Optimizing Building Energy Modeling (BEM) Workflow through Integrating HVAC systems Design and Building Information Modeling (BIM) using gbXML Schema Version 6.01

Heating, Ventilation, and Air-conditioning (HVAC) systems have been adopted in architecture since 1972 for maintaining a comfortable indoor thermal environment. It plays a crucial role in occupants’ thermal comfort, health, and productivity. However, most of the office buildings today have a low thermal satisfaction rate (Huizenga, C. et al., 2006), and the energy consumed by HVAC systems for maintaining the indoor environment consists of 48% of the total energy consumption (Pérez-Lombard, L. et al., 2008).

One major cause of the issue is that most HVAC systems are poorly designed based on engineering experience, without enough building information input from architects or without enough HVAC systems data output to energy engineers for performance simulation. Mainstream architecture design and HVAC engineering design workflow is prescriptive and code compliant. With the precedent development of sustainable design standards, performance-based design process integration is expected. The main communication issue between architects and engineers is now being handled by Building Information Modeling (BIM) and Building Energy Modeling (BEM) technology. 

This paper proposed an BIM-HVAC-BEM gbXML workflow to enable smooth communication between architects and engineers. Green Building XML (gbXML) is one of the most prevalent BIM data models. It enables interoperability between BIM to BEM. BEM is mainly used for building performance analysis. However, through interviewing twenty engineers and building energy modeling professionals in the industry, it turned out that most gbXML files are only used for importing and exporting building geometry information. Information such as HVAC systems and internal loads are rarely handled due to the lack of functionality in current BIM software. Taking account of more than 15% of the total energy consumption in the US is used by HVAC systems (DOE 2011), it is crucial to enable seamless HVAC data exchange between BIM software and BEM software. 

Firstly, through a detailed data mapping of ASHRAE baseline variable air volume and reheat system between IDF data model (EnergyPlus version 9.0) to corresponding elements in gbXML schema (version 6.01), interoperability issues were discovered and were concluded into three categories: missing components, the difficulty of decoding performance curves, and complex data mapping rules. Secondly, through redefining data mapping rules in current gbXML schema in terms of HVAC systems, the ASHRAE baseline system type seven is coded as a gbXML file. Finally, the gbXML file is validated through a medium office building case study. Revit 2020.1 Architecture is used in this study as the BIM tool. OpenStudio 2.9.1 and EnergyPlus 9.2.0 are used as the BEM tool. They are open-source and cross-platform, being adopted by lots of mainstream building performance analysis software.

Based on the result of this study, current gbXML schema version 6.01 is capable of defining HVAC systems data, but the data mapping rules need to be documented and presented. Redefined data mapping rules and improvement suggestions are proposed to the current gbXML schema in terms of HVAC systems. The improved interoperability will eliminate the duplicate generation of HVAC data and allows a bidirectional information update between BIM and BEM software, supporting a more accurate and efficient building performance analysis process, thus improving indoor thermal comfort and building energy usage.