Component-based, gray-box modeling of ductless multi-split heat pump systems

Multi-split ductless heat pumps are touted as an energy efficient option for air conditioning and heating in residential and small commercial applications. However, their use of multiple indoor units with variable speed compressor and fans and complicated controls leads to difficulties in understanding their characteristics by experiments only. A method to model the system using gray-box component models and an empirical control algorithm is described in this paper. The model was constructed from experimental results of an 8 kW R410A dual-unit ductless heat pump system having an accumulator. Strategies for modeling systems with accumulators and saturated control inputs are presented. Parametric studies with varying indoor air temperature, individual indoor unit load ratio, indoor fan speed and total indoor load were conducted. The results show that the model predicts system performance accurately and can help to understand the effect of saturated control inputs and changes of accumulator status on system performance.