# Designing a Conceptual Digital Twin Architecture for High-Temperature Heat Upgrade Systems
## 3 Digital Twin Architecture for Heat Upgrade Systems  
While the digital twins emerged as an industry 4.0 concept and proved to be successful on the smart manufacturing stage, their popularity in other areas such as energy systems is evolving significantly. As seen in Section 2 of this paper, there is no consensus on digital twin architecture and implementation methodology or tools. While there are similarities between the several analyzed architectures, most of them are very specific to a single use case or are defined by the technologies used to instantiate it. In this section, we propose a digital twin architecture that incorporates specific requirements identified for the industrial heat upgrade systems but is also general enough to allow instantiation for other domains with small or no changes. An overview of the digital twin layered architecture for a high-temperature heat upgrade system, together with its main components and basic architectural details, is presented in Figure 1. Next, we describe each of the layers and components of our proposed architecture. On the lowest level layer, we have all the individual Physical Equipment assets involved in the high-temperature heat upgrade system that will be interfaced and connected with the digital world through the Gateway layer, detailed below. The physical assets consist of heat exchangers, heat pumps, renewable energy collectors, thermal energy storage, including their sensors and actuators such as motors pumps and valves for mass flow and pressure control. The next layer is represented by the Hardware Control Units. We have identified several types of hardware control units, depending on their compatibility with IoT technologies and availability of support for digital control and communication. This layer contains control devices that are placed physically close to the physical equipment, usually in the same building or facility. There are three types of control units:  
* Default control units--these units represent and control physical equipment that already has an IoT compatible controller or there is an API available. Interfacing and controlling these devices will require low effort and financial costs due to their digital readiness. * Dedicated control units--we expect that some of the physical equipment have a closed-source controller, or a controller without communication capabilities. This type of device requires the augmentation of the specific physical unit with a dedicated controller. Usually, this is the case when the physical equipment is old, and replacing it with a new one that has IoT capabilities is not economically feasible. This is usually costly in terms of time and effort, as it requires domain knowledge about the device and additional sensors and computing hardware to acquire the device state. * Master controller unit--while the previous two types of control units are responsible for single physical devices, this control unit is responsible for reactive, time-critical operations, supervision, and orchestration of the system in case connection to higher control levels is not available temporarily due to a disruption in the network. The Gateway layer realizes the bidirectional connection of the DT, bridging lower-level edge devices with higher-level software layers, which are deployed to a private or public cloud based on application requirements. In complex IoT systems, devices and controllers use diverse protocols, such as HTTP, MQTT, OPC-UA, or Modbus. The Gateway interfaces diverse systems providing a unified communication and easy integration of both legacy or proprietary devices and modern devices. The Gateway is also a central point where connected devices are maintained through a device management interface. This allows for device discovery, enrollment, and provisioning. Real-time monitoring such as device status and health checking procedures are also present.