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Cut away diagram of a rhombic drive beta configuration Stirling engine design
Pink - Hot cylinder wall
Dark grey - Cold cylinder wall (with coolant inlet and outlet pipes in yellow)
Dark green - Thermal insulation separating the two cylinder ends
Light green - Displacer piston
Dark blue - Power piston
Light blue - Flywheels
Not shown: external heat-source, and external heat-sinks. In this design the displacer piston is used without a regenerator.
In the family of heat engines, \'Stirling engine\' defines a closed-cycle regenerative hot air engine. In this context; "hot air" may be taken to include other permanent gases, "closed-cycle" to mean the working fluid is permanently contained within the system, and "regenerative" to refer to the use of an internal heat exchanger - the regenerator.
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In the conversion of heat into mechanical work, the Stirling engine has the potential to achieve the highest efficiency of any real heat engine, theoretically up to the full Carnot efficiency, though in practice this is limited by non-ideal properties of the working gas and engine materials, such as friction, thermal conductivity, tensile strength, creep, melting point, etc. The Stirling engine can run on any heat source, including solar, chemical and nuclear. There are many possible implementations of the Stirling engine most of which fall into the category of reciprocating piston engine.
In contrast to internal combustion engines, Stirling engines have the potential to be more energy efficient, quieter, and more reliable with lower maintenance requirements. They are preferred for certain niche applications that value these unique advantages, particularly in cases where the primary objective is not to minimize the capital cost per unit power ($/kW), but rather to minimize the cost per unit energy generated by the engine ($/kWh). Compared to an internal combustion engine of a given power rating, Stirling engines currently have a higher capital cost and are usually larger and heavier; therefore, the engine technology is rarely competitive on this basis alone. For some applications, a proper cost-benefit analysis can favor a Stirling engine over an internal combustion engine.
In recent years, the advantages of Stirling engines have become increasingly significant, given the general rise in energy costs, energy shortage and environmental concerns such as climate change. These growing interests in Stirling technology have fostered the ongoing research and development of Stirling devices. The applications include water pumping, space-based astronautics, and electrical generation from plentiful energy sources that are incompatible with the internal combustion engine, such as solar energy, agricultural waste and domestic refuse.
Another useful characteristic of the Stirling engine is that the cycle is reversible, meaning that if supplied with mechanical power, it can function as a heat pump. Experiments have been performed using wind power driving a Stirling cycle heat pump for domestic heating and air conditioning. In the late 1930s, the Philips Corporation of the Netherlands successfully utilized the Stirling cycle in cryogenic applications.Hargreaves, Clifford M.. "Chapter2, Section 4", The Philips Stirling Engine, 63. ISBN 0-444-88463-7.
Stirling\'s air engine (as it is referred to in early textbooks) was invented by Reverend Dr. Robert Stirling and patented by him in 1816. It followed earlier attempts at making an air engine and it 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. When the name became simplified to Stirling engine is not known, but may be as recently as the mid twentieth century when the Philips company began to experiment with working fluids other than air. The main subject of Stirling\'s original patent was a heat exchanger which he called the "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 an air engine, in which application it is now commonly 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 pressurisation which by 1845 had sufficiently increased the power output for it to drive all the machinery at a Dundee iron foundry.
As well as conserving fuel, the inventors sought to create a safer alternative to the steam engines of the time whose boilers frequently exploded with dire consequences. 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.
Though it failed as a competitor to the steam engine as an industrial scale prime mover, during the latter nineteenth and early twentieth centuries smaller engines of the Stirling/hot air type were produced in large numbers, finding application wherever a reliable source of low to medium power was required, such as raising water. These generally operated at lower temperatures so as not to tax available materials, and thus tended to be rather inefficient. Their major selling point was that they could be operated safely by anybody capable of managing a fire. As the century progressed, this role was eventually usurped by the electric motor and small internal combustion engines, and by the late 1930s the Stirling engine was a largely forgotten scientific curiosity represented only by toys and a few small ventilating fans.
