Numerical Study for the Design of a Thermal Energy Storage System with Multiple Tunnels Based on Phase Change Material: Case Study Mining in Chile (Thermal Storage in Off-Grid Industrial Applications)  
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1 Escuela de Ingenieria Quimica, Pontificia Universidad Catolica de Valparaiso, Valparaiso 2340025, Chile 1  
[PERSON]  
2 Escuela de Ingenieria Mecanica, Pontificia Universidad Catolica de Valparaiso, Valparaiso 2430120, Chile 2  
[PERSON]  
2 Escuela de Ingenieria Mecanica, Pontificia Universidad Catolica de Valparaiso, Valparaiso 2430120, Chile 2  
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Thermal energy storage (TES) has emerged as one of the cheapest and most practical alternatives [8; 9; 10]. In these cases, its low cost per kWh of capacity and duration of storage is essential, reaching up to 16 continuous hours in some designs [11]. The operation was based on heating a material medium, and later, the TES was used directly to generate electrical energy [12]. It is currently being applied in various industries such as industrial heating, domestic hot water, chemicals, food, and Concentrated Solar Power (CSP) [13; 14; 15]. The latter has allowed accelerated development and research in TES, considering various types and classifications to achieve the best cost-efficiency ratio [16; 17]. The most used and commercially applied configuration consists of a sensible heat storage system, distributing the cold heat transfer fluid (HTF) in one tank and the hot one in another, constituting two separate tanks [18]. However, this system incurs high capital costs for its construction materials, operation on large volumes of HTF, and maintenance of multiple devices [19]. For this reason, particular interest has been taken in developing systems that use a single tank, where the hot and cold HTF are naturally separated into fluid layers due to the buoyant force, giving rise to a temperature stratification called thermocline [20]. This configuration can be achieved through a porous bed thermocline formed between the HTF and a solid particulate fill material or a thermocline with structured solid material. On the other hand, the TES mechanism can be used in three ways: sensible, latent, or thermochemical [18]. Until now, thermochemical heat storage has been investigated in laboratories, while sensible heat storage has been used in industrial applications [21]. However, latent heat storage is a superior technology due to the high energy density of the phase change materials (PCMs) used, as shown in [22]. Combining a structured thermocline configuration and a PCM storage material offers high potential for developing small-scale, compact, high-efficiency off-grid thermal storage systems. In this sense, one of the most promising alternatives for designing a thermal storage system is a porous bed thermocline tank [20], combining HTF and inert granular material. Indeed, it introduces a considerable cost reduction by minimizing the volume occupied by the HTF. However, despite its proven potential, a critical concern has prevented its implementation in commercial plants and compromises the system's structural integrity [23]. This challenge is related to the phenomenon known as \"thermal ratcheting\", caused by the settlement of solid particle fill within the tank. Structured thermocline tanks were proposed as a viable solution to this problem.