Stirling engine

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Thermodynamic cycles
Atkinson cycle Brayton/Joule cycle Carnot cycle Combined cycle Crower cycle Diesel cycle Ericsson cycle Hirn cycle Kalina cycle Lenoir Cycle Linde-Hampson cycle Miller cycle Mixed/Dual Cycle Otto cycle Porter/Brayton cycle Rankine cycle Scuderi cycle Stirling cycle Two-stroke cycle Wankel cycle
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The Stirling engine is a heat engine of the external combustion piston engine type whose heat-exchange process allows for near-ideal efficiency in conversion of heat into mechanical work.

Contents 1 History 2 Functional Description 3 Engine 4 Configurations 5 Heat sources 6 History and development 7 The Stirling Cycle 8 Advantages of Stirling engines 9 Disadvantages of Stirling engines 10 Applications 10.1 Combined heat and power applications 10.2 Solar power generation 10.3 Stirling cryocoolers 10.4 Heat pump 10.5 Marine engines 10.6 Nuclear power 10.7 Aircraft engines 10.8 Geothermal energy 11 References 12 Further reading 13 External links 13.1 How it works 13.2 History 13.3 Academic and technical studies 13.4 Patents 13.5 Societies and conferences 13.6 Hobbyists and enthusiasts 13.7 Commercial manufacturers 13.8 Directories and indexes

[edit] History

Invention of the Stirling engine is credited to the Scottish clergyman Rev. Robert Stirling who, in 1816, made significant improvements to earlier designs and took out the first patent. He was later assisted in its development by his engineer brother James Stirling.

The inventors sought to create a safer alternative to the steam engines of the time, whose boilers often exploded due to the high pressure of the steam and the inadequate materials. Stirling engines will convert any temperature difference directly into movement.

[edit] Functional Description

The Stirling engine works by the repeated heating and cooling of a sealed amount of working gas, usually air or other gases such as hydrogen or helium. This is accomplished by moving the gas between hot and cold heat exchangers, the hot heat exchanger being a chamber in thermal contact with an external heat source, e.g. a fuel burner, and the cold heat exchanger being a chamber in thermal contact with an external heat sink, e.g. air fins or, in space, a thermal radiator.

The gas follows the behaviour described by the gas laws which describe how a gas's 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 recompress the gas on the return stroke, giving a net gain in power available on the shaft. The working gas flows cyclically between the hot and cold heat exchangers.

When one side of the piston is open to the atmosphere, the operation of the cold cycle 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, the atmosphere does work on the gas and "compresses" it. Atmospheric pressure, which is greater than the cooled sealed volume of gas, pushes on the piston.

The working gas is sealed within the piston cylinders, so there is no exhaust gas (other than that incidental to heat production if combustion is used as the heat source). No valves are required, unlike other types of piston engines.

To summarize, the Stirling engine uses the potential energy difference between its hot end and cold end to establish a cycle of a fixed amount of gas expanding and contracting within the engine, thus converting a temperature difference across the machine into mechanical power.

The greater the temperature difference between the heat source and cold source, the easier it is for the Stirling engine to operate and the less efficient the design has to be for the engine to run. But small demonstration engines have been built which will run on a temperature difference of around 15 °C, e.g. between the palm of a hand and the surrounding air, or between room temperature and melting water ice. (Ref) , (Ref -pdf)

[edit] Engine

Some Stirling engines use a separate displacer piston to move the working gas back and forth between cold and hot reservoirs. Others rely on interconnecting the power pistons of multiple cylinders to move the working gas, with the cylinders held at different temperatures.

In true Stirling engines a regenerator, typically a mass of wire, is located between the reservoirs. As the gas cycles between the hot and cold sides, its heat is transferred to and from the regenerator. In some designs, the displacer piston is itself the regenerator. The displacer piston does not need a seal, because with tight tolerances a small air gap acts as a regenerator. This regenerator contributes to the efficiency of the Stirling cycle. Not clearly shown in the next images is that the fluid from the cold and the heat source flow from the upper and lower end of the engine towards the middle where they will have reached the same temperature. These features both belong to the class of reverse flow exchangers, which enable high efficiency in technology and nature.

