stirling engine

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STIRLING ENGINE

ABSTRACT

The quest of human beings to develop engines with high power, high torque, less vibration and most essentially with no pollution is on since the discovery and development of engine. Stirling engine is just one step forward towards the creation of a noise free and pollution less engine. The Stirling engine is the engine, which uses a fixed amount of gas sealed inside a cylinder. The expansion and contraction of the gas, using heat from external source, creates the useful work. The main advantage of this engine is its capability to use any type of fuel and the emission of no exhaust gases. Due to this pollution free and use of any type of fuel characteristics the Stirling engine has greater potential over any other type of engine existing today. Hence this engine is highly preferred in automobile sector finding its application in submarines to hybrid cars. Due to the above specified advantages the Stirling engine is giving a cutting edge to all other engine existing today and is viewed as an answer to the existing energy crisis

Dept. of Mechanical Engineering

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STIRLING ENGINE

INTRODUCTION
"…These imperfections have been in a great measure removed by time and especially by the genius of the distinguished Bessemer. If Bessemer Iron or steel had been known thirty five or forty years ago there is a scarce doubt that the air engine would have been a great success … It remains for some skilled and ambitious mechanist in a future age to repeat it under more favorable circumstances and with complete success…" (Written in the year 1876 by Dr. Robert Stirling [1790-1878])

Figure 1 : Sketch of Robert Stirling of his invent The Stirling Engine was invented by Robert Stirling. This device was born as a competence to the vapor machine, since a Stirling Engine works with smaller pressures than the device created by Watt and it did not require a qualified train engineer. At the end of s.XIX with the development of the internal combustion engine and the appearance of electric engines, the machine of this study was forgotten. Nowadays the technology that involves the invention of Robert Stirling is in completely development because of the fact that now very useful applications are available. 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.

Dept. of Mechanical Engineering

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STIRLING ENGINE

Or more specifically, 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 is the inclusion of a regenerator that differentiates the Stirling engine from other closed cycle hot air engines. 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. The Stirling engine is noted for its high efficiency compared to steam engine, 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.

Dept. of Mechanical Engineering

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STIRLING ENGINE

HISTORY
The Stirling Engine is one of the hot air engines. It was invented by Robert Stirling (1790-1878) and his brother James. His father was interesting in engine and he inherited it. He became a minister of the church at Scotland in 1816. At this period, he found the steam engines are dangerous for the workers. He decided to improve the design of an existing air engine. He hope it wound be safer alternative. After one year, he invented a regenerator. He called the “Economiser” and the engine improves the efficiency. This is the earliest Stirling Engine. It is put out 100 W to 4 kW. But the internal combustion engine substituted for it quickly. The Ericsson invented the solar energy in 1864 and did some improvements for after several years. Robert’s brother, James Stirling, also played an important role in the development of Stirling engines.

Figure 2 : Earliest Stirling engine

The original patent by Reverend Stirling was called the "economizer", for its improvement of fuel-economy. The patent also mentioned the possibility of using the device in an engine. Several patents were later determined by two brothers for different configurations including pressurized versions of the engine. This component is now commonly known as the "regenerator" and is essential in all high-power Stirling devices.

Dept. of Mechanical Engineering

REC Hulkoti

STIRLING ENGINE

Figure 3 : Stirling Engine’s principle of operation

Stirling engine of the second generation began in 1937.The Philips of Holland used new materials and technology to ascend a very high level. The knowledge about the heat transfer and fluid physical, which is a great significance to improving of the structure and raised the stability.

Dept. of Mechanical Engineering

REC Hulkoti

STIRLING ENGINE

PRESENTATION OF STIRLING ENGINES I.
Stirling thermodynamic cycle
The Stirling engine cycle is a closed cycle and it contains, most commonly a fixed mass of gas called the "working fluid" (air, hydrogen or helium). The principle is that of thermal expansion and contraction of this fluid due to a temperature differential. So the ideal Stirling cycle consists of four thermodynamics distinct processes acting on the working fluid: two constant-temperature processes and two constant volume processes.

Figure 4 : A pressure/volume graph of the ideal Stirling cycle

Dept. of Mechanical Engineering

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STIRLING ENGINE

1. Isothermal expansion: The expansion space is heated externally, and the gas undergoes near-isothermal expansion.

2. Constant-volume (known as isovolumetric or isochoric) heat removal: The gas is passed through the regenerator, thus cooling the gas, and transferring heat to the regenerator for use in the next cycle.

3. Isothermal compression: The compression space is intercooled, so the gas undergoes near isothermal compression

4. Constant-volume heat addition: The compressed air flows back through the regenerator and picks up heat on the way to the heated expansion space.

