Modification of the phase change transfer model for underwater vehicles: A molecular dynamics approach

The thermophysical properties of phase change material (PCM) directly affect the performance of underwater vehicles. The accuracy of the phase change transfer model is an important index for evaluating the performance of PCM as well. In this paper, a molecular dynamics approach is proposed to obtain the thermophysical properties of PCM under different pressures. On such a basis, the modified models of “temperature-pressure-density,” “temperature-pressure-thermal conductivity” and “pressure-specific heat capacity” are established. By comparing the simulation results with the unmodified phase change transfer model and the laboratory experimental results, it can be seen that the modified phase change transfer model has higher accuracy. Furthermore, an ocean thermal energy conversion system for underwater vehicles is deployed in the South China Sea to test the utilization performance. By the contrast between the simulated calculation of the modified model and the sea trial results, it is found that the root mean square (RMS) of accumulator pressure between the simulation and sea trial is 0.2565, and the energy storage volume RMS is 1.6868. When the accumulator pressure reaches 205 bar, the time error is 0.58%, and the energy storage volume error is 3.16%. The results indicate that the modified model is effective in practical application.     

Research progress on heat transfer enhancement technology of phase change energy storage

相变储能是通过相变材料吸/放热过程来实现能量储存的技术,它能够解决热量供需时间、空间和强度上的不匹配,并以其高储能密度成为储能领域的研究热点,但由于相变材料的热导率较低,使其应用受到限制。针对相变储能材料熔化/凝固过程中热导率低引起的传热速率慢的问题,从优化储能设备结构、添加剂提高相变材料热导率以及联合强化传热技术三方面综述国内外相变材料储能强化传热技术的最新进展。通过比较各种强化传热方式的优劣,实验和模拟均显示复合强化传热即可解决相变材料热导率低,又增大传热面积,从而提高相变材料的传热性能;多孔金属作为导热添加剂增强导热效果更好;并提出了相变储能强化传热技术未来需要解决的相关技术难题。     

复合相变蓄能材料的制备及性能研究

综述了相变蓄能材料的研究进展,介绍了相变蓄能材料的种类和特点。阐述了溶胶一凝胶法的基本原理和特性,并用这一方法制备了两种有机-无机复合相变蓄能材料。对实验制备的新材料进行了差示扫描量热分析（DSC）、扫描电镜分析（SEM）,分析结果表明：经过有机-无机复合的相变蓄能材料具有较高的蓄热能力。两种新材料的相变热分别达到147.577 J/g和253.407 J/g,同时,该材料具有较好的热稳定性,是令人满意的复合相变材料,可广泛应用于太阳能利用、工业废热、余热回收系统。     

利用单水浴法测量相变贮热材料及其构件的贮热能力

发展了一种用于测量相变贮热材料及其构件贮热能力的单水浴法。这种方法使用一个其中盛有一定量水的恒温水浴，将水温从T1均匀加热到T2,利用热流片测出通过水浴各壁面的热损，用消耗的电能扣除热损后即得到加热水所消耗的电能Qw；之后将待测相变材料或其构件浸没在水中，将水浴温度调到T1,并从该温度均匀加热到T2，同时利用热流片测出通过水浴各壁面的热损，消耗的总电能扣除相应的热损后即得加热水和相变材料或其构件所消耗的电能Qw＋pom;Qw＋pom减去Qw即为相奕材料或其构件从温度T1升高到T2所吸收的热能，即相变材料或其构件在温度区间[T1，T2]上的贮热能力。利用该方法对一种定形相变材料的贮热能力的测试结果与DSC分析的结果相差不到2％，还利用该方法对颗粒状定形相变材料与混凝土共混成型的贮热构件的贮热能力进行了测试。     

New challenge in advanced thermal energy transportation using functionally thermal fluids

The present review article presents the current status of some researches on thermal energy transportation using functionally thermal fluid, which is a mixture of heat transfer medium like water and other material with or without phase change like a paraffin wax as a latent heat storage material. This functionally thermal fluid offers attractive opportunities for thermal energy transportation and heat transfer enhancement of heat exchanger. This article describes classification and characteristics of functionally thermal fluids and their application. Referring to functionally thermal fluid for the usage of sensible heat, some visco-elastic fluids for flow drag reduction in a thermal energy transport system such as aqueous polymer solution and surfactant solution are mentioned. On the other hand, this article describes heat transfer and hydrodynamic characteristics of some phase change slurries like ice slurry, phase change microemulsion slurry, phase change microencapsule slurry, clathrate slurry and shape-stabilized paraffin and polyethylene pellets as functionally thermal fluids using latent heat between solid and liquid phases. Finally, it leads to the conclusion that some functionally thermal fluids are very useful for the advanced thermal energy transportation and heat exchanger systems.     

