Technical Field
[0001] This invention relates to Stirling-cycle engines, to other regenerative thermal machines,
and more particularly to a new method for the construction of the regenerator element
common to all such machines. The new method involves the deliberate incorporation
of certain anisotropic materials such as pyrolytic graphite to improve the heat transfer
and storage performance characteristics of the regenerator. This will enhance the
overall performance of regenerative thermal machines, especially those which embody
a practical approximation to the well known Stirling thermo-dynamic cycle in the production
of both mechanical power (i.e. prime movers, compressors, fluid pumps) and refrigeration
(i.e. refrigerators, air conditioners, heat pumps, gas liquefiers).
[0002] A Stirling-cycle engine is a machine which operates on a closed regenerative thermo-
dynamic cycle, with periodic compression and expansion of a gaseous working fluid
at different temperature levels, and where the flow is controlled by volume changes
in such a way as to produce a net conversion of heat to work, or vice versa. The regenerator
is a device which in prior art takes the form of a porous mass of metal in an insulated
duct. This mass takes up heat from the working fluid during one part of the cycle,
temporarily stores it within the machine until a later part of the cycle, and subsequently
returns it to the working fluid prior to the start of the next cycle. Thus the regenerator
may be thought of as an oscillatory thermodynamic sponge, alternately absorbing and
releasing heat with complete reversibility and no loss.
[0003] A reversible process for a thermodynamic system is an ideal process, which once having
taken place, can be reversed without causing a change in either the system or its
surroundings. Regenerative processes are reversible in that they involve reversible
heat transfer and storage; their importance derives from the fact that idealized reversible
heat transfer is closely approximated by the regenerators of actual machines. Thus
the Stirling engine is the only practical example of a reversible heat engine which
can be operated either as a prime mover or as a heat pump.
Background
[0004] The Stirling-cycle engine was conceived and reduced to practice in Scotland 164 years
ago. A hot-air, closed-cycle prime mover based on the principle was patented by the
Reverend Robert Stirling in 1817 as an alternative to the explosively dangerous steam
engine. Incredibly, this event occurred early in the Age of Steam, long before the
invention of the internal combustion engine and several years before the first formal
exposition of the Laws of Thermodynamics.
[0005] Air was the first and only working fluid in early 19th century machines, whereas
hydrogen and helium have been the preferred working fluids for modern machines. In
Great Britain, Europe, and the United States thousands of regenerative hot air prime
movers in a variety of shapes and sizes were widely used throughout the 19th century.
The smaller engines were reliable, reasonably efficient for their time, and most important,
safe compared with contemporary reciprocating steam engines. The larger engines were
less reliable, however, because they tended to overheat and often succumbed unexpectedly
to premature material failure.
[0006] Toward the end of the 19th century the electric motor and the internal combustion
engine were developed and began to replace not only the Stirling-cycle engines, but
also the reciprocating steam engines of that era. These new machines were preferred
because they could produce greater power from more compact devices and because they
were more economical to manufacture. The limitations of early, as well as those of
current Stirling engines are in part directly attributable to the design and performance
characteristics of the regenerator element. Both the specific power capacity and the
overall thermal efficiency of regenerative thermal machines are direct consequences
of the inherent performance characteristics and heat transfer properties of the regenerator.
[0007] Since World War II there have been unprecedented advances in the general technologies
of machine design, heat transfer, materials science, system analysis and simulation,
manufacturing methods, and Stirling engine development. Today, in comparison to their
conventional internal combustion counterparts, all modern Stirling-cycle prime movers
are external combustion engines which consistently demonstrate (in the laboratory)
higher efficiency, multifuel capability, lower exhaust emissions, quieter operation,
equivalent power density, and superior torque characteristics.
[0008] Nevertheless, none of these engines is mass produced for any commercial application
anywhere in the modern world. The reason for this is that contemporary Stirling engines
have been developed largely by adapting traditional methods and designs from the more
familiar internal combustion engine technology base. Patchwork adaptation of the old
as a shortcut to the new is a process which exorably produces a hodge-podge arrangement
of excessive mechanical complexity and which inevitably results in high production
costs.
