Quasi-analytical study of dynamic performance of the thermocline heat storage system under time-varying inlet flow and temperature environment

Abstract

The dynamic thermal performance of single-tank thermocline (STTC) thermal energy storage (TES) system under oscillating boundary conditions (BCs) during cloud cover or start-up period is critical to the safety and efficiency of the whole concentrating solar power (CSP) plant. In this study, the approximate analytical solution of the STTC TES system under oscillating velocity and temperature BCs are derived from the one dimensional-two phase (1D-2P) models, which can be applied in arbitrary realistic BCs with Fourier decomposition method. After model validation with dynamic experimental data, the effects of typical sinusoidal oscillating parameters on packed bed transport phenomena and TES efficiency were investigated. It is found both of the oscillating inlet velocity and temperature lead to degradation of thermal stratification, such as outlet temperature fluctuation, while it also causes enhanced heat transfer effect and increased charging/discharging cut-off efficiency. Cut-off efficiency increases with amplitude of oscillating inlet velocity or oscillating inlet temperature, while for oscillating frequency there is optimum value. On that basis, the general optimum equation of cut-off efficiency and oscillating frequency is proposed, and the relative error is smaller than 1.0% when outlet temperature is larger than 0.011 for the discharging process with oscillating inlet velocity BC and smaller than 3.2% for oscillating inlet temperature BC. Moreover, the thermocline region movement velocity increases with inlet velocity for oscillating inlet velocity BC and decreases with oscillating frequency for oscillating inlet temperature BC.