train, according an embodiment of the present invention; 
         FIG. 18  is a schematic diagram of a multi-shaft gas turbine architecture including a rear-end drive power turbine and further including a stub shaft and a speed-reducing mechanism to reduce the speed of forward stages of a compressor, at least one low-loss bearing with a low-loss lubricant, and at least one rotating component made of a low-density material in use with the power train, according to an embodiment of the present invention; and 
         FIG. 19  is a schematic diagram of a multi-shaft, front-end drive gas turbine architecture including a low pressure compressor section coupled to a low pressure turbine section via a low-speed spool and a high pressure compressor section coupled to a high pressure turbine section via a high-speed spool, and further including at least one low-loss bearing with a low-loss lubricant and at least one rotating component made of a low-density material in use with the power train, and optionally including a torque-altering mechanism, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As mentioned above, many gas turbine architectures deployed in power-generating plants use slide bearings in conjunction with a high viscosity lubricant (i.e., oil) to support the rotating components of the turbine, the compressor, and the generator. High viscosity oil bearings have high maintenance interval costs and cause excessive viscous losses in the power train, which in turn can adversely affect overall output of a power-generating plant. There are also costs associated with the oil skids that accompany the high viscosity oil bearings. 
     Low-loss bearings—including bearings having a low-loss lubricant—are one alternative to the use of high viscosity oil bearings. However, certain gas turbine architectures used in a power train of a power-generating plant (i.e., plants with outputs of 50 megawatts (MW) or greater) are difficult applications for the use of low-loss bearings. Specifically, plants with outputs of 50 megawatts (MW) or greater typically require greater gas turbine sizes. As gas turbine sizes increase, the supporting bearing pad area increases as a square of the rotor shaft diameter, while the weight of the power train architecture increases as a cube of the rotor shaft diameter. Therefore, the increase in bearing pad area and the increase in weight should be proportional as gas turbine sizes increase. In power train architectures producing outputs of less than 50 MW (i.e., smaller power trains), it is contemplated that low-loss bearings may still be accommodated in rotating components, albeit with reduced performance, operation, and/or efficiency. However, for power train architectures producing outputs of 50 megawatts (MW) or greater, implementing low-loss bearings (including low-loss lubricant bearings) in gas turbine architectures posts greater challenges, such that for the low-loss bearings are suitable for supporting gas turbine architectures in high output plants, requirements for the proportional increase in bearing pad area and the increase in weight need to be met. It is hypothesized, in the current disclosure, that a combination of incorporating light-weight or low-density materials for the power train, particularly in locations where low-loss bearings are implemented, would help promote such proportionality, thereby creating a power train architecture having a weight supportable by low-loss bearings. 
     In addition to creating a power train architecture having a weight supportable by low-loss bearings, the use of lighter weight or low-density materials can also promote the ability to produce greater airflows. Conventionally, to produce greater airflow rate, rotating blades with longer blade lengths are used. However, centrifugal loads or pulls placed on the rotating blades during operation of a gas turbine increase with the longer blade lengths, making it difficult to generate a higher airflow rate. For example, the rotating blades in the forward stages of a multi-stage axial compressor used in a gas turbine are larger than the rotating blades in both the mid and aft stages of the compressor. Such a configuration makes the longer, heavier rotating blades in the forward stages of an axial compressor more susceptible to being highly stressed during operation due to large centrifugal pulls induced by the rotation of the longer and heavier blades. 
     More particularly, large centrifugal pulls are experienced by the blades in the forward stages due to the high rotational speed of the rotor wheels, which, in turn, stress the blades. The large attachment stresses that may arise on the rotating blades in the forward stages of an axial compressor become problematic as it becomes more desirable to increase the size of the blades in order to produce a compressor that can generate a higher airflow rate as demanded by certain applications. 
     It would be desirable, therefore, to provide a power train architecture for a power-generating plant, which incorporates one or more low-loss bearings (including low-loss lubricant bearings), as applied in gas turbines, steam turbines, or generators. In some embodiments, such low viscosity or low-loss bearings are used in conjunction with components made of low-density materials. Such architectures can provide greater power output with fewer viscous losses, thereby increasing the overall efficiency of the power-generating plant. 
     With the embodiments of the instant disclosure, both the efficiency and power output of the power train architecture be further improved by allowing rotating components of larger radial length to be used. As discussed earlier, the challenge with producing rotating components of larger lengths has been that their weight makes them incompatible with low-loss lubricant bearings. However, the use of low-density materials for one or more of the rotating components permits the fabrication of components of the desired (longer) lengths without a corresponding increase in the airfoil pulls and rotor wheel diameter. As a result, a greater volume of air may be employed in producing motive fluid to drive the gas turbine, and low-loss lubricant bearings may be used to support the power train section in which the low-density rotating components are located. 
     Various embodiments of the present invention are directed to providing power train architectures that have a gas turbine with low viscosity fluid bearings and low-density materials as part of a power-generating plant. 
     As used herein, the phrase “power train architecture” refers to an assembly of moving parts, which can include the rotating components of one or more of a generator, a compressor section, a turbine section, a reheat turbine section, a power turbine section, and a steam turbine, which collectively communicate with one another in the production of power. The power train architecture is a subset of the overall power plant equipment used in a power-generating plant. The phrases “power train architecture” and “power train” may be used interchangeably. 
     As used herein, a “low-loss bearing” is a bearing assembly having one or more primary bearing units, which has a working fluid that has a low or very low viscosity. The “primary bearing unit” may be a journal bearing, a thrust bearing, or a journal bearing adjacent a thrust bearing. A “low-loss lubricant bearing” or a “low-loss bearing including a low-loss lubricant” is a bearing assembly in which the working fluid is a low-loss lubricant or a very low viscosity fluid and which requires no additional secondary bearing. In certain embodiments, the “low-loss lubricant bearing” or the “low-loss bearing including a low-loss lubricant” includes a hydrodynamic bearing in which the working fluid is a low-loss lubricant or a very low viscosity fluid. 
     As used herein, a hydrodynamic bearing or hydrodynamic bearing(s) may refer to a type of fluid bearings including, but not limited to, fluid film bearings that rely on a film of oil or air to create a clearance between moving and stationary elements. A hydrodynamic (or fluid dynamic) bearing may support a load on a thin film of fluid (oil or air), and there is no direct contact between the hydrodynamic bearing and a moving surface of the part the hydrodynamic bearing supports. As discussed earlier, it is discovered that a combination of incorporating light-weight or low-density materials for a power train architecture, particularly in locations where low-loss bearings are implemented, would promote a proportionality needed for creating the power train architecture having a weight supportable by low-loss bearings. It is further discovered, that by providing hydrodynamic bearing with the low-loss lubricant, and using it in conjunction with a rotating component including the low-density material in a given section of the power train, benefits of hydrodynamic bearings can be realized, while mitigating risks of high load for hydrodynamic bearings during start up. In certain embodiments, each of a plurality of hydrodynamic bearings of the present disclosure requires no secondary bearing, such as a roller bearing element. 
     The phrase “low-loss lubricants,” as used in the present low-loss bearings, refers to fluids having a viscosity much greater than water (i.e., 1 centipoise at 20° C.) and preferably having a viscosity of between approximately VG8 and approximately VG20, where VG represents viscosity grade in centistokes (cSt) at 40° C. on the ISO scale developed by the International Standards Organization. Per ISO standards (set forth in ISO 3448 published in 1992), each viscosity grade is designated by the nearest whole number to its midpoint kinematic viscosity in mm 2 /second or centistokes (cSt) at 40° C. (1 mm 2 /second=1 cSt). It is to be understood that for a specified value of viscosity grade or midpoint kinematic viscosity in a form centistokes (cSt), a range of +/−10 percent of the value is permitted. For example, a viscosity grade of VG8 may correspond to a midpoint kinematic viscosity in about 8 mm 2 /second or about 8 cSt at 40° C., which may correspond to a range of kinematic viscosity between about 7 cSt and about 9 cSt. Similarly, a viscosity grade of VG  18  may correspond to a midpoint kinematic viscosity in about 18 mm 2 /second or about 18 cSt at 40° C., which corresponds to a range of kinematic viscosity between about 16 cSt and about 20 cSt. In certain embodiments, the low-loss lubricant may be a mineral oil-based lubricant having a viscosity grade between (and including the viscosity grade values of) VG8 and VG20, and a midpoint kinematic viscosity between about 8 centistokes and about 20 centistokes at 40° C., wherein VG represents the viscosity grade in centistokes at 40° C. In some embodiments, the low-loss lubricant may be a mineral oil-based lubricant having a viscosity grade between (and including the VG values of) VG16 and VG20, and a midpoint kinematic viscosity between about 16 centistokes and about 20 centistokes at 40° C. In some embodiments, the low-loss lubricant may be a mineral oil-based lubricant having a viscosity grade of about VG18, and a midpoint kinematic viscosity of about 18 centistokes at 40° C. Specific examples of low-loss lubricants having a viscosity in the range above include mineral oil-based lubricants in the API base oil group III; synthetic-based polyalphaolefins (PAOs) in the API base oil group IV; and certain polyalkylene glycols (PAGs). In contrast, “high viscosity” oils (also referred to herein as conventional oils) used in industrial gas turbines may have a viscosity equals to or greater than about VG32 for high-temperature environments. In certain embodiments, the “high viscosity” oils have a viscosity of greater than VG45. 
     As used herein, a “mono-type low-loss bearing” is a bearing assembly having a single primary bearing unit, which has a very low viscosity working fluid and which is accompanied by a secondary bearing that is a roller bearing element. As used herein, a “hybrid-type low-loss bearing” is a bearing assembly having two primary bearing units accompanied by a secondary bearing. Examples of “roller bearing elements” used as the secondary or back-up bearings in mono-type or hybrid-type low-loss bearings include spherical roller bearings, conical roller bearings, tapered roller bearings, and ceramic roller bearings. 
     U.S. patent application Ser. No. 14/460,576, entitled “POWER GENERATION ARCHITECTURES WITH MONO-TYPE LOW-LOSS BEARINGS AND LOW-DENSITY MATERIALS”, which is incorporated by reference herein, provides more details on the use of mono-type bearings in power generation architectures. U.S. patent application Ser. No. 14/460,595, entitled “POWER GENERATION ARCHITECTURES WITH HYBRID-TYPE LOW-LOSS BEARINGS AND LOW-DENSITY MATERIALS”, which is incorporated by reference herein, provides more details on the use of hybrid-type bearings in power generation architectures. 
