Numerical investigation of thermal performance of geometrically modified spherical ice capsules during the discharging period

Ice thermal energy storage (ITES) is a significant option for shifting cooling load from peak hours to off-peak hours in order to reduce the cooling cost and equipment capacity. Encapsulated ITES system is one of the most preferred types of ITES systems, because it is easier to apply and has lower initial cost than other types of ITES systems. Capsule geometries have great impact on the thermal performance of the encapsulated ITES system. Spherical capsules are widely used in the application because they provide better thermal performance due to the ratio of its volume to its surface area. Also, modified capsule geometries are used for increasing the thermal performance of the ITES system. This paper focuses on a numerical study in order to determine the effect of the geometrically modified spherical capsule on the thermal performance during the discharging period. Seven geometrically modified spherical capsules were investigated and compared with the ordinary spherical capsule. The FLUENT 17.1 code was used for numerical analyses. At the end of the study, it was found that geometrically modified capsules provided higher thermal performance than an ordinary spherical capsule in the equal capsule volume. Capsule D provided the longest discharging period of 2118 seconds, when V-HTF,V- in = 0.001 m/s and T-HTF,T-in = 275.5 K. The lowest discharging time of 464 seconds was obtained with Capsule A, when V-HTF,V- in = 0.01 m/s and T-HTF,T- in = 283 K. Among the studied modified capsules, Capsule F performed the worst thermal performance. Energy efficiency values were high, and they did not give comparable difference between the capsule models and working conditions. While all exergy values were 98.50% and higher, exergy efficiency values changed between 85.12% and 51.91%. Exergetically, Capsule D provided the best thermal performance during the discharging period