Who invented the Stirling engine

Stirling engine

Lexicon> Letter S> Stirling engine

Definition: a hot gas engine with a working gas in an enclosed space

Alternative term: Stirling engine

More general terms: hot gas engine, heat engine

English: Stirling engine

Categories: engines and power plants, heating and cooling

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 08/03/2015; last change: 03/14/2020

URL: https://www.energie-lexikon.info/stirlingmotor.html

The Stirling engine is an older invention than the engines with internal combustion that are predominant today.

A Stirling engine (or Stirling engine) is a heat engine in the form of a hot gas engine, in which the working gas, unlike most internal combustion engines, is in a closed room, i.e. it is not constantly exchanged. This type of engine was invented in 1816 by Robert Stirling, who worked full-time as a pastor, and partly replaced the use of steam engines. This was of interest because it avoids the risk of accidents from machines operated with high-pressure steam.

Compared to the types of internal combustion engines (gasoline engines and diesel engines) with internal combustion that are very dominant today, Stirling engines have important advantages - in particular the potential for using very different heat sources - but also significant disadvantages, which will be discussed further below. There are various similarities with the later developed Ericsson motor.

Working principle

In order to make the functional principle of the Stirling engine easy to understand, conceptually simpler, although practically hardly feasible versions of engines are discussed below.

Theoretical single cylinder engine

The conceptually simplest variant of a Stirling engine is a reciprocating piston engine with an externally closed working cylinder, in which the volume of a working gas is constantly reduced and increased again by a piston that moves periodically up and down. (For this purpose, the piston is typically connected to a rotating crankshaft via a crank drive.) When the gas expands, it does work on the piston, thus contributing to the drive power; however, the later compression of the gas again requires mechanical energy which must be supplied to the piston. In order to keep this compression work as small as possible, the temperature of the working gas in the compression phase is kept as low as possible by cooling the cylinder head; this gives the gas a lower pressure. In the expansion phase, on the other hand, the temperature is increased as much as possible by supplying heat from outside. This has to be done by conduction. Such a motor can only work with this supply and removal of heat.

Although this basic principle is simple, it is difficult to implement in practice. In particular, it is not practical to constantly switch back and forth between heating and cooling of the working gas.

Version with two pistons

The aforementioned difficulty can be solved by performing the compression and expansion with two separate pistons, as shown in Figure 2.

In order to work out the basic idea clearly, we assume a discontinuous movement of the two pistons without paying attention to the necessary mechanics. A work cycle then runs as follows:

The central idea of ​​the Stirling engine is to carry out the expansion and compression of the gas in zones of different warmth.
  • The expansion of the working gas takes place in the upper hot zone. The upper piston moves upwards while the lower piston stands still. In the ideal case, the cooling of the working gas due to the expansion is largely prevented by the fact that heat is absorbed from the outside.
  • When the upper piston moves down again later, the lower piston also goes down in parallel. The volume of the working gas does not change, which is why no mechanical energy is supplied or withdrawn. (At least the contributions of the individual pistons balance each other out in this regard.) However, the gas is transferred from the upper hot zone to the lower cold zone, flowing through the regenerator (see below).
  • Finally, the working gas is compressed again by moving the lower piston upwards while the upper piston stands still. Since the gas is cooler in this case than during expansion because of heat dissipation to the cold wall, its pressure is lower and the compression work to be applied is less than the expansion work performed by the gas in the work cycle. That is why the gas does net work during each work cycle, which can be used (after deducting friction losses, etc.).

If these processes are carried out slowly enough, in principle approximately the theoretically simplified (ideal) Stirling cycle realize:

  • isothermal expansion in the work cycle, d. H. the temperature drop due to expansion is completely prevented by the supply of heat
  • isochoric cooling (i.e. at constant volume)
  • isothermal compression
  • isochoric heating

For the highest possible efficiency (i.e. high energy efficiency) of the process, the temperature difference between the hot and cold zone should be as high as possible. (More precisely, the ratio of the absolute temperatures counts.) It is therefore advantageous to use heat at the highest possible temperature level, as long as this is not limited by the load capacity of the engine parts.

Most of the time, the heat required for operation is generated by a burner. One uses hereby one outercombustion in contrast to internal combustion such as B. in a gasoline engine. The continuous combustion process is much easier to control and a much wider range of fuels can be used; see below for more details.

Function of the regenerator

The regenerator is of great importance for the efficiency of the engine.

