【作者】 夏莉；

【导师】 张鹏；

【作者基本信息】 上海交通大学 ， 制冷及低温工程， 2011， 博士

【摘要】 潜热储能是利用物质在相变过程中,吸收或放出相变潜热来进行能量储存与释放的技术。潜热储能的储能密度高,而且,在储/放能过程中,材料在发生相变时,吸收/放出大量的热,但温度近似恒定。相变储能技术已经经历了几十年的发展,实践证明它是传统显热储能的潜在替代者之一。然而相变储能的规模应用仍面临一些问题:几乎所有的相变材料(PCM)都存在热导率低、相变过程中的传热性能差的问题;相变传热理论缺乏透彻的分析,现有的分析方法和技术在相变问题的研究上还存在较大的难度;相变储能装置的数值研究方法还不完善,求解精度和耗费计算资源间很难兼顾;相变储能系统运行的实验数据缺乏。基于此,本文开展了以下研究工作:(1)以PCM为基体材料,在其中分别添加具有高热导率的纳米粒子和具有多孔结构的膨胀石墨(EG)来强化PCM的导热性能。首先,制备以石蜡为基体材料、纳米碳粉和碳纳米管作为分散相的纳米复合PCM。添加5%碳纳米管的复合PCM的导热系数增加了26.26%。通过测试得知添加分散粒子的方法对PCM热导率改善的效果并不理想。因此决定选用具有多孔结构的膨胀石墨(EG)去改善石蜡的换热情况。在制备的EG/石蜡复合PCM中EG的质量配比由1wt%连续的变化到10wt%。随着EG添加量的增加,致密的EG网络逐渐在复合物中形成。正是致密的EG网络为PCM内部的热传导提供了路径。与纯石蜡相比,当复合物中添加10wt%的EG时,热导率提高了近10倍以上。EG的出现引起了复合物中石蜡相变温度的迁移。复合物中石蜡的熔点和凝固点与纯石蜡相比最大偏差为1.2℃,而峰值相变温度的最大偏差为5.6℃。随着EG配比的增加,复合PCM的相变潜热是先增大后减小的。在储能单元的充放热测试中,EG(7)/石蜡(93)和EG(10)/石蜡(90)复合PCM储热周期比纯石蜡的分别减少42.8%和48.9%,放热周期分别减少64.1%和66.5%。为满足较高温度的储能需求,选用乙酰胺(AC)作为蓄热材料,这里同样采用EG来提升AC的热导率。10wt% EG的添加可以使EG/AC复合PCM的热导率达到AC的6倍以上。EG/AC复合物的熔点和凝固点分别由纯AC的65.91℃和42.46℃变化到66.95℃和65.52℃,EG的添加可明显的改善AC中的过冷。EG的添加使复合物的潜热降低到163.71 kJ/kg,比纯AC的194.92 kJ/kg降低了近30 kJ/kg。在充放热测试中,EG/AC复合PCM为储能材料时,充/放热周期分别比采用AC时减少45%和76%。(2)求解VOF(Volume of Fluid)与焓–多孔介质耦合模型,模拟出具有自由表面的一类相变过程,该模型可以较完整的反映出材料相变过程中的各方面特征,并利用可视化实验对模型的准确性进行了验证。结果表明:石蜡内部的自然对流在石蜡的融化过程中起到非常重要的作用,在自然对流的旺盛期,石蜡的最大融化速率为每秒0.002005%,而到融化快结束时,自然对流减弱,融化速率为每秒0.000395%;融化过程对自然对流也有影响,石蜡中的温差及相界面位置决定了自然对流的发展,在液体石蜡内部的最大流速先增大后减小,液体石蜡中的流速在融化进行150 s左右达到最大值6.08×10-3 m/s。石蜡在整个融化过程中体积膨胀了近10%。另外,利用该模型研究了描述材料糊状区形态的系数C对相变过程的影响。C值越大的材料融化得越慢;而且融化过程中糊状区的范围越小;固体下沉引起的接触融化在C值较小的材料中作用较明显。(3)建立一个可以详细研究潜热储能床内部的流动与换热的数学模型,并利用文献中的实验结果对模型的精确度进行了验证。该有效储能床模型具有优于其它模型的诸多特点:模型具有很强的通用性,适于各种储能床的研究;该模型可以反映出储能床内部详细的流动信息和相变单元内部详细的温度梯度。另外,还利用有效储能床模型进行一些参数的研究,这些参数研究很难用其它模型实现。数值结果显示:1.对于采用随机排列的潜热储能床,其热释放率要高于规则排列时的情况;2. PCM的封装会对系统的换热产生不同程度的影响。在放热过程中,采用不锈钢封装的相变单元的放热时间要比采用聚乙烯的减少近15%。聚乙烯封装材料的厚度对整个潜热储能床的换热性能具有较明显的影响,而不锈钢封装的厚度对系统的换热性能却没有明显的影响。实践证明有效储能床模型能够成为一种潜热储能床设计与操作条件优化的重要工具。(4)构建了壳管式潜热储能实验系统,并分别选用石蜡和EG(7)/石蜡(93)复合材料作为该储能系统中的蓄热材料。相变储能水箱的容积为166 L,储能材料填充的体积百分比为55.3%。在热释放过程中,填充了复合PCM的水箱出口温度保持在50℃以上的时间较填充石蜡的水箱长了近1000 s,并且换热流体的出口温度在此期间最高可较填充石蜡的高出8℃。间歇式取热模式下工作,纯石蜡内部的温度变化和复合材料内部的温度变化有着非常明显的差别。但在这种取热模式下工作,两者的换热流体出口温度却相差不大。本论文研制了具有高热导率的复合PCM,并对PCM的相变换热进行了深入分析,解决了现有储能系统充放热效率低的问题,这对太阳能等低品位热能潜热储存的广泛应用起到了积极的作用。更多还原

【Abstract】 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.更多还原