###### Abstract  
Shell-and-tube latent heat thermal energy storage units employ phase change materials to store and release heat at a nearly constant temperature, deliver high effectiveness of heat transfer, as well as high charging/discharging power. Even though many studies have investigated the material formulation, heat transfer through simulation, and experimental studies, there is limited research dedicated to the storage unit design methodology. This study proposes a comprehensive methodology that includes the material assessment with multi-attribute decision-making and multi-objective decision-making tools, epsilon-NTU method, and cost minimization using Genetic Algorithm. The methodology is validated by a series of experimental results, and implemented in the optimization of a storage unit for solar absorption chiller application. A unit cost of as low as USD 8396 per unit is reported with a power of 1.42 kW. The methodology proves to be an efficient, reliable, and systematic tool to fulfill the preliminary design of shell-and-tube LHTES before the computational fluid dynamics or detailed experimental studies are engaged. s +
Footnote †: journal: Applied Sciences  
Among various LHTES configurations, the shell-and-tube type is widely studied [15]. Due to its advantages, such as simple design, low cost, low pressure drop [16; 17], large heat transfer area, high discharging power, and high effectiveness [5], the shell-and-tube type of LHTES is the most employed configuration [18]. The design of a shell-and-tube LHTES unit encompasses a wide range of topics. Many past studies have been dedicated to heat transfer enhancement by inclusions of material additives or metal objects into the PCMs [19; 20]. Others focused on the refinement of storage configuration to achieve satisfactory melting and solidification performance [21; 22]. However, very few studies have been found to answer the question of what specific steps to follow in searching for both the best storage material and the corresponding schematic design of a shell-and-tube LHTES unit. [PERSON] et al. [23] proposed a flowchart that describes different design stages of LHTES, from material research to commercial product. However, since this flowchart involves the full life span of the design process, it does not specify the detailed design method, for example, how to obtain material and geometrical properties. No optimization was mentioned in the flowchart. [PERSON] et al. [24] described the design process of the shell-and-tube LHTES system for CSP tower plants, as illustrated in Figure 1. The design process covers PCM selection, storage volume estimation, selection of geometric parameters, and optimizing storage volume with the given design alternatives. It pioneers the design methodology of the shell-and-tube LHTES system even though a few limitations still exist. The PCMs selection, according to this study, adopts the melting temperature and the availability of detailed properties of the materials to filter the database. This practice is commonly seen in most studies and engineering cases. For example, in the methodology developed by [PERSON] et al. [18] to design a cascade PCM storage unit, three PCMs were selected based on the temperature restrictions and material properties. However, when dealing with a broad spectrum of materials with detailed property data available, researchers should spend a significant amount of effort choosing the best alternatives. Another limitation found in this design process and other commonly used design processes is the employment of numerical simulations, which is complicated by the high demand for computational resources, especially for optimizing many geometric parameters that need dozens to hundreds of iterations. To replace the time-consuming computational fluid dynamics (CFD) simulations, [PERSON] et al. [25] developed an analytical approximation of a shell-and-tube LHTES unit's melting time. The analytical approximation was validated by CFD with modest errors (from 10.9% to 13.6%).