Abstract:
A turbine generator uses a common bearing carrier to support a high-speed spool and a low-speed spool, thereby enabling a more axially compact turbine generator design. The high-speed spool includes a high-speed compressor assembly mechanically powered by a high-speed turbine, which receives power from a combustion chamber between the high-speed turbine and the high-speed compressor assembly. The low-speed spool includes a pre-combustion power turbine aerodynamically powered by the compressor assembly and upstream of the combustion chamber. The pre-combustion turbine mechanically powers an electrical generator, which includes coreless stators and a rotor assembly. The stators have a planar configuration to receive axial flux from magnets of the rotor assembly. Optionally, the low-speed spool may include a post-combustion turbine that is arranged downstream of the combustion chamber and that mechanically powers the electrical generator.

Description:
BACKGROUND 
     This invention relates generally to gas turbine generators, and more particularly to small-scale gas turbine generators, also known as microturbines. 
     A turbine generator converts stored chemical energy into electrical power to charge a battery or to run an electricity-consuming device. Turbine generators typically combust a chemical fuel such as natural gas, propane, gasoline, or diesel, which heats a working fluid such as air that flows through a turbine. This airflow through the turbine transfers energy from the air into rotational mechanical energy, which drives a generator that converts mechanical energy into electrical energy. Gas turbines are commonly used in stationary power plant facilities, where economic considerations motivate turbine design choices. Hence, many turbine generators are bulky, heavy, and expensive. These characteristics are disadvantageous for vehicular applications, where ideal turbine generators are light, compact, powerful relative to weight, and suitable for mass-production. 
     SUMMARY 
     In one embodiment, a turbine generator includes a combustion chamber, a high-speed spool, a low-speed spool, and a common bearing carrier. A compressor assembly on the high-speed spool pressurizes a flow, which expands across a pre-combustion power turbine on the low-speed spool to produce mechanical work. A low-speed shaft connects the pre-combustion turbine to the electrical generator, which consumes mechanical energy from the pre-combustion power turbine. A combustion chamber burns fuel mixed with the pressurized flow from the pre-combustion power turbine. The resulting high-temperature flow expands across a high-speed core turbine on the high-speed spool to transfer mechanical energy to a high-speed shaft, which mechanically powers the compressor assembly. 
     In one embodiment, the low-speed and high-speed spool are supported by a common bearing carrier. The common bearing carrier is attached to a low-speed bearing assembly, which supports the low-speed spool, and a high-speed bearing assembly, which supports the high-speed spool. The high-speed bearing assembly includes a bearing cartridge fastened to the common bearing carrier by a retaining pin. Because they are supported by the common bearing carrier, the high-speed spool and the low-speed spool have a common axis of rotation. The high-speed bearing assembly may be arranged radially inside the common bearing carrier and the low-speed bearing assembly may be arranged radially outside the common bearing carrier. This configuration enables the high-speed bearing assembly and low-speed bearing assembly to be arranged at approximately the same axial location. 
     In one embodiment, the turbine generator includes an electrical generator axially located between the high-speed compressor assembly and the high-speed core turbine. The electrical generator includes a rotor assembly operatively coupled to the pre-combustion turbine on the low-speed spool. The rotor assembly may include one or more permanent magnet assemblies and return irons. The electrical generator also includes one or more coreless generator stators electrically excited by an axial magnetic flux received from the rotor assembly. These stators may be fabricated on printed circuit boards. 
     In an alternative embodiment, the low-speed spool includes a post-combustion power turbine upstream of the high-speed core turbine. The post-combustion power turbine is powered by a high-temperature, pressurized flow from the combustion chamber. The post-combustion power turbine is operatively coupled via the low-speed shaft to the electrical generator to power the electrical generator. 
     In an alternative embodiment, the pre-combustion power turbine is omitted. The low-speed spool may include a rotating diffuser upstream of the combustion chamber and downstream of the compressor impeller. As an alternative to the rotating diffuser, a stationary diffuser may be placed between the combustion chamber and the compressor impeller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual flow diagram of a two-spool turbine system with a pre-combustion turbine on a common low-speed spool with an electrical generator. 
         FIG. 2  is a cross-sectional diagram illustrating a two-spool turbine system with a pre-combustion turbine operationally coupled to a low-speed electrical generator. 
         FIG. 3  is an enlarged cross-sectional diagram illustrating an electrical generator on a common low-speed spool with the pre-combustion turbine of the two-spool turbine system. 
         FIG. 4  is a conceptual flow diagram of an alternative embodiment of a two-spool turbine system with a post-combustion turbine on a common low-speed spool with the pre-combustion turbine and the electrical generator. 
