Stirling Engine - Wikipedia
Transcript of Stirling Engine - Wikipedia
Stirling engine
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Alpha type Stirling engine. The expansion cylinder (red) is maintained at a high temperature
while the compression cylinder (blue) is cooled. The passage between the two cylinders contains
the regenerator.
A Stirling engine is a heat engine that operates by cyclic compression and expansion of air or
other gas, the working fluid, at different temperature levels such that there is a net conversion of
heat energy to mechanical work.[1]
The engine is like a steam engine in that all of the engine's heat flows in and out through the
engine wall. This is traditionally known as an external combustion engine in contrast to an
internal combustion engine where the heat input is by combustion of a fuel within the body of
the working fluid. Unlike the steam engine's use of water in both its liquid and gaseous phases as
the working fluid, the Stirling engine encloses a fixed quantity of permanently gaseous fluid such
as air or helium. As in all heat engines, the general cycle consists of compressing cool gas,
heating the gas, expanding the hot gas, and finally cooling the gas before repeating the cycle.
Originally conceived in 1816 as an industrial prime mover to rival the steam engine, its practical
use was largely confined to low-power domestic applications for over a century.[2]
The Stirling
engine is noted for its high efficiency, quiet operation, and the ease with which it can use almost
any heat source. This compatibility with alternative and renewable energy sources has become
increasingly significant as the price of conventional fuels rises, and also in light of concerns such
as peak oil and climate change. This engine is currently exciting interest as the core component
of micro combined heat and power (CHP) units, in which it is more efficient and safer than a
comparable steam engine.[3][4]
Contents
1 Name and definition
2 Functional description
3 History
4 Theory
5 Analysis
6 Applications
7 Other recent applications
8 Alternatives
9 Photo gallery
10 See also
11 References
12 Bibliography
13 Further reading
14 External links
[edit] Name and definition
Robert Stirling was the inventor of the first practical example of a closed cycle air engine, and
though it had been suggested as early as 1884 by Fleeming Jenkin that all such engines should
therefore generically be called Stirling engines, this nomenclature found little favour, and the
various types on the market continued to be known by the name of their individual designers or
manufacturers, e.g. Rider's, Robinson's or Heinrici's (hot) air engine. Then, in the 1940s, the
Philips company was searching for a suitable name for its own version of the 'air engine', which
by that time had already been tested with other gases, eventually settling on 'Stirling engine' in
April 1945.[5]
However, nearly thirty years later Graham Walker was still bemoaning the fact that
such terms as 'hot air engine' continued to be used interchangeably with 'Stirling engine' which
itself was applied widely and indiscriminately. The situation has now improved somewhat, at
least in academic literature, and it is now generally accepted that 'Stirling engine' should refer
exclusively to a closed-cycle regenerative heat engine with a permanently gaseous working fluid,
where closed-cycle is defined as a thermodynamic system in which the working fluid is
permanently contained within the system and regenerative describes the use of a specific type of
internal heat exchanger and thermal store, known as the regenerator. An engine working on the
same principle but using a liquid rather than gaseous fluid existed in 1931 and was called the
Malone heat engine.[6]
It follows from the closed cycle operation that the Stirling engine is an external combustion
engine that isolates its working fluid from the energy input supplied by an external heat source.
There are many possible implementations of the Stirling engine most of which fall into the
category of reciprocating piston engine.
[edit] Functional description
The engine is designed so that the working gas is generally compressed in the colder portion of
the engine and expanded in the hotter portion resulting in a net conversion of heat into work.[7]
An internal Regenerative heat exchanger increases the Stirling engine's thermal efficiency
compared to simpler hot air engines lacking this feature.
[edit] Key components
Cut-away diagram of a rhombic drive beta configuration
Stirling engine design:
Pink – Hot cylinder wall
Dark grey – Cold cylinder wall (with coolant inlet
and outlet pipes in yellow)
Dark green – Thermal insulation separating the
two cylinder ends
Light green – Displacer piston
Dark blue – Power piston
Light blue – Linkage crank and flywheels
Not shown: Heat source and heat sinks. In this design the
displacer piston is constructed without a purpose-built
regenerator.
As a consequence of closed cycle operation the heat that drives a Stirling engine must be
transmitted from a heat source to the working fluid by heat exchangers and finally to a heat sink.
A Stirling engine system has at least one heat source, one heat sink and up to five heat
exchangers. Some types may combine or dispense with some of these.
[edit] Heat source
Point focus parabolic mirror with Stirling engine at its center and its solar tracker at Plataforma
Solar de Almería (PSA) in Spain
The heat source may be combustion of a fuel and, since the combustion products do not mix with
the working fluid (that is, external combustion) and come into contact with the internal moving
parts of the engine, a Stirling engine can run on fuels that would damage other (that is, internal
combustion) engines' internals, such as landfill gas which contains siloxane.
Some other suitable heat sources are concentrated solar energy, geothermal energy, nuclear
energy, waste heat, or even biological. If the heat source is solar power, regular solar mirrors and
solar dishes may be used. Also, fresnel lenses have been advocated to be used (for example, for
planetary surface exploration).[8]
Solar powered Stirling engines are becoming increasingly
popular, as they are a very environmentally sound option for producing power. Also, some
designs are economically attractive in development projects.[9]
[edit] Recuperator
An optional heat exchanger is the recuperator used when high efficiency is desired from
combustion fuel input to mechanical power output. As the heater of a fuel-fired engine with high
efficiency must operate at a nearly uniform high temperature, there is considerable heat loss from
the combustion gases exiting the burner unless this can be cooled by preheating the air needed
for combustion. Engines used within combined heat and power systems can instead cool the
exhaust gases at the "cold" side of the engine.
[edit] Heater
In small, low power engines this may simply consist of the walls of the hot space(s) but where
larger powers are required a greater surface area is needed in order to transfer sufficient heat.
Typical implementations are internal and external fins or multiple small bore tubes
Designing Stirling engine heat exchangers is a balance between high heat transfer with low
viscous pumping losses and low dead space. With engines operating at high powers and
pressures, the heat exchangers on the hot side must be made of alloys retaining considerable
strength at temperature and also not corrode or creep.
[edit] Regenerator
Main article: Regenerative heat exchanger
In a Stirling engine, the regenerator is an internal heat exchanger and temporary heat store placed
between the hot and cold spaces such that the working fluid passes through it first in one
direction then the other. Its function is to retain within the system that heat which would
otherwise be exchanged with the environment at temperatures intermediate to the maximum and
minimum cycle temperatures,[10]
thus enabling the thermal efficiency of the cycle to approach the
limiting Carnot efficiency defined by those maxima and minima.
The primary effect of regeneration in a Stirling engine is to greatly increase the thermal
efficiency by 'recycling' internally heat which would otherwise pass through the engine
irreversibly. As a secondary effect, increased thermal efficiency promises a higher power output
from a given set of hot and cold end heat exchangers (since it is these which usually limit the
engine's heat throughput), though, in practice this additional power may not be fully realized as
the additional "dead space" (unswept volume) and pumping loss inherent in practical
regenerators tends to have the opposite effect.
