Preliminary Analysis of an Innovative Rotary Displacer Stirling Engine

Stirling engines are an external combustion heat engine that converts thermal energy into mechanical work that a closed cycle is run by cyclic compression and expansion of a work fluid (commonly air or Helium) in which, the working fluid interacts with a heat source and a heat sink and produces network. The engine is based on the Stirling cycle which is a subset of the Carnot cycle. The Stirling cycle has recently been receiving renewed interest due to some of its key inherent advantages. In particular, the ability to operate with any form of heat source (including external combustion, flue gases, alternative (biomass, solar, geothermal) energy) provides Stirling engines a great flexibility and potential benefits since it is convinced as engines running with external heat sources. However, several aspects of traditional Stirling engine configurations (namely, the Alpha, Beta, and Gamma), specifically complexity of design, high cost, and relatively low power to size and power to volume ratios, limited their widespread applications to date. This study focuses on an innovative Stirling engine configuration that features a rotary displacer (as opposed to common reciprocating displacers), and aims to utilize analytical and numerical analysis to gain insights on its operation parameters. The results are expected to provide useful design guidelines towards optimization. The present study starts with an overview of the Stirling cycle and Stirling engines including both traditional and innovative rotary displacer configurations, and their major advantages and disadvantages. The first approach considers an ideal analytical model and implements the well-known Schmidt analysis assumptions for the rotary displacer Stirling engine to define the effects of major design and operation parameters on the performance. The analytical model resulted in identifying major variables that could affect the engine performance (such as the dead volume spaces, temperature ratios and the leading phase angle). It was shown that the dead volume could have a drastic effect over the engine performance and the optimum phase angle of the engine is 90o. The second approach considers a non-ideal analytical model and aims to identify and account the main sources of energy losses in the cycle to better represent the engine performance. The study showed that the ideal efficiency and the non-ideal efficiency could have 15% difference that could have as an enormous effect on the engine performance.