Patent Application
20130180238
(Not Applicable)
(Not Applicable)
This invention relates to free piston Stirling engines and more particularly relates to a new manner of controlling the output power of such engines that gives the advantageous results of reducing manufacturing costs by easing alignment and concentricity tolerance requirements and of providing a new engine power output modulation characteristic by which a load modulates the output power of the engine with the characteristic that the power output from the engine is an increasing function of the difference between a desired load operating limit and the current load.
Stirling machines have been known for nearly two centuries but in recent decades have been the subject of considerable development because they offer important advantages. Modern versions have been used as engines and heat pumps for many years in a variety of applications.
The prior art has numerous examples of prior art, free piston Stirling engines including U.S. Pat. Nos.: 7,640,740; 7,692,339; 7,775,041; 7,827,789; and 8,011,183. Explanation can be found at http://www.sunpower.com/services/technology/stirling.php.
Until about 1965, Stirling machines were constructed as kinematically driven machines meaning that the piston and displacer are connected to each other by a mechanical linkage, typically connecting rods and crankshafts. The free piston Stirling machine was then invented by William Beale. In the free piston Stirling machine, the pistons are not connected to each other by a mechanical drive linkage. A free-piston Stirling machine is a thermo mechanical oscillator and the displacer 14 is, in the prior art, driven by the working gas pressure variations and pressure differences in spaces or chambers within the machine. The piston 16 is either driven by a reciprocating prime mover when the Stirling machine is operated in its heat pumping mode or drives a reciprocating mechanical load 24 when the Stirling machine is operated as an engine. Free piston Stirling machines offer numerous advantages including the ability to control their frequency, phase and amplitude, the ability to be hermetically sealed from their surroundings and their lack of a requirement for a mechanical fluid seal between moving parts to prevent the mixing of the working gas and lubricating oil.
The function of the displacer 14 is to alternately displace working gas in the expansion space 10 and the compression space 12 and thereby cyclically vary the proportion of working gas in each of those spaces. A characteristic of a free piston Stirling engine is that the power out produced by the engine is an increasing function of the amplitude of the displacer reciprocation. The greater the amplitude of the displacer reciprocation, the greater the heat that is transferred into and out of the working gas during each operating cycle and therefore the greater the heat energy input to the engine and the mechanical energy output from the machine during each cycle.
The recognized and preferred manner of driving the displacer 14 in reciprocation is based upon the pressure differential that exists because a connecting rod 26 typically extends through a cylindrical bore 28 in the piston 16 and into a back space 30 and sometimes to a planar spring 32 which centers the connecting rod 26. The back space 30 usually has a large volume relative to the working space so that the cyclical gas pressure variations in the back space 30 are relatively small compared to the cyclical gas pressure variations in the working space. The gas pressure in the working space applies a cyclically varying force on the displacer 14 and its connecting rod 26. The instantaneous value of that force is equal to the product of the instantaneous pressure in the working space multiplied by the cross sectional area of the connecting rod 26. The gas in the back space 30 applies an oppositely directed force that is equal to the product of the instantaneous pressure in the back space 30 multiplied by the cross sectional area of the connecting rod 26. Consequently, the net force driving the displacer is the difference between the two oppositely directed forces. That is true whether the back space 30 pressure never appreciably varies or the back space has a smaller volume that acts like a gas spring and therefore undergoes pressure variations. In addition to the pressure differential method for driving the displacer, displacers have also been driven by a separate prime mover and by a mechanical drive linkage connected to the piston but both of these methods have been largely discarded in current technology.
The use of the pressure differential method for driving the displacer not only requires a displacer connecting rod and a bore, which usually extend into and most commonly through the piston, but also requires that the connecting rod have a cross sectional area that is large enough to provide a sufficient drive force for driving the displacer at desired amplitudes. These requirements add considerably to the cost of manufacturing a free piston Stirling engine because of the extremely close tolerance manufacturing and assembly that are required. One reason for the increased cost is the need for a tight gas seal between the connecting rod and the piston bore through which the rod extends. Consequently, both must be precision machined to minimize the clearance between them and yet permit their relative reciprocation. Additionally, the rod and bore also introduce substantial concentricity and alignment problems. Both the piston bore and the displacer rod not only must be precisely concentric in their position but also they must be aligned precisely coaxially along the central axis of the displacer cylinder and the piston cylinder. Otherwise they would interfere during reciprocation. Furthermore, as the connecting rod diameter becomes larger, the concentricity and alignment problems become larger. In summary, the combination of the need for a close fit of the rod within the bore through the piston and the need for precise alignment and concentricity make both manufacturing and assembly a significant part of the cost of fabricating such free piston Stirling engines.