The regenerator works like a thermal capacitor. The ideal regenerator has very high thermal
capacity, very low thermal conductivity, almost no volume, and introduces no friction to the
working fluid. As the regenerator approaches these ideal limits, Stirling engine efficiency
increases.[11]
The design challenge for a Stirling engine regenerator is to provide sufficient heat transfer
capacity without introducing too much additional internal volume ('dead space') or flow
resistance, both of which tend to reduce power and efficiency. These inherent design conflicts
are one of many factors which limit the efficiency of practical Stirling engines. A typical design
is a stack of fine metal wire meshes, with low porosity to reduce dead space, and with the wire
axes perpendicular to the gas flow to reduce conduction in that direction and to maximize
convective heat transfer.[12]
The regenerator is the key component invented by Robert Stirling and its presence distinguishes
a true Stirling engine from any other closed cycle hot air engine. However, many engines with no
apparent regenerator may still be correctly described as Stirling engines as in the simple beta and
gamma configurations with a 'loose fitting' displacer, the surfaces of the displacer and its
cylinder will cyclically exchange heat with the working fluid providing a significant regenerative
effect particularly in small, low-pressure engines. The same is true of the passage connecting the
hot and cold cylinders of an alpha configuration engine.
[edit] Cooler
In small, low power engines this may simply consist of the walls of the cold space(s), but where
larger powers are required a cooler using a liquid like water is needed in order to transfer
sufficient heat.
[edit] Heat sink
The heat sink is typically the environment at ambient temperature. In the case of medium to high
power engines, a radiator is required to transfer the heat from the engine to the ambient air.
Marine engines can use the ambient water. In the case of combined heat and power systems, the
engine's cooling water is used directly or indirectly for heating purposes.
Alternatively, heat may be supplied at ambient and the heat sink maintained at a lower
temperature by such means as cryogenic fluid (see Liquid nitrogen economy) or ice water.
[edit] Configurations
There are two major types of Stirling engines that are distinguished by the way they move the air
between the hot and cold sides of the cylinder:
1. The two piston alpha type design has pistons in independent cylinders, and gas is driven
between the hot and cold spaces.
2. The displacement type Stirling engines, known as beta and gamma types, use an
insulated mechanical displacer to push the working gas between the hot and cold sides of
the cylinder. The displacer is large enough to thermally insulate the hot and cold sides of
the cylinder and displace a large quantity of gas. It must have enough of a gap between
the displacer and the cylinder wall to allow gas to easily flow around the displacer.
[edit] Alpha Stirling
An alpha Stirling contains two power pistons in separate cylinders, one hot and one cold. The
hot cylinder is situated inside the high temperature heat exchanger and the cold cylinder is
situated inside the low temperature heat exchanger. This type of engine has a high power-to-
volume ratio but has technical problems due to the usually high temperature of the hot piston and
the durability of its seals.[13]
In practice, this piston usually carries a large insulating head to
move the seals away from the hot zone at the expense of some additional dead space.
[edit] Action of an alpha type Stirling engine
The following diagrams do not show internal heat exchangers in the compression and expansion
spaces, which are needed to produce power. A regenerator would be placed in the pipe
connecting the two cylinders. The crankshaft has also been omitted.
1. Most of the working gas is in contact with the hot
cylinder walls, it has been heated and expansion has
pushed the cold piston to the bottom of its travel in the
cylinder. The expansion continues in the hot cylinder,
which is 90° behind the cold piston in its cycle,
extracting more work from the hot gas.
2. The gas is now at its maximum volume. The hot
cylinder piston begins to move most of the gas into the
cold cylinder, where it cools and the pressure drops.
3. Almost all the gas is now in the cold cylinder and
cooling continues. The cold piston, powered by flywheel
momentum (or other piston pairs on the same shaft)
compresses the remaining part of the gas.
4. The gas reaches its minimum volume, and it will now
expand in the hot cylinder where it will be heated once
more, driving the hot piston in its power stroke.
The complete alpha type Stirling cycle
[edit] Beta Stirling
A beta Stirling has a single power piston arranged within the same cylinder on the same shaft as
a displacer piston. The displacer piston is a loose fit and does not extract any power from the
expanding gas but only serves to shuttle the working gas from the hot heat exchanger to the cold
heat exchanger. When the working gas is pushed to the hot end of the cylinder it expands and
pushes the power piston. When it is pushed to the cold end of the cylinder it contracts and the
momentum of the machine, usually enhanced by a flywheel, pushes the power piston the other
way to compress the gas. Unlike the alpha type, the beta type avoids the technical problems of
hot moving seals.[14]
[edit] Action of a beta type Stirling engine
Again, the following diagrams do not show internal heat exchangers or a regenerator, which
would be placed in the gas path around the displacer.
1. Power piston (dark
grey) has compressed
the gas, the displacer
piston (light grey) has
moved so that most of
the gas is adjacent to
the hot heat exchanger.
2. The heated gas
increases in pressure
and pushes the power
piston to the farthest
limit of the power
stroke.
3. The displacer
piston now moves,
shunting the gas to
the cold end of the
cylinder.
4. The cooled gas is
now compressed by
the flywheel
momentum. This takes
less energy, since
when it is cooled its
pressure dropped.
The complete beta type Stirling cycle
[edit] Gamma Stirling
A gamma Stirling is simply a beta Stirling in which the power piston is mounted in a separate
cylinder alongside the displacer piston cylinder, but is still connected to the same flywheel. The
gas in the two cylinders can flow freely between them and remains a single body. This
configuration produces a lower compression ratio but is mechanically simpler and often used in
multi-cylinder Stirling engines.
[edit] Other types
Other Stirling configurations continue to interest engineers and inventors. Tom Peat conceived of
a configuration that he likes to call a "Delta" type, although currently this designation is not
widely recognized, having a displacer and two power pistons, one hot and one cold.[15]
There is also the rotary Stirling engine which seeks to convert power from the Stirling cycle
directly into torque, similar to the rotary combustion engine. No practical engine has yet been
built but a number of concepts, models and patents have been produced, such as the Quasiturbine
engine.[16]
Another alternative is the Fluidyne engine (Fluidyne heat pump), which use hydraulic pistons
to implement the Stirling cycle. The work produced by a Fluidyne engine goes into pumping the
liquid. In its simplest form, the engine contains a working gas, a liquid and two non-return
valves.
The Ringbom engine concept published in 1907 has no rotary mechanism or linkage for the
displacer. This is instead driven by a small auxiliary piston, usually a thick displacer rod, with
the movement limited by stops.[17]
[edit] Free piston engines
Various Free-Piston Stirling Configurations... F."free cylinder", G. Fluidyne, H. "double-acting"
Stirling (typically 4 cylinders)
"Free piston" Stirling engines include those with liquid pistons and those with diaphragms as
pistons. In a "free piston" device, energy may be added or removed by an electrical linear
alternator, pump or other coaxial device. This sidesteps the need for a linkage, and reduces the
number of moving parts. In some designs friction and wear are nearly eliminated by the use of
non-contact gas bearings or very precise suspension through planar springs.
