Optimization of a Phase Change Thermal Storage Unit

Abstract

Several options exist to collect thermal energy from the sun for domestic use. This study examines a system integrating evacuated tube collectors with heat pipes with a storage unit using melted paraffin wax to store thermal energy. A shell-and-tube heat exchanger is embedded within the paraffin wax storage with a volume of 0.23 m3 . The heat exchanger includes two loops: one for glycol to transfer heat to the paraffin and one for water to extract heat from the melted paraffin. Although the paraffin has the benefit of being inexpensive and nontoxic, it has low thermal conductivity. Therefore, the heat exchanger has large brazed copper fins to extend areas of high thermal conductivity into the wax reservoir. To determine the benefit of the fins, wax and working fluid temperature data are taken from a constructed thermal energy storage unit and then used to verify a finite-difference one-dimensional analytical model of the unit. The maximum operating temperature of the glycol/water mix heat transfer fluid was approximately 65 °C when the fluid flowed at 3.78 l/min. City water at approximately 11.34 l/min was used to test the water heating capabilities of the unit. The one dimensional model proved useful in predicting the heat storage mode of the system. Due to its form, which was specifically developed for the unit in the study, the model could be adjusted to calculate thermal performance of similarly constructed thermal storage units.