State-of-the-art of the digital twin concept in HVAC+R systems for thermal management of buildings

The drive for sustainable energy solutions and decarbonization has become crucial, particularly in the building sector. Heating, ventilation, air conditioning, and refrigeration (HVAC + R) systems represent the largest share of energy use in buildings, especially during peak load periods. The digital twin concept has emerged as a pivotal approach for enhancing the energy efficiency of HVAC + R systems through real-time monitoring, predictive performance, and system optimization. This paper presents a comprehensive state-of-the-art review of the digital twin concept and its applications in HVAC + R systems. It systematically examines simulation tools, middleware approaches, software interfaces that enable the development of digital twin architectures, alongside real-world implementation of the digital twin concept, and its key services, such as (a) performance and operation monitoring, (b) fault detection and diagnosis, (c) control and operation optimization, and (d) flexibility provision. The review identifies the main challenges and future opportunities for the secure integration of the digital twin concept into HVAC + R systems. The following are the notable challenges. Modeling bottleneck on the data level: data harmonization and ontologies that are the foundation for efficient digital twin integration; at the process level: Modeling for the behavior prediction of the system and service integration towards efficient decision-making; and at integration level: the lack of standardization of digital twin architectures, data acquistion and utilization to improve the control and even the design of next-generation HVAC + R systems. Addressing these challenges systematically is crucial for advancing the adoption and deployment of digital twin concepts in the building sector. • Reviews the role of digital twins in enhancing energy efficiency of HVAC + R systems. • Summarizes promising simulation tools, software interfaces for the development of digital models. • Discusses and summarizes the state-of-the-art of HVAC digital twin in real-world implementations. • Identifies major challenges in digital twin application for HVAC + R systems and energy management. • Highlights future opportunities for advancing digital twin adoption in sustainable building systems.