Intermittent Refrigerant Mass Flow Rate Sensor for Enhanced Performance Monitoring of Heat Pumps
Heat pumps (HPs) are a key technology for the decarbonization of the building sector. To ensure their long-term contribution to climate goals, it is essential to evaluate their real-world performance - specifically their coefficient of performance (COP) and heating capacity - and enable robust diagnosis and control strategies. A critical parameter for this purpose is the refrigerant mass flow rate. While several non-intrusive on-field methods exist to estimate this quantity using limited measurements, their accuracy can degrade over time due to changes in performance of the HP components. This study presents the development of a novel sensor system designed to intermittently measure the refrigerant mass flow rate in a HP circuit. Its objective is to provide a reliable reference measurement that can be used to verify and correct the output of available mass flow rate estimation methods. The device consists of an electrical heater applied to a refrigerant line, introducing a controlled thermal load into the HP circuit. By measuring the resulting temperature difference upstream and downstream of the device, the refrigerant flow rate can be calculated. As this process slightly disturbs normal operation of the HP, it is intended for periodic rather than continuous use. The first part of this paper presents the thermal model of the sensor behind the mass flow rate measurement and the uncertainty calculation method. In the second part, the experimental set-up used for testing the sensor system is described. Finally, the results of the experimental validation are presented, highlighting the accuracy of this sensor system with a relative error ranging between 2.5 and 7 %, and an expanded uncertainty between 5 and 6 %. This device enhances the robustness of online COP estimation methods and supports more accurate, real-time monitoring of heat pump performance under actual operating conditions.