METHOD FOR IMPROVING GAS BEARING FUNCTION AT LOW THERMAL COOLING POWER

Abstract

A method for increasing working gas flow rate through gas bearings of a free piston, gamma configured Stirling heat pump to avoid failure of the gas bearings while maintaining thermal cooling power. The Stirling heat pump lifts heat from a storage chamber and has pistons that are driven in reciprocation at an operating frequency by linear electric motors. A temperature control maintains a steady state storage chamber temperature by sensing storage chamber temperature and modulating piston amplitude. The invention comprises (a) driving the pistons with linear electric motors that are driven by a variable frequency, AC power source; (b) sensing the pistons' amplitude of reciprocation; and (c) if the sensed piston amplitude is less than a selected piston activation amplitude, increasing the frequency of the AC power source to increase the Stirling heat pump's operating frequency. That decreases thermal cooling power which causes the temperature control to increase piston amplitude.

Claims

1. A method for increasing working gas flow rate through gas bearings of a free piston, gamma configured Stirling heat pump in order to avoid a failure of the gas bearings, the Stirling heat pump being thermally connected to a storage chamber of a cooler, the Stirling heat pump having a displacer and having pistons driven in reciprocation at an operating frequency by linear electric motors, the cooler also having a temperature control that maintains a steady state storage chamber temperature by sensing storage chamber temperature and increasing piston amplitude at temperatures above the steady state storage chamber temperature and decreasing piston amplitude at temperatures below the steady state storage chamber temperature, the method comprising: (a) driving the pistons with linear electric motors that are driven by a variable frequency, AC power source; (b) sensing the pistons' amplitude of reciprocation; and (c) if the sensed piston amplitude is less than a selected piston activation amplitude, increasing the frequency of the AC power source to increase the Stirling heat pump's operating frequency; wherein the increase in the operating frequency reduces the Stirling heat pump's thermal cooling power causing the temperature control to increase the Stirling heat pump's thermal cooling power by increasing piston amplitude and thereby increasing working gas pressure amplitude to increase working gas flow rate through the gas bearings.

2. The method according to claim 1 wherein the steps of claim 1 are cyclically repeated.

3. The method according to claim 2 wherein the Stirling heat pump has a gas bearing failure threshold R.sub.0 at a gas bearing failure amplitude X.sub.P0 and the selected piston activation amplitude is a piston amplitude X.sub.P1 that is greater than the gas bearing failure amplitude X.sub.P0 by a selected margin of safety.

4. The method according to claim 3 wherein the step of increasing the Stirling heat pump's operating frequency is not repeated less than a selected thermal inertia time delay following a previous increase of the operating frequency.

5. The method according to claim 4 wherein the thermal inertia time delay is in the range of 15 to 30 minutes.

6. The method according to claim 4 wherein steps of increasing the Stirling heat pump's operating frequency are incremental frequency steps.

7. The method according to claim 6 wherein the incremental frequency steps are in the range of 0.1 Hz to 2 Hz.

8. The method according to claim 4 where each step of increasing the Stirling heat pump's operating frequency is a smoothly continuous increase.

9. The method according to claim 2 wherein the AC power source frequency is increased sufficiently to reduce the thermal cooling power to substantially zero.

10. The method according to claim 2 wherein the method further comprises: if the Stirling heat pump is operating at an increased frequency and the piston amplitude exceeds a selected piston deactivation amplitude (X.sub.P2), reducing the frequency of the AC power source to reduce the Stirling heat pump's operating frequency

11. The method according to claim 10 wherein: (a) the Stirling heat pump has a gas bearing failure threshold R.sub.0 at a gas bearing failure amplitude X.sub.P0 and the selected piston activation amplitude is a piston amplitude X.sub.P1 that is greater than the gas bearing failure amplitude X.sub.P0 by a selected margin of safety; and (b) the selected piston deactivation amplitude (X.sub.P2) is greater than the selected piston activation amplitude X.sub.P1.

12. The method according to claim 11 and further comprising: the step of reducing the frequency of the AC power source to reduce the Stirling heat pump's operating frequency reduces the frequency of the AC power source by an amount equal to the net sum of all the increases of the frequency of the AC power source.