The motor as described above could in principle also function without a regenerator. However, the amount of heat required per work cycle and the amount of waste heat to be dissipated can be reduced with the help of a regenerator, which significantly increases the efficiency of the motor.

The regenerator can, for example, be a metal part with fine bores or a metal mesh through which the working gas can flow. (In Figure 1 it is indicated by the interrupted structure between the two pistons.) It works through its heat capacity: When the working gas flows from the bottom up, it is preheated in the regenerator, and vice versa, it gives off heat to the regenerator when it later flows down again.

Suitable working gases

In the simplest case, air can be selected as the working gas of a Stirling engine. However, higher performance and efficiency are possible by using a lighter gas such as hydrogen or helium. On the other hand, this increases not only the costs, but also the effort for the seal, so that such gases can escape through small leaks much more easily than air, and in some cases can even diffuse through airtight materials.

Real variants of the Stirling engine

Actually used Stirling engines are built a little more complicated and deviate a little from the functional principle explained above.

In practice, a sinusoidal movement of the pistons is practically always used, even if this is not entirely ideal in terms of energy.

The discontinuous movement of the pistons described above would be impractical. For this reason, a continuously rotating crankshaft was usually used, which allowed the two pistons to move up and down sinusoidally, with a phase shift of 90 °, which was achieved by suitably attaching the connecting rods to the crankshaft. For example, while the upper piston is at its bottom dead center, the lower piston is moving upwards at maximum speed. Because the processes described above are no longer carried out strictly one after the other, but in a temporally overlapping manner, the efficiency of the machine unfortunately decreases somewhat.

The two pistons can also be a little further apart than shown above; the regenerator then becomes a longer tube, which can be beneficial for heat transfer. However, the dead volume in such parts should be kept as small as possible, as this would otherwise reduce the efficiency. On the other hand, the gas should be able to flow through the regenerator without excessive friction losses.

The heat transfer via the cylinder walls alone is not practical, since the available surface is too small. For faster heat transfer, additional heat exchangers with a larger surface are installed in the expansion or compression space. Unfortunately, this has the consequence that the harmful dead volume is increased (see below).

There are also construction methods (the Beta type), in which the two pistons (a working piston and a displacement piston) move in the same cylinder. The displacement piston can also take over the function of the regenerator (heat storage), especially in engines with low output.

There are also a large number of other designs, for example with working pistons driven on both sides and with several cylinders.

Advantages of the Stirling engine

The most important advantage of the Stirling engine is that a large number of different heat sources can be used for the drive:

The fuel requirements for the Stirling engine are far lower than for engines with internal combustion.
  • A burner can generate heat at a high temperature level with the aid of a fuel. Because the combustion can run continuously, it can be controlled much more easily than the internal combustion, e.g. B. in a gasoline engine or diesel engine - even with low quality fuels. Here, a very high exhaust gas quality is also possible without complex forms of exhaust gas aftertreatment z. B. achievable with a catalytic converter; For example, a Stirling engine powered by natural gas can produce far less nitrogen oxides than a gasoline engine with a regulated catalytic converter. In addition, a large number of fuels can be used that are not suitable for conventional combustion engines, for example wood (also in the form of wood pellets) or other biomass, such as agricultural waste such as straw, or with lean gases such as sewage gas or landfill gas, even with a low methane content of e.g. . B. well below 20%, which would not be useful for a conventional gas engine. The climate-damaging methane slip can also be very low.
Heat from renewable sources can also be used; however, it should be available at the highest possible temperature level.
  • Heat from solar energy can also be used (Solar thermal); Sunlight concentrated with mirrors is usually used for this, which hits a compact absorber. (Ordinary solar collectors are less suitable for this, as the temperatures that can be achieved with them are quite low.)
  • Another way of using renewable energy is to use geothermal energy. In this case, however, the usable temperature difference and thus also the efficiency is quite limited.
  • Otherwise, waste heat z. B. can be used from an industrial process.
  • In special cases, heat from a radionuclide battery is used to drive a Stirling engine.

Further advantages result from the fact that certain problems with other internal combustion engines are avoided, in particular the contamination of engine parts and the contamination of the lubricating oil by combustion residues or unburned fuel; the Stirling engine works with a working gas that is sealed off from the environment. This also makes Stirling engines very low-maintenance; they can achieve well over 10,000 operating hours without maintenance. The consumption of lubricating oil is very low or even zero in engines with dry running.