     In either mono-type or hybrid-type low-loss bearings, the working fluid(s) may be very low viscosity fluids. In certain embodiments, in hydrodynamic bearings, the working fluid(s) may be very low viscosity fluids. Examples of “very low viscosity” fluids used in the present low-loss bearings have a viscosity less than water (i.e., 1 centipoise at 20° C.) and may include, but are not limited to: air (e.g., in high pressure air bearings), gas (e.g., in high pressure gas bearings), magnetic flux (e.g., in high flux magnetic bearings), and steam (e.g., in high pressure steam bearings). In a gas bearing, the gaseous fluid may be an inert gas, hydrogen, carbon dioxide (CO 2 ), nitrogen dioxide (NO 2 ), or hydrocarbons (including methane, ethane, propane, and the like). In some embodiments, power generation architectures may include a hydrodynamic bearing including very low viscosity fluid(s). 
     In hybrid-type low-loss bearings, the first primary bearing unit includes a magnetic bearing having magnetic flux as the working fluid. The second primary bearing unit includes a foil bearing supplied with a high pressure fluid having a very low viscosity, examples of which are provided above. 
     For clarity in illustrating the various power train architectures, the bearings (regardless of type) are represented with a rectangular symbol and the number  140 . Generally speaking, the working fluid provided by a bearing fluid skid to each primary bearing unit is illustrated by an arrow. In some embodiments, one or more of the primary bearing units may include a hydrodynamic bearing. To represent hybrid-type low-loss bearings, the working fluids provided by the bearing fluid skid to the two primary bearing units are represented in the Figures by two lines with different-shaped arrows. In particular, an arrow with a closed head represents piping delivering the magnetic fluid, while an arrow with an open head represents piping delivering one of the above-mentioned very low viscosity fluids. 
     Although the Figures may illustrate the hybrid-type low-loss bearings being used in most or all of the sections of the power train architectures, it is not necessary that all of the bearings be hybrid bearings. For example, a combination of low-loss lubricant bearings may be used in conjunction with conventional oil bearings, the low-loss lubricant bearings being used in some locations and the conventional oil bearings being used in other locations. Alternately or in addition, one or more of the bearings may include very low viscosity fluids in either mono-type or hybrid-type low-loss bearings, as long as at least one bearing is a low-loss lubricant bearing. In scenarios where a conventional oil bearing is used at a particular location, it would receive a single fluid (oil) supplied from the bearing fluid skid. In some embodiments where a hydrodynamic bearing is used at a particular location, it would receive a single fluid (oil) supplied from the bearing fluid skid. In scenarios where a mono-type bearing (containing a very low viscosity fluid) is used, such bearing would likewise receive a single fluid from the bearing fluid skid. Thus, the use of two arrows to each bearing in the accompanying Figures is merely illustrative and is not intended to limit the scope of the disclosure to any particular arrangement (e.g., one using only hybrid-type bearings). 
     As used herein, a “low-density material” is material that has a density that is less than about 0.2 lbm/in 3 . Examples of a low-density material that is suitable for use with rotating components (e.g., blades  130  and  135 ) illustrated in the Figures and described herein include, but are not limited to: composite materials, including ceramic matrix composites (CMCs), organic matrix composites (OMCs), polymer glass composites (PGCs), metal matrix composites (MMCs), carbon-carbon composites (CCs); beryllium; titanium (such as Ti-64, Ti-6222, and Ti-6246); intermetallics including titanium and aluminum (such as TiAl, TiAl 2 , TiAl 3 , and Ti 3 Al); intermetallics including iron and aluminum (such as FeAl); intermetallics including platinum and aluminum (such as PtAl); intermetallics including cobalt and aluminum (such as CbAl); intermetallics including lithium and aluminum (such as LiAl); intermetallics including nickel and aluminum (such as NiAl); and nickel foam. 
     Use of the phrase “the low-density material” in the present application, including the Claims, should not be interpreted as limiting the various embodiments to the use of a single low-density material, but rather can be interpreted as referring to components including the same or different low-density materials. For example, a first low-density material could be used in one section of an architecture while a second (different) low-density material could be used in another section. In another example, a first low-density material could be used in one stage of a section (e.g., the turbine section), while a second (different) low-density material could be used in a second stage of the same section (e.g., the turbine section). 
     In the Figures, the use of low-density materials is represented by a dashed line in the respective section of the power train where such low-density materials may be used. To represent the use of low-density material within the rotating components of the generator, cross-hatched shading is used. Although the Figures may illustrate the low-density materials being used in most or all of the sections of the power train architectures, it should be understood that the low-density materials may be confined to only those sections supported by low-loss bearings. In some embodiments, the low-density materials may be confined to only those sections supported by hydrodynamic bearings. 
     In contrast to the low-density materials described above, a “high-density material” is a material that has a density that is greater than about 0.2 lbm/in 3 . Examples of a high-density material (as used herein) include, but are not limited to: nickel-based superalloys (such as alloys in single-crystal, equi-axed, or directionally-solidified form, examples of which include INCONEL® 625, INCONEL® 706, and INCONEL® 718 (Special Metals Corporation, New Hartford, N.Y.); steel-based superalloys (such as wrought CrMoV and its derivatives, GTD-450, GTD-403 Cb, and GTD-403 Cb+, General Electric Company, Schenectady, N.Y.); and all stainless steel derivatives (such as 17-4PH® stainless steel, AISI type 410 stainless steel, and the like). 
     The technical effects of having power train architectures with combined features of low-loss lubricant bearings including hydrodynamic bearings and low-density materials as described herein are that these architectures: (a) provide the ability to use low-loss bearings, for example, hydrodynamic bearings, in a power train that would otherwise be too heavy to operate; (b) provide the ability to operate the bearings at acceptable temperatures, while carrying heavy loads, without prematurely degrading the low-loss lubricant bearing fluid; (c) deliver a high output load while reducing viscous losses that are typically introduced into the power train through the use of high viscosity oil-based bearings; and (d) allow a reduction in the flow and volume of lubricant used by each bearing, thereby permitting a corresponding reduction in the size of the associated lubricant reservoirs, pumps, and the like. 
     Delivering a larger quantity of airflow by using rotating blades in the gas turbine that include low-density materials translates to a higher output of the gas turbine. As a result, gas turbine manufacturers can increase the size of the rotating blades to generate higher airflow rates, while at the same time ensuring that such longer blades keep within the prescribed inlet annulus (AN 2 ) limits to obviate excessive attachment stresses on the blades, even when the blades are made from low-density materials. Note that AN 2  is the product of the annulus area A (in 2 ) and rotational speed N squared (rpm 2 ) of a rotating blade, and is used as a parameter that generally quantifies power output rating from a gas turbine. 
       FIGS. 1 through 13  illustrate various power train architectures including gas turbines, steam turbines, and/or generators, which may include multiple bearing locations.  FIGS. 14 through 19  illustrate various gas turbine architectures, which may include multiple bearing locations. Low-loss bearings  140  (especially including low-loss lubricant bearings) may be used in any location throughout the power train, as desired, regardless of the power output of the power-generating architecture. In power train architectures producing 50 MW or more of electricity, it may be advisable to use low-density materials in conjunction with low-loss bearings, since the larger component size and associated increases in weight with high-power-generating plants may require the use of low-density materials. In power train architectures producing outputs of less than 50 MW (i.e., smaller power trains), it is contemplated that low-loss bearings may be used without low-density materials in the rotating components, although improved performance, operation, and/or efficiency may be achieved by using low-density materials for at least some of the rotating components. 
     In those cases where low-loss bearings are used to support a particular section of the power train architecture, low-density materials may be used in the particular rotating components of that particular section of the power train where the low-loss bearings are used to support that particular section. For example, if the low-loss bearings are supporting a compressor section, low-density materials can be used in one or more of the stages of rotating blades within the compressor section (as indicated by dashed lines). Similarly, if the low-loss bearings are supporting a generator, low-density materials can be used in the rotating components of the generator (as indicated by cross-hatching). In some embodiments, the low-loss bearings include a hydrodynamic bearing having the low-loss lubricant. 
     The term “rotating component” is intended to include one or more of the moving parts of a compressor section, a turbine section, a reheat turbine section, a power turbine section, a steam turbine, or a generator, such as blades (also referred to as airfoils), coverplates, spacers, seals, shrouds, heat shields, and any combinations of these or other moving parts. For convenience herein, the rotating blades of the compressor and the turbine will be referenced most often as being made of a low-density material. However, it should be understood that other components of low-density material may be used in addition to, or instead of, the rotating blades. 
     Although the descriptions that follow with respect to the illustrated power train architectures are for use in a commercial or industrial power-generating plant, the various embodiments are not meant to be limited solely to such applications. Instead, the concepts of using low-loss bearings and rotating components of low-density material are applicable to all types of combustion turbine or rotary engines, including, but not limited to, a stand-alone compressor such as a multi-stage axial compressor arrangement, aircraft engines, marine power drives, and the like. 
     Referring now to the Figures,  FIG. 1  is a schematic diagram of a single-shaft, simple cycle power train architecture  100  with a gas turbine  10  and a generator  120 . At least one low-loss lubricant bearing and at least one rotating component made of a low-density material are used with the power train of the gas turbine, according to an embodiment of the present invention. 
     Briefly, as shown in  FIG. 1 , the gas turbine  10  comprises a compressor section  105 , a combustor section  110 , and a turbine section  115 . The gas turbine  10  is in a front-end arrangement with generator  120  such that the generator is located proximate the compressor section  105 . Other architectures for the gas turbine  10  may be used, many of which are illustrated in the following Figures, including  FIGS. 16, 17, and 19 . 
       FIG. 1  and  FIGS. 2-19  do not illustrate all of the connections and configurations of the compressor section  105 , the combustor section  110 , and the turbine section  115 . However, these connections and configurations may be made pursuant to conventional technology. For example, the compressor section  105  can include an air intake line that provides inlet air to the compressor. A first conduit may connect the compressor section  105  to the combustor section  110  and may direct the air that is compressed by the compressor section  105  into the combustor section  110 . The combustor section  110  combusts the supply of compressed air with a fuel provided from a fuel gas supply in a known manner to produce the working fluid. 
