Beata Pytlik

The paper presents the experimental study of the zeolite heat storage unit discharging process in a laboratory scale. The Authors focused on the discharging process, which utilizes adsorption of water, in the form of steam, on zeolite, because the adsorption process is considered as more challenging in terms of reaction kinetics and heat transfer. The Authors designed and built a laboratory stand with a sorption heat storage unit filled with 13X zeolite and with a separated heat transfer fluid system, where air was used for discharging. Dynamic parameters including the temperature of inlet and outlet air and the temperature distribution inside the zeolite bed during the discharging process were investigated. The gathered measurement data were used to determine the heat fluxes and to compute dynamic heat balance of the thermal storage unit including internal and external heat losses. It was demonstrated that the applied design and scale of the thermal storage unit allows to reach the thermal power over 300 W and heat the discharging air from 40 °C to over 110 °C. The innovative aspect of the study is the improvement of operational stability of the sorption heat storage unit through the implementation of a heat exchanger design that separates the heat transfer fluid from the zeolite bed, as well as a control system with a neural network layer for predicting the mass flow rate of steam.

Heat storage is an emerging field of research, and, therefore, new materials with enhanced properties are being developed. Examples of phase change materials that provide high heat storage are inorganic salts and salt mixtures. They are commonly used for industrial applications due to their high operational temperature and latent heat. These parameters can be modified by combining different types of salts. This paper presents the experimental study of the impact of the composition of binary salts on their thermophysical properties. Unlike the literature data, this article provides a detailed analysis of the phase change process in both directions: solid–liquid and liquid–solid. The results indicate that the highest latent heat was observed for a 70% NaNO₃ content in the NaNO₃–KNO₃ mixture. Therefore, when this salt is used for heat storage, the most favorable choice is a 70:30 ratio, which provides the highest heat storage density and the lowest phase transition temperature. In the case of the NaNO₃–NaNO₂ mixture, the highest value of latent heat occurs for a ratio of 80:20, resulting in phase transition temperatures of 267.0 °C for the solid–liquid transition, and 253.5 °C for the liquid–solid transition. For heat storage applications, it is recommended to use pure NaNO₂ salt instead of the NaNO₃–NaNO₂ mixture.

This paper presents the effect of baffle geometry on the charging process of a low-temperature heat storage unit. Four different geometry variants were considered for this purpose. Each of them was simulated and the results were compared. The following parameters were selected as comparison criteria: the charging time of the heat storage unit, the change in the liquid and solid fractions of the phase change material, and the change in its temperature over time. The analysis showed that, independent from the heat transfer fluid velocity, the use of baffles did not significantly affect the charging time. Furthermore, the application of baffles of all studied types did not bring an essential decrease in charging time. It was found that the optimal solution was to use the simplest construction. Tuning of the HTF flow by the use of baffles is applicable to shell and tube heat exchangers; however, it adds no significant effects in the case of heat storage units of the proposed design. The abovementioned effect has been explained by the heat flux analysis, which shows that the heat transfer in the PCM is radically less intense, when comparing to the working fluid. Therefore, it is expected that enhancing the heat transfer between HTF and PCM material is possible by modifying the PCM–side design.