The noise development of a Stirling engine is usually much lower than that of a gasoline engine, mainly because the enormous pressure fluctuations in the exhaust tract are avoided.

Incidentally, a Stirling engine can be built relatively easily; For example, it does not require any valves, including the often complex valve control.

Disadvantages of the Stirling engine

Dead space effects are difficult to minimize because of certain conflicting goals.

A fundamental technical problem is that the supply and removal of heat in the Stirling engine must take place through heat conduction. This makes it difficult to convert large amounts of heat in a short time without large temperature gradients occurring. For example, it is difficult to make the working gas nearly as cold during compression as the coolant if the compression is not performed very slowly. The efficiency of the regenerator also decreases at high operating speeds. Relatively high efficiencies therefore require low engine speeds, which on the one hand results in low engine power (or a high power-to-weight ratio) and on the other hand increases the influence of disruptive effects (e.g. undesirable heat conduction phenomena). Typical speeds are a few hundred revolutions per minute, which is significantly lower than in engines with internal combustion.

So-called Dead spaces with their dead volume, as they reduce the pressure fluctuations. For example, the dead volume of the regenerator cannot be reduced as desired, since otherwise there would be high flow losses of the working gas, for example. There are similar problems with the heat exchangers for the heat supply in the hot zone and the heat dissipation in the cold zone: There is a trade-off between the maximization of the heat exchanger surface and the minimization of the dead volume.

For the reasons mentioned, the efficiencies that can be achieved with Stirling engines are significantly below the theoretically possible efficiency of a heat engine (the Carnot efficiency). However, this also applies - albeit for completely different reasons - to engines with internal combustion. For example, well-optimized Stirling engines as well as gasoline engines with outputs of a few kilowatts achieve efficiencies slightly above 30% (at full load). For powers above 100 kW, values ​​in the region of 35% are also possible.

The supply and removal of heat cannot be controlled as quickly in a Stirling engine as in an engine with internal combustion. Because of this, the power output cannot be adapted so quickly to the respective needs. This would be a major problem, for example, when used to drive automobiles.

In a number of applications, the production costs of Stirling engines are problematic. In principle, the design can be simpler than for engines with internal combustion, but more material is required due to the high power-to-weight ratio. Above all, however, the costs are relatively high due to the small number of units. Conventional combustion engines, which are built in huge numbers for use in vehicles, for example, benefit far more from the advantages of mass production and also from an enormously optimizing development that is only possible for large numbers. On the other hand, since their invention, Stirling engines have only found relatively limited applications that do not allow mass production.

Stirling engine applications

vehicles

Stirling engines are not very suitable for driving vehicles.

In most cases, Stirling engines are less suitable for driving vehicles than internal combustion engines. Operation with a high-quality fuel such as gasoline or diesel fuel would not make much sense, since the advantages of the Stirling engine (e.g. favorable exhaust gas behavior) would be less important than the disadvantages - in particular a high power-to-weight ratio and the sluggish response to changed performance requirements. (The latter problem could best be solved when used in an electric hybrid drive, which of course further increases the costs.) However, other heat sources (e.g. the combustion of wood), which could only be used with a Stirling engine, are for use in vehicles is not very practical.

Combined heat and power

More suitable areas of application are those in which, on the one hand, heat sources that cannot be used with other motors can be used and, on the other hand, the typical disadvantages are less relevant. In general, the power-to-weight ratio is less important for stationary applications, and a not too high mechanical efficiency is more acceptable where there are hardly any practicable alternatives.

Where less easily controllable fuels such as B. If wood is to be used, the advantages of the Stirling engine are particularly evident.

One example is the cogeneration of heat and power using wood as a fuel. This is possible both with larger systems (e.g. using a fluidized bed furnace) and with small systems that work with wood chips or wood pellets. Instead of using firewood only for pure heat generation, at least a small part of the generated energy (e.g.20%) can be converted into mechanical energy, which is then mostly used to generate electrical energy in a generator.

A Stirling engine is also sometimes used for high-quality fuels such as natural gas, which in principle could be used in an Otto gas engine. Its advantages such as B. the long service life, the low maintenance requirement, the smooth running as well as the excellent exhaust quality can be significant here. A Stirling engine is an interesting solution, especially for small electrical outputs of a few kilowatts.

