Improved thermocline initialization through optimized inlet design for single-tank thermal energy storage systems

Single tank thermal energy storage systems based on the thermocline concept have attracted large interest in the last years at both, scientific and industrial levels, as cost-effective alternative to the commercially available and proven molten salt double tank storage system. Recently, many experimental and modeling results of these systems validating the technology and addressing guidelines to optimize its thermal performance have been reported. However, high cyclic efficiencies predicted in simulation have not been reported to be achieved experimentally. This paper addresses one of the reasons for this discrepancy, i.e., the fact that the additional temperature stratification volume generated by the fluid distribution at the tank's inlet is typically not considered in theoretical models. This paper demonstrates that assuming idealized flow distribution in the tank's cross-section will not represent reality adequately. Using this approach, the thermocline thickness at the beginning of the charge operation can be misestimated by more than 50%. Furthermore, a sensitivity analysis of the geometric characteristics of a specific radial fluid distributor type is included in this work. A theoretical model is developed for this purpose and validated with experimental data. The results of the analysis show that the fluid distributor should always be located in the upper-most part of the tank to minimize the initial thermocline thickness. Besides, the analysis of the cross-sectional area occupied by the distributor has been analyzed. It is shown that an optimal value exists for this parameter in order not to inhibit thermal stratification and to create further temperature gradients by subdividing the tank and thereby creating fluid-dead volumes.