Rotating Stirling engine in Alpha configuration

Abstract

The invention relates to a rotating Stirling engine in an Alpha configuration for cooling the rotor coils of superconducting drives. The inner part of the Stirling engine is rotatably mounted in a flow tube (208) and can be thermally connected to a superconducting rotor. The pistons (302, 402) are arranged concentrically and are each driven by a linear motor (500A, 500B) and guided by a gas bearing. The air gaps of the linear motors form an interface for energy transfer between the rotating inner part of the Stirling engine and the stationary flow tube. A fan (228) is also located in the flow tube, which convectively dissipates the waste heat from the coils of the linear motors as well as the waste heat from the ambient heat exchanger (222) of the Stirling engine.

Description

Application area

[0001] The invention relates to free-piston Stirling engines and in particular to a Stirling cryocooler in an Alpha arrangement with concentrically arranged pistons for cooling the rotor coil of high-power-density superconducting electric motors. State of the art

[0002] Superconducting electric motors are considered a key technology for the electrification of heavy transportation vehicles such as ships, trains, trucks, and aircraft. A major challenge lies in cooling the superconducting rotor coils. Unlike the stator coils, these cannot be cooled with liquid hydrogen because no rotary coupling is currently available that functions reliably at high speeds and cryogenic temperatures. Previous rotor cooling systems used stationary thermosiphons with a cryogenic refrigerant, sealed by ferrofluid gaskets. To enable higher rotational speeds, rotationally symmetrical Stirling or pulse tube cryocoolers have been mounted directly on the rotor to cool the rotor coils by heat conduction during rotation. However, the suitability of pulse tube cryocoolers for aerospace applications is questionable, as their efficiency depends on their orientation during operation.In contrast, Stirling cryocoolers offer higher efficiency than pulse tube cryocoolers due to their orientation independence, particularly at low temperatures. However, to date, only commercially available Stirling cryocoolers have been used for cooling superconducting rotor coils, and these were never intended to rotate at high speeds on a shaft. Therefore, they cannot meet the requirements regarding weight, cooling capacity, and reliability. A rough description of a free-piston Stirling engine in an alpha configuration with concentrically arranged pistons, suitable for cooling superconducting rotor coils, was disclosed in US 8857173B2. According to this patent, an inner piston is slidably mounted in the central bore of a hollow cylindrical outer piston, the lower section of the inner piston having a large mass and the upper section being substantially hollow to reduce axial heat conduction.It is mentioned that both pistons can be driven by a linear motor. However, in the case of the inner piston, the moving part of the linear motor must necessarily be axially separated from the lower, larger section. This shifts the center of mass of the inner piston outside the area guided by the outer piston, resulting in large leverage forces acting on the guide. This is particularly disadvantageous if the guide is a gas bearing, whose load-bearing capacity is negatively affected by the leverage forces. The moving part of the linear motor essentially consists of a magnetic ring assembly. Documents US7692339B2, US6694730B2, US5642088A, and US20090001823A1 describe the obvious prior art in this regard, but do not include a method for a detachably mounted magnetic ring assembly whose outer diameter is no larger than that of the outer piston.US7692339B2 shows a magnetic ring arrangement that is flush with the outer diameter of the piston; however, the inner stator is also mounted to the piston, and its mass adversely amplifies the leverage forces described above. US5642088A proposes mounting the magnets using an adhesive bond. However, volatile substances can outgas from the adhesive, which then condense in the regenerator and thus reduce the efficiency of the Stirling engine. US6694730B2 describes clamping the magnets between magnet retaining rings, which are joined using thin rods and spot welds. However, the heat input involved can demagnetize the neodymium magnets, which are preferably used, if their Curie temperature of approximately 80 °C is exceeded.To meet the high reliability requirements of aerospace applications, it remains advantageous to guide the pistons of the Stirling engine using a non-contact gas bearing. The design of existing gas bearings for Stirling engines has been disclosed in US7600464B2, US6293184B1, and US6694730B2. US6293184B1 describes a gas-bearing working piston of a free-piston Stirling engine in a beta configuration. The working piston consists of an outer sleeve into which a hollow cylindrical core is press-fitted. According to US7600464B2, the core can also be multi-part. The core includes large radial grooves that serve as a high-pressure reservoir, small radial grooves that act as a throttle, and axial grooves connecting the radial grooves. A check valve in the working piston pressurizes the high-pressure reservoir with pressure from the working chamber during each piston stroke.The outer diameter of the working piston is guided smoothly within a cylinder, while the inner diameter guides a displacer piston rod. Radial bores in the outer sleeve and core, with the aid of small radial grooves, connect the high-pressure reservoir to the bearing gaps. Due to the throttling effect of the radial grooves, the pressure in the bearing gaps immediately behind the throttling elements becomes a function of the gap height, thus exerting a self-centering force on the working piston. A disadvantage of this type of gas bearing is that the radial bores must be at least as long as the outer sleeve or core is thick, because the throttling occurs at the interface between these two parts. The dead volume in the radial bores increases the gas bearing's tendency towards self-excited vibrations.Furthermore, the working piston in this type of gas bearing is relatively massive and is therefore preferably made of aluminum to reduce weight. However, this necessitates that the displacer piston be manufactured from multiple materials. The piston rod of the displacer piston, guided by the gas bearing, would also have to be made of aluminum to prevent jamming due to differing thermal expansion, while its dome is preferably made of stainless steel to reduce parasitic axial heat conduction. This is particularly problematic if the displacer piston consists of only one cylinder that simultaneously serves as the piston rod and the dome. Moreover, both the outer sleeve and the core of the working piston must be manufactured with high precision to meet the tight tolerances required by the gas bearing. This results in high manufacturing costs.Another variant of a gas bearing for a linear compressor is described in US7247007B2, in which throttling grooves are located in the outer surface of a cylinder liner that is pressed into a guide cylinder. The throttled working gas then enters the bearing gap through radial bores in the cylinder liner. However, since only a single piston needs to be guided in a linear compressor, this variant cannot be readily transferred to a Stirling engine with two concentrically arranged pistons. Object of the invention

