System dynamics based models for selecting HVAC systems for office buildings: a life cycle assessment from carbon emissions perspective

摘要

This study aims to explore the life cycle environmental impacts of typical heating ventilation and air condition (HVAC) systems including variable air volume (VAV) system, chilled beam system and underfloor air distribution (UAD) system through a case study based on an RMIT office building. Life cycle assessment (LCA) is employed to evaluate the environmental impacts associated with different life cycle phases of HVAC systems. Using carbon emissions as the environmental indicator, ranges of impact for each HVAC system in different life cycle stages are calculated and compared based on the information uncovered in the literature review and relevant LCA inventory database. The system designs on case study are outlined based on the characteristics of a case building. The building was originally equipped with VAV system and was then designed with the chilled beam system and UAD system for comparison purposes. The lists of materials and products used in these three HVAC systems are illustrated, forming the basis of the life cycle assessment. Inventory analyses diagrams have been established based on the activities of HVAC systems in four life cycle stages: manufacturing, construction, operation & maintenance, and demolition. Calculation methods for carbon emissions are established. In particular, inventory data have been developed for manufacturing of HVAC products as well as transportation. An integrated LCA approach based on the system dynamics model is proposed for measuring the environment impacts of HVAC systems, which can give not only explanations of these dynamic complexities but also the simulation of total environmental burden in the service life cycle. The environmental impacts between these three HVAC systems are found to be of different magnitude in different life cycle phase. For instance, the embodied energy of UAD system is the lowest in manufacture stage while that of the chilled beam system is the highest. However, chilled beam system has much more energy saving potential than the other two air conditioning systems in operation stage. The system dynamics model prioritizes evaluating carbon emissions in refurbishments and replacements so as to calculate the total life cycle carbon emission of HVAC systems in a typical office building’s 50 year service life. The results indicate that UAD system may not be the best option due to its relative higher operational energy consumption compared with chilled beam system. From a life cycle perspective, the chilled beam system has the lowest environmental impact over a 50 year time frame, and VAV system has the highest environmental impact among the three HVAC systems compared. Maintaining the energy efficient HVAC services to meet the tenants’ demands is a challenge that must be met using environmentally-conscious business practices. Given the magnitude of the challenge, this research has not only contributed to the knowledge on environmental impacts associated with different HVAC systems, but also improved the utility of the conventional LCA by integrating the system dynamics model.