Experimental investigation on the energy storage/discharge performance of xylitol in a compact spiral coil heat exchanger

The main challenge ahead of widespread application of renewable energy sources is their availability intermittence preventing continuous power supply. In order to circumvent the temporal mismatch between their supply and demand, thermal energy storage systems can be utilized. In this study, a compact spiral coil thermal storage unit was investigated to analyze the impact of operating parameters such as inlet temperature and flow rate of the heat transfer fluid and explain the physics of heat transfer during the phase change process of xylitol. A concentric double spiral coil was inserted into a storage unit to ensure an improved heat transfer performance. Using experimental data, average temperature variation, heat storage/discharge rate and liquid fraction of xylitol in the storage unit were calculated. It was found that when the PCM melted, the lower density liquid PCM created buoyancy forces resulting in natural convection. On the other hand, discharging process was mainly governed by conduction. In the present storage unit, xylitol stored 450 kJ of heat in 35 min for an inlet temperature and flow rate of 130 °C and 2.5 lpm during charging and discharged 345 kJ in 50 min at an inlet temperature and flow rate of 45 °C and 2.5 lpm. The outcomes of this analysis are expected to be greatly applicable for the design of phase change material based spiral coil units to be used for instance in solar heating systems.