[0003] Therefore, there is a need for an improved Stirling cryocooler in an Alpha configuration with concentrically arranged pistons, designed for high-speed rotation, whose gas bearing does not tend to self-excited vibrations and is not adversely affected by leverage forces, whose outer piston has a lightweight stainless steel structure and is cost-effective to manufacture, and whose magnetic ring arrangement is detachably mounted and has an outer diameter that is no larger than that of the outer piston.

[0004] The problem is solved by a Stirling engine with the features of claims 1-3. Advantageous embodiments and further developments are the subject of the dependent claims.

[0005] The invention relates to a rotating Stirling engine in an Alpha configuration for cooling the rotor coils of superconducting drives. The inner part of the Stirling engine is rotatably mounted in a flow tube and can be thermally connected to a superconducting rotor. The pistons are arranged concentrically and are each driven by a linear motor and guided by a gas bearing. The air gaps of the linear motors form an interface for energy transfer between the rotating inner part of the Stirling engine and the stationary flow tube. A fan is also located in the flow tube, which convectively dissipates the waste heat from the coils of the linear motors as well as the waste heat from the ambient heat exchanger of the Stirling engine. Example description

[0006] The invention is described and explained below with reference to the illustrations. These illustrations depict: Fig. Figure 1 is a partial sectional view of a superconducting motor with a rotating cryogenic cooler according to the present invention, mounted on the rotor. Fig. Figure 2 is an enlarged cross-sectional view of the rotating cryocooler made of Fig. 1. Fig. Figure 3 is an enlarged section from Fig. 2 and shows the function of the gas bearing using a throttle insert mounted in the outer piston. Fig. 4 is a sectional view of the rotating cryocooler in Fig. 2 along line 4 - 4 and shows the cross-section of a linear motor. Fig. Figure 5 is a perspective front view (from above) of the outer piston in Fig. 2. Fig. Figure 6 is a perspective front view (from below) of the outer piston in Fig. 2. Fig. Figure 7 is a sectional view of the outer piston in Fig. 5 along line 7 - 7. Fig. Figure 8 is a sectional view of the outer piston in Fig. 5 along line 8 - 8. Fig. Figure 9 is a sectional view of the outer piston in Fig. 7 along line 9 - 9. Fig. Figure 10 is a sectional view of the outer piston in Fig. 7 along line 10 - 10. Fig. Figure 11 is a sectional view of the outer piston in Fig. 7 along line 11 - 11. Fig. 12 is a perspective front view (from below) of the throttle insert in Fig. 3. Fig. 13 is a sectional view of the throttle insert in Fig. 12 along line 13 - 13. Fig. Figure 14 is a perspective front view (from below) of an alternative embodiment of the throttle insert in Fig. 3. Fig. 15 is a sectional view of the throttle insert in Fig. 14 through line 15 - 15. Fig. Figure 16 is a perspective front view (from below) of a distance insert in Fig. 2. Fig. 17 is a sectional view of the distance insert in Fig. 16 along line 17 - 17. Fig. Figure 18 is an exploded view of the outer piston assembly in Fig. 2.

[0007] Fig. Figure 1 shows a schematically represented sectional view of a superconducting motor 100, whose rotatably mounted rotor is cooled by a rotating Stirling engine 200 according to the invention in an Alpha configuration. The rotor has a rotor housing 116 with a rotor shaft 114, as well as a superconducting rotor coil 122, which is preferably coated with a high-temperature superconductor such as REBCO and generates a static magnetic field. The superconducting rotor coil 122 is further connected to the cooling housing of the inner piston 204 (also called the cold head) of the Stirling engine 200 by a highly thermally conductive connection 126. Thermal insulation of the rotor 118 reduces the heat input and thus the heat to be dissipated by the Stirling engine 200.

