Optimization of a Phase Change Thermal Storage Unit

Abstract

When collecting the energy of the sun for domestic use, several options exist, one being the use of evacuated tube collectors with internal heat pipes. This study proposes a system integrating these collectors with a storage unit using the phase change of paraffin wax to store energy. The storage unit makes use of a finned heat exchanger, with paraffin wax on the shell side and glycol on the tube side as the heat transfer fluid. The heat exchanger is embedded within the storage paraffin wax with a volume of 2 ft3. The heat exchanger also includes a separate loop for water to flow through and receive thermal energy from the melted wax. Although the wax has the benefit of being inexpensive and nontoxic, it has the problem of low thermal conductivity. Therefore, the heat exchanger has large copper fins brazed to it to extend areas of high thermal conductivity into the wax reservoir. The unit used in this study contains 14 fins. The use of fins will help to speed up the melting of the wax while solar energy is collected, since there is more heat transfer area. When most of the wax is melted, heat can be exchanged to water for domestic use. To determine the benefit of the fins, wax and working fluid temperature data will be taken from a constructed thermal energy storage unit, and then it is used to verify a finite-difference analytical model of the thermal operating characteristics. The maximum operating temperature of the glycol/water mix heat transfer fluid was approximately 65° C when the fluid flowed at 1 gallon per minute. The storage unit was able to store melted wax overnight with a 2–3°C temperature drop with the ambient temperature approximately at 30°C. City water at approximately 3 gpm was used to test the freezing side. The one dimensional model proved useful in predicting the heat storage mode of the system but had some error in predicting the heat release mode of the unit. The model also points to the fact that there are several considerations to be taken when simulating phase change energy storage processes.