Performance Characteristics of an Operating Supercritical CO₂ Brayton Cycle

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. Sandia National 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 °F/615 K), shaft speed (52,000 rpm), pressure ratio (1.65), flow rate (2.7 kg/s), 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 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.