     A second conduit can conduct the working fluid away from the combustor section  110  and direct it to the turbine section  115 , where the working fluid is used to drive the turbine section  115 . In particular, the working fluid expands in the turbine section  115 , causing the rotating blades  135  of the turbine  115  to rotate about the rotor shaft  125 . The rotation of the blades  135  causes the rotor shaft  125  to rotate. In this manner, the mechanical energy associated with the rotating rotor shaft  125  may be used to drive the rotating blades  130  of the compressor section  105  to rotate about the rotor shaft  125 . The rotation of the rotating blades  130  of the compressor section  105  causes it to supply the compressed air to the combustor section  110  for combustion. The rotation of the rotor shaft  125 , in turn, causes coils of the generator  120  to generate electric power and produce electricity. 
     A common rotatable shaft, referred to as rotor shaft  125 , couples the compressor section  105 , the turbine section  115 , and the generator  120  along a single line, such that the turbine section  115  drives the compressor section  105  and the generator  120 . As shown in  FIG. 1 , the rotor shaft  125  extends through the turbine section  115 , the compressor section  105 , and the generator  120 . In this single-shaft arrangement, the rotor shaft  125  can have a compressor rotor shaft part, a turbine rotor shaft part, and a generator rotor shaft part coupled pursuant to conventional technology. 
     Coupling components can couple the turbine rotor shaft part, the compressor rotor shaft part and the generator rotor shaft part of rotor shaft  125  to operate in cooperation with bearings  140 . The number of coupling components and their locations along rotor shaft  125  can vary by design and application of the power-generating plant in which the gas turbine architecture operate. In some instances in the Figures, a vertical line through the shaft may be used to represent a joint between segments of the rotor shaft  125 . 
     One representative load coupling element  104  is illustrated in  FIG. 1  (between the gas turbine  10  and the generator  120 ), by way of example. Alternately, a clutch  108  may be used as the load coupling element, as shown in  FIG. 5  (between the steam turbine  40  and the generator  120 ). In this manner, the respective rotor shaft parts that are coupled to the coupling members are rotatable thereto by respective bearings  140 . 
     The compressor section  105  can include multiple stages of blades  130  disposed in an axial direction along the rotor shaft  125 . For example, the compressor section  105  can include forward stages of blades  130 , mid stages of blades  130 , and aft stages of blades  130 . As used herein, the forward stages of blades  130  are situated at the front or forward end of compressor section  105  along rotor shaft  125  at the portion where airflow (or gas flow) enters the compressor via inlet guide vanes (that is, distal to the combustor section  110 ). The mid and aft stages of blades are the blades disposed downstream of the forward stages along the rotor shaft  125  where the airflow (or gas flow) is further compressed to an increased pressure (that is, proximate to the combustor section  110 ). Accordingly, the length of the blades  130  in the compressor section  105  decreases from forward to mid to aft stages. 
     Each of the stages in the compressor section  105  can include rotating blades  130  arranged in a circumferential array about the circumference of the rotor shaft  125  to define moving blade rows extending radially outward from the rotatable shaft. The moving blade rows are disposed axially along rotor shaft  125  in locations that are situated in the forward stages, the mid stages, and the aft stages. In addition, each of the stages can include a corresponding number of annular rows of stationary vanes (not illustrated) extending radially inward towards rotor shaft  125  in the forward stages, the mid stages, and the aft stages. In one embodiment, the annular rows of stationary vanes can be disposed on the compressor&#39;s casing (not illustrated) that surrounds the rotor shaft  125 . 
     In each of the stages, the annular rows of stationary vanes can be arranged with the moving blade rows in an alternating pattern along an axial direction of the rotor shaft  125  parallel with its axis of rotation. A grouping of a row of stationary vanes and a row of moving blades defines an individual “stage” of the compressor  105 . In this manner, the moving blades in each stage are cambered to apply work and to turn the flow toward the axial direction, while the stationary vanes in each stage are cambered to turn the flow toward the axial direction, preparing it for the moving blades of the next stage. In one embodiment, the compressor section  105  can be a multi-stage axial compressor. 
     The turbine section  115  can also include stages of blades  135  disposed in an axial direction along rotor shaft  125 . For example, the turbine section  115  can include forward stages of blades  135 , mid stages of blades  135 , and aft stages of blades  135 . The forward stages of blades  135  are situated at the front or forward end of the turbine section  115  along rotor shaft  125  at the portion where a hot compressed motive gas, also known as a working fluid, enters the turbine section  115  from the combustor section  110  for expansion. The mid and aft stages of blades are the blades disposed downstream of the forward stages along the rotor shaft  125  where the working fluid is further expanded (that is, distal to the combustor section  110 ). Accordingly, the length of the blades  135  in the turbine section  115  increases from forward to mid to aft stages. 
     Each of the stages in the turbine section  115  can include rotating blades  135  arranged in a circumferential array about the circumference of the rotor shaft  125  to define moving blade rows extending radially outward from the rotatable shaft. Like the stages for the compressor section  105 , the moving blade rows of the turbine section  115  are disposed axially along the rotor shaft  125  in locations that are situated in the forward stages, the mid stages, and the aft stages. In addition, each of the stages can include annular rows of stationary vanes extending radially inward towards the rotor shaft  125  in the forward stages, the mid stages, and the aft stages. In one embodiment, the annular rows of stationary vanes can be disposed on the turbine&#39;s casing (not illustrated) that surrounds the rotor shaft  125 . 
     In each of the stages, the annular rows of stationary vanes can be arranged with the moving blade rows in an alternating pattern along an axial direction of the rotor shaft  125  parallel with its axis of rotation. A grouping of a row of stationary vanes and a row of moving blades defines an individual “stage” of the turbine section  115 . In this manner, the moving blades in each stage are cambered to apply work and to turn the flow toward the axial direction, while the stationary vanes in each stage are cambered to turn the flow toward the axial direction, preparing it for the moving blades of the next stage. 
     As described herein, at least one of a plurality of the rotating components (e.g., blades  130  and  135 ) in one of the compressor section  105  and the turbine section  115  may be formed from a low-density material. Those skilled in the art will appreciate that the number and placement of rotating blades  130  and  135  that include a low-density material can vary by design and application of the power-generating plant in which the gas turbine architecture operates. For example, some or all of rotating blades  130  and  135  of a particular section (i.e., compressor section  105  or turbine section  115 ) can include a low-density material. In instances where rotating blades  130  and  135  in one or more rows or stages are formed of a low-density material, then rotating blades  130  and  135  in other rows or stages may be formed from a high-density material. By way of example, it may be desirable to form the blades  130  in the forward stages of the compressor section  105  and/or the blades  135  in the aft stages of the turbine section  115  from a low-density material, since these blades are the longest and would otherwise be the heaviest. In some embodiments, a plurality of hydrodynamic bearings  140  is used to support the plurality of rotating components (e.g., blades  130 ), where each of the plurality of hydrodynamic bearings  140  includes a low-loss lubricant and supports a respective section where a corresponding one of the plurality of rotating components (e.g., blades  130 ) including the low-density material is disposed. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. 
     Referring back to  FIG. 1 , the bearings  140  support the rotor shaft  125  along the power train. For example, a pair of bearings  140  can each support the turbine rotor shaft part, the compressor rotor shaft part, and the generator rotor shaft part of rotor shaft  125 . In one embodiment, each pair of bearings  140  can support the turbine rotor shaft part, the compressor rotor shaft part, and the generator rotor shaft part at their respective opposite ends of rotor shaft  125 . However, those skilled in the art will appreciate that the pair of bearings  140  can support the turbine rotor shaft part, the compressor rotor shaft part, and the generator rotor shaft part at other suitable points. 
     Moreover, those skilled in the art will appreciate that each of the turbine rotor shaft part, the compressor rotor shaft part, and the generator rotor shaft part of rotor shaft  125  is not limited to support by a pair of bearings  140 . The bearing  140  shown between the compressor section  105  and the turbine section  115  (that is, beneath the combustors  110 ) may be optional; that is, in some configurations, the gas turbine may be readily supported by bearing supporting the gas compressor section  105  and the turbine section  115  without an intermediate bearing. 
     In the various embodiments described herein, at least one of bearings  140  can be described as a low-loss bearing including a low-loss lubricant (i.e., “a low-loss lubricant bearing”). In one embodiment, all of the bearings  140  are low-loss lubricant bearings. In such a configuration, a bearing fluid skid  150  having a single fluid (i.e., a low-loss lubricant) is used. Bearings including a low-loss lubricant use a significantly smaller volume of fluid than conventional, high-viscosity oil bearings, thereby permitting the reservoirs, pumps, and other accessories in the bearing fluid skid  150  to be down-sized for the smaller fluid volume. Such an arrangement simplifies the bearing fluid skid  150  and reduces start-up and maintenance costs, when compared to conventional oil bearings. 
     Additionally, mono-type or hybrid-type low-loss bearings (as described herein) include a roller bearing element as a back-up to the primary bearing unit(s). These back-up bearings increase the length of the rotor shaft  125  connecting the sections of the power train, thereby increasing the manufacturing costs of the rotor shaft  125 . Thus, the incumbent costs of mono-type and hybrid-type low-loss bearings (when used in conjunction with low-loss lubricant bearings) are weighed against the output and efficiency benefits afforded by the reduced viscous losses such low-loss bearings provide. 
     Accordingly, to mitigate the risks of incumbent costs, in one embodiment, another of the bearings  140  may be a mono-type low-loss bearing having a very low viscosity fluid. In other embodiments, another of the bearings  140  may be a hybrid-type bearing including a first primary bearing unit supplied with magnetic flux and a second primary bearing unit supplied with a very low viscosity fluid. In some embodiments, it may be desirable to use conventional high viscosity oil bearings with the low-loss lubricant bearings and, optionally, mono-type and/or hybrid-type bearings with very low viscosity fluids. Thus, in some arrangements, a combination of bearing types may be used, in which one or more bearings include very low viscosity fluids, while at least one bearing includes a low-loss lubricant. In such combinations, the bearings  140  having very low viscosity fluids may be mono-type or hybrid-type bearings. Alternatively, in some embodiments, a combination of hydrodynamic bearings having different working fluids may be used, which includes a plurality of hydrodynamic bearings each including a low-loss lubricant, and one or more hydrodynamic bearings including a very low viscosity fluid. 
