Numerical and experimental development of a medium temperature thermal energy storage (Erythritol) system for the hot side storage in a of LiBr/H_2O air conditioning system

摘要

The effect of the axisymmetric assumption to ignore the thermal conductivity of a Phase Change Material (PCM) in the direction of the heat transfer fluid flow has been experimentally demonstrated using temperature gradients (dT / dx - temperature differences per metre) in the axial, radial and angular directions. The experiment was conducted using a shell and tube system with a medium temperature phase change thermal energy storage material, Erythritol with a melting point of 117.7°C. The results show that the temperature gradient in the axial and angular directions were 1.2% and 10.2% respectively of that recorded in the radial direction. This explains why most previous models for PCM shell and tube thermal storage systems neglected the thermal conductivity in the axial direction (direction parallel to the heat transfer fluid flow) and as such solutions only apply to simple system geometries. A one-dimensional finite difference numerical model was developed to simulate the shell and tube system. Results from the model predictions were validated using measured experimental results from the melting of the medium temperature phase change material. The overall average prediction showed a 3.1% difference from the measured experimental data. The deviation occurred in two areas; at the beginning of the phase change where the model over predicted heat conduction into the PCM and at the end of the phase change where the effect of natural convection within the PCM was not simulated.