Conclusions

A promising case for the use of distributed solar-thermal-electric generation was out­lined in this dissertation, based on low temperature-differential Stirling engine technology in conjunction with state-of-the-art solar thermal collectors. Although the predicted ef­ficiencies are modest, the estimated cost in $/W for large scale manufacturing of these systems is quite attractive in relation to conventional photovoltaic technologies. Consider­ing that the solar to thermal energy conversion is a mature technology, the main purpose of this dissertation was to understand the operation of the Stirling thermodynamic cycle at moderate temperatures and to identify the associated challenges.

A low-power single-phase Gamma-type free-piston Stirling engine engine prototype was designed and fabricated as part of the conducted research work. This prototype incor­porates an electrically driven displacer, which is actuated independent of the power piston, hence the name free-piston. It is a resonant mass-spring system and the stiffness is provided by a magnetic spring system. The magnetic spring system is based on permanent mag­nets. The specific implementation method, significantly reduces the eddy currents and the associated power dissipation which, in turn, improves the quality factor. In addition, this spring exhibits a highly linear stiffness characteristic over its full stroke. A linear stiffness characteristic is essential to avoid the complexities associated with frequency tuning. In this prototype, the power piston is not mechanically linked to the displacer piston. It forms a mass-spring resonating subsystem with the gas spring and has resonant frequency matched to that of the displacer. The displacer piston, cylinders, and heat exchangers frame are fabricated from plastic materials. The fluid flow friction and the gas spring hysteresis losses were identified as the major dissipation sources in this system. Extensive experimentation on individual component of the fabricated engine confirmed the theoretical models and design considerations, providing a sound basis for higher power Stirling engine designs.

Existing commercial Stirling engines are used for various applications such as NASA’s deep-space missions, submarine power systems, solar dish-Stirling systems, cryocooling, etc. They are mostly high-temperature single-phase Beta-type machines. Multi-phase Stir­ling engine systems are particularly interesting because they are comprised of Alpha-type Stirling engines and, hence, eliminate the need for a displacer piston and the associated de­sign and control challenges. A detailed dynamical model of multi-phase free-piston Stirling engine systems was discussed in this dissertation. The mathematical model proved that such a system is capable of starting automatically at a minimum temperature difference that is dependent on the system internal dissipation and physical dimensions. A symmetric three-phase Stirling engine prototype was fabricated and tested in this research to validate the developed mathematical models. The use of diaphragm pistons and nylon flexure were exercised on the fabricated system as possible easy-to-manufacture components. A re­verser was introduced to modify the dynamics of a multi-phase system, mainly to lower the

Corresponding gas spring hysteresis dissipation. Mathematical modeling and experimental results were discussed for the three-phase system that is equipped with a reverser.

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