     The bearings  140  include fluids supplied by a bearing fluid skid  150 , which is illustrated in  FIG. 1 . The bearing fluid skid  150  is marked with the letters “LLL” (for low-loss lubricant), “A” (for air), “G” (for gas), “F” (for magnetic flux), “S” (for steam), and “O” (for high viscosity oil) to represent the variety of fluids that may be used, although it should be understood that one or a combination of these fluids may be used to supply the multiple bearings  140  in the power train. In the present invention, an architecture having at least one bearing with a low-loss lubricant (LLL) is used. In these architectures, the bearings  140  are of a low-loss type—that is, bearings including a low-loss lubricant, as described above. If desired, combinations of low-loss lubricant bearings, mono-type type, hybrid-type bearings, and/or conventional high viscosity oil bearings may be employed. 
     The bearing fluid skid  150  may include equipment standard for bearing fluid skids, such as reservoirs, pumps, accumulators, valves, cables, control boxes, piping, and the like. The piping necessary to deliver the fluid(s) from the bearing fluid skid  150  to the one or more bearings  140  is represented in the Figures by arrows from the bearing fluid skid  150  to each of the bearings  140 . In some instances, it may be possible for the bearing fluid skid  150  to provide two or more different types of fluids (such as oil and one or more of the low-loss lubricants or very low viscosity fluids described above). Alternately, if two or more different bearing types or bearing fluids are used, bearing skids  150  for each fluid type may be employed. It is also possible to employ different bearing fluid skids  150  for different sections of the architecture. 
     Those skilled in the art will appreciate that the selection of low-loss bearings used for bearings  140  can vary by design and application of the power-generating plant in which the power train architecture operates. For example, some or all of bearings  140  can be low-loss lubricant bearings. Additionally, one or some of the bearings  140  can be mono-type or hybrid-type bearings having a very low viscosity fluid. It is desirable for at least one bearing  140  to include a low-loss lubricant, regardless of the bearing fluids or bearing types of the other bearings  140  in the power train. In some embodiments, the low-loss bearings include a plurality of hydrodynamic bearing each having the low-loss lubricant. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. 
     In addition, the power generating architecture  100  may include a combination of low-loss lubricant bearings with conventional oil bearings. In those sections where the rotor shaft part is supported by low-loss lubricant bearings (instead of conventional oil bearings), it may be preferred to incorporate low-density materials in the respective section to create a section whose weight is more easily supported and rotated. For example, the low-loss bearings may include a plurality of hydrodynamic bearing each including the low-loss lubricant and supporting a respective section of the rotor shaft where the low-density material is disposed. Likewise, those sections supported by mono-type or hybrid-type bearings including very low viscosity fluids benefit from the use of low-density materials in those sections. In some embodiments, power train  100  may include a second bearing supporting a section different from the section supported by each of the first plurality of hydrodynamic bearings. The second bearing may include the very low viscosity fluid, where the very low viscosity fluid has a viscosity grade (VG) less than VG 1 . In some embodiments, the second bearing is a mono-type bearing or a hybrid-type bearing. In certain embodiment, the second bearing may be a hydrodynamic bearing. 
     In addition, those skilled in the art will appreciate that, for clarity, the power train architecture shown in  FIG. 1 , and those illustrated in subsequent  FIGS. 2-19 , only show those components that provide an understanding of the various embodiments of the invention. Those skilled in the art will appreciate that there are additional components other than those that are shown in these figures. For example, a gas turbine and generator arrangement could include secondary components such as gas fuel circuits, a gas fuel skid, liquid fuel circuits, a liquid fuel skid, flow control valves, a cooling system, etc. 
     In a power train architecture such as those illustrated herein, which includes multiple bearings, the balance-of-plant (BoP) viscous losses are reduced in each location where a low-loss lubricant bearing is substituted for a conventional viscous fluid (oil) bearing. Thus, replacing multiple—if not all—of the viscous fluid bearings with low-loss bearings, as described, significantly reduces viscous losses, thereby increasing the efficiency of the power train at a base load of operation and a part load of operation. 
     Below are brief descriptions of the power train architectures illustrated in  FIGS. 2-13 . Specific gas turbine architectures, which may be employed in the power train architectures shown in  FIGS. 1-13 , are illustrated in  FIGS. 14-19 . All of these Figures illustrate different types of power trains that can be implemented in a power-generating plant. Although each architecture may operate in a different manner than the configuration of  FIG. 1 , they are similar in that the embodiments in  FIGS. 2-19  can have at least one low-density rotating component (e.g., the rotating blades  130  and  135  of compressor  105  and turbine  110 , respectively). Similarly, these embodiments can use at least one low-loss lubricant bearing for bearings  140 . 
     As noted above, some or all of the rotating components  130  and  135  in one or more sections can be of a low-density material. With particular reference to blades in the compressor or turbine sections, rotating components of low-density material can be interspersed by stage with rotating components of high-density material Likewise, one, some, or all of the bearings  140  can be a low-loss bearing, particularly low-loss bearings including low-loss lubricants. In this manner, bearings of a low-loss bearing type can be interspersed with other types of bearings such as high viscosity oil bearings, mono-type low-loss bearings, and/or hybrid-type low-loss bearings. 
     Further, the use of low-density rotating components and low-loss lubricant bearings in a power train of a power-generating plant are not meant to be limited to the examples illustrated in  FIGS. 1-19 . Instead, these examples are merely illustrative of some of the possible architectures in which the use of low-density rotating components and low-loss lubricant bearings can be implemented in a power train of a power-generating plant. Those skilled in the art will appreciate that there are many permutations of possible configurations of the examples illustrated herein. The scope and content of the various embodiments are meant to cover those possible permutations, as well as other possible power train configurations that can be implemented in a power-generating plant that use a gas turbine. 
     In addition, the descriptions that follow for the various architectures with their respective generator arrangements are directed to generators capable of being driven at various speeds (measured in revolutions-per-minute, or RPMs) to operate at a desired frequency output. It is not necessary that the turbine section directly drive the generator at 3600 RPMs in order to operate at 60 Hz, although such a speed and output may be desired for many applications. For instance, multi-shaft arrangements and/or torque-altering mechanisms (as in  FIG. 19 ) may be employed to achieve the desired generator output. 
     The various embodiments of the present invention are not meant to be limited to any particular type of generator and, therefore, are applicable to a wide variety of generators, including, but not limited to, two-pole generators that rotate at a speed of 3600 RPMs for operating at 60 Hz; four-pole generators that rotate at a speed of 1800 RPMs for operating at 60 Hz; two-pole generators that rotate at a speed of 3000 RPMs for operating at 50 Hz; and four-pole generators that rotate at a speed of 1500 RPMs for operating at 50 Hz. Other speeds and frequency outputs may be desired and appropriate for power train architectures producing less than 50 MW of power output. 
       FIG. 2  illustrates a simple cycle power train architecture  200  including a rear-end drive gas turbine  12 , a generator  120 , and a bearing fluid skid  150 . In the architecture  200 , the gas turbine  12  is arranged such that the generator  120  is coupled, via load coupling  104 , to the turbine section  115  of the gas turbine, thus creating a “rear-end drive” gas turbine  12 . 
     As with the architecture  100  shown in  FIG. 1 , the power train architecture  200  includes at least one bearing  140 , which is in fluid communication with the bearing fluid skid  150 . In at least one bearing  140 , the fluid is a low-loss lubricant. At least one rotating component (such as compressor blades  130  or turbine blades  135 ) is made of a low-density material, according to an embodiment of the present invention. Since the individual components of the architecture  200  are the same as those in the architecture  100 , reference is made to the previous discussion of  FIG. 1 , and the discussion of each element is not repeated here. 
       FIG. 3  is a schematic diagram of a power train architecture  300  having a front-end drive gas turbine  14  with a reheat section  205 . As shown in  FIG. 3 , the reheat section  205  includes a second combustor section  210  and a second turbine section  215 , also referred to as a reheat combustor and reheat turbine, respectively, downstream of the first combustor section  110  and the first turbine section  115 . The power train architecture  300  includes at least one low-loss bearing  140 , which is in fluid communication with the bearing fluid skid  150  (as described above). In some embodiments, the low-loss bearings include a plurality of hydrodynamic bearing each having the low-loss lubricant. In some embodiments, at least one bearing  140  is a low-loss lubricant bearing, although mono-type and/or hybrid-type bearings having a very low viscosity may also be employed. 
     In this embodiment, both the turbine section  115  and the turbine section  215  can have rotating components (such as blades  135 ,  220 , respectively), which include at least one rotating component that includes a low-density material. In one embodiment, all or some of rotating blades  135  and/or  220  in one of, some of, or all of the turbine stages can include the low-density material. In another embodiment, the rotating components  130  in the compressor section  105  may include a low-density material. In another embodiment, at least one of the compressor section  105  and the turbine section  115  may include rotating components  130 ,  135  of a low-density material, while the rotating components  220  of the reheat turbine section  215  can be of a different type of material (e.g., a high-density material). If desired, each of the compressor section  105 , the turbine section  115 , and the reheat turbine  215  may include one or more stages of rotating components  130 ,  135 ,  220  of a low-density material. Other rotating components of a low-density material, including rotating components in the generator  120 , may be used in addition to, or instead of, the low-density rotating blades  130 ,  135 ,  220  described above. 
       FIG. 4  is a schematic diagram of a single-shaft steam turbine and generator (STAG) power train architecture  400  including a front-end drive gas turbine  10 , a multi-stage steam turbine  40 , a generator  120 , and a bearing fluid skid  150 . A first load coupling  104  is positioned between the gas turbine  10  and the generator  120 . The steam turbine  40  includes a high pressure (HP) section  402 , an intermediate pressure (IP) section  404 , and a low pressure (LP) section  406 . A second load coupling  106  connects the steam turbine  40  to the generator  120 , thereby completing the unified shaft  125 . Low-loss bearings  140  may be used to support any or all of the sections of the power train, the low-loss bearings  140  being fluidly connected to the bearing fluid skid  150 . At least one of the low-loss bearings  140  includes a low-loss lubricant. In some embodiments, at least one of low-loss bearings  140  includes a plurality of hydrodynamic bearing each having the low-loss lubricant. The power train  400  also may employ mono-type low-loss bearings, hybrid-type low-loss bearings, and/or conventional oil bearings as bearings  140 , if so desired. 
     Additionally, shown in  FIG. 4  is a heat exchanger, such as a heat recovery steam generator (or “HRSG”)  50 . The HRSG  50  converts water (W) into steam that is supplied to the high pressure section  402  of the steam turbine  40 , as indicated by dashed lines. The flow paths of the steam are indicated by dashed arrows, as steam is transferred sequentially from the high pressure section  402  to the intermediate pressure section  404  to the low pressure section  406 . Energy from a portion of the exhaust gases (“EG”) from the turbine section  115  of the gas turbine  10  is used to produce steam in the HRSG. 
