# Autonomous Cyber-Physical Systems Enabling Smart Positive Energy Districts
## 3 Emerging Technologies Enabling Positive Energy Districts
### Digital Twins  
A DT \"integrates the IoT, AI, machine learning, and analytics, to create living digital simulation models that update and change information as needed\" [20]. Therefore, a DT constitutes a model that \"_continuously learns and updates itself from multiple sources to represent its near real-time status_\" [20]. Additionally, a DT is a combined approach involving \"_new types of analysis and modelling based on big data and machine learning/artificial intelligence, which combines capacities of virtual model, data management, analytics, simulation, system controls, visualization and information sharing_\" [37]. DTs are considered to be a significant solution for designing and optimizing PEDs [20; 37]. This is because virtual modeling, analysis, and testing of buildings and districts before they are built is regarded as more resilient and more efficient, due to the fact that it enables real-time and predictive monitoring, simulation, and stakeholder collaboration. Therefore, through the continuous tracking of energy consumption, production, and storage, DTs facilitate proactive management and optimization of energy systems within districts. Effectively collecting and managing data from building information models (BIM), IoT devices, and sensors is essential to optimize energy flows, support participatory planning, and ensure sustainability in PEDs. DTs are created by using pioneering simulation models that integrate real-time data from diverse sources regarding real-time states (e.g., moisture, occupancy, energy consumption, temperature, CO\({}^{2}\) concentration, renewable production, etc.). Furthermore, DTs continuously learn and renew themselves from multiple sources and, as a result, can support the identification of diverse energy strategies (e.g., identifying the most effective way to achieve a positive energy balance). DTs are significant for achieving the aims of PEDs in terms of raising the threshold of self-produced energy, energy surplus, and climate neutrality. This is achieved through real-time energy management and collaborative planning for sustainability. Their continuous development and integration will be crucial to the successful transition to energy-positive urban environments. DTs can be used for energy supply and demand measures, carbon emissions, indoor air quality, and thermal comfort. They can also be applied to calculate the expenses for building operation, maintenance, renovation, replacement, and the payback periods of energy-saving measures. In their work, [PERSON] et al. [20] present the key components of a digital PED twin, as can be seen in Figure 1. In addition, they revealed that the energy data in a digital PED twin (aggregated through the whole life cycle) is collected and transported by diverse sensors and IoT networks from the operational data network to a data analytic center, where data is either sent to stakeholders for decision-making, or returned to the individual systems for regular operations. The different decisions that stakeholders take rely on advanced analytics that are actuated in an automated infinite-state mode of operation. The digital PED twin increases system resilience by considering inter-dependent systems and optimizing future decisions and operations. The DT environment enables collaboration, communication, and interaction between all stakeholders involved in the life cycle of PEDs. The flexibility and robustness of the system increase during operation due to the reliance on real data. Creating city and district DTs signifies a radical shift toward urban sustainability from design to construction and maintenance based on the Industry 4.0 and Construction 4.0 principles [25]. DTs utilized in PED energy management comprise diverse energy system optimization, predictive analytics, and real-time monitoring. After using DTs in two urban initiatives, [PERSON] and [PERSON] claimed that the benefits of using urban digital twin include _\"visualization, planning, and facilitating dialogue among various stakeholders\"_[38].