13. The method according to claim 12 wherein the step of reducing the frequency of the AC power source is not repeated less than a selected thermal inertia time delay following a previous decrease of the operating frequency.

14. The method according to claim 13 wherein the thermal inertia time delay is in the range of 15 to 30 minutes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 is a diagrammatic view of the principal elements of a prior art freezer.

[0025] FIG. 2 is a diagrammatic view illustrating the principal elements of the gas bearing system of the Stirling heat pump of FIG. 1.

[0026] FIG. 3 is a diagrammatic view illustrating the gas bearing plenum of pistons in the Stirling heat pump of FIG. 1.

[0027] FIG. 4 is phasor diagram illustrating the effect of drive frequency variation on the phase lead of the Stirling heat pump displacer motions ahead of the Stirling heat pump piston motions which is used in the method of the invention.

[0028] FIG. 5 is a graph illustrating the relationship, in a free piston Stirling heat pump, of piston amplitude and gas bearing effectiveness and the manner in which the method of the invention affects both.

[0029] FIG. 6 is a graph that is similar to FIG. 5 but showing an additional curve that is spaced from the dashed curve by a greatly exaggerated distance in order to illustrate a small detail in the operation of the invention that is not illustrated in FIG. 5.

[0030] FIG. 7 is a graph illustrating the manner in which the operation of a free piston Stirling heat pump is modified by the method of the invention.

[0031] FIG. 8 is a diagram of an example of a Stirling cooler that is used to practice the method of the invention.

[0032] In describing the preferred embodiment of the invention which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific term so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word connected or terms similar thereto are often used.

DETAILED DESCRIPTION OF THE INVENTION

[0033] A physical embodiment of a cooler that is used to practice the method of the invention is illustrated in FIG. 8. It's physical structure is essentially the same as the prior art cooler illustrated in FIG. 1 except that the electrical AC power source 52 that drives the pistons 18 and 20 of the Stirling heat pump 10 is a variable frequency electrical power source 52. A variable frequency AC power source, sometimes referred to as a variable frequency drive (VFD), is essentially an oscillator circuit with an electronically variable frequency and can deliver sufficient power to drive an electric motor. Such power sources are commercially available for driving rotating electric motors and alternatively can be designed by a person of ordinary skill in the art. Common component parts have the same reference numbers in FIG. 8 as are in FIG. 1 and therefore the description of them is not repeated.

[0034] The following description of the invention begins with a description of some operating characteristics and relationships that exist in a Stirling free piston heat pump of the type previously described and how those characteristics and relationships are applied in the practice of the method of the invention. Some of those characteristics and relationships were described in the foregoing background of the invention. This description will then turn to a more detailed description that applies the method of the invention.

Basic Concepts and Principles

[0035] The method of the invention applies the following operating characteristics of a free piston, gamma configured, Stirling heat pump.

[0036] The pistons 18 and 20 and the displacer 16 reciprocate periodically and therefore their motion can be represented by the phasor diagram of FIG. 4. The amplitude of the piston reciprocation is an increasing function of the AC voltage amplitude applied to the linear electric motors that drive the pistons 18 and 20. As known in the art, the most desirable operation of a Stirling heat pump is obtained when it is designed so that the displacer leads the pistons by a phase angle in the range of 45° to 65° at a frequency near the resonant frequency.

[0037] Referring briefly to FIG. 4, if the drive frequency of the linear electric motors that drive the pistons is increased, the displacer-piston phase angle ϕ decreases. That phase angle decrease causes the thermal cooling power of the

[0038] Stirling heat pump to decrease, even if the electric motor voltage amplitude and the piston amplitude remain unchanged. The reduced thermal cooling power results in a reduction in the rate at which heat is lifted from the colder mass in the storage chamber 14 to the warmer mass of the ambient air.

[0039] As previously explained, the Stirling heat pump has a gas bearing system that supplies working gas from its plenums. The plenums are charged with working gas by pressure variations of the working gas. The pressure amplitude of the working gas is an increasing function of the piston amplitude of reciprocation. Consequently, a reduction in piston amplitude reduces the gas flow rate through the gas bearings and a sufficient reduction in piston amplitude will allow the gas bearings to fail.

