## 3 Building Digital Twins
### Digital Twin Building Solutions in the Market  
Although building digital twin technology was available for over a decade, its adoption in the Danish and European markets is still limited. While there were few initiatives and projects exploring the implementation of digital twins in building applications, the focus is primarily on newly constructed, highly energy-efficient buildings. These buildings are seen as ideal candidates for digital twins as they can bring Building Information Models (BIM) to life by integrating real-time data and dynamic information, creating a live and dynamic digital twin that can support the building throughout its life cycle. Conversely, existing and older buildings often lack any form of digital models, making it challenging to implement digital twins in those contexts. One noteworthy project in this field is the EU Horizon 2020 project called the 'SPHERE: BIM Digital Twin platform.' This ambitious initiative aims to provide a BIM-based digital twin platform that can enhance the energy performance of newly constructed buildings throughout their life cycles. The platform also seeks to reduce time, costs, and the environmental impact associated with the construction process. Overall, while digital twin technology demonstrated positive impacts in terms of energy savings, cost reduction, and resource optimization, its widespread implementation in the Danish and European markets is still in progress. The current focus is primarily on new, energy-efficient buildings, with the goal of expanding the use of digital twins to existing structures in the future. Additionally, given the considerable potential demonstrated in utilizing digital twins across various fields, several commercial solutions for digital twins in the building sector emerged in recent years, offered by energy engineering and construction companies. Arup, for instance, introduced 'Neuron' [35], an intelligent digital twin platform for buildings that leverages advanced machine learning and predictive maintenance to assist asset ownersin achieving significant energy savings. By utilizing 5G and the Internet of Things, the platform collects real-time data from equipment and systems, referred to as'sense data.' Furthermore, it employs Building Information Modelling (BIM) to present these intricate datasets through a cloud-based, centralized management console. Arup also employs artificial intelligence and machine learning for the analysis, optimization, and automation of operations. Similarly, Granlund presented a comparable platform known as the 'Granlund Manager's Digital Twin' [36], which offers real-time data visualization on the building's 3D layout drawing, allowing for the monitoring of specific conditions at the building, floor, and unit levels. This digital twin aggregates data from BIMs used during the design phase, as well as from IoT and automation systems and property users. Another noteworthy development is Autodesk's 'Autodesk Tendem' [37], a cloud-based digital twin technology platform designed for buildings and facilities applications. The Tendem platform enables projects to initiate and maintain a digital presence, utilizing BIM data to generate valuable business intelligence. Through the utilization of Autodesk Tendem, construction and engineering companies can create and transfer a digital replica of a building to owners and operators, incorporating accessible, contextually relevant, and insightful data to support operational efficiency. The Tandem platform assists AEC firms in leveraging BIM data throughout the entire project lifecycle, facilitating the creation and transfer of a digital twin. This platform also helps building owners connect operational systems to the digital twin, consolidating fragmented data into actionable business intelligence. Similarly, Siemens introduced a digital twin solution called 'Building Twin' [38], which provides real-time insights into a building's performance and allows immediate adjustments to optimize efficiency.