regenerator

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A regenerator is a heat exchanger with a filling mass (storage mass) serving as a short-term heat storage medium, through which a warm and a cold gas flows alternately . Heat is first transferred from the gas to the heat accumulator in order to then be given off again to the gas that subsequently flows through.

Detailed view of a regenerator

For high temperatures, the storage mass consists of refractory bricks, while at low or only moderately high temperatures, metals or, in the meantime, ceramic elements are often used. Storage masses can rest or be moved in the case of rotary heat exchangers . In the simplest case, a stationary storage mass is operated discontinuously, that is, it is alternately charged with hot and cold gas. With a multi-chamber system, it is possible to switch between different storage tanks so that almost uninterrupted (quasi-continuous) operation is possible. The regenerator operation is time-dependent and therefore unsteady. Regenerators can either be fixed or movable.

The regenerator allows the construction of large heat exchangers with relatively simple means, in which the small but unavoidable mixing of the material flows plays a subordinate role, such as for example wind heaters , air preheaters or rotary heat exchangers. The regenerator is a characteristic element in Siemens-Martin ovens , systems for regenerative post-combustion and pulse tube coolers, among others .

Example of a regenerator based on a Stirling engine

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The pressure p as a function of the volume V in an ideal Stirling engine.
Stirling engine in beta configuration

The function of Stirling engines - especially that of the regenerator - can be explained particularly easily in the so-called beta configuration. Then there is a single working space between the heated floor and the moving piston. The regenerator, here a gas-permeable copper wire mesh of low mass, which also acts as a displacement piston, divides the working area into a hot area below and a cold area above. The regenerator can be moved very quickly with negligible expenditure of energy.

  1. Start position: The piston is at its lowest position, immediately below the regenerator. Most of the contained gas is below the regenerator, is heated, the pressure is high and the piston and regenerator move upwards and expand the gas.
  2. While the piston is near its top dead center, the regenerator is moved downwards, the hot gas flows up through the metal mesh, heats it up and is cooled down in the process, which is why the pressure drops. The temperature of the metal mesh is considerably higher at the bottom than at the top. It stores heat, which in the fourth step heats the gas again.
  3. The gas is in the cold room, is cooled there, and the pressure drops. The piston moves up and compresses it at low temperature and low pressure.
  4. Near the bottom dead center of the piston, the regenerator moves up on the piston. The cooled, compressed gas crosses the metal mesh and is preheated by it, the pressure increases. The regenerator has released the previously stored heat (point 2). Continue with point 1


In real Stirling engines, the regenerator is usually not moved jerkily, because this can be done easily with connecting rods and the mechanics are stressed more evenly. However, this structural simplification reduces the efficiency because the corners of the pV diagram are rounded and therefore the area covered, which is a measure of the work done, is reduced. In the flat-plate Stirling engine , the sudden movement of the regenerator is realized, which is probably the main reason for the small temperature difference required.

An ideal regenerator takes so much heat from the working gas that flows into the cold area that the gas has the temperature of the cold area when it leaves the regenerator. Conversely, it heats the working gas so much when it flows into the hot area that the temperature of the hot area is reached again. In such an idealized case, no exergy would be lost and the Carnot efficiency would be achievable. In other words: The regenerator should ensure the greatest possible temperature differences on the right (2) and left (4), because this is the only way to maximize the area driven around. Without an effective regenerator, the gas would swing back and forth between the two areas with too little temperature change. Then (in point 3) considerably more heat would have to be dissipated and the efficiency would be quite low.

A fundamental disadvantage of the "gentle" movement of the regenerator can be seen in the picture below : During a certain period of time, both pistons move upwards, which is why the gas is simultaneously cooled above and heated below the regenerator. This systematic error could be avoided if the regenerator were to fit closely to the piston. This structural detail is better solved in the flat-plate Stirling engine.

A real regenerator should meet the following requirements:

  • The heat capacity of the regenerator should be as large as possible so that its temperature hardly changes, although heat is given off to the working gas flowing through or is absorbed by it. The regenerator must therefore be as large as possible and consist of a material with a high specific heat capacity. The degree of gaps in the regenerator (its cavity), on the other hand, should be as small as possible.
  • The pressure loss of the gas flowing through should be small. A small regenerator with a large proportion of voids would be ideal.
  • The dead volume in the regenerator should be as small as possible. The ideal is a small, short regenerator with a small proportion of voids.
  • The regenerator must not become clogged with abrasion from the machine (e.g. the piston running surface). Therefore, the flow paths should have the largest possible free cross-sections.

As a compromise between the sometimes contradicting requirements, regenerators are often made of a porous or fibrous material (for example copper wire with a diameter of <0.03-0.2 mm) which, with a large surface area, is capable of generating a lot of heat quickly and without great flow losses to be saved and returned just as quickly. However, the many layers of fabric required are very complex to manufacture. The regenerator is therefore often the most expensive component in the Stirling engine. Usually it is dimensioned so that it can store around five times more energy than is supplied to the expansion space per work cycle. How to get the formula

the temperature difference and volume ratio must be made as large as possible. (The indices refer to the picture above right) The factor n (the amount of substance in the working gas) means that the work that can be achieved increases proportionally to the amount of active gas, which is why the internal pressure of the Stirling engine should be as high as possible.

Individual evidence

  1. ↑ a b c d e f g D. Sucker, P. Kuhn: Heat transfer in regenerators. In: Association of German Engineers, VDI Society for Process Engineering and Chemical Engineering (ed.): VDI-Wärmeatlas. Calculation sheets for heat transfer. 7th expanded edition. VDI-Verlag, Düsseldorf 1994, ISBN 3-18-401362-6 . Pp. N1-N14.
  2. ↑ Otto Carlowitz, Olaf Neese: Starting points for the conceptual and operational optimization of thermal exhaust gas cleaning systems with regenerative exhaust air preheating. In: Hazardous substances - cleanliness. Air . 65, No. 7/8, 2005, ISSN  0949-8036 , pp. 320-327.
  3. ↑ a b Hans D. Baehr, Karl Stephan : Heat and mass transfer. Springer Verlag, Berlin and Heidelberg 1994, ISBN 3-540-55086-0 , pp. 46-47.
  4. ↑ VDI 2442: 2014-02 exhaust gas cleaning; Processes and technology of thermal waste gas cleaning (Waste gas cleaning; Methods of thermal waste gas cleaning). Beuth Verlag, Berlin. P. 22.