Optimal operation of thermal energy storage units based on stratified and fully mixed water tanks

This paper refers to thermal energy storage units based on fully mixed and stratified liquid storage tanks. The stratified unit was modeled by using three partial differential equations governing the time and space dependence of hot gas, liquid storage medium and pipes wall material, respectively. For the study of fully mixed units a standard approach based on the number of thermal units concept was slightly generalized to take into account the time dependence of the heat transfer coefficients. The performance indicator adopted here is the minimum exergy destruction. All relevant irreversibility sources were considered. Every operation regime has an optimum charging time, which increases by decreasing the hot gas inlet temperature. Also, there is an optimum gas mass flow rate that makes the exergy destruction number a minimum minimorum. The stratified units are normally more effective than the fully mixed units from the point of view of exergy conservation. The standard approach based on the number of thermal units concept tends to overestimate the performance of thermal energy storage systems. Its accuracy is better in case of fully mixed units.