## 1 Introduction  
The thermal energy storage is a key element for increasing the operational flexibility of energy systems, especially when these are supplied by renewable energy sources. When the space available for storage technologies is limited, such as in urban contexts, Latent Heat Thermal Storage (LHTS) through the melting/solidification of Phase Change Materials (PCMs) can reduce the occupied volume up to five times with respect to sensible thermal storages to reach the same energy content. Therefore, an increasing amount of attention is focused on LHTS systems in order to advance their Technology Readiness Level (TRL) and market development. Currently, LHTS technologies have scarcely been introduced to market, and their TRL is lower than seven [1], which indicates that this technology has generally been demonstrated only in operational environments. In this framework, numerous LHTS modeling approaches have been proposed in the existing literature. LHTSs are commonly analyzed through detailed numerical models, which, in few cases, are validated by experimental results. In fact, the intrinsic physical complexity of the phase change phenomenon can be easily simulated by numerical models [2]. Most of the models are focusedon LHTSs design optimization with the aim of enhancing the heat transfer between the PCM and the heat transfer fluid (HTF). As reported in [3], several enhancement techniques can be pursued. Among these, numerous works investigated the optimal sizing of HTF tube fins. For instance, [PERSON] et al. [4] performed a numerical study of a shell-and-tube LHTS prototype filled with PCM salts with a full 3D model. Using commercial software based on a finite element scheme, they determined the optimal number of HTF tubes and longitudinal fins per tube that minimizes the overall discharging time. They also monitored a few performance parameters of the identified configuration, such as the melting fraction, charging/discharging times, outlet HTF temperature, and energy stored. Then, results were validated with experiments [5] showing good agreement. [PERSON] et al. [6] adopted a shape optimization strategy for Y-shaped fins with one or two bifurcations in order to enhance the performance of a shell-and-tube LHTS. A two-dimensional cross-section model, solving the underlying equations with a commercial finite volume code, was implemented. The fins optimization was performed requiring the complete discharge of the LHTS unit both in a short and long-time interval. A further application of CFD modeling to LHTS fins design optimization was presented by [PERSON] et al. [7]. They extended the investigation of the fins design through the combination of topology optimization techniques and ad hoc finite element two-dimensional and three-dimensional models. Here, only heat conduction is considered. Then, the same authors increased their model complexity by including also natural convection [8]. [PERSON] et al. [9] developed a detailed 3D in-house code to study the performance of an annular LHTS with multiple tubes. More specifically, they determined the effect of the arrangement and number of tubes on the liquid fraction and melting time. Then, in another work [10], the same model was also adopted to investigate the influence of variable operational parameters such as HTF inlet temperature and flow rate on the LHTS performance. Similarly, [PERSON] et al. [11] compared the thermal behavior of a shell-and-tube LHTS using a 2D conduction-convection heat transfer model for two different LHTS orientations (vertical and horizontal). In an attempt to simplify the simulation of LHTS systems, [PERSON] et al. [12] coupled a 1D model for the heat exchanger tubes and a reduced 3D model for the PCM and fins. This model can be used to improve the geometric and operational parameters of fin-and-tubes LHTSs with a limited computational effort, although the PCM parameters should be carefully calibrated. A different simplification approach was followed by [PERSON] et al.