It is therefore an object and feature of the present invention to provide a free piston Stirling engine that permits the connecting rod and its mating bore in the piston to be totally eliminated or, in the alternative if retained, allows them to be considerably smaller in diameter and thereby reduces the concentricity and alignment requirements and their corresponding manufacturing costs.
There are a variety of methods and apparatus for controlling a free piston Stirling engine. Many are complicated and involve sophisticated electronic circuitry typically with digital processors. Control is necessary because, if the demand of a load that is driven by a free piston Stirling engine is reduced during operation, the piston and displacer amplitude can increase to the extent that the piston and displacer collide with internal structures and become damaged or destroyed. It is an object and feature of the invention that the invention permits control by technically simple structures that are readily accessible outside the casing of the engine.
It would be desirable for some applications to have a free piston Stirling engine that has an operating characteristic that the power output from the engine is reduced as the load approaches some desirable, selected operating limit and becomes zero power output when that limit is reached. For example, that characteristic would be desirable for a domestic water supply system in which a water pump is driven by a free piston Stirling engine and the engine receives its power from a solar collector. In a domestic water system, a supply of water is pumped into a reservoir, such as a pressure tank, until a suitable upper pressure limit is reached, commonly 50 psi. It would be desirable that the Stirling engine stop operating the pump when the upper pressure limit is reached and also desirable that the drive power from the Stirling engine increase as the pressure drops below the upper pressure limit. In particular it would be desirable that the engine power output be an increasing function of the difference between the upper pressure limit and the current pressure in the system. That would provide the characteristic that the engine and pump would stop when the pressure reaches the upper pressure limit but water would be pumped at a rate that is proportional to (or some other increasing mathematical function of) the extent to which the current water pressure is below the upper limit pressure so that the greater the need for pump operation, the greater the drive power that is applied to the pump. It is an object and feature of the invention to provide an engine and load configuration that exhibits such an operating characteristic.
Although unusual or rare, free piston Stirling engines have been shown in a free casing configuration sometimes known as free-cylinder. The free casing Stirling engine offers a means by which mechanical power extraction devices (e.g. a water pump) may be coupled externally to a pressurized, sealed engine. The cyclically repetitive acceleration and deceleration of the moving displacer and piston masses, along with masses connected to them, apply f=ma forces to the engine casing. The result is usually regarded as undesirable vibration of the casing and the vibrations are often reduced by use of a vibration balancer or vibration damper. However, if the casing is mounted so that it is free to reciprocate with respect to a reference ground, a load can be linked between the casing and the ground and the casing motion used to drive the load. Unlike the free piston engine having its load coupled directly to its piston, the free casing configuration has the ability to start at any value of load. In the free casing configuration of the prior art, the load is coupled externally between the engine casing and a reference ground. So far as known, a free piston Stirling engine has never had its power output controllably modulated, or otherwise controlled, by controlling the amplitude of the casing reciprocation.
In fact, if a water pump is driven by a free casing, and the free piston Stirling engine has its displacer driven in the usual manner by means of a displacer connecting rod using the above described pressure differential to drive the displacer, an increase in the water pressure causes an increase in power applied to drive the displacer and a consequent increase in piston and displacer amplitude and therefore in the force applied to the pump. If the pump pressure rises high enough to entirely stop the casing motion, the displacer and piston reciprocate at an excessive amplitude that is likely to cause internal collisions. Conventional free casing configurations generate more thermodynamic power as a result of more casing constraint. If the load increases enough to completely restrain the casing and stop casing motion, the additional thermodynamic power must go somewhere. The additional power is not transferred to the load if the load motion is stopped. So that additional thermodynamic power is transferred to self-destructive internal behavior such as increased amplitude of piston and displacer motion. The present invention does the opposite. With the invention, as the reciprocation is restrained, the generated thermodynamic power decreases. Therefore, if the invention is used for the water pump example, as the pump pressure rises, the amplitudes of reciprocation of the displacer and piston are reduced, thereby avoiding damaging collisions.
In a free piston Stirling engine, the power output from the engine is an increasing function of the amplitude of the displacer's amplitude of reciprocation. The basic concept of the invention is that the displacer is sprung to the casing so that the displacer is driven, at least in dominant part, by the motion of the casing and the power output from the free piston Stirling engine is controlled by controlling the casing amplitude of reciprocation and thereby controlling the displacer amplitude of reciprocation. The displacer is desirably driven only by power coupled through the spring from the casing in which case a displacer connecting rod can be entirely eliminated along with the concentricity and alignment problems it causes. Alternatively, a displacer rod of smaller diameter can be used and the displacer driven in minor or inconsequential part by the above-described pressure differential used in the prior art. With the invention, because the casing applies the principal proportion of the displacer drive power, the displacer motion can be controlled by controlling the casing motion (e.g. by a brake). Power output can be taken from the reciprocating piston or from the reciprocating free casing. Regardless of whether power output is from the piston, the casing or both, engine power output is still controlled by controlling casing amplitude and thereby controlling displacer amplitude.
