Analysis and Comparison of Different Types of Exergy Loads in HVAC Systems

The exergy loads of the HVAC system are analyzed to see the variation patterns under different influencing factors. The state of outdoor air is selected as the reference state and the equations for exergy load calculations are derived, including exergy loads of sensible heat and latent heat for indoor air and fresh air, respectively. Then, different types of air conditioning loads and the corresponding exergy loads are analyzed for a standard floor of an office building in Changsha as a case study, which is equipped with an air-cooled system or a water-cooler system under the intermittent operation condition. The results show that the significant differences exist between the air conditioning loads and the exergy loads with respect to variation range and change trend. The energy quality of air-conditioning loads is very low, so it is more suitable to use low-grade energy sources to drive air conditioning systems. The exergy load of latent heat of fresh air is the highest in all exergy loads because of its large amount of exergy destruction of condensed water. The maximum indoor exergy load of sensible heat is observed 4 hours later than its air conditioning part. Moreover, the exergy loads in the water-cooled system are considerably less than those in the air-cooled system.