Patent Abstract:
A Stirling engine which utilizes an inner and outer dual shell pressure containment system surrounding the high pressure and temperature engine components. The space between the shells is filled with a pressure backup gas and an insulation material with the backup gas being in communications with the working fluid. The backup gas and insulation provide a time varying pressure field, driven by the pressure variations in the Stirling engine working fluid, which cancels the pressure differential on the heat transfer tubing and allows an averaging of pressures during each cycle of engine operation. In one embodiment the backup gas is placed inside the inner shell.

Full Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates, generally, to pressure chambers. More particularly, the invention relates to Stirling engines with a dual shell pressure chamber. 
   2. Background Information 
   The maximum Stirling engine efficiency is related to the Carnot efficiency which is governed by the ratio of maximum working fluid temperature relative to the minimum fluid temperature. Improvements in technologies which increase the margin between the two temperature extremes is beneficial in terms of total cycle efficiency. The lower working fluid temperature is typically governed by the surrounding air or water temperature; which is used as a cooling source. The main area of improvements result from an increase in the maximum working temperature. The maximum temperature is governed by the materials which are used for typical Stirling engines. The materials, typically high strength Stainless Steel alloys, are exposed to both high temperature and high pressure. The high pressure is due to the Stirling engines requirement of obtaining useful power output for a given engine size. Stirling engines can operate between 50 to 200 atmospheres internal pressure for high performance engines. 
   Since Stirling engines are closed cycle engines, heat must travel through the container materials to get into the working fluid. These materials typically are made as thin as possible to maximize the heat transfer rates. The combination of high pressures and temperatures has limited Stirling engine maximum temperatures to around 800° C. Ceramic materials have been investigated as a technique to allow higher temperatures, however their brittleness and high cost have made them difficult to implement. 
   U.S. Pat. No. 5,611,201, to Houtman, shows an advanced Stirling engine based on Stainless Steel technology. This engine has the high temperature components exposed to the large pressure differential which limits the maximum temperature to the 800° C. range. U.S. Pat. No. 5,388,410, to Momose et al., shows a series of tubes, labeled part number 22 a through d, exposed to the high temperatures and pressures. The maximum temperature is limited by the combined effects of the temperature and pressure on the heating tubes. U.S. Pat. No. 5,383,334 to Kaminiishizono et al, again shows heater tubes, labeled part number 18, which are exposed to the large temperature and pressure differentials. U.S. Pat. No. 5,433,078, to Shin, also shows the heater tubes, labeled part number 1, exposed to the large temperature and pressure differentials. U.S. Pat. No. 5,555,729, to Momose et al., uses a flattened tube geometry for the heater tubes, labeled part number 15, but is still exposed to the large temperature and pressure differential. The flat sides of the tube add additional stresses to the tubing walls. U.S. Pat. No. 5,074,114, to Meijer et al., also shows the heater pipes exposed to high temperatures and pressures. 
   The Stirling engine disclosed in the inventor&#39;s U.S. Pat. No. 6,041,598 overcomes the limitations and shortcomings of the above prior art by providing a dual shell pressure chamber. An inner shell surrounds the heat transfer tubing and the regenerator. The portion surrounding the heat transfer tubing contains a thermally conductive liquid metal to facilitate heat transfer from a heat source to the heat transfer tubing and also to transmit external pressure to the heat transfer tubing. An outer shell that acts as a pressure vessel surrounds the inner shell and contains a thermally insulating liquid between the inner and outer shells. Pressure of the working fluid as it flows through the regenerator is transmitted through the inner shell to the insulating liquid and back across the inner shell to the liquid metal surrounding the heat transfer tubing. This system tends to balance the pressure across the heat transfer tubing and the inner shell, thereby allowing the engine to operate with the working fluid at a high pressure to generate significant power while keeping the wall of the heat transfer tubing thin to facilitate heat transfer. 
   The preferred material for the insulating liquid is a salt or glass such as Boron Anhydride or a mixture of Boron Anhydride and Bismuth Oxide. Those materials are fairly viscous when liquid, but still allow significant convection currents. A filler material such as ceramic fiber or similar material is placed in the liquid salt region to minimize convective currents. While this can work very well to transmit and balance the pressure across the inner shell and across the heat transfer tubing, combining the filler material and the liquid salt and installing it between the shells in a manner that does not produce voids can be difficult. Also, before the salt melts it does not transmit pressure. Therefore, significant preheating must be done to thoroughly melt the salt before the engine can be run with significant pressure in the working fluid. 
   The present invention improves on the dual shell pressure chamber and overcomes the difficulties in using the insulating liquid between the shells by using gas instead of a liquid. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a longitudinal vertical cross sectional view showing the overall arrangement for a complete Stirling engine system; 
       FIG. 2  is a detailed view of the circled portion of  FIG. 1  illustrating an aperture in the inner shell and an insulating gas backup medium between the shells; 
       FIG. 3  is a detail view similar to  FIG. 2  showing an annular gas backup chamber; 
       FIG. 4  is a detailed view similar to  FIG. 2  showing an annular gas backup chamber and an insulation protection wall; and 
       FIG. 5  is a partial longitudinal vertical cross sectional view of the upper portion of the Stirling engine showing the placement of a gas backup chamber within the inner shell above the heat transfer tubing. 
