Microturbine cooling system

A microturbine engine core with gas bearings used to support a rotating group is provided, including air flow control devices downstream of the gas bearings to maintain high pressures. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to ascertain quickly the subject matter of the technical disclosure. The abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 C.F.R. 1.72(b).

BACKGROUND OF THE INVENTION
 This invention relates generally to microturbine power generation systems
 and more particularly to a microturbine construction and method for
 providing improved stability and heat transfer through the microturbine
 engine core.
 Microturbines are multi-fuel, modular distributed power generation units
 having multiple applications. They offer the capability to produce
 electricity at a lower cost per kilowatt than do central plants, and they
 do not require the installation of expensive infrastructure to deliver
 power to the end users. Thus, in parts of the world lacking the
 transmission and distribution lines of a basic electric infrastructure,
 commercialization of microturbines may be greatly expedited. In the United
 States and other countries already having a suitable electric
 infrastructure, distributed generation units will allow consumers of
 electricity to choose the most cost-effective method of electric service.
 In addition to primary power generation, microturbines also offer an
 efficient way to supply back-up power or uninterruptible power. Other
 applications for microturbines exist as well.
 Structurally, engine cores of present-day microturbine power generating
 systems include a compressor, a turbine for converting gaseous heat energy
 into mechanical energy, and an electrical generator for converting the
 mechanical energy produced by the turbine into electrical energy. The
 electrical generator includes a rotor and a stator. The rotor is
 mechanically coupled to wheels of the turbine and the compressor. While
 some proposed designs for microturbines include oil-lubricated ball
 bearings, microturbines can advantageously incorporate gas bearings
 instead. As used herein, "air bearings" are a subset of gas bearings--for
 example, gas bearings in which the operating medium is air obtained from
 the environment surrounding the microturbine.
 If gas bearings are used in a microturbine, the above-described combination
 of rotor, compressor and turbine are rotatably supported by the gas
 bearings. The gas bearings in a common configuration include fluid film
 journal and thrust bearings. A microturbine engine core that uses gas
 bearings includes a single moving part, which allows for low technical
 skill maintenance and a high level of reliability.
 Because unwanted heat can be generated by the engine core of a microturbine
 power generating system, it is desirable to include design features that
 allow for cooling of the electrical generator components, including the
 stator and the electrical conductor therein (e.g., stator wires). When the
 stator is of conventional, multi-tooth design, one method for cooling the
 stator involves passing cooling fluid, such as water or glycol, through a
 sleeve that surrounds the stator to transfer stator heat to the fluid. The
 fluid then may be cooled in a heat exchanger and passed back through the
 cooling sleeve surrounding the stator. Alternatively, a continuous supply
 of cool water may be used and, after it is heated by the unwanted stator
 heat, passed outside the microturbine power generating system for other
 uses. However, while the use of a fluid-cooled conventional stator offers
 design opportunities, it also presents certain problems, including
 problems associated with a microturbine that uses gas bearings.
 Specifically, cooling of air bearings, a rotor, and a stator end turn
 becomes problematic. Furthermore, stator end turn cooling typically
 requires special cooling flow components.
 In the present microturbine cooling system, however, air bearing cooling
 flow--which is already required--performs the secondary function of stator
 end turn cooling. Using the existing cooling flow system for stator end
 turn cooling results in a simpler, lower cost microturbine.
 Additionally, it is well known that air bearing damping and load capacity
 are a function of their operating pressure. The design arrangement of the
 present invention operates the air bearings at or near their highest
 possible pressures, resulting in a significant improvement in rotor
 dynamic stability because of improved bearing damping and load capacity.
 The present invention offers several other advantages as well.

DESCRIPTION OF THE INVENTION
 A microturbine power generating system that uses the present invention has
 a microturbine core 10 that includes a compressor 12, a turbine 14 for
 converting gaseous heat energy into mechanical energy, and an electrical
 generator 16 for converting the mechanical energy produced by the turbine
 into electrical energy. The electrical generator includes a rotor 18 and a
 stator 20. The rotor, which may advantageously be a rare earth permanent
 magnet rotor, is mechanically coupled to wheels of the turbine and the
 compressor; in the embodiment depicted in FIG. 1, these components are
 linked by shaft 15. The engine core also includes thrust disk 19, which is
 either integral to (part of) the shaft or a separate piece secured to the
 shaft. As used in the claims, the phrase "the shaft includes a thrust
 disk" applies to each of the embodiments in the foregoing sentence. The
 rotor, compressor and turbine are rotatably supported by gas bearings,
 including forward journal bearing 30, aft journal bearing 32, and thrust
 bearings 34 and 36. The thrust disk rotates between the thrust bearings.
 In the microturbine cooling system disclosed and claimed herein, flow
 control devices 40, 42, 44, and 46 are employed to keep the air bearing
 pressures high. These flow control devices include known devices for
 controlling or restricting the flow of gas or air, such as orifices,
 seals, and valves. As shown in FIG. 1 by the lines and directional arrows
 depicting cooling flow 50, all the flow control devices are arranged
 downstream of the air bearing components, which results in the air bearing
 cooling system operating at or near the full discharge pressure of the
 compressor. Compressor discharge flow 52 is also depicted in the figure by
 the line and directional arrow extending from the compressor. The cooling
 circuit can advantageously include a cooling device 62 for removing some
 of the heat of the compressed air before the compressed air flows through
 the air bearings. The cooling device is chosen from the group that
 includes, for instance, air-to-air heat exchangers and liquid-to-air heat
 exchangers. The temperature of the compressed air that becomes cooling
 flow 50 is lowered by the transfer of heat to the cooling medium (not
 shown) of cooling device 62. The cooling medium in a liquid-to-air heat
 exchanger is commonly water, glycol or oil, although other liquids can be
 used. The cooling medium flows through an open loop or closed loop, and
 can also function to remove heat from other portions of a microturbine
 power generating system.
 In addition to the downstream arrangement of the flow control devices in
 the air cooling circuit of the present invention, the air cooling circuit
 is constructed so that much of the cooling air that is routed to forward
 stator end turn 22 does not pass over--and consequently is not heated
 by--other sources of power loss such the thrust and journal bearings or
 rotor windage. This is done by splitting the flow of the cooling air and
 redirecting some of the cooling air from the main cooling air supply line
 (either internally or externally) and routing it to the vicinity of the
 forward end turn of the stator. The air flow is split when it reaches one
 or more junctions 54 in the cooling circuit. Similarly, cooling air flow
 to aft stator end turn 24 is only exposed to one thrust bearing heat
 source but no others.
 The split cooling flow circuit of the present invention also ensures that
 none of the air bearing cooling flow entrances 30a, 32a, 34a and 36a are
 exposed to potentially contaminated cooling air that has passed through
 the stator end turns or the rotor gap 28. Thus, the potential for
 contamination of the air bearings by foreign objects coming from the
 permanent magnet generator, stator, insulation, varnish, wire ties, or
 other permanent magnet generator is substantially reduced, in turn
 reducing the probability of air bearing failure.
 On the aft journal bearing another unique design feature has been
 incorporated into the microturbine cooling system. Instead of allowing the
 heated air bearing cooling flow to exhaust into and mix with the main
 compressor inlet process flow, a labyrinth shaft seal 60 is employed to
 redirect that flow and vent it overboard. Reducing the amount of heat that
 enters the process flow of compressor inlet 12a serves to maximize the
 performance of the microturbine core.
 The invention now having been described in detail, those skilled in the art
 may recognize modifications and substitutions to the specific embodiments
 disclosed herein. Such modifications and substitutions are within the
 scope and intent of the present invention, as set forth in the following
 claims.