[0008] A cavity within the rotor 124 preferably contains an insulating vacuum to prevent any heat input through convection and to prevent gas components from condensing within the rotor. The superconducting motor 100 further comprises a stator, consisting of a stator housing 102, and a superconducting stator coil 112, which preferably consists of a superconducting MgB2 stranded cable and generates a dynamic magnetic field. Since superconductors exhibit electrical resistance when an alternating voltage is applied (though this resistance is significantly lower than that of normal conductors), the heat to be dissipated by the superconducting stator coil 112 is higher than that of the superconducting rotor coil 122. Therefore, cooling is preferably achieved with a cryogenic liquid 108, such as hydrogen at 21 K, which can be supplied via a supply line 106 in the stator housing 102.Thermal insulation of the stator 104 reduces heat input and thus the heat that the cryogenic fluid 108 needs to dissipate. The connection between the Stirling engine 200 and the superconducting motor 100 is achieved by two flange connections. A rotating mounting flange 214 on the rotating part of the Stirling engine 200 is sealed to the rotor housing 116, and a stationary mounting flange 212 on the stationary part of the Stirling engine 200 is connected to the stator housing 102.

[0009] Fig. Figure 2 shows a sectional view of the rotating Stirling engine according to the invention in Alpha configuration 200. The Alpha configuration is characterized in that a piston is arranged in both the compression chamber 242 and the expansion chamber 244, and both act simultaneously as working and displacer pistons. This means that both pistons contribute to the periodic compression and expansion of the working gas. In the compression chamber 242, an outer piston 302 of an outer piston assembly 300 is arranged, which has the form of a hollow cylinder and whose outer surface 304 (see Figure 2) Fig. 5 - 11) is guided in a housing of the outer piston 202. In the expansion chamber 244, an inner piston 402 of an inner piston assembly 400 is arranged, which is guided in the inner surface 306 of the outer piston 302. The compression chamber 242 is connected to the expansion chamber 244 by an ambient heat exchanger 222, a regenerator 226, and a cold heat exchanger 218. According to the known Stirling principle, the heat exchangers 218 and 222 ensure heat absorption and heat release to the environment, respectively. The regenerator 226 increases the efficiency of the process by increasing the temperature difference between the compression chamber 242 and the expansion chamber 244 through the intermediate storage of heat. Furthermore, the inner piston 402 moves concentrically to a cylinder liner 206, which spatially separates the compression chamber 242 from the expansion chamber 244 and directs the gas flow between these chambers through the heat exchangers 222 and 218 and the regenerator 226.A leak at the sealing gap between cylinder liner 206 and inner piston 402 does not cause a pressure loss in the Stirling process, but only a short-circuit flow between the hot and cold sides, or between compression chamber 242 and expansion chamber 244. Since a sufficiently small sealing gap also acts as a regenerator, the sealing gap between cylinder liner 206 and inner piston 402 can be made larger than the sealing gap between outer piston 302 and inner piston 402. This reduces manufacturing costs and simultaneously prevents the inner piston 402 from seizing. The outer piston 302 and inner piston 402 are guided without contact by means of a self-sustaining gas bearing, which utilizes the compression pressure of the pistons and will be described in detail later.The two pistons 302 and 402 are each driven by separate linear motors 500A and 500B, preferably moving magnet motors, which are already known in the prior art. In an advantageous embodiment, identical linear motors are used to reduce manufacturing costs. Driving both pistons offers the advantage that the frequency of the piston movement can be precisely matched to the resonance frequency and the phase angle between the two pistons can be precisely controlled. This ensures high efficiency. Furthermore, the required drive power for the compression and expansion of the working gas is distributed across two linear motors, whose outer diameter can then be reduced. This is particularly advantageous in a rotating Stirling engine to keep centrifugal forces low even at high speeds.The linear motors each generate a magnetic flux in the outer stator 510, which is formed from stator segments 512, using a coil 506 (see . Fig. 4), as well as in the inner stator 502, both of which are made of a ferromagnetic material. The stator segments 512 are preferably made of stacked electrical steel sheets with a thickness between 0.3 and 0.5 mm. Alternatively, the use of stators made of soft magnetic composites is also possible, which can be manufactured, for example, from the material Somaloy® from Höganäs AB. In the air gap between the outer stator 510 and the inner stator 502 is a magnetic ring assembly 520, which is attached to the outer piston 302 and the inner piston 402 by a snap-fit ​​mechanism. The magnetic ring assembly 520 consists of a radially magnetized ring magnet 522, a magnet carrier 528, and a retaining ring 548. The ring magnet 522 can be divided into several individual segments and is preferably made of neodymium. The magnet carrier 528 is preferably made of a material with low magnetic permeability, such as stainless steel or titanium.The air gaps