At this time Philips wanted to expand sales of its radios in areas where mains electricity was unavailable and the supply of batteries uncertain. Philips’ management decided that a low-power portable generator was needed, and tasked a group of engineers at the company research lab (the Nat. Lab) in Eindhoven to evaluate the situation. Reviewing various prime movers, each was rejected until the Stirling engine was considered. Inherently quiet and capable of running from any heat source (common lamp oil was favoured), it seemed promising. Encouraged by their first experimental engine, which produced 16 watts of shaft power from a bore and stroke of 30x25mm, a development program was begun. This work continued throughout World War II and by the late 1940s they had an engine – the Type 10 – which was sufficiently developed to be handed over to Philips’ subsidiary Johan de Witt in Dordrecht to be ‘productionised’ and incorporated into a generator set. The set progressed through three prototypes (102A, B, and C), with the production version, rated at 200 watts electrical output from a bore and stroke of 55x27 mm, being designated MP1002CA (known as the \'Bungalow set\'). Production of an initial batch began in 1951, but it became clear that they could not be made at a price that the market would support, and the advent of transistor radios with their much lower power requirements meant that the whole raison d\'être for the set was disappearing. Though the MP1002CA may have been a dead end, it represents the start of the modern age of Stirling engine development.
Philips went on to develop the Stirling engine for a wide variety of applications including vehicles, but only achieved any commercial success with the \'reversed Stirling engine\' cryocooler. They did take out a large number of patents and amass a wealth of information relating to Stirling engine technology, which was later licensed to other companies.
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. 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 behavior 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 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, the atmosphere does work on the gas and "compresses" it. Atmospheric pressure, which is greater than the cooled working gas, pushes on the piston.
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 expanding and contracting within the engine, thus converting thermal energy into mechanical power. The greater the temperature difference between the hot and cold sources, the greater the potential Carnot cycle efficiency.
Video showing the compressor and displacer of a very small Stirling Engine in action
Small demonstration engines have been built which will run on a temperature difference of as little as 7 °C, for example between the palm of a hand and the surrounding air, or between room temperature and melting water ice.Palm Top Stirling Engine Quote: "...This engine is running on PALMTOP! by using heat of Palm. Then temperature difference of it is 7K..."Pasco model SE-8575: The visible Stirling engine (pdf)Working cardboard model of a stirling engine (German website translated with translate.google.com)
Most high performance Stirling engines are pressurized. That is, the mean pressure of the working fluid is above atmospheric pressure. This increases the mass of working fluid processed per cycle. Thus, all other things being equal, the engine produces more power. Unfortunately all other things seldom are equal, and to realise the potential of pressurization larger heat exchangers (including the regenerator) are required. This inevitably increases dead space and possibly gas flow resistance, both of which tend to reduce power output. Like most aspects of Stirling engine design, optimization of this aspect is a delicate balancing act between often conflicting requirements. It was experimenting with pressurization which initially led Philips to move from atmospheric air to other gases for the working fluid. At high temperatures and pressures, the oxygen in air tended to combine with any lubricating oil that made its way past the piston seals, giving problems with clogging the heat exchangers or even the possibility of an explosion. It was later found that some gases, particularly hydrogen and helium, offered other advantages over air.
A Stirling engine and generator set with 55 kW electrical output, for combined heat and power applications. Click image for detailed description.
The idealized or "text book" Stirling cycle is a thermodynamic cycle with two isochores (constant volume) and two isotherms (constant temperature). It is the most efficient thermodynamic cycle capable of practical implementation in an engine - its theoretical efficiency equaling that of the hypothetical Carnot cycle. However 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 favored over a more complex mechanism needed to replicate the idealized cycle. See also Stirling cycle
In a Stirling engine, the regenerator is an internal heat exchanger and temporary 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, thus enabling the thermal efficiency of the cycle to approach the limiting Carnot efficiency defined by those maxima and minima.
The primary effect of regeneration in a Stirling engine is to greatly increase the thermal efficiency by \'recycling\' internally heat which would otherwise pass through the engine irreversibly. As a secondary effect, increased thermal efficiency promises a higher power output from a given set of hot and cold end heat exchangers (since it is these which usually limit the engine\'s heat throughput), though, in practice this additional power may not be fully realized as the additional dead space and pumping loss inherent in practical regenerators tends to have the opposite effect.
A regenerator is difficult to design. The ideal regenerator would be: a perfect insulator in one direction, a perfect conductor in another, have no internal volume yet infinite flow area and infinite surface area. As with the hot and cold exchangers, achieving a successful regenerator is a delicate balancing act between high heat transfer with low viscous pumping losses and low dead space. These inherent design conflicts are one of many factors which limit the efficiency of practical Stirling engines. A typical embodiment might consist of 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.
The regenerator is the key feature invented by Robert Stirling and its presence or otherwise should be used to distinguish a true Stirling engine from other closed cycle hot air engines.
