The Stirling Cycle

The history of the Stirling Cycle


The Stirling cycle is a thermodynamic closed cycle that was invented in 1816 (!) by a Scottish minister, Robert Stirling.

It was used in the so-called hot air engine, which was considered at the time to be capable of replacing the steam engine. This was partly because the boilers used in early steam engines were prone to explosion. The counterpart of the hot air motor, the refrigerator, was first recognized in 1832. Both machines experienced high and low points during the nineteenth century. Scottish minister, Robert Stirling

The principle behind the machines was almost condemned to obscurity after the invention of the internal combustion engine (gas-, petrol-, and diesel motors) and compressor refrigerators with external evaporation.

In 1938 Philips Research Laboratories was looking for a means of generating electricity to power radios in remote areas where there was no electricity supply. The practically-forgotten hot air motor attracted attention. In 1946 Philips started studying the cooling techniques used in the Stirling cycle. The result was the development of the cold gasrefrigerator.

This machine, the cryogenerator, marked the start of significant cryogenic activities at Philips. So even though the Stirling hot air motor never became a commercial success, the Stirling cryogenerator is incorporated in equipment used from Antarctica to the North Pole. 

In 1990, Philips’ cooling-related activities became independent and eventually continued under the name of Stirling Cryogenics BV. Thanks to continual innovation and considerable investment in R&D, the Stirling cryogenerator is now used in advanced technological machinery for cooling gases and liquids to extremely low temperatures
(200 K to 20 K).

Applications with Stirling cryogenerators are used in a wide range of applications, including the production of liquid gases, cooling gases and liquids, and cooling during (industrial) processes.

The Stirling Cryogenerator

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Stirling Cryogenerator Stirling cycleThe central element in all equipment made by Stirling Cryogenics is the Stirling cryogenerator, operating according to the principles of the Stirling cycle. This cycle is remarkable because it enables the cryogenerator to produce extremely low temperatures (less than 20 K), and allows virtually all gases and liquids to be cooled.

The Stirling cycle is a closed cycle, which means that the cryogenerator working gas (which is Helium gas) never comes into contact with the substance (gas or liquid) being cooled. This also eliminates contamination of the working gas, which results in greater operational safety. The closed Stirling cycle also brings more advantages:

The Stirling cryogenerator is extremely environmentally friendly: it does not cause ozone layer depletion in any way, does not contribute to the greenhouse effect, and does not discharge any harmful or toxic gases.

The Stirling cryogenerator is extremely efficient, especially when compared to other cryogenic processes. Stirling is the only company in the world that success -fully produces Stirling cycle- based cryogenerators with cooling power of 1,000-4,000 watt (at 77 K).

The Stirling cycle involves alternately compressing and expanding a fixed quantity of a nearly perfect gas (also known as ideal gas) in a closed cycle. Helium is being used for this. The compression takes place at room temperature to facilitate the discharge of heat caused by compression, whereas the expansion is performed at the required low temperature.

Ins and Outs 

The Stirling cycle alternately compresses andWorking stirling cryogenerators expands a fixed quantity of a nearly perfect gas (also know as an ideal gas) in a closed cycle (Helium). The compression takes place at room temperature to facilitate the discharge of heat, caused by compression, whereas the expansion is performed at the required low temperature.

The annular channel F connects spaces D and E, and contains three heat exchangers: the regenerator G, the cooler H and the freezer J. In position 1 most of the gas is in space D and at room temperature.

Phase 1: The gas is compressed by piston B.Stirling cycle: From ambient to 20K

Phase 2: The gas is displaced by means of the displacer from space D to space E, which is already at a lowtemperature. During this displacement the gas passes through the heat exchangers. The cooler dissipates the heat caused by compression through cooling water. The regenerator cools the gas almost to the temperature prevailing in space E.   

Phase 3: In this phase the actual cold production takes place, namely by expanding the gas through movement ofthe displacer and piston together.

Phase 4: The gas is returned to space D. While passing the freezer its cold is dissipated to the ambient environment, and in the regenerator it is now a bit colder and the cycle can start over again.

It is clear that a large temperature The two stage stirling cycle stirling cryogeneratordifference will occur between the compression space and the expansion space. The way in which this temperature difference is established, and what influence the regenerator has in this, is shown in Figure d. The working gas in both the compression and expansion space is initially at ambient temperature. During the first working cycle the gas is successively cooled by the cooler and by the expansion to temperature T1. When the expanded gas returns to the compression space, a temperature gradient is established in the regenerator. This means that, after the second compression stroke, theworking gas is slightly pre-cooled in the regenerator before it is expanded in the expansion spaces to reach temperature T2. After a significant number of strokes the temperature gradient in the regenerator reaches equilibrium, which meansthat the working gas reaches its lowest temperature, T3, after expansion. It is obvious that the regenerator is the most important component in this cooling process.

Two-stage Stirling cycle

The principles of the two-stage Stirling cycle are the same as for the one-stage cycle. The difference is that the working gas after being cooled in cooler H, now passes two regenerators (G1 and G2) and two freezers (J1 and J2) and is expanded twice (first in expansion space E1, second inE2). As the gas expands it cools to 80 K at the first stage and to 20 K at the second stage there are two temperature gradients between the compression space and the two expansion spaces. The influence of the two regenerators in this two-stage expansion Stirling cycle can be clearly seen.

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Video Philips Cryogenics: The Stirling cycle part 1

And old, but very interesting, movie from our predecessor Philips Cryogenics about the development and technology of the Stirling engine / cryogenerator.

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