of the linear motors 500A and 500B also form an interface for energy transfer between the rotating and stationary parts of the Stirling engine 200. The magnetic flux is transferred from the stationary outer stator 510 through the housing of the outer piston 202 to the rotating inner stator 502. The housing of the outer piston 202 is preferably made of a material with low magnetic permeability and also has the smallest possible wall thickness, since the magnetic resistance in the magnetic circuit increases significantly with increasing air gap. A small wall thickness of the pressurized housing of the outer piston 202 is primarily made possible by a small outer diameter, which in turn is achieved by arranging the outer stator 510 outside the pressure vessel.In the de-energized state, the two pistons 302 and 402 are in one of two detent positions, in which the ring magnet 522 is centered on one of the two poles of the outer stator 510. Therefore, no additional support for the two pistons 302 and 402 is required. When an alternating voltage is applied to the coil 506, the ring magnet 522 causes the linear motors 500A and 500B to oscillate the two pistons 302 and 402. The phase angle between the voltages applied to the two linear motors 500A and 500B corresponds to the phase angle between the movement of the two pistons 302 and 402. To maintain the optimal phase angle for the Stirling process while ensuring high efficiency, it is necessary for both pistons to have a resonant frequency that is as similar as possible.According to the invention, this is achieved by ensuring that the spring stiffnesses of the working gas in the compression chamber 242 and in the expansion chamber 244 are as similar as possible to the spring stiffness of the working gas in the buffer chamber 246. For this purpose, in a preferred embodiment, the volume of the buffer chamber 246 is at most four times the combined volume of the working chamber (consisting of 242, 244 and the cavities in 218, 222 and 226), and in a particularly preferred embodiment, at most three times the combined volume of the working chamber (consisting of 242, 244 and the cavities in 218, 222 and 226). Different spring stiffnesses of the working gas due to temperature differences in the compression chamber 242 and in the expansion chamber 244 can be partially compensated for by differences in the effective axial piston area of ​​the two pistons 302 and 402.Remaining differences in the spring stiffness of the working gas are compensated for by the control of the linear motors 500A and 500B. To prevent the movement of the inner piston 402 from dividing the buffer chamber 246 into two separate gas chambers with different spring stiffnesses, axial through-holes 412 are provided in the underside of the inner piston 402. Furthermore, a check valve 404 on the underside of the inner piston 402 ensures that working gas flows from the cavity 408 of the inner piston 402 into the buffer chamber 246 as soon as it expands. This helps to increase the pressure in the buffer chamber 246 and thus to adjust its spring stiffness, despite its slightly larger volume, to the spring stiffnesses in the compression chamber 242 and the expansion chamber 244.The check valve 404 is preferably designed so that it does not seal completely in the blocking direction, in order to allow filling with working gas after evacuation of the working chamber. This can be achieved, for example, by a Tesla valve (see US1329559A). The rotating part of the Stirling engine 200 is supported within the stationary part by means of rolling bearings 232A and 232B. Furthermore, an airflow between the rotating and stationary parts of the Stirling engine 200 is generated by a flow tube 208 with radial inlet openings 248 and a fan 228. This convectively dissipates the waste heat from a heat sink 224, which is thermally connected to the ambient heat exchanger 222, as well as the waste heat from the coil 506 in the linear motors 500A and 500B. To cool the coil 506, the airflow is directed through cavities 514 between the outer stator segments 512. The cavities 514 are located in . Fig. Figure 4 shows that the stator segments, which are made from stacked electrical steel sheets, are arranged radially. In the area of ​​the rolling bearings 232A and 232B, the airflow is guided through axial ventilation openings 252A and 252B in the housing of the outer piston 202. The heat sink 224 is connected to the rotating part of the Stirling engine 200 and preferably consists of an extruded profile with axial ribs made of a material with high thermal conductivity, such as aluminum or copper. Alternatively, heat exchanger tubes with helically brazed or welded ribs can also be used. The helix is ​​preferably designed like a multi-start thread with a high pitch, which, when the rotating part of the Stirling engine 200 turns, creates an airflow that relieves the fan 228 and thus reduces its energy consumption.The ambient heat exchanger 222 is preferably connected to the housing of the outer piston 202 by brazing without thermal distortion and additionally has a radial groove for receiving a snap ring 234. The snap ring 234 serves as a stop for a union flange 216, which is connected to the housing of the inner piston 204. Screws 238 are preferably used for this purpose, although other connection types are possible. For example, the union flange 216 could include a threaded pipe section that is screwed directly into a mating thread on the housing of the inner piston 204. A seal 236 preferably seals the working gas from the environment by means of a metal C-ring or an O-ring.