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.
The casing 36 is drivingly linked to the displacer 40 by a double acting gas spring 54. This gas spring 54 is formed by a gas spring piston 56 that sealingly reciprocates within an enclosed cylinder 58. A gas spring piston connecting rod 60 extends slidingly through the displacer 40 to a perforate support plate 62 in the compression space 46. The perforate support plate 62 extends to and is fixed to an interior wall of the engine, such as directly to the casing 36. The support plate 62 has openings or perforations to permit working gas to flow through it. Consequently, the gas spring piston 56, its piston rod 60 and the support plate 62 are all stationery with respect to the casing 36. Therefore, when the displacer 40 reciprocated, it is sprung to the casing by the gas spring 54.
During operation, this gas spring 54 applies a drive force to the displacer 40 from the reciprocation of the casing which drives the displacer 40 in reciprocation. In this manner drive power is applied to the displacer from the casing. In the embodiment of
As stated above, the power output from a free piston Stirling engine is an increasing function of the amplitude of the displacer reciprocation. A power output control 64 is linked to the casing 36 and, in the embodiment of
The power output control that is a variable reciprocation restraint used with the invention acts on the casing to variably and controllably restrain the reciprocation of the casing. The variable reciprocation restraint is drivingly linked to the casing, usually between the casing and the reference ground. Typically a reciprocation restraint is a reciprocation damper that has variable damping so that the quantity of power that it absorbs is variable. Another example of a variable reciprocation restraint is an electromagnetic generator or alternator with an energy absorbing load such as a variable resistance. A characteristic of the variable restraint is that it varies the restraining force it is capable of exerting on the casing so that it restrains the casing more tightly against reciprocation. As a reciprocation restraint becomes capable of exerting more force, it first exerts more force and becomes more effective in reducing casing reciprocation. As casing reciprocation is reduced, the displacer drive power and displacer amplitude are reduced and therefore engine power decreases. As engine power is reduced, the force and power output from the engine is reduced so the force actually applied by the variable restraint is reduced. Another example of a reciprocation restraint is a fluid pump that pumps fluid through a variable orifice allowing a controllably variable fluid flow resistance. Since an alternator and a motor are both essentially a moving magnet that reciprocates within, or is otherwise magnetically coupled to an armature coil, a linear motor/alternator may be connected in a circuit that can be controlled to alternatively consume power in a variable resistance or other load or operate the linear motor/alternator as a motor to drive the casing in reciprocation to increase the amplitude of reciprocation of the casing and thereby increase the power output of the Stirling engine. In other words, the reciprocation restraint can be a variable power absorber or a variable power transducer. Another type of reciprocation restraint is an apparatus which can vary the mass drivingly linked to the casing. Varying a mass that is mechanically connected to the casing increases or decreases the restraining force applied to the casing by the mass because F=ma.
Because increasing and decreasing the restraint to control the reciprocation amplitude of the casing decreases or increases the power output of the Stirling engine, the load, which is being driven by the Stirling engine and to which its output power is coupled, can be driven by the casing, by the piston or by both. The designer has the options to position a load, such as an alternator, inside the casing and connected to the piston or outside the casing and driven by the casing or have a load in both locations. If the load is only outside the casing and driven by the casing, the piston can be fixed to or itself have a sizeable mass and operate as an inertia reactor. Optionally, the designer may also want to include a spring 68 between the casing 36 and the reference ground 38.
An example of a useful application of the self controlling load concept is a solar powered domestic water system illustrated in
In that manner, the invention generally provides the capability of an engine operating characteristic for any load for which the drive force, which is required to drive the load, increases as the load approaches closer to a selected operating limit and for which load it is desirable to decrease the drive power applied to that load as the load approaches and reaches that operating limit. In other words, this concept is useful when it is desirable to have the power output from the engine be an increasing function of the difference between the operating limit and the current load.
Referring to
Although the casing drive power should be dominant (that is, greater than the sum of any other displacer drive power applied to the displacer more than 50%) for many applications that can benefit from the advantage of controlling power output by controlling the casing amplitude of reciprocation, the casing drive power should be a considerably higher proportion of the total displacer drive power. It is preferred that the displacer drive power from casing control be at least 70% of the total casing drive. An example would be a displacer drive that has a 50 watt drive power from the casing and 2 watts drive power from the pressure differential drive. Most preferred for some applications is that the proportion of drive power from the casing be greater than 99% so that the displacer drive power from the differential pressure variations is an inconsequential or trivial proportion of the total displacer drive power.
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.
This application claims the benefit of U.S. Provisional Application No. 61/586,280 filed 13 Jan. 2012.
| Number | Date | Country | |
|---|---|---|---|
| 61586280 | Jan 2012 | US |
