# Toward a Practical Digital Twin Platform Tailored to the Requirements of Industrial Energy Systems
## 2 State of the Art
### Digital Twin Architectures
#### 2.5.1 Existing Digital Twin Concepts, Architectures and Platforms  
The first DT concept was published by [PERSON] [14] in 2014, defining the three basic aspects of (i) the physical space, (ii) the virtual space and (iii) the connection between them to exchange data and information. These three main dimensions are found in most other concepts and architectures. Concurrently, they are often organized into physical (i), computing (ii) and network layers (iii) [67; 76]. [PERSON] three-dimensional approach is very minimalistic, and consequently, many extensions have been proposed in recent years to reflect the growing complexity of DT concepts. [PERSON] et al. [79] argued that [PERSON] three dimensions do not indicate the extent to which services and features can be provided, and also, the extent to which information can be retrieved and data can be processed is not explicitly presented. These shortcomings in combination with the rapid development of enabling technologies, omnipresent availability of data, and the need for services led [PERSON] et al. [24; 80] to propose a 5D-DT concept, adding (iv) DT data and (v) DT services to the three dimensions proposed by [PERSON] [14]. In the 5D-DT concept, especially the service dimension is emphasized as an important part. The functionality of the DT is encapsulated into standardized services with user-friendly interfaces for easy and on-demand usage. [PERSON] et al. [81] propose a \"DT 8-dimension model\" to provide a template for planning DTs; however, they admit that further research is needed toward a reference model for the implementation of a DT. [PERSON] et al. [31] presented DT models and a four-layer networked architecture of cloud-side-end collaboration for battery management systems. This architecture contains four layers from the perspective of hardware functionality, namely \"edge computing layer\", \"data access layer\", \"data storage and analysis layer\" and \"data-based application layer\". [PERSON] et al. [37] proposed a five-component DT architecture for robust power plant operation, consisting of (1) a physics-based dynamic system model, (2) an anomaly detection model, (3) a sensor database, (4) a digital thread model and (5) localized in-depth simulations. While this architecture is specific regarding purpose and possible modeling techniques for physical components, the authors circumvent the important topics of data storage and processing and connections within the DT. Furthermore, no standardization aspects are considered to ensure the scalability and openness of the architecture. In [21], a \"Reference Framework for Digital Twins\" is presented that specifies the structure and interrelations of the main DT building blocks, which were identified based on a literature review. Interestingly, the blocks are almost identical to the dimension in the 5D-DT concept by [PERSON] et al. [72; 80] except for the notable absence of an equivalent to the connection dimension. The concept for a \"Cognitive Twin Toolbox\" was presented in [82] with a special focus on the process industry. The toolbox is organized into the five layers, which again have some similarity to the 5D-DT [72]. The \"Data Ingestion and Preparation Layer\" and the \"Service Management Layer\" are of a similar design as the data and service dimension in the 5D-DT. The \"Model Management Layer\" serves a similar function as the virtual entity. Additionally, the toolbox also has a \"Twin Management Layer\", to handle synchronization of the DT structure, and a \"User Interaction Layer\". Just like the \"Reference Framework for Digital Twins\", the \"Cognitive Twin Toolbox\" has no explicit connection layer. The three main building parts of a DT presented by [PERSON] [14] align with the definition of CPS, which is a concept that is very prominent in the industrial production domain. Various papers in the manufacturing field mention the use of the DT to simulate a CPS [17].