Unsteady-state thermal performance analysis of cascaded packed-bed latent thermal storage in solar heating system

Thermal storage system is crucial to economically and stably utilize solar energy to address its volatility and fluctuation. A cascaded packed-bed latent heat storage system combined with solar evacuated tube collector is constructed in this paper, and the dynamic thermal performances are numerically investigated. The solar evacuated tube collector model is established to simulate the heat transfer fluid (HTF) charging process, which is experimentally verified by comparing the inlet/outlet temperatures. A latent heat storage system in three-stage packed-bed using phase-change materials with different melting temperatures ranging from 311 K to 335 K is modeled by enthalpy-porosity method, which is validated by comparing with the experimental data of literature. Then, the solar thermal storage system is integrated, and the impacts of the mass flow rate of HTF, solar collector area and initial temperature of packed-bed on its dynamic energetic and exergetic performances are analyzed. The results and discussions are analyzed to obtain optimal operation conditions. The increase of mass flow rate by 0.04 kg/s declines the maximum temperature by 9.53%. The initial temperature of packed-bed ranging from 293 K to 300 K achieves higher HTF temperature, heat storage efficiency and charging capacity. The solar collector area is necessarily matched to the phase-change materials to obtain high efficiency and also avoid HTF vaporization.