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

We have developed a system of differential-output monitors that diagnose current and voltage in the vacuum section of a 20-MA 3-MV pulsed-power accelerator. The system includes 62 gauges: 3 current and 6 voltage monitors that are fielded on each of the accelerator's 4 vacuum-insulator stacks, 6 current monitors on each of the accelerator's 4 outer magnetically insulated transmission lines (MITLs), and 2 current monitors on the accelerator's inner MITL. The inner-MITL monitors are located 6 cm from the axis of the load. Each of the stack and outer-MITL current monitors comprises two separate B-dot sensors, each of which consists of four 3-mm-diameter wire loops wound in series. The two sensors are separately located within adjacent cavities machined out of a single piece of copper. The high electrical conductivity of copper minimizes penetration of magnetic flux into the cavity walls, which minimizes changes in the sensitivity of the sensors on the 100-ns time scale of the accelerator's power pulse. A model of flux penetration has been developed and is used to correct (to first order) the B-dot signals for the penetration that does occur. The two sensors are designed to produce signals with opposite polarities; hence, each current monitor may be regarded as a single detector with differential outputs. Common-mode-noise rejection is achieved by combining these signals in a 50-balun. The signal cables that connect the B-dot monitors to the balun are chosen to provide reasonable bandwidth and acceptable levels of Compton drive in the bremsstrahlung field of the accelerator. A single 50-cable transmits the output signal of each balun to a double-wall screen room, where the signals are attenuated, digitized (0:5-ns=sample), numerically compensated for cable losses, and numerically integrated. By contrast, each inner-MITL current monitor contains only a single B-dot sensor. These monitors are fielded in opposite-polarity pairs. The two signals from a pair are not combined in a balun; they are instead numerically processed for common-mode-noise rejection after digitization. All the current monitors are calibrated on a 76-cmdiameter axisymmetric radial transmission line that is driven by a 10-kA current pulse. The reference current is measured by a current-viewing resistor (CVR). The stack voltage monitors are also differentialoutput gauges, consisting of one 1.8-cm-diameter D-dot sensor and one null sensor. Hence, each voltage monitor is also a differential detector with two output signals, processed as described above. The voltage monitors are calibrated in situ at 1.5 MVon dedicated accelerator shots with a short-circuit load. Faraday's law of induction is used to generate the reference voltage: currents are obtained from calibrated outer-MITL B-dot monitors, and inductances from the system geometry. In this way, both current and voltage measurements are traceable to a single CVR. Dependable and consistent measurements are thus obtained with this system of calibrated diagnostics. On accelerator shots that deliver 22 MA...

Performance Characteristics of an Operating Supercritical CO2 Brayton CycieSupercritical CO2 (S-CO2) power cycles offer the potential for better overall plant economics due to their high power conversion efficiency over a moderate range of heat source temperatures, compact size, and potential use of standard materials in construction. Sandia Nationai Labs (Albuquerque, NM) and the U.S. Department of Energy (DOE-NE) are in the process of constructing and operating a megawatt-scale supercritical CO2 split-flow recompression Brayton cycle with contractor Barber-Nichols Inc. (Arvada, CO). This facility can be counted among the first and only S-CO2 power producing Brayton cycles anywhere in the world. The Sandia-DOE test-loop has recently concluded a phase of construction that has substantially upgraded the facility by installing additional heaters, a second recuperating printed circuit heat exchanger (PCHE), more waste heat removal capability, higher capacity load banks, higher temperature piping, and more capable scavenging pumps to reduce windage within the turbomachinery. With these additions, the loop has greatly increased its potential for electrical power generation, and its ability to reach higher temperatures. To date, the loop has been primarily operated as a simple recuperated Brayton cycle, meaning a single turbine, single compressor, and undivided flow paths. In this configuration, the test facility has begun to realize its upgraded capacity by achieving new records in turbine inlet temperature (650°FI615K), shaft speed (52,000rpm), pressure ratio (1.65), flow rate (2.7kgls), and electrical power generated (20 kWe). Operation at higher speeds, flow rates, pressures, and temperatures has allowed a more revealing look at the performance of essential power cycle components in a supeicritical CO2 working fluid, including recuperation and waste heat rejection heat exchangers (PCHEs), turbines and compressors, bearings and seals, as well as auxiliary equipment. In this report, performance of these components to date will be detailed, including a discussion of expected operational limits as higher speeds and temperatures are approached.

Through multi-year funding from DOE-NE, Sandia National Labs supercritical carbon dioxide (SCO2) closed Brayton cycle (CBC) research and development team have recently overseen the completion of the SCO2 CBC recompression test assembly (TA), and delivery from the development contractor's facility to Sandia, Albuquerque. The primary components of the completed TA include two turboalternator-compressors and associated motor/controllers, three printed circuit heat exchangers, and six shell-and-tube heaters and associated controllers. Principal supporting components include a cooling tower, electricity-dissipating load bank, and leakage flow management equipment. With this milestone completed, significant increase in CBC R&D is anticipated with the objective of advancing the technology readiness level of components seen by industry as immature. This report presents detailed descriptions of all components and operating software necessary to operate the recompression CBC.

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 ...