Evolution of the Stirling cycle: Emphasis on reliability, durability, and long term unattended operation

Abstract

The Stirling cycle is characterized by high efficiency coupled with the ability to effectively function on a range of heat sources. Included are industrial process waste heat, biomass, geothermal heat, and conventional combustion. Some of these sources are considered to be environmentally friendly, renewable, and have therefore interested researchers in the pursuit of pollution free or near pollution free electrical power generation. Drawbacks to the cycle include a high level of mechanism complexity which can impact reliability and durability. Further, the cycle demonstrates a relatively low power output per engine size when compared with that of other prime movers. Contemporary Stirling engine classification, Alpha, Beta or Gamma, is based on the physical layout of displacer and power cylinders with respect to a crankshaft. This report concerns a contemporary development of the Stirling cycle heat engine in which reliability, durability, and long term unattended operation are key objectives. To meet these objectives, the engine design focused on several factors which included: minimizing the number of moving parts, particularly reciprocating parts; incorporating materials not typically encountered in Stirling technology; use of liquid cooling; and, utilizing helium as the working fluid. The initial design parameters, e.g., phase angle, volume compression ratio, etc., were taken from those applicable to Gamma type engines. The literature suggests that Gamma engines represent state-of-the-art in the technology. Design efforts resulted in a working prototype with three moving parts per power cylinder. Included are a rotary displacer, a power piston and a connecting rod. Operation of the prototype demonstrated that Gamma design parameters were less than ideal for the new engine. This report summarizes the design elements of a new classification of Stirling engine and presents the results of optimization work to date. © 2012 American Society for Engineering Education.