Patent Application
20110314805
The present invention relates to heat engines and heat pumps, incorporating aspects of Stirling engines and engines with timed opening of valves for gas exchange, particularly diesel engines. The invention is useful for heat pumping, refrigeration, and also for recycling of steam latent heat in superheated steam drying.
The following Specification will teach a core invention that can be viewed from several perspectives and can be configured in a variety of ways. The invention is a heat engine and a heat pump—two “separate” terms that refer to a device and a related process that can be employed in two directions: as a heat engine to convert a “downhill” hot-to-cool flow of heat into mechanical power; and as a heat pump to convert mechanical power into an “uphill” cool-to-hot or cold-to-warm flow of heat. The new invention will teach a merger of the heat engine and heat pump aspects into a unitary whole with the elimination of several costly and wasteful intermediate energy conversion steps. Important background for the present invention is found in the technology of Stirling engines and Stirling heat pump/refrigeration systems. Yet, most “Stirling” systems are closed thermodynamic cycles, where heat is conducted in and out through the walls of a hermetic containment. In some of the literature, a “Stirling engine” is a closed hermetic system by definition. Thus, parts of the present invention will be described as “Stirling-like” components or subsystems, sharing characteristics in common with Stirling engines but differing in three important respects:
Pertinent to the present invention are examples of so-called Stirling-Diesel hybrids: systems that include valves and gas exchange with the external system or environment, operated in conjunction with reciprocating gas flow through a regenerator. Patton, in U.S. Pat. Nos. 7,219,630, 7,004,115 and earlier patents, teaches a system employing two pistons, one for intake and compression and the other for power delivery and exhaust functions, the first piston being coupled to the second through a regenerator. A parallel is seen where the core components of a Stirling engine are a displacer piston, a regenerator, and a power piston. Unlike external-combustion, hermetic Stirling engines, Patton's system resembles a Diesel engine, in that it breathes air and employs direct fuel injection into a gas that is sufficiently hot to cause immediate combustion without spark ignition. While diesel engines achieve the high air temperatures required for combustion through high compression ratios (typically 18-to-1) and the accompanying adiabatic heating, Patton's system uses low compression while most of the needed air pre-heating is accomplished with a regenerator. Timed internal combustion heat is produced, by fuel injection, in the “right place”—inside the combustion chamber, as opposed to externally—at the “right time”—when the power piston is being driven down, early in a power stroke. Heat that conducts inward through the walls of a conventional Stirling cylinder flows at all times. Idealized diagrams may show the path of incoming heat “blocked” by a moving regenerator or one or two moving pistons, but in such a situation, heat continues to flow into the sides of the regenerator, so there is little effective “timing” of the heat flow. Indeed, there is a tendency for a maximum heat flow rate to occur in a Stirling cylinder at the “wrong time”—when the cylinder temperature is at a minimum and the power piston is performing a compression stroke.
Patton provides an excellent review of the prior art, including many references giving relevant background to his own and the present inventors' teachings. In one such reference, U.S. Pat. No. 5,050,570, Thring teaches an “Open cycle internal combustion Stirling engine” incorporating two pistons with coaxial shafts and sharing a common cylinder, a typical Stirling engine configuration, also including a regenerator, but equipped with timed valves, fuel and spark ignition. In the more recent U.S. Pat. No. 5,499,605, Thring teaches a two-piston Stirling-hybrid configuration that anticipates the more advanced recent work of Patton. As will be seen, however, the present invention offers many useful, energy-saving functions not anticipated in Patton's work or the earlier work of Thring and others. In U.S. Pat. No. 5,632,255, “Regenerated engine with an improved heating stroke,” Ferrenberg teaches the use of a single moving element combining the functions of a regenerator and a displacer piston, henceforth described as a “regenerator piston” in the Specification below. Ferrenberg shows that a power piston and regenerator piston sharing a common cylinder can perform functions requiring two separate cylinders where the regenerator is a fixed component with through-flow of gas driven by a separate displacer piston. While Ferrenberg claims certain performance advantages to this unitary cylinder approach, more recent teachings of Patton (e.g., U.S. Pat. No. 7,219,630) show five valves used in conjunction with separate compressor and power pistons to accomplish a highly efficient hybrid Stirling-Diesel cycle. In U.S. Pat. No. 6,457,309, “Multifuel internal combustion Stirling engine,” Firey teaches pistons with coaxial shafts sharing a common cylinder, which he calls “displacer piston” and “compression piston.” Similar language is echoed in the Specification below, except that in place of “compression piston” the term “power piston” is used generically to describe both pistons that convert shaft power into compression power for refrigeration, and that convert pneumatic power to shaft power in heat engine. operation. Power pistons generally require sliding seals to fulfill pressure-bearing functions, though systems demonstrated, for example, by Global Cooling of Athens, Ohio, achieve power piston functionality with air bearings and no sliding seals. As will be seen in the following Specification, important functions traditionally requiring power pistons can be accomplished by purely pneumatic means, without the intervention of solid power piston components, nor of water pistons and the like (as taught for example in U.S. Pat. No. 4,676,066 by Tailer et. al., and in more recent teachings that reference Tailer.). Unlike a Stirling power piston, a Stirling displacer piston does not require tight sliding seals, but rather only a moderately close fit in the cylinder, since the low flow resistance of a typical Stirling regenerator results in little pressure difference across a displacer piston. Firey teaches a configuration wherein the displacer piston operates between the compression (or “power”) piston and the region where cylinder walls are exposed to hot combustion products. It is highly desirable for a compression or power piston to operate in a relatively cool cylinder, as this minimizes thermal expansion problems, reduces wear and results in a system that tolerates contamination and grit, including from combustion of dirty fuels (including coal in Firey's example.) This advantageous protection of heat-sensitive components by a displacer piston is carried into new and unanticipated contexts in the invention to be described below.
