肋片增强式梯级相变储热系统放热特性的三维数值

梯级相变储热技术已被证明是解决相变材料导热性能差的重要方法。已有的关于梯级相变储热系统的数值研究通常是基于一维或二维数学模型完成的,大部分的研究聚焦于系统的储热过程。本工作设计了一种肋片增强型三管式梯级相变储能系统,并建立了三维数值模型。然后研究了系统放热过程中各级PCM的性能变化规律,探究了传热流体进口温度和PCM初始温度对系统放热速率的影响规律。结果表明,在放热过程中,各级PCM相变不会同时发生,受到储热材料的相变温度和潜热的影响最大。传热流体进口流速的增大会提高相变材料的放热速率,但随着流速的进一步增加,放热速率的提高程度明显减弱。相变材料的初始温度对显热放热过程具有一定的影响,但对于潜热放热阶段影响较小。     

基于圆柱形单元的储热装置放热实验研究

蓄热水箱作为太阳能供暖系统的重要核心设备,其性能直接影响着储能系统的整体运行效率。设计一种基于圆柱形相变单元的相变储热装置,并搭建相变蓄热水箱性能测试平台,通过单一控制变量法得到储热装置放热过程的温度变化曲线。研究表明：对于空间一定的储热装置,在等质量相变材料（PCM）时,相变单元的直径对装置放热速率的影响较大;相变单元之间的间距对装置放热速率的影响较小;当增大换热流体（HTF）的入口流量及降低HTF入口温度时,能大大减少储热装置的放热时间,提高储热装置的整体性能。     

组合相变材料储热系统的储热速率研究

建立了组合式柱内封装相变材料熔化－固化循环相变储热系统的物理模型,用有限差分法进行了数值模拟求解。结果表明,与采用单一相变材料的传统储热系统相比,在给定相变材料组合方式和传热流体进口温度条件下,传热流体流量存在最佳值;选用三种石蜡作用相变材料和水作传热流体的模拟计算结果表明,相变速率可提高１５％￣２５％左右。     

Encapsulation of PCM in ceramic thermal energy storage materials

以红柱石为主要原料,采用原位生成堇青石技术制备高温性能优良的红柱石蜂窝陶瓷储热材料.再利用特制的封装剂将相变材料(PCM)封装在部分蜂窝陶瓷孔中,制备储热密度大的显热-潜热高温复合储热材料.采用SEM,EPMA,TG-DTA等测试方法对封装剂与陶瓷基体的结合性,PCM与陶瓷基体的适应性及复合储热材料的储热密度进行研究.结果表明红柱石蜂窝陶瓷能安全地封装PCM,封装质量分数为20%的K₂SO₄后的储热密度为987.70 kJ/kg(0~1080 ℃),封装质量分数为16%的NaCl复合储热密度为796.40 kJ/kg(0~810 ℃).制备的复合储热材料具有较高的储热密度,能实现高温储热.     

太阳能储热装置的蓄热性能研究

根据能量供求关系设计太阳能储热装置,将熔盐作为相变储能材料[1-2]、导热油为传热介质,解决能量供求在时间和空间上不匹配的矛盾;通过实验研究储热装置内各监测点温度随时间的变化关系;数值分析储热装置内温度场与外界工况的关系,并与实验结果分析比较,得出了Fluent可用于预测储热装置内PCM在蓄热过程中的物性变化;在此基础上改变相变换热器的结构,强化传热,改善储热装置的蓄热性能。     

Compatibility between cordierite-mullite ceramics and PCMs

以煅烧铝矾土、滑石及石英为原料,采用挤出成型,制备堇青石-莫来石复相蜂窝陶瓷,用这种显热储热陶瓷（简称基体）封装PCM制备潜热/显热复合储热材料。设计F系列封装剂配方组成,选择PCM为AlSi₂₀、Na₂SO₄,制得PCM/蜂窝陶瓷复合储热材料,热震循环（200～900 ℃）30次,每次循环在900 ℃保温12 h。利用XRD、SEM、TG-DSC等分析相组成、微观结构、封装剂与基体结合性、基体与PCM相容性、复合储热材料的相变温度及相变焓。结果表明,经1440 ℃烧成的蜂窝陶瓷,Wa、Pa、D及a轴抗压强度分别为2.38%、6.32%、2.65 g/cm3,25.77 MPa;XRD显示相组成为堇青石、莫来石及少量镁铝尖晶石;30次热震后,Wa、Pa略有升高,D及σa略有降低。封装剂剪切强度测定结果表明,封装剂F2与基体结合强度最大,达2.53 MPa,SEM显示F2与基体结合良好。热震30次后,基体断面SEM图表明,PCM无渗漏。TG-DSC结果表明,经30次热循环后,AlSi20/蜂窝陶瓷复合储热材料的相变范围为571.7～583.5 ℃,峰值为578.3 ℃,峰值温度偏移不超过0.7%。复合储热材料具有优良的储热性能。     

