US 3886744 A
A power-control system for a Stirling engine which includes a supply line by which working gas is added to the engine's compression space to increase pressure therein and power, and a dump line for discharging gas and reducing power, and a storage container having two variable-volume chambers divided by a bellows; the first chamber contains a quantity of said working gas maintained at a substantially constant temperature which is in communication with said supply and dump lines via appropriate valves, and the second chamber is sealed and contains an azeotropic mixture such as methane and butane. When the gas is added to the first chamber, the bellows moves to reduce the volume of the second chamber, with a conversion of some gas to liquid in the second chamber, and maintenance of the pressure in both chambers.
Description (OCR text may contain errors)
United States Patent 11 1 .laspers 1 1 June 3, 1975 1 POWER-CONTROL SYSTEM FOR [731 Assignee: North American Philips Corporation, New York. NY.
22 Filed: July 22.1974
Primur Evwniner-Martin P. Schwadron Assistant Evaminer-H. Burks. Sr. Attorney. Agent. or F irmFrank R. Trifari  ABSTRACT A power-control system for a Stirling engine which includes a supply line by which working gas is added to the engines compression space to increase pressure therein and power. and a dump line for discharging gas and reducing power. and a storage container having two variable-volume chambers divided by a bellows; the first chamber contains a quantity of said working gas maintained at a substantially constant temperature which is in communication with said supply and dump lines via appropriate valves. and the second chamber is sealed and contains an azeotropic mixture such as methane and butane. When the gas is added to the first chamber, the bellows moves to reduce the volume of the second chamber, with a conversion of some gas to liquid in the second chamber, and maintenance of the pressure in both chambers.
8 Claims, 6 Drawing Figures Patented y} June 3, 1975 4 Sheets-Sheet 1 Fig.
l mDOmOP SPEED Patented June 3, 1975 3,886,744
4 Sheets-Sheet 2 Fig. 3
Patented June 3, 1975 4 Sheets-Sheet Fig.5
Patented June 3, 1975 [ATM] Pressure 4 Sheets-Sheet 4 VALVE 39 MOVES EA STROKE,BOTTLE 4O STROKE,BOTTLE 4| BELLOWS DISPLACEMENT Fig. 6
POWER-CONTROL SYSTEM FOR STIRLING ENGINES BACKGROUND OF THE INVENTION This invention concerns a control system for regulating the power output of a Stirling engine by varying the pressure of the working gas in the engines working spaces. Typically this is accomplished by connecting supply and dump lines or gas conduits to the engines compression space, with valves arranged to permit gas flow into the compression space for increasing pressure therein which results in higher output torque and to permit gas flow out of the compression space for reducing pressure and torque. The gas will flow merely by having the supply line at higher pressure than the compression space which cycles between high and low cycle pressure. and having the dump line at lower pressure. Apparatus for the above system typically includes a storage container holding a quantity of working gas at high pressure, a compressor having its intake communicating with the engines dump line and its discharge communicating with the storage container, and a supply line from the container to the compression space.
While Stirling engines may be used to drive various devices such as boat propellers, automobiles, and electric power generators, the operation characteristics of the driven devices are not always readily compatible with the Stirling engine, such that certain combinations have been considered to be non-feasible, which is the subject of the present invention.
FIG. 2 of the drawings illustrates certain relevant characteristics of the subject devices, relative to a torque vs. speed diagram. Curve a-b is a typical torque curve for a driven device such as a boat propellor or an automobile. For higher speeds or power, greater torque is required, with the power demanded, P Torque x Speed X Constant. Curve c-b is the torque curve for another driven device, an electric power generator wherein speed is shown to be constant at level c; substantially constant speed is a requirement in order to provide stable frequency and voltage, regardless of the load on the system and the corresponding power demanded by the generator. In this constant speed application power will vary only with torque, since Power Torque x Constant speed x Constant. Furthermore, torque x is directly proportional to pressure in the working space; high pressure curve P3 corresponds to high torque e, as compared to low pressure curve P1 relating to lower torque f. Accordingly for a given increase in power, torque (as affected by pressure) must increase more greatly with fixed speed than where speed also increases. Thus, for electric power generation with a Stirling engine, operation over a substantial pressure range will be required.
The use of a compressor-storage tank control system for regulation over a wide pressure range has various undesirable features. A hermetic seal must be maintained throughout the working gas system, including the compressor, the storage tank, the supply and dump lines. and the working space. In order to accommodate the high pressure range, the storage tank must be very high pressure, i.e. 250 at; and the compressor intake must relate to the low pressure. i.e. 50 at. while its discharge must relate to the high pressure. 250 at. Thus a complex and expensive compressor is required. that must operate over a large pressure range: the hermetic seal problem is heightened due to high temperature in the compressor from the extreme pressure range, and also there is noise, vibration, and power drain caused by the compressor. The present invention provides a control apparatus which substantially overcomes all of' the above-described undesirable features.
