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Phase change material (PCM) based thermal energy storage (TES) is an important solution to the waste of heat and intermittency of new energy sources. However, the thermal conductivity of most PCMs is low, which severely affects the thermal energy storage performance. Oscillation of the tube bundles in a TES unit can intensify the convection of liquid PCM and, therefore, enhance heat transfer. However, the energy storage performance of bundled-tube TES systems in response to oscillation at different amplitudes and frequencies has not been well understood yet, and the optimum time to apply the oscillation during phase transition remains unexplored. To address this issue, this present study was carried out. First, the melting behaviour of PCM with oscillation starting at different times was investigated. Then, the influences of oscillation frequency and amplitude on the melting performance were explored. Finally, the solidification behaviour of PCM with oscillation starting at different times was examined. Results show that the oscillation can accelerate the phase transition process by enhancing convective heat transfer. Compared to the case without oscillation, the complete melting and solidification times are reduced by 8.2 and 6.7% for the case with oscillation starting at 200 s, respectively. The effect of oscillation frequency on the melting enhancement is negligible, while the oscillation amplitude has an important effect on the melting enhancement. [ABSTRACT FROM AUTHOR]

Summary: An appropriate design of solar thermal power plant (STPP) is important before construction. This present study attempted to deal with some fundamental questions in design of STPP. What is the relationship between the optimal design direct normal irradiance (DNI) and weather conditions? What is the optimal solar multiple for a STPP at a specific weather condition? How does the thermal energy storage (TES) affect the electricity output of STPP, and how does dispatch fraction affect the usage of stored thermal energy? To help answer these questions, a general model was built and a critical design DNI was proposed from the perspective of effective use of solar energy. There will be solar energy wasting if the design DNI is below the critical design DNI, while the power block will not be effectively used if the design DNI is above the critical design DNI. This critical design DNI can be used to determine the optimal solar multiple for a given design DNI. The effects of TES size and dispatch fraction on the critical design DNI were also discussed. A critical design direct normal irradiance was proposed in this study from the perspective of effective use of solar energy with considering weather condition, thermal energy storage, and dispatch strategy. The critical design direct normal irradiance can demonstrate solar energy usage ability of a solar thermal power plant, and it can also help to determine the solar multiple at a given weather condition. [ABSTRACT FROM AUTHOR]

The employment of annular fins and longitudinal fins in latent heat thermal energy storage unit can significantly improve its thermal performance. However, the annular fins can greatly suppress the convection and longitudinal fins lead to severe non-uniform melting. To address this issue, three different types of combinational fins were proposed and applied in a vertical shell-tube storage unit in this study. The numerical model was first built with the adoption of enthalpy-porosity method. The simulation results were then verified with experimental data. Results showed that combinational fins leads to faster melting thorough enhancing both natural convection and conduction compared with only longitudinal or annular fins. This advantage in melting enhancement is even greater with the increase of fin volume. Moreover, the enhancement effect of combinational fins is better with the same longitudinal and annular fins volume and a higher longitudinal fin at the same longitudinal fin volume. The optimum combinational fins in present work has a melting time reduction of 31.0% and 21.2% compared with only longitudinal and annular fins, respectively. The results can provide important reference for the fins design in vertical shell-tube thermal energy storage units. [ABSTRACT FROM AUTHOR]

• T-shaped fin can enhance heat conduction and convection of PCM at the bottom region. • The horizontally and vertically arranged T-shaped fins outperforms other designs. • T-shaped fins reduce about 1/3 of melting time regardless of HTF temperature and fin volume. • A length ratio of vertical to horizontal portion greater than 1.0 can better enhance the melting performance. Enhancing the melting performance of phase change material (PCM) has always been an important issue in thermal energy storage. T-shaped fin has the potential to enhance melting as it has much larger heat transfer surface away from heat source compared to traditional longitudinal plate fin. However, its enhancement performance remains unexamined. This study examined the effect of T-shaped fins on PCM melting in a horizontal shell-and-tube storage unit. The melting enhancement performance of four T-shaped fins was compared with that of eight longitudinal plate fins which have the same total fin volume with T-shaped fins. The results showed that T-shaped fins can significantly enhance the melting process by shortening the complete melting time and improving the melting uniformity regardless of the heat transfer fluid (HTF) temperature. Compared to longitudinal plate fins, horizontally and vertically arranged T-shaped fins save 34.5% melting time, while the T-shaped fins having a 45° angle with horizontal direction save only 8% melting time. The lengths of the vertical and horizontal portions were further optimized to improve the melting performance. It is found T-shaped fins with a length ratio of vertical to horizontal portion greater than 1.0 can better enhance the melting performance, and a length ratio of 1.33 saves 47.1% melting time compared to the longitudinal plate fins. [ABSTRACT FROM AUTHOR]

• Frustum tube system enhances both conduction and convection. • Geometry modification significantly affects heat transfer in pure PCM. • Combination of frustum tube and metal foam further improves melting performance. • The frustum tube system outperforms the conical shell one. In this study, seven vertical shell-tube latent heat storage (LHS) systems are built to explore the effect of geometry on the melting and solidification behavior of phase change material (PCM). The performances of these systems using pure PCM (paraffin RT82) and composite PCM (paraffin RT82 embedded in copper foam) are investigated. Two-dimensional numerical model based on the enthalpy-porosity method and two-temperature energy equations is developed and verified with the experimental data. The solid-liquid interface, isotherms, time histories of liquid fraction and stored energy are used to evaluate the thermal performance. Results show that the conical shell system enhances natural convection compared with the cylinder system, whereas the frustum tube system enhances both convection and conduction. Geometry modification has little effect on the thermal performance of composite metal foam/PCM. Compared with the cylinder LHS system, the complete melting time with geometry modification is reduced by at least 9.2% for pure PCM, and less than 5.9% for composite metal foam/PCM. Moreover, it is also found that the frustum tube system outperforms the conical shell system in both melting and solidification, regardless of metal foam and HTF material. [ABSTRACT FROM AUTHOR]

• The relative position of hot and cold portions determines the intensity of natural convection. • Hot portion has a greater effect on liquid fraction evolution compared to that of cold one. • Higher state liquid fraction is attained when natural convection is perpendicular to the cold portion. • Higher stored energy is obtained when the cold portion locates at the bottom right corner. The thermal performance of paraffin wax in a square cavity under simultaneous charging and discharging (SCD) condition is investigated. The paraffin wax is charged and discharged by partially heating and cooling walls. A computational fluid dynamics (CFD) model is developed and validated against experimental data. Numerical studies were done to explore the performance of the system with different arrangements of the hot and cold portions. Results show that heating PCM from bottom and cooling from top accelerates the melting process, and heating PCM from side and cooling from bottom increases the stored energy. In addition, the effect of positions of partially thermally active walls for each arrangement was investigated. Results show that the stronger the natural convection, the higher PCM liquid fraction. More specifically, when heating from the bottom, the best performance is achieved with the hot portion locates at the middle of bottom wall regardless of the position of cold portion; increasing the height of hot portion results in a lower liquid fraction and stored energy; increasing the height of cold portion leads to lower PCM mean temperature and then decreases the stored energy. Furthermore, it is also found the melting rate increases as the cold portion gets further away from the hot portion. [ABSTRACT FROM AUTHOR]