Yongcai Li

Review on Heat Transfer Mechanisms and Characteristics in Encapsulated PCMs

Latent heat storage (LHS) is a particularly promising technique compared with the conventional sensible heat storage (SHS) as it provides a high-energy storage density with a small volume. However, there are difficulties in practical engineering applications of LHS due to the heat releasing/absorbing, which involves phase transition and moving boundary problems and the unacceptable low thermal conductivity of the phase-change material (PCM). Furthermore, the encapsulation would affect the heat transfer characteristics of PCM significantly, depending on the parameters of various encapsulations and boundary conditions. Hence, this review analyzes heat transfer mechanisms during the phase-change process and numerical analysis for heat transfer in macroencapsulated PCMs according to the shape of containment. The effective heat capacity method and the enthalpy method, two of the most widely used numerical approaches for phase-change problems, are presented in detail. Besides numerical models for different PCM containment such as spherical, rectangular, and cylindrical containment models, PCM-based heat-sink models are reviewed, including several heat transfer enchantment technologies: finned structure and porous matrix. Finally, the challenges in numerical modeling and designing an LHS unit are also summarized in this article.

Heating storage performance of a water tank–combined phase change material:An experimental case study

Water tank is a commonly used unit which has been frequently applied for thermal energy storage units. In order to enhance the thermal performance of the water tank, the phase change material is utilized by embedding paraffin into water tank. But the key question is whether a competitive charging could be achieved. Therefore, an experimental system has been constructed. In this article, two key indices, the energy storage capacity and the energy storage intensity, are investigated. The performances of the phase change material–thermal energy storage tank during the heat charging processes are investigated experimentally, and a series of experiments are carried out under different heat transfer fluid flow rates and distance between phase change material plates. The temperature evolutions of the phase change material plates and heat transfer fluid are obtained during the experiments. The thermal performance of the phase change material–thermal energy storage tank was observed to be more effective than the conventional sensible thermal energy storage tank. It is found that water flow rate of 1.3 m3/h is taken as the optimal working condition, and the 3-cm plate distance is considered as the optimal design.

Numerical study of the influences of geometry orientation on phase change material's melting process

The acceleration of the melting process of phase change materials caused by buoyancy-driven natural convection has been widely acknowledged, especially for rectangular geometries. This acceleration phenomenon exists in the cases where phase change materials are heated at the bottom boundary or at both upper and bottom boundaries. This article reveals how the melting rate could be affected by changing the orientation of a rectangular phase change material container with a constant temperature boundary. The transient melting processes of lauric acid in a two-dimensional rectangular container with five orientations (θ = 0°, 22.5°, 45°, 67.5°, and 90°) were simulated using the computational fluid dynamics software. The computational fluid dynamics model was validated against available experimental data obtained from published literature. Results show that when the rectangular geometry is rotated from vertical direction (θ = 0°) to horizontal direction (θ = 90°), the total melting time is increased by about five times. For all investigated orientations, the heat transfer rate at the heated boundary is found to first increase at the initial stage (within about 100 min) and then decrease during the following melting process. Moreover, the total amount of thermal storage for the horizontally placed case is slightly lower than the other cases.

Thermal Storage Capacity and Night Ventilation Performance of a Solar Chimney Combined with Different PCMs

Thermal storage capacity and airflow rate of a solar chimney combined with different PCMs are numerically studied during nighttime. PCMs with phase change temperatures of 38°C, 44°C, 50°C, and 63°C are selected in this numerical study. Results show that the maximum average ventilation rate of 610 kg/m2 and maximum thermal storage of 4750 kJ/m2 are achieved at the phase change temperature of 38°C. However, for phase change temperature of 63°C, night ventilation does not occur under the identical conditions. The findings reveal that a lower phase change temperature can increase the chargeability (and therefore the dischargeability) of a solar chimney, since a higher phase change temperature demands higher solar radiation intensity and longer charging time for a solar chimney. For PCM with a phase change temperature of 44°C, most of the heat stored in PCM is lost to ambient through glass cover by radiation and only a small portion is used for heating the air within air channel.

Experimental Study of the Heat Transfer Performance of PCMs Within Metal Finned Containers

Latent heat thermal energy storages (LHTES) are particularly attractive methods owing to these factors: meet the time shift between energy supply and demand; provide a high energy storage capacity; store and release heat at a relatively constant temperature; provide constant comfort thermal environment without temperature swings when it is applied for space heating or cooling. Nevertheless, the efficiency of using the LHTES techniques is heavily affected by the low thermal conductivities of phase change materials (PCMs). This characteristic of PCMs prolongs the charging and discharging cycle and barriers the widely practical application of LHTES. Hence, researchers generated a lot of related technologies, such as metal fines, carbon fibres, metal honeycomb structure, etc, to overcome this issue and aimed to achieve reasonable thermal conductivities.