# Enhanced Solar Photovoltaic System Management and Integration: The Digital Twin Concept
## 3 Digital Twins for Solar Photovoltaics
### PV DT Technologies
#### 3.3.4 PT-DT Communications  
Figure 6 depicts a simplified overview of PV PT-DT communication. The physical resource layer draws data from different resources, such as WEC sensors and energy data from inverters, meters, data loggers, and UAVs. These sensors transmit the data via either cabled or wireless connectivity, as may be required.  
The recent inclusion of real-time image processing from unmanned aerial vehicles (UAVs) has created a need for very stable connections with wide bandwidth, such as 5G networks, to be available [103]. Note that before transmission, local repositories may exist as a temporary backup for limited historical data.  
Communications for PV-DT can be viewed from two (2) perspectives: the communicating devices and communication layers [104]. With respect to the communicating devices, we consider the following:  
* Intra and inter-subsystem (SS) communication within the DT counterpart;
* Intra and inter-subsystem (SS) communication within the PT counterpart;Each of the different forms of communication is visualized in Figure 6, where some communications are wired and others are wireless. Note that most weather and environmental conditions (WECs) sensors use wired communication from sensors to dataloggers. Measurements by energy meters are also wired and come directly from the associated inverters or electronics within the system. The dataloggers and energy meters transfer the data to cloud resources via wireless networks. The aerial and ground robots must also maintain reliable communication with their respective controllers. The aerial and ground robots often utilize standard low-latency video control link radio frequency or wireless network bands (2.4 GHz/5 GHz) or satellite communications as required. Typical communication protocols for the ARs and GRs are DJI Lightbridge/OcuSync, MAVLink, and Robot Operating System (ROS).  
The inclusion of aerial robots (ARs) and ground robots (GRs) in the overall PV-DT architecture introduces a need for higher-fidelity communication. When the robots are remotely operated by a remote pilot near the asset, control is clearer, and manual intervention is possible. However, when the piloting is conducted from a distance there is an increased need for automated/autonomous robot missions, as well as real-time image data processing, over low-latency, high-data-rate communication networks. For example, a single drone with a 4 K (UHD) camera at a 30 fps frame rate and HEVC compression will need 15-30 Mbps per camera feed. Often, multiple cameras are used, such as RGB + Infrared (for Electroluminescence), and the robots combine these camera outputs with other sensors, such as LiDAR and depth cameras, for safe navigation. When combined with simultaneous data transmission from other sensors within the physical twin (real system), the required data transmission rates on-site can increase dramatically. Furthermore, there is the need to ensure that all sensor data are received or controlled synchronously for accurate use of adaptive real-time AI/ML models, further increasing the need for reliable local communication networks. While there is a range of networks or protocols that can be applied, this work is limited to providing a system-level view as previously described.  
In this work, we consider the main network layers applicable to data transfers. From Figure 7, the network layer includes all the data transfer protocols, gateways, and any edge computing that may be required [105; 106], while the cloud layer is dedicated to the bulk of data processing, storage, and management [107; 52; 108]. The terminal layer is the interactive layer where human-in-the-loop visualization, inspection, and processing occur.  
Figure 6: Subsystem communication in PV-DT.  
Specific control actions may still be implemented via direct signal processing to some control devices and not through the cloud [109; 110]. This is to ensure failsafe or backup for instances that can result in significant hazards like fire.