Conference Year



Heat pump water heater, Stratification, Recovery efficiency, MIX number


This paper presents the performance of the Heat Pump Water Heater (HPWH) in warm-up and recovery at different draw rates and draw amount. Performance is evaluated based on COPsys and Recovery Efficiency as measure. Results show that the recovery efficiency is proportional to draw amount. With increase of draw rate, stratification worsens causing lower recovery efficiency. This paper also presents numerical investigations to validate the influence of draw rate and analyze the influence of baffle, nozzle and inlet position on tank stratification by Three-Dimensional Computational Fluid Dynamics (CFD) methods. CFD results are confirmed with the experimental data indicating that higher draw rate destroys the tank stratification. Thermal performance of HPWH systems is strongly influenced by the thermal stratification in the water tank. The simulation results also show that the baffle, nozzle and inlet position helps preserving the stratification in the water tank. A dimensionless coefficient, the MIX number, ranging from 0 to 1, with 0 representing a perfectly stratified tank and 1 representing a fully mixed tank, has been used to characterize the tank stratification level.