Stirling refrigeration systems are well known in the art, and have found particular application for cryogenic operation. In U.S. Pat. No. 4,996,841, Meijer et. al. teach a “Stirling cycle heat pump for heating and/or cooling systems” wherein a Stirling engine directly powers the rotary shaft that drives a Stirling heat pump. This extreme proximity of the fuel-powered prime mover to a heat pump might be contrasted with the Solar One and Solar Two projects by Stirling Energy Systems in California's Imperial Valley and Mojave Desert. There, solar-powered Stirling-electric generators are projected to produce over 1000 peak megawatts of electricity, whose greatest value to the utility system is to meet correlated extreme air conditioning load demands. While Meijer et. al. reduce the gap between Stirling producer and heat-pumping user from hundreds of kilometers to a fraction of a meter, it will be seen that the present invention brings these functions still closer together, in a manner that is compatible with solar power and that, solar- or fuel-driven, eliminates costly and wasteful intermediate energy conversions.
A rapidly emerging technology for drying of wet solids, ranging from grains to wood chips to sewage sludge, conducts the drying process in superheated steam, whose mass is increased as the drying wet materials give off steam. This continuously-produced steam is collected, compressed, and forced to condense at a temperature above the boiling point associated with the steam pressure inside the drying apparatus. The resulting steam condensation heat is transferred back into the drying apparatus, effectively recycling this heat energy to promote further drying. The Swedish company G.E.A. Barr-Rosin has successfully implemented this technique in multiple industrial applications, typically drying in a sealed apparatus at several atmospheres' pressure, while others have demonstrated similar techniques at lower pressures and associated boiling temperatures for use with heat-sensitive materials. These energy-saving processes are powered by a costly form of energy—electricity. As will be seen, the current invention extends naturally, in its applications, into the area of efficient drying, where superheated steam becomes the working fluid of an open-cycle Stirling-like system that interacts very directly with the drying materials. Recognizing that steam from drying materials will commonly be laden with grit, whether from laundry of sawdust being dried for fuel-wood pellet production, and recognizing that thorough filtration of grit from large volumes of steam entails costs and technical challenges, it will be appreciated that Meijer et. al. teach ways to make a Stirling engine that is tolerant of grit. The connection being suggested here was not recognized by Meijer et. al., however, nor by the growing industries that perform superheated steam drying. Effective energy-saving and cost-saving hybrid technologies of this sort are much needed, with a number of examples being taught below.
The invention to be taught below is best understood from a background of heat engine principles that have been applied, in separate contexts, to Stirling and Diesel engines. These engines are understood in a broad generalized context from the perspective of their associated idealized thermodynamic cycles. Though these cycles are amply described in the literature, the terminology and approaches differ from place to place. To establish background with a consistent approach and vocabulary the idealized Stirling and Diesel cycles will be reviewed briefly here, along with the Carnot cycle, which provides an instructive if impractical example of the best performance that can theoretically be achieved with a heat engine. The Specification will then proceed directly from these known thermal cycles to new cycles and variations, shown first as abstract graphs and then in exemplary practical embodiments of the invention.
The fundamental benchmark for heat engine efficiency was described by Carnot, in the form of equations and graphs relating to the idealized cycle bearing his name. The diagrams of
In diagram 100 of
dS=dQ/T
Here, dQ is an infinitesimal quantity of heat energy “Q” flowing into the object, and “T” is the absolute temperature, described in this Specification in Kelvin units, though Rankine units are also applicable. If an infinitesimal quantity “dQ” of heat flows from a first object at temperature T1 through a thermal flow resistance and into a second object at a lower temperature T2, then the change in entropy of the first object is dS1=−dQ/T1, a decrease since the object loses heat, while the change in entropy of the second object is dS2=dQ/T2, an increase of greater magnitude than “″dS1|” (read: “absolute value of dS1”) since denominator “T2” is smaller than denominator “T1”. Thus, the sum “dS1+dS2” is seen to increase whenever there is a temperature drop due to thermal flow resistance. If two objects are in thermal equilibrium at equal temperatures, then there will be no flow of heat. The observation that heat always flows “downhill” from higher to lower temperature, or does not flow at all for systems at equal temperatures, is re-stated in thermodynamics as a postulate, which along with very little additional information leads to the derivation of the Second Law (of Thermodynamics), namely that the entropy of a closed system always increases or remains the same. In that context,
In 100, a large heat reservoir 102 is at a high temperature, for example 900 Kelvins, as labeled. Heat flows reversibly (i.e. with no temperature differential and no increase in entropy) via 106 into a heat engine 108, represented schematically by a vertically elongated rectangle. The dot patterns in his rectangle and the square blocks representing heat reservoirs indicate temperature by analogy to density of atoms or molecules of an ideal gas at a given constant pressure. Thus, as temperature approaches zero Kelvins, the gas particles come very close together and the dot density approaches black, while at high temperatures the gas density and dot density are low. The temperature-indicating dot density in 108 represents the operating temperature range from zero to the maximum temperature of the system, here 900 Kelvins by way of example. Heat engines require a heat source and a heat sink, and in this case the heat sink is represented by thermal reservoir 118, whose dot density is higher than in 102 to represent a lower temperature, 300 Kelvins in this example. Heat flows into 118 via path 116. The vertical extent of the curly brackets represent temperatures or temperature ranges: here the high temperature of 102 by bracket 104, the lower temperature of 118 by bracket 112, and the temperature difference between 102 and 118 by bracket 110. In heat engine 108, heat flows downhill across the temperature difference 110, which in this example is two-thirds as great at the absolute temperature indicated by bracket 104. In flowing two-thirds of the distance from the maximum system temperature toward absolute zero, two-thirds of the heat energy flowing through 106 is converted to a mechanical energy output 114, while the remaining one-third of the heat energy is not recoverable as mechanical energy and dumps into the heat sink 118 via 116. The generalization is easily seen. The maximum fraction of heat energy that can be converted to mechanical energy or work in a heat engine is the source-to-sink temperature differential (110) divided by the absolute temperature (104) of the source. This is expressed by the famous Carnot equation.
