System-level analysis of a novel air-cooled condenser using spray freezing of phase change materials
Ben Xu, Swanand Bhagwat, Hongxin Xu, Arif Rokoni, Matthew McCarthy, Ying Sun
Abstract
A comprehensive system-level analysis is performed for a novel air-cooled condenser based on spray freezing of phase change materials (PCMs). This novel air-cooled condenser uses PCMs to decouple the process of steam condensation and heat rejection to air in order to significantly improve air-side heat transfer and reduce steam condensation temperature as compared to conventional air-cooled condensers (ACCs). Melting of solid PCM particles in a two-phase PCM slurry flow anchors the steam condensation temperature close to the PCM melting point regardless of the change in ambient air temperature. Spray freezing of millimeter-sized liquid PCM droplets increases the air-side heat transfer coefficient by five times compared to the finned-tubed ACCs. A multiscale model, which directly captures the melting and settling of PCM particles at the microscopic level and accounts for phase change through energy source terms at the macroscopic level, has been developed to simulate the PCM slurry flow over heated tube bundles. Using this multiscale model, the effects of particle volume fraction, Reynolds number, and particle to steam tube diameter ratio on the averaged wall Nusselt number of the steam tubes are investigated. It is found that the averaged wall Nusselt number for a PCM slurry flow of 20% solid fraction achieves a 38% enhancement over the PCM single-phase flow of same Reynolds number. On the air side, the freezing/melting of PCM droplet/particle is approximated based on a 1-D transient heat conduction model and the air-side pressure drop across the PCM droplet array is determined using a 3-D k-ε turbulence model. The performance of this spray-freezing PCM ACC is compared against a baseline ACC of a 500 MWe power plant. It is found that, for comparable footprint area and ambient conditions, the spray-freezing PCM ACC reduces the initial temperature difference to as low as 16.8 °C and provides up to 10.8 MW net power production gain compared to the baseline ACC.
Keywords
Phase change material; Dry cooling; Slurry flow and heat transfer; Power plant cooling