The ideal Stirling engine cycle has the same theoretical efficiency as a Carnot heat engine for the same input and output temperatures. The thermodynamic efficiency is higher than steam engines (or even some modern internal combustion and Diesel engines).

Action of a Beta type Stirling engine

1 Power piston (hatched) has compressed the gas, the displacer piston ( grey) has moved so that most of the gas is adjacent to the hot heat exchanger.

2 The heated gas increases its pressure and pushes the power piston along the cylinder. This is the power stroke.

3 The displacer piston now moves to shunt 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 cooled its pressure also dropped.

[edit] Configurations

Engineers classify Stirling engines into three distinct types:

• An alpha Stirling contains two separate power pistons in separate cylinders, one "hot" piston and one "cold" piston. The hot piston cylinder is situated inside the higher temperature heat exchanger and the cold piston cylinder is situated inside the low temperature heat exchanger. This type of engine has a very high power-to-volume ratio but has technical problems due to the usually high temperature of the "hot" piston and its seals. (See animation here [1])

• 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. (See animation here [2])

• 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. (See animation here [3])

Changes to the configuration of mechanical Stirling engines continue to interest engineers and inventors alike. Notably, some are in hot pursuit of the rotary Stirling engine; the goal is to convert power from the Stirling cycle directly into torque, a similar goal to that which led to the design of the rotary combustion engine. No practical engine has yet been built but a number of concepts, models and patents have been produced. ^[1]

There is also a field of "free piston" Stirling cycles engines, including those with liquid pistons and those with diaphragms as pistons. [See the book "Free Piston Stirling Cycle Engines" by G. Walker]

An alternative to the mechanical Stirling engine is the fluidyne pump, which uses the Stirling cycle via a hydraulic piston. In its most basic form it contains a working gas, a liquid and two check valves for moving parts. The work produced by the fluidyne goes into pumping the liquid.

[edit] Heat sources

Any temperature difference will power a Stirling engine and the term "external combustion engine" often applied to it is misleading. A heat source may be the result of combustion but can also be solar, geothermal, or nuclear or even biological. Likewise a "cold source" below the ambient temperature can be used as the temperature difference. (see liquid nitrogen economy). A cold source may be the result of a cryogenic fluid or iced water. Since small differential temperatures require large mass flows, parasitic losses in pumping the heating or cooling fluids rise and tend to reduce the efficiency of the cycle.

Because a heat exchanger separates the working gas from the heat source, a wide range of combustion fuels can be used, or the engine can be adapted to run on waste heat from some other process. Since the combustion products do not contact the internal moving parts of the engine, a Stirling engine can run on landfill gas containing siloxanes without the accumulation of silica that damages internal combustion engines running on this fuel. The life of lubricating oil is longer than for internal-combustion engines.

The U.S. Department of Energy in Washington, NASA Glenn Research Center in Cleveland, and Stirling Technology Co. of Kennewick, Wash., are developing a free-piston Stirling converter for a Stirling Radioisotope Generator. This device would use a plutonium source to supply heat.

[edit] History and development

Devices called air engines have been recorded from as early as 1699 around the time when the laws of gases were first set out. The English inventor Sir George Cayley is known to have devised air engines c. 1807. Robert Stirling's innovative contribution of 1816 was what he called the 'Economiser'. Now known as the regenerator, it stored heat from the hot portion of the engine as the air passed to the cold side, and released heat to the cooled air as it returned to the hot side. This innovation improved the efficiency of Stirling's engine enough to make it commercially successful in particular applications, and has since been a component of every air engine that is called a Stirling engine.

During the nineteenth century the Stirling engine found applications anywhere a source of low to medium power was required, a role that was eventually usurped by the electric motor at the century's end.

It was also employed in reverse as a heat pump to produce early refrigeration.