The fact that the process on the left of 100 is reversible implies both that there is no entropy increase and that the process can in fact be run in reverse, as the heat pump represented on the right of 100. Here, mechanical energy flows via 114 into heat engine 128, here operating as a heat pump, which uses its mechanical input energy to draw heat from a reservoir 122 at temperature 124 via path 126 into the engine. In this example, the output temperature is represented by 130, which is identical to 104, while the difference between 130 and 124 is the equivalent temperature difference 120. The quantity of output heat energy from 128 via 132 into thermal reservoir 134, at temperature 130, is seen to be the sum of heat energies entering the heat pump via paths 114 and 126, with the ratio of these two energy flows being represented by the ratio of height 120 to height 124. In the illustrated case, two-thirds of the output heat flow comes from mechanical energy flowing via 114, while the remaining one-third comes from heat energy drawn from 122 via 126. The ratio “two-thirds” is the same for the heat pump as it was as described above for the heat engine in this example, where the temperatures on the two sides match. It is not, however, necessary that the temperatures on the two sides match. Mechanical energy flowing via 114 is a “general purpose” type of energy whose use varies with context, as seen in
In diagram 150 of
As will be taught in the following Specification, it is possible to construct a highly efficient heat-powered heat pump employing a high-temperature heat source such as 102 (of 100) providing a small heat flow 106, so that a large heat flow is drawn across a relatively low temperature differential 158, drawing heat 162 from, and possibly refrigerating or air-conditioning, a heat source 160, while delivering “waste” or “useful” heat 170, and thereby heating a heat sink 172. The temperature 112 of heat sink 118 in 100 is shown as intermediate between the source and sink temperatures of system 150. In a system combining a heat engine and a heat pump to make a heat-powered heat pump, the heat sink 118 for the heat engine 100 becomes heat sink 172 of diagram 150. The high temperature heat sink 134 of diagram 100 becomes the lower temperature heat sink 172 of diagram 150, while the heat engine heat sink 118 of 100 is effectively combined with the heat pump heat sink to become the overall heat sink 172. By analogy, one can think of an electrical transformer with two terminals on the primary or power input side and two additional terminals on the secondary or power output side, a four-wire device. If one primary terminal is interconnected with one secondary terminal to a common ground, then one has a three-terminal device, or similarly, a three-wire autotransformer. The thermal system of diagram 150 abstractly describes a three-terminal thermal “step down autotransformer.” Employing analogous electrical terminology, one has a “high voltage” or high-temperature source for the heat engine component 152, not numbered separately but analogous to source 102 of diagram 100, a secondary “ground potential” or low-temperature source 160, and the “autotransformer” output terminal as heat sink 172, which receives both “waste” heat from system 152 and pumped heat from source 160. Where the intention is heating, whether for drying, space heating, distillation, or similar functions, the “waste” heat is not wasted but is part of the useful system output, combining with the pumped heat to achieve an effective system gain or Coefficient of Performance, “CoP”. In common usage, however, “CoP” refers to gain from electrical wattage input to thermal wattage output. In diagram 150, the system “CoP” is from high-temperature thermal power input to lower-temperature thermal power output.
Diagram 150 represents a theoretical possibility, not a practical implementation. As will be shown, there are means and methods for achieving usefully large fractions of the ideal heat-to-heat CoP performance or “Thermal Leverage” implied by these diagrams and the underlying Carnot equations for converting heat to work and work back to heat.
The Stirling Engine was first described by a Scotsman, the Reverend Dr. Robert Stirling, in an 1816 patent, and demonstrated in 1818, where it was used to pump water. The term “Stirling engine” has come to refer to a class of heat engines that incorporate an external heat source, a heat sink, and an internal gas cycle for producing mechanical energy. The term “Stirling heat pump” has come to refer to devices similar to Stirling engines but configured to operate in the opposite direction, employing mechanical energy to transport heat from a cooler region to a warmer region, the purpose being to refrigerate the cooler region, or heat the warmer region, or both. “Stirling cycle” refers to an idealized thermodynamic cycle that corresponds very roughly to the operation of a Stirling engine or Stirling heat pump. Similarly, “Diesel cycle” is a mathematical idealization of a lossless diesel engine, while “Carnot cycle” is a mathematical construct, representing a hypothetical engine that achieves an efficiency level that can be approached but never reached or exceeded with a real-world heat engine. These idealized cycles, known in the prior art, are reviewed briefly here, leading up to two new, non-conventional cycles that roughly characterize modes of operation of the present invention.
Graph 200 of
Graph 250 of
Observe that adiabatic expansions and compressions can, and indeed must, proceed quickly in real machines (so that there is little time for heat transfer), whereas isothermal strokes must proceed relatively slowly to minimize losses. Thus we find a shortcoming of Stirling engines. Their dependence on equilibrium heat transfer in each of the four steps means, in practice, that it is difficult to construct a Stirling engine that exhibits high specific power, that is, high power-per-weight or high power-per-volume of the machine.