[0009] The modern regenerator construction, for example, is an awkward, although servicable,
design compromise among conflicting requirements for efficient heat transfer, minimum
flow losses, and maximum packing density. The use of traditional materials and methods
offers no thoroughly satisfactory solutions to this dilemma. Despite clearly superior
technical performance characteristics, therefor, contemporary Stirling engines are
invariably not cost competitive from the standpoint of economical mass production.
[0010] DE-A-1501584 discloses a thermal regenerator filling mass comprising a roll of coated
tape, wherein the coated tape comprises a layer of a material bonded to the tape,
wherein the specific heat of the material forming a layer is high compared with that
of the tape and the thermal conductivity of the tape is low compared with that of
the said material. The filling mass has a low thermal conductivity in the direction
of flow of the medium through the regenerator.
[0011] US-A-3960204 discloses a low void volume regenerator for a Vuilleumier cryogenic
cooler comprising a shell providing conduit for working fluid, a thermal mass packing
said shell and configured to provide a passage therethrough with the highest practice
ratio of exposed surface area to cross-sectional flow area and having a thermal conductivity
in the direction of flow of said working liquid which is lower than that in the direction
normal to the direction of said flow.
Disclosure
[0012] The invention comprises fundamental concepts and mechanical components which in combination
enhance the operation yet lower the cost of Stirling-cycle machines, by means of the
use of a regenerator which employs materials of construction which have anistropic
symmetry to achieve anisotropic thermal conductivity and large specific heat capacity
in a thermal mass having the highest practicable ratio of exposed surface area to
cross-sectional flow area.
[0013] It is a primary object of the invention to provide a novel form of regenerator, designed
to incorporate certain materials such a pyrolytic graphite, which possess anisotropic
symmetry in addition to the desirable physical properties of low density and high
heat capacity, thereby inherently exhibiting a high thermal conductivity in directions
normal to the flow of working fluid and a low thermal conductivity in the direction
of the flow within the same contiguous mass.
Brief Description of Drawings
[0014] Other objects, advantages, and novel features of the invention will become readily
apparent upon consideration of the following detailed description when read in conjunction
with the accompanying drawings wherein:
FIG. 1 is an illustration of the operational sequence of events during one complete
cycle of an idealized single-acting two-piston Stirling engine used in the prime mover
mode;
FIG. 2(a) and FIG. 2(b) are schematics which illustrate the idealized pressure-volume
and temperature-entropy diagrams of the thermo- dynamic cycle of the working fluid
in the same machine depicted by FIG. 1; FIG. 2(c) is a pressure-volume diagram which
depicts the working of an actual machine; and
FIG. 3 is an illustration of the construction of a regenerator element using anisotropic
perforated disks.
Best Mode for Carrying Out Invention
[0015] Attention is directed to FIG. 1 wherein numeral 1 designates an idealized version
of a two-piston Stirling-cycle prime mover. A conceptually constant mass of pressurized
gaseous working fluid occupies the working volume between the compression piston 2
and the expansion piston 3. The total working volume is comprised by compression space
4, regenerator 5, and expansion space 6. A portion of compression space 4 is continually
cooled by cooler 7, while a portion of expansion space 6 is continually heated by
heater 8. Arrows 9 are intended to represent the input of heat by conduction, convection,
or radiation. Escape of fluid from the working volume is prevented by the piston seals
10.
[0016] During the compression stroke (between positions I and II) the working fluid is compressed
isothermally by piston 2 at the minimum temperature level of the cycle. Heat is continually
rejected at this temperature through cooler 7; the pressure rises slightly and the
total working volume decreases to a minimum. During the forward displacement (cold-side
to hot-side transfer) stroke (between positions II and III) regenerator 5 yields stored
heat to the working fluid as it is transferred to expansion space 6 with the volume
remaining constant. The temperature and pressure rise to their maximum levels.
[0017] During the expansion stroke (between positions III and IV) the working fluid expands
isothermally at the maximum temperature level of the cycle, doing work on piston 3.