[0040] As also previously explained, a Stirling cooler has a temperature control system that continuously senses the temperature in the cooler's storage chamber, increases the Stirling heat pump's thermal cooling power by increasing piston drive voltage (and therefore piston amplitude) when the sensed temperature exceeds a set point temperature and decreases the Stirling heat pump's thermal cooling power by decreasing piston drive voltage (and therefore piston amplitude) when the sensed temperature is below a set point temperature.

[0041] With the above operating characteristics in mind, the manner in which the above characteristics are applied to practice the invention can be described. In briefest outline of the invention, if the piston amplitude decreases to a piston activation amplitude (X.sub.P1) at which gas bearing operation is near failure due to a reduced gas flow through the gas bearings, the piston drive frequency is increased.

[0042] The drive frequency increase initiates a series of changes in the operation of the Stirling heat pump that causes the temperature control system to increase the piston amplitude and thereby increase the gas flow rate through the gas bearings.

[0043] Looking at the series of changes in more detail, piston amplitude is sensed continuously or repeatedly. If the piston amplitude falls near or to an amplitude at which the gas bearings are expected to fail (gas bearing failure amplitude X.sub.P0), the frequency of the electrical power source that drives the pistons in reciprocation is increased. The frequency increase of the electrical power source reduces the piston-displacer phase angle. The reduction of the piston-displacer phase angle reduces the thermal cooling power and therefore reduces the rate at which heat is lifted from the storage chamber 14. Consequently, the interior temperature of the storage chamber will likely rise. The rise of the storage chamber temperature will be sensed by the temperature control system. In response, the temperature control system will increase the piston amplitude in order to increase the rate of heat lift. The increase of piston amplitude increases the gas flow rate through the gas bearings in order to avoid gas bearing failure.

[0044] As will be seen, there are alternative ways of increasing the frequency. There are also various methods for reversing the frequency increase and for reducing the drive frequency to disengage the invention.

Applying the Method of the Invention

[0045] Referring to FIG. 8, the method of the invention increases the working gas flow rate through gas bearings of a free piston, gamma configured, Stirling heat pump 10 in order to avoid a failure of its gas bearings. Such a failure can occur if the temperature control system of the Stirling heat pump 10 reduces the piston amplitude to an amplitude that threatens to reduce the gas flow rate through the gas bearings to a flow rate that would make the gas bearings ineffective. The temperature control system is incorporated into the computer hardware of a control system 34. The temperature control system maintains a steady state storage chamber temperature by sensing storage chamber temperature and increasing piston amplitude at temperatures above a steady state, set point, storage chamber temperature and decreasing piston amplitude at temperatures below a steady state, set point, storage chamber temperature.

[0046] The potential for such a flow rate reduction occurs at a set point temperature at the higher boundary temperature of the nominal temperature operating range of a cooler of the type previously described and illustrated in FIG. 8. A Stirling heat pump 10 that employs the invention is thermally connected to a storage chamber 14 of the cooler. The Stirling heat pump has a displacer 16 and pistons 18 and 20 that are driven in reciprocation at an operating frequency by linear electric motors previously described.

[0047] In order to employ the invention, the pistons are driven with linear electric motors that are driven by a variable frequency, AC power source 52 rather than being driven at a constant frequency in the conventional manner. The pistons' amplitude of reciprocation is sensed and applied to the control system 34. The amplitude is sensed periodically or continuously as symbolized in FIG. 8 by piston amplitude recovery 54. The value of the sensed piston amplitude is used by the computer control system for both its temperature maintenance feedback control and for implementing the invention.

[0048] There are a variety of ways for sensing the piston amplitude. The most convenient manner of sensing the piston amplitude is according to the disclosure in U.S. Pub. US20220003574 because that method does not require an amplitude sensor. Alternatively, the prior art shows sensors that can be included in the free piston Stirling heat pump, such as U.S. Pat. No. 4,667,158. As another alternative, because the pressure amplitude of the pressure variations (wave) in the work space of the Stirling heat pump is an increasing function of piston amplitude, the piston amplitude could be sensed by sensing the pressure amplitude. A particular model can be laboratory tested to derive an algorithm that relates piston amplitude to the pressure wave amplitude or other parameters of Stirling heat pump operation.