Graph 300 of
Graph 350 of
Graph 400 of
Pure Stirling engines as well as Stirling hybrid and Stirling-like engine designs revolve around a critical pair of components: a regenerator and either a displacer piston or a regenerator piston, the latter combining displacement and regeneration functions in a single moving part. This component pair will be called a Stirling Subsystem throughout the following Specification. A Stirling engine generally consists of this Stirling Subsystem plus a paired heat source and heat sink with a temperature differential to thermally power the system, plus a power piston and further mechanical energy conversion means, typically including a crankshaft, driven by the power piston. A Stirling heat pump is fundamentally similar to a Stirling engine except that it is configured to work in the opposite direction, using mechanical input energy from a power piston to move heat “uphill” against an opposing temperature gradient, from a heat source to a warmer heat sink. The objective may be to refrigerate the heat source or to warm the heat sink. As with a Stirling engine, a Stirling heat pump includes a Stirling Subsystem as described. More complicated Stirling systems may include multiple Stirling Subsystems, power pistons, heat sources and sinks, and interacting pistons may sometimes combine the functions of power piston and displacer piston in single moving parts.
The invention to be taught below employs a Stirling Subsystem as defined above, but differs from a Stirling engine or Stirling heat pump in other important respects. An understanding of existing Stirling engines is important for understanding the present invention.
Focusing first on the regenerator of a Stirling Subsystem, it consists of a porous, solid, heat-resistant medium that maintains a temperature gradient and transfers heat into and out of a gas-phase working fluid. Physically, a regenerator can be a canister of fine gravel, or a fused-together mesh of crossing wires, on a ceramic honeycomb of small gas-carrying channels, or a pressed-together bundle of capillary tubes. In an efficient utilization, gas going through the regenerator is always close to thermal equilibrium with the solid material. In normal operation, the regenerator has a “hot” end and a “cool” end, where “hot” and “cool” are relative terms and both could be above boiling or below freezing. The “axial” direction of the regenerator is taken to be the direction of the cool-to-hot temperature gradient, and also the direction of reversing gas flow. The hot-end absolute temperature may be more than double the cool-end absolute temperature, as a result of which the gas properties of density, viscosity, and molar specific heat may change considerably from one end to the other. Ignoring these nonlinear aspects and speaking in approximate terms regarding average gas properties, one can attribute an approximate time constant to the thermal equilibration of gas in the regenerator pores or channels with the solid surfaces in contact with the gas. The degree of thermal equilibration of gas with the solid regenerator material can then be expressed in terms of the equilibration time constant and the average transit time for gas traveling from one end to the other. Thus, for example, if the equilibration time constant is about one millisecond and the end-to-end transit time is about ten milliseconds, then the temperature of gas emerging from (say) the hot end will be cooler than the hot-end surface by roughly 10% of the total end-to-end temperature difference. In that case, one could say that the gas thermal equilibration is about 90% efficient. If the gas flow rate is then doubled, the equilibration efficiency will drop to about 80%, and if the flow rate is halved, then the equilibration efficiency will rise to about 95%. While these characterizations are approximate in ignoring nonlinear properties, they are nevertheless useful in describing regenerator performance.
It is an object of the present invention to use a Stirling-like system, employing components typically associated with Stirling engines including a heater, a displacer, a regenerator and a pressure containment space that allows the heater, displacer and regenerator to develop pressure changes, but to couple these components intermittently, via valves, to an external system that receives pneumatic power from the Stirling-like system via a direct exchange of working fluid with the Stirling-like system. It is a related object that the valves operate so that the pneumatic power causes a one-directional flow of working fluid in the external system, so that the Stirling-like system and valves function together as a heat-powered compressor. It is a still further object that this compressor be employed to drive a Rankine Cycle, for example for pumping heat or distilling liquids or drying solids.
It is an object of the present invention to use a Stirling-like system, employing components typically associated with Stirling engines including a heater, a first displacer, a first regenerator, and a pressure containment space allowing the heater, first displacer and first regenerator to develop oscillatory pressure changes, and to couple this oscillatory pressure to a second Stirling-like system, employing a second displacer and second regenerator, the second displacer being operated in coordination with the phase of the first displacer so that the second Stirling-like system pumps heat. It is a related object to provide intermittent valved coupling between the second Stirling-like system and separate parts of an external environment, such that working fluid is drawn from part of that external environment into the second Stirling like system, heat is pumped from a cooled part of that working fluid to a heated part of that working fluid inside the second Stirling like system, waste heat is further added to the heated part of that working fluid, and the cooled and heated parts of the working fluid are returned to parts of the external environment for heating and cooling.
These and other objects will become clear from the Specification to follow.
The figures through
a is a graphic representation of the algebraic equations of Carnot describing the efficiency of an ideal heat engine and an ideal heat pump, with graphic emphasis on the symmetry of the heat engine and heat pump efficiencies.
b is a variation on
a is a pressure-volume diagram of an ideal Carnot cycle.
b is a pressure-volume diagram of an ideal Stirling cycle, with a superimposed curve representing non-ideal performance of a real Stirling engine.
a is a pressure-volume diagram of an ideal Diesel cycle.
b is a pressure-volume diagram of an ideal Otto cycle.
a illustrates components of a Stirling-like system, including a heater and a Stirling subsystem.
