Alpha type Stirling engine. There are two cylinders. 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.

Beta Type Stirling Engine. There is only one cylinder, hot at one end and cold at the other. A loose fitting displacer shunts the air between the hot and cold ends of the cylinder. A power piston at the end of the cylinder drives the flywheel.

A Stirling engine is a heat engine operating 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][2]

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.^[3]

Typical of 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. The efficiency of the process has an upper limit set by the efficiency of the equivalent Carnot cycle operating between the same hot and cold temperatures.

Like the steam engine, the Stirling engine is traditionally classified as an external combustion engine, as all heat transfers to and from the working fluid take place through a solid boundary (heat exchanger). This contrasts with an internal combustion engine where heat input is by combustion of a fuel within the body of the working fluid. Unlike a steam engine's (or more generally a Rankine cycle engine's) usage of a working fluid in both its liquid and gaseous phases, the Stirling engine encloses a fixed mass of permanently gaseous fluid such as air.

The Stirling engine is noted for its high efficiency compared to steam engines,^[4] 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.^[5][6]

Name and definition[link]

Robert Stirling was the Scottish inventor of the first practical example of a closed cycle air engine in 1816, and it was suggested by Fleeming Jenkin as early as 1884 that all such engines should therefore generically be called Stirling engines. This naming proposal 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. In the 1940s, the Philips company was seeking a suitable name for its own version of the 'air engine', which by that time had been tested with working fluids other than air, and decided upon 'Stirling engine' in April 1945.^[7] However, nearly thirty years later Graham Walker still had cause to bemoan the fact such terms as 'hot air engine' continued to be used interchangeably with 'Stirling engine', which itself was applied widely and indiscriminately.^[8] The situation has now improved somewhat, at least in academic literature, and it is now generally accepted '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.

It follows from the closed cycle operation 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.

Functional description[link]

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.^[2] An internal Regenerative heat exchanger increases the Stirling engine's thermal efficiency compared to simpler hot air engines lacking this feature.

Key components[link]

Cut-away diagram of a rhombic drive beta configuration Stirling engine design: Pink – Hot cylinder wall Dark grey – Cold cylinder wall Yellow - Coolant inlet and outlet pipes 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 driving 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^{[clarification needed]} heat exchangers. Some types may combine or dispense with some of these.

Heat source[link]

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 provided by the combustion of a fuel and, since the combustion products do not mix with the working fluid and hence do not come into contact with the internal parts of the engine, a Stirling engine can run on fuels that would damage other types of engines' internals, such as landfill gas which contains siloxane.

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 and mirrors have been advocated to be used (for example, for planetary surface exploration).^[9] 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.^[10]

Heater / hot side heat exchanger[link]

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 (unswept internal volume). With engines operating at high powers and pressures, the heat exchangers on the hot side must be made of alloys that retain considerable strength at temperature and that will also not corrode or creep.

Regenerator[link]

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,^[11] 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 increase the thermal efficiency by 'recycling' internal heat which would otherwise pass through the engine irreversibly. As a secondary effect, increased thermal efficiency yields a higher power output from a given set of hot and cold end heat exchangers. It is these which usually limit the engine's heat throughput. In practice this additional power may not be fully realized as the additional "dead space" (unswept volume) and pumping loss inherent in practical regenerators reduces the potential efficiency gains from regeneration.

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. 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. Many small 'toy' Stirling engines, particularly low-temperature difference (LTD) types, do not have a distinct regenerator component and might be considered hot air engines, however a small amount of regeneration is provided by the surface of displacer itself and the nearby cylinder wall, or similarly the passage connecting the hot and cold cylinders of an alpha configuration engine.

Cooler / cold side heat exchanger[link]

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.

Heat sink[link]

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 temperature and the heat sink maintained at a lower temperature by such means as cryogenic fluid (see Liquid nitrogen economy) or iced water.

Displacer[link]

The displacer is a special-purpose piston, used in Beta and Gamma type Stirling engines, to move the working gas back and forth between the hot and cold heat exchangers. Depending on the type of engine design, the displacer may or may not be sealed to the cylinder, i.e. it is a loose fit within the cylinder and allows the working gas to pass around it as it moves to occupy the part of the cylinder beyond.

Configurations[link]

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 insulate the hot and cold sides of the cylinder thermally and to displace a large quantity of gas. It must have enough of a gap between the displacer and the cylinder wall to allow gas to flow around the displacer easily.