The temperature level is maintained by the input of heater 8; the pressure drops and
the total working volume increases to a maximum. During the reverse displacement (hot-side
to cold-side transfer) stroke (between positions IV and I) regenerator 5 recovers
heat from the working fluid as it is transferred to compression space 4 with the volume
remaining constant. The temperature and pressure return to the starting levels of
the cycle.
[0018] A clearer understanding of the foregoing may be obtained by referring to the diagrams
of FIG. 2(a) and FIG. 2(b) wherein the same complete cycle is presented in terms of
the pressure-volume diagram and the temperature-entropy diagram for the working fluid.
For each process as depicted by the curves between the indicated position numbers
I-II, II-III, and IV-I, the area under a curve on the P-V diagram is a representative
measure of the mechanical work added to or removed from the system during the process.
Similarly, the area under a curve on a T-S diagram is a measure of the heat transferred
to or rejected from the working fluid during the process.
[0019] Actual machines differ fundamentally from the idealized versions in that the motion
of each piston is continuous and smooth, rather than discontinuous and abrupt. This
causes the indicated processes of FIG. 2(a) and FIG. 2(b) to overlap one another,
and results in P-V diagrams which are smooth continuous curves devoid of sharp corners
as shown by FIG. 2(c). Thus the piston motion of actual machines is smoothly periodic
to point of being sinusoidal, and the working fluid is likewise distributed in a periodically
time-variant manner throughout the total working volume.
[0020] As previously noted, the regenerator is a device comprised by a thermal mass so arranged
and deployed within a thermal machine that it takes up heat from the working fluid
during one part of the cycle, temporarily stores it within the machine until a later
part of the cycle, and subsequently returns it to the working fluid prior to the start
of the next cycle. As explained in prior art Patent US-A-3,960,204, it is important
to minimize longitudinal thermal conductivity of all regenerators. My concept proposes
the utilization of the unique physical property known as bulk anisotropy, which is
displayed by certain well-known materials such as pyrolytic graphite and pyrolytic
boron nitride, for the construction of an advanced regenerator in the manner illustrated
by FIG. 3.
[0021] It may be seen that regenerator 20 is nothing more than an ordered or stacked assemblage
of perforated sick elements 21 contained within a tubular duct 22 which possesses
a comparitively low thermal conductivity. The perforations 23, which may take many
differnt forms, are designed so as to maximize the ratio of the perimeter of the perforation
to the cross sectional area of the perforation. The basic purpose of this approach
is to maximize both the capacity and the rate of heat transfer with respect to the
material of the regenerator, while at the same time to minimize working fluid flow
losses and longitudinal thermal conductivity losses within the regenerator.
[0022] Pyrolytic graphite is a polycrystalline form of carbon having a high degree of molecular
orientation. It possesses no binder, has a very high purity, and may exceed 98.5%
of the theoretical density for carbon. The material is usually produced by chemical
vapor deposition onto a substrate which is maintained at an elevated temperature.
Such deposits possess great high temperature strength, exceptional thermo-physical
properties, and phenomenal anisotropic symmetry. That is, they naturally and consistently
exhibit one value for physical constants as measured in the plane of the deposit and
compared to the value for the same constant as measured across the plane of the deposit.
[0023] It is a most remarkable, but nevertheless well-known fact that the thermal conductivity
of pyrolytic graphite in the plane of the deposit is about equal to that of copper
at room temperature (4.2 watts/cm
2/°C/cm); but the conductivity across the plane of the deposit is reduced by almost
200 to 1 (0.025 watts/cm2l°C/cm). The corresponding values at 1000°C are know to be
similarly anomalous (1.25 watts/cm2/°C/cm and 0.012 watts/cm2/°C/ cm) and the value
of the specific heat at 750°C (1182°F) is known to be approximately 0.45 cal/g/ °C,
which is among the highest values for all structural engineering materials.
