An experimental method of determination and validation of latent thermal energy storage dynamic characteristic by effective enthalpy

Abstract

The paper draws attention to the problems of evaluating stored heat capacity in a latent heat thermal energy storage (LHTES) that operates with unsteady inlet temperature conditions. These problems are studied for custom-built LHTES designed to be charged/discharged at varying heat transfer rates in a chilled water system. The LHTES incorporates a typical fin and tube heat exchanger and commercially available ATP 20 phase change material (PCM). For the designed storage construction, the ratio of heat transfer area to PCM volume is 354 m2/m3, and the ratio of PCM volume to the total volume (compactness factor) is 0.87. The LHTES thermal characteristics were investigated on a test stand using inlet temperature inputs changing linearly at 1, 2 or 3 K/h. During each test, a complete phase transition in the PCM was ensured. Based on LHTES inlet and outlet temperature profiles, enthalpy distributions versus outlet temperature were evaluated. The experiments proved that these distributions are not a material property as they depend on the geometric characteristics of the LHTES and charging/discharging rates. However, they can describe the thermal behaviour of the PCM filling the LHTES and include the influence of a non-uniform temperature distribution inside the LHTES on the PCM hysteresis and temperature range of the complete melting/solidification. These effects can be modelled by the proposed function of effective enthalpy. The effective enthalpy allows for relatively simple monitoring of phase transition progression in the LHTES based solely on the parameters measurable outside the storage.