ADIABATIC HEAT PUMP WITH ADAPTIVE CONTROL

Description

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of U.S. application Ser. No. 18/790,856, entitled Adiabatic Heat Pump, and filed Jul. 31, 2024, the disclosure of which is hereby incorporated in its entirety by reference thereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure pertains to a heat engine and a method for operating the same; and, more particularly, to a heat transfer apparatus appointed to transfer heat from a heat source to a heat sink using a working fluid, the apparatus comprising two separate fluidic loops for the respective circulation principally of vapor and liquid portions of the fluid.

2. Description of the Prior Art

Closed cycle, vapor compression heat pumps are widely used in the Heating, Ventilation, and Air Conditioning (HVAC) industry. These conventional systems are based on the circulation of a refrigerant (or working fluid) in a single closed loop formed through an electrically-driven compressor and two heat exchangers, which are usually termed a condenser and an evaporator, and other ancillary components. Heat pumps may be operated in single mode either to cool or heat the interior of a building structure, but more commonly, can be reversed to cool the interior in summer and heat it in winter. Systems based on similar principles are also used in other heat transfer applications.

These systems rely on the intrinsic thermodynamic properties of the working fluid, particularly on the fluid's ability to absorb and reject a large amount of heat energy quickly, in accordance with its critical behavior as a function of the temperature and pressure to which it is subjected. In most systems, the absorption and rejection of heat is heightened by using a working fluid that is repetitively transformed between the liquid and vapor states as it traverses the heat pump loop. Each transformation is accompanied by the absorption or release of a relatively large amount of heat energy, termed the latent heat of evaporation. This is accomplished by configuring and operating the system so that the requisite temperature and pressure are found at crucial locations in the flow path.

A typical prior art, vapor compression heat pump system is shown generally at 2 in FIG. 1. The system comprises a compressor 4, first heat exchanger 14 and second heat exchanger 16, and a metering valve 6. Tubing connects these components to form a single loop through which a working fluid circulates.

The term “heat exchanger” is used herein in its conventional sense to denote generally a structure by which heat energy is transferred from one fluid to another fluid, with the two fluids being kept separate. Each of the fluids can be either a liquid, a gas, or a combination thereof. Ordinarily, a heat exchanger has a conduit that is part of a closed loop through which the first fluid flows. The exchanger may further comprise one or more heat transfer structural elements bonded to the conduit and disposed so that they increase the area in contact with the second fluid. The heat transfer structural elements are thus in thermal communication with the first fluid so that heat can be transferred while keeping the two fluids separate, i.e., without any fluidic communication between them. Numerous forms of heat exchangers are known in the art.

Conventional heat pump systems use a variety of known types of heat exchangers for the condenser and evaporator. For illustrative purposes, the heat exchangers 14, 16 in FIG. 1 are of the tube and fin type. Each comprises a central metal tube 18 and a plurality of thin metal fins 19, each having an aperture through which the central tube 18 passes. Each fin 19 is soldered, welded, or otherwise affixed to central tube 18 to provide mechanical integrity and thermal contact. The fins 19 are thus exposed to an ambient atmosphere in which the respective heat exchanger is situated and function as heat transfer thermal elements, as discussed above. The working fluid (either vapor or liquid) passes through the central tube so that thermal energy is exchanged through the fins between the working fluid and the ambient atmosphere, which functions as the second fluid. The direction of heat flow is determined by the relative temperature of the two fluids. The material, number, and configuration of fins 19 may be selected in accordance with the fluids used and operational requirements. Most commonly, a large number of fins are constructed of thin sheets of metal with good thermal conductivity to provide a large surface area from which heat is transferred. The rate of heat transfer may be enhanced further by using a fan to increase the airflow across the fins. Alternatively, central tube 18 might have a meander pattern that passes multiple times through apertures in fins 19. The heat exchanger might also have a plurality of flow tubes, each being connected to an inlet manifold and an outlet manifold, through which the working fluid is respectively received and discharged.

For cooling the interior air of a building, first heat exchanger 14 is situated outside the building and functions as the condenser, while second heat exchanger 16 is situated inside and functions as the evaporator. Refrigerant enters compressor 8 at inlet 7 and is 100% vapor at low temperature and pressure. The compressor compresses the refrigerant, increasing both its temperature and its pressure. Refrigerant flows from outlet 8 to the first heat exchanger 14, where it condenses, liberating heat that is rejected to the outside air. The refrigerant continues to metering valve 6, which controls the flow so that refrigerant enters into the second heat exchanger 16 at low temperature and pressure. Cooling of the building is effected by transferring interior heat through second heat exchanger 16 into the refrigerant, causing it to evaporate or boil, reaching saturation. The single loop is completed as refrigerant vapor exits the second heat exchanger 16 and returns to inlet 7 of compressor 8.

Heat pump system 2 can be converted to heat a building interior by suitable valving that reverses the refrigerant flow and the functioning of the heat exchangers so that first heat exchanger 14 becomes the evaporator and second heat exchanger 16 becomes the condenser. In both configurations, heat transfer is enhanced and speeded by the evaporation and condensation of the refrigerant, which respectively releases and absorbs a relatively large amount of heat energy as the latent heat of vaporization.

As noted above, conventional heat pump systems further rely on vaporizing their working fluid during the operating cycle. At a given ambient pressure, any chemical substance feasibly used as a working fluid reversibly transforms between liquid and vapor (or gas) at a temperature defined as the phase boundary temperature or boiling point. The transition is accompanied by the release or rejection of heat energy on boiling or the absorption of heat on condensation back to the liquid, the amount per unit mass being termed the specific latent heat of vaporization. The boiling point varies with the ambient pressure, with the locus of points on a phase diagram showing the variation of this temperature with pressure being termed the phase boundary curve. Liquid at a temperature below the phase boundary temperature for a given pressure is said to be subcooled. At the boundary temperature, the liquid is said to be saturated since there is an equilibrium coexistence of vapor and liquid in a closed container.

Heat pump system 2 is designed to approximate the thermodynamic Carnot refrigeration cycle as closely as possible to obtain the highest practically attainable efficiency. The Carnot refrigeration cycle is commonly referred to as a “reversed heat engine” and employs the same underlying principle of operation as the theoretical Carnot cycle. In simplest terms, the theoretical Carnot cycle involves a heat engine that transfers heat between a hot heat reservoir at temperature TH and a cold reservoir at temperature Tc using a working fluid. In a heat pump system configured for cooling, the reservoirs are the atmospheres inside and outside the building. In the first step of the Carnot cycle, the fluid is isolated from the cold reservoir but in thermal contact with the hot reservoir so that heat is transferred from the hot reservoir to the fluid. The fluid isothermally expands, drops in pressure, and does mechanical work on its surroundings. The second step is adiabatic, with the fluid being isolated from both reservoirs so it does not absorb or reject any heat. It cools and expands without change of entropy, delivering further work to the surroundings. Third, the fluid is placed in contact with the cold reservoir and compressed. Energy as heat and entropy is rejected isothermally into the cold reservoir. Finally, the fluid is again thermally isolated from the hot and cold reservoirs while it continues to be compressed adiabatically and without a change in entropy. The Carnot cycle is a theoretical construct in which: no energy is lost to friction or other mechanical dissipation, the thermodynamic processes in all four stages are assumed reversible, and the respective temperature differences between (a) the hot reservoir and the hot fluid and (b) the cold reservoir and the cold fluid are assumed infinitesimal. It is understood that the Carnot cycle represents an upper bound on the efficiency of any classical process in accordance with the Second Law of Thermodynamics.

Heat pump systems can be used for other heating or cooling purposes, e.g., in residences and commercial buildings and in industrial processes. For example, hot water could be provided by using a fluid-to-water heat exchanger as the condenser so that incoming water from the mains is heated to provide hot water for typical domestic use in bathing, washing, etc.

In recent years, numerous researchers have endeavored to improve vapor compression heat pump systems, to increase their energy efficiency, and to streamline their manufacture. Despite attempts to modify conventional heat pumps to allow them to operate efficiently over a wider range of outside ambient temperatures, both equipment structure and operational characteristics, and the thermodynamic properties of possible working fluids remain limiting factors.