The process lines in the figure above reflect the properties of an ideal gas. The main processes, like for most heat engines, are cooling, compression, heating and expansion. A Stirling engine operates through the use of an external heat source and an external heat sink having a sufficiently large temperature difference between them. The gasses used inside a Stirling engine never leave the engine. There are no exhaust valves that vent high-pressure gasses, as in a gasoline or diesel engine, and there are no explosions taking place.

II. Engine configurations

Mechanical configurations of Stirling engines are classified into three important distinct types: Alpha, Beta and Gamma arrangements. These engines also feature a regenerator (invented by Robert Stirling). The regenerator is constructed by a material that conducts readily heat and has a high surface area (a mesh of closely spaced thin metal plates for example). When hot gas is transferred to the cool cylinder, it is first driven through the regenerator, where a portion of the heat is deposited.

Dept. of Mechanical Engineering

REC Hulkoti

STIRLING ENGINE

When the cool gas is transferred back, this heat is reclaimed. Thus the regenerator “pre heats” and “pre cools” the working gas, and so improve the efficiency. But many engines have no apparent regenerator like beta and gamma engines configurations with a “loose fitting” displacer, the surfaces of the displacer and its cylinder will cyclically exchange heat with the working fluid providing some regenerative effect.

1. Alpha Stirling:
Alpha engines have two separate power pistons in separate cylinders which are connected in series by a heater, a regenerator and a cooler. One is a “hot” piston and the other one a “cold piston”. The hot piston cylinder is situated inside the high temperature heat exchanger and the cold piston cylinder is situated inside the low temperature heat exchanger. The generator is illustrated by the chamber containing the hatch lines.

Figure 5 : Alpha engine’s configuration

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. 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.

Dept. of Mechanical Engineering

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STIRLING ENGINE

Action of an alpha type Stirling engine:
The following diagrams do not show internal heat exchangers in the compression and expansion spaces, which are needed to produce power. A regenerator would be placed in the pipe connecting the two cylinders. The crankshaft has also been omitted.

1. Most of the working gas is in contact with the hot cylinder walls, it has been heated and expansion has pushed the 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.

Dept. of Mechanical Engineering

REC Hulkoti

STIRLING ENGINE

Figure 6: Action of an alpha type Stirling engine

2. Beta Stirling:
The Beta configuration is the classic Stirling engine configuration and has enjoyed popularity from its inception until today. Stirling's original engine from his patent drawing of 1816 shows a Beta arrangement. Both Beta and Gamma engines use displacer- piston arrangements. The Beta engine has both the displacer and the piston in an in- line cylinder system. The Gamma engine uses separate cylinders. The purpose of the single power piston and displacer is to “displace” the

working gas at constant volume, and shuttle it between the expansion and the compression spaces through the series arrangement cooler, regenerator, and heater.

Figure 7 : Beta engine’s configuration

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 between the hot and cold heat exchangers.

Dept. of Mechanical Engineering

REC Hulkoti

STIRLING ENGINE

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.

Action of a beta type Stirling engine:
Again, the following diagrams do not show internal heat exchangers or a regenerator, which would be placed in the gas path around the displacer

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

2. The heated gas increases in pressure and pushes the power piston to the farthest limit of thepower 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.

Figure 8: Action of an beta type Stirling engine

3. Gamma Stirling:
A gamma Stirling is simply a beta Stirling in which the power piston is mounted in a separate cylinder alongside the displacer piston cylinder, but is still connected to the

Dept. of Mechanical Engineering

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STIRLING ENGINE

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. The advantage of this design is that it is mechanically simpler because of the convenience of two cylinders in which only the piston has to be sealed. The disadvantage is the lower compression ratio.

Figure 9 : Gamma engine’s configuration

4 . Other types:
Changes to the configuration of mechanical Stirling engines continue to interest engineers and inventors who create a lot of different version of the Stirling engine. There is also a large field of "free piston" Stirling cycles engines, including those with liquid pistons and those with diaphragms as pistons. For example, as 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 non-return valves. The work produced by the fluidyne goes into pumping the liquid.

Dept. of Mechanical Engineering

REC Hulkoti

STIRLING ENGINE

ANALYSIS
Comparison with internal combustion engines:
In contrast to internal combustion engines, Stirling engines have the potential to use renewable heat sources more easily, to be quieter, and to be more reliable with lower maintenance. They are preferred for applications that value these unique advantages, particularly if the cost per unit energy generated is more important than the capital cost per unit power. On this basis, Stirling engines are cost competitive up to about 100 kW. 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. Their lower maintenance requirements make the overall energy cost comparable. The thermal efficiency is also comparable (for small engines), ranging from 15% to 30%. 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 Gotlandclass submarines. 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.