带相变蓄热小型热泵冷凝热回收性能实验研究

通过实验初步研究了采用光管螺旋相变蓄热器替代传统水蓄热器的小型家用热泵冷凝热回收系统的性能,对相变蓄热和水蓄热的冷凝热回收过程进行了对比实验,分析并得到了两类冷凝热回收系统的性能参数及综合能效系数。实验数据表明:与水蓄热系统相比,光管螺旋相变蓄热器体积减小,并且系统运行较传统水蓄热工况下更加平稳,但存在传热效果较差、热回收率低的缺点,回收率仅为15%,系统综合能效系数2.9。可知,相变蓄热器的内部结构对系统综合能效系数影响很大。     

Phase change microcapsules in thermal Energy applications:A critical reviewCSCD

Phase change microcapsules can carry large amounts of heat and be dispersed into other mediums either as a solid composite or as slurry fluids without changes to their appearance or fluidity. These two standout features make phase change microcapsules ideal for use in thermal energy applications to enhance the efficiency of energy utilisation. This review paper includes methods used for the encapsulation of phase change materials, especially the method suitable for large scale productions, the trends of phase change microcapsule development and their use in thermal energy applications in static and dynamic conditions. The effect of phase change microcapsules on convective heat transfer through addition to thermal fluids as slurries is critically reviewed. The review highlighted that so far the phase change microcapsules used mainly have polymeric shells, which has very low thermal conductivities. Their enhancement in convective heat transfer was demonstrated in locations where the phase change material experiences phase change. The phase change results in the slurries having higher apparent local specific heat capacities and thus higher local heat transfer coefficients. Out of the phase change region, no enhancement is observed from the solid microcapsule particles due to the low specific heat capacity and thermal conductivity of the phase change microcapsules compared to that of water, which is normally used as slurry media in the test. To further the research in this area, phase change microcapsules with higher specific heat capacity, higher thermal conductivity and better shape stability need to be applied.     

Electrospun polyethylene glycol/cellulose acetate phase change fibers with core–sheath structure for thermal energy storage

The ultrafine phase change fibers (PCFs) with core–sheath structure based on polyethylene glycol/cellulose acetate (PEG/CA) blends were fabricated successfully via coaxial electrospinning for thermal energy storage. SEM and TEM images show that cylindrical and smooth phase change fibers are obtained and PEG as a phase change ingredient is encapsulated completely by CA sheath. The morphology of the composite fibers before and after thermal treatment indicates that the prepared fibers are form stable phase change materials (PCMs). The results from DSC demonstrate that the composite fibers impart balanced and reversible phase change behaviors, and phase transition enthalpies of the composite fibers increase with the increasing of PEG content in the fibers, while the phase transition temperatures of the fibers are similar with those of pure PEG. The stress–strain curves show that the ultimate strength and ultimate strain of the composite fibers are lower than those of CA fibers, and they decrease with the increase of PEG content. The PEG/CA composite fibers have extensive applications as a smart material for thermal energy storage and temperature regulation.     

Review of thermal energy storage for air conditioning systems

Thermal energy storage is very important to eradicate the discrepancy between energy supply and energy demand and to improve the energy efficiency of solar energy systems. Latent heat thermal energy storage (LHTES) is more useful than sensible energy storage due to the high storage capacity per unit volume/mass at nearly constant temperatures. This review presents the previous works on thermal energy storage used for air conditioning systems and the application of phase change materials (PCMs) in different parts of the air conditioning networks, air distribution network, chilled water network, microencapsulated slurries, thermal power and heat rejection of the absorption cooling. Recently, researchers studied the heat transfer enhancement of the thermal energy storage with PCMs because most phase change materials have low thermal conductivity, which causes a long time for charging and discharging process. It is expected that the design of latent heat thermal energy storage will reduce the cost and the volume of air conditioning systems and networks.     

