| The utilization of renewable energy resources is one of the most effective ways to cope with the energy and environment crisis that the world faces. Solar energy with the characteristics of being abundance, cost-free and clean to the environment is a prospective renewable energy resource, but the extensive utilization of solar energy is mainly limited due to its intermittence and instability. Therefore, effective and reliable solar energy storage has become more attractive recently. The latent thermal energy storage(LTES) system using phase change materials(PCMs) as energy storage medium, which is with high energy storage density and isothermal heat storage/retrieval characteristics, has shown good potential over the years for energy management, particularly in solar energy storage systems. However, the performance of LTES system is highly limited by low thermal conductivities of pure PCMs. The addition of porous structures to PCM can provide additionally thermal conduction paths in pure PCM and consequently enhances the heat transfer in the inner part of the PCM. However, the fabrication of the composite PCMs, the determination of the thermo-physical properties, the heat transfer analysis in LTES has not been studied thoroughly. The main contents involved in the present thesis are:Paraffin/carbon foam composite PCMs and paraffin/metal foam composite PCMs were prepared with vacuum impregnation method in the present study, and the structure and thermo-physical properties of the composite PCMs were studied extensively. A steady-state test system which considered the thermal contact resistance(TCR) between the specimen and adjacent surfaces was constructed to measure the effective thermal conductivities of the composite PCMs, which were also theoretically calculated based on the correlations and models from the literature. The results showed that the thermal conductivities measured with steady-state method showed good agreement with the theoretical predictions, and the thermal conductivities of the composite PCMs were drastically enhanced, e.g., the thermal conductivities of the paraffin/copper foam composite PCMs fabricated by the copper foams with the porosities of 96.95%, 92.31%, 88.89% and pore size of 25 PPI were about thirteen, thirty-one, forty-four times larger than that of pure paraffin, respectively. The effective thermal conductivity increases as the porosity decreases, and no noticeable changes of effective thermal conductivity were detected by varying the pore size of the metal foam for the same porosity. The ratios of TCR to the total thermal resistances of the composite PCMs with the thickness of about 20.0 mm were in the ranges of 15.0~50.0% in the experiments. The result with a differential scanning calorimeter(DSC) showed that the solid-solid phase change of the composite PCMs becomes unapparent, which is due to the high thermal conductivity of the skeleton and the effect of the thermal non-equilibrium of porous materials. The solid-liquid phase change temperatures of the composite PCMs show no significant shift.Sodium nitrate, potassium nitrate and their mixture were used as the base materials, and expanded graphite(EG) with high thermal conductivity and thermo-chemical stability was used as an additive to enhance the thermal conductivity. EG with various mass fractions was added to the base materials to form composite PCMs, and the composite PCMs were cold-compressed to form shape-stabilized PCMs at room temperature. The thermal conductivities of the composite PCMs fabricated by cold-compression were investigated at different temperatures by the steady state method. The results showed that the addition of EG considerably enhanced the thermal conductivities, e.g. the thermal conductivities of pure nitrates(Na NO3/KNO3=1:1) were measured to be 0.63~0.76 W/(m·K), and the thermal conductivities of Na NO3/KNO3=1:1/10 wt. % EG and Na NO3/KNO3=1:1/20 wt. % EG composite PCMs were enhanced by about six times and nine times, respectively. The thermal conductivities of pure nitrates and nitrates/EG composite PCMs in solid state showed the behavior of temperature dependant, indicating the decrease with the increase of the temperature. The phase change temperature and latent heat of pure nitrates and nitrates/EG composite PCMs were both measured by DSC, and the phase diagram of nitrates-EG was shown accordingly. It was found that the presence of EG could improve the stability of the nitrates/EG mixture PCMs to some extent, so the melting and freezing processes will finish under a smaller temperature variation in terms of solar energy storage behavior.Eutectic molten salt can be used as the LTES medium in the middle-temperature solar energy applications, but the investigation of local flow and thermal phenomena of eutectic salt is insufficient. Molten salt(50 wt. % Na NO3, 50 wt. % KNO3) with a melting temperature of about 220 oC were encapsulated in a vertically circular LTES unit firstly. The VOF and enthalpy-porosity coupled model were adopted to numerically investigate the ascent of salt/air interface and the evolution of solid/liquid interface during the melting process. The numerical investigation was verified by the