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1. (WO2007003927) PRESSURISED GENERATOR
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PRESSURISED GENERATOR

Technical Field of the Invention

The invention relates to a generator, particularly for use with a Stirling engine.

Background of the Invention

Stirling engines offer advantages of multi-fuel capabilities (geothermal, solar, bio-, fossil- and nuclear fuel), very low NOx and HC emissions when burning fossil fuels, very high total efficiency (particularly when used with combined heat and power or CHP) and very low maintenance compared to internal combustion engines.

More and more focus is directed towards micro combined heat and power units in order to produce and distribute energy in a more efficient manner. This is becoming relevant for local energy production, especially for households that heat their homes by means of a gas boiler. Since Stirling cycle engines in combination with an electric generator can achieve efficiencies up to 90% (compared to an industrial combined heat and power plant which can obtain approx. 50% efficiency) there is a huge market potential for this type of power generation. In addition, the reduction of produced CO2 from this type of energy conversion is quite substantial and is one way to contribute to the reduction of global warming.

In order to achieve a high performance from a Stirling unit it is quite common for the unit to be pressurised to e.g. 80 bar; sometimes even higher. This is due to the fact that the power output is in general proportional to the working Stirling cycle pressure. There are instances where this kind of unit is pressurized up to 120 bar.

Operating pressures as mentioned will then require the pressure vessel (i.e.
crankcase) to have an increased wall thickness due to the high operating pressures. In addition flanges and threaded connections will increase in size due to high load conditions and deflections placed upon these areas.

US-5755100 discloses a hermetically sealed Stirling engine with a generator, in which the generator with stator/rotor, flywheel, starter rotor and starter stator are located within a hermetically sealed pressurized housing. A starter rotor is fastened to the crankshaft by way of a flywheel. A starter stator is fastened by means of screws to a bell housing, which in turn is fastened to the crankcase of the Stirling engine. The generator armature is screwed directly to the flywheel and the generator stator is fastened to the bell housing. Finally the generator housing is attached and sealed to the bell housing.

This system is complicated and incorporates several costly solutions. In addition, the size of the connecting flange is quite large, since the flange must always be larger than the generator stator.

Since modern micro co-generating units will penetrate the market in the near future there is a need for a lightweight and production friendly Stirling engine design.

Disclosure of the Invention

The invention provides a generator for use with a Stirling engine, the generator having a rotor shaft for connection to a Stirling engine crankshaft, a generator rotor and a generator stator, in which the rotor shaft, generator rotor and generator stator are placed within a pressure containing outer housing, characterized in that the generator stator is fastened within the outer housing, and in which the pressure containing outer housing is configured for connection to a crankcase for a Stirling engine in a pressure communicating relationship, whereby to eliminate the need for a pressure resistant rotary seal between the crankcase and the pressure containing outer housing.

It is preferred that the pressure containing outer housing is formed of two portions joined by a peripheral weld.

It is further preferred that the pressure containing outer housing is generally spherical.

It is still further preferred that the pressure containing outer housing comprises two hemispheres welded together.

It is yet still further preferred that the generator stator is fastened within just one of the outer housings; and such fastening may be by means of a shrink fit.

The generator outer housing may be of a composite material.

The power connection to the generator may be through the end cap of the outer housing via a gas tight connection.

In one preferred form there may be a cooling shroud with cooling inlet and cooling outlet which surrounds part of all of the generator outer housing.

The invention includes a generator as described above, in combination with a Stirling engine having a crankcase.

The flywheel of the Stirling engine may be located within the generator outer housing and fastened to the rotor shaft.

The generator outer housing may have a flange that is attached to the Stirling engine crankcase by means of fasteners.

The generator outer housing may have threads that are in direct threaded
engagement with the Stirling engine crankcase end cap.

The generator flange may be welded directly to the Stirling engine crankcase-mating surface.