Many engines which have no apparent regenerator can still with some justification be called Stirling engines as, in the simple beta and gamma configurations with a \'loose fitting\' displacer, the surfaces of the displacer and its cylinder will cyclically exchange heat with the working fluid providing some regenerative effect. This is most often seen in small model and LTD types where the additional flow losses and dead space associated with a separate regenerator could actually prove counterproductive and the \'no regenerator\' approach is probably near optimal.
Also see: Economiser
Engineers classify Stirling engines into three distinct types. The Alpha type engine relies on interconnecting the power pistons of multiple cylinders to move the working gas, with the cylinders held at different temperatures. The Beta and Gamma type Stirling engines use a displacer piston to move the working gas back and forth between hot and cold heat exchangers in the same cylinder.
The following diagrams do not show a regenerator, which 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 90o 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. |
Alpha type Stirling. Animated version. |
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. |
A beta Stirling has two pistons within the same cylinder both connected to the same crankshaft. One of these is the tightly fitted power piston and the other a loosely fitted displacement piston.
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Beta type Stirling. Animated version. |
Changes to the configuration of mechanical Stirling engines continue to interest engineers and inventors. Notably, some are in pursuit of the rotary Stirling engine; the goal here 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.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 2006Rotary piston array machine Concept from Gangolf Jobb . Accessed August 2007
An alternative to the mechanical Stirling device is the Fluidyne engine or heat pump, which use hydraulic piston(s) 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.
There is also a field of "free piston" Stirling cycles engines, including those with liquid pistons and those with diaphragms as pistons. Energy may be extracted from a free piston by electromagnetic means, making it especially suitable for some types of solar power electricity generation.
A recent development of Stirling devices are the Thermoacoustic engine and Thermoacoustic refrigerator. These devices are mechanically very different than Stirling devices, although the individual path traveled by each working gas molecule does follow a real Stirling cycle. High-amplitude acoustic standing waves cause compression and expansion analogous to a Stirling power piston, while out-of-phase acoustic traveling 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.
Virtually any temperature difference will power a Stirling engine. The heat source may be derived from fuel combustion, hence the term "external combustion engine", although the heat source may also be solar, geothermal, waste heat, nuclear or even biological. Likewise a "cold sink" can be used in lieu of a heat source, if it is below the ambient temperature (see also: liquid nitrogen economy). A cold source may be the result of a cryogenic fluid or ice water. In the case where a small temperature differential is used to generate a significant amount of power, large mass flows of heating and cooling fluids must be pumped through the external heat exchangers, thus causing parasitic losses that tend to reduce the efficiency of the cycle.
In all external heat engines, a heat exchanger separates the working gas from the heat source, so a wide range of heat sources can be used, including any fuel or 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.
Most Stirling devices do not use lubricating liquids like oil, because they would tend to foul the regenerator. Instead, any sliding surfaces will use materials with a low coefficient of friction, such as PTFE or graphite. Alternatively, sliding surfaces can be avoided altogether by using diaphragms for sealed pistons. These are some of the factors that allow Stirling engines to often have lower maintenance requirements and longer life than internal-combustion engines.
The U.S. Department of Energy in Washington, NASA Glenn Research Center in Cleveland, and Infinia Corporation 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.
Los Alamos National Laboratory has developed an "Acoustic Stirling Heat Engine"Los Alamos National Laboratory: Acoustic Stirling Heat Engine Home Quote: "...More Efficient than Other No-Moving-Parts Heat Engines..." 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".
Think Nordic, an electric car company in Norway, is working with inventor Dean Kamen on plans to install Stirling engines in the Think City, an otherwise all-electric vehicle that will be commercially available at the end of 2007 in Europe.
Since 1988,Kockums\' pages on Stirling engines Kockums shipyards have equipped submarines with Stirling engines. They are currently used on submarines of the Gotland and Södermanland classes. These engines use diesel fuel and liquid oxygen and are called Stirling AIP for air-independent propulsion.
WhisperGen, a New Zealand-based company has developed stirling engines that can be powered by natural gas or diesel. Recently an agreement has been signed with Mondragon Corporación Cooperativa, a Spanish firm, to produce WhisperGen\'s microCHP and make them available for the domestic market in Europe. Some time ago E.ON UK announced a similar initiative for the UK. Stirling engines would supply the client with hot water, space heating and a surplus electric power that could be fed back into the electric grid.