Alpha Stirling[link]

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.

Action of an alpha type Stirling engine[link]

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 hot piston to the bottom of its travel in the cylinder. The expansion continues in the cold cylinder, which is 90° behind the hot 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

Beta Stirling[link]

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]

Action of a beta type Stirling engine[link]

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 its pressure drops when it is cooled.
The complete beta type Stirling cycle

Gamma Stirling[link]

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.

Other types[link]

Other Stirling configurations continue to interest engineers and inventors.

The hybrid between piston and rotary configuration is a double acting engine. This design rotates the displacers on either side of the power piston

Top view of two rotating displacer powering the horizontal piston. Regenerators and radiator removed for clarity

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 for example the Quasiturbine engine.^[15]

Another alternative is the Fluidyne engine (Fluidyne heat pump), which uses 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.^[16][17]

The two-cylinder stirling with Ross yoke is a two-cylinder stirling engine (not positioned at 90°, but at 0°) connected with a special yoke. The engine configuration/yoke setup was invented by Andy Ross (engineer)^{[disambiguation needed ]}.^[18]

The Franchot engine is a double acting engine invented by ‘Franchot’ in the nineteenth century. A double acting engine is one where both sides of the piston are acted upon by the pressure of the working fluid. One of the simplest forms of a double acting machine, the Franchot engine consists of two pistons and two cylinders and acts like two separate alpha machines. In the Franchot engine, each piston acts in two gas phases, which makes more efficient use of the mechanical components than a single acting alpha machine. However, a disadvantage of this machine is that one connecting rod must have a sliding seal at the hot side of the engine, which is a difficult task when dealing with high pressures and high temperatures^{[citation needed]}.

Free piston Stirling engines[link]

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 avoids 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.

Four basic steps in the cycle of a “Free piston” Stirling engine,

1. The power piston is pushed outwards by the expanding gas thus doing work. Gravity plays no role in the cycle.
2. The gas volume in the engine increases and therefore the pressure reduces, which will cause a pressure difference across the displacer rod to force the displacer towards the hot end. When the displacer moves the piston is almost stationary and therefore the gas volume is almost constant. This step results in the constant volume cooling process which reduces the pressure of the gas.
3. The reduced pressure now arrests the outward motion of the piston and it begins to accelerate towards the hot end again and by its own inertia, compresses the now cold gas which is mainly in the cold space.
4. As the pressure increases, a point is reached where the pressure differential across the displacer rod becomes large enough to begin to push the displacer rod (and therefore also the displacer) towards the piston and thereby collapsing the cold space and transferring the cold, compressed gas towards the hot side in an almost constant volume process. As the gas arrives in the hot side the pressure increases and begins to move the piston outwards to initiate the expansion step as explained in (1).

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.^[19] 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.^[20] G.M. Benson also made important early contributions and patented many novel free-piston configurations.^[21]

What appears to be the first mention of a Stirling cycle machine using freely moving components is a British patent disclosure in 1876.^[22] 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.

Thermoacoustic cycle[link]

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.

History[link]

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.^[23] 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.^[24] 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^[25] 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.^[26]

Though it has been disputed^[27] 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,^[28] whose boilers frequently exploded causing many injuries and fatalities.^[29][30] 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^[31]) — for example, the Dundee foundry engine was replaced by a steam engine after three hot cylinder failures in four years.^[32]

Later nineteenth century[link]

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^[33] 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.^[34] 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.^[35] 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.^[36]

Twentieth century revival[link]

During the early part of the twentieth century the role of the Stirling engine as a "domestic motor"^[37] 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.^[38]

At this time Philips was seeking to expand sales of its radios into parts of the world where mains electricity was unavailable and the supply of batteries uncertain. Philips' management decided that offering 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 alternative ways of achieving this aim. After a systematic comparison of various prime movers, the team decided to go forward with the Stirling engine, citing its 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).^[39] 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.^[40]

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,^[41] various development models were produced in a program which continued throughout World War II. By the late 1940s the 'Type 10' was ready to be handed over to Philips' subsidiary Johan de Witt in Dordrecht to be productionised and incorporated into a generator set as originally planned. The result, rated at 180/200 W electrical output 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 besides which 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.^[42] Some found their way into university and college engineering departments around the world^[43] 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. However, they filed a large number of patents and amassed 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.^[44]

