Patent Application
20130042607
This invention relates generally to free-piston Stirling engines, heat pumps and coolers and more particularly relates to improving the performance of a gamma configured free-piston Stirling machine with opposed power pistons by providing improved control of its output in a manner that can be more precisely adapted to and optimized for the operating conditions encountered by the Stirling machine. In the invention, a displacer has a connecting rod extending past the power pistons to an electromagnetic linear transducer. The linear transducer controls the amplitude and phase of the displacer's reciprocation allowing the linear transducer to control a Stirling cooler/heat pump in a manner that delivers a maximum rate of heat transfer or maximum efficiency over the entire range of operating temperatures and to control a Stirling engine in a manner that matches the power output of the engine to the load power demand while maximizing efficiency and stability over the entire range of operating temperatures and within the limits of the machine.
Fundamental Stirling Principles
As well known in the art, in a Stirling machine a working gas is confined in a working space that includes an expansion space and a compression space. The working gas is alternately expanded and compressed in order to either do mechanical work or to pump heat from the expansion space to the compression space. The working gas is cyclically shuttled between the compression space and the expansion space as a result of the motion of one or more power pistons and, in some machines a displacer. The compression space and the expansion space are connected in fluid communication through a heat accepter, a regenerator and a heat rejecter. The shuttling cyclically changes the relative proportion of working gas in each space. Gas that is in the expansion space, and gas that is flowing into the expansion space through a heat exchanger (the accepter) between the regenerator and the expansion space, accepts heat from surrounding surfaces. Gas that is in the compression space, and gas that is flowing into the compression space through a heat exchanger (the rejecter) between the regenerator and the compression space, rejects heat to surrounding surfaces. The gas pressure is essentially the same in the entire work space at any instant of time because the expansion and compression spaces are interconnected through a path having a relatively low flow resistance. However, the pressure of the working gas in the work space as a whole varies cyclically and periodically. When most of the working gas is in the compression space, heat is rejected from the gas. When most of the working gas is in the expansion space, the gas accepts heat. This is true whether the machine is working as a heat pump or as an engine. The only requirement to differentiate between work produced or heat pumped, is the temperature at which the expansion process is carried out. If this expansion process temperature is higher than the temperature of the compression space, then the machine is inclined to produce work so it can function as an engine and if this expansion process temperature is lower than the compression space temperature, then the machine will pump heat from a cold source to a warm heat sink.
As also well known in the art, there are three principal configurations of Stirling machines. The alpha configuration has at least two pistons in separate cylinders and the expansion space bounded by each piston is connected through a regenerator to a compression space bounded by another piston in another cylinder. These connections are arranged in a series loop connecting the expansion and compression spaces of multiple cylinders. The beta configuration has a single power piston, usually referred to simply as the piston, arranged within the same or a concentric cylinder as a displacer piston, usually referred to a simply a displacer. A gamma Stirling machine also has a displacer and at least one power piston but the piston is mounted in a separate cylinder alongside and sufficiently far from the axis of the displacer cylinder that the displacer and piston will not collide.
Stirling machines can operate in either of two modes to provide either: (1) an engine having its piston or pistons driven by applying an external source of heat energy to the expansion space and transferring heat away from the compression space and therefore capable of being a prime mover for a mechanical load, or (2) a heat pump having the power piston or pistons (and sometimes a displacer) cyclically driven by a prime mover for pumping heat from the expansion space to the compression space and therefore capable of pumping heat energy from a cooler mass to a warmer mass. The heat pump mode permits Stirling machines to be used for cooling an object in thermal connection to its expansion space, including to cryogenic temperatures, or for heating an object, such as a home heating heat exchanger, in thermal connection to its compression space. Therefore, the term Stirling “machine” is used generically to include both Stirling engines and Stirling heat pumps.
A Stirling machine that pumps heat from its expansion space is sometimes referred to as a cooler when its purpose is to cool a mass in thermal connection to its expansion space and sometimes is referred to as a heat pump when it purpose is to heat a mass in thermal connection to its compression space. They are fundamentally the same machine to which different terminology is applied Both “pump” (transfer) heat from an expansion space to a compression space. Working gas expansion in the expansion space absorbs heat from the interior walls surrounding the expansion space of the Stirling machine and working gas compression in the compression space rejects heat into the interior walls of the Stirling machine surrounding the compression space. Consequently, the terms cooler/heat pump, cooler and heat pump can be used equivalently when applied to fundamental machines.
Similarly a Stirling engine and a Stirling cooler/heat pump are basically the same power transducer structures capable of transducing power in either direction between two types of power, mechanical and thermal.
