Thermofluid design and off design performance of an air-cooled heat exchanger system for a 50 MWe sCO2 CSP Plant

Despite the increasing interest in supercritical carbon dioxide (sCO2) cycles in place of Rankine cycles for power generation, there is limited information available on the design and off-design performance of mechanical air-cooled heat rejection systems for sCO2 power cycles. In this work, the conceptual design and thermofluid system modelling of an air-cooled heat exchanger (ACHE) system for a 50 MWe sCO2 CSP plant is presented. The design includes the selection of major components for the ACHE system including the piping, fan, and finned-tube bundle, as well as the development of an in-house thermofluid model of the system using Python. A model of the system was also developed using the commercial code Flownex to simulate off-design conditions. To limit auxiliary power consumption, sCO2 ACHEs require deep finned-tube bundles with forced-draft fans that operate at a relatively high static pressure rise, and a relatively low volume flow rate. The results show that flow bypass combined with fan on-off control is an effective method to maintain a bulk outlet sCO2 temperature. However, to prevent local over-cooling to temperatures below the critical point, fan speed control is required. Additionally, employing fan speed control significantly lowers auxiliary power consumption relative to bypass control.