Starting in 1986, Infinia Corporation began developing both highly reliable pulsed free-piston Stirling engines, and thermoacoustic coolers using related technology. The published design uses flexural bearings and hermetically sealed Helium gas cycles, to achieve tested reliabilities exceeding 20 years. As of 2010, the corporation had amassed more than 30 patents, and developed a number of commercial products for both combined heat and power, and solar power.^[45] More recently, NASA has considered nuclear-decay heated Stirling Engines for extended missions to the outer solar system.^[46]

Theory[link]

A pressure/volume graph of the idealized Stirling cycle

The idealised Stirling cycle consists of four thermodynamic processes acting on the working fluid:

1. 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.
2. 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.
3. 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
4. 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, 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 is a long way from representing what is actually going on inside a practical Stirling engine and should only be regarded as a starting point 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.^[47][48]

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. A question that often arises is whether the ideal cycle with isothermal expansion and compression is in fact the correct ideal cycle to apply to the Stirling engine. Professor C. J. Rallis has pointed out that it is very difficult to imagine any condition where the expansion and compression spaces may approach isothermal behavior and it is far more realistic to imagine these spaces as adiabatic.^[49] An ideal analysis where the expansion and compression spaces are taken to be adiabatic with isothermal heat exchangers and perfect regeneration was analyzed by Rallis and presented as a better ideal yardstick for Stirling machinery. He called this cycle the 'pseudo-Stirling cycle' or 'ideal adiabatic Stirling cycle'. An important consequence of this ideal cycle is that it does not predict Carnot efficiency. A further conclusion of this ideal cycle is that maximum efficiencies are found at lower compression ratios, a characteristic observed in real machines. In an independent work, T. Finkelstein also assumed adiabatic expansion and compression spaces in his analysis of Stirling machinery ^[50]

Operation[link]

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.^[51]

In a displacer type stirling engine you have one piston and one displacer. A temperature difference is required between the top and bottom of the large cylinder in order to run the engine. In the case of the low-temperature difference (LTD) stirling engine, temperature difference between your hand and the surrounding air can be enough to run the engine. The power piston in the displacer type stirling engine, is tightly sealed and is controlled to move up and down as the gas inside expands. The displacer on the other hand is very loosely fitted so that air can move freely between the hot and cold sections of the engine as the piston moves up and down. The displacer moves up and down to control the heating and cooling of the gas in the engine.
There are two positions,

1) When the displacer is near the top of the large cylinder.
• Inside the engine most of the gas has been heated by the heat source and it expands. This causes the pressure to increase which forces the piston up.

2) When the displacer is near the bottom of the large cylinder.
• Most of the gas in the engine has now cooled and contracts causing the pressure to decrease, which in turn allows the piston to move down and compress the gas.

Pressurization[link]

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.^[52] A difficulty of pressurization is that while it improves the power, the heat required increases proportionately to the increased power. This heat transfer is made increasingly difficult with pressurization since increased pressure also demands increased thicknesses of the walls of the engine which, in turn, increase the resistance to heat transfer.

Lubricants and friction[link]

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.^[53]

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.

Analysis[link]

Comparison with internal combustion engines[link]

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.^[54]

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.^[55] Their lower maintenance requirements make the overall energy cost comparable. The thermal efficiency is also comparable (for small engines), ranging from 15% to 30%.^[54] 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 are also used as a marine engine in Swedish Gotland-class submarines.^[56] 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.^[57][58]

Advantages[link]

• 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 those emissions associated with the intermittent combustion processes of a reciprocating internal combustion engine 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. Crude Stirling engines can be made using common household materials.^[59]
• 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 release 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.

Disadvantages[link]

Size and cost issues[link]

• 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. 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.^[53]
• 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.^[60]

Power and torque issues[link]

• 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).^[52] Compared with internal combustion engines, this makes it more challenging for the engine designer to transfer heat into and out of the working gas. Because of the Thermal efficiency the required heat transfer grows with lower temperature difference, and the heat exchanger surface (and cost) for 1 kW output grows with second power of 1/deltaT. Therefore the specific cost of very low temperature difference engines is very high. 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.

Gas choice issues[link]

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,^[61] but the oxygen in a highly pressurized air engine can cause fatal accidents caused by lubricating oil explosions.^[53] 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 high temperature differential (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 if released from the engine.
• 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.^[62] 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.

