Department of Mechanical and Mechatronic Engineering

Permanent URI for this community

https://scholar.sun.ac.za/handle/10019.1/109

Browse

Browsing Department of Mechanical and Mechatronic Engineering by browse.metadata.advisor "Bell, A. J."

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Theoretical simulation, manufacture and experimental evaluation of a free piston stirling engine electric generator.
    (Stellenbosch : Stellenbosch University, 2019-12) De La Bat, Jean Gerard; Harms, T. M.; Dobson, R. T.; Bell, A. J.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: A Stirling engine is a suitable candidate for heat and electrical power co-generation when integrated with a parabolic-dish solar concentrator. Such solar systems are envisaged as being a viable option for micro off-grid co-generation using smart energy control strategies in developing countries rich in solar irradiance. This dissertation presents the theoretical simulation, fabrication and experimental evaluation of a novel prototype free-piston Stirling engine electric generator. The theoretical model was developed from first principles, by discretising the working fluid into a network of one-dimensional control volumes. By applying the conservation of mass, momentum and energy and applicable property functions to each control volume, a system of non-linear time-dependent partial differential equations was generated. These equations were solved sequentially using a fully-explicit numerical method with linear upwind-differencing and by employing a backwardsstaggered velocity scheme. A transient electromagnetic finite element analysis of the linear generator was performed and is represented in the theoretical simulation model through generator characteristic functions. A novel experimental engine prototype was manufactured that includes a linear motor attached to the displacer shaft. Finite element analyses were used to design the strength and safety-critical mechanical components of the engine. The electromechanical, flexure spring stiffness and displacer and piston mechanical friction characterising equations of the theoretical model were matched to the as-built engine by direct experimentation. Seven selected experimental test runs were used to establish whether the theoretical model is capable of emulating the behaviour of the as-manufactured engine, with hot-end temperatures varied between 300 and 600 C and working fluid charge pressures between 1.0 and 2.0MPa. Self-sustained, closed-circuit operation of the engine was achieved at a hot-end temperature of 470 C and charge pressure of 1.7MPa absolute, with a peak electrical output power of approximately 73W. A good correlation between experimental and simulation results was demonstrated for several operating conditions, thereby validating the theoretical simulation model. It is concluded that it is critical to ensure proper alignment of the moving parts so that internal sliding friction may be minimised. To achieve continuous and sustained operation without piston-casing collisions, it is recommended that an electronic feedback control system be integrated to the existing engine.