[0049] If the sensed piston amplitude is less than a selected piston activation amplitude (X.sub.P1), the operating frequency of the AC power source is increased to increase the frequency of reciprocation of the pistons 18 and 20. The increase in the Stirling heat pump's operating frequency reduces the Stirling heat pump's thermal cooling power. The reduction of the Stirling heat pump's thermal cooling power will allow the temperature in the storage chamber to rise. The rise in the storage chamber temperature is sensed by the temperature control system which causes the temperature control system to increase the Stirling heat pump's thermal cooling power by increasing piston amplitude. The increase in piston amplitude increases the working gas pressure amplitude. The increased working gas pressure amplitude increases the working gas flow rate through the gas bearings to make the gas bearings more effective.

[0050] The method steps described above are cyclically repeated during operation of the cooler in order to invoke operation of the method whenever the piston amplitude decreases enough to make gas bearing operation insufficiently effective. More specifically, whenever the piston amplitude falls to or below the selected piston activation amplitude (X.sub.P1), the frequency of the AC power source is increased to initiate the above sequence of cause and effect steps. The frequency may be increased multiple times to higher and higher frequencies. Multiple increases may occur if the heat leakage from the storage chamber became small enough to prevent the storage chamber temperature from rising or permitting it to fall further.

[0051] FIG. 5 illustrates the effects of performing the above method on associated Stirling heat pump operating parameters. The dashed curve 57 of FIG. 5 illustrates, in a slightly simplified manner, the relationship between piston amplitude and piston radial displacement. Piston radial displacement is a measure of the effectiveness of the gas bearings of the Stirling heat pump. As the gas bearings become more effective, the pistons are maintained nearer the radial center of their cylinders. If the piston radial displacement increases to the cylinder radius, the piston is contacting and rubbing on the cylinder wall so the gas bearings are ineffective.

[0052] The dashed curve 57 is a characteristic operating curve of the free piston Stirling heat pump. There is a gas bearing failure threshold R.sub.0 at which the piston radial displacement equals the cylinder radius so the piston contacts the cylinder. The intersection 50 of the dashed curve with the gas bearing failure threshold R.sub.0 occurs at the gas bearing failure amplitude X.sub.P0. The gas bearing failure amplitude X.sub.P0 is the piston amplitude at which the gas flow through the gas bearings has decreased sufficiently to permit the piston to contact its cylinder.

[0053] The basic invention is to sense the piston amplitude and, if and when the sensed piston amplitude is less than a selected piston activation amplitude, the frequency of the AC power source is increased to increase the frequency of reciprocation of the pistons. The invention can be performed using the gas bearing failure amplitude X.sub.P0 as the selected activation amplitude which causes the piston drive frequency to be increased. However, in order to better avoid piston-cylinder wear at the gas bearing failure amplitude X.sub.P0, it is preferable to provide a safety margin that avoids a decrease in piston amplitude all the way to the gas bearing failure amplitude X.sub.P0 before activating the frequency increase. The size of that safety margin is a matter for selection according to the professional judgment of the designing engineer. Therefor a gas bearing failure amplitude is preferably selected at a piston amplitude that is greater than the gas bearing failure amplitude X.sub.P0 by a safety margin selected by the designing engineer. FIG. 5 shows a selected activation amplitude X.sub.P1 which intersects an activation threshold R.sub.1 at point 60. The radial distance between points 50 and 60 is the radial displacement safety margin.

[0054] If piston amplitude decreases (moving leftward) along the curve 57 all the way to the selected activation amplitude X.sub.P1, that activation amplitude X.sub.P1 is sensed, the drive frequency in increased and the above cause and effect sequence occurs to cause the temperature control system to increase the piston amplitude. The amplitude increase by the temperature control system moves the operation along the curve 57 to the operating point 62. As seen in FIG. 5, at the operating point 62 the free piston Stirling heat pump begins operating at a higher piston amplitude and a lower piston radial displacement. Consequently, at the operating point 62 the gas bearing are operating more effectively. The higher piston amplitude caused by the temperature control has increased the gas flow rate through the gas bearings resulting in the piston being held closer to the central axis of its cylinder (i.e. a smaller radial displacement).

