An Approach to Turbomachinery for Supercritical Brayton Space Power Cycles

摘要

Closed Brayton cycles using supercritical working fluids have the potential to reach high efficiencies at lower turbine inlet temperatures, using extremely compact turbomachinery. This general concept has many potential applications, including light-weight energy-dense electrical power systems for space propulsion and life-support. Hardware development for this technology has been led by Sandia National Laboratories, where supercritical CO_2 turbomachinery running on gas foil thrust and journal bearings is being operated. One of the primary challenges in supercritical fluid turbomachinery design has involved the development of high pressure strategies for bearings and seals; the bulk of this paper outlines a strategy that has been employed to address these issues, leading to successful testing of S-CO_2 power turbines. These turbines are hermitically-sealed and sized at 125kWe, therefore the approaches taken are commensurate with design concerns for hardware for space power systems. Space power systems have different requirements than Earth-based systems. In particular, this is apparent in fact that CO_2 is favored as a working fluid in Earth's atmosphere due the proximity of its critical temperature (304K) to ambient conditions. In space, fluids with critical temperatures near 400K or higher are favored in order to maximize the average heat rejection temperature and minimize radiator mass. Still, the lessons learned from supercritical CO_2 apply, and designs developed are a good starting point for working with other supercritical fluids. Experience with CO_2 at Sandia has been generalized to show that seals and bearings performance depends on fluid pressure and density, above all. This study will summarize designs for bearings and seals technology for the Sandia supercritical CO_2 turbine, and in doing so, identify and propose solutions to the challenges for supercritical working fluids for advanced space power systems.