Applications[link]

Applications of the Stirling engine range from heating and cooling to underwater power systems. A Stirling engine can function in reverse as a heat pump for heating or cooling. Other uses include: combined heat and power, solar power generation, Stirling cryocoolers, heat pump, marine engines, and low temperature difference engines

Alternatives[link]

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.

Photo gallery[link]

• Rider-hot-air-engine.jpg

  Preserved examples of antique Rider hot air engines - an alpha configuration Stirling

See also[link]

• Beale Number
• Cogeneration
• Distributed generation
• Fluidyne engine
• Quasiturbine
• Relative cost of electricity generated by different sources
• Schmidt number
• Stirling radioisotope generator
• Thermomechanical generator
• West Number

References[link]

Bibliography[link]

Further reading[link]

• 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. Bibcode 1979ASAJ...66.1508C. DOI:10.1121/1.383505. 
• P. Fette. "About the Efficiency of the Regenerator in the Stirling Engine and the Function of the Volume Ratio V_max/V_min". 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. 
• D. Haywood. "An Introduction to Stirling-Cycle Analysis" (PDF). http://www.mech.canterbury.ac.nz/documents/sc_intro.pdf. Retrieved 2009-01-19. ^{[dead link]}
• Z. Herzog (2006). "Stirling Engines". Mont Alto: Pennsylvania State University. http://mac6.ma.psu.edu/stirling/. Retrieved 2009-01-19. 
• F. Kyei-Manu; A. Obodoako (2005). "Solar Stirling-Engine Water Pump Proposal Draft" (PDF). http://www.engin.swarthmore.edu/academics/courses/e90/2005_6/E90Proposal/FK_AO.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. ^{[dead link]}
• N.P. Nightingale (1986). "NASA Automotive Stirling Engine MOD II Design Report" (PDF). NASA. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880002196_1988002196.pdf. Retrieved 2009-01-19. 
• D. Phillips (1994). "Why Aviation Needs the Stirling Engine". http://www.airsport-corp.com/fourpartstirling.html. Retrieved 2009-01-19. 

External links[link]

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
• Explanations stirling engine and demos
• Shockwave3D models: Beta Stirling and LTD

Coordinates: 56°07′02″N 3°56′23″W / 56.117222°N 3.939722°W / 56.117222; -3.939722

Stirling
Scottish Gaelic: Sruighlea
Stirling City Centre
Stirling  Stirling shown within the Stirling council area
Population	33,710 (2008)
OS grid reference	NS795935
Civil parish	Stirling
Council area	Stirling
Lieutenancy area	Stirling and Falkirk
Country	Scotland
Sovereign state	United Kingdom
Post town	STIRLING
Postcode district	FK7-FK9
Dialling code	01786
Police	Central Scotland
Fire	Central Scotland
Ambulance	Scottish
EU Parliament	Scotland
UK Parliament	Stirling
Scottish Parliament	Stirling
	Mid Scotland and Fife
List of places: UK • Scotland •

Stirling (Gaelic: Sruighlea pronounced [ˈs̪t̪ruʝlə]) is a city and former ancient burgh in Scotland, and is at the heart of the wider Stirling council area. The city is clustered around a large fortress and medieval old-town beside the River Forth. Historically it was strategically important as the "Gateway to the Highlands", with its position near the boundary between the Scottish Lowlands and Highlands, and its crossing of the Forth, the nearest to the river mouth.

Once the capital of Scotland, Stirling contains the Great Hall (restored 1999) and the Renaissance Palace (restoration completed 2011) within the Castle that rivalled any building in Europe at the time. Stirling also has its medieval parish church, The Church of the Holy Rude, where King James VI was crowned King of Scots on 29 July 1567. The Holy Rude still functions as living church with a service every Sunday.

Stirling is a centre for local government, higher education, retail, and light industry. Its population in 2008 was 33,710,^[1] for Stirling itself, the wider urban area including Bridge of Allan and Bannockburn has a population of 45,750.^[2] This makes it the smallest city in Scotland: indeed it is smaller than many of Scotland's larger towns.

One of the principal royal strongholds of the Kingdom of Scotland, Stirling was created a Royal burgh by King David I in 1130, which it remained until 1975, when the county of Stirlingshire was absorbed into Central Region. In 2002, as part of Queen Elizabeth II's Golden Jubilee, Stirling was granted city status.