Thermal properties of paraffin/graphite composite phase change materials in battery thermal management system

Abstract

The thermal properties of paraffin/graphite composite phase change materials for power nickel metal hydride batteries were experimentally investigated. Two different modes for heat dissipation were designed in this experimental study: air cooling and cooling with phase change materials. Paraffin/graphite composite phase change thermal energy storage materials were prepared and tested by differential scanning calorimetry. It appeared that the battery thermal management system with phase change materials had better performance than air cooling, especially when the scale of paraffin/graphite composite material approximates 4:1.     

Ocean thermal energy utilization process in underwater vehicles: Modelling,temperature boundary analysis,and sea trail

Autonomous underwater vehicles (AUVs) play an important role in the ocean observation and marine scientific research. The utilization of marine environmental energy is a desirable way to enhance the sailing range and endurance of AUVs. In this paper, a detailed model with four sub-models is established to describe the ocean thermal energy utilization process in AUVs, where the factors of system pressure, material's thermal-pressure-physical properties and grid nodes are considered. Based on the model, performance of the utilization system under constant and variable temperature boundaries is analyzed. At last, an AUV driven by ocean thermal energy is deployed in South China Sea to test the utilization performance, and the model is validated by the practical trail data. It is proved that thermal-driven AUV saves about 50% energy of buoyancy-driven process in sea trail. The simulation results are in good agreement with the trial data and prove the correctness and effectiveness of the model. The utilization system shows a better performance in the temperature condition with a higher surface temperature and a greater gradient of thermocline. When reaching the designed depth, AUVs can obtain plenty of time for phase change. The conclusions drawn in the paper provide some guidance for development of the thermal-driven AUV.     

Harvesting of PEM fuel cell heat energy for a thermal engine in an underwater glider

The heat generated by a proton exchange membrane fuel cell (PEMFC) is generally removed from the cell by a cooling system. Combining heat energy and electricity in a PEMFC is highly desirable to achieve higher fuel efficiency. This paper describes the design of a new power system that combines the heat energy and electricity in a miniature PEMFC to improve the overall power efficiency in an underwater glider. The system makes use of the available heat energy for navigational power of the underwater glider while the electricity generated by the miniature PEMFC is used for the glider's sensors and control system. Experimental results show that the performance of the thermal engine can be obviously improved due to the high quality heat from the PEMFC compared with the ocean environmental thermal energy. Moreover, the overall fuel efficiency can be increased from 17 to 25% at different electric power levels by harvesting the PEMFC heat energy for an integrated fuel cell and thermal engine system in the underwater glider.     

Theoretical study on a novel phase change process

This paper mainly deals with a novel homogeneous phase change process in materials (HPCP). The HPCP is analysed in detail and the expressions for one‐dimensional HPCPs are derived. It is concluded that, compared with the conventional phase change processes, the complete phase change time of HPCPs can be decreased by 60% for a spherical phase change material (PCM), 50% for a cylindrical PCM and 33% for a flat plate PCM, respectively, and the application of HPCPs to thermal energy storage systems can charge or discharge thermal energy with constant rates. Possible applications of HPCPs to thermal energy storage are simulated and further discussed using composite flat plate PCMs. Copyright © 1999 John Wiley & Sons, Ltd.     

正交各向异性相变材料的无网格法传热模型及应用

利用无网格迦辽金(EFG)法建立正交各向异性相变材料的传热计算模型,基于该模型编程完成各向异性材料太阳能相变蓄热水箱和管壳式相变蓄热单元的相变传热分析,并探讨热导率因子和材料方向角对复合材料相变传热特性的影响.研究表明:在相同节点布置下EFG法的温度场和相界面计算精度均高于有限元法,EFG法在动态相界面追踪方面具有明显...     