Problem to Which the Invention is Directed
As is well known, free-piston Stirling engines and coolers (FPSE/C) of the beta and gamma configurations employ two major moving parts, viz. the displacer and the piston or pistons as in opposed piston gamma configurations. The internally generated pressure variations of the working gas drives the displacer. This requires that the forces on the displacer be very carefully balanced so as to obtain the proper dynamic operation of the displacer. These forces consist of the spring forces, the inertia force, the pressure drop force and the differential pressure force across the displacer rod. The motion of the displacer directly controls the function of the machine, whether the machine is a cooler/heat pump, in which case the controlled function is the thermal lift, or the machine is an engine (prime mover), in which case the controlled function is the delivered mechanical power. The degree of lift or delivered power is determined by the relative phase angle between the displacer and piston motions and the amplitude of the motions of the displacer.
The essential problems and difficulties with driving the displacer with gas pressures alone are that:
a. In heat pumps, the maximum possible efficiency (or coefficient of performance) is not maintained at all operating conditions. The machine will therefore have increasingly compromised performance depending on how far the operating condition is from the design point.
b. In prime movers or engines, the problem is more severe in that it is often the case that stable operation with a changing load is only possible with an electronic controller between the load and the engine. This electronic controller needs a power capability at least as high as the maximum power delivered and a response time at least greater than the response time of the engine. There is also the problem of extracting the maximum efficiency at different operating conditions as in point (a).
It is therefore an object and feature of the invention to provide full but independent displacer control while minimizing added mass and dead volume in an opposed piston gamma configuration.
A further object of the invention is to provide an improved controllable free-piston Stirling configuration for opposed piston gamma type engines to control the displacer motions in order to change the power curve of the engine so that a variable but stable operating point is always established by assuring that the engine power curve grows with piston amplitude slower than the load curve does.
A further object of the invention is to provide an improved controllable free-piston Stirling configuration for opposed piston gamma type engines and heat pumps whereby the displacer motions are adjusted in order to maximize the efficiency or coefficient of performance depending on whether the device is operating as an engine or a heat pump.
A still further object of the invention is to provide an improved controllable free-piston Stirling configuration for opposed piston gamma type heat pumps in which the displacer phase may be reversed in order to pump heat in either direction through the machine.
The invention is an improvement of an opposed piston gamma type Stirling machine and results in improved operating stability, optimization of efficiency or coefficient of performance and allows a Stirling cooler/heat pump to pump heat in either direction. The improvement is a linear electromagnetic transducer that is drivingly linked to the displacer, located on the opposite side of the power piston's axis of reciprocation from the displacer (preferably in any bounce space) and is controlled by an electronic control. The invention allows independent control of the displacer's amplitude and phase. The location of the linear transducer avoids the need for design compromises and modifications that would negatively affect the efficiency, cost and performance of the Stirling machine. The control of the displacer is independent in the sense that the displacer amplitude and phase can be whatever the designer wants so long as sufficient power is applied by the electromagnetic transducer to the displacer at an appropriate phase that a desired resultant amplitude and resultant phase will result. That is true whether the drive power of the electromagnetic transducer that is drivingly linked to the displacer is the sole source of displacer drive power or the displacer drive power is supplemented by simultaneous application of displacer drive power in the conventional manner. For a Stirling cooler/heat pump, the electronic control can also be capable of driving the displacer at (1) a phase angle that pumps heat in one direction through the machine or (2) at another phase angle that pumps heat in the opposite direction through the machine and also allows selectively switching between the heat pumping directions.
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.
Published U.S. patent application, Pub. No. US 2011/0005220 A1, Ser. No. 12/828,387, published Jan. 13, 2011 and having the identical inventor as the present invention, is hereby incorporated by reference. The present invention may be applied to multiple piston gamma arrangements disclosed in that US Patent application.
Terminology and Definitions
Although the terms used in this description are understood by those skilled in the art, it is desirable that some of them be briefly explained in order to facilitate understanding of the description and the invention.
“Electromagnetic linear transducers”. As known in the art, both an electric motor and an alternator are the same basic device. They are electromagnetic transducers that have a stator, ordinarily having an armature winding, and a rotating or reciprocating member that includes magnets, usually permanent magnets. They convert power in either direction between electrical power and mechanical power. A motor/alternator structure can be mechanically driven by a prime mover to generate electrical power output or a motor/alternator can be driven by a source of alternating electrical power to operate as a motor providing a mechanical output.
Consequently, both a Stirling machine and a motor/alternator structure are energy transducers that can each be operated in either of two modes. They can be drivingly connected together with one operating as the prime mover and the other performing work, either generating electrical power or transferring heat.