The generator may serve as a starting motor for the Stirling engine, or may serve as an electric drive for a pressurized Stirling cryocooler.

Brief Description of the Drawings

FIG. 1 is a simplified cross section showing a proposed Stirling engine with a first embodiment of a pressurised generator.
FIG. 2 is a simplified cross section of a second embodiment of a pressurised generator assembly.
FIG. 3 is an external view of the pressurised generator assembly shown in Figure 2,

FIG. 4 is a detailed cross section of a fastening method used in that assembly.
FIG. 5 is a cross section of another pressurised generator assembly.
FIG. 6 is a detailed cross section of a flange connection used in that assembly.
FIG. 7 is a view partly in section of the pressurised generator assembly illustrated in

Figures 2 to 4, when mounted together with a Stirling engine.

Detailed Description

FIG 1 depicts a cross section of one proposal for a pressurised β-type Stirling engine, generally designated as SA. In this proposal, the generator is placed within a pressurised container. This arrangeinent creates no need for advanced and expensive seals or penetrations that would be necessary in order to seal a drive shaft extending from within the pressurized container. Since the working gas in Stirling engines is usually helium, and sometimes even hydrogen, it would be almost impossible to keep a gas tight connection. Keeping a tight seal around a rotating shaft is difficult, especially when this shaft is stationary while the unit is not operating.

The arrangement in Figure 1 has a crankshaft C5 rotor shaft R, flywheel S, generator rotor 3 and generator stator 5. An electrical connection extends from the stator 5 to a gas tight connection G on generator shell 8. With the flywheel S positioned as shown in figure 1, the mating flanges 1, 1' and generator shell 8 are relatively large, because this proposal has included the flywheel S within the pressurized engine crankcase.

FIG. 2 is a cross section of a another pressurised generator assembly. The outer part of the assembly consists of two outer shell halfs 6 and 8 that are connected by a seam weld W. Outer shell 8 also includes a flange 1 with an O-Ring groove 4.
Within the shell there is a generator assembly. The generator assembly consists of rotor shaft R, flywheel S, generator rotor 3, and generator stator 5. In order to support the generator assembly during its operation a bearing arrangement 9 is pressed into the shell half 8. The inner race of said bearing 9 is also shrunk fitted onto rotor shaft R. The generator stator 5 is fastened to outer shell 8 by means of e.g. a weld Wl. In order to transmit current from within the pressurized generator to the grid or another consumer supply, leads L are connected to the generator stator 5 and soldered to a gas tight hermetic coupling G. This coupling then serves as a connection between atmospheric conditions and the pressurized space where the power is generated.

The outer shells 6 and 8 can be fabricated from sand castings, investment castings, forgings, machined from solid, or fabricated from a composite material such as carbon fibre.

The assembly process is as follows; the flywheel S is either fixed to the rotor shaft R by means of e.g. a keyway or it is shrunk fitted in its correct position. Next the generator rotor 3 is fitted to the rotor shaft R by means of e.g. a keyway; it is shrunk fitted in its correct position or a locknut LN is used to hold it in place. Said lock nut is in threaded engagement at the end of the rotor shaft R and exerts a defined axial force in order to secure the generator rotor 3 by means of friction. The flywheel S will hinder axial displacement during tensioning of locknut LN. Next, the bearing 9 is fitted into outer shell 8. There is an interference fit in order to secure the bearing into place within the outer shell 8. Now the complete rotating assembly, comprising rotor shaft R, flywheel S, generator rotor 3 and lock nut LN is fixed in place through the bearing 9. This is also performed as a light interference fit in order to avoid any axial displacement of the rotating assembly. Now, the generator stator 5 is positioned concentrically with respect to the rotating assembly and the outer shell 8. When the positioning is correct, the generator stator 5 is fixed to the inner surface of the outer shell 8 by means of welding e.g. TIG, MIG, laser or electron beam. Thereafter the leads L are soldered to the hermetic coupling G making sure the leads are not permitted to come into conflict with the rotating assembly. The hermetic coupling G is then screwed in place by means of e.g. lock nut. The last fabrication process involves positioning outer shell 6 accurately with respect to the other outer shell 8. When this is achieved both shells 6 and 8 are bonded together by means of e.g. TIG, MIG, laser or electron beam welding.

