High-Temperature Split-Flow Recompression Brayton Cycle Initial Test Results
Supercritical CO 2 (S-CO 2 ) power plants offer the potential for better economics because of their small size, use of standard materials, and improved electrical power conversion efficiency at modest temperature (400–750°C). Sandia National Laboratories (SNL or Sandia) and the U.S. Department of Energy Office of Nuclear Energy (DOE-NE) are operating a supercritical CO 2 Brayton cycle power system—the Generation IV (Gen IV) split-flow S-CO 2 compressor test loop—currently located in Arvada Colorado, at Barber Nichols, Inc., under contract to Sandia. A photograph of the upgraded loop is shown in Figure E-1. This system is one of the first S-CO 2 power-producing Brayton cycles operating in the world. This report provides a summary of the newly installed hardware and briefly describes some of the test results that were performed in the upgraded split-flow Brayton loop during June and July of 2011. The Gen IV S-CO 2 split-flow Brayton loop was reconfigured to operate as a simple heated recuperated Brayton loop for the testing in this report period. The test loop had just concluded a phase of construction that substantially increased the capability of the loop by adding heaters, a high-temperature (HT) recuperator, more waste heat removal capability, high-power load banks, larger diameter piping with more bends to reduce thermal stress, and more capable scavenging pumps to reduce windage and friction within the turbomachinery and provide greater cooling capabilities. With these additions, the loop greatly increased its capacity for electrical power generation (30–80 kWe per generator, depending on the loop configuration) and its ability to reach high temperatures (up to 810 K [1000°F]). In recent weeks of testing, the test facility began to realize this potential by achieving new records in turbine inlet temperature (615 K [650°F]), shaft speed (52,000 rpm), pressure ratio (1.65), flow rate (2.7 kg/s), and electrical power generated (20 kWe). These operating conditions still remain short of design conditions for each turbine—an inlet temperature of 810 K (1000°F), a pressure ratio of 1.8, a shaft speed of 75,000 rpm, a flow rate of 3.5 kg/s, and a maximum generator power of 125 kWe—but they are beginning to be approached. Not all planned upgrades were implemented due to the limited funding profile of FY2010. Only 520 kW of heater power were installed—not the total 780 kW. Furthermore, because one of the heater controllers was not functioning, only 390 kW of heater power were available. Similarly, the 3-in. piping was installed only in the high-pressure leg of the loop, not in the low-pressure leg, as was originally planned. We also had to rely on the new load bank capability for rapid shutdown situations, and were not able to install a dedicated shutdown resistor. Nevertheless, in spite of these limitations, we operated the loop at record operating conditions and were largely limited only by the facility and support equipment—not the turbomachinery.