In the early 1960s, W.T. Beale invented a free piston version of the Stirling engine in order to
overcome the difficulty of lubricating the crank mechanism.[18]
While the invention of the basic
free piston Stirling engine is generally attributed to Beale, independent inventions of similar
types of engines were made by E.H. Cooke-Yarborough and C. West at the Harwell Laboratories
of the UKAERE.[19]
G.M. Benson also made important early contributions and patented many
novel free-piston configurations.[20]
What appears to be the first mention of a Stirling cycle machine using freely moving components
is a British patent disclosure in 1876.[21]
This machine was envisaged as a refrigerator (i.e., the
reversed Stirling cycle). The first consumer product to utilize a free piston Stirling device was a
portable refrigerator manufactured by Twinbird Corporation of Japan and offered in the US by
Coleman in 2004.
[edit] Thermoacoustic cycle
Thermoacoustic devices are very different from Stirling devices, although the individual path
travelled by each working gas molecule does follow a real Stirling cycle. These devices include
the thermoacoustic engine and thermoacoustic refrigerator. High-amplitude acoustic standing
waves cause compression and expansion analogous to a Stirling power piston, while out-of-
phase acoustic travelling waves cause displacement along a temperature gradient, analogous to a
Stirling displacer piston. Thus a thermoacoustic device typically does not have a displacer, as
found in a beta or gamma Stirling.
[edit] History
Illustration to Robert Stirling's 1816 patent application of the air engine design which later came
to be known as the Stirling Engine
The Stirling engine (or Stirling's air engine as it was known at the time) was invented and
patented by Robert Stirling in 1816.[22]
It followed earlier attempts at making an air engine but
was probably the first to be put to practical use when in 1818 an engine built by Stirling was
employed pumping water in a quarry.[23]
The main subject of Stirling's original patent was a heat
exchanger which he called an "economiser" for its enhancement of fuel economy in a variety of
applications. The patent also described in detail the employment of one form of the economiser
in his unique closed-cycle air engine design[24]
in which application it is now generally known as
a 'regenerator'. Subsequent development by Robert Stirling and his brother James, an engineer,
resulted in patents for various improved configurations of the original engine including
pressurization which had by 1843 sufficiently increased power output to drive all the machinery
at a Dundee iron foundry.[25]
Though it has been disputed[26]
it is widely supposed that as well as saving fuel the inventors
were motivated to create a safer alternative to the steam engines of the time,[27]
whose boilers
frequently exploded causing many injuries and fatalities.[28][29]
The need for Stirling engines to
run at very high temperatures to maximize power and efficiency exposed limitations in the
materials of the day and the few engines that were built in those early years suffered
unacceptably frequent failures (albeit with far less disastrous consequences than a boiler
explosion[30]
) - for example, the Dundee foundry engine was replaced by a steam engine after
three hot cylinder failures in four years.[31]
[edit] Later nineteenth century
A typical late nineteenth/early twentieth century water pumping engine by the Rider-Ericsson
Engine Company
Subsequent to the failure of the Dundee foundry engine there is no record of the Stirling brothers
having any further involvement with air engine development and the Stirling engine never again
competed with steam as an industrial scale power source (steam boilers were becoming safer[32]
and steam engines more efficient, thus presenting less of a target to rival prime movers).
However, from about 1860 smaller engines of the Stirling/hot air type were produced in
substantial numbers finding applications wherever a reliable source of low to medium power was
required, such as raising water or providing air for church organs.[33]
These generally operated at
lower temperatures so as not to tax available materials, so were relatively inefficient. But their
selling point was that, unlike a steam engine, they could be operated safely by anybody capable
of managing a fire.[34]
Several types remained in production beyond the end of the century, but
apart from a few minor mechanical improvements the design of the Stirling engine in general
stagnated during this period.[35]
[edit] Twentieth century revival
During the early part of the twentieth century the role of the Stirling engine as a "domestic
motor"[36]
was gradually taken over by the electric motor and small internal combustion engines.
By the late 1930s it was largely forgotten, only produced for toys and a few small ventilating
fans.[37]
At this time Philips was seeking to expand sales of its radios into areas where electricity
was unavailable and the supply of batteries uncertain. Philips' management decided that a low-
power portable generator would facilitate such sales and tasked a group of engineers at the
company's research lab in Eindhoven to evaluate alternatives.
After a systematic comparison of various prime movers, the Stirling engine's quiet operation
(both audibly and in terms of radio interference) and ability to run on a variety of heat sources
(common lamp oil – "cheap and available everywhere" – was favoured), the team picked
Stirling.[38]
They were also aware that, unlike steam and internal combustion engines, virtually
no serious development work had been carried out on the Stirling engine for many years and
asserted that modern materials and know-how should enable great improvements.[39]
Philips MP1002CA Stirling generator of 1951
Encouraged by their first experimental engine, which produced 16 W of shaft power from a bore
and stroke of 30mm × 25mm,[40]
Philips began a development program. This work continued
throughout World War II and by the late 1940s handed over the Type 10 to Philips' subsidiary
Johan de Witt in Dordrecht to be "productionised" and incorporated into a generator set. The
result, rated at 200 W from a bore and stroke of 55 mm x 27 mm, was designated MP1002CA
(known as the "Bungalow set"). Production of an initial batch of 250 began in 1951, but it
became clear that they could not be made at a competitive price and the advent of transistor
radios with their much lower power requirements meant that the original rationale for the set was
disappearing. Approximately 150 of these sets were eventually produced.[41]
Some found their
way into university and college engineering departments around the world[42]
giving generations
of students a valuable introduction to the Stirling engine.
Philips went on to develop experimental Stirling engines for a wide variety of applications and
continued to work in the field until the late 1970s, but only achieved commercial success with
the 'reversed Stirling engine' cryocooler. They did however take out a large number of patents
and amass a wealth of information which they licensed to other companies and which formed the
basis of much of the development work in the modern era.[43]
Towards the end of the century, several companies developed research prototypes of medium-
power engines and in some cases small production series. A mass market was never achieved
because the unit costs were very high and some technical problems remained unsolved. Now in
the twenty-first century, some commercial success is starting to become feasible, notably with
combined heat and power units.
In the field of low-power engines, many plans, kits and finished engines are available
commercially. Apart from traditional small models and some larger machines for real use, a new
type was introduced in the 1980s: the low-temperature flat plate type.
[edit] Theory
Main article: Stirling cycle
A pressure/volume graph of the idealized Stirling cycle
The idealised Stirling cycle consists of four thermodynamic processes acting on the working
fluid:
Isothermal Expansion. The expansion-space and associated heat exchanger are
maintained at a constant high temperature, and the gas undergoes near-isothermal
expansion absorbing heat from the hot source.