[0010] The following section will first describe the structure of the outer piston 302, which is located in Fig. 5 - 11 is shown in different views. The outer piston 302 is preferably made of the same material as the housing of the outer piston 202, such as stainless steel, so that the bearing gap of the gas bearing 332 (see Fig. 3) is not affected by different coefficients of thermal expansion. Furthermore, the outer piston 302 has an annular high-pressure reservoir 326, which is formed by axial blind bores 314 and 316. The axial blind bores 314 are inserted from the side facing the compression chamber 242, and the axial blind bores 316 from the side facing the buffer chamber 246. The axial blind bores 314 and 316 are both located on a pitch circle with preferably the same diameter, and their number is preferably identical. They are also arranged rotationally offset from each other, which allows them to connect inside the outer piston 302 to form the annular high-pressure reservoir 326. First, a spacer insert 340 is inserted into each of the axial blind bores 314 and 316, which is Fig. 16 and Fig. Figure 17 shows that the spacer inserts 340 are preferably made of a low-density plastic, such as polyethylene, and primarily serve to generate a defined gas volume in the annular high-pressure reservoir 326, independent of the axial blind bores 314 and 316. To keep the weight of the outer piston 302 low, it is advantageous to make the axial blind bores 314 and 316 as large as possible, which would also allow the annular high-pressure reservoir 326 to be larger than required for the gas bearing. Beyond a certain volume of the annular high-pressure reservoir 326, the load-bearing capacity of the gas bearing is no longer improved. However, if the volume of the annular high-pressure reservoir 326 becomes too large, it takes longer for the supporting gas film to build up after the machine starts, which can lead to increased wear.To adjust the volume of the annular high-pressure reservoir 326 to a defined value, the size of the central bore 342 and the size and number of the radial grooves 344A and 344B can be varied. The central bore 342 and the radial grooves 344A and 344B also ensure that a continuous flow channel is maintained within the annular high-pressure reservoir 326. The outer diameter of the spacer inserts 340 is formed by the outer surfaces 346A, 346B, and 346C and is preferably designed such that, together with the diameter of the axial blind bores 314 and 316, a clearance fit is achieved. Furthermore, the spacer inserts 340 serve as depth stops for the subsequently inserted throttle inserts 350A and 350B.The spacer inserts 340 in the axial blind bores 314 abut in the center of the annular high-pressure reservoir 326 with the spacer inserts 340 in the axial blind bores 316, thus limiting the depth to which the throttle inserts 350A and 350B can be inserted from both sides into the axial blind bores 314 and 316. At the same time, the spacer inserts 340 facilitate the removal of the throttle inserts 350A and 350B. By striking a mounted throttle insert 350A or 350B with a punch, the two opposing throttle inserts are driven out (since each spacer insert 340 rests against two spacer inserts 340 on the opposite side due to the rotational offset of the axial blind bores 314 and 316). The throttle inserts 350A and 350B, which are in . Fig. The throttle inserts 350A and 350B, shown in Figures 12-15, are inserted into the axial blind bores 314 and 316 after the spacer inserts 340 with an interference fit, thus sealing the annular high-pressure reservoir 326 to the compression chamber 242 and the buffer chamber 246. The throttle inserts 350A and 350B are preferably made of aluminum. This prevents a significant increase in the weight of the outer piston assembly 300 and ensures that the forming forces during pressing in the throttle inserts 350A and 350B have a greater effect on the aluminum than on the stainless steel of the outer piston 302, which must be manufactured with high precision for gas bearing. The throttle inserts 350A have a check valve 352 with a flow direction as indicated by arrow 354 (see Figure 12-15). Fig. 3) in the direction of the annular high-pressure reservoir 326. The throttle inserts 350B do not have a check valve, but are otherwise identical in construction to the throttle inserts 350A. The throttle inserts 350A can be installed either only on the side of the outer piston 302 facing the compression chamber 242, or only on the side facing the buffer chamber 246, or on both sides. In a preferred embodiment, the throttle inserts 350A are installed on both sides of the outer piston 302 in every second axial blind bore 314 and 316. The throttle inserts 350B are installed in the remaining axial blind bores 314 and 316. The check valves 352 are preferably designed as ball check valves with an outer diameter of 5.5 mm.Many small check valves are preferable to a few large check valves to prevent inertial forces from negatively affecting the firing behavior due to the oscillating piston movement. The function of the check valves 352 is to ensure an internal pressure supply for the gas storage by means of the compression pressure from the combined movement of the outer piston 302 and the inner piston 402.