     
    
    
     The figures depict various embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
     DETAILED DESCRIPTION 
       FIG. 1  is a conceptual flow diagram of a two-spool turbine system  100  with a pre-combustion turbine  30  on a common low-speed spool with an electrical generator  34 . A flow of a working fluid (typically a gas such as air) enters a compressor assembly  4  through a compressor inlet duct  2 . Using mechanical energy, the compressor assembly  4  increases the pressure of the flow, which exits the compressor assembly  4  through the compressor outlet duct  6 . In one embodiment, the flow&#39;s pressure ratio between the compressor outlet duct  6  and the compressor inlet duct  2  is approximately six-to-sixteen. A high-speed shaft  24  operatively connects the compressor assembly  4  to a high-speed turbine  16  and transfers rotational mechanic energy to the compressor assembly  4  from the high-speed turbine  16 . 
     The compressor outlet duct  6  also serves as an inlet duct to the pre-combustion turbine  30 , which is operatively connected to an electrical generator  34  by a low-speed shaft  36  to transfer mechanical energy. In one embodiment, the low-speed shaft  36  rotates at approximately one-quarter to one-half the rate of the high-speed shaft  24 . Running the low-speed spool at a lower speed than the high-speed spool reduces the tip speed of the pre-combustion turbine  30 , which reduces tensile stresses in the turbine components and enables manufacturing these components using lightweight materials (e.g., aluminum, titanium, polymer) to improve the turbine generator&#39;s specific power. The flow expands through the pre-combustion turbine  30  while generating mechanical energy, which decreases the pressure of the flow exiting through the pre-combustion turbine outlet duct  44 . In one embodiment, the flow&#39;s total pressure ratio between the pre-combustion turbine outlet duct  44  and the compressor outlet duct  6  is approximately two-to-three. Overall, the flow&#39;s pressure ratio between the pre-combustion turbine outlet duct  44  and the compressor inlet duct  2  is approximately three-to-six, in one embodiment. 
     The pressurized flow in the pre-combustion turbine outlet duct  44  enters a heat recovery device  8 , which the pressurized working fluid exits through the combustor inlet duct  10 . The heat recovery device  8  may be a recuperator or a regenerator, for example. A high-temperature flow enters the heat recovery device  8  through the turbine outlet duct  14  and exits the heat recovery device  8  through the exhaust port  18 . While in the heat recovery device  8 , the high-temperature flow transfers heat to the pressurized flow through a thermally conductive material that physically separates the flows. As illustrated, the high-temperature and pressurized flows circulate through the heat recovery device  8  in a countercurrent arrangement for improved heat transfer, but another arrangement (e.g., cross flow) may be used instead. 
     The pressurized flow from the combustor inlet duct  10  mixes with fuel from the fuel inlet  20  in the combustor  12 , which ignites the mixture to increase the flow&#39;s temperature. The flow exits the combustor  12  through the high-speed turbine inlet duct  22 , which directs the flow into the high-speed turbine  16 . The hot, pressurized flow expands across the high-speed turbine  16 , producing mechanical work that the high-speed shaft  24  transfers to the compressor assembly  4 . The flow exits the high-speed turbine  16  through the high-speed turbine outlet duct  14 , which discharges the high-temperature flow into the heat recovery device  8 . 
     Two-Spool Turbine System 
       FIG. 2  is a cross-sectional diagram illustrating a two-spool turbine system with a pre-combustion turbine  30  operationally coupled to a low-speed electrical generator  34 . A flow from the compressor inlet duct  2  enters the compressor assembly  4 , where the compressor impeller  38  (e.g., a centrifugal impeller) pressurizes the flow. The high-speed shaft  24  operatively connects the components of the high-speed spool, including the compressor impeller  38  and the high-speed turbine impeller  88  of the high-speed turbine  16 . 
     The compressor impeller  38  discharges the pressurized, high-velocity flow into the compressor vaneless space  40 , which serves as the compressor outlet duct  6 . The flow enters the pre-combustion turbine impeller  42  of the pre-combustion turbine  30 , where the flow expands and generates mechanical energy in the pre-combustion turbine impeller  42 . In this way, the compressor assembly  4  powers the aerodynamically coupled pre-combustion turbine  30 . The pre-combustion turbine impeller  42  discharges the flow through the pre-combustion turbine outlet duct  44 . The low-speed shaft  36  operatively connects the components of the low-speed spool, including the pre-combustion turbine impeller  42  and the electrical generator  34 . Locating the electrical generator  34  axially between the compressor impeller  4  and the high-speed turbine impeller  88  enhances the system&#39;s compact configuration. 
     The pressurized flow enters the high-speed turbine  16  through the high-speed turbine inlet duct  22  and expands across the high-speed turbine impeller  88 , generating mechanical energy for the high-speed spool. The high-speed turbine impeller  88  discharges the still hot flow through the high-speed turbine outlet duct  14 . It is noted that the high-speed spool may include a starter motor/generator (not illustrated) to initiate rotation of the high-speed shaft  24  when starting the two-spool turbine system  100 , and/or to generate part of the electrical load during operation. 