Constant-Volume (known as isovolumetric or isochoric) heat-removal. The gas is passed
through the regenerator, where it cools transferring heat to the regenerator for use in the
next cycle.
Isothermal Compression. The compression space and associated heat exchanger are
maintained at a constant low temperature so the gas undergoes near-isothermal
compression rejecting heat to the cold sink
Constant-Volume (known as isovolumetric or isochoric) heat-addition. The gas passes
back through the regenerator where it recovers much of the heat transferred in 2 to 3,
heating up on its way to the expansion space.
Theoretical thermal efficiency equals that of the hypothetical Carnot cycle - i.e. the highest
efficiency attainable by any heat engine. However, though it is useful for illustrating general
principles, the text book cycle it is a long way from representing what is actually going on inside
a practical Stirling engine and should not be regarded as a basis for analysis. In fact it has been
argued that its indiscriminate use in many standard books on engineering thermodynamics has
done a disservice to the study of Stirling engines in general.[44][45]
Other real-world issues reduce the efficiency of actual engines, due to limits of convective heat
transfer, and viscous flow (friction). There are also practical mechanical considerations, for
instance a simple kinematic linkage may be favoured over a more complex mechanism needed to
replicate the idealized cycle, and limitations imposed by available materials such as non-ideal
properties of the working gas, thermal conductivity, tensile strength, creep, rupture strength, and
melting point.
[edit] Operation
Since the Stirling engine is a closed cycle, it contains a fixed mass of gas called the "working
fluid", most commonly air, hydrogen or helium. In normal operation, the engine is sealed and no
gas enters or leaves the engine. No valves are required, unlike other types of piston engines. The
Stirling engine, like most heat engines, cycles through four main processes: cooling,
compression, heating and expansion. This is accomplished by moving the gas back and forth
between hot and cold heat exchangers, often with a regenerator between the heater and cooler.
The hot heat exchanger is in thermal contact with an external heat source, such as a fuel burner,
and the cold heat exchanger being in thermal contact with an external heat sink, such as air fins.
A change in gas temperature will cause a corresponding change in gas pressure, while the motion
of the piston causes the gas to be alternately expanded and compressed.
The gas follows the behaviour described by the gas laws which describe how a gas' pressure,
temperature and volume are related. When the gas is heated, because it is in a sealed chamber,
the pressure rises and this then acts on the power piston to produce a power stroke. When the gas
is cooled the pressure drops and this means that less work needs to be done by the piston to
compress the gas on the return stroke, thus yielding a net power output.
When one side of the piston is open to the atmosphere, the operation is slightly different. As the
sealed volume of working gas comes in contact with the hot side, it expands, doing work on both
the piston and on the atmosphere. When the working gas contacts the cold side, its pressure
drops below atmospheric pressure and the atmosphere pushes on the piston and does work on the
gas.
To summarize, the Stirling engine uses the temperature difference between its hot end and cold
end to establish a cycle of a fixed mass of gas, heated and expanded, and cooled and compressed,
thus converting thermal energy into mechanical energy. The greater the temperature difference
between the hot and cold sources, the greater the thermal efficiency. The maximum theoretical
efficiency is equivalent to the Carnot cycle, however the efficiency of real engines is less than
this value due to friction and other losses.
Video showing the compressor and displacer of a very small Stirling Engine in action
Very low-power engines have been built which will run on a temperature difference of as little as
0.5 K.[46]
[edit] Pressurization
In most high power Stirling engines, both the minimum pressure and mean pressure of the
working fluid are above atmospheric pressure. This initial engine pressurization can be realized
by a pump, or by filling the engine from a compressed gas tank, or even just by sealing the
engine when the mean temperature is lower than the mean operating temperature. All of these
methods increase the mass of working fluid in the thermodynamic cycle. All of the heat
exchangers must be sized appropriately to supply the necessary heat transfer rates. If the heat
exchangers are well designed and can supply the heat flux needed for convective heat transfer,
then the engine will in a first approximation produce power in proportion to the mean pressure,
as predicted by the West number, and Beale number. In practice, the maximum pressure is also
limited to the safe pressure of the pressure vessel. Like most aspects of Stirling engine design,
optimization is multivariate, and often has conflicting requirements.[47]
[edit] Lubricants and friction
A modern Stirling engine and generator set with 55 kW electrical output, for combined heat and
power applications
At high temperatures and pressures, the oxygen in air-pressurized crankcases, or in the working
gas of hot air engines, can combine with the engine's lubricating oil and explode. At least one
person has died in such an explosion.[48]
Lubricants can also clog heat exchangers, especially the regenerator. For these reasons, designers
prefer non-lubricated, low-coefficient of friction materials (such as rulon or graphite), with low
normal forces on the moving parts, especially for sliding seals. Some designs avoid sliding
surfaces altogether by using diaphragms for sealed pistons. These are some of the factors that
allow Stirling engines to have lower maintenance requirements and longer life than internal-
combustion engines.
[edit] Analysis
[edit] Comparison with internal combustion engines
In contrast to internal combustion engines, Stirling engines have the potential to use renewable
heat sources more easily, to be quieter, and to be more reliable with lower maintenance. They are
preferred for applications that value these unique advantages, particularly if the cost per unit
energy generated ($/kWh) is more important than the capital cost per unit power ($/kW). On this
basis, Stirling engines are cost competitive up to about 100 kW.[49]
Compared to an internal combustion engine of the same power rating, Stirling engines currently
have a higher capital cost and are usually larger and heavier. However, they are more efficient
than most internal combustion engines.[50]
Their lower maintenance requirements make the
overall energy cost comparable. The thermal efficiency is also comparable (for small engines),
ranging from 15% to 30%.[49]
For applications such as micro-CHP, a Stirling engine is often
preferable to an internal combustion engine. Other applications include water pumping,
astronautics, and electrical generation from plentiful energy sources that are incompatible with
the internal combustion engine, such as solar energy, and biomass such as agricultural waste and
other waste such as domestic refuse. Stirlings have also been used as a marine engine in Swedish
Gotland class submarines.[51]
However, Stirling engines are generally not price-competitive as an
automobile engine, due to high cost per unit power, low power density and high material costs.
Basic analysis is based on the closed-form Schmidt analysis.[52][53]
[edit] Advantages
Stirling engines can run directly on any available heat source, not just one produced by
combustion, so they can run on heat from solar, geothermal, biological, nuclear sources
or waste heat from industrial processes.
A continuous combustion process can be used to supply heat, so most types of emissions
can be reduced.
Most types of Stirling engines have the bearing and seals on the cool side of the engine,
and they require less lubricant and last longer than other reciprocating engine types.
The engine mechanisms are in some ways simpler than other reciprocating engine types.
No valves are needed, and the burner system can be relatively simple.
