Free Piston Engines: Thermoacoustic Stirling Engine
Free PistonEngines:ThermoacousticStirling Engine
April 4
2009Brought to you by- Ritesh Bhusari
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Free Piston Engines: Thermoacoustic Stirling Engine :1
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In February 1999, the International Academy of Engineering convened an
expert panel to select the technologically outstanding achievements of the 20th
century & its no surprise that the I.C. Engine Technology topped the list. But,
Pollution concerns, global warming and shrinking fossil fuel reserves have
focused world attention on how engines generate electrical and mechanical
power in a better way.
“The free piston engine is an attempt to combine the high thermal
efficiency of a reciprocating engine with high power/weight ratio of a rotary
turbine. It is a combination of a reciprocating engine and a rotary turbine.
The quest for increased power from a given cylinder size has resulted in a
long process of development. Important steps in this process of development are
improvements in the fuels used and in the design of various components for
higher efficiencies and lower cost and weight. However, a different approach in
the direction of using different cycles of operation or modifications of existing
cycle, has also been pursued with great interest.”
In a step towards exploiting existing power cycles, scientists at the U.S
Department of Energy's Los Alamos National Laboratory have developed a
remarkably simple, energy-efficient engine which works on ‘ Stirling Cycle ‘ and
has no oscillating pistons, oil seals or lubricants, known as the
“Thermoacoustic Stirling Engine”.
Sound waves in "thermoacoustic" engines can replace the pistons and
cranks that are typically built into conventional engines & hence in true sense
thermoacoustic stirling engine can be termed as advancement in free piston
engines.
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Free Piston Engines: Thermoacoustic Stirling Engine :2
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Basic Stirling Cycle:
Stirling Engine, type of engine that derives mechanical power from the
expansion of a confined gas at a high temperature. The stirling cycle [Fig. 1] was
patented in 1816 by the Scottish clergyman Robert Stirling and was used as a
small power source in many industries during the 19th and early 20th centuries.
The need for automobile engines with low emission of toxic gases has revived
interest in the Stirling engine, and prototypes have been built with up to 500
horsepower and with efficiencies of 30 to 45 percent.
The cycle that provides the work is called the Stirling cycle; it consists in
its simplest form of the compression of a fixed amount of so-called working gas
(hydrogen or helium) in a cool chamber. This cool compressed gas is transferred
to a hot chamber, which is heated by an external burner, where the gas expands
and drives a piston that delivers the work. The expanded hot gas is then cooled
and returned to the cold chamber, and the cycle begins again. Stirling also
conceived the idea of a regenerator (a solid with many holes running through it,
which he called the “economiser”) to store thermal energy during part of the cycle
and return it later [Fig. 2]
The engine is able to transform heat into work because the expansion of
the gas at high temperature delivers more work than is required to compress the
same amount of gas at low temperature.
The heat for the expansion chamber is provided by an external
continuous burner that can operate on gasoline, alcohol, natural gas, propane,
butane, or solar energy and the exhaust generated has very low free carbon and
toxic gas levels. The Stirling engine runs smoothly because pressure variations in
the compression and expansion chambers are sinusoidal, that is, relatively
gradual, rather than explosive as in internal-combustion cycles.
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Free Piston Engines: Thermoacoustic Stirling Engine :3
Stirling engines are unique heat engines because their theoretical
efficiency is nearly equal to their theoretical maximum efficiency, known as the
Carnot Cycle efficiency.
Figure 1: PV & TS Representation Of Stirling Cycle
Figure 2: Stirling Cycle
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Free Piston Engines: Thermoacoustic Stirling Engine :4
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Thermoacoustics is the study of the thermoacoustic effect and the
attempt to harness the effect as a useful heat engine. A thermoacoustic prime
mover uses heat to create sound. Simply put, thermoacoustic effect is the
conversion of heat energy to sound energy or vice versa. Utilizing the
Thermoacoustic effect, engines & refrigerators are developed that use heat as an
energy source and have no moving parts!
Transformation of Heat Energy into intense Acoustic Energy:
Thermoacoustic device [Fig. 3] consists, in essence, of a gas-filled tube
containing a “stack” (top), a porous solid with many open channels through
which the gas can pass. Resonating sound waves (created, for example, by a
loudspeaker) force gas to move back and forth through openings in the stack.