含碳二元系相变储热材料储热性能分析选择北大核心CSCD

相变储热技术与聚光太阳能发电技术相结合可以提高太阳能的利用率,减缓化石燃料燃烧带来的环境压力。本文通过分析相变储热材料的选择标准,对筛选出具有研究价值的含碳二元系相变储热材料的性能特别是热物理性能进行分析。研究发现,硅、硼、铝、铬、铁单质材料与碳元素形成的二元化合物或固溶体具有较高的熔点,形成的含碳二元系相变储热材料在高温相变储热领域应用前景广阔。在含碳二元系相变储热材料中,Fe-C二元合金可满足高温相变储热系统1100~1500℃的相变储热要求,当合金为含碳4.3%的Fe-C共晶成分时,Fe-C二元合金的相变潜热理论值为611 kJ/kg,热导率约为(40±16)W/(m·K),相变温度为1148℃,具有相对其他合金成分更为优异的综合储热性能可用于聚光太阳能热发电系统储热。     

储热材料研究进展

本文结合储热材料的分类、特点、应用及存在的问题对储热材料的最新研究进展进行了综述,主要包括有机相变储热材料、熔融盐类相变储热材料、合金相变储热材料及复合类储热材料。探讨了储热材料成分组成、制备工艺及性能特点,进一步介绍了其最新研究进展,并对储热材料的下一步研究进行了展望,提出开发高性能纳微复合结构储热材料是未来研究的重点。     

基于梯度孔隙率金属泡沫的复合相变单元储热性能数值模拟北大核心CSCD

向相变材料中添加金属泡沫可以解决相变材料低导热率引起的换热效果较差等问题,提高系统的整体蓄热效率。然而,复合相变材料的传热性能受金属泡沫孔隙率分布的影响较显著,为进一步提高相变储能单元的传热性能,本工作基于低孔隙率金属泡沫-相变材料(PCM)复合储能系统,建立了一种新的梯度孔隙率金属泡沫结构,通过数值模拟方法,对蓄热单元熔化过程中的熔化率、储能速率、储能总量进行分析,系统研究了孔隙率沿加热方向负梯度分布、正梯度分布对复合相变材料熔化速度和储热性能的影响。研究结果表明,负梯度孔隙率结构可以进一步提高储能系统的储热效率,其中,孔隙率梯度为0.12(案例S-6)时增强效果最显著。在熔化周期的不同阶段,负梯度孔隙率对复合材料的传热均有不同程度增强,对于S-6,在1000 s、2000 s、2600 s时,熔化率相较于均匀孔隙率结构分别增加了0.67%、2.31%、9.90%;随着孔隙率梯度的增加,相变材料的热性能提高越显著,与均匀孔隙结构相比,改进的负梯度孔隙率结构其完全熔化时间最高可缩短7.32%,储热速率可提高8.02%。对于正梯度孔隙率结构,其对熔化速度没有显著影响,但是储热总量可提高0.49%。     

Recent progress in thermal energy storage materials

本文结合储热材料的分类、特点、应用及存在的问题对储热材料的最新研究进展进行了综述,主要包括有机相变储热材料、熔融盐类相变储热材料、合金相变储热材料及复合类储热材料。探讨了储热材料成分组成、制备工艺及性能特点,进一步介绍了其最新研究进展,并对储热材料的下一步研究进行了展望,提出开发高性能纳微复合结构储热材料是未来研究的重点。     

Effects of different multiple PCMs on the performance of a latent thermal energy storage system

The present study presents a theoretical model for the performance of a shell and tube latent thermal energy storage (LTES) unit using multiple phase change materials (PCMs). The model is based on the enthalpy method. Numerical simulations are carried out to investigate the effects of different multiple PCMs on the melted fraction, stored thermal energy and fluid outlet temperature of the LTES unit. Numerical results indicate that PCMs’ fractions and melting temperatures play an important role in the performance of the LTES unit. As a result, appropriate choosing of multiple PCMs is very significant for the performance improvement of the LTES unit.     