A Stirling engine uses a single-phase working fluid which maintains an internal pressure
close to the design pressure, and thus for a properly designed system the risk of explosion
is low. In comparison, a steam engine uses a two-phase gas/liquid working fluid, so a
faulty relief valve can cause an explosion.
In some cases, low operating pressure allows the use of lightweight cylinders.
They can be built to run quietly and without an air supply, for air-independent propulsion
use in submarines.
They start easily (albeit slowly, after warmup) and run more efficiently in cold weather,
in contrast to the internal combustion which starts quickly in warm weather, but not in
cold weather.
A Stirling engine used for pumping water can be configured so that the water cools the
compression space. This is most effective when pumping cold water.
They are extremely flexible. They can be used as CHP (combined heat and power) in the
winter and as coolers in summer.
Waste heat is easily harvested (compared to waste heat from an internal combustion
engine) making Stirling engines useful for dual-output heat and power systems.
[edit] Disadvantages
[edit] Size and cost issues
Stirling engine designs require heat exchangers for heat input and for heat output, and
these must contain the pressure of the working fluid, where the pressure is proportional to
the engine power output. In addition, the expansion-side heat exchanger is often at very
high temperature, so the materials must resist the corrosive effects of the heat source, and
have low creep (deformation). Typically these material requirements substantially
increase the cost of the engine. The materials and assembly costs for a high temperature
heat exchanger typically accounts for 40% of the total engine cost.[48]
All thermodynamic cycles require large temperature differentials for efficient operation.
In an external combustion engine, the heater temperature always equals or exceeds the
expansion temperature. This means that the metallurgical requirements for the heater
material are very demanding. This is similar to a Gas turbine, but is in contrast to an Otto
engine or Diesel engine, where the expansion temperature can far exceed the
metallurgical limit of the engine materials, because the input heat source is not conducted
through the engine, so engine materials operate closer to the average temperature of the
working gas.
Dissipation of waste heat is especially complicated because the coolant temperature is
kept as low as possible to maximize thermal efficiency. This increases the size of the
radiators, which can make packaging difficult. Along with materials cost, this has been
one of the factors limiting the adoption of Stirling engines as automotive prime movers.
For other applications such as ship propulsion and stationary microgeneration systems
using combined heat and power (CHP) high power density is not required.[54]
[edit] Power and torque issues
Stirling engines, especially those that run on small temperature differentials, are quite
large for the amount of power that they produce (i.e., they have low specific power). This
is primarily due to the heat transfer coefficient of gaseous convection which limits the
heat flux that can be attained in a typical cold heat exchanger to about 500 W/(m2·K), and
in a hot heat exchanger to about 500–5000 W/(m2·K).
[47] Compared with internal
combustion engines, this makes it more challenging for the engine designer to transfer
heat into and out of the working gas. Increasing the temperature differential and/or
pressure allows Stirling engines to produce more power, assuming the heat exchangers
are designed for the increased heat load, and can deliver the convected heat flux
necessary.
A Stirling engine cannot start instantly; it literally needs to "warm up". This is true of all
external combustion engines, but the warm up time may be longer for Stirlings than for
others of this type such as steam engines. Stirling engines are best used as constant speed
engines.
Power output of a Stirling tends to be constant and to adjust it can sometimes require
careful design and additional mechanisms. Typically, changes in output are achieved by
varying the displacement of the engine (often through use of a swashplate crankshaft
arrangement), or by changing the quantity of working fluid, or by altering the
piston/displacer phase angle, or in some cases simply by altering the engine load. This
property is less of a drawback in hybrid electric propulsion or "base load" utility
generation where constant power output is actually desirable.
[edit] Gas choice issues
The used gas should have a low heat capacity, so that a given amount of transferred heat leads to
a large increase in pressure. Considering this issue, Helium would be the best gas because of its
very low heat capacity. Air is a viable working fluid,[55]
but the oxygen in a highly pressurized
air engine can cause fatal accidents caused by lubricating oil explosions.[48]
Following one such
accident Philips pioneered the use of other gases to avoid such risk of explosions.
Hydrogen's low viscosity and high thermal conductivity make it the most powerful
working gas, primarily because the engine can run faster than with other gases. However,
due to hydrogen absorption, and given the high diffusion rate associated with this low
molecular weight gas, particularly at high temperatures, H2 will leak through the solid
metal of the heater. Diffusion through carbon steel is too high to be practical, but may be
acceptably low for metals such as aluminum, or even stainless steel. Certain ceramics
also greatly reduce diffusion. Hermetic pressure vessel seals are necessary to maintain
pressure inside the engine without replacement of lost gas. For HTD engines, auxiliary
systems may need to be added to maintain high pressure working fluid. These systems
can be a gas storage bottle or a gas generator. Hydrogen can be generated by electrolysis
of water, the action of steam on red hot carbon-based fuel, by gasification of hydrocarbon
fuel, or by the reaction of acid on metal. Hydrogen can also cause the embrittlement of
metals. Hydrogen is a flammable gas, which is a safety concern, although the quantity
used is very small, and it is arguably safer than other commonly used flammable gases.
Most technically advanced Stirling engines, like those developed for United States
government labs, use helium as the working gas, because it functions close to the
efficiency and power density of hydrogen with fewer of the material containment issues.
Helium is inert, which removes all risk of flammability, both real and perceived. Helium
is relatively expensive, and must be supplied as bottled gas. One test showed hydrogen to
be 5% (absolute) more efficient than helium (24% relatively) in the GPU-3 Stirling
engine.[56]
The researcher Allan Organ demonstrated that a well-designed air engine is
theoretically just as efficient as a helium or hydrogen engine, but helium and hydrogen
engines are several times more powerful per unit volume.
Some engines use air or nitrogen as the working fluid. These gases have much lower
power density (which increases engine costs), but they are more convenient to use and
they minimize the problems of gas containment and supply (which decreases costs). The
use of compressed air in contact with flammable materials or substances such as
lubricating oil, introduces an explosion hazard, because compressed air contains a high
partial pressure of oxygen. However, oxygen can be removed from air through an
oxidation reaction or bottled nitrogen can be used, which is nearly inert and very safe.
Other possible lighter-than-air gases include: methane, and ammonia.
[edit] Applications
It has been suggested that this section be split into a new article titled applications of the
Stirling engine. (Discuss)
A desktop alpha Stirling engine. The working fluid in this engine is air. The hot heat exchange is
the glass cylinder on the right, and the cold heat exchanger is the finned cylinder on the top. This
engine uses a small alcohol burner (bottom right) as a heat source
[edit] Heating and cooling
If supplied with mechanical power, a Stirling engine can function in reverse as a heat pump for
heating or cooling. Experiments have been performed using wind power driving a Stirling cycle
heat pump for domestic heating and air conditioning. In the late 1930s, the Philips Corporation
of the Netherlands successfully utilized the Stirling cycle in cryogenic applications.[57]
[edit] Combined heat and power
Thermal power stations on the electric grid use fuel to produce electricity, however there are
large quantities of waste heat produced which often go unused. In other situations, high-grade
fuel is burned at high temperature for a low temperature application. According to the second
law of thermodynamics, a heat engine can generate power from this temperature difference. In a
CHP system, the high temperature primary heat enters the Stirling engine heater, then some of
the energy is converted to mechanical power in the engine, and the rest passes through to the
cooler, where it exits at a low temperature. The "waste" heat actually comes from engine's main
cooler, and possibly from other sources such as the exhaust of the burner, if there is one.
