The famous dutch Philips research laboratory developed experimental Stirling engines for a wide variety of applications and continued to work in the field until the late 1970s, but only achieved commercial success with the reversed Stirling engine.
Philips constructed a refrigerator based on the Stirling cycle, which came to know as the Philips refrigerator.
The basic type of Philips-Stirling-type cooler. From left to right it consists of a piston, a compression space, and heat exchanger (all at ambient temperature Ta), a regenerator, a heat exchanger, an expansion space, and a piston (all at the low-temperature TL).
Left and right the thermal contact with the surroundings at the temperatures Ta and TL is supposed to be perfect so that the compression and expansion, are used to reduce the total input power, usually, helium is used as the working fluid.
The cooling cycle is split into 4 steps. The cycle starts when the two pistons are in their most left positions:
- The warm piston moves to the right and the cold piston is fixed. The compression at the hot end is isothermal (by definition), so heat Qa is given off to the surroundings at ambient temperature (Ta).
- The two pistons move to the right. The volume between the two pistons is kept constant. The hot gas enters the regenerator with temperature (Ta) and leaves it with temperature TL. The gas gives off heat to the regenerator material.
- The cold piston moves to the right while the warning piston is fixed. The expansion is isothermal and heat QL is taken up. This is useful cooling power.
- The two pistons move to the left while the total volume remains constant. The gas enters the regenerator with low-temperature TL and leaves it with high temperature (Ta) so heat is taken up from the regenerator material. At the end of this step, the state of the cooler is the same in the beginning.
The corresponding cycle consists of two isotherms and two isochoric (constant volume process). The volume (V) is the volume between the two pistons. In practice, the cycle is not divided into discrete steps as described above. Usually, the motions of both pistons are driven by common rotary axes which make the motions harmonic. The phase difference between the motions of the two pistons is about 90. In the ideal case, the cycle is reversible so the COP (the ratio of the cooling power and the input power) is equal to the Carnot COP.
It is not so practical to have a cold piston, as described above, so, in many cases, a displacer is used instead of the cold piston. A displacer is a solid body that moves back and forth in the cold head driving the gas back and forth between the water and the cold end of the cold head via the regenerator.
In Philips-Stirling refrigerators that reach temperatures of 17K and produce 1 W of refrigeration at 25 K, the compressed working fluid (Helium) is expanded in two or three stages at temperatures intermediate between room temperature and the lowest temperature reached. There is only a single piston for compression.
- Extremely environmentally friendly
- Does not cause ozone layer depletion in any way
- Does not contribute to the greenhouse effect
- Extremely efficient compared to other cryogenic cooling principles.
- Does not emit any harmful or toxic product
Philips-Stirling refrigerators or cooler is used in a wide range of applications, including the production of liquid gases and the cooling of gases, liquid and industrial processes.