Sandia National Laboratories is investigating advanced Brayton cycles using supercritical working fluids for use with solar, nuclear or fossil heat sources. The focus of this work has been on the supercritical CO 2 cycle (S-CO2) which has the potential for high efficiency in the temperature range of interest for these heat sources, and is also very compact, with the potential for lower capital costs. The first step in the development of these advanced cycles was the construction of a small scale Brayton cycle loop, funded by the Laboratory Directed Research & Development program, to study the key issue of compression near the critical point of CO 2 . This document outlines the design of the small scale loop, describes the major components, presents models of system performance, including losses, leakage, windage, compressor performance, and flow map predictions, and finally describes the experimental results that have been generated. 4

This Sandia supported research project evaluated the potential improvement that "condensing" supercritical carbon dioxide (S-CO 2 ) power cycles can have on the efficiency of Light Water Reactors (LWR). The analytical portion of research project identified that a S-CO 2 "condensing" re-compression power cycle with multiple stages of reheat can increase LWR power conversion efficiency from 33-34% to 37-39%. The experimental portion of the project used Sandia's S-CO 2 research loop to show that the as designed radial compressor could "pump" liquid CO 2 and that the gas-cooler's could "condense" CO 2 even though both of these S-CO 2 components were designed to operate on vapor phase S-CO 2 near the critical point. There is potentially very high value to this research as it opens the possibility of increasing LWR power cycle efficiency, above the 33-34% range, while lowering the capital cost of the power plant because of the small size of the S-CO 2 power system. In addition it provides a way to incrementally build advanced LWRs that are optimally designed to couple to S-CO 2 power conversion systems to increase the power cycle efficiency to near 40%. 4 Executive SummaryThis "late start" LDRD project evaluated the potential improvement that "condensing" supercritical carbon dioxide (S-CO 2 ) power cycles can have on the power conversion efficiency of Light Water Reactors (LWR). The research was performed over a period of about 3-4 months and consisted of both analysis and experiments. The analytical portion of research project identified that a S-CO 2 "condensing" re-compression power cycle with multiple stages of reheat can increase LWR efficiency to ~37-39%, according to computational models. Typical LWRs using steam turbines operate closer to 33-35%. The experimental portion of this project used Sandia's S-CO 2 research loop to show that the as-designed radial compressor could efficiently "pump" liquid CO 2 and that the gas cooler could "condense" CO 2, even though both of these components were designed to operate using single phase CO 2 near the critical point.There is potentially very high value to this research, as it opens the possibility of increasing LWR power cycle efficiency above the 33-35% range, while lowering the capital cost of the power plant due to the small size of the S-CO 2 power system . In addition this provides a way to incrementally build advanced LWRs that are optimally designed to couple to S-CO 2 power conversion systems, to further increase the power cycle efficiency beyond 40%.The research project consisted of two portions, an analysis portion and an experimental portion. The first portion performed a series of power cycle analysis to assess the potential of S-CO 2 power systems to increase the efficiency in LWRs. The power cycle that appears most suitable for LWRs is the "condensing" re-compression cycle with multiple stages of reheat. The second effort performed a series of experimental tests using the Sandia S-CO 2 compression test-loop to validate the ability of these power ...

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The DOE Office of Nuclear Energy and Sandia National Labs are investigating supercritical CO2 Brayton cycles as a potentially more efficient and compact power conversion system 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 both high efficiency, in temperature range (400–750 C), and for reduced capital costs due to very compact turbomachinery. The cycle achieves high efficiency due to 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. 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. Other portions of the program include modifications to the compression loop to operate it as a simple heated Brayton loop by adding a small turbine and a heater, but with no recuperator. The early testing of this simple Brayton cycle is under way. 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 exchangers. The re-compression (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 current compression loop uses a main compressor that is identical to the main compressors 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 tools 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 tests are providing the first measurements and information on these important supercritical CO2 power conversion system questions. Some of this data is presented in this report. 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. Compressor inlet fluid densities have been varied from 14% to 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 loops have 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. These results have positive implications for the ultimate success of the S-CO2 cycle. In general the main compressor shows no adverse behavior while operating over a wide range of normal operating conditions. It operates reliably and with performance values that are very near the predicted results. Future efforts will focus on operating the Brayton cycle loop at sufficiently high temperatures that electrical power can be produced near the end of 2009. The compression-loop hardware is now the test bed for confirming the remaining parameters to support the next stage of development — which is the 1 MW heater-class split-flow or re-compressor Brayton cycle.

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