Comparative Evaluation of Circular Truncated-Cone and Paraboloid Shapes for Thermal Energy Storage Tank based on Thermal Stratification Performance

Abstract

In solar water heating systems, geometrical configurations of thermal energy storage (TES) play a crucial role in the enhancement of system performances. Upon conducting a thorough study on the influence of different shapes and aspect ratios of TES tanks, the present study focuses on the choice of suitable TES tank geometry based on the key performance parameters, viz. Richardson number, stratification number, storage efficiency and the initial thermocline thickness. Thermal stratification characteristics are investigated and analyzed in detail for three chosen shapes of TES, namely truncated-cone, paraboloid, and cylinder, using a two-dimensional unsteady numerical model. Results of the numerical model on cylindrical TES is found to have good agreement with the findings of the in-house experiments. With increasing aspect ratio (AR) of the paraboloid and truncated cone TES tanks, initial thermocline thickness and the energy losses increase, whereas, Richardson number and average stratification number decrease, indicating better stratification performance in storage tanks with lower AR. However, there is a limit on reducing the AR. At every AR investigated for the two shapes of TES, Richardson number for the paraboloid TES is greater than that of the truncated-cone TES, qualitatively suggesting a better temperature stratification in the paraboloid TES. The effect of flow rate on the thermal stratification characteristics is also studied, which suggests that increasing the flow rate enhances mixing and deteriorates stratification. The findings can be used as guidelines for designing TES with improved thermal performance.