Additional challenges have come from environmental requirements. A number of refrigerants that have attractive properties for the heat pump itself have been discovered to be environmentally detrimental in that they are believed to cause ozone depletion and affect global warming. Both national and international regulations have thus been imposed that limit the use of certain refrigerants or ban them entirely. Substitutes have been developed, but in some instances, existing HVAC equipment does not perform as efficiently or reliably with them. In others, existing equipment is incapable of running with the new refrigerants. Some of the proposed substitutes are also undesirable because of toxicity or flammability.

Accordingly, there remains a significant need for systems that exhibit improved operational efficiency and can accommodate different and approved refrigerants that are more environmentally friendly.

SUMMARY OF THE INVENTION

One aspect of the present disclosure provides a heat transfer apparatus, comprising:

• • a) a compressor-expander having a cylindrical bore and a piston movable within the bore along a central axis thereof, the piston hermetically separating the bore into first and second volume portions that are fluidically connected through an intervening check valve that is configured to permit the transfer of a working fluid from the first volume portion to the second volume portion; and wherein the cylindrical bore has: a liquid compressor inlet and a liquid compressor outlet situated respectively to receive working fluid into, and discharge working fluid out of, the first volume portion; and a vapor compressor outlet situated to discharge working fluid from the second volume portion;
  • b) a prime mover capable of being connected to an external power source and configured to drive the piston in reciprocating motion along the central axis;
  • c) a reservoir having a vapor return inlet, a liquid return inlet, and a liquid reservoir outlet that is in fluidic communication with the liquid compressor inlet through an intervening supply valve;
  • d) a vapor return providing fluidic communication between the vapor compressor outlet and the vapor return inlet controllable by an intervening regulating valve; and
  • e) a liquid return providing fluidic communication between the compressor-expander and the reservoir controllable by an intervening return valve.

Another aspect provides a method for transferring heat energy from a heat source to a heat sink, comprising: providing a reservoir containing subcooled working fluid, a compressor-expander having a total volume variably apportioned between first and second volume portions, a vapor return, and a liquid return, wherein the first volume portion is fluidically connected to the reservoir through the vapor return and the second volume portion is fluidically connected to the reservoir through the liquid return, and carrying out repetitively a cycle comprising the steps of:

• • a) transferring a preselected amount of the subcooled working fluid from the reservoir to the first volume portion;
  • b) expanding the first volume portion to reduce adiabatically the pressure therein, whereby the working fluid is cooled and at least a portion thereof is converted to vapor;
  • c) contracting the second volume portion to increase adiabatically the pressure therein above a target pressure setpoint, whereby any working fluid therein is heated, and thereafter releasing the pressurized fluid to pass through the vapor return to the reservoir, whereby heat is rejected from the working fluid through the vapor return to the heat sink and pressure in the second volume portion is reduced below the target pressure setpoint;
  • d) thereafter, expanding the second volume portion and contracting the first volume portion;
  • e) equalizing the pressures of the working fluid between the first and second volume portions by fluidically connecting the first and second volume portions so that the temperature of working fluid remaining in the second volume portion is increased; and
  • f) thereafter, emptying the first volume portion by isobarically transferring the working fluid remaining therein through the liquid return to the reservoir, while removing ambient heat into the working fluid in the liquid return.

A further aspect of the method of the present disclosure provides for adaptive control of the amount of working fluid introduced into the compressor-expander during the course of the apparatus's operation. The control urges the temperature of the working fluid at a preselected point within the fluidic path to a target value. The amount of fluid may be adjusted dynamically in proportion to the deviation of the measured temperature from a control temperature setpoint. The adaptive control may improve the stability of the system and permit it to operate more efficiently.

In certain embodiments, the present heat transfer apparatus and method of operation afford significant advantages of performance over conventional vapor compression heat pump systems, both in configurations appointed solely for heating or cooling and in those capable of being reversed to provide both modes of operation. In suitable configurations, the systems may save electrical power required for operation while providing increased efficiency ratings as denoted by industry-recognized metrics. With the capability for adaptive operation, they are capable of minimizing temperature lift to further improve efficiency. Systems may also be designed to operate over a wider ambient temperature range compared to existing systems, which are often limited for heating in cold winter climates. They may be configured with a wide range of working fluids chosen for reasons of environmental effects, toxicity, and flammability.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood, and further aspects and advantages will become apparent when reference is had to the following detailed description of certain preferred embodiments of the invention and the accompanying drawings, wherein like reference numerals denote similar elements throughout the several views and in which:

FIG. 1 depicts schematically a prior art vapor compression heat pump system;

FIG. 2 depicts schematically and in cross-sectional view an implementation of a heat transfer apparatus of the present disclosure;

FIGS. 3A-3F depict schematically various stages in an operating cycle of an implementation of the present heat transfer apparatus system, specifically showing the coordination of piston movement and valve sequencing through the stages of the operating cycle;

FIG. 4 provides a timeline schematically depicting the sequencing of valves in an embodiment of the disclosed apparatus during a cycle of the compressor-expander;

FIG. 5A depicts schematically and in cross-sectional view a reversing valve used with the present heat transfer apparatus, with the internal passages of the valve in one configuration;

FIG. 5B depicts schematically the same reversing valve as shown in FIG. 5A, but with its internal passages set to direct fluid flow in an opposite direction;

FIG. 6 depicts schematically another implementation of the present heat transfer apparatus, with the capability of being reversed to operate alternatively in both heating and cooling modes; and

FIGS. 7A-7C depict an algorithm for adaptively controlling the heat transfer system of the present disclosure.

DETAILED DESCRIPTION

Various aspects of the present disclosure relate to the construction and use of a heat transfer apparatus capable of transferring heat energy from a heat source to a heat sink.

Heat Transfer Apparatus

An implementation of the heat transfer apparatus of the present disclosure is depicted generally at 10 in FIG. 2. It comprises vapor expansion heat pump 12, vapor heat exchanger 14, and liquid heat exchanger 16, each being denoted generally by dashed rectangles. The heat exchangers 14, 16 and heat pump 12 are in fluidic communication through conduits such as tubes of metal or other suitable material.

The apparatus employs a working fluid that is compressed, expanded, and circulated through the conduits, as further described below. Preferably, the working fluid has thermodynamic characteristics such that it can reversibly undergo transformation from liquid to vapor at temperatures and pressures that are compatible with desirable operating characteristics of the present system. Some suitable fluid substances, including many single-phase substances, undergo a true thermodynamic phase change at specified temperatures and pressures, with an associated and relatively large latent heat of vaporization. However, other azeotropic and zeotropic agents that do not have uniquely defined phase transition conditions are also usable in suitably configured systems in accordance with the present disclosure. Such materials also gain and release comparably large amounts of heat associated with evaporation and condensation, but over a small range of temperatures and pressures rather than at specific, well-defined combinations of pressure and temperature. They also add additional design flexibility, permitting the present apparatus to be employed over a wider range of ambient temperature conditions.

Heat pump 12 comprises compressor-expander 20 and reservoir 40. In the implementation depicted, compressor-expander 20 has an internal cavity that includes a generally cylindrical bore. Piston 26 is adapted to be moved within the bore of that cavity in a cyclic, reciprocating motion upward and downward along the central bore axis. The drive mechanism for piston 26 includes a prime mover. In an embodiment, the prime mover is a rotating electric machine 27, which drives a rotary crank 31 and a connecting rod 33 to convert rotary motion to reciprocating linear motion. The crank 31 is attached to a rotating shaft of the machine and thus driven directly, as depicted in FIG. 2; alternatively, crank 31 can be driven indirectly through a belt, chain, or gear mechanism. The wall of compressor-expander 20 includes an aperture with a suitable hermetic rotary seal through which the rotating shaft passes. Connecting rod 33 articulates at one end with crank 31 and at the other either directly with piston 26 or, as shown in FIG. 2, with an actuating shaft 35 attached to the piston, possibly as a unitary structure. Rotation of crank 31, as shown by the dashed arrow, thus causes actuating shaft 35 to drive piston 26 in a linear, reciprocating motion along the cylinder axis. Alternatively, crank 26 and connecting rod 33 might be situated outside the compressor expander, with actuating shaft 35 slidably passing through an aperture in the top wall of the compressor expander with a suitable linear shaft seal. In another alternative, the entire drive system, including machine 27, could be housed within the compressor-expander 20 to eliminate the need for any moving shaft to penetrate.