“Resonating” means that a spring is linked or connected to a body and the spring and the mass of the body have characteristics that form a resonant system that has a resonant frequency. The spring constant, force constant or torsion coefficient of the spring is related to the total mass of a body so that they have a natural frequency of oscillation, either angular oscillation (for rotationally oscillating body) or linear (reciprocating) oscillation. The resonant frequency of the bodies in the invention is the operating frequency of the Stirling machine. When describing the oscillating motion of one or more bodies in a resonant system, the principal structure, such as the displacer, is sometimes referred to as being resonated. It should be understood, however, that the effective mass of a body in a resonant system includes the mass of all structures that are attached to and move with it. As known in the prior art, a resonant system is commonly used to balance the inertial forces of a displacer and other reciprocating bodies.
“Springs” are used in the present invention to resonate the oscillating and reciprocating masses. The term “spring” includes mechanical springs (such as coil springs, leaf springs, planar springs, spiral or involute springs), gas springs, such as formed by a piston having a face moving in a confined volume, electromagnetic springs and other springs as known in the prior art or a combination selected from them. Gas springs also include the working gas in the work space in a Stirling machine and, in some implementations, can also include the back space because the gas applies a spring force to a moving wall of a confined space as the volume of the space changes. As known to those in the art, generally a spring is a structure or a combination of structures that applies a force to two bodies that is proportional to the displacement of one body with respect to the other. The proportionality constant that relates the spring force to the displacement is referred to as the spring constant, force constant or torsion coefficient.
“Drive rod” and “connecting rod”. A “connecting rod” connects two or more bodies so that they move together as a unit, usually with one body being driven through the connecting rod by another body. A “drive rod” in a Stirling machine is a rod that functions to cause a drive force to be applied to a displacer. Conventionally, a displacer is driven in reciprocation by the varying working gas pressure. A drive rod is connected to extend from the displacer through a mating cylindrical wall into a bounce space, sometime called a back space. The bounce space is a confined space that is not connected in communication with the working space. Consequently, the pressure in the bounce space does not vary as a result of working space pressure variations. The drive rod functions as a piston with the net driving force applied to that piston, and therefore to the displacer, being the result of the differential pressure applied to the cross sectional area of the drive rod in one direction by the gas in the working space and in the opposite direction by the gas in the bounce space. A drive rod can additionally function as a connecting rod as a result of its being connected to another body in addition to its extending through a cylindrical wall with differing pressures at opposite ends of the cylindrical wall. Consequently, the term “rod” can be used to refer to a rod that has only a connecting function or only a driving function or both functions. However, the term “rod” in this context of Stirling machines and the present invention, is not limited to a solid or a cylindrical rod. A connecting rod can be hollow and can have other cross-sectional shapes so long as it is capable of mechanically connecting two bodies. Although a cylindrical cross-sectional shape is by far the most practical for a drive rod, other configurations can be used.
A displacer 18 is mounted in a displacer cylinder 20 for reciprocation along a displacer axis of reciprocation 22 for cyclically varying the proportional distribution of a working gas between the expansion space and the compression space. A pair of power pistons 24 and 26 are mounted within piston cylinders 28 and 30 on opposite sides of the displacer axis of reciprocation 22 for reciprocation along a piston axis of reciprocation 32. Each piston is connected to an electromagnetic transducer that is not associated with the invention. This electromagnetic transducer is of conventional construction having circularly arranged magnets 33 that are fixed to the pistons 24 and 26 and reciprocate with the pistons 24 and 26 within a stator having armature windings 35 that are also arranged in a circular configuration around the magnets 33. The electromagnetic transducers that are connected to the pistons function as a linear motor for driving the Stirling machine and operating it as a cooler/heat pump or function as a linear alternator if the Stirling machine is operated as an engine.