b provides a second perspective view of the regenerator of
a through 22q are small iconic diagrams of the dual-Stirling engine of
The summary begins with a brief continuation of the abstract ideal cycles discussed previously. In graph 500 of
In graph 600 of
The term “Stirling-like” is used throughout this Specification to describe thermodynamic cycles that employ a regenerator to capture useable heat energy, develop pressure change, and perform pneumatic work against a load. In related usage, the present invention provides for direct pneumatic power production and pneumatic power-to-pumped-heat conversion through novel uses of the “Stirling Subsystem” as described in the above “Background . . . ” section as “ . . . a regenerator and either a displacer piston or a regenerator piston, the latter combining displacement and regeneration functions in a single moving part.” To complete a heat engine or heat pump, one needs at least two heat reservoirs, either drawn upon collectively as a source of power in a heat engine, or heat-pumped from the lower to the higher temperature reservoir in the case of a heat pump that is driven by an external source of mechanical input power. Diagram 150 of
Recalling the electrical transformer or autotransformer analogy, the electrical system invented by Nicolai Tesla and deployed by Westinghouse required alternating current “AC” electric power. The term “AC” will be used below in a generic sense to include oscillatory pneumatic power, which delivers energy in pulsatile fashion but in a one-way direction when pneumatic pressure and volume flow oscillate together in-phase. As in electrical systems, “reactive power” describes a situation with no net one-way flow of energy when pressure and volume flow-rate are in quadrature phase. Reactive pneumatic power is usually counterproductive and to be minimized. Regarding sources of “AC” pneumatic power, timed internal combustion is an excellent example of heat flow in pulses that are timed to cause in-phase variation of pressure and volume displacement. Traditional Stirling engines suffer because it is difficult to modulate the flow of input heat for optimal timing, but the regenerator largely overcomes this limitation. The Stirling Subsystem, including the regenerator and displacer means, is an effective thermal-to-pneumatic DC-to-AC converter, producing an oscillatory pressure variation with flow for volume displacement. A Stirling heat pump is a pneumatic-to-thermal AC-to-DC converter. Thus, we see the beginnings of a thermal DC-to-DC converter that employs the Stirling Subsystem as the necessary intermediary for efficient thermal energy conversion from one temperature differential to another, realizing the “promise” implied by diagram 150.
With minor exceptions, embodiments of the present invention use no mechanical piston, avoiding sliding seals, connecting rods and crankshafts and related components. The only mechanical part undergoing large motions is a displacer piston, which may optionally incorporate a regenerator into the moving piston itself and be called a regenerator piston, or which may be a non-permeable piston that drives gas through a separate fixed regenerator. The piston, generally driven by a low-power electric motor, incorporating or working in conjunction with a regenerator, responds to heat from a heater to produce oscillatory pneumatic power. This power may be used in two ways.
In a first power use, the gaseous working fluid of the Stirling-like core system may be a gas to be compressed, and compression may be accomplished through rectification of the oscillatory pneumatic power, typically employing two valves per compressor stage. Two or more Stirling Compressors may be cascaded to handle larger ratios of load pressure. This Stirling compression drives a Rankine cycle including evaporation and condensation with associated uptake and release of heat. An obvious application is Rankine cycle heat pumping, using a closed refrigerant cycle, for example as applied to space heating and air conditioning. Propane is a viable working refrigerant fluid for this purpose, being a viable but far from ideal Stirling working fluid. Extreme high temperatures cause excessive decomposition of propane and must be avoided. An important working fluid in the realm of Stirling compression is water vapor, which is not subject to high-temperature decomposition in typical Stirling-like applications. Applications of water vapor compression include superheated steam drying, distillation, and concentration of solutions, for example, of maple sap to make syrup. An already well developed field is superheated steam drying with electrically driven compression and recovery of the heat of forced condensation. The new invention eliminates the electrical step and the subsequent mechanical steps of the compression process, going straight to thermally-driven pneumatic compression, for example, of water vapor. Among many uses to be contemplated are heat-driven heat-recycling clothes dryers, lumber kilns, apparatus for drying wood chips and other biofuel components, grains and other foods, and manure and sludge. Systems can be powered flexibly by fossil fuels, biofuels, and concentrating solar collectors. The thermal-to-pneumatic energy conversion efficiency is usually moderately low, but the other side of the equation is often an offsetting high Coefficient of Performance or CoP in converting the pneumatic power into complementary processes of evaporation with closely coupled condensation and heat transfer to drive further evaporation. The large fraction of “waste” input heat from this process is mostly retained and used for evaporation and to overcome system heat losses. Overall heat-in to heat-out gains typically range from two to five, with the figures being strongly dependent on design, application, and operating conditions.
When gas is exchanged through a Stirling compressor, heat is carried out of the system by gas convection, eliminating the usual performance-limiting Stirling bottleneck of heat elimination by conduction out of a sealed enclosure. The intermittently-coupled external Rankine system becomes the extended heat sink for the Stirling Compressor. As is suggested in graphs 500 and 600 and specifically the exhaust and intake cycles of 516, 518, 614 and 616, extra convective cooling of the Stirling subsystem, when its valves are open, provides improved heat removal and enhanced performance. Even though atmospheric pressure steam is subjectively “hot,” it is nevertheless much cooler than combustion temperatures, while the Stirling-related thermodynamic properties of steam are moderately good.
In a second category of use of pulsating pneumatic power from a heat-driven Stirling subsystem, the “AC” pressure variation is exploited directly, without valve rectification to a unidirectional flow. Specifically, conventional Stirling heat pumps use cyclic mechanical compression, in phased coordination with regenerator motion, to move heat. Typical existing applications entail large ratios of absolute temperature and take advantage of the high heat capacity ratio or “gamma” of helium gas. For space heating and air conditioning applications, however, the needed ratios of absolute temperature are small, which relaxes the technical requirements of the system design. Air, with its slightly lower heat capacity (7/5, as opposed to 5/3 for helium), varies less in temperature over a given volume compression ratio, but the higher specific heat of air (compared to helium) partly compensates for the smaller temperature fluctuation. Particularly “low lift” (i.e., low absolute temperature ratio) heat pump applications can use a particularly simple coupled-cylinder dual-Stirling design, as will be taught. Higher “lift” applications are performed effectively with a more integrated dual-Stirling design in which a heat engine regenerator piston travels in the same cylindrical space as the heat pump regenerator, in overlapping ranges of motion so that the effective dead volume of the system is extremely low.
