Optimization and Analysis of a Heat Exchanger with Encapsulated Phase Change Material

Abstract

In this study, the performance of a heat exchanger (HX) with encapsulated phase change material (EPCM) undergoing alternating melting and freezing is studied with the intent of analyzing the thermal behavior and optimizing its performance. The specific application of the EPCM HX is to cool the exiting hot water stream from the steam condenser in a power plant in lieu of a traditional cooling tower. The HX consists of a series of stacked flattened tubes with encapsulated phase change material (PCM) and arranged in lateral sections that are alternately exposed to orthogonal streams of the hot water and the ambient air drawn by a fan. A systems-level code was developed to predict the quasi-steady state and transient performance of the HX. Using large-eddy simulations, new heat transfer and pressure drop correlations for in-line flattened tube banks were developed to predict the HX performance. The capital cost of the HX is minimized using the genetic algorithm for a specific cooling duty and coefficient of performance (COP). The effect of various design parameters on the capital costs is also studied. The study shows that using PCMs with different melting temperatures along the water flow direction enhances the overall heat transfer. It is observed that more PCM is required to meet the desired COP and cooling duty for EPCM HX with a smaller footprint or height due to lower allowable air velocity, thus demanding a higher tube surface area. The transient simulations reveal that the PCM undergoes sensible heat transfer locally due to spatial nonuniformities in heat transfer distribution.