The electric machine is capable of being energized by an external power source 37, such as the electrical grid or a battery (not shown). The machine configuration and design preferably render it capable of operating either as a motor or as a generator, i.e., a machine capable of either: (a) converting electrical energy from source 37 to mechanical work to rotate the crank 31 or (b) converting mechanical energy to electrical energy returnable to source 37 if the piston is driven by action of the working fluid. For example, a permanent magnet-type DC motor is one such device and source 37 might be energized from the electrical grid, as well as including a battery or other electrical storage device to store energy returned during part of the operation when the machine is converting the mechanical power back to electrical. In some implementations, a flywheel (not shown) is associated with the rotating crank to reduce vibration, better stabilize the rotating motion and the instantaneous rotational speed during each cycle, and mechanically store energy during portions of each cycle. Preferably, the flywheel is directly attached to a shaft of the rotating crank or is otherwise formed integrally therewith. For the sake of clarity and consistency, the terms “cycle” and “piston cycle,” as used herein with respect to the reciprocating motion of piston 26 refer to the action of a single up-and-down motion of the piston and are marked as the beginning at the down most extremum of the piston's motion (conventionally termed “bottom dead center,” or “BDC”), continuing through the half-way point with the piston at its top extremum (“top dead center,” or “TDC”), and ending at the next BDC.

The total volume of the cylindrical bore, apart from piston 26 and any enclosed drive system components, is apportioned between the first volume portion 22 below piston 26 and the second volume portion 24 above. The volume of the cylindrical bore is thus exchanged between portions 22 and 24 as piston 26 moves up and down. To minimize or preferably eliminate any incidental or undesired flow of working fluid between volume portions 22 and 24, piston 26 is either dimensioned to fit the cylinder bore very closely or suitable seals (not shown) are provided. Preferably, the cylinder and piston 26 are configured so that the volume of first volume portion 22 is reduced as much as possible when piston 26 is at BDC, to minimize retention of the working fluid. In various embodiments, the opposing bottom surfaces of piston 26 and the cylinder may be either planar or have mating domed concave and convex shapes, respectively. A check valve 28 (V3) permits the controllable transfer of a vapor portion of the working fluid from the first volume portion 22 to the second volume portion 24. Depending on how the prime mover and piston drive are arranged, the minimum volume of portion 24 (reached when piston 26 is at its topmost extremum) will ordinarily be larger than the minimum in portion 22. Compressor-expander 20 has a liquid compressor inlet 30 and a liquid compressor outlet 32 situated respectively to receive working fluid into and discharge working fluid out of first volume portion 22. Compressor-expander 20 further comprises a vapor compressor outlet 34 situated to discharge working fluid out of second volume portion 24.

Reservoir 40 comprises a volume configured to receive working fluid discharged from heat exchangers 14, 16. Reservoir 40 further comprises a vapor return inlet 44, a liquid return inlet 42, and a liquid reservoir outlet 48. As depicted, inlets 42 and 44 are fluidically connected external to the reservoir, joining to form a single reservoir inlet 46 into the volume of the reservoir. Alternatively, inlets 42 and 44 might be situated so working fluid from each can enter the volume directly, thus eliminating the need for reservoir inlet 46. Reservoir 40 is sized to contain an adequate amount of working fluid throughout the operation of the apparatus, including during each portion of each operating cycle.

Although not required, the volume of reservoir 40 is typically much larger than the volume of compressor-expander 20, even including the volume of the heat exchangers and associated connecting piping. In some embodiments, the volume ratio is at least 2:1, 5:1, 10:1, 20:1, 30:1, or 50:1. The Choice of a large volume so that the reservoir contains much more fluid than is circulated through the two loops during each cycle minimizes thermal changes within the reservoir as the fluid is returned at the end of each cycle. A large ratio is also believed to minimize the number of cycles required after start-up to reach a steady state.

Compressor-expander 20 and reservoir 40 are fluidically connected such that two loops are provided for separately circulating working fluid in two streams. More specifically, a vapor loop is completed by a vapor return that provides fluidic communication between vapor compressor outlet 34 and vapor return inlet 44, with flow controllable by regulating valve 54 (V4). In some embodiments, the vapor return further comprises a vapor heat exchanger 14 through which working fluid flows. In FIG. 2, vapor compressor outlet 34 is connected to the inlet of vapor heat exchanger 14, and vapor return inlet 44 is connected to the outlet of vapor heat exchanger 14 through regulating valve 54. Some implementations include a vapor loop bypass 21 with a bypass shutoff valve 23. As depicted, valve 23 is shut off; opening it permits fluid to be shunted directly from vapor compressor outlet 34 to vapor return inlet 44 when there is minimal or no need for heat to be extracted. A liquid loop is completed by a liquid return that provides fluidic communication between liquid compressor outlet 32 and liquid return inlet 42. In the implementation of FIG. 2, the liquid return comprises liquid heat exchanger 16, whose inlet and outlet are respectively connected to liquid compressor outlet 32 and liquid return inlet 42. Control of fluid flow in the liquid return is provided by return valve 52. FIG. 2 shows return valve 52 situated at liquid compressor outlet 32, but it might be placed elsewhere in the return, either ahead of liquid heat exchanger 16 or on the reservoir side. When the heat transfer system configuration and operating parameters are optimally set, the fluid exiting the two return loops and entering the reservoir at the respective liquid and vapor return inlets is predominantly liquid, but it will be understood that there may be some residual vapor in each stream. A skilled artisan will further recognize that the use of the terms “liquid” and “vapor” in denominating the two loops and various inlets and outlets of components of the present system, such as compressor-expander 20 and reservoir 40, is not intended to mean that exclusively liquid or vapor passes therethrough, but rather to more clearly and descriptively delineate the two loops that carry different streams of the working fluid flow. In the implementation of FIG. 2, a portion of the liquid and vapor loops is shared as a fluidic connection from liquid reservoir outlet 48 to compressor inlet 30.

In the practice of the present invention, any suitable alternative may be used for each of the heat exchangers. The particular type, configuration, and sizing for each may be selected depending on the operational requirements. In the implementation depicted in FIG. 2, the heat exchangers 14, 16 are of the tube and fin type, as described above. This type is conveniently used when heat energy is to be exchanged with the ambient atmosphere. Some such implementations include fans (not shown) for both heat exchangers; these are disposed to blow the ambient atmosphere across the fins to increase the transfer of thermal energy. A tube-in-tube heat exchanger may be used if a liquid, such as water, is to be heated or cooled.

The heat transfer apparatus of the present invention operates by the circulation of working fluid through two loops. The fluid flows are controlled by an appropriate valving arrangement. In the implementation of FIG. 2, the flows are controlled by check valve 28 between first and second volume portions 22, 24 of compressor-expander 20; liquid inlet valve 50 (V1) between liquid reservoir outlet 48 and liquid compressor inlet 30; liquid outlet return valve 52 (V2) between liquid compressor outlet 32 and liquid heat exchanger 16; and vapor inlet regulating valve 54 between vapor heat exchanger 14 and vapor return inlet 44. In FIG. 2, all of these valves are shown in the closed position, but during operation, they are opened and closed in a suitable sequence based on the reciprocating motion of piston 26.

Furthermore, in the configuration of FIG. 2, the various valves are shown as being disposed in specific positions relative to other system components. For example, check valve 28 is shown as being integral with piston 26, inlet valve 50 and return valve 52 are located near the inlet 30 and outlet 32 of the first volume portion 22 of compressor-expander 20, and regulating valve 54 is at inlet 44 of reservoir 40. A skilled person will recognize that each of these valves governs the transfer of working fluid from one part of system 10 to another so that the same functions can be accomplished with one or more of the valves disposed in alternative positions along the relevant fluidic path. For example, check valve 28 could alternatively be disposed in a separate fluidic connection, such as a bypass tube, that permits working fluid to flow from first volume portion 22 to second volume portion 24 when required. Similarly, return valve 52 could be located downstream of heat exchanger 16. However, the locations shown in FIG. 2 are preferred. Other forms of construction might also be possible, such as embodiments in which check valve 28 is unitarily incorporated in piston 26. Valves 50, 52, and 54 might also include check valve protection to ensure that only the requisite unidirectional working fluid flow is possible. Any type of valve having suitable functionality may be used in the present heat transfer apparatus.