If the temperature difference along the stack is made sufficiently large,
sound can compress and warm a parcel of gas (a), but it remains cooler than the
stack and thus absorbs heat. When this gas shifts to the other side and expands
(b), it cools but stays hotter than the stack and thus releases heat. Hence, the
parcel thermally expands at high pressure and contracts at low pressure, which
amplifies the pressure oscillations of the reverberating sound waves,
transforming heat energy into acoustic energy. A device that creates sound from
heat is called a thermoacoustic heat engine.
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Free Piston Engines: Thermoacoustic Stirling Engine :5
Figure 3: Working Principle of a Thermoacoustic device
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Introduction:
The thermoacoustic Stirling heat engine [Fig. 4 & 5] developed by the
LANL scientist’s converts heat to intense acoustic power in a simple device that
comprises only pipes and conventional heat exchangers and has no moving
parts. The acoustic power can be used directly in acoustic refrigerators or pulse-
tube refrigerators to provide heat-driven refrigeration, or it can be used to
generate electricity via a linear alternator or other electroacoustic power
transducer. Already the engine's 30% efficiency and high reliability may make
medium-sized natural-gas liquefaction plants (with a capacity of up to a million
gallons per day) and residential cogeneration economically feasible.
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Free Piston Engines: Thermoacoustic Stirling Engine :6
The power production process is environmentally friendly and up to 30
percent efficient while typical internal combustion engines are 25 to 40 percent
efficient.
Because the thermoacoustic Stirling heat engine contains no moving
parts and is constructed of common materials, it requires little or no
maintenance, can be manufactured inexpensively, and is expected to have many
future uses.
Figure 4: Thermoacoustic Stirling Engine (TASHE)
Figure 5: Thermoacoustic Stirling Engine (TASHE)
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Free Piston Engines: Thermoacoustic Stirling Engine :7
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Scaled drawing of the “TASHE”, used in the measurements is shown in
Figure 6. Essentially, it is composed of a 1/4-wavelength resonator filled with 30-
bar helium. The torus-shaped section contains the heat exchangers, regenerator
and other duct work necessary to force the he-lium to execute the Stirling cycle.
The rest of the hardware past the resonator junction forms the resonator and
variable acoustic load.
Loud Speaker: It is used to generate sound waves in the resonator tube.
Cold Heat exchanger: It is a simple shell & tube type heat exchanger
with tubes arranged in parallel to acoustic displacement.
Regenerator: The regenerator is a mesh of fine wires or sintered metal
structure sealed within the tube The function of the regenerator is to abstract and
hold heat from working gas flowing from hot space to cold space and return it
back to working gas flowing from cold space to hot space thus increasing
thermal efficiency.
Hot Heat exchanger: It is similar in construction to the cold heat
exchanger. Its location is chosen so as to not disturb the flow in the thermal
buffer tube.
Thermal buffer tube: The thermal buffer tube (TBT) is a tapered tube &
provides a thermal buffer between the hot heat exchanger and room
temperature.
Flow Straightener: It ensures that the flow entering the bottom of the
TBT is spatially uniform, not a jet flow due either to the geometry of the
secondary cold heat exchanger or to flow separation at the resonator junction.
Clockwise farther around the torus are the resonator junction, feedback
inertance, and compliance. The inertance and compliance provided by these
components act (respectively) like inductance and capacitance in an analogous
electrical circuit (bottom), which introduce phase shifts (between voltage and
current in an electrical network and between gas pressure and velocity in an
acoustic network). Although pressure and gas velocity are 90 degrees out of
phase within the main standing-wave resonator
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Free Piston Engines: Thermoacoustic Stirling Engine :8
Jet Pump: It is used to stop streaming problems known as gudgeon
streaming.
Acoustic Load: Here the sound energy is converted to useful work.
