Supercritical CO2 Brayton Cycle Power Generation Development Program and Initial Test Results

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.