In some implementations of the present apparatus, any one or more of valves 23, 28, 50, 52, and 54 may be actuated electrically. Without limitation, solenoid valves driven electromagnetically or by a motor (e.g., a stepper motor) may be used to minimize the response time for opening and closing after an actuation signal is given. Regulating valve 54 is optionally of a type that permits its opening pressure to be adjustable either mechanically or electrically.

All of the foregoing valves are preferably operated by a suitably connected electronic control device (ECD). Devices having the requisite functionality to be able to coordinate the sequencing of the valves with the motion of piston 26 are known in the art by a number of names, including an electrical or electronic controller, a microprocessor, an embedded microcontroller, a programmable controller, a general-purpose computer, or a system combining a plurality of such devices.

The various units or components described herein, such as the electronic control device, the prime mover, the various sensors, and some or all of the valves, may be coupled to one another via a wireless transmission and may consequently comprise transceiver circuitry and one or more antennas. Additionally or alternatively, the units described herein may be coupled to one another via a wired or optical link for transmitting digital information and may consequently comprise interface circuitry (such as a Universal Serial Bus (USB) socket). They may also be connected for transmission of analog signals. It should be appreciated that the units described herein may be coupled to one another via any combination of optical, wired, and wireless links.

The various units described herein may comprise any suitable circuitry to cause the performance of the methods described herein and as illustrated in the Figures. The modules may comprise devices commonly termed firmware, including, without limitation: at least one application-specific integrated circuit (ASIC); and/or at least one field programmable gate array (FPGA); and/or single or multi-processor architectures; and/or sequential (Von Neumann)/parallel architectures; and/or at least one programmable logic controllers (PLCs); and/or at least one microprocessor; and/or at least one microcontroller; and/or a central processing unit (CPU); and/or a graphics processing unit (GPU), to perform the methods.

The various units may comprise and/or be in communication with one or more memories, e.g., non-transitory computer-readable storage mediums, that store the data described herein and/or that store software (computer readable instructions) for performing the processes described herein. Without limitation, the storage mediums may store the information permanently or in a form that may be altered by imposing on them suitable electrical, magnetic, or optical impulses.

Electronic control devices of any of the delineated types will typically include an internal system clock from which user-programmable timers may be derived and synchronized. Also, the devices usually include suitable interfaces for receiving or transmitting digital or analog signals usable for processing sensor inputs and controlling components such as the various valves and motors used in the present heat transfer apparatus.

Ordinarily, such a control device is programmed to control one or more of the apparatus's valves based on input from sensors responsive to various conditions, such as ambient temperature and pressure (inside and/or outside) and operating conditions at one or more points within the components of the system. For example, and without limitation, the control device may respond to current and voltage in the prime mover and temperatures and pressures of the working fluid, and temperatures inside and outside the building or other controlled space.

The control device may also rely on information from a sensor that indicates the position of the piston, either directly or indirectly, using electromechanical, electro-optical, magnetic, electromagnetic, or other suitable sensing mechanisms. This sensor could sense the position of the piston directly. Alternatively, and especially in implementations wherein the piston is driven by a crank arrangement, the position can be inferred indirectly, e.g., from the angular position of the crank. In many implementations, the crank 31 is driven directly by rotating machine (motor) 27, so a suitable sensor could be mounted anywhere on the common shaft. For example, the system might employ a sensor adapted to signal each time the crank reaches a definite, preselected angular position within its rotation, such as the point corresponding to piston TDC or BDC. Intermediate positions of the piston could then be inferred, corresponding to a fraction of the total cycle duration, measured relative to successive triggering of the crank position sensor. However, such a system would lack any ability to account for variations in the instantaneous rotational speed within each cycle. Thus, in still another alternative, the position might be ascertained using an angular encoder attached to the crank that is capable of giving a precise angular position of the crank with a signal that may be communicated to the ECD by either digital or analog means.

One preferred type of high-resolution angular encoder divides each shaft revolution into a large, predetermined number of equal increments and outputs a digital pulse as the shaft advances by each increment. For example, an encoder might generate 1000 such pulses per 3600 rotation or one pulse for every 0.36° motion. The encoder may also separately provide a synchronizing pulse at one or more predetermined angular positions, such as a pulse at 0° and/or a pulse at 180°. These pulses may be employed to provide a definite indication to the ECD when piston 26 reaches BDC during each cycle to permit accurate synchronization. Piston position sensors of other types may also be used, provided that they provide requisite information to allow control of the sequencing of the valves in the present apparatus.

Alternatively, in other crank-based implementations, the system might employ a cam arrangement that mechanically actuates one or more of the valves at the requisite point in the reciprocating motion of piston 26.

The sequencing of the valves, in turn, regulates the thermodynamic cycle of the working fluid in each loop. For example, the duration of inlet valve 50 being opened determines how much fluid is introduced into compressor-expander 20 during the piston upstroke. In some preferred embodiments, the sequencing of the valves is done adaptively, meaning that the timing at which one or more of the valves opens and closes may be adjusted by the ECD in response to any of the ambient internal or external conditions, the critical behavior of the particular working fluid used, or a desired mode of operation, such as the rate at which a desired conditioned air temperature is to be obtained after startup or a limitation on the rate of electrical energy consumption is needed.

Heat pump 12 may further comprise sensors for variables such as temperature and pressure that are capable of providing information by which the operation of the heat transfer apparatus may be adaptively controlled. The sensors may be disposed to measure the temperature and pressure of the working fluid at one or more locations within the liquid and/or vapor loops or to measure the temperature of the ambient air either within a building space that is to be heated or cooled or the outside air. Different implementations may include various combinations of such temperature and pressure sensors. The sensors may be of any type, including, without limitation, electromechanical, electro-optical, magnetic, electromagnetic, or other suitable sensing types that provide electrical or optical signals that can be received and processed by the electronic control device. The sensors may be connected directly to the control device, for example, by wire or fiber optic link, or indirectly by wireless transmission, using any suitable analog or digital protocol.

The implementation shown in FIG. 2 includes sensors 92 and 94 for the pressure P₁ and temperature T₁, respectively, near liquid reservoir outlet 48 of reservoir 40; sensors 96 and 97 for the pressure P₂ and temperature T₂, respectively, near outlet port 32 of compressor-expander 20; and sensors 98 and 99 for the pressure P₃ and temperature T₃, respectively, of the liquid working fluid within lower portion 43 of reservoir 40. The present heat transfer apparatus may further comprise temperature sensors (not shown) appointed to measure T₄, the temperature inside the building or other space to be controlled, or T₅, the outside air temperature experienced by heat exchanger 14. The T₄ sensor is often incorporated in a thermostat, capable of sensing the ambient temperature relative to a setpoint and signaling the need for heating or cooling based on the difference.

Heat Transfer Method

In another aspect, the present disclosure provides a method for transferring heat energy from a heat source to a heat sink.

A skilled person will recognize that the operation of a practical heat engine is typically described by the thermodynamic processes that occur repetitively during steady-state operation. That is, immediately after startup, a number of cycles have to be completed before the operation can be described as having reached a “steady state.” At that point, characteristics of the system and its working fluid, such as temperatures and pressures at particular locations in the apparatus, vary only slightly between the values attained at comparable instants within each subsequent cycle. A skilled person will further recognize that in the practice of the present method, within each cycle, one or more of the constituent steps may be carried out during time intervals that overlap or that some of the steps may be carried out simultaneously, depending on the particulars of the equipment being used. For example, in implementations that employ a compressor-expander that includes a reciprocating piston that delimits the first and second volume portions, expanding the volume of either portion is necessarily concomitant with the contraction of the other portion on the opposite side of the piston.

Thermodynamic processes that occur during the operation of a heat engine are often characterized using terms such as “adiabatic,” “isothermal,” “isobaric,” and “reversible.” A skilled artisan will recognize that these terms are idealized in that no practical process ever fully attains the conditions implied by such terms. The artisan will thus understand that processes described herein by these and similar thermodynamic terms will ordinarily deviate slightly but insubstantially from the idealized definition.