In a combined heat and power (CHP) system, mechanical or electrical power is generated in the
usual way, however, the waste heat given off by the engine is used to supply a secondary heating
application. This can be virtually anything that uses low temperature heat. It is often a pre-
existing energy use, such as commercial space heating, residential water heating, or an industrial
process.
The power produced by the engine can be used to run an industrial or agricultural process, which
in turn creates biomass waste refuse that can be used as free fuel for the engine, thus reducing
waste removal costs. The overall process can be efficient and cost effective.
Disenco, a UK based company are going through the final stages of development of their
HomePowerPlant. Unlike other m-CHP appliances coming to market the HPP generates 3 kW of
electrical and 15 kW of thermal energy, making this appliance suitable for both the domestic and
SME markets.
WhisperGen, a New Zealand firm with offices in Christchurch, has developed an "AC Micro
Combined Heat and Power" Stirling cycle engine. These microCHP units are gas-fired central
heating boilers which sell unused power back into the electricity grid. WhisperGen announced in
2004 that they were producing 80,000 units for the residential market in the United Kingdom. A
20 unit trial in Germany started in 2006.[58]
[edit] Solar power generation
Placed at the focus of a parabolic mirror a Stirling engine can convert solar energy to electricity
with an efficiency better than non-concentrated photovoltaic cells, and comparable to
Concentrated Photo Voltaics. On August 11, 2005, Southern California Edison announced[59]
an
agreement to purchase solar powered Stirling engines from Stirling Energy Systems over a
twenty year period and in quantity (20,000 units) sufficient to generate 850 MW of electricity.
These systems, on a 4,500 acre (19 km2) solar farm, will use mirrors to direct and concentrate
sunlight onto the engines which will in turn drive generators.
[edit] Stirling cryocoolers
Any Stirling engine will also work in reverse as a heat pump; when a motion is applied to the
shaft, a temperature difference appears between the reservoirs. The essential mechanical
components of a Stirling cryocooler are identical to a Stirling engine. In both the engine and the
heat pump, heat flows from the expansion space to the compression space; however, input work
is required in order for heat to flow against a thermal gradient, specifically when the compression
space is hotter than the expansion space. The external side of the expansion-space heat
exchanger may be placed inside a thermally insulated compartment such as a vacuum flask. Heat
is in effect pumped out of this compartment, through the working gas of the cryocooler and into
the compression space. The compression space will be above ambient temperature, and so heat
will flow out into the environment.
One of their modern uses is in cryogenics, and to a lesser extent, refrigeration. At typical
refrigeration temperatures, Stirling coolers are generally not economically competitive with the
less expensive mainstream Rankine cooling systems, even though they are typically 20% more
energy efficient. However, below about −40 ° to −30 °C, Rankine cooling is not effective
because there are no suitable refrigerants with boiling points this low. Stirling cryocoolers are
able to "lift" heat down to −200 °C (73 K), which is sufficient to liquefy air (oxygen, nitrogen
and argon). They can go as low as 40–60 K, depending on the particular design. Cryocoolers for
this purpose are more or less competitive with other cryocooler technologies. The coefficient of
performance at cryogenic temperatures is typically 0.04–0.05 (corresponding to a 4–5%
efficiency). Empirically, the devices show a linear trend, where typically the COP = 0.0015 ×
Tc – 0.065, where Tc is the cryogenic temperature. At these temperatures, solid materials have
lower values for specific heat, so the regenerator must be made out of unexpected materials, such
as cotton.[citation needed]
The first Stirling cycle cryocooler was developed at Philips in the 1950s and commercialized in
such places as liquid air production plants. The Philips Cryogenics business evolved until it was
split off in 1990 to form the Stirling Cryogenics BV, The Netherlands. This company is still
active in the development and manufacturing of Stirling cryocoolers and cryogenic cooling
systems.
A wide variety of smaller size Stirling cryocoolers are commercially available for tasks such as
the cooling of electronic sensors and sometimes microprocessors. For this application, Stirling
cryocoolers are the highest performance technology available, due to their ability to lift heat
efficiently at very low temperatures. They are silent, vibration-free, and can be scaled down to
small sizes, and have very high reliability and low maintenance. As of 2009, cryocoolers are
considered to be the only commercially successful Stirling devices.[citation needed]
[edit] Heat pump
A Stirling heat pump is very similar to a Stirling cryocooler, the main difference being that it
usually operates at room temperature and its principal application to date is to pump heat from
the outside of a building to the inside, thus cheaply heating it.
As with any other Stirling device, heat flows from the expansion space to the compression space;
however, in contrast to the Stirling engine, the expansion space is at a lower temperature than the
compression space, so instead of producing work, an input of mechanical work is required by the
system (in order to satisfy the second law of thermodynamics). When the mechanical work for
the heat pump is provided by a second Stirling engine, then the overall system is called a "heat-
driven heatpump".
The expansion side of the heat pump is thermally coupled to the heat source, which is often the
external environment. The compression side of the Stirling device is placed in the environment
to be heated, for example a building, and heat is "pumped" into it. Typically there will be thermal
insulation between the two sides so there will be a temperature rise inside the insulated space.
Heat pumps are by far the most energy-efficient types of heating systems. Stirling heat pumps
also often have a higher coefficient of performance than conventional heat pumps. To date, these
systems have seen limited commercial use; however, use is expected to increase along with
market demand for energy conservation, and adoption will likely be accelerated by technological
refinements.
[edit] Marine engines
The Swedish shipbuilder Kockums has built 8 successful Stirling powered submarines since the
late 1980s.[51]
They carry compressed oxygen to allow fuel combustion whilst submerged that
provides heat for the Stirling engine. They are currently used on submarines of the Gotland and
Södermanland classes. They are the first submarines in the world to feature a Stirling engine air-
independent propulsion (AIP) system, which extends their underwater endurance from a few
days to two weeks.[60]
This capability has previously only been available with nuclear powered
submarines.
A similar system also powers the Japanese Sōryū class submarine.[61]
[edit] Nuclear power
There is a potential for nuclear-powered Stirling engines in electric power generation plants.
Replacing the steam turbines of nuclear power plants with Stirling engines might simplify the
plant, yield greater efficiency, and reduce the radioactive byproducts. A number of breeder
reactor designs use liquid sodium as coolant. If the heat is to be employed in a steam plant, a
water/sodium heat exchanger is required, which raises some concern as sodium reacts violently
with water. A Stirling engine eliminates the need for water anywhere in the cycle.