Both categories of application share the same driving system, which is a Stirling subsystem producing oscillatory pressure variation and then opening valves to a cooling heat exchange environment or coupled system.
Going through various embodiments of the invention,
The valve 860 and distillation system 800 on the left of
Drawing 1000 of
The working fluid for a Rankine Cycle space heating of this sort must have appropriate thermodynamic properties, especially a critical temperature in the right range, and be environmentally acceptable. The fluid must also withstand the highest temperatures of the Stirling Subsystem without excessive decomposition. Propane is an example from a very short list of potential working fluids. Most other refrigerants that might be used in the Rankine Cycle have poorer properties from the Stirling Subsystem standpoint, and there are problems of high-temperature decomposition. Even propane has limits beyond which an excess of non-condensing decomposition products will degrade system performance—those include ethylene, methane and hydrogen, while propylene will be produced but will cycle to some extent with the propane. The equilibrium concentration of the byproducts increases with temperature, setting a practical upper limit to the hot-side temperature of this system. A most detailed examination shows that solar-heat-driven systems of this sort have good potential, while combustion-powered systems cannot take maximum advantage of the high temperatures that are readily provided in an efficient burner. As is seen in other examples, water vapor as a Rankine Cycle working fluid is tolerant of high temperatures and has better thermodynamic properties than propane, from a Stirling Subsystem viewpoint.
System 1200 of
Subsystem 1300 of
Subsystem 1400 of
System 1500 of
Normally, power generation in a Stirling engine with one power piston requires a separate displacer piston moving in a different oscillatory phase, typically with roughly a 90 degree phase difference. This limitation is overcome in interconnected multi-cylinder configurations, where phase-shifted interactions among cylinders give rise to self-oscillation of the combined displacer/power pistons. It normally does not work, however, to have a single piston with a wide piston head acting as a displacer piston or regenerator piston and also as a power piston. As illustrated here, the lower part of the piston assembly is a thick piston shaft, whose opposite end travels into a region of relatively constant pressure, potentially acting as a power piston, if the pressure were in the correct phase. For ideal lossless regenerator action, pressure varies in phase with piston displacement, a “reactive” phase delivering no net power. For driving a self-oscillating regenerator piston with a thick shaft rising from and sinking into a region of relatively constant pressure, the pressure phase must shift away from reactive and toward a power phase, in-phase with velocity rather than displacement. When pneumatic power is drawn from the system as described, however, and specifically when valves open after some initial pressure change, allowing flow that inhibits or stops that pressure change from continuing in the same direction, then this variety of specific loading conditions causes the wall-penetrating “power piston area” to deliver power through the piston shaft to the generating apparatus. Thus, for example, when passive check valves rectify the fluid flow into a fluid load with roughly constant back pressure, the check valves remain closed following the start of piston motion in a given direction, while their delayed opening “clamps” the pressure profile against further significant pressure increase until the displacer piston comes to a stop and the opened valve re-closes. The piston motor/generator will require electric power input until conditions are achieved that provide an appropriate combination of load back pressure, regenerator-produced pressure oscillation, and valve openings following pressure change, increasing or decreasing. With energy storage for starting, as with a battery, a system of this type can start from battery power, establish conditions for power generation, recharge the battery, and continue to produce surplus electric power. In fact, an appropriate adaptive level of electric power consumption must then be maintained, in order to prevent excess oscillation amplitude and banging against mechanical limits. On the other hand, an appropriate kind of pressure loading of the inlet and outlet valves is required in order for there to be any self-oscillation. There are known alternative approaches to preventing excess oscillation, for example the small un-numbered end feature on the top of piston 1406 and the receiving dash-pot feature that the piston feature pushes into near top-dead-center. Since the generator function of the motor/generator is required for starting and establishing regenerative piston oscillation, however, a small incremental expense brings about the advantage of generating from excess piston power rather than dissipating that excess.
This hybrid Diesel-Stirling system for delivering both pneumatic and electric power has the advantage of forced convective removal of heat via the lower-left valve 1506, which functions as an exhaust valve, while the lower left valve, shown closed here, functions as the intake valve. Note that there is little or no compression stroke in this cycle, depending on operation of the thick center shaft. P-V diagram 500 of
System 1600 of
System 1700 of
An optional operating cycle for system 1800 is illustrated in diagram 1800 of
Drawing 1900 of
The timing diagram for 1900 is similar to the one for 1700 with one exception. Respective numbers 2002, 2004, 2010, 2020, 2030, 2040 and 2050 correspond to numbers 1802, 1804, 1810, 1820, 1830, 1840 and 1850 of the earlier diagram, while the start of the next operating cycle is delayed by an extra time interval 2060, with both regenerator pistons down and the space above them filled with heated air. At this time of maximum gas temperature and pressure, cam 1914 opens the upper valve to the hot air source, for example the air-circulating concentrating solar collector, exchanging fresh heat into the system. The valves then close, isolating the system from gas in the solar collector or other hot air source. Thus, the hot air source operates systematically at above-ambient pressure. When the lower four valves open in time region 2030, the system pressure is not far from ambient pressure and is restored to ambient pressure by the opening of the valves. The overall action is pump chilled air out the bottom of 1900 and warmed air out the middle.
