基于高导热定形相变储热复合材料的多粒径填充床热设计及储/放热特性研究

Medium-low temperature solid-liquid phase change thermal energy storage in packed-bed has technical bottlenecks of low effective heat storage density and poor heat charge/discharge power, suffering from the limitations of the intrinsic porosity of packed-bed and poor thermal conductivity of phase change materials. This project proposes to conduct applied fundamental researches on the preparation of high thermally conductive expanded graphite-based form-stable phase change thermal energy storage composites (EG-FSPCMs), as well as the thermal design and heat storage/release characteristics of multi-granularity packed-bed based on the EG-FSPCMs. Firstly, multiple influences of composition, adsorption ratio and density on the heat storage/release, mechanics and leakage characteristics of EG-FSPCMs are explored, based on which high thermally conductive spherical phase change thermal energy storage units are built. Meanwhile, regulatory mechanisms of the nonuniform distribution of storage unit sizes on the local porosity of packed-bed are revealed, with which optimal configurations of multi-granularity packed-bed are obtained. Based on the above studies, a thermal energy storage system employing multi-granularity packed-bed based on high thermally conductive EG-FSPCMs is set up. Both experimental and numerical investigations are conducted to investigate the flow and heat transfer coupling mechanisms and storage/release characteristics of the system under different heat transfer fluids and operation conditions. On this basis, the packed-bed structure and operational mode of the storage system are optimized, taking tech-economy in large-scale applications as the orientation. The planned work aims to realize a medium-low temperature solid-liquid phase change thermal energy storage technology with high heat storage density, high heat charge/discharge power and low cost, which can provides theoretical and practical foundations of thermal energy storage for the network utilization of low-grade industrial waste heat and solar heating in high-cold regions.

中低温填充床固-液相变储热受填充床固有孔隙率及相变材料导热系数低的限制，存在有效储热密度小、储/放热功率低的技术瓶颈。本项目针对高导热膨胀石墨定形相变储热复合材料的构建及基于此的多粒径填充床热设计与储/放热特性开展应用基础研究。探索膨胀石墨定形相变储热复合材料的组分、吸附率及成形密度对其储/放热、力学及泄露特性的多元影响规律，构建高导热球形相变储热单元；揭示储热单元粒径的非均匀分布对填充床局部孔隙率的调控机制，获得多粒径填充床的优化构型；基于以上研究，建立高导热定形相变多粒径填充床储热系统，采用实验与数值模拟相结合的方法探究其流动与传热耦合规律及在不同传热介质与工况下的储/放特性；在此基础上实现以大规模应用技术经济性为导向的填充床构型及系统运行模式优化。本项目旨在实现高密度、高功率、低成本的中低温固-液相变储热技术，为低品位工业余热网络化利用及高寒地区太阳能采暖提供储热理论及实践依据。

填充床固-液相变储热作为一项可广泛应用于太阳能热利用、建筑节能、电网削峰填谷及废热回收利用等领域的储热技术，目前仍存在储、放热功率低的技术瓶颈。本课题以高功率密度相变储热的应用为目标，针对高导热膨胀石墨定形相变储热复合材料的构建及其填充床热设计与储-放热调控机理进行了研究。采用熔融共混法制备了膨胀石墨-硬脂酸复合相变储热材料，结果显示，硬脂酸可通过熔融吸附和毛细作用稳定地吸附在膨胀石墨表面形成稳定的复合相变材料；复合相变材料具有清晰的石墨骨架高导热通道。采用准冷等静压法构建了膨胀石墨-硬脂酸复合相变储热单元；在最优成型密度（980 kg m-3）和膨胀石墨质量分数（20 wt.%）下，储热单元的周向和径向导热系数分别为13.4 W m-1 K-1和3.8 W m-1 K-1。对比分析了不同石墨质量分数下的相变储热单元在储-放热循环过程中截面上的瞬态温度分布变化规律。研究结果表明，相变储热单元的储、放热过程随膨胀石墨质量分数的增加而加快，在最优构型下相变储热单元可实现分钟级储、放热过程。建立了基于高导热膨胀石墨-硬脂酸复合相变储热单元随机堆叠的高功率填充床储热系统实验装置，并在空气流量为30 m3/h和储/放热温度区间为27-86 ℃的工作条件下，研究了系统的瞬态储、放热特性。在标准储-放热循环中，系统的储热容量利用因子为90.3%，储-放热循环效率达93.5%。基于焓方法建立了三维瞬态相变储热系统传热仿真计算模型，利用MATLAB构建了可模拟规模化相变储热单元储、放热瞬态过程的计算程序。利用该计算程序对百千瓦相变储热单元进行了参数化性能研究，揭示了不同运行条件和换热结构对储能单元功率和热效率的影响规律。同时，还对该储热单元应用于建筑供暖和生活热水供应两个应用场景进行了模拟。结果显示，100 kWh储热单元可满足195.7 m2空间16小时连续供热，或能提供2500升40 ℃热水，供25名居民日常使用6小时。本研究提供了一种高效的相变储热单元设计方法，并开发了相变储热过程仿真计算工具，有助于推动我国储热行业的发展。