# Designing a Conceptual Digital Twin Architecture for High-Temperature Heat Upgrade Systems
## 2 Related Work  
### Architecture  
The seminal paper on digital twins, authored by [PERSON] [12], introduces a broad architectural description of the concept that contains only three components: the real physical space, the virtual space, and the connection between the two. To better differentiate between different manifestations of the digital twin, depending on the current stage of its lifecycle, [PERSON] also defined the following concepts: digital twin prototype, digital twin instance, and digital twin environment. These distinctions are important for designing digital twins in industrial energy systems, where prototypes can be tested before deployment, and instances can evolve as operational conditions change. In the context of a digital twin driven product design and manufacturing, the authors of [13; 14] propose a new conceptual architecture by adding two more components, data and services. The literature refers to this proposal as the 5D-DT architecture due to its inspiration from the 5C architecture for cyber-physical systems, which focuses on connection, conversion, cyber, cognition, and configuration for smart manufacturing [15]. Going forward, the popularity of digital twins grows significantly, and the literature contains multiple proposals for digital twin architectures, from very abstract ones to very specific ones in terms of application area or technology used. In this section, we will review and describe the most recent and relevant ones. For industrial energy systems, a generic digital twin architecture has been proposed in [16], called EDTA, and consists of six layers: asset (physical components), integration (connectivity), communication (data transfer), information (data management), functional (operations), and business (value generation). Inspired by the Reference Architecture Model Industry 4.0 (RAM14.0) [17], it uses the same names for its six layers to achieve consistent naming and understanding and it targets the instance phase of the twinned entity, as defined by RAM14.0 life cycle. A proof-of-concept instantiation of EDTA was made using semantic web technologies. RDF and existing OWL ontologies are extended to represent the resources and services of the instantiated digital twin. The twinned entity was a packed-bed thermal energy storage device with the main digital twin service being the simulation of thermodynamic behavior. A concrete architecture that defines specific requirements for industrial energy systems is presented in [18]. Its components and layers are based on both 5D-DT and EDTA to address the need for modularity, scalability, and resilience in long-operating industrial systems. While some of the defined requirements represent the core of the digital twin concept, some are specific to the industrial energy systems: DT should be modular and scalable due to the physical entity's long operation lifespan and continuous evolution. In addition, the DT should be robust to physical parameters degradation due to harsh and unexpected physical environment changes. In addition to the architecture descriptions, the authors also provide in-depth details of technologies used to implement their proposed architecture: MQTT protocol in the connection layer, XAMControl for the physical layer, FMU (Functional Mock-up Unit) for the virtual layer, the data dimension is modeled using OWL ontologies, while the service layer is executing in a microservice framework using Docker. OpenTwins architecture presented in [19] and further extended with FMI [20] (Functional Mock-up Interface) simulation and AI capabilities in [21], focuses mainly on the technical implementation aspects. OpenTwins core framework is built around open-source packages: Eclipse Hono and Ditto for IoT device communication, Kafka, and Telegraffor data ingestion into InfluxDB, which is a time series database. Data visualization and end-user interaction panels are created using Grafana, where a Unity WebGL plugin was integrated specifically for 3D model visualization.