     Low-density materials may be used for the rotating components of at least one of the compressor section  105  of the gas turbine  10 , the turbine section  115  of the gas turbine  10 , the high pressure section  402  of the steam turbine  40 , the intermediate pressure section  404  of the gas turbine  40 , the low pressure section  406  of the steam turbine  40 , and the generator  120 . The use of low-density materials (e.g., in blades  130 ,  135 ) reduces the weight of the stage, stages, or components being rotated, thus facilitating the use of low-loss bearings  140  for the corresponding section of the power train architecture  400 . In some embodiments, low-loss bearings  140  includes a plurality of hydrodynamic bearings each having the low-loss lubricant and supporting the corresponding section of the power train architecture  400  where a corresponding one of the plurality of rotating components including the low-density material is disposed. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. 
       FIG. 5  illustrates a power train architecture  500 , which is a variation of the power train architecture  400  shown in  FIG. 4 . In  FIG. 5 , a single-shaft steam turbine and generator (STAG) is provided with a front-end drive gas turbine  10 , a generator  120 , a clutch  108 , a multi-stage steam turbine  40 , a heat exchanger  50 , and a bearing fluid skid  150 . In this architecture  500 , the generator  120  is coupled, via load coupling  104 , to the front end (i.e., compressor section  105 ) of the gas turbine  10  and is further coupled, via the clutch  108 , to the steam turbine  40 . Steam supplied from the heat exchanger  50  is directed to the high pressure section  402  of the steam turbine  40 , the steam being subsequently routed through the intermediate pressure section  404  and the low pressure section  406  (as indicated by dashed arrows). 
     Low-density materials may be used for the rotating components of at least one of the compressor section  105  of the gas turbine  10  (e.g., in blades  130 ), the turbine section  115  of the gas turbine  10  (e.g., in blades  135 ), the high pressure section  402  of the steam turbine  40 , the intermediate pressure section  404  of the steam turbine  40 , the low pressure section  406  of the steam turbine  40 , and the generator  120 . The low-density materials may be used in one or more stages, for example, in an individual section of the gas turbine  10  or steam turbine  40 . 
     Low-loss lubricant bearings  140  may be used to support one or more sections of the power train architecture  500  where a corresponding rotating component including the low-density material is disposed. In some embodiments, low-loss bearings  140  includes a plurality of hydrodynamic bearings each having the low-loss lubricant. Other bearing types (including conventional oil bearings, mono-type low-loss bearings, and/or hybrid-type low-loss bearings) may be used in sections of the power train  500 , in addition to at least one low-loss lubricant bearing. The bearings  140  are fluidly connected to the bearing fluid skid  150 , as described previously, from which at least one of the bearings  140  receives a low-loss lubricant. In some embodiments, bearings  140  are fluidly connected to bearing fluid skid  150 , from which at least one of the bearings  140  receives a very low viscosity fluid. 
       FIG. 6  illustrates a power train architecture  600 , which is another alternate arrangement of the power train architecture  400  shown in  FIG. 4 . In  FIG. 6 , a single-shaft steam turbine and generator (STAG) is provided with a rear-end drive gas turbine  12 , a generator  120 , a multi-stage steam turbine  40 , a heat exchanger  50 , and a bearing fluid skid  150 . In this architecture  600 , the generator  120  is coupled, via a first load coupling  104 , to the rear end (i.e., turbine section  115 ) of the gas turbine  12  and is further coupled, via a second load coupling  106 , to the steam turbine  40 . Steam supplied from the heat exchanger  50  is directed to the high pressure section  402  of the steam turbine  40 , the steam being subsequently routed through the intermediate pressure section  404  and the low pressure section  406  (as indicated by dashed arrows). 
     Low-density materials may be used for the rotating components of at least one of the compressor section  105  of the gas turbine  12  (e.g., in blades  130 ), the turbine section  115  of the gas turbine  12  (e.g., in blades  135 ), the high pressure section  402  of the steam turbine  40 , the intermediate pressure section  404  of the steam turbine  40 , the low pressure section  406  of the steam turbine  40 , and the generator  120 . The low-density materials may be used in one or more stages, for example, in an individual section of the gas turbine  12  or steam turbine  40 . 
     Low-loss lubricant bearings  140  may be used to support one or more sections of the power train architecture  600 . In certain embodiments, steam turbine  40  may include a second plurality of hydrodynamic bearings  140  supporting a steam turbine rotor shaft part within high pressure section  402 , intermediate pressure section  404 , and low pressure section  406 , each of the second plurality of hydrodynamic bearings including the low-loss lubricant. In some embodiments, each of the second plurality of hydrodynamic bearings including the low-loss lubricant supports a section in which a corresponding one of the rotating component including the low-density material is disposed. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. 
     Other bearing types (including conventional oil bearings, mono-type low-loss bearings, and/or hybrid-type low-loss bearings) may be used in sections of the power train  600 , in addition to at least one low-loss lubricant bearing. In some embodiments, power train  600  may include a second bearing supporting a section different from the section supported by each of the first plurality of hydrodynamic bearings. The second bearing may include the very low viscosity fluid, where the very low viscosity fluid has a viscosity grade (VG) less than VG 1 . In embodiments, the second bearing is a mono-type bearing or a hybrid-type bearing. In certain embodiment, the second bearing may be a hydrodynamic bearing. The bearings  140  are fluidly connected to the bearing fluid skid  150 , as described previously, from which at least one of the bearings  140  receives a low-loss lubricant. In some embodiments, steam turbine bearing fluid skid  150  delivers the low-loss lubricant to each one of the first plurality of hydrodynamic bearings and the second plurality of hydrodynamic bearings. 
       FIG. 7  illustrates a power train architecture  700 , which is still another alternate arrangement of the power train architecture shown in  FIG. 4 . In  FIG. 7 , a single-shaft steam turbine and generator (STAG) is provided with a front-end drive gas turbine  14  with a reheat section  205 , a generator  120 , a multi-stage steam turbine  40 , a heat exchanger  50 , and a bearing fluid skid  150 . In this arrangement, the generator  120  is coupled, via a first load coupling  104 , to the front end (i.e., compressor section  105 ) of the gas turbine  14  and is further coupled, via a second load coupling  106 , to the steam turbine  40 . Steam supplied from the heat exchanger  50  is directed to the high pressure section  402  of the steam turbine  40 , the steam being subsequently routed through the intermediate pressure section  404  and the low pressure section  406  (as indicated by dashed arrows). 
     Low-density materials may be used for the rotating components of at least one of the compressor section  105  of the gas turbine  14  (e.g., in blades  130 ), the turbine section  115  of the gas turbine  14  (e.g., in blades  135 ), the reheat turbine section  215  of the gas turbine  14  (e.g., in blades  220 ), the high pressure section  402  of the steam turbine  40 , the intermediate pressure section  404  of the steam turbine  40 , the low pressure section  406  of the steam turbine  40 , and the generator  120 . The low-density materials may be used in one or more stages, for example, in an individual section of the gas turbine  14  or steam turbine  40 . 
     Low-loss lubricant bearings  140 , for example, each of a plurality of hydrodynamic bearings including the low-loss lubricant, may be used to support one or more sections of the power train architecture  700  in which a corresponding one of the rotating components made of low-density materials is disposed. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. In certain embodiments, other bearing types (including conventional oil bearings, mono-type low-loss bearings, and/or hybrid-type low-loss bearings) may be used in sections of the power train  700 , in addition to at least one low-loss lubricant bearing. The bearings  140  are fluidly connected to the bearing fluid skid  150 , as described previously, from which at least one of the bearings  140  receives a low-loss lubricant. In some embodiments, bearing fluid skid  150  may deliver the very low viscosity fluid to at least one of bearings  140 . 
       FIG. 8  is a schematic diagram of a two-on-one ( 2 : 1 ) combined cycle power train architecture  800  including two front-end drive gas turbines  10  (each with its own generator  120 , heat exchanger  50 , and bearing fluid skid  150 ) and one multi-stage steam turbine  40  with its own generator  120  and bearing fluid skid  150 . As shown, the gas turbines  10  may be oriented in parallel to one another, although such configuration is not required. 
     In this architecture  800 , each gas turbine  10  operates on its own shaft  125  and is coupled, via a first load coupling  104 , to a generator  120 . In one or both gas turbines  10 , low-density materials may be used as the rotating components in the compressor section  105  (e.g., in blades  130 ) or the turbine section  115  (e.g., in blades  135 ) or in other areas (e.g., in the generator  120 , as indicated by cross-hatching). The bearings  140  supporting the generator  120  and various sections of the gas turbine  10  may be low-loss lubricant bearings including a plurality of hydrodynamic bearings, as described herein. In certain embodiments, architecture  800  also may include mono-type low-loss bearings, hybrid-type low-loss bearings, and/or conventional oil bearings, as long as at least one bearing  140  is a low-loss lubricant bearing. The bearings  140  are fluidly connected to the bearing fluid skid  150  associated with the respective gas turbine  10 . 
     Exhaust products from the turbine section  115  of each gas turbine  10  are directed to a respective heat exchanger  50  (e.g., a HRSG), which produces steam for the high pressure section  402  of the steam turbine  40 . Steam is subsequently routed through the intermediate pressure section  404  and the low pressure section  406  of the steam turbine  40  (as indicated by dashed arrows). The steam turbine  40  is coupled, via a shaft  126 , to a corresponding generator  120 . A load coupling  106  may be included between the steam turbine  40  and the generator  120 . 
     Low-density materials may be used as the rotating components in the high pressure section  402  of the steam turbine  40 , the intermediate pressure section  404  of the steam turbine  40 , the low pressure section  406  of the steam turbine  40 , or in other areas (e.g., in the generator  120  associated with the steam turbine  40 ). The low-density materials may be used in one or more stages, for example, in an individual section of the steam turbine  40  or may be used in all stages of one or more sections of the steam turbine  40 . 