visualization experiment. Natural convection played an important role during the melting of the molten salt, and the maximum melting rate could reach 0.0646% per second in the intensive period of the natural convection. It was observed that that the free surface of molten salt ascended, and volume expansion was about 10.0%. The phenomenon of solid sinking was also found during the melting of molten salt. The previous eutectic salt and salt/metal foam composites were encapsulated in a LTES unit as the heat storage media, and the heat transfer characteristics during heat storage and retrieval were experimentally and numerically investigated. Natural convection was weakened in the case of salt/metal foam composites as the heat storage medium due to the flow resistance of the metal foam. Heat retrieval process dominated by heat conduction was largely accelerated in the case of salt/metal foam composites due to the reason that the thermal conductivity was significantly enhanced. A three-dimensional model considering thermal non-equilibrium between the salt and metal foam was established to describe the heat transfer characteristics inside the LTES unit. Two-temperature energy equations were used, and the laminar flow of liquid molten salt in porous structure is resembled as flow around cylinder. The temperature difference between the salt and metal foam was distinct due to the high thermal conductivity of metal skeleton, e.g. the maximum temperature difference between the salt and copper skeleton during heat storage was 6.8 ?C, while that between the salt and nickel skeleton was 4.4 ?C.A shell-tube LTES system for solar hot water supply using paraffin/EG composite PCM was built in order to improve the slow charging and discharging rates of the LTES system with pure paraffin, and the thermal behaviors of the system were experimentally studied. Various experiments were conducted with different heat fluid(HTF) temperatures and flow rates for heat storage and retrieval, respectively. The temperature evolutions at different heights showed different features in the shell-tube tank because of the stratification phenomenon. The utilization of paraffin/EG composite PCM greatly enhanced the heat storage/retrieval rates of the LTES system. The time-duration of the composite PCM during the discharging process was reduced more than that during the charging process. Both the inlet temperature and flow rate of the HTF affected the time-duration for heat storage and retrieval prominently. The higher flow rate was, the higher charging and discharging rate would be. The large temperature difference between the HTF and the initial state of PCM would accelerate the charging and discharging processes. While no significant difference was found for different initial temperatures during the discharging process. The maximum charging power in the LTES system was 10.78 k W, whereas the maximum discharging power from the LTES system was 13.62 k W. In all, the efficiency of the LTES system was improved with the application of the composite PCM. A three-dimensional computational fluid dynamical model, which was based on enthalpy method and considered the apparent specific heat incorporating latent heat and equivalent thermal conductivity of liquid paraffin, was developed to investigate the charging and discharging characteristics of the LTES system. It is shown that the model can accurately predict the thermo-fluidic behaviors of the LTES system during heat storage and retrieval. The model allowed for thermal gradients inside the PCM tubes. The thermal gradients inside the PCM tubes was determined with a UDF file, and the heat transfer coefficients between the HTF and PCM tubes at various heights and moments were estimated accordingly. The thermal gradient of the tube encapsulated with pure paraffin was more prominent than those of the composite, due to the low thermal conductivity of pure paraffin. The heat transfer coefficients for pure paraffin were much lower than those of paraffin/EG composites correspondingly, e.g., the heat transfer coefficients during the discharging process were about 20~50 W/(m2·K) for pure paraffin, while those for paraffin/7 wt. % EG composite and paraffin/10 wt. % EG composite were 140~180 W/(m2·K) and 170~210 W/(m2·K), respectively.The salt powder was encapsulated in the LTES unit by heat sealing firstly, and a LTES system was built accordingly. The charging process of the system was conducted considering the slow temperature increase of solar collector, which indicated the heat storage process with alternating temperature, and the total energy stored can be reached to 110 MJ?Due to the low thermal conductivity of pure salt, nickel foam was applied to improve the thermal performance of the system. The heat storage/retrieval powers of the system with the salt/nickel foam composite were greatly improved, compared with those of pure salt, i.e. when the mass flow rate of oil was 0.06 kg/s, the mean power during heat storage was 1.81 k W; and while the mass flow rate of oil was 0.03 kg/s, the mean power during heat retrieval was 4.54 k W, and the energy efficiency could come to about 80.04%. |
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