In the late 1940s, the Philips Electronics company in The Netherlands was searching for a versatile electricity generator to enable worldwide expansion of sales of its electronic devices in areas with no reliable electricity infrastructure. The company put a huge R&D research effort into Stirling engines building on research it had started in the 1930s and which lasted until the 1970s. The only lasting commercial spin-off from this for Philips was its reversed Stirling engine: the Stirling cryocooler (see below).

Los Alamos National Laboratory has developed an "Acoustic Stirling Heat Engine" [4] 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 electroacoustic power transducer".

[edit] The Stirling Cycle

The ideal stirling cycle consists of four thermodynamic processes acting on the working fluid:

• 1. Isothermal Compression
• 2. Constant-Volume (or isometric) heat-addition
• 3. Isothermal Expansion
• 4. Constant-Volume (or isometric) heat-removal

This ideal stirling cycle is commonly known as a "squared-cycle", because the transitions between the processes are discontinuous; so when the cycle is graphed on a Pressure-Volume plot, the shape of the cycle contains corners. A real stirling cycle in a stirling engine, requires relatively smooth motion which is commonly sinusoidal or quasi-sinusoidal. In this case the shape of the PV-plot is elliptical. Also in a real engine cycle, the heat transfer performance of the heat exchangers ranges from 100% effectiveness in an isothermal process, to 0% effectiveness in an adiabatic process (no heat transfer). The compression and expansion processes can be modeled as a polytropic processes [5]

,

where k is constant, and n is bounded by:

.

where c_V is the specific heat capacity at constant volume (J/kgK) and c_p is the specific heat capacity at constant pressure (J/kgK)

Compared to the ideal cycle, the efficiency of a real engine is reduced by irreversibilities, friction, and the loss of short-circuit conducted heat, so that the overall efficiency is often only about half of the ideal (Carnot) efficiency. [6]

[edit] Advantages of Stirling engines

• The heat is external and the burning of a fuel-air mixture can be more accurately controlled.
• They can run directly on any available heat source, not just one produced by combustion, so they can be employed to run on heat from solar, geothermal, biological or nuclear sources.
• A continuous combustion process can be used to supply heat, so emission of unburned fuel can be greatly reduced.
• Most types of Stirling engines have the bearing and seals on the cool side; consequently, they require less lubricant and last significantly longer between overhauls than other reciprocating engine types.
• The engine as a whole is much less complex than other reciprocating engine types. No valves are needed. Fuel and intake systems are very simple.
• They operate at relatively low pressure and thus are much safer than typical steam engines.
• Low operating pressure allows the usage of less robust cylinders and of less weight.
• They can be built to run very quietly and without air, for use in submarines or in space.
• They start easily and run more efficiently in cold weather, features lacking in their internal combustion cousins.
• A Stirling engine which is pumping water can be configured so that the pumped water cools the cool side. This is, of course, most effective when pumping cold water.
• They are extremely flexible. They can be used as CHP (Combined Heat and power) in winters and as coolers in summers (cryocooling).

[edit] Disadvantages of Stirling engines

• Some Stirling engine designs require both input and output heat exchangers, which must contain the pressure of the working fluid, and which must resist any corrosive effects due to the heat source. These increase the cost of the engine, especially when they are designed to the high level of "effectiveness" (heat exchanger efficiency) needed for optimizing fuel economy. Fuel economy may not be an issue with the advantages of using unlimited but unusual fuel sources that a Stirling engine can make use of.

• Stirling engines that run on small temperature differentials are quite large for the amount of power that they produce, due to the heat exchangers. Increasing the temperature differential (and pressure) allows smaller Stirling engines to produce more power.

• Dissipation of waste heat is especially complicated because the coolant temperature is kept as low as possible to maximize thermal efficiency. This drives up the size of the radiators markedly, which can make packaging difficult. This has been one of the factors limiting the adoption of Stirling engines as automotive prime movers. (Conversely, it is convenient for domestic or business heating systems where combined heat and power (CHP) systems show promise. ref)

• A "pure" 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 shorter for Stirlings than for others of this type such as steam engines. Stirling engines are best used as constant run, constant speed engines.

