Performance Improvement in a Stirling Cooler Using Methane in Helium-Methane Mixture for Refrigeration at 130K

1. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K Prof. S. L. Bapat Indian Institute of Technology Bombay Powai, Mumbai – 400 076 email : slbapat@iitb.ac.in

2. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K Actual system and schematic of β Configuration Stirling Liquefier

3. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K • Use of condensable working fluid component: It is expected that the condensable component (Methane) will condense inside the regenerator; at the cold temperature end. The condensate will be drawn in to the expansion space along with the gaseous fluid (He) and will work as carrier fluid for the condensate. Some quantity of condensable fluid will expand isentropically to the lowest pressure level in the system; during expansion stroke of the displacer. Heat load on the condenser head causes it to evaporate in the atmosphere of carrier gas (He). The condensable component will evaporate only till the expansion space becomes saturated with the vapour.

4. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K • Use of condensable working fluid component (Contd.) The isentropic expansion process will provide work output which will reduce the net work input for the system. During the return stroke of the displacer, the condensable fluid vapours will be pushed back in to the regenerator and will re-condense at the cold end of the regenerator. Due to presence of the condensable fluid vapours, the mass of carrier fluid (He) in the system reduces. This results in the reduction in contribution from expansion of gaseous fluid towards the cooling effect against the normal Stirling cycle.

5. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K Table 1. Geometric and operating parameters of the cooler

6. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K

7. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K

8. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K Frequency = 55 Hz Mean pressure = 20.5 bar Helium-Methane Mixture Helium Figure 1. Variation in refrigerating capacity

9. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K Figure 2. Variation of the power requirement at different refrigeration temperatures

10. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K Figure 3. The simultaneous effect of the variations in the cooling capacity and power requirement as a function of refrigeration temperature required

11. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K Figure 4. The simultaneous variation of coefficient of performance (C.O.P) and Methane molar composition with respect to the refrigeration temperature

12. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K Figure 5. The variation of Methane molar concentration versus refrigeration temperature

13. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K • Conclusions • When the in-situ re-condensation of Methane is considered, one of • the main options is to use the closed cycle Stirling cycle cryo-cooler. • With reference to C.O.P. with Helium as the working fluid, the use of • Methane-Helium mixture as the working fluid in the same cooler, • without absolutely any change in hardware shows an increase in • refrigeration capacity of around 180% at 130 K further increasing to • more than 350% at higher refrigeration temperature of 160 K. • The use of condensable fluid in a Stirling cryo-cooler for Methane or • natural gas re-condensation shows great promise and that too • without any hardware modifications.

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15. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K Comparison of Carnot Cycle and Stirling Cycle on P-V and T-S Charts

16. Performance improvement in a Stirling cooler with Methane as a condensable component in Helium-Methane mixture as working fluid for refrigeration temperature about 130 K Schematic representation of state points in ideal Stirling engine, Time-displacement diagram of ideal Stirling engine