# Toward a Practical Digital Twin Platform Tailored to the Requirements of Industrial Energy Systems
## 2 State of the Art
### Requirements on Digital Twins in the Energy Sector  
Since most DT implementations are realized following a specific goal without any architectural template [21], it is impossible to overcome the implementation barriers given in Section 2.3 in a standardized manner. However, a DT that is tailored to the specific characteristics of IES (i.e., one that meets their requirements) could be an enabler to integrate digital applications on a single platform and thus make IES significantly more efficient. While having mainly different scopes of applications in mind than the energy sector, some researchers directed their research on establishing common focus points in DT development and servization. Furthermore, they listed requirements they deemed as necessary DT capabilities. These requirements are summarized in the following paragraph and critically reviewed to provide a foundation for our assessment in regard to IES. [PERSON] et al. [67] highlighted requirements on DTs in the construction sector. Demands for smart factory systems were established in Ref. [68]. [PERSON] et al. [69] studied DT definitions in manufacturing and clustered them into requirements (1) derived from the literature, (2) derived from DTs in use today, and (3) applications in the near future. [PERSON] et al. [70] formulated requirements for the architecture of a knowledge exploration platform of industrial data for integration into digital services. [PERSON] et al. specified functional and non-functional requirements for a DT service framework derived in a literature review [22] and clustered these requirements based on three RAMI 4.0 layers (information, functional and business). [PERSON] et al. [71] collected requirements for ontological modeling in industrial applications. [PERSON] et al. [72] proposed eight rules for DT modeling, which are, in their original short denomination, (1) data and knowledge based, (2) modularization, (3) light weight, (4) hierarchy, (5) standardization, (6) servitization, (7) openness and scalability, and (8) robustness. [PERSON] et al. [73] summarized and identified four features of digital twins based on the literature, including digital modeling, analytical support, timeliness update and control. [PERSON] et al. [37] stated seven requirements for their DT architecture for power plants. Based on this literature about DT requirements and considering the specific characteristics of IES and technical barriers impeding the DT development outlined in this subsection, we established a set of key requirements for DTs in the energy domain. These requirements are listed in Table 3 together with references that provided motivation for them. Requirements R1 and R3 express the need for bidirectional automated information exchange between physical and virtual entities, which is considered the most distinctive feature of a DT [19]. Especially for highly dynamic systems, the DT must be capable of informing and warning human operators (R2). Both requirements R5 and R7 address the issue that the DT must not only provide recommendations for action but to trigger these actions, hence optimizing the operation of IES. Since IES and their corresponding DTs should be in operation for very long time-spans, IES are continuously evolving; i.e., components are added and environment variables change. Therefore, DTs should be modular (R4), scalable (R6) and build on standardized technologies (R12), thus facilitating the maintenance and continuous development of the DT platform. Especially in IES, parameters of physical units can change significantly, e.g., due to degradation in harsh conditions. Hence, the DT must be robust in that it can automatically adapt to behavioral changes (R9). This reinforces also requirement R10, which is a typical DT requirement also in other domains.