**1.3 Literature survey**

PCMs are integral to TES systems, celebrated for their ability to store heat energy efficiently in compact designs through the significant enthalpy changes during phase transitions. Ideal PCMs exhibit high latent and specific heat capacities, excellent thermal conductivity, and minimal volume change, and are nontoxic and noncorrosive, ensuring effective heat transfer [9]. TES, utilizing PCMs, is crucial for balancing thermal energy supply and demand, leading to the development of compact and efficient Latent Heat Thermal Energy Storage Systems (LHTESS) [1, 10]. These systems operate within a limited temperature range, focusing on the phase transition temperature [11]. PCMs have broad applications across various domains, including underfloor heating [12], building energy management [13], solar power [14], waste heat recovery [15], domestic water heating [16], electronics thermal management [17], passive cooling [18], and thermal insulation for unmanned underwater vehicles [19], showcasing their versatility and effectiveness in enhancing energy efficiency and management.

Nanofluids have enhanced TES by incorporating nanoparticles to enhance specific heat capacity [20]. PCMs are grouped into organics (e.g., ice, paraffin), inorganics (e.g., salt hydrates [LiNO3·3(H2O)]), and eutectics [21]. Organic PCMs, for instance,

paraffin wax, offer uniform freezing and reliability but have low storage capacity (i.e., needs larger quantities) and poor power ratings due to low thermal conductivity. Solutions like finned tubes and metal matrices have been explored to enhance power ratings [22, 23]. Inorganic PCMs like salt hydrates have high latent heat capacity but suffer from subcooling, phase segregation, degradation, poor nucleation, significant volume changes, and corrosiveness [24, 25]. Efforts to address these issues have been ineffective and costly. Eutectic Phase Change Materials (EPCMs), such as Na2CO3∙10H2O-Na2HPO4∙12H2O, offer stable PCM with high enthalpy and durability. EPCMs can achieve unique phase change temperatures, by adding stearic acid. However, EPCMs also face challenges like low thermal conductivity, phase separation, supercooling, and corrosion due to their diverse PCM blend [26, 27].

Subcooling or supercooling poses challenges to the power rating and reliability of thermal energy storage (TES) systems by delaying solidification in phase change materials (PCMs), crucial for time-sensitive applications. The phenomenon is quantified by the difference between the phase transition and nucleation temperatures, typically assessed using the T-History method [28]. Factors like nucleation types, surface finish, and experimental conditions significantly impact subcooling. To mitigate subcooling in salt hydrates, researchers emphasize the importance of nucleation methods. Seeding techniques, introducing nucleating agents with compatible lattice structures [29], and the Cold Finger Technique, which initiates nucleation by creating a cold spot [30], are explored alongside dynamic methods employing high-pressure and shock waves to prompt PCM solidification, enhancing the thermal performance of Latent Heat Thermal Energy Storage Systems (LHTESS) [31].

The CFT addresses subcooling by leaving part of the PCM unmelted for nucleation, with controlling the melt fraction posing a challenge. ML, particularly through an artificial neural network (ANN) employing radial basis functions, offers a robust solution. It predicts the PCM's melt fraction more accurately than traditional methods, improving thermal capacity utilization and mitigating subcooling. This approach is effective across various conditions, given sufficient data for model training [32].