In order to implement the invention, a displacer connecting rod 34 is fixed to and extends from the displacer 18 through the space between the pistons 24 and 26 and beyond the piston axis of reciprocation 32. An electromagnetic linear transducer 36 is drivingly connected to the displacer connecting rod 34 at a position that is on the opposite side of the piston axis of reciprocation 32 from the displacer 18 and outside all space occupied by the pistons during their reciprocation. Preferably, as illustrated, the linear transducer 36 is located in an extended bounce space 38. By locating the linear transducer 36 at the bounce space 38, implementation of the transducer 36 does not affect the work space or require an increase of dead space in the work space. That location also does not require any compromising tradeoffs or modifications of the structures near the regenerator, heat rejecting heat exchanger, heat accepting heat exchanger or the pistons. For simplicity, the linear transducer shown is of the moving magnet type such as illustrated in U.S. Pat. No. 4,602,174. The linear transducer 36 has magnets 40 that are connected to the end of the displacer connecting rod 34 for reciprocating with the connecting rod 34 and the displacer 18. The linear transducer 36 has a stator 42, with coil windings 44, that is attached to the casing 10 so that relative motion between the displacer 18 and the casing 10 will result in the same relative motion between the magnets 40 and the stator 42. The connecting rod 34 is connected to a piston 46 that reciprocates in its mating cylinder for extracting power from the cycle as a result of the differential pressure applied to opposite ends of the piston 46 and delivering that power to the displacer in the manner well known in the art. In this manner, the piston 46 is a relatively short segment of drive rod that functions as a conventional drive rod but only supplements the drive power applied to drive the displacer 18 by the linear transducer 36. A conventional drive rod having the same diameter as the piston 46 along its entire length can be substituted for the connecting rod 34 and the piston 46. However, the illustrated arrangement with the smaller diameter connecting rod 34 is preferred because less space is occupied by the connecting rod between the reciprocating pistons 24 and 26 and therefore less dead space is included in the working space. Reduced dead space results in increased efficiency. In conventional free-piston Stirling machinery, the diameter of a drive rod is sized so that sufficient power is provided to the displacer in order to drive the displacer 18 with the appropriate amplitude and phase relative to the pistons 24 and 26. In the invention, the piston 46 is sized to provide supplemental displacer drive power, the remainder of the necessary power being provided by the electromagnetic linear transducer 36. The linear transducer 36 in the bounce space 38 provides the additional power needed for proper motion and in some cases may subtract power in order to alter the displacer dynamic motion for a particular outcome such as efficiency maximization or response to a load change on the output of an engine.
Planar mechanical springs 48 are utilized to balance the inertial forces of the displacer 18, as in the prior art. Typically, this spring has a spring constant so that the combined mass of the displacer, the rod and any other mass fixed to them is a resonant system at the nominal designed operating frequency of the Stirling machine. The presence of these springs 48 reduces the maximum force that needs to be delivered by the linear transducer 36 for driving the displacer. The practical result of keeping these forces low is that the linear transducer may be made smaller and can be operated with smaller currents for a given voltage.
An electronic control 49 provides power or extracts power as necessary from the linear transducer 36 and controls its motion in response to the demands of one or more outputs from the machine. The control 49 has an output connected to the stator coil 44 of the linear transducer 36 for controlling and adjusting at least one of the frequency, the phase and the amplitude of the displacer 18 as a function of parameters of machine operation that are sensed in real time and input to the control. As known in the art, this control is accomplished by controllably adjusting one or more of the amplitude, phase and frequency of the voltage applied to the stator coil 44 of the linear transducer 36. The sensed parameters used as the input or inputs for embodiments of the invention typically include one or more of several parameters depending upon the purposes of the embodiment. The typical sensed parameters include the amplitude of the pistons and their time of top-dead-center (TDC), displacer amplitude and its time of TDC and/or the temperature of an object, or container for an object, that is being cooled or heated by a Stirling cooler/heat pump. The prior art has many examples of apparatus for sensing in real time the value of these parameters. As known in the electronic control art, a set point input may also be an input to enable control for operating the machine at a set point by means of human control, such as for setting a desired temperature, pressure or voltage, or by means of another control system. The electronic control applies electrical power to the linear transducer for driving the displacer in reciprocation or absorbing electrical power from the transducer for reducing the amplitude of reciprocation of the displacer. Representative examples of electronic controls for embodiments of the invention are discussed in greater detail in a later portion of this description.
As is readily apparent,
For stable operation two things are required, (a) the power generated by the Stirling engine prime mover must match the power absorbed by the load having the linear characteristic and (b) the power absorbed by that load must increase faster with increasing piston amplitude than power generated by the Stirling engine. In
Referring to
Other stable operating points for matching greater or lesser load power demands and Stirling engine outputs are now simply a matter of shifting the power curve 232 up or down the load curve, for example to provide power curves 234 and 236. The power curve 232 is shifted down the load curve, for example to 236, by reducing the displacer amplitude in order to reduce engine power output to a lower steady state power operating point and then controlling the displacer amplitude at the new operating point so that the displacer's amplitude of reciprocation is a decreasing function of the power piston's amplitude of reciprocation. Consequently, the engine power curve is shifted in this manner along a continuum that extends along the compressor power curve.