A hermetically sealed generator assembly is now complete and ready to be mounted to a Stirling engine. This generator is "sealed for life" and is not intended to be opened during its lifetime. The only part subjected to wear is the ball bearing 9. Said bearing is dimensioned conservatively and it is greased for life. If there is a failure, it would be cost effective to change the complete unit instead of cutting the welds and refurbishing with a new bearing.

FIG. 3 is an external view of the pressurised generator assembly. This view shows how fasteners F can be fitted through the generator assembly flange 1. The flange can be equipped with e.g. six drilled holes. In addition the circumferential weld seam W is shown. On the outer part of the generator assembly there is a cooling shroud 10. Cooling water enters the shroud* through inlet nozzle 13 and exits through outlet nozzle 12. The shroud 10 covers as much as possible of the outer part of the generator housing. It may be visualised as a shroud that is swept 360 degrees around the circumference of outer shells 6 and 8. For clarity reasons this is not shown in FIG 3, as the shroud covers the weld seam W completely. Cooling hoses are omitted for clarity. The shroud forms a cavity and is dimensioned according to the heat flux that arises during operation. Said shroud permits the cooling of the generator assembly. During operation there will always be a power loss through the generator stator windings. Since this loss is manifest as heat it must be transferred away from the system or else the generator may over heat. In addition it has been observed that if the generator housing can be kept at a constant temperature, the Stirling cycle will operate at a steady rate.

Another benefit of using the cooling shroud is the ability to recover lost heat from the process. The recovered heat is transferred to the cooling system, which in turn is transferred to a heat exchanger within a thermal store and used within a central heating system. An increase in the system thermal efficiency should be achievable.

FIG. 4 is a detailed cross section of a fastening method. In this figure another varient of the invention is disclosed. Instead of using a flange coupling with fasteners F as shown in Fig 3, a threaded connection 1" is utilized. If the threads are of good quality there will be no need for an O-Ring groove or O-Ring seal against the mating surface. This solution reduces the total number of components and simplifies assembly.

FIG. 5 is a cross section of another pressurised generator assembly. This assembly consists of a generator assembly placed within a hermetically sealed container or canister. The outer canister consists of shell 8 and dome 6 welded together with weld 7. The generator stator 5 is either shrink fitted to outer shell 8 or joined to outer shell by other means e.g. welding or adhesive. The outer shell 8 has a cooling shroud 10 that covers most of its outer surface. Said shroud has a cavity 4 which permits cooling water to circulate and transfer heat generated during operation. The cooling shroud is fitted with inlet and outlet nozzles 13 and 12 for cooling water.

The rotating generator assembly consists of a rotor shaft R, generator rotor 3, flywheel S and bearing 9. If the rotor 3 is too heavy for one bearing, another bearing 9A is placed on the opposite side for extra support as shown in FIG 5. Two bearing supports are provided; one support 11 is fastened to outer shell 6/8, the other bearing support is part of outer shell 8 forming a boss to the left of the threads 1" as shown in FIG 5.

The hermetic coupling with leads is omitted for clarity. The hermetic coupling assembly would be identical to the one described in relation to FIG 2.