[0055] Although FIG. 5 accurately depicts the operation of the invention for practical purposes, there is a slight refinement that is illustrated in FIG. 6. The decrease in the relative phase angle between the pistons and the displacer, which results from the frequency increase, causes a slight increase in the pressure amplitude although the piston amplitude has not immediately increased. This is because at the lower displacer phase angle, the pressure contributions due to the piston and displacer motions become more in phase resulting in a higher pressure amplitude for an unchanged piston amplitude. That means that, upon the frequency increase and resulting phase angle decrease, the free piston Stirling heat pump begins to operate along a characteristic operating curve that is shown as a solid line curve 58. However, in reality the curve 58 is so close to the curve 57 that it cannot be accurately illustrated as separate from the curve 57 at the scale of the drawing. Therefore, in FIG. 6 the distance between the dashed curve 57 and the solid line curve 58 has been greatly exaggerated so the curves can be seen as separate in order to illustrate the operation. For practical purposes the two curves 57 and 58 are overlying.

[0056] As described in connection with FIG. 5, upon sensing that the piston amplitude has decreased to the piston activation amplitude X.sub.P1 the drive frequency is increased, the temperature control system increases piston amplitude resulting in operation of the free piston Stirling heat pump moving from point 60 to point 62. As explained in connection with FIG. 5, at the point 62, the free piston Stirling heat pump is operating at a higher piston amplitude and a lower piston radial displacement. Consequently, at the operating point 62 the gas bearings are operating more effectively. The higher piston amplitude has increased the gas flow rate through the gas bearings resulting in the piston being held closer to the central axis of its cylinder (i.e. a smaller radial displacement).

Thermal Inertia

[0057] Although the control systems of a cooler are able to respond to sensed parameters within seconds, milliseconds or less, a temperature change in the storage chamber of most coolers typically requires several minutes because of their efficient insulation. After increasing the piston drive frequency, the method of the invention causes the cooler's temperature control system to increase piston amplitude in order to improve gas bearing effectiveness. That increase in piston amplitude is initiated by an increase in the temperature of the cooler's storage chamber. Because a relatively long time interval is usually required before the storage chamber temperature increases, it is usually desirable to have a time delay before the piston's drive frequency is again increased. The time delay should be long enough to allow the storage chamber temperature to increase sufficiently that a temperature increase can be sensed. The length of the time delay is dependent upon the thermal inertia of the particular cooler. In the absence of a suitable time delay, once the piston activation amplitude has been sensed, the repeated application of the method of the invention before the storage chamber temperature has had time to increase could result in an excessive increase in the drive frequency. Based on experience in the field of ultra-low temperature freezers, an appropriate thermal inertia time delay should at least be in the range of between 5 minutes and one hour but preferably in the range of 15 to 30 minutes. Of course laboratory testing of a prototype cooler can be used to determine the amount of time delay that is appropriate for a particular model of a cooler and its expected conditions such as ambient temperature and expected usage.

[0058] FIG. 7 illustrates a result of increasing the piston drive frequency according to the invention. FIG. 7 shows a characteristic solid line operating curve 66 that relates piston amplitude to thermal cooling power of the Stirling heat pump before increasing the drive frequency. The temperature control system modulates the piston amplitude along the curve 66 around an operating point 67. The thermal cooling load is the cooling power that maintains the storage chamber at a set point temperature. The operating point 67 is associated with the set point temperature that was selected by the user. Before increasing the drive frequency, as the storage chamber temperature rises and falls, the temperature control system operates the Stirling heat pump along the characteristic curve 66 and around the operating point 67. When the piston drive frequency is increased according to the invention and therefore the thermal cooling power is decreased, the temperature control system increases the piston amplitude as previously described. The effect of the frequency increase and resulting phase decrease is to shift the characteristic operating curve 66 to the dashed line operating curve 68. After that shift, the temperature control system modulates the piston amplitude along the curve 68 around an operating point 69. The important feature is that the Stirling heat pump then maintains the thermal cooling power at the same cooling load line but does so at a higher piston amplitude.

Frequency Increase

[0059] The step of increasing the frequency of the AC power source in order to increase the Stirling heat pump's operating frequency can be performed in a smoothly continuous manner or can be performed in incremental steps, such as stepwise incremental steps. Regardless of which way the frequency is increased, the Stirling heat pump will have electrical, mechanical and thermal inertia. Consequently, the actual operating frequency increase will not occur instantaneously following an increase in the electric motor drive frequency. If the control system increases the frequency in increments, it is believed that a preferred increment is in the range of 0.1 Hz to 2 Hz. Of course laboratory testing of particular coolers can reveal a frequency increment that is preferred for the particular cooler.