Heat transfer analysis of encapsulated phase change materials

Solar energy is receiving a lot of attention recently since it is a clean, renewable, and sustainable energy. Solar energy is used for space heating, power generation and other applications. A major limitation however is that it is available for only about 2000 h a year in many places. Therefore it is critical to find ways to store solar thermal energy for the off hours. Sensible heat of material has been used for storing thermal energy but due to material properties this type of thermal storage has limitations. Using encapsulated phase change materials is potentially a better way to store thermal energy with the associated reversible heat transfer. The present work deals with certain aspects of storing solar thermal energy in high temperature phase change materials with melting points above 400 °C. The objective is the storage of large amounts of solar energy (～600 MWh). Two kinds of encapsulated capsules are considered; zinc encapsulated in nickel and eutectic salt mixtures (57 mol% NaCl and 43 mol% MgCl₂) in stainless steel encapsulation. Diffusion and phase change computations are reported here in the form of temperature profiles of the phase changing and encapsulated materials for spherical capsules. The time for heating and melting during charging (storage of thermal energy into capsulated phase change material) and the time for cooling and solidification during discharging (retrieval of thermal energy) are presented for both zinc–nickel and salt–stainless steel systems. As per expectations, the time for heat transfer is much shorter for liquid heat transfer media compared to those for gases. Moreover, the heat transfer times are shorter with smaller sizes of capsules.     

铝/石蜡复合相变材料蓄热性能的数值模拟

相变储能材料由于其具有周期性储存和释放能量的特点,在电池热管理、太阳能发电等领域应用广泛,然而由于导热系数低的原因限制了其进一步的应用.高导热率泡沫材料的添加为解决这一不足提供了一种有效的方法.采用三周期性极小曲面(triply periodic minimal surface,TPMS)生成泡沫铝骨架,基于孔隙尺度数...     

Polyurethanes as solid–solid phase change materials for thermal energy storage

Polyurethane polymers (PUs) have been synthesized as solid–solid phase change materials for thermal energy storage using three different kinds of diisocyanate molecules and polyethylene glycols (PEGs) at three different molecular weights. PEGs and their derivatives are usually used as phase change units in polymeric solid–solid phase change materials due to the hydroxyl functional groups. 1000, 6000, and 10,000 g/mol number average molecular weight PEGs are used as working element as hexamethylene, isophorone, and toluene diisocyanates are used as hard segment at the backbone. The effects of molecular weight of PEG and type of diisocyanate on the thermal energy storage properties have been discussed. Only two of the produced polymers show solid–liquid phase change as the rest show solid–solid phase transitions. The produced PUs with a solid–solid phase transitions have potential to be used in thermal energy storage systems.     

Thermal energy storage properties of a capric acid/stearic acid binary system and a 48# paraffin/liquid paraffin binary system

In this paper, the phase change temperature, latent heat and thermal stability of a capric acid/stearic acid binary system and a 48# paraffin/liquid paraffin binary system were experimentally studied. The experimental results showed that the phase change temperature and phase change latent heat change with the content of the component. The phase change temperatures of binary mixtures change in a wide range, so they can be used in different fields by adjusting the mixing ratio. The phase change latent heat of fatty acid mixtures is higher than that of paraffin mixtures. The thermal stability of fatty acid mixtures is better than that of paraffin mixtures. The mixtures used in the phase change material wall or the phase change material floor as energy storage materials were given in the paper.     

The micro‐/nano‐PCMs for thermal energy storage systems: A state of art review

With advancement in technology—nanotechnology, various thermal energy storage (TES) materials have been invented and modified with promising thermal transport properties. Solid‐liquid phase change materials (PCMs) have been extensively used as TES materials for various energy applications due to their highly favourable thermal properties. The class of PCMs, organic phase change materials (OPCMs), has more potential and advantages over inorganic phase change materials (IPCMs), having high phase change enthalpy. However, OPCMs possess low thermal conductivity as well as density and suffer leakage during the melting phase. The encapsulation technologies (ie, micro and nano) of PCMs, with organic and inorganic materials, have a tendency to enhance the thermal conductivity, effective heat transfer, and leakage issues as TES materials. The encapsulation of PCMs involves several technologies to develop at both micro and nano levels, called micro‐encapsulated PCMs (micro‐PCM) and nano‐encapsulated PCMs (nano‐PCM), respectively. This study covers a wide range of preparation methods, thermal and morphological characteristics, stability, applications, and future perspective of micro‐/nano‐PCMs as TES materials. The potential applications, such as solar‐to‐thermal and electrical‐to‐thermal conversions, thermal management, building, textile, foam, medical industry of micro‐ and nano‐PCMs, are reviewed critically. Finally, this review paper highlights the emerging future research paths of micro‐/nano‐PCMs for thermal energy storage.     

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。