# Towards a Digital Twin Warehouse through the Optimization of Internal Transport
## 1 Introduction  
Digital twins are an innovative solution that, with low investment, allows determining the behavior of a product, equipment, or facility in a realistic environment. Their transversality means that they can be used in numerous applications, in particular, in the Industry 4.0 context [1]. Studies focused on production environments point out how these tools could be implemented on cyber-physical systems, as clearly identified by [PERSON] et al. [2] in their literature review or in plants with a demand for a high variety of products [3]; in production lines, efficiency and manufacturing improvements are achieved after the construction and analysis of digital twins [4]. Storage, as one of the production support systems, received special attention because of its high asset investment [5]. A current trend is to provide companies with structured warehousing solutions, in which the layout of the locations will remain constant throughout the life cycle of the warehouse. This approach is also transferred to the choice of one or more positioning logics that optimize internal transport based on fixed working conditions, obtained, at the best of times, by historical analysis of storage needs and different approaches as to improve the space through warehouse management [6]; to determine the best alternative for warehouse storage location assignment through a multi-criteria evaluation [7]; to define a systematic methodology of storage selecting assignments and a picking order system [8]; to analyze order picking strategies finding their relationship withproductivity [9]; and to analyze the alternatives of storage location assignment through a Pareto-optimal method considering the efficiency of a warehouse, the required space, and the picking order distance [10]. All these approaches provide solutions to particular problems. However, these static methods and their solutions applied to storage problems restrict the ability to adapt to environments where conditions are or may be changing. [PERSON] et al. [11] have already expressed the need to provide warehouses with flexibility as a way to face environmental changes. There is currently a growing demand for recycled and by-products, among other reasons, due to a higher concern for environmental issues [12]. These types of resources, and also the raw materials produced by other production methods, are and probably will be increasingly scarce. The difficulties in terms of regularity of supply and the homogeneity of the formats in which this material is supplied make it necessary for certain types of companies to propose storage and logistics solutions that are adaptable to these new conditions. All this is accentuated when the stored material deteriorates easily, which requires particular inventory management [13] and a definition of the strategy during storage [14], including when these materials are used as packaging. Unlike raw materials, recycled materials are not always presented in a stackable or even homogeneous form. The use of digital twins as a management tool, both when assessing the operational variables of the process or redefining strategies according to the evolution of reality, makes it necessary to use increasingly complex simulation models, which consider not only variations in the operating conditions of the scenarios but also the possibility of physical redistribution of the locations and/or their storage logics. In the business world, especially within small and medium-sized companies, this ability to adapt to the environment will be essential for their survival, with alternatives in logistics such as green packaging [15] or internal transport and warehouse management with RFID (Radio Frequency Identification) tags and optimization of picking activities [16].