Evaluation on the Discharging Rate and State of Health of Paraffin/HDPE after 10,000 Thermal Cycling under Various Charging Speed

Abstract

Fast-charging mode thermal energy storage (TES) can be obtained by improving the average heat transfer rate of the system. However, it leads to the risk of performance deterioration for the long-term operation. The impact of charging rate for paraffin and FS-Paraffin (form-stable paraffin) is evaluated in the present work. The heating/cooling process is repeated for 10,000 cycles with different charging rate (1℃/min, 5℃/min and 10℃/min). The performance is focused on the discharging rate and thermal decomposition. It shows that the fast-mode treatment causes a severe deceleration in the discharging speed. The lowest discharging rate of paraffin is obtained at 0.64℃/min. The FS-Paraffin is generally better to withstand from thermal deterioration. It has a suitable discharging rate (0.82℃/min) with the lowest state of health (SoH) percentage of 97.3%. It implies that the presence of polymer additive (20 wt% high-density polyethylene) for FS-Paraffin reduces the impact of fast-charging operation. Further evaluations are discussed in the article, including the impact on SoH, thermal conductivity and storing performance. The overall condition indicates that thermal deterioration likely occurred during the fast-mode operation and requires extensive protection to maintain the long-term operation of the TES material.