Abstract

The concept of sustainability applied to the building industry requires that engineers consider the reduction of both energy use and the related environmental impacts during both design and commissioning phases. According to estimates of the end-use energy consumption and related greenhouse gas (GHG) emissions, issued by National Resources Canada, the commercial building sector contributed by about 14% to the total national end-use energy consumption and by 13% to the total national GHG emissions in 2003, while GHG emissions associated with space heating and cooling accounted for more than half of total emissions (NRCan 2005). Therefore, the increase of the energy efficiency of Heating, Ventilating and Air-Conditioning (HVAC) systems in commercial buildings and the reduction of their associated environmental impacts are extremely important. Two types of Variable Air Volume (VAV) systems applied to an existing office building in Montreal are analyzed in this research. The first uses a constant temperature for the supply air from an air handling unit (AHU), while the second varies the temperature of the supply air in terms of the highest instantaneous cooling load of all zones. In order to estimate the energy performance of the two VAV systems, thermodynamic analysis based on both first and second laws of thermodynamics is applied, and mathematical models are developed to simulate the system operation. The Engineering Equation Solver (EES) environment is used to implement and solve the mathematical models. At the system level, the annual system Coefficient of Performance (COP) of the first system is 0.99, while the COP of the second system is 1.24. However, the annual energy efficiency of both VAV systems drops by about 34% when the power plant and delivery systems are taken into consideration. The hourly maximum exergy efficiency of both VAV systems is less than 0.16, indicating that a large potential of the improvement of exergy efficiency exists for both VAV systems. The annual GHG emissions due to the off-site primary energy use by both VAV systems are 43 tons/yr and 33.5 tons/yr, respectively. The study outlines that the most exergy destruction within both VAV systems is due to heat sources, i.e., electric boilers. Therefore, the largest improvement of exergy efficiency can be obtained by changing the heat source from an electric boiler to renewable energy sources such as solar or geothermal energy. Moreover, this change may also lead to the reduction of the GHG emissions due to the operations of HVAC systems