Optimization of the coefficient of performance of a heat pump with an integrated storage tank – A computational fluid dynamics study

The performance of a heating system consisting of a heat pump with an integrated storage tank was evaluated by means of Computational Fluid Dynamics (CFD) calculations. The aim of the investigations was to elucidate how thermal stratification in the tank would influence the COP of the system. Differently designed storage tanks were investigated under typical operation conditions.

CFD models of heat storage tanks were developed and validated by experiments on a test tank. Calculations with the validated models were carried out in order to find the optimal tank design and system settings for achieving the highest possible COP. The investigated parameters were volume flow rate, tank geometry, diffuser design and tank wall material.

Two different system COPs were defined: one based only on charge operation and the other on both charge and discharge operations. It was found that there is a direct connection between the thermal stratification inside the tank and the COP of the heating system. Higher degree of stratification leads to higher COP.

For the ideal case where there is no mixing, no vertical thermal conduction in the tank and no heat loss from the tank, the system COP after a charge-discharge cycle is 3.69. When the effect of mixing is taken into account, the obtained COP of the system is decreased by approximately 9%. If heat transfer to the tank wall and vertical thermal conduction are also taken into account, the obtained system COP is lowered by another 5%.

Moreover, the COP of a heating system with high flow rates can be significantly increased by a diffuser plate installed in the tank with a small distance to the inlet/outlet. The best performing system studied consisted of an insulated tall and slim tank with a h/d ratio of 3.64 and a perforated plate diffuser, giving a COP of 3.23 which was 32% higher compared to a case without a diffuser.

The performance of the suggested system was not affected by variations of flow rates in the range from 0.12 to 0.24 kg/s.