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Feasibility Study of a Heat-powered Liquid Piston Stirling Cooler
Langdon-Arms, Samuel Blair
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This thesis presents a feasibility study of the proposed liquid piston Stirling cooler or LPSC, which is a 4-cylinder double-acting alpha-type Stirling machine capable of utilising a heat input to produce a cooling effect. The novel aspects of the work include: (a) the first known experimental results of the two heater-LPSC configuration, and the associated performance predictions from a validated third order computer model; (b) development of two independent methods for obtaining the system’s operational frequency; (c) evidence and quantification of the Rayleigh-Taylor instability in reciprocating liquid pistons, and the mitigation of this instability via the use of piston floats; and finally (d) the development of design constraints and recommendations specific to the LPSC. The LPSC is a tri-thermal, thermo-mechanical, free-piston system based on the Siemens-Stirling configuration first proposed in the late 1970s. A test-rig was constructed and used as part of an experimental investigation into understanding the complex multi-degree of freedom system and identifying its feasibility for heat-powered cooling applications. In its current, un-optimised state, it has proven to be functional over a wide range of mean gas pressures (1 bar–6 bar) and relatively low heat source temperatures (80°C–150°C). A third order model of the LPSC was constructed in the computer modelling software, Sage. The model encompasses the complex system dynamics and heat transfer characteristics and is validated against experimental results. The model was used to predict a more optimised piston geometry which led to the first tangible cooling effect in subsequent experiments, in the range of 5°C below ambient. Evidence of a liquid piston acceleration limit, likely resulting from the Rayleigh-Taylor (RT) instability phenomenon, is consistently observed during the experiments. The use of submerged polyethylene piston floats is found to increase surface stability and enable maximum accelerations of 25 m/s² to 30 m/s². The two heater configuration, with two adjacent heated spaces connected in series with two adjacent absorber spaces, is shown to be the optimal configuration for cooling performance, with relative phase angles close to 90° and a conversion ratio of heater gas pressure amplitude to absorber gas pressure amplitude of 1.02 to 1. When considering test-rig performance at 150°C heater temperature and a maximum charge pressure of 6 bar, the Sage model predicts a thermal COP of 0.53 for the current set-up (19.3 mm diameter, 94 cm long liquid water pistons). This rises to 0.59 with the installation of 23 mm diameter, 3 m long liquid pistons. When the working gas in the model is replaced with hydrogen, the performance of the LPSC increases significantly: the COP increases by 2.7%, to 0.6, the cooling capacity increases by 59.1%, the cold-side temperature in the primary absorber is decreased from 13.1°C to 7°C, and the second law efficiency increases from 5.3% to 9.9%. A number of design considerations for the LPSC are explored. The existence of a potential piston acceleration limit imposed by the RT instability imposes a constraint on the minimum piston length. A crude capacity evaluation is also conducted, which indicates that a LPSC capable of generating a cooling effect of approximately 5 kW is feasible using 15 L liquid water pistons. Based on the research findings, it is deduced that such an LPSC system is feasible both technically and commercially, although to what extent in either regard is still unknown.