Modeling the performance of electronic expansion valves in heat pumps under soft faults

Heat pumps can have soft faults that cause them to keep operating but not as efficiently as they should. Detecting them is essential for proper maintenance. Nowadays, heat pumps usually include as expansion device electronic expansion valves (EEV), which generally control the superheat (SH) at the outlet of the evaporator. When soft faults occur, such as an undercharge or liquid line restrictions, the EEV opens to continue controlling the SH but becomes fully open at some point. Therefore, its behavior becomes a fixed orifice, and it can no longer continue to control. This study models the performance of the EEV for different conditions in heat pumps under soft faults, and it is validated with an experimental campaign in which the EEV opening is measured at different operating conditions and refrigerant charge levels. The evolution of the main system variables affected by the imposed fault levels of undercharge, overcharge, and liquid line restrictions are also analyzed.