Analytical and experimental investigations of nanoparticles embedded phase change materials for cooling application in modern buildings

This paper presents the analytical and experimental investigations of the phase change heat transfer characteristics and thermodynamic behavior of spherically enclosed phase change material (PCM) with dispersion of nanoparticles for latent thermal energy storage (LTES) system in buildings. In this study, the heat transfer characteristics in terms of the transient temperature variations, moving interface positions, complete rate of solidification and melting were analyzed for the six different PCMs considered in pure form and with dispersed nanoparticles as well. The heat transfer characteristics of the PCMs considered were analytically modeled and experimentally evaluated for the steady state and transient conditions for various heat generation parameters during freezing and melting cycles of the LTES system. The experimental results infer that for the same thermal load conditions the rate of solidification for the PCMs decreased with the increased mass fractions of nanoparticles while compared to the pure PCMs. For the same operating conditions of the LTES system, similar heat transfer characteristics were observed for the six PCMs considered. In this paper, the analytical model solutions and experimental results for the 60% n-tetradecane: 40% n-hexadecane PCM are presented. The solidification time for the 60% n-tetradecane: 40% n-hexadecane PCM embedded with the aluminium and alumina nanoparticles were expected to reduce by 12.97% and 4.97% than at its pure form respectively. Besides, the test results indicate that by increasing the mass fraction of the nanoparticles beyond the limiting value of 0.07 the rate of solidification was not significant further. Furthermore, the rate of melting was improved significantly for the PCMs embedded with the dispersed nanoparticles than the pure PCMs. The analytical solutions obtained for the pure and dispersed nanoparticles based PCMs were validated using the experimental results. The deviations observed between the analytical solutions and the experimental results were in the range of 10%-13%. Based on the analytical and experimental results the present nanoencapsulated LTES system can be regarded as a potential substitute for the conventional LTES system in buildings for achieving enhanced heat transfer characteristics and energy efficiency.     

Experimental Investigation on Heat Storage/Retrieval Characteristics of a Latent Heat Storage System

The charging and discharging characteristics of a latent thermal energy storage (LTES) system were experimentally studied. Pure paraffin and paraffin/expanded graphite (EG) composite containing 7% and 10% mass fraction of EG were used as the phase-change materials (PCMs). Various experiments were conducted with different heat transfer fluid (HTF) temperatures and flow rates for heat storage and retrieval, respectively. The time durations of the charging and discharging processes, the mean power, and the energy efficiency of the system, which are the important factors of the LTES system, were discussed. The results showed that natural convection played a crucial role in the heat transfer during the charging process of paraffin, but heat conduction was the main heat transfer mechanism during the discharging process of paraffin. The higher the flow rate was, the higher the charging and discharging rate would be. Large temperature difference between the HTF and the initial state of PCM would accelerate the charging and discharging processes. During the charging process, the large temperature difference would result in the accelerated phase-change process due to the enhanced natural convection that could be seen clearly when the PCM was paraffin. While no significant difference was found for different initial temperatures during the discharging process. The performance of the LTES was affected prominently by the PCMs, HTF temperatures, and flow rates. The energy efficiency was higher for the 10 wt% EG PCMs, and the mean power during the discharging process was larger accordingly.     

Analysis and optimization of a latent thermal energy storage system with embedded heat pipes

Latent thermal energy storage system (LTES) is an integral part of concentrating solar power (CSP) plants for storing sun’s energy during its intermittent diurnal availability in the form of latent heat of a phase change material (PCM). The advantages of an LTES include its isothermal operation and high energy storage density, while the low thermal conductivity of the PCM used in LTES poses a significant disadvantage due to the reduction in the rate at which the PCM can be melted (charging) or solidified (discharging). The present study considers an approach to reducing the thermal resistance of LTES through embedding heat pipes to augment the energy transfer from the heat transfer fluid (HTF) to the PCM. Using a thermal resistance network model of a shell and tube LTES with embedded heat pipes, detailed parametric studies are carried out to assess the influence of the heat pipe and the LTES geometric and operational parameters on the performance of the system during charging and discharging. The physical model is coupled with a numerical optimization method to identify the design and operating parameters of the heat pipe embedded LTES system that maximizes energy transferred, energy transfer rate and effectiveness.     