Comparison from economic point:
As said above the Stirling engine is a kind of external combustion engine, and it can use a variety of fuels. It can be estimated that combustible gases are the best material, including gasoline, diesel, propane, sunshine and salad oil; even cow dung can be run on as fuels. A cup of coffee cannot become a cup of gasoline, but it can be also used as a Stirling engine driver. There is a famous experiment that a Stirling engine can easily run on a cup of coffee. The Stirling engine is a kind of piston engine. In the external heating sealed chamber, the expansion of gases inside the engine promotes the pistons work. After the expanded gases cooling down in the air-conditioned room, next process is

Dept. of Mechanical Engineering

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STIRLING ENGINE

taking on. As long as a certain value of the temperature difference exists, a Stirling Engine can be formed.

Figure 10. Stirling Engine working on a cup of coffee

This experiment shows that only a very small power operation can carry out a Stirling engine, which contributes a lot to energy conservation. This characteristic especially shows out on economy point. The benefits obtained from the Stirling engine are definitely far beyond the costs.

So once solar is used to produce energy for the Stirling engine, the cost would surely be cut down for quite a lot. As long as there is sunshine, the Stirling engine will run on and on. Of course it costs much to manufacture a Stirling engine, as it requires a high level of the materials and manufacturing processes. Nowadays, more and more countries have recognized that a society with sustainable development should be able to meet the needs of the community without endangering future generations. Energy problem is a worldwide one, and it is sooner or later to get into the transition-to-new-energy period. Because of its sustainability, renewably and efficiency, the Stirling engine is just the very one being consistent with the requirements of the times.
Dept. of Mechanical Engineering REC Hulkoti

STIRLING ENGINE

REASONS TO USE STIRLING ENGINES
There are several reasons to use a Stirling Engine:


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.



Some types of Stirling engines have the bearings and seals on the cool side of the engine, where they require less lubricant and last longer than equivalents on 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.



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 overpressure relief valve can cause an explosion.

 

In some cases, low operating pressure allows the use of lightweight cylinders. They can be built to run quietly and without an air supply, for air-independent propulsion use in submarines.



They start easily (albeit slowly, after warmup) and run more efficiently in cold weather, in contrast to the internal combustion which starts quickly in warm weather, but not in cold weather.



A Stirling engine used for pumping water can be configured so that the water cools the compression space. This is most effective when pumping cold water.



They are extremely flexible. They can be used as CHP (combined heat and power) in the winter and as coolers in summer.



Waste heat is easily harvested (compared to waste heat from an internal combustion engine) making Stirling engines useful for dual-output heat and power systems.

Dept. of Mechanical Engineering

REC Hulkoti

STIRLING ENGINE

DISADVANTAGES


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.



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.



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.



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, and hence not flammable. Helium is relatively expensive, and must be supplied as bottled gas.



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.

Dept. of Mechanical Engineering

REC Hulkoti

STIRLING ENGINE

APPLICATIONS OF THE STIRLING POWER
1. Cars
In the ages of 1970s and 1980s several automobile companies like “General Motors” or “Ford” were researching about Stirling Engine. This device is good for a constant power setting, but it is a challenge for the stop and go of the automobile. A good car can change the power quickly. One possibility to obtain this important characteristic is design a power control mechanism that will turn up or down the burner. This is a slow method of changing power levels because is not enough to accelerate crossing an intersection. The best solution in spite of these difficulties in automobiles is hybrid electric cars where Stirling Engine could give enough power to make long trips where could get burn gasoline or diesel, depending on which fuel was cheaper. The batteries could give the instant acceleration that drivers are used to. This invention makes the car silent and clean running.

2. Aircraft engines
Stirling engines may hold theoretical promise as aircraft engines, if high power density and low cost can be achieved. They are quieter, less polluting, gain efficiency with altitude due to lower ambient temperatures, are more reliable due to fewer parts and the absence of an ignition system, produce much less vibration (airframes could last longer) and use safer, less explosive fuels. However, the Stirling engine often has low power density compared to the commonly used Otto engine and Brayton cycle gas turbine. This issue has been a point of contention in automobiles, and this performance characteristic is even more critical in aircraft engines.