The assembly procedure for the solution in FIG 5 would is as follows: the flywheel

5 is pressed on to the rotor shaft R. Flywheel S can be shrink fitted or fixed by means of a keyway. Next, the generator rotor 3 is fitted to the rotor shaft R by means of e.g. a keyway or it is shrunk fitted in its correct position. Then the bearing 9 is fixed into its recess within outer shell 8. The outer shell 8 already has shroud 10 with inlet and outlet nozzles 13 and 12 prefabricated. Now the rotating generator assembly is fixed into place through the bearing 9. This is a light interference fit to avoid any axial movement of said. assembly. Thereafter the generator stator 5 is pressed into outer shell 8 and positioned axially with respect to the generator rotor 3. If the design requires an extra bearing 9 A, a bearing support 11 is fastened within the opening of outer shell 8. This bearing support is machined circular and has a concentric tolerance that will line up against the inner surface of outer shell 8 and generator rotating assembly. Lastly, the end cap or dome 6 is positioned and welded in place together with bearing support 11.

FIG. 6 is a detailed cross section of a flange connection. The flange 1 shown in FIG.

6 is an alternative to the threaded connection as shown in FIG. 5. The flange consists of e.g. six holes H that accommodate fasteners (not shown), and an O-Ring groove 4 to allow a O-Ring seal to achieve a gas tight connection.

FIG. 7 is a side view of the pressurised generator from FIG 2 and FIG 3 mounted together with a Stirling engine SA. The pressurised generator is bolted to an end cap 11 that is fastened to the Stirling engine crankcase by means of fasteners (not shown). The rotor shaft R is e.g. fitted with a spline that mates to a corresponding spline on the Stirling engine crankshaft 2. This figure shows that the Stirling engine crankcase volume can be reduced compared to the proposed configuration shown in FIG 1. This is because the Stirling engine's flywheel S has been moved from the crankcase and placed within the pressurized generator housing. The necessary volume within the Stirling engine crankcase then becomes dependent upon the size and throw of the power and displacer connecting rods.

In FIG 7 the flywheel is placed in a separate chamber i.e. the generator housing, so reducing the size of the flanged or threaded diameter. This reduces the area acted upon by the internal pressure within the Stirling engine.

For asynchronous and synchronous generators is it a requirement that the speed variation during each revolution is minimised. With a low speed variation the generator will achieve a higher electrical efficiency. Additionally, the efficiency of the Stirling cycle will be improved. In order to achieve this efficiency a high inertia flywheel is required. The larger the diameter of the flywheel, the more inertia one can attain. By placing the flywheel into the pressurised generator housing, it is feasible to attain a large inertia flywheel and at the same time reduce the flange diameter and size because the pressure exerted on the flange has been reduced compared to the other proposed solution.

Comparing FIG 7 to FIG 1 one can clearly see the difference in the diameter or area of the O-Ring that is reacted upon by the pressure within the Stirling engine. From FIG 1 it is clear that the O-Ring and groove diameter must be larger than the Stirling engine flywheel diameter. From FIG 7 it is clear that the O-Ring and groove diameter must be larger than the Stirling engine crankshaft diameter. Using traditional strength analysis one can show that the flange that is sized just above the crankshaft diameter will be more compact than the solution presented in FIG 1.

The solution from FIG 7 shows that the flange on the pressurised generator must be smaller and lighter than the solution from FIG 1.

A second fastening method is to use the threaded flange solution as shown either in FIG 4 or FIG 5. This would be a so-called "spin on" solution. The Stirling engines' end cap 11 would then be fitted with female threads that would engage with threaded flange 1".

A third fastening possibility would be to weld the flange 1, 1" directly to the Stirling engine crankcase. This would eliminate all fasteners, 0-Rings, O-Ring grooves and also minimise any feasible leakages from said joint. This solution would be suitable when the Stirling engine and generator design is well proven and documented through field trials.

Even though this description has been related to a Stirling engine, the solutions presented in this application can also utilize this design as an electric drive for a hermetically sealed Stirling cooler. The generator would then function as an electric motor and drive the Stirling unit as a cryocooler.

The Stirling engine may be hermetically sealed, and the generator may be used to function as a starter motor; or may produce electricity and heat. Including the generator within the sealed assembly eliminates leakage problems.