Impact of Startup and Defrosting on the Modeling of Hybrid Systems in Building Energy Simulations

The use of hybrid system (HS) consisting of an air-source heat pump (HP) and gas-fired boiler is spreading, because their installation can mitigate the problems related to HP systems in existing buildings. The efficiency of the HS is strongly affected by the control strategy and size of the heat generators as well as the building characteristics. A coupled building-HVAC simulation is the most effective way to study the influence of these factors. However, the HS model must be simple enough but relevant to actual physical behavior to be fit for purpose. In this regard, reduced generators’ performance during the startup phase and COP degradation due to defrost cycles are often overlooked in energy simulations of building and HVAC systems. This paper introduces new, reliable, and easy-to-compute experimentally derived correlations to consider performance losses due to defrosting cycles and startup phases in building energy simulations. Unlike previous studies, the new correlations consider the dependency of startup losses and duration on the operating conditions. The model is validated by experimental simulation of two load profiles. Experimental data show that the HP under some conditions can take up to 20 min before it comes up to capacity, and during this time the COP degrades to about 15% from the nominal COP.