The system of
A potential advantage of the system embodiment of 21 is the flexibility gained by not requiring a moving regenerator. A displacer piston can be very lightweight, simplifying the linear actuation process. Also, less expensive options are available for fixed regenerators, particularly where weight is not an issue. As will be seen in following a description of 2100, a performance issue is dead volume. The dual regenerator pistons of 1700 and 1900 can move through overlapping ranges, exploiting all the available volume and thus enhancing performance. Working within given manageable high temperature limits, the system of 2100 cannot achieve as large an engine-mode pressure swing to drive the heat-pump-mode components. This limitation joins together with the lower “gamma” limitation of using diatomic air versus monatomic helium, along with related issues of the much higher molecular weight and molecular size of air molecules, compared to hydrogen or helium, these unalterable issues leading to much lower thermal conductivity. A dimensional analysis of regenerator function shows that combined efficiency and volume flow rate improve strongly with increasing regenerator area, while thickness is a minor issue, within limits and provided that appropriate finer materials are available for construction of thinner regenerators. With this consideration plus desire for a compact layout, the regenerators of 2100 are shown occupying the entire “floor space” of the working cylinders. This choice of shape was made independent of any consideration of a possible material. This layout also makes for a clear conceptual illustration, which will now be laid out verbally.
The detailed description of 2100 begins with naming and describing the essential components, by number, in functional categories. There are two mirror-image subassemblies, the cylindrical housings being joined by central bridging segment of pipe 2106. In addition to this center fluid interconnection, there are two gas input/output pipes, left and right at 2108 and 2110 across the top, and a single manifold pipe 2112 bridging across the bottom. The ends of 2112 terminate with servo valves, 2134 on the left and connecting into a high-temperature heater, for example connecting pipe 2104 of the concentrating solar collector drawn and referenced earlier. The opposite valve 2136 on the lower right connects to a gas source at an intermediate or warm temperature, for example from the cold air duct return of a home heating system. Near the left end, just inside valve 2134, the manifold pipe connects to the outside of a plenum consisting of a U-channel, circumferentially wrapped to capture an annular space that couples pneumatically into the left end of 2112. Holes 2140 punched through the walls of the cylinder allow gas to couple from the cylinder's left end into the annular plenum and connecting pipe. The mirror image of this structure is found on the right end of the cylinder with annular plenum 2154 coupled via holes 2146 to the interior end of the cylinder while the plenum connects on the outside to the right end of manifold pipe 2112, just inside valve 2136. The center of this manifold pipe couples via servo valve 2138 and radial connecting pipe 2122 into the bridging segment of pipe 2106, as previously described. The interior of this system is divided into four regions whose relative volumes vary due to the motion of displacer pistons. Hot and warm regions 2102 and 2126 are separated by moving regenerator 2118, driven by motor system 2114 and driving working fluid either through regenerator 2156 or between the internal system and external systems, depending on valve settings. Similarly warm and cold regions 2104 and 2128 are separated by moving regenerator 2120, driven by motor system 2116 and driving working fluid either through regenerator 2158 or between the internal system and external systems, again depending on valve settings.
The system of 2100 has five valves, among which two pairs are functionally joined to operate in the same way at the same time, whether by common electrical control of mechanical connection. Thus, hot air or other hot gas originating from 1202 and 1204, the numbered end components of the solar collector of drawing 1200, are controllably isolated from or connected to the variable-volume hot interior region 2102 by the simultaneous opening or closing of valves 2130 and 2134. Actuation of displacer piston 2118 by the motor and rack and pinion mechanism of 2114 causes volume 2102 to expand or contract under control. With valves 2130 and 2134 open and the remaining valves closed, as drawn, the piston action causes gas exhaust from cylinder region 2102 and complementary intake into cylinder region 2126 as the piston moves to the right, and the reverse as the piston moves left. Thus, slightly cooled hot air in 2102 can be almost totally exchanged for a fresh charge of hot air with a single stroke-right and stroke-left of 2118 driven by 2114. This action accomplishes the same kind of external heating gas exchange that takes place in time period 2060 of graph 2000, where in that case the gas exchange was mediate by blowers and gas flow through momentarily opened valves. System 2100 needs no blowers, although the continuations of passageways 1202 and 1204 toward the heating source might be brought together into a rectifying one-way check valve system that causes the air circulation at a more distant point, as in a solar collector tube, to always move in one direction.
When 1202 and 1204 close, then if valves 2132 and 2136 open as a pair, motion of regenerator piston 2120, powered via motor and rack and pinion mechanism 2116, can cause gas exchange between right-hand interior regions 2104 and 2128, with piston 2120 starting either toward the left as shown, or fully left, to maximize volume 2104, or with 2120 starting on the far right, to maximize volume 2128. Unlike the double-ended working fluid source of 1202 and 1204, optionally representing the connecting ends of a continuous solar collector tube, the fluid sources feeding into paired and commonly actuated valves 2132 and 2136 are not symmetric, being a cool or cold source feeding via 2132 into region 2128 (here almost minimized) and a warm source feeding via 2136 into region 2104 (here almost maximized). Because valves 2134 and 2136 share a common passageway 2124 and it is generally desired to lose high temperature heat via short-circuit to the warm fluid circuit, 2134 and 2136 are opened only during separate non-overlapping time periods. A back-and-forth stroke of piston 2120, driven by motor, rack and pinion mechanism 2116, therefore accomplishes an intake and exhaust stroke for one input, and in reverse order an exhaust and intake stroke for the other input, thereby exchanging heated warm air or chilled cool air with their respective sources. If there is a long connecting passageway for either or both of these air sources, it might be advantageous optionally to create a loop with one-way check valves to the right of valves 2132 and or 2136 to cause gas circulation from separated points and minimize re-breathing of gas.
