热管式蓄冷器的设计与热动力学分析

为强化相变蓄冷材料传热,将脉动热管技术与相变蓄冷技术相结合,设计一套脉动热管相变蓄冷装置。采用脉动热管可强化传热,提高形核率,加快结晶速率,减少蓄冷时间。该装置将脉动热管蓄冷装置与相变材料相耦合,使脉动热管相变蓄冷器的传热得到强化。相变蓄冷系统的蓄放冷性能与相变材料的相变过程、传热方式密切相关,本研究对脉动热管传热机理进行探析,建立了精确、通用性较高的理论模型,揭示了传热机理,为脉动热管蓄冷装置的模拟和实验提供指导。     

相变蓄冷技术在公共建筑太阳能空调系统中的应用研究

为提高公共建筑太阳能空调系统的太阳能保证率,在传统太阳能空调蓄冷系统的基础上,将相变蓄冷技术应用在公共建筑太阳能空调系统中,并建立相应的数学模型。经软件模拟和实验测试,表明太阳能空调相变蓄冷系统数学模拟与实测数据吻合较好,验证了太阳能空调相变蓄冷系统的可行性和有效性。在该文案例中,与传统水蓄冷系统相比,相变蓄冷系统可使系统制冷季太阳能保证率提高17.5%。     

相变储能材料蓄放能过程热力学分析

通过对相变储能系统的蓄放能过程进行热力学分析,以期找到新的节能方向和措施。以水溶液复配1%的聚丙烯酸钠(PAAS)以及0.1%多壁碳纳米管(MWCNT)制得新型相变储能材料,然后对填充该材料的系统进行热物性测试实验并将所得数据结合热力学模型加以分析。结果表明,随着材料终温的降低,系统蓄冷量变大,但(?)效率却在逐步降低,而初温对材料(?)效率的影响不大。因此,通过对蓄冷材料蓄冷终温进行控制等措施对节能和提高能量利用率具有一定实际意义。     

相变蓄冷装置内传热及相变特性研究

建立了三种不同蓄冷球球径堆叠方式的相变蓄冷装置模型,对其进行了数值模拟以研究其内部传热及相变特性。结果表明：随着流速的提高、蓄冷球直径的减小,蓄冷结束后三种方案中装置内蓄冷球的凝固率提升显著;双球径方案与单球径方案装置内蓄冷球凝固率随时间变化的规律在蓄冷过程初段相类似,但两方案中相同球径部分蓄冷球相变结束,双球径方案中发生相变的蓄冷球由大直径转变为小直径时,其凝固率随时间变化的速度逐渐超过单球径方案。该研究可为相变蓄冷装置的实际设计及性能优化提供参考     

空调蓄冷材料及蓄冷球内非固定融化问题的研究

通过实验方法研究了空调蓄冷材料的相变温度、相变潜热和比热等热学性能。并对该种蓄冷材料制成的蓄冷球进行了释冷特性的研究,探讨了蓄冷球内固液密度差引起的固态物上浮对蓄冷球融化释冷的影响,得到了融化率随时间的变化关系。结果表明:该蓄冷材料融解热为149.4kJ/kg,相变温度为7.9℃,该相变材料可作为空调蓄冷材料,蓄冷球的直径及固液密度差对融化释冷特性有较大影响。     

填充开孔泡沫铝的固液相变蓄冷器设计与实验研究

为进一步满足固液相变蓄冷器需求,在计算固液相变蓄冷器漏热的基础上进行了相应优化,使用开孔泡沫铝作为固液相变蓄冷器填充材料,搭建实验台进行不同负载功率下的液氮固体蓄冷系统性能实验研究。实验结果表明：所设计的固体蓄冷系统能够在0.5 W的热负载下工作4.23 h; 2 W热负载实验条件下,固体蓄冷系统能够在温度小于65 K条件下工作1 h。     

小型蓄冷空调系统的实验研究

本文描述了一套小型蓄冷调控系统,并采用有机相变材料进行了性能实验,得到了系统各参数随时间的变化规律;在此分析基础上,对小型蓄冷空调系统的设计提出了建议。     

盐溶液除湿冷却系统的潜能蓄冷研究

研究了一种新型的太阳能驱动的空调系统———溶液除湿蒸发冷却空调系统气液相变蓄冷机理及其潜力。以LiCl水溶液为蓄冷工质,在理论上和实验上分别对这种蓄冷模式进行了蓄冷特性的探讨,并与传统的蓄冷模式进行对比,结果表明此种蓄冷模式在太阳能液体除湿空调系统中有独特的优势和很好的应用前景。     

