Thermal and economic evaluation of thermocline combined sensible-latent heat thermal energy storage system for medium temperature applications

Abstract

In the current work a new thermocline combined sensible-latent heat thermal energy storage configuration is proposed as an alternative to the currently used thermal storage systems; containing solid rod structures of cheap naturally occurring material with phase change material capsules impregnated between the rods. An energy balance method coupled with an enthalpy based technique are used to develop a comprehensive transient numerical model. The numerical simulations are performed to compare the performance and the cost of the three types of thermal energy storage systems. The three thermal energy storage systems are; sensible rod structure, encapsulated phase change material and combined sensible-latent heat. The influence of different evaluation indexes on economic feasibility and the performance of thermal energy storage such as capital cost, capacity cost per kWh, axial temperature distribution, pumping work, thermocline degradation, effective discharging time and effective discharging efficiency; are analyzed. The results show that effective discharging efficiency and capacity costs for encapsulated phase change material, combined sensible-latent heat, sensible rod structure are 95%, 87%, 76% and $42/kWh, $37/kWh, $35/kWh, respectively. Moreover, the hybrid configuration exhibits a storage capacity of 78.5 kWh/m3, which is 26% higher than sensible rod structure and 22% lower than encapsulated phase change material configuration. The results of the comparative study indicate that the combined sensible-latent heat TES system seems to be a more viable option among the considered configurations due to its optimized performance and comparatively low cost. Also due to the reasons that thermal ratcheting of the storage tank is avoided and it provides stable fluid outlet temperature.