This method of operating a Stirling cooler/heat pump and reversing the direction of pumping the heat is applicable to other Stirling machines utilizing a displacer. The method comprises driving the power piston in cyclic reciprocation with a prime mover and driving the displacer in cyclic reciprocation with an electromagnetic linear transducer driven at a selected phase angle relative to the phase angle of the power piston. At times the selected phase angle is controlled to be a first phase angle that causes a first space within the working space to operate as an expansion space for cooling an object and the second space to be a compression space for rejecting heat from the Stirling machine. At other times the selected phase is changed to a second phase angle that causes the first space to be a compression space for heating an object and a second space to be an expansion space for accepting heat. The first phase angle should be in the range from substantially 40° to substantially 70° and the second phase angle should be in the range from substantially −110° to substantially −140°. Most preferably, the first phase angle is substantially 60° and the second phase angle is substantially −120°. The linear transducer that drives the displacer is ordinarily driven by an alternating current and the method of controlling it further comprises adjusting the frequency and voltage of the alternating current.
Electronic Controls that can be used with the present invention are illustrated by examples in
In the present invention, the electromagnet linear transducer that is mechanically connected to the displacer will, in most applications, operate at times under some operating conditions as a linear motor that is driven by an alternating power source applied from its control to apply drive power to the displacer and maintain or increase the amplitude of reciprocation of the displacer. The same electromagnetic linear transducer in the embodiment can operate at other times under different operating conditions as a linear alternator to absorb power from the displacer and reduce its amplitude of reciprocation. In some embodiments the electromagnetic linear transducer that is mechanically connected to the displacer can be the sole source of power for driving the displacer in reciprocation and in other embodiments it can be a supplemental source of displacer drive power with the displacer also receiving drive power in the manner that is well known and conventional in the prior art.
As explained previously, compressor loads are linear with respect to piston amplitude while the power produced by the Stirling engine is approximately according to the square of piston amplitude. For simplicity, the head temperature Th is assumed to be held constant by the heat input controller 581. Demand for heating (or cooling) is determined by a thermostat 583. Since there are no linear alternators or motors on the pistons, it is necessary to determine their motions by separate transducers 585 and 587, typically small position sensors. The displacer linear transducer 588, may be used as a position sensor or, alternatively, a separate position sensor 589 may be used. The control logic 590 provides inputs to the displacer controller 591 which, in turn, determines the inputs to the displacer driver/load 592. Once Th is sufficiently warm, the machine is started by the displacer controller 591 which provides a starting AC voltage and initial frequency to the displacer driver/load 592. The piston and displacer motion sensors determine the amplitudes and top-dead-centers (TDCs) of the moving parts. The control logic first tests whether the displacer or pistons have exceeded their maximum amplitudes and if so, signals the displacer controller to reduce the displacer drive voltage. If the amplitudes are within their limits, then the phase between the displacer and the pistons is determined (the pistons always move in phase, i.e., both move outwards or inwards as the case may be). If the phase is greater than the design point, typically around 40°, then the control logic signals the displacer controller to reduce the displacer driver frequency. If the phase is less than the design phase, then the control logic signals the displacer controller to increase the displacer driver frequency. Voltage to the displacer driver is controlled by the demand set by the thermostat 583. It is understood that the various rates required to increase or decrease the driver voltage and frequency are critical to the stability of the system. However, the essential requirement of providing sufficient power input to the compressors at all conditions is established by the displacer controller and control logic.
Advantages of the invention include: (1) improved control of the Stirling machine because of the independent control of the displacer that is made possible with the invention and therefore allows improved stability and efficiency; (2) a reduction in dead volume (dead space) which also improves efficiency; and (3) a mechanical topology or configuration that, because the linear magnetic transducer of the invention is placed on the opposite side of the piston axis of reciprocation from the displacer, allows more freedom to design and construct the transducer based upon its desired characteristics without compromises or constraints dictated by locating the transducer in other locations within the Stirling machine.
The invention is applicable to the gamma configuration of a Stirling machine wherein two or more pistons are arranged at right angles to the displacer motion. In order to minimize dead volume, the displacer drive area is provided on the displacer spring, which is mounted beyond the pistons so that the pistons do not have to accommodate the displacer drive or connecting rod as in conventional beta machines. This arrangement achieves substantial but incomplete balancing. The displacer remains unbalanced but is generally of low mass compared to the overall machine mass of the machine so that the residual motion is actually quite small and in many cases, acceptable.
The current invention provides an electromagnetic linear transducer attached to the aft end of the displacer located within the bounce space. Since this space is free to configure and has no significant effect on the performance of the machine, the linear transducer may be sized according to its own terms of efficiency and required power level while minimizing the moving mass without the compromises that would be needed if the linear transducer were positioned elsewhere in the Stirling machine.
Design compromises that are avoided include:
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.