双层相变材料组合式蓄冷装置的性能研究CSCD

实验测试了两种相变温度分别为-5.4℃和-9.6℃、相变焓分别为132 kJ/kg和173 kJ/kg的十三烷和十二烷在不同组合模式下对蓄冷装置温度变化的影响。结果表明,十三烷在外层、十二烷在内层的组合模式,尽管总相变潜热在所有组合中仅排第三,但蓄冷装置将内壁温度控制在0℃以下的时间最长,与双层同为十二烷的工况相同,高于双层同为十三烷及十三烷在内十二烷在外层的模式。相变温度更高的材料在外层,可以降低环境热量传递至蓄冷装置内部的速率,延长蓄冷时间,有利于提高蓄冷装置的性能。     

土壤蓄冷与释冷过程的模拟与试验验证

结合土壤耦合热泵技术与冰蓄冷技术的特点,并在土壤蓄冷与土壤耦合热泵集成系统构想的基础上,建立了考虑土壤周期性冻融相变的土壤蓄冷、释冷过程的数学模型,并采用数值方法进行求解。通过建立模拟土壤蓄冷、释冷过程的砂箱试验台并由试验测试结果表明:所建模型的计算结果与试验数据吻合较好,二者偏差在10%以内。应用本文所建数学模型可对土壤蓄冷系统的蓄冷、释冷运行特性进行计算与分析,并为该集成系统的应用提供理论支持与技术储备。     

严寒地区单体建筑太阳能-相变墙系统蓄热特性研究

利用太阳能对水加热并通入相变墙进行蓄热,对减少严寒地区单体建筑供热能耗有重要意义。以大庆市某单体建筑为例,结合该地区太阳能分布特点及建筑热负荷大小,对适用于该地区的太阳能-相变墙系统进行集热与储热能力计算,并采用CFD方法研究单一工况下该系统的热工变化规律及不同热水参数、换热管规格对相变墙蓄热特性的影响。结果表明：该相变墙热稳定性良好,但受自然对流影响,底部相变材料熔化较慢;管径DN25、入口流速0.3m/s、供水温度310.15K、回水温度309.15K、管间距107mm可使相变材料在4小时内完成蓄热,平均节能率为31.8%。研究结果可望为降低严寒地区建筑供热能耗提供新思路。     

Form-stable phase change materials for thermal energy storage

The present paper considers the state of investigations and developments in form-stable phase change materials for thermal energy storage. Paraffins, fatty acids and their blends, polyethylene glycol are widely used as latent heat storage component in developing form-stable materials while high-density polyethylene (HDPE), styrene-butadiene-styrene (SBS) triblock copolymer, Eudragit S, Eudragit E, poly (vynil chloride) (PVC), poly (vynil alcohol) (PVA) and polyurethane block copolymer serve as structure supporting component. A set of organic and metallo-organic materials with high transition heat in solid-solid state is considered as perspective for-stable materials to store thermal energy. Another perspective class of form-stable materials are the materials on the basis of such porous materials as expanded perlite and vermiculite impregnated with phase change heat storage materials. The technology of producing new form-stable ultrafine heat storage fibers is developed. It opens availability to produce the clothers with improved heat storage ability for extremely cold regions. The perspective fields of application of form-stable materials are discussed. The further directions of investigations and developments are considered.     

Energy and exergy analysis of a latent heat storage system with phase change material for a solar collector

Analysis of energy and exergy has been performed for a latent heat storage system with phase change material (PCM) for a flat-plate solar collector. CaCl₂·6H₂O was used as PCM in thermal energy storage (TES) system. The designed collector combines in single unit solar energy collection and storage. PCMs are stored in a storage tank, which is located under the collector. A special heat transfer fluid was used to transfer heat from collector to PCM. Exergy analysis, which is based on the second law of thermodynamics, and energy analysis, which is based on the first law, were applied for evaluation of the system efficiency for charging period. The analyses were performed on 3 days in October. It was observed that the average net energy and exergy efficiencies are 45% and 2.2%, respectively.     

Experimental study of the phase change and energy characteristics inside a cylindrical latent heat energy storage system: Part 2 simultaneous charging and discharging

