Transient performance of a thermocline energy storage system using the two-energy equation model

Abstract

This work investigates the thermal behavior of the charging cycle of a thermal energy storage system for a concentrated solar power plant filled with solid porous material. A transient model that describes turbulent flow in a hybrid medium (porous/clear) with both forced and natural convection was used. The mean flow macroscopic equations are developed based on the concept of double-decomposition. Governing equations were discretized using the SIMPLE method and the system of algebraic equations was relaxed by the SIP procedure. The k-ε turbulence model was used for modeling turbulence. The two-energy equation model was used to evaluate heat transfer between the solid and fluid phases. Simulations are based on an axisymmetric tank with external convection, hot fluid inlet at the top and distributor regions at the top and bottom of the tank. Effects of Reynolds number (Ret), porosity (ϕ) and permeability (K) were investigated. Additionally, storage (ηst) and charging (ηchg) efficiencies were compared to quantify the effectiveness of the charge. Consequently, a new thermal charge efficiency (ηth) was proposed to evaluate the charging cycle. Finally, it was found that the most efficient charging parameters were the ones with higher porosity and lower permeabilities. Moreover, increasing Ret values increased the thermal charge efficiency up to a maximum and thereafter presented a stabilized trend with further increasing of Ret values.