• Power output of a Stirling is constant and hard to change rapidly from one level to another. 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 mass of entrained working fluid (generally helium or hydrogen). This property is less of a drawback in hybrid electric propulsion or base load utility generation where a constant power output is actually desirable.

• Hydrogen's lowest molecular weight makes it the best working gas to use in a Stirling engine, but as a tiny molecule, it is very hard to keep inside the engine. Typically, auxiliary systems need to be added to maintain the proper quantity of working fluid. These systems can be as simple as a gas storage bottle or as complicated as a gas generator. In any event, they add weight, increase cost, and introduce some undesirable complications. Some engines use air as the working fluid which is less thermodynamically efficient but avoids loss problems. 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 of hydrogen with fewer of the material containment issues. Hydrogen is also a very flammable gas, while helium is inert.

[edit] Applications

[edit] Combined heat and power applications

The principal use of Stirling engines today is as an economical source of electrical power often utilising a heat source from an industrial process. 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 power back into the electricity grid. They 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.

[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 photovoltaic cells. On August 11, 2005, Southern California Edison announced an agreement to purchase solar powered Stirling engines from Stirling Energy Systems[7] over a twenty year period and in quantity (20,000 units) sufficient to generate 500 megawatts 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: i.e. when a motion is applied to the shaft, a temperature difference appears between the reservoirs. One of their modern uses is in refrigeration and cryogenics.

The essential mechanical components of a Stirling cryocooler are identical to a Stirling engine. The turning of the shaft will compress the working gas causing its temperature to rise. This heat will then be dissipated by pushing the gas against a heat exchanger. Heat would then flow from the gas into this heat exchanger which would probably be cooled by passing a flow of air or other fluid over its exterior. The further turning of the shaft will then expand the working gas. Since it had just been cooled the expansion will reduce its temperature even further. The now very cold gas will be pushed against the other heat exchanger and heat would flow from it into the gas. The external side of this heat exchanger would be inside a thermally insulated compartment such as a refrigerator. This cycle would be repeated once for each turn of the shaft. Heat is in effect pumped out of this compartment, through the working gas of the cryocooler and dumped into the environment. The temperature inside the compartment will drop because its insulation prevents ambient heat from coming in to replace that pumped out.

As with the Stirling engine, efficiency is improved by passing the gas through a “Regenerator” which buffers the flow of heat between the hot and cold ends of the gas chamber.

The first Stirling-cycle cryocooler was developed at Philips in the 1950s and commercialized in such places as liquid nitrogen production plants. The Philips Cryogenics business evolved until it was split off in 1990 to form the Stirling Cryogenics & Refrigeration BV, Stirling The Netherlands. This company is still active in the development and manufacturing Stirling cryocoolers and cryogenic cooling systems.

A wide variety of smaller size Stirling cryocoolers are commercially available for tasks such as the cooling of sensors.

Thermoacoustic refrigeration uses a Stirling cycle in a working gas which is created by high amplitude sound waves.

[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, heat-pump".

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 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

Kockums[8], the Swedish shipbuilder, had built at least 10 commercially successful Stirling powered submarines during the 1980s. As of 2005 they have started to carry compressed oxygen with them. (No endurance stated.)

[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 by-products. 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 obviates 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 cold source is space itself.

[edit] Aircraft engines

They hold theoretical promise as aircraft engines. They are quieter, less polluting, gain efficiency with altitude (internal combustion piston engines lose efficiency), 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. (see below "Argument on why the Stirling engine can be applied in aviation" or "Why Aviation Needs the Stirling Engine" by Darryl Phillips, a 4-part series in the March 1993 to March 1994 issues of Stirling Machine World)

[edit] Geothermal energy

Some believe that the ability of the Stirling engine to convert geothermal energy to electricity and then to hydrogen may well hold the key to replacement of fossil fuels in a future hydrogen economy.^{[citation needed]}

[edit] References

1. ^ Rotary Stirling Engines This site is intended to assist and support all enthusiasts who work to advance the cause of the Stirling Cycle engine. Accessed October 2006

• Van Wylan, Gordon J. and Sontag, Richard F. (1976). Fundamentals of Classical Thermodynamics SI Version 2nd Ed.. New York: John Wiley and Sons. ISBN 0-471-04188-2.
• Walker, G. (1985). Free Piston Stirling Cycle Engines. Springer-Verlag. ISBN 0-387-15495-7.
• Hargreaves, C. M. (1991). The Philips Stirling Engine. Elsevier Publishers. ISBN 0-444-88463-7.

