## 1 Introduction  
Thermal energy storage (TES) is the most common method to store energy and shift the electricity peak load. The main advantage of TES systems is to reduce installation and maintenance costs, and also to reduce environmental impacts over the chemical-based technologies. Energy storage allows extra thermal energy to be used for future consumption. For instance, energy demand can be balanced between day and night or heat can be stored in summer for using in winter or vice versa. Generally, thermal energy storage can be applied in two different ways including sensible heat and latent heat. The energy density is much higher for energy storage by latent heat and the efficiency of such systems is generally higher too. A detailed description of different ways to store thermal energy has been proposed by [PERSON] and [PERSON] [1]. Conventional central air-conditioning systems of commercial building represent about 50% of the electrical demand, leading to a high electricity price. Air-conditioning using TES is currently considered as an efficient way to solve this issue. Cooling load demand during the warmest hours of summer days leads to an increase in the electrical energy request, intensifying the peak load. Thus, to reduce electrical energy consumption during peak hours, cold energy storage systems are used. One of the most common adopted methods is using ice on coil energy storage system. Ice thermal storage systems for office building applications have received great interest over recent decades due to their lower first cost, lower operating cost, and their ability to eliminate CFC (chlorofluorocarbons) refrigerants. Low electrical rates during nighttime (6 times lower than during daytime in average) results in designing a system operated at night which significantly decreases the operating costs and provides flexibility to control the peak of electrical rates [2]. Ice storage systems are divided into two different types including static type and dynamic type. External melt ice-on-coil type, internal melt ice-on-coil type, and encapsulated ice type are static systems while dynamic type includes ice slurry type and ice harvesting type. Internal melt ice-on-coil cold storage system are currently widely available in heating, ventilation, and air-conditioning (HVAC) applications. In internal melt ice-on-coil cold storage systems, the heat transfer fluid (HTF), which is usually a glycol solution, flows through coils in both charging (solidification) and discharging (melting) modes. Analyzing these two processes is crucial to design properly HVAC cold storage systems. Although extensive research has been conducted on the thermal characteristics of ice-on-coil cold storage systems, especially in the field of external melt, few studies have considered the internal melt ice-on-coil tanks. In this field, [PERSON] and [PERSON] [3] investigated the thermal performance of an internal melt ice-on-coil ice storage system by both experimental and numerical studies. Two different heat transfer enhancement methods including thin rings and annular fins around tubes were utilized to increase the ice formation and melting and accordingly increase the performance of the cold storage system. Three different types of tube including tube without rings, tube with diagonal rings, and tube with fins were considered. Their results indicated that tube with annular fins offers the best overall performance. At a specific distance between annular fins, the maximum ice formation was achieved. A novel paraffin-based cold storage system was investigated experimentally by [PERSON] et al. [4]; 1450 kg of paraffin were placed in a shell around 18 spiral-shaped coils. The results showed that during the melting process, the natural convection within the phase change material (PCM) is the dominant heat transfer mechanism. The better overall efficiency was obtained at the lowest supplied temperatures and mass flow rates. [PERSON] et al. [5; 6] performed an experimental analysis of an internal melt-ice-on-coil system.