Preparation Of Composite Phase Change Material And Study On The Thermo-Physical Phenomena In The Latent Thermal Energy Storage

Latent thermal energy storage (LTES) using phase change material (PCM) completes the energy storage and retrieval in the phase change of PCM. LTES is one of the most preferred forms of energy storage since it can provide high energy storage density, and nearly isothermal heat storage/retrieval processes. For such energy storage system, solid-liquid transition is most preferred because of the small variation in volume, unlike liquid-gas or solid-gas transitions. The LTES technique has been developed and researched for many years, which has been an outstanding candidate for the sensible thermal energy storage. However, the LTES was not used extensively for the following reasons: almost all the PCMs meet with a drawback of lower thermal conductivity which leads to the lower rate of heat storage and retrieval; it is difficult to accurately study and clearly analyze the theory of heat transfer in the process of phase change by using the previous theoretical method; the previous packed bed models all fail in accurately accounting for the details of the LTES system and the accuracy and time-consuming could not be properly compromised, which limited their extensive utilization; moreover, the experimental data of LTES system was still insufficient. The main objective of this thesis is to improve the thermal properties of the natural PCM, deeply analyze the mechanism of heat transfer in phase change, probe an effective numerical method to research the performance of the LTES system, and investigate experimentally the storage and retrieval of a real LTES system. The main contents involved in this thesis are:(1) The thermal conduction in PCM is enhanced by the addition of nano-particles and expanded graphite (EG). Firstly, the carbon nano-tube and carbon nano-particle were dispersed into paraffin to prepare phase change composites with high thermal conductivity, respectively. The addition of 5wt% carbon nano-tube can result in a 26.26% increase in the thermal conductivity. The test result indicated: the effect of thermal-conductivity enhancement was not as good as requirement by spreading the particles into PCM. EG/paraffin composites, with mass fraction of EG varying from 0 to 10 wt%, were prepared and characterized. Polarizing optical microscope investigation showed that compact EG networks formed gradually with increase in the mass fraction of EG. These networks provided thermal conduction paths which enhanced the thermal conductivity of the composite PCMs, e.g., an addition of 10 wt% EG resulting in a more than 10 fold increase in the thermal conductivity compared to that of pure paraffin. Thermal characterization of the composite PCMs with a differential scanning calorimeter (DSC) revealed the effect of the porous EG on the phase change behavior of paraffin. The shifts in the phase change temperatures were observed. The maximum deviation of the melting/freezing points of the composite PCMs from that of pure paraffin was 1.3℃whereas that of the peak melting/freezing temperature was 5.6℃. The DSC investigation also showed an anomaly in the latent heat of the paraffin in the composite PCMs in that it first increased and then decreased with increase in the EG fraction. Heat storage/retrieval tests of the composite PCMs in a LTES system showed that the heat storage/retrieval durations for EG(10)/paraffin(90) composite were reduced by 48.9% and 66.5%, respectively, compared to pure paraffin, which indicated a great improvement in the heat storage/retrieval rates of the system. Moreover, for the requirement of heat storage with a higher temperature, acetamide (AC) can be a potential candidate PCM. Its utilization is however hampered by its poor thermal conductivity. EG/AC composite with 10wt% EG was prepared. Transient hot-wire tests showed that the addition of 10wt% EG led to about five-fold increase in thermal conductivity. The melting/freezing points shifted from 66.95/42.46℃for pure AC to 65.91/65.52℃for EG/AC composite, and the latent heat decreased from 194.92 to 163.71 kJ/kg. In addition, heat storage/retrieval tests in a LTES unit showed that the heat storage/retrieval durations were reduced by 45% and 76%, respectively. Further numerical investigations demonstrated that the less improvement in heat transfer rate during the storage could be attributed to the weakened natural convection in liquid(melted) AC because of the presence of EG.(2) The thermo-physical phenomena in the melting process of paraffin, which included volume expansion, thermal conduction in solid paraffin, thermal convection and conduction in the melted paraffin and variation of phase-change interface, were numerically investigated. The VOF (Volume of Fluid) and enthalpy-porosity coupled model were adopted to simulate the ascent of paraffin/air interface which was caused by volume expansion and the variation of paraffin phase-change interface. The model was verified by the visualization experiment. The numerical results indicated: on the one hand, the natural convection has played an important role during the melting of the paraffin and the maximum melting velocity of the paraffin reaches 0.002005% per second in the intensive period of the natural convection; on the other hand, the extent of melting also has an influence on the intensity of the natural convection and the maximum flow velocity of the paraffin occurs at 150 s with its value climbing to 6.08×10-3 m/s. In the whole melting process, the volume expansion of 10% has been observed. By using this model, the effect of the mushy zone constant, C, on the phase change process was investigated. With increase of C, the velocity of melting has become lower, and the region of mushy zone has become narrower. In addition, the contact melting was enhanced with increase of C.(3) An effective packed bed model was developed, which could investigate the flow field as the fluid flows through the voids of the PCM units, and at the same time could account for the thermal gradients inside the PCM spheres. The proposed packed bed model was validated experimentally and found to accurately describe the thermo-fluidic phenomena during heat storage and retrieval. The effective packed bed model is superior to other models in that it is versatile for various packed bed LTES systems and it is capable of showing in detail the flow field in the packed bed and the thermal gradients inside the PCM spheres. The proposed model was then used to do a parametric study on the influence of the arrangement of the PCM spheres and encapsulation of PCM on the heat transfer performance of LTES bed, which was difficult to perform with the previous packed bed models. The results indicated: that random packing is more favorable for heat storage and retrieval as compared to special packing; the encapsulation of the PCM has a significant influence on the heat transfer of the LTES system. The freezing duration of the PCM spheres with stainless steel encapsulation was nearly 15% shorter than that with polyolefin encapsulation. The thickness of the polyolefin encapsulation has significant influence on the heat transfer performance of the LTES system, whereas it is not evident as for the stainless steel encapsulation. The effective packed bed model would be one of the most preferred tools to optimize the design and operation of the packed bed LTES systems.(4) The EG(7)/paraffin(93) composite PCM and the paraffin were used in a shell and tube LTES system and the performance of the LTES system was experimentally investigated. The volume of the LTES tank was 166 L with 55.3% volume filled with PCM. It is indicated the stored thermal energy can be rapidly and intensively released in the system filled with EG/paraffin composite, which was significant for the utilization of the LTES system. In the LTES system filled with EG/paraffin composite, the outlet temperature of water could maintain a high level in a longer duration than that with paraffin, such as the outlet temperature of water of the LTES system filled with paraffin/EG composite could be maintained above 50℃for another more 1000 s than that with paraffin, and the temperature difference of water between these two systems in the outlet can arise to 8℃. There was a large difference between the temperature evolutions of the pure paraffin and paraffin/EG composite PCM in the step-by-step heat retrieval mode, whereas the temperature evolutions of water in the outlet of the two LTES systems were almost the same with each other.It is expected that the studied work here can be used widely in future to store the low grade thermal energy, such as solar thermal or waste heat, as the technology can well address the problems to ensure both large energy storage density and good heat transfer performance.