     The bearings  140  supporting the generator  120  and various sections of the steam turbine  40  are fluidly connected to the bearing fluid skid  150  associated with the steam turbine  40 . A low-loss lubricant bearing  140  may be used to support one or more sections of the steam turbine  40  and/or its generator  120 , in addition to or instead of the low-loss lubricant bearing  140  being used in one or both of the gas turbine-generator trains. In some embodiments, each of a plurality of bearings  140  supporting one or more sections of the steam turbine  40  and/or its generator  120 , in addition to or instead of the low-loss lubricant bearing  140  being used in one or both of the gas turbine-generator trains is a hydrodynamic bearing including the low-loss lubricant. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. Alternately, or in addition, the bearings  140  supporting the steam turbine  40  and its associated generator  120  may include mono-type low-loss bearings, hybrid-type low-loss bearings, and/or conventional oil bearings. 
       FIG. 9  is a schematic diagram of a two-on-one (2:1) combined cycle power train architecture  900  including two rear-end drive gas turbines  12  (each with its own generator  120 , heat exchanger  50 , and bearing fluid skid  150 ) and one multi-stage steam turbine  40  with its own generator  120  and bearing fluid skid  150 . As shown, the gas turbines  12  may be oriented in parallel to one another, although such configuration is not required. 
     In this architecture  900 , each gas turbine  12  operates on its own shaft  125  and is coupled, via a first load coupling  104 , to a generator  120 . In one or both gas turbines  12 , low-density materials may be used as the rotating components in the compressor section  105  (e.g., in blades  130 ) or the turbine section  115  (e.g., in blades  135 ) or in other areas (e.g., in the generator  120 , as indicated by cross-hatching). The bearings  140  supporting the generator  120  and various sections of the gas turbine  12  may be low-loss lubricant bearings, including a plurality of hydrodynamic bearings, as described herein. In certain embodiments, architecture  900  also may include mono-type low-loss bearings, hybrid-type low-loss bearings, and/or conventional oil bearings, as long as at least one bearing  140  is a low-loss lubricant bearing. The bearings  140  are fluidly connected to the bearing fluid skid  150  associated with the respective gas turbine  12 . 
     Exhaust products from the turbine section  115  of each gas turbine  12  are directed to a respective heat exchanger  50  (e.g., a HRSG), which produces steam for the high pressure section  402  of the steam turbine  40 . Steam is subsequently routed through the intermediate pressure section  404  and the low pressure section  406  of the steam turbine  40  (as indicated by dashed arrows). The steam turbine  40  is coupled, via a shaft  126 , to a corresponding generator  120 . A load coupling  106  may be included between the steam turbine  40  and the generator  120 . 
     Low-density materials may be used as the rotating components in the high pressure section  402  of the steam turbine  40 , the intermediate pressure section  404  of the steam turbine  40 , the low pressure section  406  of the steam turbine  40 , or in other areas (e.g., in the generator  120  associated with the steam turbine  40 ). The low-density materials may be used in one or more stages, for example, in an individual section of the steam turbine  40  or may be used in all stages of one or more sections of the steam turbine  40 . 
     The bearings  140  supporting the generator  120  and various sections of the steam turbine  40  are fluidly connected to the bearing fluid skid  150  associated with the steam turbine  40 . A low-loss lubricant bearing  140  may be used to support one or more sections of the steam turbine  40  and/or its generator  120 , in addition to or instead of the low-loss lubricant bearing  140  being used in one or both of the gas turbine-generator trains. In some embodiments, each of a plurality of bearings  140  supporting one or more sections of the steam turbine  40  and/or its generator  120 , in addition to or instead of the low-loss lubricant bearing  140  being used in one or both of the gas turbine-generator trains is a hydrodynamic bearing including the low-loss lubricant. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. Alternately, or in addition, the bearings  140  supporting the steam turbine  40  and its associated generator  120  may include mono-type low-loss bearings, hybrid-type low-loss bearings, and/or conventional oil bearings. 
       FIG. 10  is a simplified schematic diagram of a three-on-one (3:1) combined cycle power train architecture  1000 , which includes three rear-end drive gas turbines  12  (each with its own generator  120 , heat exchanger  50 , and bearing fluid skid  150 ) and one multi-stage steam turbine  40  with its own generator  120  and bearing fluid skid  150 . As discussed above, low-density materials may be used in the rotating components of at least one of the compressor section  105  of at least one gas turbine  12 , the turbine section  115  of at least one gas turbine  12 , the generator section  120  of at least one gas turbine  12 , the high pressure section  402  of the steam turbine  40 , the intermediate pressure section  404  of the steam turbine  40 , the low pressure section  406  of the steam turbine  40 , and the generator  120  associated with the steam turbine  40 . Advantageously, for the reasons provided herein, at least one of the sections of the power train architecture  1000  that includes the low-density materials in some or all of its rotating components is supported by at least one low-loss bearing  140  having a low-loss lubricant (as illustrated in the previous Figures). In some embodiments, each at least one low-loss bearings  140  supporting at least one of the sections of power train architecture  1000  that includes the low-density materials in some or all of its rotating components is a hydrodynamic bearing including the low-loss lubricant. 
       FIG. 11  is a schematic diagram of a multi-shaft, combined cycle power train architecture  1100 , which includes a front-end drive gas turbine  10  coupled on a first shaft  125  to a first generator  120  and having a first bearing fluid skid  150 . A first load coupling  104  may be used to connect the gas turbine  10  to the generator  120 . The power train architecture  1100  further includes a multi-stage steam turbine  40  coupled on a second shaft  126  to a second generator  120  and having a second bearing fluid skid  150 . A second load coupling  106  may be used to connect the steam turbine  40  to its corresponding generator  120 . A heat exchanger  50  is fluidly connected to both the gas turbine  10  and the steam turbine  40 , as previously discussed. In this architecture  1100 , the steam from the heat exchanger  50  is provided to the high pressure section  402  of the steam turbine  40  and is subsequently routed through the intermediate pressure section  404  of the steam turbine  40  and the low pressure section  406  of the steam turbine  40 . 
     Again, the rotating components in the compressor section  105  of the gas turbine  10 , the turbine section  115  of the gas turbine  10 , the generator  120  associated with the gas turbine  10 , the high pressure section  402  of the steam turbine  40 , the intermediate pressure section  404  of the steam turbine  40 , the low pressure section  406  of the steam turbine  40 , and/or the generator  120  associated with the steam turbine  40  may be produced from low-density materials. The low-density materials may be used to produce blades  130  in the compressor section  105  or blades  135  in the turbine section  115 , for example. The low-density material may be used for some or all of the rotating components in a given section of the power train architecture  1100 . 
     Low-loss lubricant bearings  140 , for example, a plurality of hydrodynamic bearings including the low-loss lubricant, may be used to support some or all of rotating components in the given section of the power train architecture  1100  in which rotating components made of low-density materials are disposed. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. In certain embodiments, other bearing types (including conventional oil bearings, mono-type low-loss bearings, and/or hybrid-type low-loss bearings) may be used in sections of the power train  1100 , in addition to at least one low-loss lubricant bearing. The bearings  140  are fluidly connected to the bearing fluid skid  150 , as described previously, from which at least one of the bearings  140  receives a low-loss lubricant. In some embodiments, bearing fluid skid  150  may deliver the very low viscosity fluid to at least one of bearings  140 . 
       FIG. 12  is a schematic diagram of a multi-shaft, combined cycle power train architecture  1200 , which is a variation of the architecture  1100  shown in  FIG. 11 . In  FIG. 12 , the architecture  1200  includes a rear-end drive gas turbine  12  coupled on a first shaft  125  to a first generator  120  and having a first bearing fluid skid  150 . A first load coupling  104  may be used to connect the gas turbine  12  to the generator  120 . 
     The power train architecture  1200  further includes a multi-stage steam turbine  40  coupled on a second shaft  126  to a second generator  120  and having a second bearing fluid skid  150 . A second load coupling  106  may be used to connect the steam turbine  40  to its corresponding generator  120 . A heat exchanger  50  is fluidly connected to both the gas turbine  12  and the steam turbine  40 , as previously discussed. In this architecture  1200 , the steam from the heat exchanger  50  is provided to the high pressure section  402  of the steam turbine  40  and is subsequently routed through the intermediate pressure section  404  of the steam turbine  40  and the low pressure section  406  of the steam turbine  40 . 
     As before, one or more of the rotating components in the compressor section  105  of the gas turbine  12 , the turbine section  115  of the gas turbine  12 , the generator  120  associated with the gas turbine  12 , the high pressure section  402  of the steam turbine  40 , the intermediate pressure section  404  of the steam turbine  40 , the low pressure section  406  of the steam turbine  40 , and/or the generator  120  associated with the steam turbine  40  may be produced from low-density materials. The low-density materials may be used to produce blades  130  in the compressor section  105  or blades  135  in the turbine section  115 , for example. The low-density material may be used for some or all of the rotating components in a given section of the power train architecture  1200 . 
     Low-loss lubricant bearings  140 , for example, a plurality of hydrodynamic bearings including the low-loss lubricant, may be used to support some or all of the rotating components in the given section of power train architecture  1200  in which rotating components made of low-density materials are disposed. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. In certain embodiments, other bearing types (including conventional oil bearings, mono-type low-loss bearings, and/or hybrid-type low-loss bearings) may be used in sections of the power train  1200 , in addition to at least one low-loss lubricant bearing. The bearings  140  are fluidly connected to the bearing fluid skid  150 , as described previously, from which at least one of the bearings  140  receives a low-loss lubricant. In some embodiments, bearing fluid skid  150  may deliver the very low viscosity fluid to at least one of bearings  140 . 
       FIG. 13  is a schematic diagram of a multi-shaft, combined cycle power train architecture  1300 , which is a variation of the architecture  1100  shown in  FIG. 11 . In  FIG. 13 , the architecture  1300  includes a front-end drive gas turbine  14  with a reheat section  205  coupled on a first shaft  125  to a first generator  120  and having a first bearing fluid skid  150 . A first load coupling  104  may be used to connect the gas turbine  14  to the generator  120 . 
     The power train architecture  1300  further includes a multi-stage steam turbine  40  coupled on a second shaft  126  to a second generator  120  and having a second bearing fluid skid  150 . A second load coupling  106  may be used to connect the steam turbine  40  to its corresponding generator  120 . A heat exchanger  50  is fluidly connected to both the gas turbine  14  and the steam turbine  40 , as previously discussed. In this architecture  1300 , the steam from the heat exchanger  50  is provided to the high pressure section  402  of the steam turbine  40  and is subsequently routed through the intermediate pressure section  404  of the steam turbine  40  and the low pressure section  406  of the steam turbine  40 . 
