Modeling and analysis of air-cooled heat exchanger integrated with supercritical carbon dioxide recompression Brayton cycle

Abstract

This paper investigates the thermal performance of the recompression supercritical carbon dioxide power cycle integrated with a direct air-cooled heat exchanger. The desirable attributes for economical concentrated solar power plants are their integration ability with thermal energy storage and to accommodate dry cooling. The expressively influencing parameters such as compressor inlet temperature, pressure, and split mass fraction have been investigated for cycle maximum efficiency under arid climatic conditions. The cooling process of supercritical carbon dioxide, unlike steam condensation, is sensible heat transfer with the non-linear variation of thermophysical properties. Effective, efficient, and affordable heat exchanger technology is crucial for the deployment of supercritical carbon dioxide Brayton power cycles in concentrated solar power plants. A MATLAB code has been developed for an air-cooled heat exchanger with multi-pass sub heat exchanger approach to overcome the inevitable temperature variations at the extreme end of the gas cooler. A two dimensional discretization methodology is used to model the heat exchanger to accommodate the rapidly varying isobaric heat capacity in the critical temperature region. The heat transfer and pressure drop performance are calculated using empirical correlations for the Nusselt number and friction factor for each sub-heat exchanger. With the increased contact time of cooling and process fluid, the desired gas cooler outlet temperature of 33.5 °C is achieved by rejecting 823.76 kW, 359.5 kW, and 219.46 kW amount of heat from each sub-heat exchanger, respectively. The results of this work are of significance for the design of the air-cooled heat exchanger for the supercritical carbon dioxide power generation system.