Determination of the heat capacity of a water-ice reservoir as a lower heat source for heat pump - Numerical analysis

Abstract

The paper presents the design of a heat exchanger immersed in a water-ice reservoir and the determination of its heat capacity as a lower heat source for the heat pump. This is an innovative solution, the first project on this scale in Poland. Heat absorption from the water-ice tank took place in three stages: from water at a temperature range of 20oC to 0oC, from the water-ice phase change at 0oC, and from ice at a temperature range of 0oC to 10oC. The CFD (Computational Fluid Dynamics) analysis of a heat exchanger performance was performed. It required simulation of water natural convection, water-ice phase change, and heat transfer from the ground. The heat flux absorbed in the designed exchanger was calculated based on the current glycol temperature and the implemented COP (Coefficient of Performance) characteristic of the heat pump. This was done via the user-defined function (UDF) available in Ansys FLUENT. The compiled internal software subroutine was defined based on the DEFINE_ADJUST macro. Moreover, the thermal resistance of ice forming on the pipes was included. The numerical analysis indicated that 66097 kWh of heat would be absorbed from the reservoir in 500 hours of exploitation. The volume fraction of water at the end of the simulation was equal to 26.7% and the volume fraction of ice was equal to 73.3%. The CFD simulation confirmed the heat capacity value of the water-ice storage tank which fulfilled the design requirements.