Virtual Refrigerant Mass Flow and Power Sensors for Variable-Speed Compressors

The use of variable-speed compressors in heat pumps and air conditioners has increased in recent years in order to improve comfort and energy efficiency. At the same time, there is a trend towards embedding more sensors in this type of equipment to facilitate real-time energy monitoring and diagnostics. Although compressor mass flow rate and power consumption are useful indices for performance monitoring and diagnostics, they are expensive to measure. The virtual variable-speed compressor sensor (VVC) was developed to estimate mass flow rate and power consumption using inexpensive temperature sensors and embedded models. The mass flow and power consumption at maximum compressor speed and rated superheat are first correlated with suction and discharge pressure. These pressures are estimated using virtual pressure sensors that use condensation and evaporation temperatures measured on return bends within the heat exchangers. The mass flow and power predictions are corrected for different speeds and inlet superheat heats using additional correlations. The VVC can predict mass flow rate and power consumption within RMS errors of ± 5% and ± 3%, respectively, under normal (no fault) conditions. The VVC also works well under conditions where faults in the system exist, since the VVC isolates these two estimates from other possible external faults such as condenser or evaporator fouling. One of the inputs to the model is compressor frequency, which can be difficult to measure for technicians in the field. However, for an embedded application, the compressor frequency would generally be available from the motor controller. Alternatively, the VVC could be used to estimate compressor frequency if mass flow rate or power consumption was provided as an input from another independent virtual sensor.