Supercritical CO2 Compression Loop Operation & Test Results

by Steven A. Wright1, Paul S. Pickard1, Robert Fuller2, Ross F. Radel1, & Milton E. Vernon1
1Sandia National Laboratories
2Barber-Nichols Inc.

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The DOE Office of Nuclear Energy and Sandia National Labs are investigating supercritical CO2 Brayton cycles because they may offer potentially more efficient and compact power conversion systems for advanced nuclear reactors, and other heat sources including solar, geothermal, and fossil or bio-fuel systems. The focus of this work is on the supercritical CO2 Brayton cycle which has the potential for high efficiency, in temperature range (400 - 750°C), and for reduced capital costs due to very compact turbomachinery1. The cycle achieves high efficiency because of the non-ideal behavior of supercritical CO2, and it achieves extremely high power density because the fluid in the turbomachinery is very dense, 10% - 60% the density of water.

Sandia and its contractor Barber Nichols Inc.2 have fabricated and are operating a supercritical CO2 (S-CO2) compression test-loop to investigate the key technology issues associated with this cycle. The compression loop is part of a multi-year phased development program to develop a megawatt (MW) heater class closed S-CO2 Brayton cycle to demonstrate the applicability of this cycle to heat sources above 400°C. A more ambitious effort is currently constructing a recompression cycle Brayton loop (1) which is some times called a split-flow Brayton cycle. This cycle is used to increase the efficiency of the system by providing large amounts of recuperation using printed circuit heat exchangers3. The recompression (or split flow) Brayton cycle is designed to operate at 1000°F (538°C) and produce up to 250 kWe with a 1.47” OD radial compressor and a 2.68” OD radial turbine. The compression loop uses a main compressor that is identical to the main compressor in all the Brayton cycles that are being developed at Sandia.

The key issues for the supercritical Brayton cycle include the fundamental issues of compressor fluid performance and system control near the critical point. Near the critical point very non-ideal fluid behavior is observed which means that standard models for analyzing compressor performance cannot be used. Thus, one of the goals of the program is to develop data that can be used to validate the tools and models that are used to design the turbomachinery. Other supporting technology issues that are essential to achieving efficiency and cost objectives include bearing type, thrust load and thrust load balancing, bearing cooling, sealing technologies, and rotor windage losses. The current S-CO2 compression tests are providing the first measurements and information on these important supercritical CO2 power conversion system questions.

In the testing to date, the turbomachinery has reached maximum speeds of 65,000 rpm, peak flow rates of over 9 lb/s and pressure ratios of just over 1.65, at compressor inlet densities near 70% the density of water. Although the data from these tests are only the first results to be analyzed, they indicate that the basic design and performance predictions are sound. The compression loop has operated the turbo-compressor on the liquid and vapor side of the saturation curve, very near the critical point, above the critical point and even on the saturation dome. We have also operated the compressor near the choked flow regime and even in surge. At the current operating speeds and pressures, the observed performance map data agrees extremely well with the model predictions and the results are presented in this report. These results have positive implications for the ultimate success of the S-CO2 cycle.

This work was supported by Sandia National Laboratories. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.