[edit] Further reading

• P. H. Ceperley (1979). "A pistonless Stirling engine — The traveling wave heat engine". J. Acoust. Soc. Am. 66: 1508–1513.

• Beale Number, estimating the power output of a Stirling Engine

[edit] External links

Wikimedia Commons has media related to: Stirling engine

[edit] How it works

• videos and photos zoom on personals stirling engines, into french
• How Stuff Works: Stirling-engine A good description with animated diagrams.

• How it works by Amitabha Mukerjee
• Animations:
  • Alpha type machine,
  • Beta type machine,
  • Gamma type machine
  • Gamma type machine with Ross yoke,
  • Stirling fly motor animation

[edit] History

• Amitabha Mukerjee: Stirling Engine History,

[edit] Academic and technical studies

• David Haywood University of Canterbury NZ: "Introduction to Stirling-Cycle Analysis" (PDF)
• Stirling-Cycle Research Group, University of Canterbury NZ
• Ohio University
• Israel Urieli: Stirling Engine Simple Analysis,
  • Alpha Stirlings,
  • Beta Stirlings,
  • Gamma Stirlings
• Peter Fette: Stirling Engine Researcher, mirror
  • Animation,
  • Regenerator efficiency and simulation
  • Stirling Engine with 8 cylinders, twice double acting
• Argument on why the Stirling engine can be applied in aviation, mirror
• 15 pages (pdf) regarding design of a Fluidyne pump
• The rotary piston array machine
• SNAP - Stirling Numerical Analysis Program

[edit] Patents

• US Patent 20050242232 for a solar powered aircraft using the suns heat to drive a Stirling engine and propeller.

[edit] Societies and conferences

• The Stirling Engine Society
• Stirling News - newsletter published quarterly in the UK by The Stirling Engine Society
• Programme of the 10th Stirling Engine Conference 2001

[edit] Hobbyists and enthusiasts

• Build a simple stirling engine using aluminum cans
• Webring for Stirling engine Hobbyists
• Simple Do-It-Yourself Stirling engine This only requires a temperature difference of 8 °C to run. A hot hand and/or an ice cube is enough to keep it running.
• Japanese hobbyist's Stirling engine home page
• How to build a Stirling engine from a test-tube
• Melbourne Society of Model & Experimental Engineers Journal A novel Stirling cycle hot air engine to build
• Will Rausch - Stirling engine enthusiast Many links
• Robert Sier - Stirling engine enthusiast Many links
• Rotary Stirling Engines enthusiast website
• Polish website about Stirling engines a lot of photos and animations

[edit] Commercial manufacturers

• Kontax Stirling Engines - Hand-held Low Temperature differential Stirling engines.
• Thermal Engine Corporation - Stirling-powered wood stove fan.
• American Stirling Company - Power Producing Engines
• Stirling Technology, Inc. - Energy recovery ventilator technology.
• STM Power - Combined heat and power Stirling engine-generator sets.
• QRMC - Stirling engine manufacturer.
• AIM Infrared Modules - Stirling cooler manufacturer.
• Carl Aero GmbH - Maker of working model Stirling engines.
• WhisperGen - Domestic Stirling-engine central heating boilers which generate electricity.
• Sunpower - "the world's leading developer of free-piston machines."
• Stirling Energy Systems - Building a 29.4% efficient, 500 MW Stirling energy plant (expandable to 850 MW), using satellite dish compound mirrors, north of Los Angeles. Larger than all other U.S. solar power projects combined.

[edit] Directories and indexes

• Stirling Engines at the Open Directory Project