[0011] The operating principle of gas storage is most easily explained in Fig. 3. This process is to be understood and will be described below. As soon as the pressure in the compression chamber 242 or in the buffer chamber 246 is greater than in the annular high-pressure reservoir 326 due to the oscillating piston movement, the check valves open and working gas flows under high pressure into the annular high-pressure reservoir 326 in the flow direction indicated by arrow 354. From there, the working gas passes through radial through-bores 356A and 356B into radial throttle grooves 362A and 362B, which are formed in the outer surfaces 358A and 358B of the throttle inserts 350A and 350B. From the radial throttle grooves 362A and 362B, the working gas then passes into helical throttle grooves 366A and 366B. From the helical throttle slots 366A and 366B, the working gas enters further radial throttle slots 364A and 364B.The radial throttling grooves 364A and 364B are located, in the assembled state, at the same level as radial through-holes 312 on the side of the outer piston 302 facing the compression chamber 242, or at the same level as radial through-holes 308 on the side of the outer piston 302 facing the buffer chamber 246. The division into two radial and one helical throttling groove is primarily for manufacturing purposes; however, other paths establishing a throttling gas connection between the radial through-holes 356A and 356B and the radial through-holes 308 or 312 would function equally well. The throttled working gas passes through the radial through-bores 308 and 312 into the bearing gap 332 between the outer piston 302 and the housing of the outer piston 202, and into the bearing gap 328 between the outer piston 302 and the inner piston 402.The length and cross-section of the throttle grooves 362A, 362B, 364A, 364B, 366A, and 366B are preferably designed such that the pressure drop during flow through the throttles is the same as during the outflow of the working gas from the bearing gaps 328 and 332. Because working gas continuously flows out of the annular high-pressure reservoir 326, the maximum pressure there never reaches the same level as in the compression chamber 242. Nevertheless, a load-bearing gas film can be maintained continuously, since the working gas flows alternately through the bearing gaps 328 and 332 into the chamber where the pressure is currently lower, as indicated by arrows 334 and 336.For example, if the combined movement of the outer piston 302 and the inner piston 402 causes compression in the compression chamber 242, then the opposite buffer chamber 246 is automatically expanded, causing the working gas of the gas storage to preferentially flow into the buffer chamber 246 and thereby create a supporting gas film. Conversely, if the combined movement of the outer piston 302 and the inner piston 402 causes expansion in the compression chamber 242, then the opposite buffer chamber 246 is automatically compressed, causing the working gas of the gas storage to preferentially flow into the compression chamber 242 and thereby create a supporting gas film. As in . Fig. As can be seen in Figure 2, the gas bearing guides the inner piston assembly 400 only in the central area, while a large part of the moving mass is concentrated in the magnetic ring arrangement 520 on the side facing the buffer chamber 246. Therefore, the inner piston 402 has a counterweight 406 on the side facing the expansion chamber 244. The counterweight 406 serves to shift the center of gravity of the moving mass of the inner piston assembly 400 to the center of the area guided by the gas bearing when both pistons 302 and 402 are in the neutral position. This prevents the inner piston 402 from being tilted by leverage forces, which could negatively affect the load-bearing capacity of the gas bearing. The area guided by the gas bearing is in Fig. 2 is marked by the dimension L and the center of the area guided by the gas storage is marked by the dimension L / 2.