     A common bearing carrier  54  is attached to a high-speed bearing assembly  52  radially inside the common bearing carrier  54  and to a low-speed bearing assembly  56  radially outside the common bearing carrier  54 . The high-speed bearing assembly  52  supports the high-speed shaft  24  of the high-speed spool and includes a high-speed bearing cartridge  84  fastened to the common bearing carrier  54  by a high-speed bearing retaining pin  86 . The low-speed bearing assembly  56  supports the low-speed shaft  36  of the low-speed spool. Because of the common bearing carrier  54 , the low-speed spool and the high-speed spool rotate around a common axis of rotation. The common bearing carrier  54  is attached to the low-speed bearing assembly  56  and the high speed-bearing assembly  52  at approximately the same axial location. The use of the common bearing carrier  54  beneficially enables nesting the high-speed spool radially inside the low-speed spool but at approximately the same axial location. This configuration results in a compact configuration of the two-spool turbine system  100 . 
     Electrical Generator 
       FIG. 3  is an enlarged cross-sectional diagram illustrating the electrical generator  34  on a common low-speed spool with the pre-combustion turbine  30  of the two-spool turbine system  100 . In one embodiment, the electrical generator  34  is a multi-stage axial flux brushless design including coreless generator stators  62  and  66 , which are secured to the housing of the system  100 . The coreless generator stators  62  and  66  are lighter than stators with an iron core; additionally, the use of coreless stators  62  and  66  reduces audible noise by eliminating torque cogging from stator teeth. The electrical generator  34  also includes a rotor assembly including a front rotor  72 , a middle rotor  74 , and a back rotor  76 . A front permanent magnet assembly  60  is attached adjacent to a front return iron  58  on the front rotor  72 . A middle permanent magnet assembly  64  is attached to the middle rotor, and a back permanent magnet assembly  68  is attached adjacent to a back return iron  70  on the back rotor  74 . The return irons  58  and  70  are made from a material with high magnetic permittivity relative to air to provide a return path for the magnetic field of the permanent magnet assemblies  60  and  68 , respectively. 
     The rotor assembly, including the permanent magnet assemblies  60 ,  64 , and  68  and the return irons  58  and  70 , is operatively coupled to the low-speed shaft  36 . When the flow expands across the pre-combustion turbine  30 , the low-speed shaft  36  turns the rotor assembly to electrically excite the coreless generator stators  62  and  66  through an axial magnetic flux. The number of rotors, magnet assemblies, and stators may be varied; for example, the middle permanent magnet assembly  64 , middle rotor  74 , and coreless generator stator  66  may be omitted or duplicated. In one embodiment, the coreless generator stators  62  and  66  have a planar configuration that enables fabrication on printed circuit boards to reduce manufacturing costs. In an alternative embodiment, the electrical generator  34  may be arranged to electrically excite the electrical generator&#39;s stators through a radial magnetic flux. Such a configuration may use iron core stators in place of coreless generator stators  62  and  66 . 
     Post-Combustion Turbine 
       FIG. 4  is a conceptual flow diagram of an alternative embodiment of the two-spool turbine system  100  with a post-combustion turbine  80  on a common low-speed spool with the pre-combustion turbine  30  and the electrical generator  34 . The alternative embodiment is similar to that illustrated in  FIG. 1  except for the addition of a post-combustion turbine  80 , which is operatively coupled to the pre-combustion turbine  30  and electrical generator  34  via the low-speed shaft  36 . The speed of the low-speed spool including the post-combustion turbine  80  is lower than the speed of the high-speed spool, which reduces the tip speed of the post-combustion turbine  80  and consequently reduces tensile stresses enables manufacturing from lightweight materials to improve the turbine generator&#39;s specific power. The pressurized flow from the combustion chamber  12  enters the post-combustion turbine  80  through the post-combustion turbine inlet duct  78 . The pressurized flow expands in the post-combustion turbine  80 , which generates mechanical work transferred to the electrical generator  34 . The post-combustion turbine  80  discharges the flow through the high-speed turbine inlet duct  22 . 
     In one embodiment, the post-combustion turbine  80  is manufactured from a ceramic material to withstand the high-temperatures of the combusted flow. As an alternative to ceramic material, the post-combustion turbine  80  may be manufactured from a metallic material and cooled using flow diverted from the pre-combustion turbine outlet duct  44 . The cooling flow may be used to internally cool, transpiration cool, or film cool the post-combustion turbine  80 . In an alternative embodiment, the system  100  of  FIG. 4  includes a rotating diffuser in place of the pre-combustion turbine  30 . The rotating diffuser may be operatively coupled to the low-speed shaft  36  to allow the rotating diffuser to rotate at a lower speed than the compressor impeller  38 . As an alternative to the rotating diffuser, a stationary diffuser may reduce the flow velocity from the compressor outlet duct  6  to the pre-combustion turbine outlet duct  44 . 
     Summary 
     The foregoing description of the embodiments of the invention has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. 
     Additionally, the language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.