History[link]

Stirling Castle (southwest aspect)

Stirling was originally a Stone Age settlement as shown by the Randolphfield standing stones and Kings Park prehistoric carvings that can still be found south of the town.^[3][4] The city has been strategically significant since at least the Roman occupation of Britain, due to its naturally defensible crag and tail hill (latterly the site of Stirling Castle), and its commanding position at the foot of the Ochil Hills on the border between the Lowlands and Highlands, at the lowest crossing point of the River Forth. It remained the river's lowest crossing until the construction of the Kincardine Bridge further downstream in the 1930s. It is supposed that Stirling is the fortress of Iuddeu or Urbs Giudi where Oswiu of Northumbria was besieged by Penda of Mercia in 655, as recorded in Bede and contemporary annals.

A ford, and later bridge, of the River Forth at Stirling brought wealth and strategic influence, as did its port. The town was chartered as a royal burgh by King David in the 12th century, with charters later reaffirmed by later monarchs (the town then referred to as Strivelyn). Major battles during the Wars of Scottish Independence took place at the Stirling Bridge in 1297 and at the nearby village of Bannockburn in 1314 involving William Wallace and Robert the Bruce respectively. There were also several Sieges of Stirling Castle in the conflict, notably in 1304. Sir Robert Felton, governor of Scarborough Castle in 1311, was slain at Stirling in 1314.

The Wallace Monument

The origin of the name Stirling is uncertain, but folk etymology suggests that it originates in either a Scots or Gaelic term meaning the place of battle, struggle or strife. Other sources suggest that it originates in a Brythonic name meaning "dwelling place of Melyn".^[5] The town has two Latin mottoes, which appeared on the earliest burgh seal of which an impression of 1296 is on record:^[6]

Hic Armis Bruti Scoti Stant Hic Cruce Tuti (The Britons stand by force of arms, The Scots are by this cross preserved from harms) and

Continet Hoc in Se Nemus et Castrum Strivilinse (The Castle and Wood of Stirling town are in the compass of this seal set down.)

Standing near the castle, the Church of the Holy Rude is one of the town's most historically important buildings. Founded in 1129 it is the second oldest building in the city after Stirling castle. It was rebuilt in the 15th century after Stirling suffered a catastrophic fire in 1405, and is reputed to be the only surviving church in the United Kingdom apart from Westminster Abbey to have held a coronation.^[7] On 29 July 1567 the infant son of Mary, Queen of Scots, was crowned James VI of Scotland here.^[7] Musket shot marks that may come from Cromwell's troops during the Wars of the Three Kingdoms are clearly visible on the tower and apse.^[7] Another important historical religious site in the area is the ruins of Cambuskenneth Abbey, the resting place of King James III of Scotland and his queen, Margaret of Denmark.^[8] During the Wars of the Three Kingdoms, the Battle of Stirling also took place in the centre of Stirling on 12 September 1648.

The tomb of James III, King of Scots at Cambuskenneth Abbey

The fortifications continued to play a strategic military role during the 18th century Jacobite Risings. In 1715, the Earl of Mar failed to take control of the castle. On 8 January 1746, the army of Bonnie Prince Charlie seized control of the town but failed to take the Castle. On their consequent retreat northwards, they blew up the church of St. Ninians where they had been storing munitions; only the tower survived and can be seen to this day.^[9]

Economically, the city's port supported overseas trade, including tea trade with India and timber trade with the Baltic. The coming of the railways in 1848 started the decline of the river trade, not least because a railway bridge downstream restricted access for shipping. By the mid 20th century the port had ceased to operate.

Famous residents have included Mary, Queen of Scots; King James VI of Scotland; Sir Henry Campbell-Bannerman; documentary film pioneer John Grierson; film music composer Muir Mathieson; animation pioneer Norman McLaren; TV presenter Kirsty Young; and footballers Billy Bremner (captain of Leeds United and Scotland) and Frank Beattie (captain of Kilmarnock).

The Barnwell brothers, Frank and Harold, worked at Grampian Motors in Causewayhead, and in 1909 they designed and flew the first powered aircraft in Scotland. Frank Barnwell went on to design aircraft including the Bristol Blenheim. A small monument to the brothers' pioneering achievement has been erected at Causewayhead roundabout.^[10]

Geography[link]

Neighbouring towns, villages and places.

