Virtual Refrigerant Charge Sensor for Booster Refrigeration Systems

In this paper a virtual refrigerant charge sensor (VRCS) for a booster refrigeration system with a receiver unit is developed and proposed. The VRCS estimates the liquid charge in the receiver unit by utilizing a combined mass and energy balance equation for mapping the change in the receiver unit's time constant to the liquid volume in the receiver unit. The time constant for the receiver unit is found by utilizing the commonly available data in an online subspace identification method. The parameters estimated by the online subspace identification method are recast in a linear fractional transformation structure to identify parameter changes due to liquid volume change. Hence, the VRCS functions in a plug and play framework where only the initial refrigerant charge in percentage is required to be known. Results from utilizing the VRCS on experimental data from a charge test conducted on a test setup located at Danfoss A/S are shown, where the VRCS shows an accurate estimation of the liquid charge in the receiver. The results from the VRCS can be employed for: early prediction of low charge in the system, early detection of a too large leakage rate, estimation of the current charge on a daily basis, and estimation of the amount of charge lost.