Experimental and numerical investigation of the melting process and heat transfer characteristics of multiple phase change materials

The melting and heat transfer characteristics of multiple phase change materials (PCMs) are investigated both experimentally and numerically. Multiple PCMs, which consist of three PCMs with different melting points, are filled into a rectangle-shaped cavity to serve as heat storage unit. One side of the cavity is set as heating wall. The melting rate of multiple PCMs was recorded experimentally and compared with that of single PCM for different heating temperatures. A two-dimensional mathematical model to describe the phase change heat transfer was developed and verified experimentally. The properties of multiple PCMs, including the effect of the melting point difference (combined type), thermal conductivity, and latent heat, on the heat transfer performance of the PCM were analyzed numerically. The results show that, the melting time decreases before it increases, with an increasing melting point difference for the multiple PCMs. In addition, the melting point decreases with increasing distance from the heating wall. Most of these types of multiple PCMs melt faster than the single PCM, and the multiple PCMs, with the melting point arranged as 322 K/313 K/304 K, has the shortest melting time in this study. The melting rate of the multiple PCMs, 322 K/313 K/304 K, accelerates faster than for the single PCM as the thermal conductivity, latent heat, and heating wall temperature increase. Finally, generalized results are obtained using a dimensionless analysis for both single and multiple PCMs.     

相变微胶囊悬浮液传热特性实验研究

相变微胶囊(microencapsulated phase change material,MPCM)在建筑节能领域应用广泛,为研究其传热特性,搭建了以水为换热流体(heat transfer fluid,HTF),微胶囊悬浮液为储能介质的潜热储能(latent thermal energy storage,LTES)系统。在实验过程中,通过改变换热流体的进口初始温度以及搅拌器的搅拌速率,获得了MPCM悬浮液的温度变化规律并计算了MPCM悬浮液的平均充放冷速率。实验结果表明:在充冷过程中,MPCM相变时温度变化速率减缓,相变温度区间较大,而在放冷过程中,MPCM相变时温度保持恒定,相变温度区间较小;未搅拌时,MPCM悬浮液中温度梯度较大,传热能力较差;搅拌时,MPCM悬浮液混合均匀,其温度梯度很小,传热能力较强;增加搅拌器的搅拌速率及水与相变微胶囊悬浮液的温差均可以提高MPCM的充放冷速率。     

组合式相变材料蓄热系统中相变温度分布研究

建立了组合式相变材料蓄热系统物理模型，在忽略显热的假设条件下研究了相变温度呈线性分布的组合式相变材料蓄热系统传热特性，得出了最优线性相变温度分布，并采用考虑显热的数值计算（有限差分法）证实了理论分析得出的最优相变温度与实际相变温度分布几乎相同。     

Numerical heat transfer analysis of the packed bed latent heat storage system based on an effective packed bed model

Due to the complexity of the fluid flow and heat transfer in packed bed latent thermal energy storage (LTES) systems, many hypotheses were introduced into the previous packed bed models, which consequently influenced the accuracy and authenticity of the numerical calculation. An effective packed bed model was therefore developed, which could investigate the flow field as the fluid flows through the voids of the phase change material (PCM), 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 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; both the material and the thickness of the encapsulation have the apparent effects on the heat transfer performance of the LTES bed.     

Study on the efficiency of effective thermal conductivities on melting characteristics of latent heat storage capsules

Improvement of the thermal conductivity of a phase change materials (PCM) is one effective technique to reduce phase change time in latent heat storage technology. Thermal conductivity is improved by saturating porous metals with phase change materials. The influence of effective thermal conductivity on melting time is studied by analyzing melting characteristics of a heat storage circular capsule in which porous metal saturated with PCM is inserted. Numerical and approximate analyses were made under conditions where there are uniform or non-uniform heat transfer coefficients around the cylindrical surface. Four PCMs (H₂O, octadecane, Li₂CO₃, NaCl) and three metals (copper, aluminum and carbon steel) were selected as specific materials. Porosities of the metals were restricted to be larger than 0.9 in order to keep high capacity of latent heat storage. Results show that considerable reduction in melting time was obtained, especially for low conductivity PCMs and for high heat transfer coefficient. Melting time obtained by approximate analysis agrees well with numerical analysis. A trial estimation of optimum porosity is made balancing the desirable conditions of high latent heat capacity and reduction of melting time. Optimum porosity decreases with increase in heat transfer coefficient.