Heat Transfer in a Form-Stable Direct-Contact Latent Thermal Energy Storage Unit

Abstract

This study investigated various aspects of thermal storage concept, including material characterization and analysis of form-stable, unencapsulated phase-change materials (PCM) that underwent solid-solid phase transition and was in direct contact with the working fluid. The study focused on temperature range between 100-140°C (212-284°F). Mathematical heat transfer models were developed to examine the operating characteristics of the thermal energy storage unit, identify key parameters influencing storage, and conducted parametric studies. Both single-phase and phase-change working fluids were considered in the models. Experiments were conducted using a packed bed of PCM pellets and a single-phase working fluid (tri-ethylene glycol) to evaluate and demonstrate the heat storage concept during charging and discharging. The experimental results aligned well with the heat transfer models, validating their accuracy. Parametric studies explored a wide range of parameters not feasible in laboratory experiments, shedding light on charging, discharging, and thermal storage characteristics. These models facilitated the development and implementation of optimization algorithms for packed bed latent heat storage units. The findings indicated that form-stable latent heat units utilizing commercially available polymers undergoing solid-solid phase transition can serve as long-term stable thermal storage candidates for use with several single-phase working fluids as well as two-phase steam.