The time mismatch between energy availability and energy demand with solar domestic hot water (SDHW) systems is often solved using energy storage. Energy storage systems typically employ water for thermal energy storage, however, water storage takes up considerable space and weight due to the large volumes required under certain conditions. A latent heat energy storage system (LHESS) may provide a valuable solution to the space and weight issue, while also correcting the energy mismatch by storing energy in phase change materials (PCMs) when it is available, dispensing energy when it is in demand, and acting as a heat exchanger when there is supply and demand simultaneously. PCMs are advantageous as energy storage materials due to their high energy density which reduces the space requirements for energy storage. However, heat transfer problems arise due to the inherently low thermal conductivity of PCMs. Simultaneous charging and discharging has not been addressed in literature making questionable the ability of a LHESS to operate as a heat exchanger during the mode of operation. The main objective of this research is to study the heat transfer processes and phase change behavior of a PCM during simultaneous charging and discharging of a LHESS.In Part 2 of this paper, experiments are performed using a vertical cylindrical LHESS which is charged and discharged simultaneously to replicate latent heat energy storage paired with a SDHW system with simultaneous energy supply and demand. Dodecanoic acid is used as the PCM. Experimental results for simultaneous operations are presented, under various scenarios and flow rates for both the hot and cold heat transfer fluids. The ability of the system to directly transfer heat between the hot and cold heat transfer fluids is studied, and the results found during consecutive, or separate, charging and discharging, presented in Part 1 of this paper, are compared to the results found during simultaneous charging and discharging. It was found that natural convection in the melted PCM clearly provides an advantage towards direct heat exchange between the hot and cold heat transfer fluid; while the low thermal conductivity of solid PCM provides a barrier to this direct energy exchange.     

Review on storage materials and thermal performance enhancement techniques for high temperature phase change thermal storage systems

Designing a cost-effective phase change thermal storage system involves two challenging aspects: one is to select a suitable storage material and the other is to increase the heat transfer between the storage material and the heat transfer fluid as the performance of the system is limited by the poor thermal conductivity of the latent heat storage material. When used for storing energy in concentrated solar thermal power plants, the solar field operation temperature will determine the PCM melting temperature selection. This paper reviews concentrated solar thermal power plants that are currently operating and under construction. It also reviews phase change materials with melting temperatures above 300 °C, which potentially can be used as energy storage media in these plants. In addition, various techniques employed to enhance the thermal performance of high temperature phase change thermal storage systems have been reviewed and discussed. This review aims to provide the necessary information for further research in the development of cost-effective high temperature phase change thermal storage systems.     

Experimental assessment of heat storage properties and heat transfer characteristics of a phase change material slurry for air conditioning applications

A new microencapsulated phase change material slurry based on microencapsulated Rubitherm RT6 at high concentration (45% w/w) was tested. Some heat storage properties and heat transfer characteristics have been experimentally investigated in order to assess its suitability for the integration into a low temperature heat storage system for solar air conditioning applications. DSC tests were conducted to evaluate the cold storage capacity and phase change temperature range. A phase change interval of approximately 3 °C and a hysteresis behaviour of the enthalpy were identified. An experimental set-up was built in order to quantify the natural convection heat transfer occurring from a vertical helically coiled tube immersed in the phase change material slurry. First, tests were carried out using water in order to obtain natural convection heat transfer correlations. Then a comparison was conducted with the results obtained for the phase change material slurry. It was found that the values of the heat transfer coefficient for the phase change material slurry were higher than for water, under identical temperature conditions inside the phase change interval.     

Numerical analysis of charging and discharging performance of an integrated collector storage solar water heater

This work aims to evaluate the energy and the exergy performance of an integrated phase change material (PCM) solar collector with latent heat storage in transient conditions. A theoretical model based on the first and the second laws of thermodynamics is developed to predict the thermal behaviour of the system. The effect of natural convection on heat during the melting process is taken into account using an effective thermal conductivity. Influence of PCM thicknesses on the melt fraction, on the energy stored and on the exergy destroyed are studied during charging and discharging processes. Results indicate that the complete melting time is shorter than the solidification time. The latent heat storage system increases the heating requirements at night and reduces the exergy efficiency.     

Heat transfer enhancement in water when used as PCM in thermal energy storage

Efficient and reliable storage systems for thermal energy are an important requirement in many applications where heat demand and supply or availability do not coincide. Heat and cold stores can basically be divided in two groups. In sensible heat stores the temperature of the storage material is increased significantly. Latent heat stores, on the contrary, use a storage material that undergoes a phase change (PCM) and a small temperature rise is sufficient to store heat or cold. The major advantages of the phase change stores are their large heat storage capacity and their isothermal behavior during the charging and discharging process. However, while unloading a latent heat storage, the solid–liquid interface moves away from the heat transfer surface and the heat flux decreases due to the increasing thermal resistance of the growing layer of the molten/solidified medium. This effect can be reduced using techniques to increase heat transfer. In this paper, three methods to enhance the heat transfer in a cold storage working with water/ice as PCM are compared: addition of stainless steel pieces, copper pieces (both have been proposed before) and a new PCM-graphite composite material. The PCM-graphite composite material showed an increase in heat flux bigger than with any of the other techniques.