	Callander	Crieff	Perth
	Loch Lomond		Alloa, Dunfermline
		Stirling    East
	Glasgow, Kirkintilloch	Cumbernauld, Denny	Falkirk, Edinburgh

Governance[link]

In terms of local government, the city of Stirling is a part of the wider Stirling Council area, which governs on matters of local administration as set out by the Local Government etc (Scotland) Act 1994. Elections to the council take place every four years. The Council is currently controlled by the Scottish National Party.^[11] The Provost of Stirling is Fergus Wood.^[12]

In terms of national government, Stirling forms part of county constituency of Stirling constituency of the House of Commons, electing one Member of Parliament (MP) to the House of Commons of the parliament of the United Kingdom by first past the post system. Anne McGuire of the Labour Party is the MP for Stirling constituency of the House of Commons.^[13]

For the purposes of the Scottish Parliament, Stirling forms part of the Stirling constituency of the Scottish Parliament constituency. The Stirling Scottish Parliament (or Holyrood) constituency created in 1999 is one of nine within the Mid Scotland and Fife electoral region. Each constituency elects one Member of the Scottish Parliament (MSP) by the first past the post system of election, and the region elects seven additional members to produce a form of proportional representation. The constituency is represented by Bruce Crawford, MSP of the Scottish National Party.^[14]

As Scotland comprises a single European Parliament Constituency, Stirling participates in electing six MEPs using the D'Hondt method of proportional representation every four years.

Geography[link]

A map of Stirling from 1945

Stirling is renowned as the Gateway to the Highlands and is generally regarded as occupying a strategic position at the point where the flatter, largely undulating Scottish Lowlands meet the rugged slopes of the Highlands along the Highland Boundary Fault.^[15][16] The starkness of this contrast is evidenced by the many hills and mountains of the lower Highlands such as Ben Vorlich and Ben Ledi which can be seen to the northwest of the city. On the other hand, the Carse of Stirling, stretching to the west and east of the city, is one of the flattest and most agriculturally productive expanses of land in the whole of Scotland.

The land surrounding Stirling has been most affected by glacial erosion and deposition. The city itself has grown up around its castle which stands atop an ancient quartz-dolerite sill, a major defensive position which was at the lowest crossing point on the River Forth. Stirling stands on the Forth at the point where the river widens and becomes tidal. To the east of the city the Ochil Hills dominate the skyline with the highest peak in the range being Ben Cleuch, although Dumyat is more noticeable from Stirling. The Ochils meet the flat carse (floodplain) of the River Forth to the east of the distinctive geographical feature of Abbey Craig, a crag and tail hill upon which stands the 220 ft (67m) high Wallace National Monument.^[17]

The climate of Stirling differs little from that of much of the rest of central Scotland. The warm Gulf Stream air current from the Atlantic Ocean is the predominant influence, with a prevailing southwesterly wind. That said, the areas round Stirling Town Centre encounter significantly less snow in Winter than many of its very close neighbours such as Denny and Dunblane. Although this could be said as being anecdotal, it is likely to be because it is at a lower level and could be said to have its own microclimate.

Areas of Stirling[link]

Top of the Town[link]

Top of the Town consists of Broad Street, Castle Wynd, Ballengeich Pass, Lower Castle Hill Road, and St Mary's Walk. These streets all lead up to Stirling Castle and are the favourite haunt of tourists who stop off at the Old Town Jail, Mar's Wark, Argyll's Lodging and the castle. Ballengeich Pass leads to the graveyard at Ballengeich and the Castle Wynd winds past the old graveyard. The Top of the Town from Broad Street upwards is renowned for its cobblestoned roads, and cars can be heard rattling over the cobblestones on the way down. Craft shops and tourist-focused shops are evident on the way up and once at the top, panoramic views are available across Stirling and beyond.

