## 3 Building Digital Twins  
### Digital Twin Concept and Role  
Digitalization plays a crucial role in enabling smart buildings to effectively interact with users and understand the internal and external environment. Central to establishing this interaction is the abundance of data available. Just two decades ago, building managers only needed to connect a few dozen data points within a building. However, this landscape drastically changed, and today we are dealing with tens of thousands, and sometimes even hundreds of thousands, of data points in some facilities. Consequently, smart buildings now generate vast amounts of data from various building services, components, and levels within the built environment. In buildings equipped with advanced devices such as Internet of Things (IoT) devices, the number of sensors, meters, and devices significantly increased, leading to a greater need for efficient, automated, and user-friendly platforms for data storage, management, and sharing. The primary advantage lies in the immediate access to data, which provides a comprehensive view of a building's performance. This empowers owners and operators to manage assets and facilities more effectively in a timely manner, leading to enhanced operational efficiency. The concept of 'digital twin' is a fundamental aspect and a key element of digitalization. It refers to a digital representation of physical assets, systems, devices, or processes that serves various purposes. A digital twin goes beyond being a mere digital model of a component or system; instead, it is a dynamic operational model where real-time data are integrated with the virtual replica to create a comprehensive digital twin. This integration enables continuous monitoring of the actual state of the system and facilitates informed decision-making for future states and applications. In recent years, the scope of digital twin applications expanded, and successful implementations were witnessed across different industries and stages, including design, production, manufacturing, and maintenance. These applications are proven to be particularly valuable in areas such as understanding customer needs, predicting weaknesses and faults, optimizing processes to adapt to dynamic changes, and providing timely maintenance alerts to prevent failures. Conceptually, a digital twin consists of three main components: (A) the physical component, representing the real-world component in physical space, (B) the virtual component, representing the digital counterpart in virtual space, and (C) data and information, which act as the bridge between the physical and virtual components. As illustrated in Figure 3, the connection and exchange of information between the physical component and the virtual replica are facilitated through data in various forms and at different levels. The successful implementation of digital twin solutions in the manufacturing and production sectors paved the way for exploring their application at the building and facility levels. A building digital twin offers a holistic view of the operational building, bridging the physical and digital realms. It combines a virtual model of the building with real-time data collected from meters, sensors, and IoT devices to analyze, optimize, and inform decision-making throughout the building's life cycle. Consequently, a digital twin holds value at every stage of the building value chain, including conception, design, construction, commissioning, operation, and maintenance. As more data are integrated into the digital twin, it opens up opportunities for scenario planning and what-if analyses using intelligent algorithms. By leveraging organized data, a digital twin provides real-time insights into the building's performance, enabling immediate adjustments to optimize performance at various levels. This leads to more energy-efficient, cost-effective, and sustainable smart buildings and facilities. Figure 3: The main components of a standard digital twin.