   

   DETAILED DESCRIPTION 
   U.S. Pat. No. 6,041,598 granted Mar. 28, 2000, and hereby incorporated by reference, discloses a dual shell pressure chamber as used with a Stirling engine. Referring to  FIG. 1 , a cylinder  10  is provided with an expansive bellows  11 , a working fluid, such as Helium, is contained in cylinder  10  above power piston  12  and is shuttled through heat transfer tubing  14 , regenerator  16 , and cooling pipes  18  by the action of displacer piston  20 . Lower housing  22  has an inner area  24  which acts as a reservoir for the working fluid and is in fluid communication with the working fluid in cylinder  10  through throttle ports in cylinder  10 . 
   The inner shell  30  surrounds the heat transfer tubing  14  and regenerator  16 . The upper portion  32  of inner shell  30  contains a liquid metal region  34  filled with a thermally conductive liquid metal, such as silver, which surrounds the heat transfer tubing  14 . The regenerator  16  is preferably a coiled annulus of thin material disposed between cylinder  10  and inner shell  30 . Outer shell  40  surrounds inner shell  30  and acts as a pressure vessel. The inner shell  30 , outer shell  40  and flange  36  bound a pressure backup region  42 . The pressure backup region is filled with a material to provide pressure backup against inner shell  30  and consequently through liquid metal region  34  to heat transfer tubing  14 . It is also desirable that the pressure backup region  42  contain an insulating material  44 , as depicted in  FIG. 2 , to minimize the heat transfer between the hot elements (heat transfer tubing  14 , upper portion  32  of the inner shell, and the upper portion of regenerator  16 ) and cold elements (lower portion of regenerator  16 , and flange  36 ) and to minimize the overall heat loss through the outer shell  40 . 
   As an alternative to using an insulating liquid in the pressure backup region  42 , as disclosed in U.S. Pat. No. 6,041,598, the present invention uses a gas, preferably the same gas as the working fluid, such as helium, in the pressure backup region  42 , preferably in conjunction with the insulating material  44  such as carbon fiber mat or cloth, or ceramic fiber mat or cloth. In the alternative a lower conductivity gas such as Argon could be used as long as the gas in the backup region is not allowed to mix with the working fluid in cylinder  10 . The insulating material  44  prevents significant convection current flow in the gas, thereby significantly reducing heat transfer through pressure backup region  42  as would occur with the use of gas alone. Since the gas is compressible, it does not transmit pressure like a liquid, so it will not transfer the transient pressure from the working fluid in the regenerator  16  to the liquid metal region  34 , and consequently to the heat transfer tubing  14 , like the liquid will when the engine is running. However, the gas does provide a fairly uniform backup pressure against the outside of the inner shell  30  which is transmitted to the liquid metal region  34  and consequently to the heat transfer tubing  14 . 
   During engine operation with a heat source of approximately 2000 degrees F., pressure fluctuates inside cylinder  10  over a range of approximately 1000 psi during each cycle of the power piston  12 . By pressurizing pressure backup region  42  to a desired amount, inner shell  30  and heat transfer tubing  14  can see only tensile, only compressive, or a combination tensile and compressive load. For example if the nominal pressure of the working fluid inside cylinder  10  is 1000 psi, during operation the pressure will range between 500 and 1500 psi. If the pressure in backup region  42  is set at 1500 psi, shell  30  and heat transfer tubing  14  see only a 0–1000 psi compressive load. This may be desirable to prevent any tensile cracking from occurring in those structures. In that case shell  30  may be compressed against regenerator  16  which may detrimentally effect the regenerator. Alternatively, the backup pressure may be set at 500 psi such that shell  30  and heat transfer tubing see only a 0–1000 psi tensile load, thus preventing any compression of shell  30  against the regenerator, but requiring shell  30  and heat transfer tubing  14  to have sufficient tensile strength. Setting the backup pressure at 1000 psi results in a ±500 psi tensile and compressive load across shell  30  and heat transfer tubing  14 . The inventor believes this is the best mode of operation because it subjects the structures to the lowest absolute load. 
   Using the gas pressure backup in this manner, the pressure of the working fluid can be raised to any desirable level to produce significant power in the engine while the loads on the heat transfer tubing  14  and the inner shell  30  are kept low. The upper bounds of the pressure is limited only by safety and manufacturing considerations for the outer shell  40  and the lower housing  22 , which function as a pressure vessel against the atmosphere. Lower housing  22  can be designed to enclose an electrical generator connected to the output shaft  43  of the dual shell Stirling engine, thereby eliminating the need for any external high-pressure seal against a rotating shaft extending through the lower housing. 