Figure 6: Apparatus of TASHE
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Free Piston Engines: Thermoacoustic Stirling Engine :9
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In a nutshell, a thermoacoustic engine converts heat from a high-
temperature source into acoustic power while rejecting waste heat to a low-
temperature sink. A thermoacoustic refrigerator does the opposite, using acoustic
power to pump heat from a cool source to a hot sink. Thermoacoustic Stirling
engine designed at Los Alamos National Laboratory (top) weighs 200 kilograms
and measures 3.5 meters long. The regenerator (middle, dark red) sits in one of
two channels that connect the main helium-filled resonator with a “compliance
volume” (dark blue); the other connection is through a narrow pipe, or “inertance
tube” (dark green). The inertance and compliance provided by these components
act (respectively) like inductance and capacitance in an analogous electrical
circuit (bottom), which introduce phase shifts (between voltage and current in an
electrical network and between gas pressure and velocity in an acoustic network).
The phase shift created by the inertance-compliance network at the left creates a
small pressure difference across the regenerator, driving gas through it. This flow
increases and decreases in phase with the rise and fall of pressure in the main
resonator. These conditions ensure that the regenerator provides more gain than
loss, thus amplifying the acoustic oscillations within the engine [Fig. 7a]
The thermal energy injected at the hot end of the regenerator is
transformed efficiently into acoustic energy, which can be used, for example, to
drive a reciprocating electric generator or to power a refrigerator. One such
device under development for commercial application is intended to liquefy
natural gas. These devices perform best when they employ noble gases as their
thermodynamic working fluids. Unlike the chemicals used in refrigeration over the
years, such gases are both nontoxic and environmentally benign. Another
appealing feature of thermoacoustics is that one can easily flange an engine onto
a refrigerator, creating a heat-powered cooler with no moving parts at all.
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Free Piston Engines: Thermoacoustic Stirling Engine :10
The sound levels generated in such devices, using specialised speakers, are extreme:
in one case the levels reach 190 dB, about 10 million times as intense as the front row levels at
a rock concert and 300 times the intensity needed to ignite human hair. However, the sound
levels outside the rigid pressure vessel are acceptable. They are not noisy because the casing is
a quarter of an inch thick. You hear only a low frequency hum. A prototype refrigerator has
already been built and uses sound to "pump" heat from a lower temperature to a higher. The
engine has an efficiency of 30 per cent, which is comparable with that of a car engine (25-40
per cent).
So far, most machines of this variety reside in laboratories. But prototype
thermoacoustic refrigerators have operated on the Space Shuttle and aboard a
Navy warship. And a powerful thermoacoustic engine has recently demonstrated
its ability to liquefy natural gas on a commercial scale.
Figure 7a: Apparatus of TASHE
Figure 7b: Equivalent electrical Circuit
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Free Piston Engines: Thermoacoustic Stirling Engine :11
Electrical Analogy:
The pressure and velocity of acoustic waves in a gas have rough
analogies in AC electric circuits: The pressure resembles the voltage, and the
velocity the current [Fig. 7b]
The regenerator produces an amount of acoustic power that is
proportional to the product of the oscillating pressure of the gas and the
oscillating velocity of the gas. The power wasted in the regenerator is proportional
to the square of the oscillating velocity. This loss is analogous to the power
dissipated in an electrical resistor, which is proportional to the square of the
current that flows through it.
Faced with such losses—say, from the resistance of the wires in a
transmission line—electrical engineers long ago found an easy solution: Increase
the voltage and diminish the current so that their product (which equals the power
transferred) remains constant. So if the oscillatory pressure could be made very
large and the flow velocity made very small, in a way that preserved their product,
we could boost the efficiency of the regenerator without reducing the power it
could produce.
Traveling acoustic waves, in contrast, have their pressure and velocity
in phase with each other. Peter Ceperley of George Mason University noted 20
years ago that when traveling waves pass through a regenerator, the
thermodynamic cycle of compression, heating, expansion, and cooling that the
gas undergoes is the same as in a Stirling engine, where mechanical pistons
establish the proper phasing of the gas motion. With gas velocity and pressure in
phase, a traveling wave acoustic engine can use a reversible, much more
efficient heat transfer process. Viscous dissipation and other losses have plagued
the experimental implementation of traveling wave engines, and the high
expectations for these engines are only now beginning to be realized.