SUMMARY OF THE INVENTION The invention is a new power regulating system in combination with a Stirling engine. The working gas space of the engine is in communication via a dump line and a supply line and appropriate valves with a storage container of working gas. This container includes a bellows separating a closed space containing a binary liquid-vapor system, and a second space containing working gas which can be conducted to and from said working space via said supply and dump conduits respectively. The liquid-vapor system is preferably an azeotropi'c mixture, such as methane and butane which will develop a pressure greater than 100 atmospheres at room temperature of approximately F. With this container structure the working gas supply is maintained at a substantially constant pressure regardless of whether gas is added to or removed from the gas space within the container. By using two such containers at different high and low pressures, it is possible to control the power of a Stirling engine over a large pressure range, such as 3:1, even while maintaining constant engine speed, as is required in electric power generation systems. With this invention numerous advantages result as compared to prior art control systems which utilize a compressor and a plain storage container having only a cavity for working gas, and optional spring means tending to maintain the gas pressure even when same has been discharged. More specific features and advantages of the invention are developed in the description of the preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a conventional powercontrol system with a Stirling engine.
FIG. 2 is a diagram plotting the relationship of speed vs torque pressure and power for a typical Stirling engine.
FIG. 3 is a schematic view of a power control system of the invention with a Stirling engine.
FIG. 4 is a schematic view of a storage container of the invention.
FIG. 5 is a schematic view similar to FIG. 3, with two storage containers of gas at high and low pressures respectively.
FIG. 6 is a schematic view showing bellows displacement relative to pressure, for a power control system using two storage containers at high and low pressures respectively.
DESCRIPTION OF THE PREFERRED EMBODIMENT A conventional power control system shown in FIG. I shows the working space 10 of a Stirling engine communicating with a supply line 11 and a dump line 12. Check valve 13 permits gas flow into space 10 when the gas pressure therein drops below the pressure of gas in the supply line; check valve 14 permits gas flow out of space 10 when the pressure therein exceeds the dump line pressure. The gas flow in dump line 12 can be directed via valve back to the supply line 11, or via valve 16 to the compressor 17 whose intake 18 has the lowest pressure of the system. By operation of the compressor the gas pressure is raised sufficiently high to recharge the storage container 19 from the compressor discharge 20. Valve 17 allows high pressure gas from the container to flow via supply line 1 1 back to working space 10. The variation of mean pressure experienced by the working space is 50 to 150 atmospheres. while the pressure range experienced by the compressor between intake and discharge would be 65 to 250 atmospheres, the latter pressure being the maximum storage container pressure. A hermetic seal 21 is maintained about the working gas system, including the compressor, the working space, and the storage container. Automatic control means 22 opens and closes valves 15, 16, and 17 as required.
FIG. 2 provides various torque curves showing the relationships of speed, torque, pressure, and power for a Stirling engine and for certain types of driven devices, this Figure having been discussed above in the Background of the Invention.
A basic embodiment of the invention is shown in FIG. 3 where a Stirling engine working space 23 has its pressure varied via supply and dump lines 24 and 25 respectively, using valves 26, 27, 28, 27a and 28a, as in FIG. 1. However, container 29 is totally different from the simple pressurized tank 19 of FIG. 1; this new container 29 is shown in detail in FIG. 4, where within heat-insulated housing 30 is a bellows 31 separating gas space 32 from liquid-vapor space 33. A binary liquidvapor system is established by using mixture of methane and butane, such as 54 percent liquid methane 46 percent liquid butane, which will provide at room temperature of about 70F (21C) a pressure of 108 at. This is a convenient and useful temperature and pressure range, as opposed to using water-water vapor mixture, which would require a temperature of over 300C to develop the desired pressure over 100 at.
In operation chamber 32 can supply gas at a substantially constant pressure despite variations in the quantity of gas present, because as gas is discharged via port 34 to the supply line 24, and when the pressure in space 32 tends to drop, the vapor pressure in space 33 will urge the bellows to move and reduce the volume of space 32 which will bring the pressure there back up, while in space 33 additional liquid will vaporize to bring the pressure therein back to nominal. A layer of gauze 35 or equivalent on the inside wall of chamber 33 hastens vaporization of the mixture when there is a rapid increase in volume of space 33. To protect the bellows from overtravel should the container be discharged excessively, a support 30a is provided to arrest movement of the bellows beyond the supports end surface; on contact of the bellows with this support the container opening becomes sealed to prevent further gas discharge. To further hasten vaporization an electric heater 30b may be incorporated into the containers wells 30, but such is not necessary for the basic operation.
In operation of the engine, when supply line valve 28 is opened, gas at higher pressure from chamber 32 will flow to engine compression space 23; and when dump line valve 27 is opened gas can flow from the space 23 into chamber 32, with corresponding movement of the bellows into space 33, and condensation of vapor therein. A single container 29 as shown in FIG. 3 is adequate for a Stirling engine to operate with a 2:1 pressure variation in the working space.