United States government labs have developed a modern Stirling engine design known as the
Stirling Radioisotope Generator for use in space exploration. It is designed to generate electricity
for deep space probes on missions lasting decades. The engine uses a single displacer to reduce
moving parts and uses high energy acoustics to transfer energy. The heat source is a dry solid
nuclear fuel slug and the heat sink is space itself.
[edit] Automotive engines
It is often claimed that the Stirling engine has too low a power/weight ratio, too high a cost, and
too long a starting time for automotive applications. They also have complex and expensive heat
exchangers. A Stirling cooler must reject twice as much heat as an Otto engine or Diesel engine
radiator. The heater must be made of stainless steel, exotic alloy or ceramic to support high
heater temperatures needed for high power density, and to contain hydrogen gas that is often
used in automotive Stirlings to maximize power. The main difficulties involved in using the
Stirling engine in an automotive application are startup time, acceleration response, shutdown
time, and weight, not all of which have ready-made solutions. However, a modified Stirling
engine has been recently introduced that uses concepts taken from a patented internal-
combustion engine with a sidewall combustion chamber (U.S. patent 7,387,093) that promises to
overcome the deficient power-density and specific-power problems, as well as the slow
acceleration-response problem inherent in all Stirling engines.[62]
However, it could be possible
to use these in co-generation systems that use waste heat from a conventional piston or gas
turbine engine's exhaust and use this either to power the ancillaries (eg: the alternator) or even as
a turbo-compound system that adds power and torque to the crankshaft.
At least two automobiles exclusively powered by Stirling engines were developed by NASA, as
well as earlier projects by the Ford Motor Company and American Motors Corporation. The
NASA vehicles were designed by contractors and designated MOD I and MOD II. The MOD II
replaced the normal spark-ignition engine in a 1985 4-door Chevrolet Celebrity Notchback. In
the 1986 MOD II Design Report (Appendix A) the results show that highway gas mileage was
increased from 40 to 58 mpg and urban mileage from 26 to 33 mpg with no change in vehicle
gross weight. Startup time in the NASA vehicle maxed out at 30 seconds,[citation needed]
while
Ford's research vehicle used an internal electric heater to jump-start the vehicle, allowing it to
start in only a few seconds.
[edit] Electric vehicles
Many people believe that Stirling engines as part of a hybrid electric drive system can bypass all
of the perceived design challenges or disadvantages of a non-hybrid Stirling automobile.
In November 2007, a prototype hybrid car using solid biofuel and a Stirling engine was
announced by the Precer project in Sweden.[63]
The Manchester Union Leader reports that Dean Kamen has developed a series plug-in hybrid
car using a Ford Think.[64]
DEKA, Kamen's technology company in the Manchester Millyard,
has recently demonstrated an electric car, the DEKA Revolt, that can go approximately 60 miles
(97 km) on a single charge of its lithium battery.[64]
[edit] Aircraft engines
Stirling engines may hold theoretical promise as aircraft engines, if high power density and low
cost can be achieved. They are quieter, less polluting, gain efficiency with altitude due to lower
ambient temperatures, are more reliable due to fewer parts and the absence of an ignition system,
produce much less vibration (airframes last longer) and safer, less explosive fuels may be used.
However, the Stirling engine often has low power density compared to the commonly used Otto
engine and Brayton cycle gas turbine. This issue has been a point of contention in automobiles,
and this performance characteristic is even more critical in aircraft engines.
[edit] Low temperature difference engines
A low temperature difference Stirling Engine by American Stirling Company shown here
running on the heat from a warm hand
A low temperature difference (Low Delta T, or LTD) Stirling engine will run on any low
temperature differential, for example the difference between the palm of a hand and room
temperature or room temperature and an ice cube. A record of only 0.5 K was achieved in 1990.
See[65]
which also shows an animated drawing of this type. Usually they are designed in a
gamma configuration, for simplicity, and without a regenerator, although some have slits in the
displacer typically made of foam, for partial regeneration. They are typically unpressurized,
running at pressure close to 1 atmosphere. The power produced is less than 1 W, and they are
intended for demonstration purposes only. They are sold as toys and educational models.
Larger (typically 1 m square) low temperature engines have been built for pumping water using
direct sunlight - not or only slightly concentrated.[66]
[edit] Other recent applications
[edit] Acoustic Stirling Heat Engine
Los Alamos National Laboratory has developed an "Acoustic Stirling Heat Engine"[67]
with no
moving parts. It converts heat into intense acoustic power which (quoted from given source) "can
be used directly in acoustic refrigerators or pulse-tube refrigerators to provide heat-driven
refrigeration with no moving parts, or ... to generate electricity via a linear alternator or other
electro-acoustic power transducer".
[edit] MicroCHP
WhisperGen, a New Zealand based company has developed stirling engines that can be powered
by natural gas or diesel. Recently an agreement has been signed with Mondragon Corporación
Cooperativa, a Spanish firm, to produce WhisperGen's microCHP and make them available for
the domestic market in Europe. Some time ago E.ON UK announced a similar initiative for the
UK. Stirling engines would supply the client with hot water, space heating and a surplus electric
power that could be fed back into the electric grid.
However the preliminary results of an Energy Saving Trust review of the performance of the
WhisperGen microCHP units suggested that their advantages were marginal at best in most
homes.[68]
However another author shows that that Stirling engined microgeneration is the most
cost effective of various microgeneration technologies in terms of reducing CO2.[58]
[edit] Chip cooling
MSI (Taiwan) recently developed a miniature Stirling engine cooling system for personal
computer chips that uses the waste heat from the chip to drive a fan.[69]
[edit] Alternatives
Alternative thermal energy harvesting devices include the Thermogenerator. Thermogenerators
allow less efficient conversion (5-10%) but may be useful in situations where the end product
needs to be electricity and where a small conversion device is a critical factor.
[edit] Photo gallery
Preserved examples of antique Rider hot air engines - an alpha configuration Stirling
[edit] See also
Thermomechanical generator
Beale Number
West Number
Schmidt number
Fluidyne engine
Stirling radioisotope generator
Relative cost of electricity generated by different sources
Distributed generation
[edit] References
1. ^ "Stirling Engines", G. Walker (1980), Clarenden Press, Oxford, page 1: "A Stirling engine is a
mechanical device which operates on a *closed* regenerative thermodynamic cycle, with cyclic
compression and expansion of the working fluid at different temperature levels."