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Free Piston Engines: Thermoacoustic Stirling Engine :12
The intense acoustic energy generated in the resonator of
thermoacoustic Stirling engine is fed to the Thermoacoustic electric generator
which acts as the acoustic load. A thermoacoustic magnetohydrodynamic
electrical generator comprises of a magnet having a magnetic field, an elongated
hollow housing containing an electrically conductive liquid and a thermoacoustic
structure positioned in said liquid, heat exchange means thermally connected to
said thermoacoustic structure for inducing said liquid to oscillate at an acoustic
resonant frequency within said housing, said housing being positioned in said
magnetic field and oriented such that the direction of said magnetic field and the
direction of oscillatory motion of said liquid are substantially orthogonal to one
another, first and second electrical conductor means connected to said liquid on
opposite sides of said housing along an axis which is substantially orthogonal to
both the direction of said magnetic field and the direction of oscillatory motion of
said liquid, whereby an alternating current output signal is generated in said
conductor means at a frequency corresponding to the frequency of said
oscillatory motion of said liquid.
.
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The thermo-acoustic technology development was led by the Los
Alamos National Laboratory's Material Science Technology Division. Praxair, Inc.
has acquired the assets and licenses to acoustic heat engines and acoustic
refrigerators. Assets acquired by Praxair include pilot plants, commercial
demonstration equipment, exclusive patent rights, licenses and development
programs. The prototype demonstration and validation previously was conducted
by Chart Industries. Praxair will continue to work with these agencies to
commercialize thermo-acoustic technology.
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Free Piston Engines: Thermoacoustic Stirling Engine :13
At the most efficient operating point, the engine delivers 710 W to its
resonator with an efficiency of 0.30 which corresponds to 41% of the Carnot
efficiency. At the most powerful operating point, the engine delivers 890 W to its
resonator with an efficiency of 0.22
Thermoacoustic engines and refrigerators were already being
considered a few years ago for specialized applications, where their simplicity,
lack of lubrication and sliding seals, and their use of environmentally harmless
working fluids were adequate compensation for their lower efficiencies. This latest
breakthrough, coupled with other developments in the design of high-power,
single-frequency loudspeakers and reciprocating electric generators, suggests
that thermoacoustics may soon emerge as an environmentally attractive way to:
Ø Power hybrid electric vehicles
Ø Capture solar energy
Ø Refrigerate food
Ø Air condition buildings
Ø Liquefy industrial gases
Ø Residential Co-generation
Ø Navy Warships
Ø Space Shuttles
and serve in other capacities that are yet to be imagined.
In 2099, the International Academy of Engineering probably will again
convene an expert panel to select the outstanding technological achievements of
the 21st century. We hope the machines that our unborn grandchildren see on
that list will include thermoacoustic devices, which promise to improve
everyone’s standard of living while helping to protect the planet
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Free Piston Engines: Thermoacoustic Stirling Engine :14
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1. Mathur. M. I and Sharma. R. P., Internal Combustion Engines.
2. Technical Guidance- Scott Backhaus, U. S. Dept.’s Los Alamos National
Laboratory.
3. Backhaus, S., and G. W. Swift. 2000. A thermoacoustic Stirling heat
engine. Journal of the Acoustical Society of America 107:3148–3166,
June 2000.
4. S. L. Garrett and S. Backhaus. The power of sound. American Scientist, 88
(6), 516-525, Nov.-Dec. 2000.
5. Ceperley, P. H. 1979. A pistonless Stirling engine—The traveling wave
heat engine. Journal of the Acoustical Society of America 66:1508–1513.
6. S. Backhaus and G. W. Swift, "A thermoacoustic-Stirling heat engine,"
Nature, 399: 335-338, May 1999.
7. Swift, G. W. 1988. Thermoacoustic engines. Journal of the Acoustical
Society of America 88:1145–1180.
8. Swift, G. W. 1997. Thermoacoustic natural gas liquefier. Proceedings of
the DOE Natural Gas Conference, Morgantown, West Virginia: Federal
Energy Technology Center.
9. Swift, G. W. 1997. Thermoacoustic engines and refrigerators. In
Encyclopedia of Applied Physics 21:245–264, ed. G. L. Trigg. New York:
Wiley-VCH.
10. Yazaki, T., A. Iwata, T. Maekawa and A. Tominaga. 1998. Traveling wave
thermoacoustic engine in a looped tube. Physical Review Letters
81:3128–3132.
11. Journal, The stirling Machine World, USA.
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