# Enhanced Solar Photovoltaic System Management and Integration: The Digital Twin Concept
## 2 Digital Replicas in Energy Systems  
While there are claims of several practical DTs by industrial pioneers, there is very little evidence within the scientific community in terms of published evidence to support these claims. [PERSON] et al. [10] reflected that scarcity of the practical case studies indicates that DTs are still in their early stages. As previously mentioned, the term DT has often been loosely used by several researchers and commercial companies. To provide some perspective on the DT concept, we present concise definitions of the four (4) closely related terms for various digital replicas (DRs) considered in this work and their potential value. The terms that are taken from [4; 5; 11; 12; 13] are as follows:  
**Digital models (DM)**--A digital representation of a physical component/system where communication has to be implemented manually based on mathematical models to conceptualize the processes/functions of an object/device. **Digital shadow (DS)**--A digital representation where there is automatic data flow from the physical component/system to the digital model while data flow in the reverse direction only occurs manually when required. This is mostly applied for historical or'real-time monitoring' and reporting. Note that a DS is not designed to simulate or predict future states or to suggest corrective actions. It provides a snapshot of the current state. **Digital twin (DT)--**A digital representation [11] with automated real-time bidirectional dataflow. This'real-time interaction' is applied for predictive analytics, simulation, and performance optimization or control of a physical component or system as may be required. Cyber-physical system (CPS)--This is the combined PT-DT system with automated bidirectional data flows including actual control of the physical twin component. The DT makes recommendations based on updates for control. However, a cyber-physical system performs actual control of the physical system via actuators. While a CPS is very similar to a DT, there are certain differences based on system complexity. DTs can be part of a CPS, assisting the system to act in a coordinated manner [14; 15]. The architecture and objectives of a CPS and DT are often the same and may essentially perform the same functions. However, in this work, we will consider the differentiation as more closely related to the previous definitions and the control structure in agreement with the definitions provided in [16; 17]. A DT is often model or data-driven, such that it provides an accurate basis for predictions and forecasting to optimize a system's performance, while the CPS is more sensor and actuator-control-driven, such that it focuses more on the continuous real-time control of different parts within the system [16; 17]. While each digital replica may be considered as a different system design based on their utilization, they can also be considered as varying levels of automation or system abstraction. In Figure 1, we present a visual representation of the terms defined in this section, similar to the proposition in [17; 18] describing levels of DT. Note that while there are functional differences in the three (3) categories of digitization highlighted in Figure 1, the DM, DS, and DT can also be considered as different levels of integration of a fully digitized system. The development of a PV installation begins  
Figure 1: Digital replica developmental levels: (**a**) digital model, (**b**) digital shadow, (**c**) digital twin, and (**d**) cyber–physical system. with the design of the system in a multi-physics platform. This is considered as a digital model (DM). Historical weather data in the location can then be used to estimate the average annual yield of the system. This can be done for a yet-to-be-installed system or an existing system using manual measurements (or datasheet values) of the physical system parameters. A step further is the automated extraction of data from the physical installation to a monitoring software to provide a'snapshot of the current state'.