Note that the three-way symmetry of passage 2108 on the upper right, 2110 on the upper left, and 2112 at the bottom center, suggest three rather than four external fluid connections might be feasible, with just one connecting line to hot source 1202, one to the warm gas, and one to the cool gas. This doesn't quite work, at least for the operating cycle to be described below, for reasons that are illuminating of the system function. To replenish hot air, it is desirable to open valves 2130 and 2134, expel the slightly cooled hot gas from the system, and draw fresh hot gas into a fully expanded region 2102, maximizing the quantity of hot gas on the hot left side of the regenerator when regenerator action resumes. However, when 2102 is full expanded, this represents the above-atmospheric (or above ambient for the warm and cool gases being used) condition of the heat pump, whose interior pressure should be brought back down to a near-match with the warm and cold inputs before those valves are opened. If the system has only three valves, assigned to hot, warm and cool gases then one cannot open the hot valve and the warm valve simultaneously for an exhaust-intake double stroke and still maintain pressure continuity with the closed-valve heat-pumping pressure cycle. Thus we see by counterexample that efficient operation calls for four valves, the left-hand pair connecting for circulation into and out of the hot gas source at high pressure, and for circulation into and out of the warm and cool gas sources at a lower pressure, for example atmospheric pressure. The hot volume 2102 is therefore maximized before opening valves 2130 and 2134, then the displacer piston moves right to displace gas out via 2130 and in via 2134, and finally the displacer moves back left to leave a maximum volume of hot gas in expanded region 2102. The system then cycles, with all valves closed, to a low pressure condition with piston 2118 shifted, hot volume 2102 minimized, and warm volume 2126 maximized. Refreshing the warm and cool or cold gases can start with displacer piston in any position, though a middle position may be desired for leaving equal refreshed half-volumes of warm and cold gas with each refresh stroke. Alternating between a full warm volume and a full cool or cold volume is feasible but entails worse pressure mismatches when valves are opened.
a through 22q follow an example of the operational sequence of the system illustrated in
22
a The system starts at warm/cold ambient pressure, piston 2118 on the right, minimizing hot gas volume and pressure, with warm air filling the expanded space 2126, and with all valves closed. Piston 2120 is centered.
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b Valves 2132 and 2136 open and piston 2120 travels full left, expelling cold air and drawing in warm air.
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c Piston 2120 travels full-right, expelling warm air and drawing in cold air.
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d Piston 2120 returns to center, leaving half-volumes of refreshed warm and cold air.
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e Valves 2132 and 2136 close and valve 2138 opens to the channel 2122 leading to central passageway 2106 between the regenerators.
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f Piston 2156 moves full-left, heating gas via the regenerator and filling the hot volume, thus raising the pressure.
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g Valve 2138 closes, isolating the regenerators, and valves 2130 and 2134 open, exposing the hot side to the external heat source via 1202 and 1204.
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h Piston 2118 strokes right, momentarily filling expanded space 2126 with hot gas.
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i Piston 2118 strokes back left, re-filling volume 2102 with fresh hot gas.
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j Valves 2130 and 2134 close and regenerator valve 2138 opens. Air on the enclosed right side is now pressurized and at above-average temperature.
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k Piston 2120 strokes right, expanding cold volume 2128 and driving above-average-temperature warm air into the warmer left side of regenerator 2158, thus tending to heat that warm side.
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l Piston 2118 now strokes right, pushing hot air into the left side of regenerator 2156 while emerging warm air from the opposite side loops past open valve 2138 into filling region 2126 on the left of the piston. The system pressure is now low and the gas on the right side is cooled adiabatically.
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m Piston 2120 strokes left, pushing the cooled cold volume into the cold right side of regenerator 2158, thus further cooling that side of the regenerator, while warm air emerges from the other side of the regenerator and loops through valve 2138 into expanding region 2104.
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n Piston 2118 strokes left, raising pressures and temperatures. Observe that in the last few steps, pistons 2118 and 2120 have been moving in a quadrature sequence, one piston motion leading the other. It is this quadrature phasing of piston motions that pumps heat. This could be accomplished with continuous quadrature-phase sinusoidal motions, but non-overlapping full-stroke motions of the two pistons are more effective. This quadrature sequence could optionally continue for further cycles before the air exchange sequence to follow in this description and in the figures.
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o Piston 2120 returns to center position, pushing a half-stroke of compression-heated warm air into the left side of regenerator 2158.
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p Piston 2118 strokes right, cooling the system and lowering the pressure to near-ambient.
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q Center valve 2138 closes. All valves are now closed, and the system has returned to the state described with reference to
The above examples illustrate the core principles of the invention in differing contexts. It will be recognized that many other particular contexts and variations are possible, falling within the teaching provided in the above Specification and further by the following claims.
U.S. Provisional Patent Application No. 61/209,921, dated 12 Mar. 2009, “Stirling engine for direct mechanical compression,” by the inventor Seale named in the present application, is incorporated here by reference. The more recent U.S. Provisional Patent Application No. 61/336,494, dated 22 Jan. 2010, “Heat engine with regenerator and timed gas exchange” by inventors Seale and Bergstrom of the present application, is further incorporated by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2010/000758 | 3/12/2010 | WO | 00 | 9/8/2011 |
| Number | Date | Country | |
|---|---|---|---|
| 61209921 | Mar 2009 | US | |
| 61336494 | Jan 2010 | US |