A representative implementation of the present method of operation may be accomplished using the heat transfer apparatus illustrated in FIG. 2. The method entails the repetition of an operating cycle that comprises steps described schematically in FIGS. 3A-3F. The method is further elucidated by FIG. 4, which depicts a timeline that shows in relative terms the sequencing of the valves through a complete piston cycle. For clarity of description herein, the cycle is regarded as beginning at BDC, continuing through the half point at TDC, and finishing at the next BDC. For each valve, the portion of the cycle during which that valve is open or closed is denoted by a bar that is open or filled, respectively. The transition points “A” to “F” (identified hereinbelow with reference to FIGS. 3A-3F) are indicated on the time axis of FIG. 4.

During each cycle, piston 26 reciprocates once within the bore of compressor-expander 20. Transition points occur at the extrema of the reciprocating motion of piston 26 and at certain intermediate points; at each transition point, the valves are sequenced, as depicted schematically in FIGS. 3A-3F and 4, and further described below. Each of FIGS. 3A-3F shows the heat pump 12 of FIG. 2 generally at one of the transition points, and more particularly, the schematic location of piston 26 at that particular point and the state of the valves 28, 50, 52, and 54 in the interval within the cycle between that point and the subsequent transition point. (For clarity of illustration, heat exchangers 14, 16, and certain elements of compressor-expander 20 and reservoir 40 and their fluidic connections are omitted in FIGS. 3A-3F.) The cycle depicted by the sequence of FIGS. 3A to 3F is repeated while the apparatus is in operation.

The present method is preferably carried out using an apparatus that includes an electronic control device (ECD), as discussed above. The ECD sequences the valves, ordinarily by sending commands to open and close the various valves at the requisite times. It correlates the timing based on the position of the piston, which is inferred from information from a motor shaft angular encoder. The angular position of the motor shaft is, in turn, tied to the piston cycle through the crank arrangement shown schematically in FIG. 2.

In principle, the sequencing of the valves could be specified either by time points or by the angular position of the crank, both being denominated relative to each piston cycle, e.g., by counting from the point the piston reaches each BDC. The functional equivalence of time and angle presumes that the crank maintains a constant rotational speed, both within a given piston cycle and from cycle to cycle. If the motor has reserve power capability sufficient to overcome variations in loading or a sufficiently large flywheel is included, the shaft rotational speed will remain substantially constant despite variations in loading as the piston moves through the various stages in its cycle. However, in practice, it is found that both the instantaneous and the long-term average speed of the motor may vary somewhat. Consequently, it is preferred that the coordination between valve sequencing as measured by the ECD system clock and the encoder-based angular position be dynamically updated and adjusted so that the amount of fluid transferred does not inadvertently change. Depending on the particular type of valves used, it also may be desirable to account for the amount of time required for each valve to effectively open or close, after a command signal is transmitted.

The operational sequence is thus set forth, beginning at transition point A, seen in FIG. 3A and FIG. 4. Piston 26 is located at the bottom extremum of its reciprocating motion, i.e., BDC, at which the second volume portion 24 is at its maximum and the first volume portion 22 is at its minimum. Reservoir 40 contains subcooled liquid working fluid 43, as indicated by the wavy surface line delimiting working fluid 43 from headspace 41 above. Preferably, headspace 41 is pressurized, e.g., by an inert gas, to overpressure the working fluid and thereby assure a steady flow of working fluid as required during the cycle, to minimize cavitation in the liquid fluid and/or to establish the temperature of the subcooled working fluid in reservoir 40. Valves 28, 52, and 54 are closed, and inlet valve 50 is opened. Thereafter, the prime mover causes piston 26 to begin to move upward, as indicated by the upwardly directed dotted arrow. Subcooled liquid working fluid at high pressure is thus transferred adiabatically from reservoir 40 into the first volume portion 22, whose volume increases commensurately. Vapor in the second volume portion 24 is at high pressure.

Transition point B (FIG. 3B and FIG. 4) occurs when a preselected amount of working fluid has been metered into first volume portion 22. In some implementations, the amount of fluid is adjusted based on the ambient temperatures at the heat exchangers and the desired amount of heating or cooling to be accomplished. To attain good efficiency, a larger amount of fluid is ordinarily required when the system is operating in a cooling mode than a heating mode, but the amount should not exceed half the cylinder volume during steady-state operation. Inlet valve 50 is then closed and piston 26 continues to move upward. The volume in first volume portion 22 increases, reducing the pressure therein. The process is adiabatic, with at least some of the working fluid being vaporized so that both it and any remaining liquid are cooled. The volume of the vaporized working fluid in second volume portion 24 (together with the volume within heat exchanger 14 and associating piping) decreases, thereby increasing its pressure and temperature. The change in internal energy, as this part of the working fluid becomes subcooled, thus stores all of the energy supplied from the prime mover.

At transition point C (FIG. 3C and FIG. 4), the vapor in second volume portion 24 reaches a target pressure. Regulating valve 54 impedes the flow through the vapor loop, allowing pressure to increase so that heat can be released. In some embodiments, regulating valve 54 is of a type that is fully closed until a specific target pressure is reached. In such implementations, regulating valve 54 opens when the pressure in the second portion 24 and the vapor return loop exceeds the pressure in reservoir 40 by the target amount. The opening allows this working fluid to flow through heat exchanger 14 and back into reservoir 40. The target pressure setpoint of regulating valve 54 is preferably selected so that the condensation temperature of the working fluid is above the ambient temperature of the heat exchanger 14 to ensure that heat energy is transferred and the benefit of the latent heat associated with the condensation is obtained. Substantially all the heat thus rejected is transferred by heat exchanger 14 to the atmosphere surrounding it. The configuration of the vapor return loop and the operating conditions of temperatures and pressures are preferably arranged so that substantially all of the initial vapor in portion 24 condenses to liquid that is returned to reservoir 40. Some portion of the working fluid in first volume portion 22 may remain as non-vaporized liquid, as indicated by the wavy surface line. Piston 26 continues its upward movement. Without being bound by any theory, it is believed that having a relatively large total volume in the combination of the top portion of compressor-expander 20, heat exchanger 16, and the associated connecting tubing promotes the efficiency of the present system, by increasing the volume of the fluid in which energy is stored in the fluid during the part of the cycle between transition points B and C.

Transition point D (FIG. 3D and FIG. 4) occurs when piston 26 reaches its top extremum, i.e., TDC, and begins to move downward. Regulating valve 54 is closed. Initially, the energy stored within the fluid of second volume portion 24 is converted to mechanical work that acts to urge the downward motion of piston 26. Ordinarily, that work is sufficient that no energy needs to be supplied from the prime mover for this motion; instead, the prime mover becomes a source generating electrical energy that can be returned to the source and preferably stored in an electrical storage device such as a battery. The amount of converted energy diminishes as this point of the cycle continues. At some point after transition point D, the pressure of the vaporized working fluid in first volume portion 22 exceeds that in second volume portion 24, causing check valve 28 to open and allow flow back into portion 24.

During the portion of the cycle after transition point D, the compression of fluid in first volume portion 22 stores some amount of energy within the fluid (thus heating it) and transfers it back into second volume portion 22 through check valve 28 in preparation for the next cycle.

The downward motion of piston 26 continues to transition point E (FIG. 3E), marked by equalization of pressure between first and second volume portions 22, 24, whereupon check valve 28 closes.

Soon thereafter, transition point F (FIG. 3F and FIG. 4) is reached with at most a slight fraction of first volume portion 22 remaining as vapor. Return valve 52 is opened to permit the remaining working fluid in first volume portion 22 to flow through liquid heat exchanger 16 and back into reservoir 40. The downward motion of piston 26 continues until the piston cycle is complete at BDC, with first volume portion 22 having been emptied of as much of the remaining working fluid as possible. Ambient heat is removed from the surroundings of the system into the working fluid in the liquid return. This extraction is enhanced in embodiments that include a heat exchanger in the liquid return loop. The cycle will then repeat, beginning again at transition point A and continuing indefinitely thereafter.

The present method and apparatus can be practiced with a wide variety of working fluids known in the heating and refrigerating arts. The choice of fluid is made to best satisfy the required performance requirements for a given end-use application and the ambient conditions expected to be encountered in a given locale while being consistent with other safety, regulatory, and marketing requirements.