2. ^ T. Finkelstein; A.J. Organ (2001), Chapters 2&3
3. ^ Sleeve notes from A.J. Organ (2007)
4. ^ F. Starr (2001)
5. ^ C.M. Hargreaves (1991), Chapter 2.5
6. ^ "A new Prime Mover", J.F.J. Malone, Journal of the Royal Society of Arts, June 12, 1931,
reprinted with further material as "Secrets of the Malone Heat Engine, Richard A. Ford (1983),
Lindsay Publications, Bradley IL
7. ^ W.R. Martini (1983), p.6
8. ^ W.H. Brandhorst; J.A. Rodiek (2005)
9. ^ B. Kongtragool; S. Wongwises (2003)
10. ^ A.J. Organ (1992), p.58
11. ^ Y. Timoumi; I. Tlili; S. Ben Nasrallah (2007)
12. ^ K. Hirata (1998)
13. ^ M.Keveney (2000a)
14. ^ M. Keveney (2000b)
15. ^ D.Liao (a)
16. ^ Quasiturbine Agence (a)
17. ^ "Ringbom Stirling Engines", James R. Senft, 1993, Oxford University Press
18. ^ "Free-Piston Stirling Engines", G. Walker et al.,Springer 1985, reprinted by Stirling Machine
World, West Richland WA
19. ^ "The Thermo-mechanical Generator...", E.H. Cooke-Yarborough, (1967) Harwell
Memorandum No. 1881 and (1974) Proc. I.E.E., Vol. 7, pp. 749-751
20. ^ G.M. Benson (1973 and 1977)
21. ^ D. Postle (1873)
22. ^ R. Sier (1999)
23. ^ T. Finkelsteinl; A.J. Organ (2001), Chapter 2.2
24. ^ English patent 4081 of 1816 Improvements for diminishing the consumption of fuel and in
particular an engine capable of being applied to the moving (of)machinery on a principle entirely
new. as reproduced in part in C.M. Hargreaves (1991), Appendix B, with full transcription of text
in R. Sier (1995), p.??
25. ^ R. Sier (1995), p. 93
26. ^ A.J. Organ (2008a)
27. ^ Excerpt from a paper presented by James Stirling in June 1845 to the Institute of Civil
Engineers. As reproduced in R. Sier (1995), p.92.
28. ^ A. Nesmith (1985)
29. ^ R. Chuse; B. Carson (1992), Chapter 1
30. ^ R. Sier (1995), p.94
31. ^ T. Finkelstein; A.J. Organ (2001), p.30
32. ^ Hartford Steam Boiler (a)
33. ^ T. Finkelstein; A.J. Organ (2001), Chapter 2.4
34. ^ The 1906 Rider-Ericsson Engine Co. catalog claimed that "any gardener or ordinary domestic
can operate these engines and no licensed or experienced engineer is required".
35. ^ T. Finkelstein; A.J. Organ (2001), p.64
36. ^ T. Finkelstein; A.J. Organ (2001), p.34
37. ^ T. Finkelstein; A.J. Organ (2001), p.55
38. ^ C.M. Hargreaves (1991), pp.28–30
39. ^ Philips Technical Review Vol.9 No.4 page 97 (1947)
40. ^ C.M. Hargreaves (1991), Fig. 3
41. ^ C.M. Hargreaves (1991), p.61
42. ^ Letter dated March 1961 from Research and Control Instruments Ltd. London WC1 to North
Devon Technical College, offering "remaining stocks...... to institutions such as yourselves..... at a
special price of £75 nett"
43. ^ C.M. Hargreaves (1991), p.77
44. ^ T. Finkelstein; A.J. Organ (2001), Page 66 & 229
45. ^ A.J. Organ (1992), Chapter 3.1 - 3.2
46. ^ "An Introduction to Low Temperature Differential Stirling Engines", James R. Senft, 1996,
Moriya Press
47. ^ a b A.J. Organ (1997), p.??
48. ^ a b c C.M. Hargreaves (1991), p.??
49. ^ a b WADE (a)
50. ^ Krupp and Horn. Earth: The Sequel. p. 57
51. ^ a b Kockums (a)
52. ^ Z. Herzog (2008)
53. ^ K. Hirata (1997)
54. ^ BBC News (2003), "The boiler is based on the Stirling engine, dreamed up by the Scottish
inventor Robert Stirling in 1816. [...] The technical name given to this particular use is Micro
Combined Heat and Power or Micro CHP."
55. ^ A.J. Organ (2008b)
56. ^ L.G. Thieme (1981)
57. ^ C.M. Hargreaves (1991), p.63
58. ^ a b by: admin (2008-11-06). "What is Microgeneration? And what is the most cost effective in
terms of CO2 reduction | Claverton Group". Claverton-energy.com. http://www.claverton-
energy.com/what-is-microgeneration.html. Retrieved 2009-07-24.
59. ^ Pure Energy Systems (2005)
60. ^ "The Kockums Stirling AIP system - proven in operational service". Kockums.
http://www.kockums.se/submarines/aipstirling.html. Retrieved 2009-11-12.
61. ^ http://www.janes.com/news/defence/naval/jni/jni071206_1_n.shtml
62. ^ J. Hasci (2008)
63. ^ Precer Group (a)
64. ^ a b S.K. Wickham (2008)
65. ^ http://www.animatedengines.com/ltdstirling.shtml
66. ^ http://www.bsrsolar.com/core1-1.php
67. ^ S. Backhaus; G. Swift (2003)
68. ^ Carbon Trust (2007)
69. ^ MSI (2008)
http://www.tweaktown.com/news/9051/msi_employs_stirling_engine_theory/index.html
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[edit] Further reading
R.C. Belaire (1977). "Device for decreasing the start-up time for stirling engines", US
patent 4057962. Granted to Ford Motor Company, 15 November 1977.
P.H. Ceperley (1979). "A pistonless Stirling engine—The traveling wave heat engine".
Journal of the Acoustical Society of America 66 (5): 1508–1513. doi:10.1121/1.383505.
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of the Volume Ratio Vmax/Vmin". http://home.germany.net/101-276996/etatherm.htm.
Retrieved 2009-01-19.
P. Fette. "A Twice Double Acting α-Type Stirling Engine Able to Work with Compound
Fluids Using Heat Energy of Low to Medium Temperatures".
http://home.germany.net/101-276996/english.htm. Retrieved 2009-01-19.
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Z. Herzog (2006). "Stirling Engines". Mont Alto: Pennsylvania State University.
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(PDF).
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pdf. Retrieved 2009-01-19.
Lund University, Department of Energy Science: Division of Combustion Engines.
"Stirling Engine Research". http://www.vok.lth.se/~ce/Research/stirling/stirling_en.htm.
Retrieved 2009-01-19.
N.P. Nightingale (1986). "NASA Automotive Stirling Engine MOD II Design Report"
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D. Phillips (1904). "Why Aviation Needs the Stirling Engine". http://www.airsport-
corp.com/fourpartstirling.html. Retrieved 2009-01-19.
[edit] External links
Wikimedia Commons has media related to: Stirling engine
Stirling engine at the Open Directory Project
I. Urieli (2008). Stirling Cycle Machine Analysis 2008 Winter Syllabus
Simple Performance Prediction Method for Stirling Engine
v • d • e
Thermodynamic cycles
Cycles normally with
external combustion
Gas cycles without phasechange -
hot air engine cycles
Bell Coleman cycle ·
Brayton/Joule cycle; (Externally
heated) · Carnot cycle · Ericsson
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