[0024] It is therefore an important specific teaching of this invention that a number of
perforated disks 21 may be made of this or similar material to have a comparatively
large transaxial thermal conductivity (i.e., in the plane of the disk), yet to have
a comparatively small axial- thermal conductivity (i.e., across the thickness of the
disk). The indicated assemblage of said perforated disks 21 would therefore comprise,
when placed within the insulative cyclindrical container 22, a remarkably efficient
regenerator. It should be apparent that such a device would quickly and effectively
transfer and store large amounts of heat to and from a fluid flowing within the internal
duct formed by the superimposed perforations 23 due to the favorable thermal properties
in the transaxial (or radial) direction, but would maintain a high temperature gradient
in the direction of flow because of the low value of thermal conductivity in that
direction.
[0025] Pyrolytic graphite also has a great difference in linear thermal expansion coefficients
between the directions within the plane of the deposit and the direction perpendicular
to the plane of the deposit. The average coefficient of linear thermal expansion from
room temperature to 1000°C is know to be 1.3 x 10
6cm/cm/C in the plane of deposit and 22.0 x 10-rcm/cmPC across the plane of deposit.
The latter value should be matched by the wall of the containing vessel, in order
to preclude or minimize thermal stresses; fortunately, it is reasonably close to that
of many structural alloys of interest, including certain alloys of aluminum, manganese,
and copper.
[0026] Since the closed cycle Stirling prime mover operates solely on the basis of the difference
in temperature in the working fluid between the hot expansion space and the cold compression
space, the development of useful power output is not specific to the source of heat
available for use. Therefore, the design of the heat source can be any one of a large
variety of possible types. A rather simple combustion system can be produced, for
example, which will cleanly and efficiently burn various kinds of both liquid fuels
and gaseous fuels without any modification whatsoever. Thus it will be appreciated
by those familiar with the art that a single prime mover may be made to operate on
regular or premium gasoline, diesel oil, alcohol, crude oil, lubricating oil, olive
oil, vegetable oil, propane, butane, natural gas, and synthetic coal gas.
[0027] It is important at this point to re-emphasize the fact that each small segment of
a well-designed regenerator transfers heat to and from the working fluid with minimal
temperature differences. Thus all stages in the regenerator are reversible in an actual
thermodyamic sense. Therefore, the entire machine cycle is reversible in function;
it is, the direction of flow of heat and work can be reversed. The Stirling engine
is truly unique in that it is the only practical example of a thermodynamically reversible
machine.
[0028] It should be thoroughly understood, therefore, that many of the design concepts disclosed
herein for Stirling prime movers are also applicable to the design and development
of Stirling refrigerators, heat pumps, air conditioners, and the like. It is another
important specific teaching of this invention that machines of this kind would be
appreciably more efficient than conventional vapor cycle reciprocating refrigerators
or thermally-activated absorption refrigerators, with a substantial savings in size
and weight. In addition, a hybrid device obtained from the combination of a Stirling
prime mover mechanically coupled to a Stirling heat pump will permit both multifuel
and non-fuel powered refrigeration units to be developed and applied to specialized
applications.
[0029] In view of the foregoing it should be readily apparent to those skilled in the art
that the operation of the present invention may be accomplished by means of and in
the context of an enormous variety of diverse applications. In fact, virtually every
market in the world which is currently occupied by the application of a reciprocating
internal combustion prime mover, or by the application of a conventional vapor cycle,
absorption, or other type of refrigeration device, is subject to improvement by virtue
of the diligent application of the teachings of this invention.
[0030] These include but are by no means limited to the following: automotive prime movers,
marine prime movers, aeronautical prime movers, industrial prime movers, military
prime movers, agricultural prime movers, multifuel prime movers, nonfuel prime movers,
portable prime movers, bio-medical prime movers, refrigerators, air conditioners,
cryogenic cooling machines, residential heat pumps, industrial heat pumps, military
heat pumps, water coolers, air compressors, other gas compressors, remote electric
generators, portable electric generators, stationary electric generators, hydroelectric
power converters, nuclear power converters radioisotope power converters, solar power
converters, geothermal power converters, ocean thermal power converters, biomass power
converters, solid waste power converters, small cogeneration power plants, large congeneration
power plants, remote fluid pumps, portable fluid pumps, stationary fluid pumps, remote
power tools, portable power tools, outdoor power tools, underwater power tools, toys
and novelties.