Broad Street at the heart of Stirling's Old Town area (called Top of the Town by locals)

Other areas[link]

• Abbey Craig
• Airthrey
• Allan Park
• Bannockburn
• Borestone
• Braehead
• Broomridge
• Burghmuir
• Cambusbarron
• Cambuskenneth
• Causewayhead
• Chartershall
• Corn Exchange
• Cornton
• Coxethill
• Craigmill
• Craig Leith
• Cultenhove
• Forthbank
• Gillies Hill
• Gowan Hill
• Hillpark
• Kenningknowes
• Kildean
• King's Park
• Laurelhill
• Livilands
• Loanhead
• Mercat Cross
• The Raploch
• Randolphfield
• Riverside
• Spittal Hill
• Springkerse
• St. Ninians
• Torbrex
• Whins of Milton
• Viewforth
• Wolfcraig

Demography[link]

The city of Stirling had a population of 33,710, in 2008,^[1] the wider urban area including Bridge of Allan and Bannockburn had a population of 45,750.^[2] The entire Stirling Council area had a population of 88,400 in 2008. The city is reputed to be the third fastest growing area of Scotland in terms of population.^[18] According to the 2001 census, 52.7% of the population was female compared to 47.2% male. Stirling had both a smaller proportion of under 16s, at 16.7% compared to the Scottish average of 19.2%, and a smaller proportion of those of pensionable age – 17.8% – compared to the Scottish average of 18.6%.^[19] The highest proportion of the population, at 24.3%, was concentrated in the 16–29 age group. Stirling also had a higher proportion of non-Scottish born residents at 16.5%, compared to the Scottish average of 12.8%. The population was also slightly younger than the Scottish average of 37 – the median age for males was 34; and the median age for females was 36, to the national average of 39. The population peaks and troughs significantly when the students come and go from the city.

Economy[link]

At the centre of a large rural agricultural hinterland that encompasses some of the flattest and most productive land in Scotland, Stirling principally functioned as a market town, symbolised by its Mercat cross, with farmers coming to sell their products and wares in the large agricultural market that was held in the town. Today, agriculture still plays a part in the economic life of Stirling, given its focus at the heart of a large rural area, but to a much lesser extent than previously.

With Stirling's development as a market town and its location as the focus of transport and communications in the region, it has developed a substantial retail sector serving a wide range of surrounding communities as well as the city itself. Primarily centred on the city centre, there are a large number of chain stores, as well as the Thistles shopping centre. However this has been augmented by out-of-town developments such as the Springkerse Retail Park on the city bypass to the east of Stirling, and the development of a large Sainsbury's in the Raploch.

A major new regeneration project on the site of the former port area and the 40-acre (160,000 m²) former Ministry of Defence site, adjacent to Stirling Railway Station, is currently underway.^[20] Known as Forthside, it has the aim of developing a new waterfront district linked to the railway station via a new pedestrian bridge. The development comprises retail, residential and commercial elements, including a conference centre, hotel and Vue multiplex cinema, that will ultimately expand the city centre area, linking it to the River Forth, which has been cut off from the city centre area since the construction of the A9 bypass under the railway station in the 1960s.^[21] For the first time in 100 years, local people will have access to the banks of the River Forth in the city centre with landscaped public areas, footpaths, cycleways and an improved public transport network.

In the service sector, financial services as well as tourism are the biggest employers. The financial services and insurance company Prudential have a large and well-established base at Craigforth on the outskirts of Stirling. In terms of tourism, the presence of such historical monuments as Stirling Castle, the National Wallace Monument and other nearby attractions like Blair Drummond Safari Park, the key role which Stirling has played in Scottish history, as well as the scenery of the area, has bolstered Stirling's position as an important tourist destination in Scotland.

The University of Stirling and Stirling Council are two of the biggest employers in the area. Knowledge related industries, research and development as well as life sciences have clustered around the university in the Stirling University Innovation Park, close to its main campus. Other public sector agencies that are major employers in the city include Central Scotland Police, Scottish Prison Service, NHS Forth Valley and the Scottish Environment Protection Agency.

Stirling is home to national construction companies Ogilvie Group, chaired by Duncan Ogilvie, who is listed in the Times Rich List as being worth £35 million.

