Conceptual Design Assessment of Megawatt-Class Multi-Cylinder Free-Piston Stirling Engines

Abstract

[Abstract] Free-piston Stirling engines (FPSE) with integral linear alternators (FPSE) have been refined to a high degree in recent years by Infinia Corporation and others. Long-life maintenance-free operation has been demonstrated for up to 11 years. All prior FPSE were of a single-cylinder configuration, but a recent innovation has demonstrated the viability of a multi -cylinder free-piston configuration. This retains all the life and reliability advantages of existing FPSE, but also enables higher power density, simplification by elimination of the displacer, and better scalability to high power levels. This paper describes a megawatt-class multi-cylinder FPSE conceptual design study performed with the primary goals of maximizing volumetric power density and maintaining high efficiency at partial power levels. Various topologies were considered, and a 6-cylinder module configuration was selected. This consists of two mechanically and electrically coupled but thermodynamically independent 3-cylinder engines configured in either of two modes for fully balanced operation. Size and performance trade studies were conducted using the SAGE thermodynamic analysis program, and modular system power levels of 1.5 and 4.2 MW were selected for preliminary conceptual designs. System packaging designs were completed and performance projections, with efficiency at about 60% of Carnot, were made for power levels of 5% to 100% of nominal power. Part load performance was outstanding, in part because of the design driver for power density and compact arrangements. Conceptual design of an engine controller/inverter that rectifies the 3-phase AC engine output to DC and provides power output control through voltage regulation of the alternator to establish piston amplitude was completed as well. This advanced IGBT based inverter provides active rectification of the 3-phase alternator output and allows engine operation from minimum stroke to full stroke while maintaining a constant DC bus voltage. The entire 6-cylinder engine and control system dynamics were modeled in SimplorerTM, and extensive runs were conducted to evaluate operation over a wide range of conditions. The model projected good system response times and stability, even in the case of destabilizing parameter variations from one cylinder to another. This design concept is quite innovative, and represents a major extrapolation of power level beyond the existing FPSE experience base. While manufacturing to precision tolerances at this size level and maintaining those tolerances with long term stability is quite challenging, no fundamental concerns were identified that would preclude practical application of such a machine.