Where a higher pressure change, such as 3:1 is required. the embodiment of the invention shown in FIG. 5 may be used. Here a Stirling engine workspace 36 communicates via supply and dump lines 37, 38 through valve 39 with two storage containers 40, 41 similar to that of FIG. 4, but at two different high and low pressures respectively, such as and atmospheres. The high pressure container will be operable with the dump line, and the low pressure container with the supply line, with valve 39 operable automatically to select the appropriate container. The working space 36 has a variation of mean pressure between 50 atm and I50 atm, and FIG. 6 shows schematically the pressure cycle by which the valve 39 switches the connection between the two bottles.
Upon command from the electronic control 42 to decrease power, valve 43 is opened. At this instant, the pressure in the dump line 38 is the maximum cycle pressure, 207 atm, since the engine is at full mean pressure. Simultaneously, valve 44 is opened, changing the pressure phase, while gas flow through valve 43 lowers the mean pressure in the engine by gas transfer to the storage system, until the desired low power output level is reached. With the pressure level in the dump line at 207 atm and the pressure level in container 40 at 110 atm, helium is transferred through the dump valve into container 40 as indicated in FIG. 6. Moving from left to right in FIG. 6, the maximum cycle pressure or dump line pressure decreases until it almost equals the 1 10 at. pressure in container 40. To descend to still lower pressures valve 39 switches the dump line to container 41 while sealing off container 40. Now dump line at a pressure slightly above H0 at. communicates with container 41 whose pressure is 65. Gas transfer continues into container 41 until the maximum cycle pressure has decreased to about 65 atm. Then the bypass 44 and dump 43 valves are closed, and the mean cycle pressure, as indicated in FIG. 6, has decreased from atm to 50 atm, a pressure change of 3:1.
To raise the pressure, and through it the power output, the process is similar except that the operation is performed on the supply line 37 through valve 45. Starting at the lowest mean pressure, valve 45 is opened connecting container 41 at 65 atm to the supply line 37 which is at the minimum cycle pressure of 35 atm. Gas is thus transferred from container 41 into the supply line and into the engine. This continues until the minimum cycle pressure approaches 65, at which time valve 39 switches the supply line to bottle 40 at 110 at; then pressure rises until the minimum cycle pressure is about 110, with the maximum cycle pressure at 207.
The response time in going from full power to idle will be about 0.1 see (with the aid of the bypass valve 44). After about 5.0 sec, gas transfer to the storage system will be complete and the bypass valve closed. The response time in going from idle to full power is estimated to be 0.5 sec.
The invention as described above has numerous im portant advantages over the prior art control systems using a compressor and a plain storage tank. With the elimination of the compressor. we greatly reduce cost. noise, maintenance and seal problems of this relatively complex mechanism; the result is a cheaper more reliable control-system, which operates with lower gas storage pressures.
A further advantage of this invention concerns sudden stoppage of the engine while operating with high pressure in the working space. in prior art control systems, with a compressor hermetically sealed into the working gas system, a stopped engine at high pressure remained in that condition, and subsequently it was extremely difficult to overcome this pressure upon the compression piston when trying to re-start the engine. With the present invention the above-described problem will not occur. At any time the dump line valve can be opened to one of the storage containers; regardless of whether the engine is running. The quantity of mixture in the storage container space 33 is selected such that upon full extension of the bellows into space 33, substantially all of the mixture will be condensed, and the bellows will then be supported by liquid and protected from damage due to overextension.
1. In a Stirling engine which includes a working space where a working gas is cycled between high and low pressures, the improvement in combination therewith of a power regulating system comprising a storage con- 7 tainer with a bellows therein separating first and second chambers, the first chamber containing working gas, the second chamber containing a binary liquid-vapor mixture, the power-regulating system further comprising a supply conduit for conducting gas from said first chamber to said working space, and a dump conduit for discharging gas from said working space to said first chamber, and valve means for selectively regulating said gas flows in said supply and dump conduits, said gas in the first chamber being maintained at a substantially constant pressure since said mixture in the second chamber will vaporize and condense respectively according to the decrease or increase of gas in the first chamber.
2. Apparatus according to claim 1 wherein said liquid-vapor system comprises an azeotropic mixture of methane and butane.
3. Apparatus according to claim 2 wherein said mixture comprises 54 percent liquid methane and 46 percent liquid butane by molecular weight.
4. Apparatus according to claim 2 wherein said mixture will have a pressure greater than 100 atmospheres at room temperature of approximately F.
5. Apparatus according to claim 1 wherein said container is a generally cylindrical housing with a closed end and an open end defining a gas port therein, said first chamber communicates with said port, and said second chamber is bounded by said closed end and said bellows.
6. Apparatus according to claim 5 wherein said bellows is movable between an extended position into said second chamber when working gas is added to the first chamber and a compressed position into said first chamber when working gas is discharged from said first chamber, and wherein said container further comprises support means in said first chamber and adjacent said bellows for restraining movement of the bellows beyond said compressed position, during excessive discharge of working gas.
7. Apparatus according to claim 6 further comprising means for closing said port when said bellows moves to its compressed position.
8. Apparatus according to claim 1 further comprising a second working gas storage container similar to said first container, the liquid-vapor system of one container providing a working gas pressure at room temperature substantially greater than the other system, and control valve means for selectively connecting said dump conduit and supply conduit to the higher or lower pressure container.
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