[0012] Fig. Figure 18 shows an exploded view of the outer piston assembly 300, including the magnetic ring assembly 520, which will be described below. The magnetic ring assembly 520 is preferably identical in design to both the outer piston assembly 300 and the inner piston assembly 400 and is therefore described only using the outer piston assembly 300 as an example. The magnetic ring assembly 520 is mounted on the outer piston assembly 300 after the spacer inserts 340 and the throttle inserts 350A and 350B have been inserted as described previously. The ring magnet 522 is then slid over the magnet carrier 528 until the lower axial surface 526 of the ring magnet 522 engages in an axial groove 532 in the magnet carrier 528.In this process, tabs 538 on the magnet carrier 528 are bent inwards. These tabs are formed by axial slots 536 and have locking lugs 542 at their open ends, with chamfers 546 facing the open end and chamfers 544 facing the closed end. A further function of the axial slots 536 is to reduce the eddy currents induced in the magnet carrier 528 by the linear motors 500A and 500B, thus increasing the efficiency of the linear motors 500A and 500B. The locking lugs 542 also have a radial retaining ring groove 534 on their inner side for receiving a retaining ring 548. For assembly, the magnet carrier 528 with the mounted ring magnet 522 is then moved over a stop bar 322 (see figure 3). Fig. 6 or Fig. 8) pushed on the outer piston 302. In doing so, the chamfer 546, which faces the open end, bends the tabs 538 inwards until the locking lugs 542 have passed the stop bar 322 and are then bent outwards again by spring forces. However, in the assembled state, the tabs 538 do not return completely to their initial position, but only until the chamfer 544, which faces the closed end, abuts a similar chamfer 324 on the stop bar 322. The remaining spring force of the tabs 538 is generated by the inclined plane on which the chamfers 324 and 544 meet (see Fig. 2) a force component that pulls the magnet carrier 528 towards the outer piston 302 until the upper axial surface 524 of the ring magnet 522 rests against the stop bar 322 without play. For manufacturing reasons, the stop bar 322 has round recesses 318 (see Fig. 6 or Fig.7), which are created during the production of the axial blind bores 316. To prevent the magnet carrier 528 from becoming loose due to the oscillating piston movement, a retaining ring 548 is additionally inserted into the radial retaining ring groove 534, preferably using a Hoopster® ring from Smalley. The radial retaining ring groove 534 is designed to be deep enough that the installed retaining ring 548 is flush with the inner diameter of the magnet carrier 528. This allows for a small air gap between the linear motors 500A and 500B and also prevents the magnetic ring assembly 520 from colliding with the inner stator 502 during the oscillating piston movement.To allow for a small air gap, it is further advantageous if the tubular part of the magnet carrier 528, which is located between the axial groove 532 and the locking lugs 542, has the smallest possible wall thickness, preferably not greater than 0.5 mm. Reference symbol list 100 superconducting motor 102 Stator housings 104 Thermal insulation of the stator 106 Supply line 108 liquid hydrogen 112 superconducting stator coil 114 Rotor shaft 116 Rotor housings 118 Thermal insulation of the rotor 122 superconducting rotor coil 124 cavity 126 thermally conductive compound 200 Stirling cryocoolers 202 Housing of the outer piston 204 Inner piston housing 206 Cylinder liner 208 Flow tube 212 stationary mounting flange 214 rotating mounting flange 216 Throw flange 218 cold heat exchanger 222 Ambient heat exchangers 224 Heat sink 226 Regenerator 228 Fan 232A Rolling bearing 232B Rolling bearing 234 Snap ring 236 Seal 238 screw 242 Compression chamber 244 Expansion area 246 Buffer space 248 radial ventilation opening 252A axial ventilation opening 252B axial ventilation opening 300 outer piston assembly 302 outer piston 304 outer surface 306 inner surface area 308 radial through hole 312 radial through hole 314 axial blind hole drilling 316 axial blind hole drilling 318 round recess 322 Stop bar 324 phase 326 High-pressure reservoir 328 Bearing gap 332 Bearing gap 334 Flow direction 336 Flow direction 340 Distance deployment 342 central bore 344A radial groove 344B radial groove 346A outer surface 346B outer surface 346C outer surface 350A throttle insert with check valve 352 Check valve 354 Flow direction of the check valve 356A radial through hole 358A outer surface 362A radial throttle groove 364A radial throttle groove 366A helical throttle groove 350B Throttle insert without check valve 356B radial through hole 358B outer surface 362B radial throttle groove 364B radial throttle groove 366B helical throttle groove 400 inner piston assembly 402 inner piston 404 Check valve 406 Counterweight 408 Cavity 412 axial ventilation opening 414 Center of mass of the inner piston 500A linear motor of the outer piston 500B inner piston linear motor 502 inner stator 504 coil 510 outer stator 512 outer stator segment 514 Cavity 520 magnetic ring arrangement 522 Ring magnet 524 axial area 526 axial area 528 magnetic carriers 532 axial groove 534 Retaining ring groove 536 axial slot 538 tab 542 Rastnase 544 phase 546 phase 548 retaining ring QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] US 8857173B2

[0002] US 7692339B2

[0002] US 6694730B2

[0002] US 5642088A

[0002] US 20090001823A1

[0002] US 7600464B2

[0002] US 6293184B1

[0002] US 7247007B2

[0002] US 1329559A

[0009]