     The rotating components in the compressor section  105  of the gas turbine  14 , the turbine section  115  of the gas turbine  14 , the reheat turbine section  215  of the gas turbine  14 , the generator  120  associated with the gas turbine  14 , the high pressure section  402  of the steam turbine  40 , the intermediate pressure section  404  of the steam turbine  40 , the low pressure section  406  of the steam turbine  40 , and/or the generator  120  associated with the steam turbine  40  may be produced from low-density materials. The low-density materials may be used to produce blades  130  in the compressor section  105 , blades  135  in the turbine section  115 , or blades  220  in the reheat turbine section  215 , for example. The low-density material may be used for some or all of the rotating components in a given section of the power train architecture  1300 . 
     Low-loss lubricant bearings  140 , for example, a plurality of hydrodynamic bearings including the low-loss lubricant, may be used to support some or all of the rotating components in the given section of power train architecture  1300  in which rotating components made of low-density materials are disposed. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. In certain embodiments, other bearing types (including conventional oil bearings, mono-type low-loss bearings, and/or hybrid-type low-loss bearings) may be used in sections of the power train  1300 , in addition to at least one low-loss lubricant bearing. The bearings  140  are fluidly connected to the bearing fluid skid  150 , as described previously, from which at least one of the bearings  140  receives a low-loss lubricant. In some embodiments, bearing fluid skid  150  may deliver the very low viscosity fluid to at least one of bearings  140 . 
       FIGS. 14 through 19  illustrate various gas turbine architectures that may be incorporated into the power train architectures illustrated in  FIGS. 1 through 13 . For convenience, the generator  120 , the bearing fluid skid  150 , the heat exchanger  50 , and the steam turbine  40  (if applicable) are omitted from this set of Figures. 
       FIG. 14  is a schematic diagram of a multi-shaft gas turbine architecture  1400 , including a rear-end drive gas turbine  16  having a compressor section  105 , a combustor section  110 , and a turbine section  115  on a first shaft  310 . The gas turbine  16  further includes a power turbine section  305  on a second shaft  315 , which is downstream of the turbine section  115 . The gas turbine  16  of  FIG. 14  may be substituted for the gas turbine  12  in the power train architecture  200  of  FIG. 2 , the power train architecture  600  of  FIG. 6 , the power train architecture  900  of  FIG. 9 , the power train architecture  1000  of  FIG. 10 , and the power train architecture  1200  of  FIG. 12 . 
     In this embodiment, a rear-end drive arrangement is provided, in which the single shaft (as shown in the gas turbine  12  of  FIG. 2 ) has been replaced with a multi-shaft arrangement. In particular, a first single rotor shaft  310  extends through the compressor section  105  and the turbine section  115 , while a second single rotor shaft  315 , separated from the shaft  310 , extends from the power turbine section  305  to the generator  120  (not shown, but indicated by the legend “To Gen”). 
     In operation, the first rotor shaft  310  can serve as the input shaft, while the second rotor shaft  315  can serve as the output shaft. In one embodiment, the output speed of the rotor shaft  315  spins at a constant speed (e.g., 3600 RPMs) to ensure that the generator ( 120 ) operates at a constant frequency (e.g., 60 Hz), while the input speed of the rotor shaft  310  may be different than that of the rotor shaft  315  (e.g., may be greater than 3600 RPMs). 
     Bearings  140  can support the various gas turbine sections on the rotor shaft  310  and the rotor shaft  315 . In one embodiment, bearings  140  may include a plurality of low-loss bearings having a low-loss lubricant, for example, a plurality of hydrodynamic bearings including the low-loss lubricant, as described herein. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. In certain embodiments, other bearings  140  can be mono-type low-loss bearings, hybrid-type low-loss bearings, or conventional oil bearings, as needs dictate. The bearings  140  are in fluid communication with the bearing fluid skid  150 , as shown, for example, in  FIG. 2 . 
     In one embodiment, the power turbine  305  can have at least one rotating component  405  (e.g., a blade) that is made of a low-density material.  FIG. 14  shows that the rotating blades  130  of the compressor section  105 , the rotating blades  135  of the turbine section  115 , and the rotating blades  405  of the power turbine section  305  can include one or more stages of low-density blades. This is one possible implementation and is not meant to limit the scope of architecture  1400 . As mentioned above, there can be any combination of low-density blades with blades made from other materials (e.g., high-density blades), as long as there is at least one rotating blade used in the power train that includes a low-density material. 
     Alternately or in addition, rotating components other than blades  130 ,  135 ,  405  may be made from low-density material; thus, the disclosure is not limited to an arrangement where only the blades are made from low-density material. Preferably, the low-density rotating components  105 ,  135 , and/or  405  are used in a section of the gas turbine  1400  that is supported by bearings  140  that are low-loss bearings, for example, a plurality of hydrodynamic bearings. In one embodiment, at least one low-loss bearing  140  includes a low-loss lubricant. In some embodiments, bearing fluid skid  150  may deliver the very low viscosity fluid to at least one of bearings  140 . 
       FIG. 15  is a schematic diagram of a multi-shaft, rear-end drive gas turbine architecture  1500  having a gas turbine  18  with a power turbine section  305  and a reheat section  205 . As with  FIG. 14 , the gas turbine  18  of  FIG. 15  may be substituted for the gas turbine  12  in the power train architecture  200  of  FIG. 2 , the power train architecture  600  of  FIG. 6 , the power train architecture  900  of  FIG. 9 , the power train architecture  1000  of  FIG. 10 , and the power train architecture  1200  of  FIG. 12 . 
     The gas turbine architecture  1500  further includes at least one low-loss bearing  140  including a low-loss lubricant, for example, a plurality of hydrodynamic bearings including the low-loss lubricant, and at least one rotating component made of a low-density material in use with the power train of the gas turbine, according to an embodiment of the present invention. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. In certain embodiments, other bearings  140  can be mono-type low-loss bearings, hybrid-type low-loss bearings, or conventional oil bearings, as needs dictate. The bearings  140  are in fluid communication with the bearing fluid skid  150 , as shown, for example, in  FIG. 2 . In some embodiments, bearing fluid skid  150  may deliver the low-loss lubricant to at least one of bearings  140 . In some embodiments, bearing fluid skid  150  may deliver the very low viscosity fluid to at least one of bearings  140 . 
     Gas turbine architecture  1500  is similar to the one illustrated in  FIG. 14 , except that the gas turbine  18  includes a reheat section  205  having a reheat combustor  210  and a reheat turbine  215 . The reheat section  205  is added to the input drive shaft  310  of the gas turbine  18 .  FIG. 15  shows that the rotating components (e.g., blades  130 ) of the compressor section  105 , the rotating components (e.g., blades  135 ) of turbine section  115 , the rotating components (e.g., blades  220 ) of the reheat turbine section  215 , and the rotating components (e.g., blades  405 ) of the power turbine section  305  can include low-density materials. This is one possible implementation and is not meant to limit the scope of architecture  1500 . 
     As mentioned above, there can be any combination of low-density components with components that include other materials (e.g., high-density materials), as long as there is at least one rotating component used in the power train that includes a low-density material. For greater efficiency, the section(s) of the architecture  1500  that are supported by low-loss bearings  140  include rotating components made of low-density material, wherein at least some of the rotating components are made of low-density material. 
       FIG. 16  is a schematic diagram of a front-end drive gas turbine architecture  1600  having a gas turbine  20  whose architecture includes a stub shaft  620  to reduce the rotating speed of forward stages  610  of a compressor  605 . The gas turbine  20  further includes at least one low-loss bearing  140  having a low-loss lubricant, for example, a hydrodynamic bearings including the low-loss lubricant, in use with the power train of the gas turbine, according to an embodiment of the present invention. The gas turbine  20  of  FIG. 16  may be substituted for the gas turbine  10  in those power train architectures having a front-end drive gas turbine, including the power train architecture  100  of  FIG. 1 , the power train architecture  400  of  FIG. 4 , the power train architecture  500  of  FIG. 5 , the power train architecture  800  of  FIG. 8 , and the power train architecture  1100  of  FIG. 11 . 
     In this embodiment, the compressor section  605  is illustrated with two stages  610  and  615 , where stage  610  represents the forward stages of compressor  605  and stage  615  represents the mid and aft stages of compressor  605 . This is only one configuration, and those skilled in the art will appreciate that compressor  605  could be configured with more stages. In any event, the rotating blades  710  associated with stage  610  are coupled to a stub shaft  620 , while the rotating blades  715  of stage  615  and the turbine section  115  are coupled along the rotor shaft  125 . In one embodiment, the stub shaft  620  can be radially outward from the rotor shaft  125  and circumferentially surround the rotor shaft  125 . In one embodiment, at least one of the rotating components (e.g., blades  710 , blades  715 , and blades  135 ) is made of a low-density material. 
     Bearings  140  are located about the compressor section  605 , the turbine section  115 , and the generator  120  (not shown) to support the various sections on the stub shaft  620  and the rotor shaft  125 . All, some, or at least one of the bearings in this configuration may be low-loss lubricant bearings, for example, a plurality of hydrodynamic bearings including the low-loss lubricant, as described herein. Such low-loss bearings  140  are particularly well-suited for supporting those sections of the architecture  1600  having rotating components made of low-density material. In some embodiments, each of the plurality of hydrodynamic bearings including the low-loss lubricant supports a section in which a corresponding one of the rotating components including the low-density material is disposed. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. In certain embodiments, other bearings  140  can be mono-type low-loss bearings, hybrid-type low-loss bearings, or conventional oil bearings, as needs dictate. The bearings  140  are in fluid communication with the bearing fluid skid  150 , as shown, for example, in  FIG. 1 . In some embodiments, bearing fluid skid  150  may deliver the low-loss lubricant to at least one of bearings  140 . In some embodiments, bearing fluid skid  150  may deliver the very low viscosity fluid to at least one of bearings  140 . 
     In operation, the rotor shaft  125  enables the turbine section  115  to drive the generator  120  (shown in  FIG. 1 , for example). The stub shaft  620  can rotate at a slower operational speed than the rotor shaft  125 , which causes the blades  710  of the forward stage  610  to rotate at a slower rotational speed than the blades  715  in the mid and aft stages of stage  615  (which are coupled to rotor shaft  125 ). In another embodiment, the stub shaft  620  can be used to rotate the blades  710  of stage  610  in a different direction than the blades  715  of stage  615 . Having the blades  710  of stage  610  rotate at a slower rotational speed and/or in a different direction than the rotating blades  715  of stage  615  can enable stub shaft  620  to slow down the rotational speed of the forward stages of blades (e.g., to approximately 3000 RPMs), while rotor shaft  125  can maintain the rotational speed of the rotating blades  135  of the turbine section  115 , and thus the speed of generator  120 , to operate at a constant speed (e.g., 3600 RPMs). 
