Optimization of the Storage Process for a Cool Thermal Storage System

Abstract

The storage process for a static-water ice-on-coil cool thermal storage system is difficult to model analytically, based on the dynamic behavior of ice production. Systems that utilize a vapor-compression cycle, with the tank acting as an evaporator, further complicate an analytical model due to the two-phase heat transfer throughout the storage tank. This analysis presents a simplified model of the storage process for a static-water ice-on-coil storage tank acting as an evaporator in a vapor-compression cycle. Specifically, the storage process is optimized by minimizing the amount of compressor work required to freeze water at 0°C. Optimization variables are refrigerant evaporating temperatures and tank heat exchanger sizing. The dynamics office production and two-phase heat transfer are simplified by assuming the overall heat transfer coefficient remains constant throughout the storage process. An average value for the overall heat transfer coefficient may be substituted and still provide useful results. A second law analysis utilizing the irreversibility developed during cool storage is also presented. The model is then used in side-by-side comparisons of compressor work, tank heat exchanger efficiency, and irreversibility, as functions of evaporating temperature, for several heat exchanger sizes.