Effect of Compressor Inlet Pressure on Cycle Performance for a Supercritical Carbon Dioxide Brayton Cycle

Supercritical carbon dioxide (sCO₂) Brayton power cycles take advantage of the high density of CO₂ near the critical point to reduce compressor power and increase cycle efficiency. However, thermophysical properties of CO₂ vary drastically near the critical point. Concerns of large property variations and liquid formation within the compressor can result in sCO₂ cycle designers selecting compressor inlet operating conditions substantially above the critical point, thereby reducing cycle performance. The Naval Nuclear Laboratory has built and tested the 100 kWe Integrated System Test (IST) to demonstrate the ability to operate and control an sCO₂ Brayton power cycle over a wide range of conditions. Since the purpose of the IST is focused on controllability, the design compressor inlet conditions were selected to be 8.2°F (4.6°C) and 270 psi (18.4 bar) above the critical point to reduce the effect of small variations in compressor inlet temperature and pressure on density. This paper evaluates the effect of design compressor inlet pressure on cycle efficiency for a simple recuperated Brayton cycle and the performance of an operating Brayton power cycle with a fixed design over a range of compressor inlet pressures.