Supercritical carbon dioxide (S-CO2) power cycles have been gaining interest in recent years due to their high efficiency and compact components. As interest in CO2 cycles grows, work is being done to test the operation and control of these cycles and model individual component and system performance. A key aspect in this work is the accuracy of the physical properties of the fluid used in the design, system operation, and model validation efforts. A commonly used physical property database is the NIST REFPROP database which implements correlations developed for CO2 by Span and Wagner.

Bechtel Marine Propulsion Corporation (BMPC) is testing an S-CO2 Brayton system at the Bettis Atomic Power Laboratory. The Integrated System Test (IST) is a two shaft recuperated closed Brayton cycle with a variable speed turbine-driven compressor and constant speed turbine-driven generator using S-CO2 as the working fluid designed to output 100 kWe. The IST utilizes coriolis mass flow meters to measure system flow rates as well as CO2 density at various points in the loop. Operational test data for CO2 temperature, pressure, and density are presented and comparisons made on the ability to accurately calculate the density of CO2 using REFPROP with temperature and pressure measurements for an operating Brayton loop. An uncertainty analysis using vendor specified instrument accuracy was performed to show that the calculated and measured properties of CO2 for the operating conditions examined are in good agreement.

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