3. Cryocooler
If It is applied mechanical energy instead of cold and heat sources by means of external engine, It is possible reach temperatures like 10 K (-263°C) in machines of high technology. The first Stirling-cycle cryocooler was developed at Philips in the 1950s and commercialized in such places as liquid nitrogen production plants. This company is still active in the development and manufacturing Stirling cryocoolers and cryogenic cooling systems.
Dept. of Mechanical Engineering REC Hulkoti

STIRLING ENGINE

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.

4. Solar Energy
Placed at the focus of a parabolic mirror a Stirling engine can convert solar energy to electricity with efficiency better than non-concentrated photovoltaic cells. On August 11, 2005, Southern California Edison announced an agreement with Stirling Energy Systems to purchase electricity created using over 30,000 Solar Powered Stirling Engines over a twenty-year period sufficient to generate 850 MW of electricity.

5. Marine engines
The Stirling engine could be well suited for underwater power systems where electrical work or mechanical power is required on an intermittent or continuous level. General Motors has done a considerable amount of work on advanced Stirling cycle engines which include thermal storage for underwater applications. United Stirling, in Malmo, Sweden, is developing an experimental four–cylinder engine using hydrogen peroxide as an oxidant in underwater power systems. The SAGA (Submarine Assistance Great Autonomy) submarine became operational in the 1990s and is driven by two Stirling engines supplied with diesel fuel and liquid oxygen.

6. Heat and power System
This device replaces traditional boilers in houses. It is an innovative system developed to provide central heating, water heating and electricity. Usually this device is called “Micro Combined Heat and Power (CHP)” and produces much less carbon dioxide than other ways of providing heat and power. Benefits: · Savings through the production of own electricity. · Reduce emissions of CO2 and other emissions. · Avoiding peak-load costs when the network is overloaded. · Allows for rapid introduction of new generation capacity. The performance is over 90% of the fuel energy resulting in a cleaner and more cost effective alternative to traditional electricity generation. Electricity generated can be fed back into the electricity grid or used in the home, reducing electricity costs even further.

Dept. of Mechanical Engineering

REC Hulkoti

STIRLING ENGINE

7. Nuclear power
Steam turbines of a nuclear plan can be replaced by Stirling engine thus reduce the radioactive by-products and be more efficient. Steam plants use liquid sodium as temperature increase so much this coolant could reacts violently with water. NASA has developed a Stirling Engine known as Stirling Radioisotope (SRG) Generator designed to generate electricity in for deep space proves in lasting missions. The heat source is a dry solid nuclear fuel slug and the cold source is space itself. This device converter produces about four times more electric power from the plutonium fuel than a radioisotope thermoelectric generator.

Figure 11 . Conceptual design of the SRG by Lockheed

8. Acoustic Stirling Heat Engine
Los Alamos National Laboratory has developed an "Acoustic Stirling Heat Engine" with no moving parts. It converts heat into intense acoustic power which (quoted from given source) "can be used directly in acoustic refrigerators or pulse-tube refrigerators to provide heat-driven refrigeration with no moving parts, or ... to generate electricity via a linear alternator or other electro-acoustic power transducer".

Dept. of Mechanical Engineering

REC Hulkoti

STIRLING ENGINE

CONCLUSION
Stirling engines qualify for “free energy” designation when they allow us to tap previously inaccessible sources of naturally occurring energy. Stirling cycle engines are very efficient for a given temperature difference between the heat source and the heat sink. Actually, steam engines (the Rankine cycle) fall into this category, too. Stirling Engines are very flexible. There are a lot of different types of engines. They can be very small and run with only a small temperature difference, they are very quiet, for example to use them in submarines or they can be used as a CHP plant. Another good point is that they can be constructed in a way that they produce no emissions. That means, in combination with solar or geothermal heat, they can be used as a renewable energy source to produce electricity. In all applications that was showed in this presentation the performance the devices are better, obviously increase the efficiency is good. Taking one with another, Stirling engine bring a tremendous revolution to human being. We think there is also a lot of potential in this area because modern industrialization should be sustained by regenerate power system. It is not a dead end but a new start.

Dept. of Mechanical Engineering

REC Hulkoti

STIRLING ENGINE

REFERENCES
In order to accomplish the current project, the following web pages have been consulted. The authors of the project would like to thank the following for their accuracy, clarity and conciseness.
.

http://en.wikipedia.org/wiki/Stirling_engine

· http://www.kockums.se · http://www.grc.nasa.gov/WWW/tmsb/index.html · http://www.infiniacorp.com/main.htm · http://www.stirlingenergy.com · http://www.whispergen.com/index.cfm · http://www.sunpower.com/index.php · www.Sterlingenergy.com · www.Stirlingengine.com

Dept. of Mechanical Engineering

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