[0060] The invention additionally allows for operation of the Stirling heat pump in an idle mode. In the prior art, when the demand for thermal cooling power is exceptionally low because heat leakage from the storage chamber is unusually low, some control systems are programmed to turn off the electrical motors, which drive the pistons of the Stirling heat pump, for an interval of time in order to reduce electrical power consumption. While energy saving are desirable, this can cause wear of the piston-cylinder interfacing surfaces. The reason is that, at the end of the shut off process and again at the beginning of restart, piston amplitude is too small to maintain the adequate operation of the gas bearings. Consequently, some wear occurs.

[0061] Because the increase in the piston drive frequency decreases the displacer-piston phase, the drive frequency can be increased enough to bring the displacer's phase lead to, or nearly to, 0°. Such a phase decrease causes the thermal cooling power to decrease to zero. In that state of operation, the pistons would be still be periodically reciprocated but would be doing no work except compressing and expanding the working gas, moving the working gas back and forth between the hot and cold work spaces and overcoming friction. Importantly, in that state the piston amplitude can be maintained above the gas bearing failure amplitude to avoid excessive wear. Although electrical power consumption would not go to zero, it would be greatly reduced and save energy because the Stirling heat pump would be doing nearly no work. This idle state is maintained until the storage chamber temperature rises above a steady state set point temperature. So this idle state provides low electrical power input consumption but the pistons are still reciprocating to maintain the effectiveness of the gas bearings.

Disengagement

[0062] As a further enhancement of the invention, there are conditions under which it can be desirable to decrease the operating frequency of the Stirling heat pump after previously increasing the frequency according to the invention. Such a frequency decrease can reverse the previous frequency increases either partially or completely. If there have been multiple frequency increases, the frequency decreases can be one step at a time, each time making no further decrease until an operating parameter, such as storage chamber temperature or piston amplitude, reaches a selected value. The frequency can be reduced by an amount equal to the net sum of all of the previous increases of the frequency of the AC power source so that the drive frequency returns to its original operating frequency in the absence of implementation of the invention.

[0063] For example, the invention can be disengaged, and the piston drive frequency returned to its original operating frequency when a piston amplitude is sensed that is greater than a selected piston activation amplitude, such as X.sub.P1. Upon such disengagement of the invention, the temperature control system would continue to control piston amplitude. Whenever piston amplitude falls below X.sub.P0 or X.sub.P1 the method of the invention is re-engaged.

[0064] Preferably, there is a selected deactivation amplitude X.sub.P2 (FIG. 5) that is a larger amplitude than the activation amplitude X.sub.P1 by a selected safety margin. If the Stirling heat pump is operating at an increased frequency as a result of activation of the invention and a piston amplitude is sensed that exceeds the selected piston deactivation amplitude X.sub.P2, the frequency of the AC power source driving the pistons is reduced. The frequency may be reduced in a manner that reduces the frequency to partially or to completely return the drive frequency to its original drive frequency.

[0065] The thermal inertia that is described above in connection with a frequency increase would also make it desirable to apply a thermal inertia time delay when reducing the drive frequency for the purpose of deactivating the method of the invention. Among other things the thermal inertia time delay would prevent unnecessary oscillation of the control system in performing the steps. For example, the step of reducing the frequency of the AC power source would not be repeated less than a selected thermal inertia time delay following a previous decrease of the operating frequency. As a more specific example, the thermal inertia time delay is in the range of 15 to 30 minutes and can be applied to partial or complete reversals of previous frequency increases.

[0066] The invention has been described in terms of increasing the piston drive frequency above the designed normal drive frequency at which the Stirling heat pump is driven when the piston amplitude has not fallen to a piston activation amplitude X.sub.P1. The further enhancement of reversing the frequency increases by reducing the drive frequency is also described immediately above. However, it is believed that there is a limit to the amount of frequency decrease. The preferred limit is that the frequency would not be reduced lower than the designed normal drive frequency although it could be reduced some amount below the designed normal drive frequency. However, it should be understood that continuing to increase the displacer phase lead by reducing the frequency would eventually result in a performance drop-off. There would also eventually be a performance drop-off from excessively increasing the frequency. Frequency changes have the effects that are described above but if the frequency change goes too far, machine performance drops off. Frequency changes do not continue to have the same desirable effects on the operation of the Stirling heat pump regardless of how much the frequency changed.