Since overall in a country like Germany much more heat than electrical energy is required, a significant contribution to electricity generation would be possible if many plants that previously only generated heat could convert at least 10 or 20% of the primary energy into electrical energy. This would be more energy efficient than pure heat generation in combination with electricity generation in power plants without combined heat and power. The additional fuel consumption for electricity generation is in fact hardly higher than the amount of electrical energy generated; thus the energy efficiency achieved is as high as if the electrical energy in a power plant were generated with an efficiency of almost 100% (which is technically impossible, of course) from the fuel.

Dish-Stirling systems

There are also machines for generating electricity with the help of solar energy, in which solar radiation is sent with the help of mirrors in a highly concentrated manner to an absorber, which in turn feeds a Stirling engine (which is sometimes called Solar motor referred to as). Such Dish-Stirling systems With a mirror surface of a few dozen square meters, they can emit an electrical output of a few kilowatts in full sunlight; Efficiencies of a little more than 30% are possible - more than with other solar thermal systems for power generation. Higher performance can be achieved simply by combining many such machines; so it is a completely modular approach.

Dish-Stirling systems are in fierce competition with photovoltaic systems, which have now become much cheaper.

Such systems naturally have to track the sun and can only use direct, but not diffuse, solar radiation. They are therefore most suitable for use in areas with a sunny, dry climate, but there, too, there is strong competition from photovoltaics. Unfortunately, Stirling Dish systems do not have nearly as dramatic degression as photovoltaics, which has now become significantly more cost-effective. In addition, photovoltaic systems are more robust in some ways. Compared to solar thermal power plants with steam turbine systems, the degree of efficiency is significantly higher and the modular approach results in a much higher level of reliability, but the often important possibility of energy storage is eliminated.

Space travel

A special application is the generation of electricity in spacecraft and space probes. While small space probes are occasionally operated with a radionuclide battery containing a thermoelectric generator, a machine with a Stirling engine is better suited for higher outputs (e.g. several kilowatts), as it allows a significantly higher degree of efficiency. However, photovoltaics is a strong alternative here too, as long as it is not about operation far from the sun.

Stirling machine as a refrigeration machine or heat pump

A machine very similar to the Stirling engine can also be operated as a heat pump. Such a machine must be mechanically driven by an electric motor, for example, and “pumps” heat from a source of lower temperature to a consumer at a higher temperature level. Similar is the already more widespread application as a refrigeration machine (especially for miniature cryocoolers), in which the aspect of heat dissipation at a low temperature level is in the foreground.

Stirling refrigeration machines and heat pumps compete with conventional refrigeration machines and heat pumps that use a phase transition of a refrigerant. They have the following advantages over them:

  • Instead of a conventional refrigerant, only a completely harmless working gas such as. B. Helium required. This is advantageous because many refrigerants pose ecological and / or health problems. For example, many of them have a high global warming potential and others are highly toxic.
  • Since there is no phase transition of the working medium, such a machine can be used in a wide range of temperatures; a high temperature lift is also possible.
In the future, Stirling refrigeration machines could occupy other fields of application.

The latter advantage is particularly important for refrigeration machines that have to generate a temperature between approx. 20 and 80 Kelvin (i.e. −253 ° C to −193 ° C). That is why Stirling engines are often used precisely for such applications; conventional chillers would be very inefficient here. In the future, however, they could also become competitive with conventional cold vapor refrigeration machines in the area of ​​significantly higher temperatures, where they have so far been more efficient. This is especially true when the waste heat is to be used at a higher temperature level; under these circumstances the efficiency of conventional refrigeration machines often suffers much more than that of Stirling machines.

Questions and comments from readers

Here you can suggest questions and comments for publication and answering. The author of the RP-Energie-Lexikon will decide on the acceptance according to certain criteria. In essence, the point is that the matter is of broad interest.

If you receive help here, you might want to return the favor with a donation with which you support the further development of the energy dictionary.

Data protection: Please do not enter any personal data here. We would not publish them anyway and we would delete them soon. See also our privacy policy.

If you would like personal feedback or advice from the author, please write to him by email.

By submitting you give your consent to publish your entries here in accordance with our rules.

literature

[1]Martin Werdich and Kuno Kübler, “Stirling machines: Basics - Technology - Applications”, Verlag Ökobuch, ISBN 978-3936896299

(Suggest additional literature)

See also: heat engine, reciprocating engine, internal combustion engine, Ericsson engine
as well as other articles in the categories of engines and power plants, heating and cooling