Supercritical CO2 Mixture Behavior for Advanced Power Cycles and Applications

by Tom Lewis, Steven Wright, & Thomas Conboy
Sandia National Laboratories

Complete Paper to be Available Following the 2011 Supercritical CO2 Power Cycle Symposium

Supercritical CO2 is considered by many to be an extremely advantageous working fluid for advanced systems with high power density requirements. The supercritical CO2 program at Sandia National Laboratories has been investigating the behavior of several supercritical CO2-based mixtures, including: SF6, neon, butane and methane in an operating closed S-CO2 compression loop to determine effects of secondary gas additions to lower or raise the supercritical state, and to evaluate compressor performance. This research effort will help establish a knowledge base for the compression behavior of supercritical CO2 fluid mixtures.  Adjustment of the CO2 mixture’s critical temperature and pressure broadens the applicability of the supercritical working fluid for a variety of possible power sources and heat sinks. By lowering the critical temperature it is possible to optimize the cycle efficiency during the colder winter months when heat sink temperatures are below 31oC (the critical temperature of CO2). Likewise, if the critical temperature is raised, the ability of dry cooling becomes a competitive alternative for arid environments and for most concentrator solar power generating systems.  

Results of the current work conducted at Sandia National Laboratories, have shown interesting agreement on non-intuitive state properties of SF6 and CO2 mixtures. These results are demonstrated by initial dropping of the critical temperature of a CO2 mixture as SF6 is added. The addition of SF6 to a CO2 dominated mixture will reduce the critical temperature until SF6 becomes the dominant component. However, the experiments with a minor addition of neon to supercritical CO2, were inconclusive as most of the measureable equation of state properties changed very little for pure CO2 due to the magnitude of the critical temperature and pressure change being difficult to quantify.    The loop’s compressor has been demonstrating stable operation while pumping supercritical mixtures, as evidenced by steady mass flow and pressure ratio runs recorded for a given compressor speed. Current mixture research will be focused on the addition of butane and methane. A brief summary of the measured results from these experiments will be presented at the conference. The work accomplished so far, though incomplete, continues to show both the promise of supercritical CO2 mixtures and the need for further research of the operation of fluids in an operating power cycle.  This presentation will discuss the recent work of CO2 mixtures, their applications in advanced power cycles and a general discussion of the supercritical closed Brayton cycle at Sandia National Laboratories.