Numerical analysis of measures to minimize the thermal instability in high temperature packed-beds for thermal energy storage systems

A transverse temperature variation may occur in packed-bed systems during the charging/discharging/standby processes due to heat losses of preferential areas, causing a lower pressure drop in these areas. In addition, the lower pressure drop in these areas results in an increased mass flow rate and further cooling, leading to an increased transverse temperature variation, in a positive feedback loop – a phenomenon called thermal instability. The thermal instability results in a difference between the maximum rock temperature of the packed-bed and the mean outlet temperature of the heat transfer fluid, which significantly can deteriorate the performance of the packed-bed. The numerical investigation presented in this paper addresses high temperature applications of large-scale, incorporating a thermal energy storage system utilizing air-rock packed-bed. As minimizing thermal instability is crucial, the study aim is to identify methods that can limit this in these storage systems. Transient 2-dimensional axisymmetric computational fluid dynamics models developed and validated in a previous work were used. The effects of different parameters, including inlet mass flow rate of the air, type of rocks, heat transfer coefficient of the packed-bed tank walls, inlet temperature of the air, porosity of the packed-bed, diameter of the rocks, and geometry of the packed-bed on the thermal instability during the discharging process initiated after one hour of standby process, were analyzed. Furthermore, the effect of various standby durations (1 h, 3 h, 5 h and 10 h) succeeded by a discharging process for two different cases – case 1 and case 2 – were carried out. Case 1 represents a benchmark case with initial geometric and operating parameters, while the parameters for case 2 were selected to minimize thermal instability in the packed-bed. The results suggest that the thermal instability, that is, the temperature difference between the maximum of the packed-bed rock and the mean of the outlet air, decreases by 41 %, 45 %, and 56 % with an increase in the inlet mass flow rate of the air from 0.25 kg/s to 0.45 kg/s, decreases in the heat transfer coefficient from 0.77 W/m2·K to 0.37 W/m2·K, and increases in the inlet temperature of air from 300 K to 400 K, respectively. Moreover, the results indicate that the thermal instability is 200 K and 29 K for case 1 and case 2, respectively, for a discharging process following 10 h of standby duration.</p

Numerical and experimental analysis of instability in high temperature packed-bed rock thermal energy storage systems

Due to heat losses of preferential areas of packed-bed energy storage systems, transverse temperature variations may occur during the charging, discharging and standby processes. Furthermore, the heat losses of preferential areas of the storage tank cause a lower pressure drop in these areas resulting in an increased mass flow rate and further cooling, and thereby an enhanced transverse temperature variation, in a positive feedback loop – a phenomenon called instability. The transverse temperature variations may deteriorate the performance and thereby the economic feasibility of packed-bed energy storage systems. In this paper, numerical and experimental investigations of an air-based packed-bed rock thermal energy storage system for large-scale high temperature applications are presented. The objective of the study is to predict the instability and to analyze the effect of different standby durations and storage size on the instability of the air-based packed-bed system. Transient axisymmetric computational fluid dynamics models were developed for the standby and discharging processes of the packed-bed thermal energy storage systems. In addition, experimental investigations were carried out at a test facility located at Stiesdal Storage, Denmark, using magnetite rocks as heat storage material and air as heat transfer fluid. The results suggest that the numerical predictions are in good agreement with the test data. The instability phenomenon is found to increase with the standby duration, resulting in a maximum difference of 161 K between the maximum rock temperature and the outlet air temperature for a standby period of 10 h followed by a discharging process. Moreover, the results indicate that the maximum difference between the rock temperature and outlet temperature is 73 K and 56 K for a reduced-scale and a full-scale system (no standby period), respectively