MSI (Taiwan) recently developed a miniature Stirling engine cooling system for personal computer chips that use the waste heat from the chip to drive a fan. http://global.msi.com.tw/index.php?func=newsdesc&news_no=591
A pressure/volume graph of the ideal Stirling cycle. In applications of the Stirling cycles (ie. Stirling engines) this cycle is quasi-elliptical, or at the very least, curved at the sharp corners.
The ideal Stirling cycle consists of four thermodynamic processes acting on the working fluid ( See diagram to right):
This ideal Stirling cycle is commonly known as a "squared-cycle", because when graphed on a Pressure-Volume plot, the rapid transitions between the processes produce a shape with corners[citation needed]. In a real Stirling engine, physical design constraints limit the net force on each engine component, and thus limit the maximum acceleration (or rate-of-change of velocity). Thus 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 quasi-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 processesDavid Haywood: An introduction to Stirling-cycle machines (pdf)
where k is constant, and n is bounded by:
where is the specific heat capacity at constant volume (J/kgK) and 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.Israel Urieli (Dr. Iz), Associate Professor Mechanical Engineering: Stirling Cycle Machine Analysis
1. The working gas is heated at a constant volume to a higher temperature. This increases its pressure. (points 4 to 1 on the graph) |
2. The working gas expands at a constant temperature to a larger volume. This decreases its pressure. The gas does work to move the piston up. (points 1 to 2 on the graph) |
2a. The gas is now fully transferred to the cool cylinder. (Point 2 on the graph) |
3. The working gas is cooled at constant volume to a lower temperature. This decreases its pressure. (Points 2 to 3 on the graph) |
4. The working gas contracts at a constant temperature to a smaller volume. This increases its pressure. (Points 3 to 4 on the graph)The Piston does work to compress the gas as it moves down. But this is less than that delivered to the piston on cycle 2 |
4a. The gas is now fully transferred to the hot cylinder. (Point 4 on the graph) |
A desktop alpha Stirling engine. The working fluid in this engine is air. The hot heat exchange is the glass cylinder on the right, and the cold heat exchanger is the finned cylinder on the top. This engine uses a small alcohol burner (bottom right) as a heat source.
Power plants on the electric grid use fuel to produce electricity, however there are large quantities of waste-heat produced which often go unused. In other situations, high-grade fuel is burned at high-temperature for a low-temperature application. According to the second law of thermodynamics, a heat engine can generate power from this temperature difference. In a CHP system, the high-temperature primary heat enters the Stirling engine heater, then some of the energy is converted to mechanical power in the engine, and the rest passes through to the cooler, where it exits at a low temperature. The "waste" heat actually comes from engine\'s main cooler, and possibly from other sources such as the exhaust of the burner, if there is one.
In a combined heat and power (CHP) system, mechanical or electrical power is generated in the usual way, however, the waste heat given off by the engine is used to supply a secondary heating application. This can be virtually anything that uses low-temperature heat. It is often a pre-existing energy use, such as commercial space heating, residential water heating, or an industrial process.
The power produced by the engine can be used to run an industrial or agricultural process, which in turn creates biomass waste refuse that can be used as free fuel for the engine, thus reducing waste removal costs. The overall process can be efficient and cost-effective.
WhisperGen, a New Zealand firm with offices in Christchurch, has developed an "AC Micro Combined Heat and Power" Stirling cycle engine. These microCHP units are gas-fired central heating boilers which sell unused power back into the electricity grid. WhisperGen announced in 2004 that they were producing 80,000 units for the residential market in the United Kingdom. A 20 unit trial in Germany started in 2006.
Placed at the focus of a parabolic mirror a Stirling engine can convert solar energy to electricity with an efficiency better than non-concentrated photovoltaic cells, and comparable to Concentrated Photo Voltaics. On August 11 2005, Southern California Edison announcedPureEnergySystems.com: World\'s largest solar installation to use Stirling engine technology an agreement to purchase solar powered Stirling engines from Stirling Energy Systemsstirlingenergy.com 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.
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. The essential mechanical components of a Stirling cryocooler are identical to a Stirling engine. In both the engine and the heat pump, heat flows from the expansion space to the compression space; however, input work is required in order for heat to flow against a thermal gradient, specifically when the compression space is hotter than the expansion space. The external side of the expansion-space heat-exchanger may be placed inside a thermally insulated compartment such as a vacuum flask. Heat is in effect pumped out of this compartment, through the working gas of the cryocooler and into the compression space. The compression space will be above ambient temperature, and so heat will flow out into the environment.
