New effective thermal conductivity model for the analysis of whole thermal storage tank

The deliberate shift from the use of fossils fuels for less pollutant energy sources has risen rapidly. This has been favored by stringent mitigation measures imposed on the use of fossil fuels which contribute highly to the emission of greenhouse gases. With the continuous advancement in the generation and use of electric power, development of efficient energy storage systems in terms of cost, reliability and storage capacity is highly desirable. Recently, latent-heat thermal energy storage systems have been identified as a promising solution. Unfortunately, the complicated nature of the phase change involved makes it hard to estimate and justify the performances of such systems.

Addressing the phase change-based problem was based on numerical analysis. Despite taking into consideration the various factors: flow transition during the melting process and the effects of natural convection, thermal and geometric parameters, a proper understanding of the phase change phenomena have not been achieved. The challenges can, however, be overcome by including the effects of the external conditions around the capsule, thermal stability on the solid-liquid interface, and natural convection.

Presently, analysis of the whole tank is of great importance in predicting the overall performance of thermal energy storage systems. To simplify the complexity of the phase changes, two models based on the assumptions: regarding two phases as a single-phase by ignoring the temperature difference between the liquid-solid interface and separately treating the liquid and solid phases have been incorporated. Regardless of the extensive analysis of the entire tanks, several limitations have been noted with both single-phase and two-phase models which compromise results accuracy. For example, it is difficult to estimate the charging and discharging performances of the systems due to the variation in the melting and solidifications depending on the capsule position.

Sejong University researchers: Min Ho Kim, Yong Tae Lee, Jiwon Gim, Abhishek Awasthi and led by professor Jae Dong Chung from the Department of Mechanical Engineering proposed a n new approach for accurate analysis of the melting and solidification behaviors in full tanks. In particular, the approach was based on effective thermal conductivity reflecting the effects of natural convection in molten phase change materials. The authors commenced by evaluating the applicability of the model over wider conditions with different capsule sizes and varying wall temperature taking into account the effects of natural convection. Eventually, the model was validated through numerical analysis and by comparing the results with the existing experimental data. The paper is published in the journal, International Journal of Heat and Mass Transfer.

The newly proposed model successfully predicted the various melting behavior depending on the position of each capsule in the tank. This was attributed to the changes in the Rayleigh number with the increase in the molten pulse change materials volume. Introduction of the effective thermal conductivity significantly decreases the calculation time thus enhancing the efficiency of the model. To actualize the study, the superiority of the present model was exhibited by creating 12-layer capsules in a vertical column. Interestingly, close agreements in the experimental and numerical results were observed. Therefore, the study by professor Jae Dong Chung and his research team will advance the design, development, and optimization of latent-heat thermal energy storage systems.