A Bank of Scotland survey in 2009 found that workers in Stirling had the highest average earnings of £716 a week.^[22]

Transport[link]

Public transport to districts within the city and to the surrounding towns, like Bridge of Allan and Alloa, is almost completely provided by buses operated principally by the First Group, although there are also railway links to Bridge of Allan, Dunblane, and, since 2008, Alloa. At the heart of Scotland's Central Belt, Stirling has direct road connections to the major cities of Glasgow, via the M80 motorway, and Edinburgh, via the M9 motorway, as well as inter-city rail links from Stirling Railway Station. Transport infrastructure in the area has been further improved with the completion of the Upper Forth Crossing and the Stirling-Alloa-Kincardine rail link, and there is an on-going (2010) upgrade of the A80 Trunk road to Motorway standards. The City of Stirling is home to a large number of commuters, with 12,000 residents commuting to work in other areas, with 13,800 workers also travelling in to the city.^[23]

Sports and recreation[link]

The headquarters of the Scottish Institute of Sport in Stirling

Stirling is home to professional league teams in football, rugby and cricket. The senior football team, Stirling Albion, play in the Scottish Football League Second Division at their home ground of the Doubletree Dunblane Stadium. In July 2010, the Stirling Albion Supporters' Trust successfully took over the running of the club buying out the long-serving chairman, Peter McKenzie, after 14 months of campaigning. This made Stirling Albion the first fully owned community club in the history of British football, after previous attempts made by Manchester United, Liverpool and Rangers.^[24] Stirling County currently play in the Scottish Premiership Division One.

The athletics team Central Athletic Club are based at University of Stirling. Stirling Wanderers Hockey Club have also moved to a brand new (international standard) pitch at Forthbank for season 2008/09.^[25] Next to this pitch there is also the ground of Stirling County Cricket Club, whose pavilion captured an architectural award in June 2009,^[26] three years after its opening.

Scotland international footballers Billy Bremner, John Colquhoun, Duncan Ferguson, and brothers Gary and Steven Caldwell were born in Stirling. So were rugby internationals Kenny Logan, Allister Hogg and Alison McGrandles, jockey Willie Carson, and cricketer Dougie Brown.

Stirling is a major centre of sports training and education in Scotland. The headquarters of the Scottish Institute of Sport is a purpose-built facility on the campus of Stirling University, which opened in 2002. Also at the university is the Scottish National Swimming Academy, as well as the Gannochy National Tennis centre, which is seen as a tennis centre of excellence.^[27]

The university has a dedicated sports studies department, which was ranked amongst the best in the United Kingdom for its provision of sports facilities, with the maximum 5-star award, shared by 16 other universities in the UK.^[28] Stirling University also currently hosts the Scottish men's lacrosse champions.

Stirling and its surrounding area has a number of 9- and 18-hole golf courses, the largest of which is the Stirling Golf Course, located in the Kings Park area of the city. The Peak, a new Sports Village, was opened in April 2009 to cater for a range of sporting activities.^[29]

Education[link]

The University of Stirling opened in 1967 on a greenfield site outside the town. Currently there are 11,544 students studying at the university, of which 8,443 are undergraduates and 3101 are postgraduates. There are 80 nationalities represented on the university campus, with 19% of students coming from overseas.^[30] It has grown into a major research centre, with a large science park – Innovation Park, located immediately adjacent to the main university campus. Innovation Park has grown since its initiation in 1993, and is now home to 40 companies engaging in various forms of research and development.^[31] In January 2008 it was announced that students from Singapore would be able to gain degrees in retail from the University of Stirling in a tie-up with the country's Nanyang Polytechnic (NYP).^[32]

Looking out over Airthrey Loch on the main campus of The University of Stirling

Stirling is also home to part of the wider Forth Valley College which was formed on 1 August 2005 from the merger of Falkirk, Stirling and Clackmannan colleges.

There are four main high schools in Stirling itself – Stirling High School, with a school roll of 964 pupils, Wallace High School with 958 pupils, St Modan's High School with 912 pupils, and Bannockburn High School in Broomridge with 752 pupils.^{[33][34][35][36]} All the city's secondary school premises have been redeveloped as a result of a Public-private partnership scheme. Stirling also has a Gaelic-medium unit situated in the city's Riverside Primary School which teaches pupils from across Stirling and Clackmannanshire through the medium of Scottish Gaelic.^[37]

Twinned cities[link]

• Villeneuve d'Ascq, France
• Dunedin, Florida, United States
• Óbuda, Hungary
• Summerside, Prince Edward Island, Canada

See also[link]

• Black Bond
• Lecropt
• List of places in Stirling (district)
• List of places in Scotland

References[link]

Mair, Craig (1990). Stirling: The Royal Burgh. John Donald Publishers. ISBN 0-85976-420-6. 

External links[link]