The apparatus and method described hereinabove with reference to FIGS. 2 and 3A-3F are intended to operate in a single mode only, wherein the atmosphere surrounding heat exchanger 14 is heated and the atmosphere surrounding heat exchanger 16 is cooled. Such a system might be used for heating the interior of any building structure by situating heat exchangers 14, 16 in the interior and exterior of the structure, respectively. A similar system could be used to cool the building interior by disposing the exchangers in opposite positions. The specific operational parameters controlling the pressure levels of the working fluid and the timing of the sequencing of the valves need to be matched to the interior and exterior air temperatures at which the system is expected to operate.

A comparable apparatus can also be used as a heat pump intended for both heating and cooling a building structure, e.g. in different seasons of the year. Implementations thus appointed may use reversing valves, such as the reversing valve depicted generally at 80 in FIGS. 5A and 5B. Valve 80 has two inlet ports, 82 and 84, and two outlet ports, 86 and 88. It includes internal passages 89, 90 that provide fluidic connection of the ports. The passages are situated in an internal structure that can be rotated by mechanical or electrical actuation. FIG. 5A shows valve 80 in one position, such that internal passage 89 connects inlet port 82 to outlet port 86 to allow fluid to flow in the direction indicated by a broken arrow. Similarly internal passage 90 connects inlet port 84 to outlet port 88, with fluid flow again shown. After rotation of the internal structure by 90°, the connections are reversed, with internal passage 89 now connecting inlet port 82 to outlet port 88 and passage 90 now connecting inlet port 84 to outlet port 86, FIG. 4B. Other reversing valve configurations that provide the needed functionality may also be used.

FIG. 5 shows a heat pump configuration that includes two such reversing valves, 80a and 80b. Valve 80a reverses connections to outlet ports 32 and 34 of compressor-expander 20, while valve 80b reverses connections to the inlet ports 42 and 44 of reservoir 40. The heat pump is depicted in FIG. 6 as set for cooling a structure, with vapor heat exchanger 14 outside and liquid heat exchanger 16 inside, and valve 80a in the setting of FIG. 5A and valve 80b in the setting of FIG. 5B. The heat pump configuration can be changed to heating by reversing the setting of valves 80a and 80b, so that the working fluid flows reverse and the heat exchangers reverse function. In some embodiments, the reversing valves 80a and 80b may be manually operated to switch between heating and cooling configurations. Alternatively, a control system may electrically actuate the valves automatically and without operator intervention. For example, the inside and outside temperatures might be compared to determine whether cooling or heating is required, with the reversing valves being set and the system operated accordingly.

Another implementation of the present heat transfer apparatus that is related to that of FIGS. 2 and 6 entails two identical compressor-expander units like unit 20. Their cylinders are coaxially aligned, with their respective pistons linked to a common actuating shaft such that they share a common prime mover and move in a push-pull fashion, i.e., as one piston moves up, the other moves down and vice versa, and the pistons reach their opposite extrema at the same instant.

The compressor-expanders 20 may have separate reservoirs but preferably share a common reservoir 40. The compressor-expanders 20 are both connected to heat exchangers 14, 16 by suitable piping and valving. During each cycle of operation, each heat exchanger receives working fluid flow twice, once from each compressor-expander 20 during the intervals apparent from FIGS. 3A-3F, as described above.

Adaptive Control

In another aspect of the present method, the operation of the heat transfer apparatus is adaptively controlled based on the measurement of the temperature and/or pressure of the working fluid at one or more locations within the fluidic path. That information is used to adjust dynamically certain operational conditions of the apparatus, including, without limitation, the amount of working fluid introduced into compressor-expander 20 during the course of the apparatus's operation. It is believed that the implementation of adaptive control permits the system to come to a steady state operation more rapidly and reliably and with higher ultimate stability and efficiency. In an implementation, the adaptive control is carried out to urge the temperature of the working fluid at a preselected point within the fluidic path, denoted herein as the control temperature, to a target value, denoted as a control temperature setpoint. The adaptive control is ordinarily implemented by adjusting the amount of working fluid introduced each cycle in proportion to the deviation of the measured control temperature from the control temperature setpoint during at least the most recent previous cycle.

In certain implementations, one of the cold side temperature (T₂) or the reservoir temperature (T₁) could be maintained as the control temperature throughout the operation. Alternatively, the system might switch between them based on a suitable criterion. For example, the system might be configured to control preferentially based on T₂ unless T2 differs from a target temperature by more than a preselected tolerance, in which case the system would revert to controlling based on T1 until T₂ was again within tolerance. In another alternative implementation, it might be advantageous to control on T₁ during initial startup and thereafter to switch to controlling on T2 once the system approaches steady state operation close to the final desired temperatures. In other implementations or conditions, either T₁ or T₂ alone might be maintained as the control throughout the operation. Ordinarily, the control is configured with a preselected tolerance range, such that the desired control temperature (e.g., T₁ or T₂) is regarded as being at the setpoint if it is within the tolerance range of that setpoint.

In an implementation, the method is carried out using an apparatus that comprises an electronic control device (as described above), which is configured to receive input from sensors that provide the requisite information by which adaptive control is directed. For example, the sensors might be any of those disposed as depicted in FIG. 2. The ECD algorithmically determines the sequencing of the valves of heat pump 12 so that an appropriate working fluid temperature is maintained as close as possible to a desired setpoint temperature. For illustrative purposes, sensors for these temperatures and pressures are located at particular positions in the system depicted in FIG. 2. A skilled artisan would readily understand that the location of these sensors is indicated only schematically and that comparable sensors could similarly be included in other implementations, including that of FIG. 6. While the sensors are used primarily for adaptive control, they may also be used for other purposes, including, without limitation, diagnostics and identification of certain system failure modes.

An exemplary algorithm for adaptive control of the heat transfer apparatus of FIG. 2 is set forth in the form of flow charts in FIGS. 7A-7C. A skilled artisan will understand these charts as using conventional representations, with rectangular operational blocks indicating program actions to be taken unconditionally and diamond decision blocks as indicating alternative program flows to be taken depending on whether a specified logical statement is true or false. It further will be appreciated that on each of these drawing figures, there are shown program nodes indicated by numerals enclosed in a circle, e.g., to indicate how the program flow proceeds directly from blocks depicted on one sheet to blocks on another sheet, the flow being connected at identically numbered nodes. As depicted, the algorithm presumes the use of the cold side temperature (T₂) as the control temperature, on which adjustment of the amount of working fluid transferred to the compressor-expander 20 is based during operation. However, as discussed above, this algorithm could readily be modified to instead use either the reservoir fluid temperature (T₁) or both these temperatures as the control during the course of operation to improve the speed of convergence, the ultimate operating efficiency, or other desirable criteria. Alternatively, the algorithm could be extended to make it capable of switching between controlling one or the other of these temperatures based on a suitable selection criterion.

The algorithm set forth in FIGS. 7A through 7C comprises three main phases: a startup operation (blocks BLK1-BLK2, a run phase (blocks BLK3-BLK11), and a shutdown operation (block BLK12). The run phase includes two sub-portions. In the first sub-portion, the program flow loops rapidly through blocks BLK3 (depicted in FIG. 7A) to BLK7 (FIG. 7B) while the piston executes a single, full up and down cycle. Key events occurring during each piston cycle are detected during this sub-portion, with appropriate actions being taken in response. After a full piston cycle is completed, the second sub-portion (blocks BLK8 through BLK11, FIG. 7C) then updates system parameters adaptively as needed for the subsequent piston cycle, including updating the appropriate flags, counters, and timing. The program reiterates through the entire run phase until automatic operation or manual intervention to shut down the system occurs, with the BLK12 (FIG. 7C) shutdown then being effected.

The present method is preferably configured to facilitate rapid and stable convergence of the heat pump system by approaching the final control temperature setpoint for cold side temperature T₂ through a plurality of stepwise increments. An incremental control temperature setpoint is associated with each of the stepwise increments. The system is allowed in each increment to converge to the current incremental control temperature setpoint before proceeding to the next increment and its control temperature setpoint. The increments may include just initial and final increments, with corresponding initial and final control temperature setpoints. But preferably, one or more intermediate increments, carried out between the initial and final increments, are also included. Each of the intermediate increments also has an associated control temperature setpoint. The inclusion of multiple increments is especially preferred if the final control temperature setpoint differs significantly from the measured value when the operation begins. For example, the algorithm might operate with incremental steps of 2, 5, or 10° F. or ° C. The same incremental change is preferable, but not necessarily, used for each step change. The program flow shown in FIGS. 7A-7C implements this stepwise technique.