One of their modern uses is in cryogenics, and to a lesser extent, refrigeration. At typical refrigeration temperatures, Stirling coolers are generally not economically competitive with the less expensive mainstream Rankine cooling systems, even though they are typically 20% more energy efficient. However, below about -40 to -30 deg.C, Rankine is not effective because there are no suitable refrigerants with boiling points this low. Stirling cryocoolers are able to "lift" heat down to -200 deg.C (73 K), which is sufficient to liquefy air (oxygen, nitrogen and argon). They can go as low as 60K - 40K, depending on the particular design. Cryocoolers for this purpose are more-or-less competitive with other cryocooler technologies. The coefficient of performance at cryogenic temperatures is typically 4-5%.[1] Empirically, the devices show a linear trend, where typically the COP = 0.0015 × Tc - 0.065 , where Tc is the cryogenic temperature. At these temperatures, solid materials have lower values for specific heat, so the regenerator must be made out of unexpected materials, such as cotton.
The first Stirling-cycle cryocooler was developed at Philips in the 1950s and commercialized in such places as liquid air production plants. The Philips Cryogenics business evolved until it was split off in 1990 to form the Stirling Cryogenics & Refrigeration BV,Stirling Cryogenics & Refrigeration BV The Netherlands. This company is still active in the development and manufacturing of Stirling cryocoolers and cryogenic cooling systems.
A wide variety of smaller size Stirling cryocoolers are commercially available for tasks such as the cooling of electronic sensors and sometimes microprocessors. For this application, Stirling cryocoolers are the highest performance technology available, due to their ability to lift heat efficiently at very low temperatures. They are silent, vibration-free, and can be scaled down to small sizes, and have very high reliability and low maintenance. As of 2008, cryocoolers are considered to be the only commercially successful Stirling devices.
Just as a Stirling engine can be reversed, a thermoacoustic engine can also be reversed to make a Thermoacoustic refrigeration device.
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 are by far the most energy-efficient types of heating systems. Stirling heat-pumps also often have a higher coefficient of performance than conventional heat-pumps. To date, these systems have seen limited commercial use; however, use is expected to increase along with market demand for energy conservation, and adoption will likely be accelerated by technological refinements.
Kockums,kockums.se the Swedish shipbuilder, had built at least 8 commercially successful Stirling powered submarines during the 1980s. As of 2005 they have started to carry compressed oxygen with them (see Gotland class submarine).
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 heat sink is space itself.
It is often claimed that the Stirling engine has too low a power/weight ratio and too long a starting time for automotive applications. They also have complex and expensive heat-exchangers that must be made of stainless steel[citation needed], and do not scale-up well to 100 kW output power[citation needed]. There have been at least two automobiles exclusively powered by Stirling engines that were developed by NASA, as well as earlier projects by the Ford Motor Company and the American Motor Company.
The main difficulties involved in using the Stirling engine in an automotive application are start-up time, acceleration response, shut-down time, and weight, not all of which have ready-made solutions. Many people believe that hybrid electric drive systems can bypass all of these setbacks. In November 2007, a prototype hybrid car using solid biofuel and a Stirling engine was announced by the Precer project in Sweden. www.precer.se (in Swedish, with an English specification sheet under the PDF link).
The NASA vehicles were designed by contractors and designated MOD I and MOD II. The MOD II replaced the normal spark-ignition engine in a 1985 4-door Chevrolet Celebrity hatchback. In the 1986 MOD II Design Report (Appendix A) the results show that the highway gas mileage was increased from 40 to 58 mpg and the urban mileage from 26 to 33 mpg with no change in gross weight of the vehicle.
Start-up time in the NASA vehicle maxed out at 30 seconds[citation needed], while Ford\'s research vehicle used an electric heater placed directly into the hot air mix to get the vehicle started in only a few seconds.
Stirling engines hold theoretical promise as aircraft engines. They are quieter, less polluting, gain efficiency with altitude, 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)
A low temperature difference Stirling Engine by American Stirling Company shown here running on the heat from a warm hand
A low temperature difference (Low Delta T, or LTD) Stirling engine will run on any low temperature differential, for example the difference between the palm of a hand and room-temperature or room temperature and an ice cube. Usually they are designed in a gamma configuration, for simplicity, and without a regenerator. They are typically unpressurized, running at near-atmospheric pressure. The power produced is less than one watt, and they are intended for demonstration purposes only. They are sold as toys and educational models.
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The references in this article would be clearer with a different or consistent style of citation, footnoting, or external linking. |
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