Charging and discharging characteristics of cascaded latent heat storage (CLHS) tank for low-temperature applications

Abstract

The cascaded latent heat storage (CLHS) system effectively enhanced the thermal and exergy performance in the low-temperature range (e.g. less than 100 °C), wherefore it was particularly significant to reveal effects of system configurations and thermophysical parameters on charging and discharging characteristics. However, few studies focused on the thorough performance analysis and optimization of the CLHS system, especially for the multi-tube and multi-stage latent heat storage tank. In this study, one-tube and four-tube CLHS tanks were built, thermophysical parameters of three kinds of paraffin were characterized, and temperature change trends of PCMs in CLHS tanks were measured. Moreover, the equivalent heat capacity model was used to simulate the three-dimensional unsteady-state CLHS system. Effects of heat transfer fluid (HTF) flow rates, HTF inlet temperatures, number of PCMs stages and layouts of HTF tubes on thermodynamic performance were studied, and the overall energy and exergy performance were evaluated. Compared with one-tube tank, the heat charge/discharge efficiency of four-tube tank increased by 13.4%/12.8%, and the exergy charge efficiency increased by 18.8%. While compared with the one-stage tank, the maximum heat charge efficiency of three-stage tank increased by 9.0%, and the maximum exergy charge efficiency increased by 20.5%. The four-tube three-stage CLHS tank under 80 °C-2 lpm and 10 °C-2 lpm were taken as the optimal heat charge and discharge condition, respectively. In addition, the four-tube three-stage tank with layout 2 had a shorter heat charge/discharge time and higher heat charge/discharge rate, and PCMs with thermal conductivity of 2 W/(m‧K) resulted in better heat charging and discharging performance. Above findings demonstrated that the four-tube three-stage CLHS system owned excellent latent heat storage potential, and provided a theoretical basis for its popularization and application.