     Slowing down the rotational speed of the forward stages of blades  710  in stage  610  in relation to the mid and aft stages of the blades  715  in stage  615  facilitates the use of larger blades in the forward stages. As a result of their larger size, the airflow (or gas flow) through compressor  605  is increased over a conventional compressor, which means that more airflow will flow through gas turbine power train  1600 . More airflow through gas turbine power train  1600  results in more output from the power train architecture. 
     Further, because the moving blades of the forward stages can operate at a reduced speed, attachment stresses that typically arise in these stages can be mitigated. As a result, if a compressor manufacturer desires to continue using blades of a high-density material in the forward stages, the slower rotational speed of the forward stage  610  permits the moving blades of the forward stages to be made in larger sizes and still remain within prescribed AN 2  limits. U.S. patent application Ser. No. 14/460,560, entitled “MULTI-STAGE AXIAL COMPRESSOR ARRANGEMENT”, which is incorporated by reference herein, provides more details on the use of a stub shaft to attain a slower rotational speed at the forward stages of a compressor. 
       FIG. 17  is a schematic diagram of a gas turbine architecture  1700  having a front-end drive gas turbine  24  with a reheat section  205 . The architecture  1700  further includes a stub shaft  620  to reduce the speed of forward stages of a compressor  605 , at least one low-loss bearing  140  with a low-loss lubricant, and at least one rotating component made of a low-density material, according to an embodiment of the present invention. In this embodiment, the reheat section  205  can be added to the configuration illustrated in  FIG. 16 . In this manner, the rotating blades  710  and  715  in stages  610  and  615 , respectively, of compressor  605 , the rotating blades  135  of the turbine  115 , and the rotating blades  220  of the reheat turbine  215  can include blades that are made of a low-density material. 
     Again, this is one possible implementation and is not meant to limit the scope of architecture  1700 . For example, there can be any number of low-density blades in combination with blades of other types of material (e.g., high-density blades) in the power train, as long as there is at least one rotating component made of a low-density material. Alternately, or in addition, rotating components other than the blades may be made of low-density materials in one or more sections. The gas turbine  24  of  FIG. 17  may be substituted for the gas turbine  14  in those power train architectures having a gas turbine with a reheat section  205 , including the power train architecture  300  of  FIG. 3 , the power train architecture  700  of  FIG. 7 , and the power train architecture  1300  of  FIG. 13 . 
       FIG. 18  is a schematic diagram of a gas turbine architecture  1800  having a rear-end drive gas turbine  22  whose architecture includes a stub shaft  620  to reduce the speed of forward stages of compressor  605 , a power turbine  905 , and at least one bearing  140  that includes a low-loss lubricant, according to an embodiment of the present invention. In this embodiment, a multi-shaft arrangement has been added to operate in conjunction with stub shaft  620 . As shown in  FIG. 18 , a first single rotor shaft  910  extends through the compressor section  605  and the turbine section  115 , while a second single rotor shaft  915 , separated from rotor shaft  910  and stub shaft  620 , extends from the power turbine section  905  to a generator  120  (as shown in  FIG. 2 ). Bearings  140  can support the rotor shaft  910 , the rotor shaft  915 , and the stub shaft  620 . In one embodiment, at least one of the bearings  140  can include a low-loss lubricant. In some embodiments, at least one of bearings  140  include a plurality of hydrodynamic bearings including the low-loss lubricant. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. In certain embodiments, low-loss lubricant bearing  140  may be used in conjunction with other bearing types (e.g., mono-type low-loss bearings, hybrid-type low-loss bearings, and/or conventional oil bearings), as needs dictate. 
     In operation, the rotor shaft  910  and the stub shaft  620  can serve as the input shafts, while the rotor shaft  915  can serve as the output shaft that drives the generator  120 . In one embodiment, the output speed of rotor shaft  915  is a constant speed (e.g., 3600 RPMs) to ensure that generator operates at a constant frequency (e.g., 60 Hz), while the input speed of the rotor shaft  910  and the stub shaft  620  is different from the speed at which the rotor shaft  915  operates (e.g., is less than the 3600 RPMs). 
       FIG. 18  shows that the rotating blades  710  and  715  of the compressor sections  610 ,  615 , the rotating blades  135  of the turbine section  115 , and the rotating blades  1005  of the power turbine section  905  can be made of low-density materials. This is one possible implementation and is not meant to limit the scope of architecture  1800 . Again, there can be any combination of low-density rotating components (e.g., blades) in use with rotating components (e.g., blades) made of different compositions (e.g., high-density materials), as long as there is at least one rotating component used in the power train that includes a low-density material. In at least one embodiment, the low-density materials are used in rotating components in the section(s) of the gas turbine architecture  1800  supported by low-loss lubricant bearings  140 . In some embodiments, low-loss lubricant bearings  140  include a plurality of hydrodynamic bearings, where each of the plurality of hydrodynamic bearings including the low-loss lubricant supports a section of gas turbine architecture  1800  in which a corresponding one of the rotating components including the low-density material is disposed. 
       FIG. 19  is a schematic diagram of a gas turbine architecture  1900  having a multi-shaft gas turbine  26  with a low-speed spool  1205  and a high-speed spool  1210 . The gas turbine  26  further includes at least one low-loss bearing  140  in use with the power train of the gas turbine, according to an embodiment of the present invention. At least one bearing  140  is a low-loss bearing including a low-loss lubricant, for example, a hydrodynamic bearing including the low-loss lubricant. The gas turbine  26  of  FIG. 19  may be substituted for the gas turbine  10  in those power train architectures having a front-end drive gas turbine, including the power train architecture  100  of  FIG. 1 , the power train architecture  400  of  FIG. 4 , the power train architecture  500  of  FIG. 5 , the power train architecture  800  of  FIG. 8 , and the power train architecture  1100  of  FIG. 11 . 
     In this embodiment, a compressor  1215  has a low pressure compressor  610  and a high pressure compressor  615  separated from low pressure compressor  610  by air. In addition, the gas turbine architecture  1900  has a turbine  1230  that includes a low pressure turbine  1250  and a high pressure turbine  1245  separated from low pressure turbine  1250  by air. The low-speed spool  1205  can include the low pressure compressor  610 , which is driven by the low pressure turbine  1250 . The high-speed spool  1210  can include the high pressure compressor  615 , which is driven by the high pressure turbine  1245 . In this architecture  1900 , the low-speed spool  1205  can drive the generator  120  at a desired rotational speed (e.g., 3600 RPMs) to operate at a desired frequency (e.g., 60 Hz), while the high-speed spool  1210  can operate at a rotational speed that is greater than that of the low-speed spool (e.g., greater than 3600 RPMs), forming a dual spool arrangement. 
     Optionally, a torque-altering mechanism  1208 , such as a gearbox, torque-converter, gear set, or the like, may be positioned along the low speed spool  1205  between the gas turbine  26  and the generator (not shown, but indicated by “To Gen”). When a torque-altering mechanism  1208  is included, the torque-altering mechanism  1208  provides output correction, such that the low-speed spool  1205  can operate at a rotational speed greater than 3600 RPMs and drive the generator at a lower rotational speed of 3600 RPMs and still achieve an operating output of 60 Hz. 
     In  FIG. 19 , at least one of the bearings  140  that support the power train  1900  can be a low-loss bearing having a low-loss lubricant. Other bearings  140  in the power train  1900  may be mono-type low-loss bearings, hybrid-type low-loss bearings, and/or conventional oil bearings, as desired. The bearings  140  are in fluid communication with the bearing fluid skid  150 , as shown in  FIG. 1 , for example. In some embodiments, bearing fluid skid  150  may deliver the low-loss lubricant to at least one of bearings  140 . In some embodiments, bearing fluid skid  150  may deliver the very low viscosity fluid to at least one of bearings  140 . 
       FIG. 19  shows that the rotating blades  1220  and  1225  of the compressor sections  610 ,  615  and the rotating blades  1235 ,  1240  of the turbine sections  1245 ,  1250  can be made of low-density materials. This is one possible implementation and is not meant to limit the scope of architecture  1900 . Again, there can be any combination of low-density rotating components (e.g., blades) in use with rotating components (e.g., blades) made of different compositions (e.g., high-density materials), as long as there is at least one rotating component used in the power train that includes a low-density material. In at least one embodiment, the low-density materials are used in rotating components in the section(s) of the gas turbine architecture  1900  supported by low-loss lubricant bearings  140 . In some embodiments, low-loss lubricant bearings  140  include a plurality of hydrodynamic bearings, where each of the plurality of hydrodynamic bearings including the low-loss lubricant supports a section of gas turbine architecture  1900  in which a corresponding one of the rotating components including the low-density material is disposed. In some embodiments, one or more of the plurality of hydrodynamic bearings may include a very low viscosity fluid. In some embodiments, each of the plurality of hydrodynamic bearings does not require a secondary bearing. 
     As described herein, embodiments of the present invention describe various power train architectures with gas turbine architectures that can use low-loss lubricant bearings and low-density materials as part of a power train in a power-generating plant. These gas turbine architectures with low-loss lubricant bearings and low-density materials can deliver a high airflow rate in comparison to other power trains that use oil bearings and high-density materials. In addition, this delivery of a higher airflow rate occurs while reducing viscous losses that are typically introduced into the power train through the use of conventional oil-based bearings. When low-loss lubricant bearings (e.g. hydrodynamic bearings including the low-loss lubricant) are used with other low-loss bearings (e.g., mono-type bearings or hybrid-type bearings having a very low viscosity fluid), maintenance costs are reduced, since components pertaining to the conventional oil bearings can be removed. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” “including,” and “having,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It is further understood that the terms “front” or “forward” and “back” or “aft” are not intended to be limiting and are intended to be interchangeable where appropriate. “About,” “approximately” and “substantially,” when applied to a particular value(s) or a range including a starting and an ending values, unless otherwise dependent on the precision of the instrument measuring the value, may include +/−10% of the particular value(s) or the starting and ending values of the range. 
     While the disclosure has been particularly shown and described in conjunction with a preferred embodiment thereof, it will be appreciated that variations and modifications will occur to those skilled in the art. Therefore, it is to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the disclosure.