Phase change material optimization for thermal storage solar vacuum tube heaters: melting point and enthalpy effects on system performance

This study aims to optimize the structural design of solar thermal storage collectors and improve their solar energy utilizati on efficiency and practicality. A novel thermal storage solar vacuum tube heater (SVTH) was designed, and the mechanisms of photothermal conversion, transfer, and storage within the heater were investigated. A theoretical model was established to explore the effects of phase change materials (PCM) on thermal characteristics of the thermal storage SVTH. The results demonstrated that the melting temperature (Tm) and enthalpy of the PCM directly affected the thermal output of the heater. Four different thermal storage SVTH were fabricated using 58# paraffin, 80# paraffin, sorbitol, and pentaerythritol as the storage materials, followed by experimental studies. As indicated, the daily effective thermal efficiency of the four thermal storage SVTH increased by 29.78%, 61.70%, 51.06%, and 31.9%, respectively, compared to the vacuum tube heaters without PCM. This is correlated with the Tm and enthalpy of the storage materials. Additionally, a solar air heater was designed by connecting ten 80# paraffin-based thermal storage SVTHs in parallel, achieving a daily thermal efficiency of 60.44%. This study provides references for design optimization of thermal storage SVTH.