Numerical modeling of transient heat transfer in a phase change composite thermal energy storage (PCC-TES) system for air conditioning applications

This paper presents a detailed numerical analysis to describe the transient heat transfer in a phase change composite-thermal energy storage (PCC-TES) system exchanging heat with a heat transfer fluid. The "PCC-TES" is precisely composed of 78% low temperature paraffin, namely n-Tetradecane (C14H30) and 22% expanded graphite) and a heat transfer fluid (namely, ethylene glycol). Such detailed numerical approach to precisely describe the heat exchange between this specifically proposed phase change composite thermal energy storage (PCC-TES) and the heat transfer fluid has not been yet addressed in literature. The mathematical modeling of the PCC-TES involves formulating two PDEs that represent conduction within the PCC (phase 1) and advection within the heat transfer fluid (phase 2). The two coupled PDEs were solved with a finite difference method. Numerics were implemented using Fortran. Results were validated using experimental data and demonstrated acceptable agreement and an accurate representation of this specific transient heat transfer problem. The variations between experimental time traces of temperature and numerically calculated data are estimated at approximately (4-9%) and attributed to the heat loss in the actual experiment. Accordingly, the slabs in the actual experiment needed a longer time to reach the temperature of the Ethylene Glycol entering the PCC-TES structure from the inlet at the top. The paper also demonstrated that a lumped-parameter ODE model can perform an incredibly good job of predicting the enthalpy uptake of the PCC precisely matching the PDE results for the enthalpy uptake of the PCC due to the high thermal conductivity of the graphite. The proposed PCC-TES system will be integrated with air conditioning systems to efficiently meet cooling demand and reduce emissions. Previous work by our research group has demonstrated that the [AC + PCC-TES] hybrid design can reduce compressor size by 50%, double the compressor efficiency (COP) during mid/off-peak hours, reduce electricity consumption by 30%, and cut CO2 emissions by 30% - all without compromising overall system performance or building comfort level.