Claims

[1] Rotating Stirling engine in alpha arrangement, having an inner piston (402) and an outer piston (302) in a concentric alpha arrangement, both of which are connected to a linear motor (500A and 500B) whose coil (506) and outer stator (510) are arranged outside the pressurized housing (consisting of 202, 222 and 204), characterized by , that the coil (504), the outer stator (510) and a heat sink (224) of an ambient heat exchanger (222) are located within a flow channel formed by radial ventilation openings (248), a flow tube (208) and a fan (228), wherein the inner piston (402) has a counterweight (406) on the side facing an expansion chamber (244) which displaces a center of mass (414) of an inner piston assembly (400) to the center of the guide of the outer piston (302) when both pistons are in the neutral position. [2] Rotating Stirling engine in Alpha configuration, comprising an outer piston (302) whose outer piston surface (304) is slidably mounted in a cylinder formed by a housing of the outer piston (202) by means of a gas bearing, wherein the outer piston (302) with its inner piston surface (306) slidably guides the inner piston (402) by means of a gas bearing, wherein the gas bearing is based on the throttling of the working gas by means of small grooves in the outer surface of a cylinder, characterized by, that the outer piston surface (304) and the inner piston surface (306) are formed by a monolithic hollow cylinder without a joining connection, wherein the outer piston (302) has an annular high-pressure reservoir (326) which is formed by axial blind bores (314 and 316) on rotationally offset bolt circles in the two axial outer surfaces of the outer piston (302), wherein the outer piston (302) has throttle inserts (350A and 350B) with which the axial blind bores (314 and 316) of the outer piston (302) are gas-tightly closed from both sides, wherein the throttle inserts (350A and 350B) have radial through bores (356A and 356B) of which throttle grooves (362A, 362B, 362A, 362B) are formed on outer cylinder surfaces (358A and 358B). 364A, 364B, 366A and 366B) exit, which open into radial through-bores (308 and 312) in the outer piston (302),to establish a connection between the annular high-pressure reservoir (326) and bearing gaps of the gas bearing (328 and 332), wherein the throttle inserts (350A and 350B) in one embodiment have check valves on the side facing a buffer chamber (246) or a compression chamber (242), which allow flow only in the direction of the annular high-pressure reservoir (326). [3] Rotating Stirling machine in Alpha configuration, which has an alternating magnetic ring arrangement (520) for the linear motors (500A and 500B), the magnet carrier (528) of which is made of a material with low magnetic permeability, such as stainless steel or titanium, has axial slots (536) to reduce eddy current losses and the ring magnet (522) of which can be made of individual segments, characterized bythat the axial slots (536) of the magnet carrier (528) are open on one side and thereby create flexible tabs (538), wherein the magnet carrier (528) has an axial groove (532) at its closed axial end for receiving a ring magnet (522), wherein the magnet carrier (528) has a radial retaining ring groove (534) at its open axial end on the inner surface for receiving a retaining ring (548), wherein the magnet carrier (528) has locking lugs (542) at its open axial end with chamfers (546) directed towards the open end, which bend the tabs (538) inwards when slid over a magnetic stop bar (322), wherein the magnet carrier (528) has locking lugs (542) at its open axial end with chamfers (544) directed towards the closed end, with which the tabs (538) after the Passing the magnetic stop strips (322) in a similar chamfer (324) they snap into place without play,wherein a retaining ring (548) is inserted into the radial retaining ring groove (534) in the magnet carrier (528) and its position is fixed without play by the spring force of the retaining ring (548) pushing the tabs (538) outwards. [4] Rotating Stirling engine in Alpha configuration according to one of the preceding claims, characterized by , that the heat sink (224) of the ambient heat exchanger (222) consists of a heat exchanger tube with helically arranged fins which, when a rotating part of the Stirling engine (200) is rotated, increase the air mass flow in the flow tube (208). [5] Rotating Stirling engine in Alpha configuration according to one of the preceding claims, characterized by , that a housing of the inner piston (204) is connected to the ambient heat exchanger (222) by means of a union flange (216), a snap ring (234) and a seal (236). [6] Rotating Stirling engine in Alpha configuration according to one of the preceding claims, characterized by , that the volume of the buffer space (246) corresponds at most to 4 times the combined volume in the working space (consisting of 242, 244 and the cavities in 218, 222 and 226). [7] Rotating Stirling engine in Alpha configuration according to one of the preceding claims, characterized by , that the inner piston (402) has a check valve (404) on the side facing the buffer chamber (246) which allows unimpeded flow only in the direction of the buffer chamber (246) and throttles the flow in the direction of a cavity (408) in the inner piston (402). [8] Rotating Stirling engine in Alpha configuration according to one of the preceding claims, characterized by , that an outer piston assembly (300) has throttle inserts (350A and 350B) which are made of a softer material than the outer piston (302). [9] Rotating Stirling engine in Alpha configuration according to one of the preceding claims, characterized by , that the outer piston assembly (300) has spacer inserts (340) which are preferably made of a very light material such as plastic. [10] Rotating Stirling engine in Alpha configuration according to one of the preceding claims, characterized by , that the outer piston assembly (300) has spacer inserts (340) which are slidably inserted from both sides into the axial blind bores (314 and 316) of the outer piston (302) and whose axial outer surfaces meet in the center of the annular high-pressure reservoir (326). [11] Rotating Stirling engine in Alpha configuration according to one of the preceding claims, characterized by, that the outer piston assembly (300) has spacer inserts (340) which serve as depth limiters when inserting the throttle inserts (350A and 350B) and whose length is designed such that the throttle inserts (350A and 350B) are flush with the axial outer surfaces of the outer piston (302) in their lowest position. [12] Rotating Stirling engine in Alpha configuration according to one of the preceding claims, characterized by , that the outer piston assembly (300) has spacer inserts (340) which, by means of central bores (342) and radial grooves (344A and 344B), maintain a continuous flow channel within the annular high-pressure reservoir (326) and simultaneously generate a defined gas volume in the annular high-pressure reservoir (326) that is independent of the axial blind bores (314 and 316). [13] Rotating Stirling engine in Alpha configuration according to one of the preceding claims, characterized by, that the magnetic ring arrangement (520) has at least one retaining ring (548) which is supplied by the company Smalley under the brand name Hoopster Ring.

Images