   Referring also to  FIG. 2 , when it is desired to operate the engine such that the backup pressure region  42  provides an average tensile and compressive load across inner shell  30 , a small aperture  50  is provided through inner shell  30 , preferably near flange  36 . The advantage of placing the aperture in a low position is that it is in the cold section of the engine and thus the metal is stronger. Aperture  50  thereby allows fluid communication between backup pressure region  42  and the working fluid contained in cylinder  10  and the working fluid reservoir in inner area  24  of lower housing  22 . When the engine is not running, all the pressures in these regions equalize. The working fluid for the engine may be charged to a desired nominal pressure, 1000 psi for example, using a single port, such as through the lower housing  22  into its inner area  24 . Pressure in cylinder  10  and in backup pressure region  42  will also equalize at that pressure. When the engine starts to run, the pressure inside cylinder  10  will fluctuate plus or minus approximately 500 psi. Because the aperture  50  is very small, preferably approximately 0.02 to 0.06 and the engine is running typically over 1000 rpm, the movement of the gas through aperture  50  will be oscillatory and rather minimal. Thus the backup pressure in backup pressure region  42  is maintained at approximately a nominal level. The use of the small aperture  50  is preferred since it allows an averaging of pressures during each cycle. The advantage is that it tracks the average pressure ratio which may change during operation. 
   As pointed out above, the gas backup provides a fairly uniform backup pressure which is of advantage if the pressure in the region  42  were to track pressure in the regenerator region  16 . As also mentioned, the aperture  50  allows an averaging of pressures during each cycle of the engine. As the size of the hole  50  increases, the pressures start to match. This is a favorable condition for stresses in the material but is detrimental to engine power which drops as more and more flow goes in and out of the port  50  with each stroke.  FIG. 3  illustrates one method of reducing the required gas flow through the port  50  which involves the use of a material in the region  44   a  which may be either a solid or only a slightly porous material. This material acts as an insulation and may comprise a cast ceramic material which is both rigid and fairly low in thermal conductivity. Filling the region  42  which such a ceramic material reduces the volume of gas required, which is restricted to the annular space  45  maintained between the ceramic insulation and the wall of the inner shell  30 . This smaller volume would be much easier to pressurize in a time varying manner. As illustrated, the annular space  45  is connected to the working fluid, i.e. the helium gas in regenerator  16  as previously described. 
     FIG. 4  illustrates still another embodiment similar to the  FIG. 3  embodiment wherein the ceramic insulation material  44   b  is spaced from the wall of the inner shell  30  with a thin stainless steel wall  46  being located on the inner border of the material  44   b . The wall  46  is spaced a slight distance from the inner shell  30 , defining a narrow annulus  45  for gas containment as previously described. In this instance, the ceramic insulator may be slightly porous for the purpose of improving its heat transfer properties. The ceramic insulator would be constructed strong enough to hold the pressure field being applied on the inside of the thin wall. This structure provides the narrow annulus which is pressurized with the gas thereby allowing a reduced volume requirement for a time varying pressure match. Aperture  50  in this instance could be larger to more closely match the pressure i.e. approximately 0.2 to 0.5 inches in diameter. Several holes  50  could be placed around the wall to provide a more balanced time varying pressure. 
     FIG. 5  illustrates still another embodiment wherein the gas backup medium may be placed above the liquid metal region  34 . The region  42  would be provided with a ceramic insulation material  44   c  as previously described, completely filling the region between the inner and outer shells. In the alternative, in this embodiment, the region  42  could be filled with an insulating liquid salt or glass as disclosed in applicant&#39;s previous patent. As shown in  FIG. 5 , a feeder pipe  47  extends from the upper portion of the cylinder  10  containing the working fluid, traverses through the liquid metal region  34  and communicates with the backup gas region  48  above the liquid metal region. As described for previous embodiments, the backup gas area  48  thus is connected to the working fluid and allows an averaging of pressures during each cycle. Although backup gas region  48  may be directly interfaced with the liquid metal region  34 , it may be desirable to place solid ceramic or metal layer such as the layer  49  between the liquid metal and the backup gas to keep the liquid metal from splashing into the inside of the engine. The backup gas arrangement in this embodiment performs substantially in the same manner as previously described in the various embodiments in allowing an averaging of pressures during each cycle or a time varying pressure dependent on the size of pipe  47 . 
   Because the backup pressure region  42  or region  48 , the working fluid area inside cylinder  10 , and the working fluid reservoir in inner area  24  of lower housing are all in fluid communication, the overall average pressure in all these areas may be adjusted upward or downward, such as through a single port in the lower housing, while the engine is running. 
   The descriptions above and the accompanying drawings should be interpreted in the illustrative and not the limited sense. While the invention has been disclosed in connection with the preferred embodiment or embodiments thereof, it should be understood that there may be other embodiments which fall within the scope of the invention.

Technology Classification (CPC): 5