In some implementations of the stepwise approach, the system proceeds immediately to the next incremental temperature control setpoint once a given current target temperature is reached. But preferably, the system is allowed to stabilize at each of the incremental control temperature setpoints before proceeding. The time the system spends during each increment, with the associated incremental control temperature setpoint, is herein termed a stabilizing interval. The duration of each stabilizing interval may be a preselected minimum time or number of piston cycles. Other suitable criteria to assure stable convergence to the desired control setpoint temperature T₂ during operation will also be apparent to a skilled artisan. For example, the system could defer incrementing T₂ for a required minimum interval after the current target T₂ is first attained. The program logic provided by the flow charts of FIGS. 7A-7C, in which a single required minimum stabilizing interval is used throughout, could also be modified to use different stabilizing intervals for the various different control temperature setpoints. For example, the stabilizing interval might be reduced or even eliminated for control temperature setpoints that are close to the final target temperature. It is believed that by allowing the system to reach a steady state condition at each incremental step before moving to a further step, the system is likely to converge more rapidly and stably over a range of possible startup conditions. However, the method disclosed herein does not require the inclusion of any intermediate steps or a stabilizing interval after any given incremental control temperature setpoint is reached.

Further, in the algorithm depicted, the system is configured to operate with the amount of working fluid introduced to compressor-expander 20 being held constant during the initial startup step, with adaptive control of the amount being enabled for the incremental steps thereafter. T₂ will necessarily begin at approximately the initial temperature T₁ of the working fluid in the reservoir and then start to drop, possibly with substantial excursions. It is believed that under some conditions, deferring adaptive control will cause a more rapid approach of T₂ to its ultimate control temperature setpoint, but enabling adaptive control with any control temperature setpoints subsequent to the first will improve stability and/or ultimate operating efficiency of the system. However, adaptive fluid control could be enabled throughout. Typically, the target steps and the stabilizing intervals are specified in the Calibration and Directives parameters.

In further detail, and referring first to FIG. 7A, the heat transfer system is turned on and initialized, as described beginning at Node 4 and proceeding through blocks BLK1 and BLK2. At block BLK1, the ECD obtains certain starting and operating parameters for system operation (Calibration and Directives). This information may be stored in memory within the ECD itself, accessible from a dedicated external memory, or manually input. Alternatively, the parameters might be located in a remote processor and accessible by the ECD through either an internal or external network, including, without limitation, the Internet. These initial operating parameters and targets are preferably based, at least in part, on current temperatures provided by sensors in communication with the ECD. The relevant temperatures may include any one or more of the working fluid reservoir temperature T1, cold side temperature T₂, hot side temperature T₃, room temperature T₄ as provided by a thermostat in the controlled space, and outside temperature T₅ provided by an outdoor sensor (preferably located proximate heat exchanger 14). Based on the T₄ setpoint, the ECD determines whether heating or cooling is required and configures the heat transfer apparatus accordingly.

In an implementation, the Calibration and Directives parameters might be stored in a lookup table that contains entries corresponding to appropriate ranges of the various temperatures that are expected to be encountered, both in the controlled space and outside. Typically, the temperature ranges in the table would encompass and reflect seasonal variations of weather and different building situations, such as differences between ordinary workdays and those after a weekend or holiday shutdown. Ordinarily, the ECD is programmed to identify the most appropriate entry in the lookup table and obtain the corresponding parameters.

While the heat transfer apparatus is operating in its normal run phase, the system may adjust one or more of the operating parameters adaptively so that the control temperature is urged to a desired setpoint and to remain thereat. While a single set of starting parameters, independent of external conditions, might be used throughout the operation, adaptive control of the system typically will cause more rapid convergence to steady-state operation if the starting parameters are tailored to the measured values of one or more of the temperatures, including T₄ and T₅.

The ECD then completes BLK1 by setting all required operational flags; zeroing and then starting a cycle timer and a stabilizing interval timer, which both are keyed to the internal ECD clock; and triggering a short initiation and calibration operation, during which inlet valve 50 and return valve 52 are opened, the opening pressure for regulating valve 54 is established, and motor 27 is energized. The flags include an initial control interval flag, which is used to mark the first operating interval, during which the system proceeds without adaptive adjustment of the opening times for inlet valve 50 and return valve 52. By initially opening both inlet valve 50 and return valve 52, motor 27 is allowed to run without any loading attributable to energy being transferred to or from working fluid through changes in pressure or the vaporization or condensation. This minimizes any changes in shaft rotational speed as loading varies and permits a first approximation the open-cycle shaft speed and period to be determined quickly. These values, in turn, are used for initial estimates for opening and closing the valves during subsequent cycles, which are updated thereafter to account for the actual loading, as set forth below in greater detail.

The initialization continues at block BLK2, wherein the full cycle time (i.e., the time to complete a single rotation of the crank and, thus, the corresponding full up and down motion of the piston) is initially predicted based on measurements taken during the motor startup. Although not required in all implementations, the algorithm described with reference to FIGS. 7A through 7C accommodates potential variability in the motor shaft speed, as manifested either in any variability of the total cycle time between piston cycles or in the instantaneous shaft speed of motor 27, as its loading varies during different portions of each individual piston cycle. As noted hereinbelow, at several points during the program flow, the current rotational speed of the motor at a given instant (and thus a current predicted full cycle time) is determined from the cycle timer, which provides the time elapsed since the last BDC, and the fraction of a full revolution completed by that instant, which is given by number of encoder pulses accumulated divided by the encoder resolution (i.e., the number of encoder pulses for a full 360°). Thus determined, the predicted cycle time, in turn, is used to determine the corresponding angular position of the crank when the valves should be opened in accordance with the desired start times and intervals for the opening of inlet valve 50 and return valve 52 during operation. Actual control of the valves can thus be based on position information provided by the shaft angular encoder. Return valve 52 is closed.

The cycle timer is used to measure the period for each revolution. Based on outputs from the shaft encoder, the proportionate number of encoder pulses corresponding to desired opening intervals for inlet valve 50 and return valve 52 is thereby calculated.

With the system thus initialized, the algorithm continues with the run phase sequence described by blocks BLK3 through BLK11 being carried out for each piston cycle for as long as the system is to be operated. The iteration in these blocks functions to adjust the amount of time for which inlet valve 50 and return valve 52 are opened, so that the measured value of the control temperature converges rapidly to the desired target value.

The first sub-portion of the main loop comprises blocks BLK3 (depicted in FIG. 7A) through BLK7 (FIG. 7B). The program rapidly loops through these steps during each piston cycle, looking for key points therein, including BDC, which marks the beginning and end of each cycle, the halfway point (TDC), and the critical times for valve sequencing, so that actions appropriate to each point can be taken. Then, at the end of each cycle, the operations of the second sub-portion (blocks BLK8 through BLK11 in FIG. 6C) are carried out to prepare for the next piston cycle. Except during initial startup, the desired opening times for inlet valve 50 and return valve 52 are updated as needed to cause the control temperature to converge to the desired steady-state operating point. The requisite counters and flags are reset as needed. Program flow at the end of the loop then returns to Node 3 for the next piston cycle, so that the entire looping through the run phase in blocks BLK3 through BLK11 continues for an indeterminate number of iterations that are carried out repetitively for as long as the heat transfer system is to be operated.

The function of blocks BLK3 through BLK7, as depicted by FIGS. 7A and 7B, is now described in greater detail. Each time block BLK3 is entered at Node 3, the system determines whether the elapsed time in the current piston cycle since the last BDC has reached the point at which inlet valve 50 is to be closed. If so, then the total expected piston cycle time is updated using the cycle timer and the portion of a full cycle indicated by the number of encoder pulses accumulated. The appointed duration for inlet valve 50 to be open is corrected based on the new cycle time, and the program proceeds to block BLK4. If the inlet valve 50 opening interval has not been reached, the updating of cycle time and inlet valve opening interval is omitted.

