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.

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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.

Supercritical 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 [1,2,3,4]. Sandia National Labs (Albuquerque, NM, US) and the US 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. [5] (Arvada, CO, US). 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 — according to models, as much as 80 kWe per generator depending on loop configuration — 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°F/615K), shaft speed (52,000 rpm), pressure ratio (1.65), flow rate (2.7 kg/s), and electrical power generated (20kWe). 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 supercritical 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.

The U.S. Department of Energy is currently focused on the development of nextgeneration nuclear power reactors, with an eye towards improved efficiency and reduced capital cost. To this end, reactors using a closed-Brayton power conversion cycle have been proposed as an attractive alternative to steam turbines. The supercritical-C02 recompression cycle has been identified as a leading candidate for this application since it can achieve high efficiency at relatively low operating temperatures with extremely compact turbomachinery. Sandia National Laboratories has been a leader in hardware and component development for the supercritical-C02 cycle. With contractor BarberNichols Inc., Sandia has constructed a megawatt-class S-CO2 cycle test-loop to investigate the key areas of technological uncertainty for this power cycle and to confirm model estimates of advantageous thermodynamic performance. Until recently, much of the work has centered on the simple S-CO2 cycle-a recuperated Brayton loop with a single turbine and compressor. However, work has recently progressed to a recompression cycle with split-shaft turbo-alternator-compressors, unlocking the potential for much greater efficiency power conversion, but introducing greater complexity in control operations. The following sections use testing experience to frame control actions made by test loop operators in bringing the recompression cycle from cold startup conditions through transition to power generation on both turbines, to the desired test conditions, and finally to a safe shutdown. During this process, considerations regarding the turbocompressor thrust state, CO2 thermodynamic state at the compressor inlet, compressor surge and stall, turbine ulc ratio, and numerous other factors must be taken into account. The development of these procedures on the Sandia test facility has greatly reduced the risk to industry in commercial development of the S-CO2 power cycle.

Small-scale supercritical CO2 demonstration loops are successful at identifying the important technical issues that one must face in order to scale up to larger power levels. The Sandia National Laboratories (Sandia) Supercritical CO2 Brayton cycle test loops are identifying technical needs to scale the technology to commercial power levels such as 10 MWe. The small demonstration loops provide a scalable approach to identify cost, technical hurdles, and future commercialization plans of commercial applications. The small size of the Sandia 1 MWth loop has demonstration of the split flow loop efficiency and effectiveness of the Printed Circuit Heat Exchangers (PCHEs) leading to the design of a fully recuperated, split flow, supercritical CO2 Brayton cycle demonstration system. However there were many problems that were encountered such as; the high rotational speeds in these units identified the need to address bearing, seals, thermal boundaries, and motor controller problems to prove a reliable power source in the 300 kWe range. Although these issues were anticipated in smaller demonstration units, we also understood that commercially scaled hardware would eliminate these problems caused by high rotational speeds at small scale. The economic viability and development of the future scalable 10 MWe solely depends on the interest of DOE and private industry. The Intellectual Property collected by Sandia proves that the ∼10 MWe Supercritical CO2 power conversion loop to be very beneficial when coupled to a 20 MWth heat source (either solar, geothermal, fossil, or nuclear). This paper will identify a commercialization plan, as well as, a roadmap from the simple 1 MWth supercritical CO2 development loop to a power producing 10 MWe supercritical CO2 Brayton loop.

Small-scale supercritical CO 2 demonstration loops are successful at identifying the important technical issues that one must face in order to scale up to larger power levels. The Sandia National Laboratories supercritical CO 2 Brayton cycle test loops are identifying technical needs to scale the technology to commercial power levels such as 10 MW e .The small size of the Sandia 1 MW th loop has demonstration of the split flow loop efficiency and effectiveness of the Printed Circuit Heat Exchangers (PCHXs) leading to the design of a fully recuperated, split flow, supercritical CO 2 Brayton cycle demonstration system. However, there were many problems that were encountered, such as high rotational speeds in the units. Additionally, the turbomachinery in the test loops need to identify issues concerning the bearings, seals, thermal boundaries, and motor controller problems in order to be proved a reliable power source in the 300 kW e range. Although these issues were anticipated in smaller demonstration units, commercially scaled hardware would eliminate these problems caused by high rotational speeds at small scale.The economic viability and development of the future scalable 10 MW e solely depends on the interest of DOE and private industry. The Intellectual Property collected by Sandia proves that the ~10 MW e supercritical CO 2 power conversion loop to be very beneficial when coupled to a 20 MW th heat source (either solar, geothermal, fossil, or nuclear). This paper will identify a 4 commercialization plan, as well as, a roadmap from the simple 1 MW th supercritical CO 2 development loop to a power producing 10 MW e supercritical CO 2 Brayton loop.

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