DEFINITIONS

[0067] Cooler—refers generically to devices that include a cooled chamber and a Stirling heat pump that pumps heat out of the chamber, such as freezers and refrigerators. [0068] Gas bearing failure threshold (R.sub.0)—the radial displacement of the piston from the central axis of the cylinder at which the piston contacts the cylinder which is the radial displacement at which the gas bearing is marginally functional. It is the clearance gap between the piston and the cylinder when the piston is centered in the cylinder. [0069] Gas bearing failure amplitude (X.sub.P0)—piston amplitude at the gas bearing failure threshold. [0070] Activation threshold (R.sub.1)—the radial displacement of the piston from the central axis of the cylinder which is less than the gas bearing failure threshold and at which the increase in the operating frequency is preferably activated according to the invention. [0071] Piston activation amplitude (X.sub.P1).—The piston amplitude at the activation threshold. The piston amplitude that the designer selects as near enough to the gas bearing failure amplitude that the invention should be activated. [0072] Piston deactivation amplitude (X.sub.P2).—The piston amplitude at which increases of the frequency of the AC power source from practicing the invention are reversed by reducing the frequency. The frequency reductions can be partial, such as in increments, or can be a complete disengagement of the net sum of the frequency increases of the invention to leave the frequency increase steps of the invention being at least temporarily unpracticed. [0073] Pressure amplitude—The amplitude of the variations of working gas pressure in the work space of the free piston Stirling heat pump. The “peak” is the “amplitude” of the alternating pressure in the heat pump. [0074] Temperature control system—The part of the control system that increases piston amplitude to increase thermal cooling power when the sensed storage chamber temperature rises above a set point operating temperature and reduces piston amplitude when the temperature falls below the set point operating temperature according to feedback control principles. [0075] Thermal inertia time delay—A time delay selected by the designing engineer that is long enough to wait for thermal inertia to allow a temperature change in the storage chamber to occur that can be sensed by the temperature sensor. This time delay prevents repeated increases of the drive frequency before thermal inertia has permitted a temperature increase to be sensed.

REFERENCE NUMBERS

[0076] 10 gamma free piston Stirling heat pump [0077] 12 freezer [0078] 13 warm end of free piston Stirling heat pump [0079] 14 storage chamber [0080] 15 cold end of free piston Stirling heat pump [0081] 16 displacer [0082] 17 access door [0083] 18 and 20 pistons [0084] 19 and 21 cylinders for pistons 18 and 20 [0085] 22 displacer connecting rod [0086] 23 optional planar springs for pistons (FIG. 2 only) [0087] 24 planar spring of displacer [0088] 25 optional connecting rods (FIG. 2 only) [0089] 26 casing [0090] 28 stator coils [0091] 30 reciprocating permanent magnets fixed to pistons [0092] 32 AC electrical power source [0093] 34 control system [0094] 36 temperature sensor [0095] 38 gas bearing plenums [0096] 40 inlet of gas bearing plenums [0097] 42 Check valve [0098] 44 radial gas bearing ports [0099] 46 work space of Stirling heat pump [0100] 48 piston cylinders [0101] 50 graphical intersection point [0102] 52 variable frequency power source [0103] 54 piston amplitude recovery (sensing) [0104] 57 principal operating curve of the free piston Stirling heat pump [0105] 58 secondary operating curve of the free piston Stirling heat pump [0106] 60 graphical intersection point [0107] 62 graphical intersection point [0108] 64 graphical intersection point [0109] 66 operating characteristic curve before increasing drive frequency [0110] 67 operating point before invention is activated [0111] 68 operating characteristic curve after increasing drive frequency [0112] 69 operating point after activation of the invention

[0113] This detailed description in connection with the drawings is intended principally as a description of the presently preferred embodiments of the invention, and is not intended to represent the only form in which the present invention may be constructed or utilized. The description sets forth the designs, functions, means, and methods of implementing the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and features may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention and that various modifications may be adopted without departing from the invention or scope of the following claims.