Then, at block BLK4, the algorithm again confirms whether the inlet valve opening time has been reached. If not, the program flow continues to Node 1, shown in FIG. 7B. If, instead, the time has been reached, corresponding to transition point B in FIG. 3B and FIG. 4, inlet valve 50 is closed, and temperature T₁ is captured. The inlet valve opening interval is again corrected in the same manner, based on the latest encoder pulse count and the cycle timer; the return valve opening interval is set to equal the inlet valve opening interval. The two opening intervals must be mutually coordinated throughout operation to ensure the compressor-expander 20 is not flooded, i.e., that all the fluid metered in during each piston upstroke is removed, either through the hot side loop or through return valve 52 in the cold side loop during the piston down stroke. In practice, the opening times are ordinarily linked throughout the operation. Opening inlet valve 50 and return valve 52 for approximately equal times ensures no flooding. After these operations, program flow proceeds to Node 1.

Referring now to FIG. 7B, the algorithm continues at Node 1. Decision block BLK5 determines whether the current cycle has reached the point for return valve 52 to be opened. If so, the cycle time (and corresponding shaft speed) are corrected as before. The updated cycle time is used to update the desired return valve opening interval as denominated by shaft angle. If not, then program flow proceeds immediately to block BLK6.

At block BLK6, the program reconfirms whether the cycle is at the point for opening return valve 52, corresponding to transition point F in FIG. 3F and FIG. 4. If so, return valve 52 is opened, the cycle time and motor speed are again updated as before, and the angular position for the return valve opening to be complete is updated. If not, the program proceeds directly to block BLK7.

While the program flow is looping through blocks BLK3 to BLK7, the piston will, at some intermediate point, reach TDC and, later in the cycle, BDC. Decision blocks BLK7A and BLK7B test for these piston positions, respectively, and cause commensurate actions to be taken. Decision block BLK7A tests whether the piston is at TDC, as indicated by the encoder pulse count or by other suitable means. If so, the first half of the cycle has been completed, corresponding to transition point D in FIG. 3D and FIG. 4; updated temperatures T₂ and T₃ and pressures P1, P₂, and P₃ are captured for possible use in the next updating of the valve sequencing and program flow proceeds. At decision block BLK7B, it is ascertained whether BDC has been reached, marking the completion of a full piston cycle. If the cycle is not yet complete, the intra-cycle looping through blocks BLK3 to BLK7 continues, with program flow returning to Node 3 (FIG. 7A). If a full cycle has been completed, then the cycle timer is reset and the program flow moves to Node 2 (FIG. 7C) and proceeds through blocks BLK8 through BLK11 for the adaptive recalculation of inlet valve and return valve opening times, as needed to converge the system to steady state and for resetting the system overhead as needed for the next piston cycle.

In particular, at block BLK8, the system first checks whether the initial control interval flag is still set. If so, flow proceeds directly to block BLK9. Otherwise, the cleared flag indicates that adaptive control is enabled, and an updated estimate for the amount of working fluid to be metered in through inlet valve 50 is thus calculated, based on the measured and target values of T_control, i.e., the temperature on which control is currently based, according to the following formulas:

Δ ⁢ T control = Target ⁢ T control - Measured ⁢ T control Updated ⁢ Volume = Current ⁢ Volume × { 1 + gain × Δ ⁢ T control Target ⁢ T control }

in which T_control is T₂ (or alternatively T₁ as discussed above); Measured T_control and Target T_control are the currently measured and setpoint values of T_control, respectively; and gain is a parameter ranging from 0 to 1. The value of gain is preselected to provide as rapid convergence as possible but without causing instability or undue oscillations. Thus, the amount of fluid for subsequent cycles (Updated Volume) is adjusted from its current value (Current Volume) directly with the deviation from the control temperature setpoint. An updated inlet valve opening time is then calculated so that the desired new volume will be transferred based on the most recently updated cycle time. A new return valve time is calculated from the updated inlet valve time. A simple implementation of adaptive control uses the value of the control temperature measured during the most recent piston cycle as Measured T_control in the foregoing calculation. A skilled artisan would recognize that other updating schemes might beneficially be used to calculate a new working fluid volume and corresponding inlet valve and return valve times to improve the stability and/or convergence of the iteration. For example, and without limitation, the updated volume might be a value calculated by using a Measured T_control based on a moving average of measured temperature values taken over several previous cycles instead of just the most recent cycle. Often, such an average is calculated, giving greater weighting to values in the immediately past cycles than those of earlier cycles. Using an average, especially a weighted average, tends to inhibit undesirable instability. In other variants, adaptive control might be enabled during certain intervals (or portions thereof) while the apparatus is operating, but the working fluid volume might not be updated for every piston cycle during a given interval. For example, and without limitation, the volume might be updated every other cycle, after another fixed, predetermined number of cycles, or regularly on some other preselected schedule. A volume change indicated by the operable formula might be limited if it exceeds a specified threshold. All of these variants are understood herein as being forms of adaptive control of the volume of working fluid introduced.

The new valve times are then used to calculate corresponding piston and shaft angular positions for the valve sequencing, taking into account any required corrections for the time it takes for the valves to actually open and close. After these calculations, the program flow proceeds to block BLK9.

The steps needed to prepare for the next piston cycle are accomplished at block BLK9, including updating the full cycle time, resetting all needed program flags and the cycle time counter, and sequencing inlet valve 50 on and return valve 52 off.

Blocks BLK10 and BLK11 establish logic for the continuing operation of the heat transfer apparatus using the aforementioned stepwise approach for attaining a target T₂ value.

As noted above, the initial control interval flag is set during startup at BLK1, signaling that adaptive control of inlet valve 50 and of the working fluid introduced during each cycle is disabled during the initial operation of the system. Block BLK10 tests whether this initial control interval flag is still set. If so, it then tests whether the initial target value of T₂ has been attained. If both conditions are true, then the initial control interval flag is cleared to indicate that adaptive control is to be performed at every iteration thereafter. Otherwise, program flow proceeds directly to block BLK11.

Block BLK11A first tests whether the stabilizing interval timer has reached its required minimum. If it has, and T₂ is at its setpoint, then the ECD acquires the next incremental T₂ control temperature setpoint, if any, from the Controls and Directives list and resets the stabilizing interval timer. If no more incremental T₂ setpoints remain, the current value is retained. Program flow then continues to block BLK11B, wherein the algorithm ascertains whether the system is to be shut off, either because the desired thermostat temperature T₄ has been attained or by either manual or other desired intervention. If the system is to continue, the program flow returns to Node 3 (FIG. 7A) for the next piston cycle.

If shutdown is to occur, the steps of block BLK12 are carried out, including powering down the motor as soon as the current piston cycle is complete with the piston at BDC, opening inlet valve 50 and return valve 52, and resetting all counters and program flags. The ECD may also be configured with an idle mode, such that when the heat transfer system is to be restarted, it directs program flow to resume at Node 4 (FIG. 7A).

Having thus described the invention in rather full detail, it will be understood that such detail need not be strictly adhered to but that further changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the invention as defined by the subjoined claims. Various components are described herein as useful in constructing the present heat transfer apparatus. These are not limiting; it is contemplated that one of ordinary skill in the relevant arts could make minor substitutions of additional components and/or structures and not substantially change the desired properties and functioning of the present heat transfer apparatus and method for the operation thereof to transfer heat energy from a heat source to a heat sink.

In the foregoing description, certain features of structure and process are described using spatially relative or directional terms such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” “up,” “down,” “top,” and “bottom,” and the like. The skilled person will understand these terms as having reference for explanatory purposes to the depiction of relevant items in the drawing figures and their relationship to other elements. Such terms may be intended to encompass different orientations of the apparatus in installation, use, or operation in addition to the orientation(s) depicted in the figures, and that structures may be differently configured in certain embodiments of the disclosure as actually installed and operated.

When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Specific dimensions, specific materials, and/or specific shapes disclosed herein are exemplary in nature and do not limit the scope of the present disclosure. The disclosure herein of particular values and particular ranges of values for given parameters are not exclusive of other values and ranges of values that may be useful in one or more of the examples disclosed herein.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The steps, processes, and operations described herein for the present methods and algorithms are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed and that the algorithms disclosed herein may be modified as needed to function with different implementations of the heat transfer system's components.

Although the descriptors “first” and “second,” etc., are used herein in reference to various elements, components, regions, layers, and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer, or section from another region, layer, or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context.