A new autonomous heat storage concept: Feasibility studies

This work proposes a novel portable device, a heat storage column, in which phase change material (PCM) capsules are packed in a closed metal cylinder filled with water to enhance heat transfer between the cylinder sidewall and the PCM capsules via natural convection. A key advantage of the proposed device is its capacity to operate autonomously for extended periods. The device can be used as a transportable heating system for households or agriculture, for example, as a supplemental night heater in greenhouses. During the day, the columns are heated by solar irradiation; at night, they are brought indoors to heat the interior. This versatility makes the device optimal for regions with significant temperature variations between day and night, such as countries with a strong continental climate. To assess the device’s efficiency, Nusselt numbers ( N u ) are obtained from numerical simulations of natural convection in packed beds. Certain assumptions were made to simplify the model, such as constant temperatures for the cylinder sidewalls and the particles, no phase change within the capsules, and no radiation effect. As a result of the simulations, contour plots of temperature and flow velocity are obtained. New Nusselt number correlations for the particle surface have been developed. The new N u relations for particle–fluid and fluid–sidewall heat transfer can be used for the prediction of the natural convection-driven heat transfer in fixed beds with a small ratio between particle diameter and bed diameter. Such new N u expressions can be used in the so-called in-particle unresolved computational fluid dynamics (CFD)-based discrete element modelling (DEM) of fixed-bed-related technologies Finally, a simple semi-empirical model for phase change processes within spherical PCM capsules was used to illustrate the melting time of PCM in such capsules inside the new autonomous heat storage column presented in this work. The results revealed that, with a temperature difference of about 20K between the capsules and the sidewall, the PCM melting time is about 1 h. This result confirms the feasibility of the main concept of this new heat storage column. • A new autonomous heat storage column has been designed. • Multiscale 3D CFD model have been used for numerical assessments. • Very good agreement with experimental data for PCM capsule melting.