Heat Pump Water Heater, Modeling, Simulation, Convection, Stratification
Heat Pump Water Heaters are becoming more and more interesting technologies for efficient sanitary hot water production. The specificities of hot water production compared to the traditional use of heat pumps for space heating are the relatively constant energy needs for different outdoor temperatures and the more rapid dynamics associated with water temperature elevation. This study focuses on the modeling and performance evaluation of an R134a air to water heat pump water heater with an external mantle heat exchanger. By nature being a thermo-hydraulic kind of system, a heat pump water heater requires both the aspects of fluid mechanics and heat transfer to be covered when modeling the global system composed of the heat pump and the thermal storage tank. Hence, a detailed thermodynamic model of the heat pump cycle is developed using Modelica covering a description of all the components of the thermodynamic cycle from compressor to evaporator and all the possible operating conditions such as heating and defrosting. This model is associated with a zonal model accounting for the convective behaviour patterns of the water observed in the storage tank at different operating conditions and boundary conditions imposed by the heat pump cycle. This dynamic model is compared against experimental data acquired from an instrumented system tested in laboratory conditions for different phases such as draw-off, standby and heating.Â Good precision (<5-10%) is attained for the heat flow rates, temperatures along the thermodynamic cycle and temperature profiles in the water tank for the different phases tested. It is shown that the water tank plays an important role in the performances of the system that is very sensitive to the operating conditions such as draw-off flow rate, heat pump operating capacity or thermal losses, that cause mixing and destruction of the thermal stratification and a reduction in the available energy for the end user.