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how it works  Figure 1 :FPSC Animation The Free Piston Stirling Cooler (FPSC) is a device which makes use of the stirling cycle and a moving magnet linear motor for cooling applications. The stirling cycle belongs to a class of thermodynamic cycles that yield the highest conversion efficiency between mechanical and thermal energy. The animation above shows how the machine works and the image below defines the components necessary to understand its operation. The stirling cycle is a reversible cycle which means that heat can be put into the machine and electric power will be produced or electric power can be put in and heat will be removed. The case of interest to us at Global Cooling is the latter of the two, the Free Piston Stirling Cooler. FPSC DYNAMICS The FPSC cycle starts with an AC input into the linear motor. This input drives a magnet ring which is rigidly attached to the piston (hence the term, moving magnet motor). The piston is the dark blue oscillating cylinder in the animantion. The white oscillating cylinder is referred to as the displacer. The difference between the displacer and the piston is that the piston has two different pressures on either of its faces whereas the pressure on either end of the displacer is the same (assuming the pressure drop through the passages between the two Figure 2 : Model M100B Image showing basic components of a FPSC. faces is negligible). This means that the pink gas in the animation cannot flow back to the other side of the piston but the blue (cold) gas on one side of the displacer is free to flow back to the pink (hot) gas face of the displacer. The piston is driven by the linear motor since it is rigidly attached to the moving magnet ring. The displacer is driven by the force which arises because of the difference in areas of its two faces.
The pressure amplitude (Pamp) is the amplitude of the pressure wave . This is not the same as the overall pressure of the machine. The FPSC is hermetically sealed and is typically pressurized 20 to 30 times atmospheric pressure. The charge port shown in figure 2 is where the helium (working fluid) is introduced into the machine. The absorber mass shown in figure 2 is a mass spring system that balances the machine. It is not shown in the animation but when the displacer and piston oscillate within the machine, the casing also oscillates. In order to mount it more easily without the transmission of vibration to a base, the absorber mass "absorbs" the vibration. FPSC THERMODYNAMICS Once the dynamics have started in a stirling cooler a very simple thermodynamic cycle ensues. To help in understanding the thermodynamics it is useful to look at a Pressure-Volume diagram. The ideal case will be as shown in figure 3.
Figure 3 : Ideal Stirling Cycle Pressure-Volume Diagram
The actual stirling cycle has many losses associated with it and does not really involve isothermal processes so it is not totally reversible. Since the FPSC involves sinusoidal motion the edges of the p-v diagram are not sharp edges as indicated in the ideal diagram in figure 3. The actual p-v diagram ends up looking more like an oval with the sharp edges of the ideal diagram rounded off. However, the ideal diagram is useful for beginning to understand the cycle. A simple second order analysis of the stirling cycle has been developed by Gustav Schmidt in 1871. The analysis has been used widely as an approximation of stirling performance. David Berchowitz and Israel Urieli give a complete description of it in their book Stirling Cycle Engine Analysis. Download the description of the Schmidt Analysis from the book. COPYRIGHT AND TRADEMARK NOTICES |
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