Heat transfer and parametric studies of an encapsulated phase change material based cool thermal energy storage system

This work investigates the transient behaviour of a phase change material based cool thermal energy storage (CTES) system comprised of a cylindrical storage tank filled with encapsulated phase change materials (PCMs) in spherical container integrated with an ethylene glycol chiller plant. A simulation program was developed to evaluate the temperature histories of the heat transfer fluid (HTF) and the phase change material at any axial location during the charging period. The results of the model were validated by comparison with experimental results of temperature profiles of HTF and PCM. The model was also used to investigate the effect of porosity, Stanton number, Stefan number and Peclet number on CTES system performance. The results showed that increase in porosity contributes to a higher rate of energy storage. However, for a given geometry and heat transfer coefficient, the mass of PCM charged in the unit decreases as the increase in porosity. The St number as well as the Ste number is also influential in the performance of the unit. The model is a convenient and more suitable method to determine the heat transfer characteristics of CTES system. The results reported are much useful for designing CTES system.

Time-Temperature Charge Function of a High Dynamic Thermal Heat Storage with Phase Change Material

A thermal heat storage system with an energy content of 40 kWh and a temperature of 58°C will be presented. This storage system is suitable for supporting the use of renewable energies in buildings and for absorbing solar heat, heat from co-generation and heat pumps or electric heat from excess wind and solar power. The storage system is equipped with a plate heat exchanger that is so powerful that even with small temperature differences between the flow temperature and the storage temperature a high load dynamic is achieved. The storage system has a performance of 2.8 kW at 4 K and 10.6 kW at a temperature difference of 10 K. Thus, large performance variations in solar thermal systems or CHP plants can be buffered very well. Further a storage charge function Q(T, t) will be presented to characterize the performance of the storage.

Numerical Study of Thermal Performance of Phase Change Material Energy Storage Floor in Solar Water Heating System

The conventional solar heating floor system contains a big water tank to store energy in the day time for heating at night,which takes much building space and is very heavy.In order to reduce the water tank volume even to cancel the tank,a novel structure of integrated water pipe floor heating system using shape-stabilized phase change materials(SSPCM)for thermal energy storage was developed.A numerical model was developed to analyze the performance of SSPCM floor heating system under the intermittent heating condition,which was verified by our experimental data.The thermal performance of the heating system and the effects of various factors on it were analyzed numerically.The factors including phase transition temperature,heat of fusion,thermal conductivity of SSPCM and thermal conductivity of the decoration material were analyzed.The results show that tm and kd are the most import influencing factors on the thermal performance of SSPCM floor heating system,since they determine the heat source temperature and thermal resistance between SSPCM plates and indoor air,respectively.Hm should be large to store enough thermal energy in the day time for nighttimes heating.The effects of kp can be ignored in this system.The SSPCM floor heating system has potential of making use of the daytime solar energy for heating at night efficiently in various climates when its structure is properly designed.

Melting and Solidification Heat Transfer Characteristics of a Phase-Change Material in a Latent Heat Storage Vessel: Effects of a Perforated Partition Plate and Metal Fiber

Today, latent heat storage technology has advanced to allow reuse of waste heat in the middle-temperature range. This paper describes an approach to develop a latent heat storage system using middle-temperature waste heat (~100oC - 200oC) from factories. Direct contact melting and solidification behavior between a heat-transfer fluid (oil) and a latent heat storage material mixture were observed. The mixture consisted of mannitol and erythritol (Cm = 70 mass %, Ce = 30 mass %) as a phase-change material (PCM). The weight of the PCM was 3.0 kg and the flow rate of the oil, foil, was 1.0, 1.5, or 2.0 kg/min. To decrease the solidified height of the PCM mixture during the solidification process, a perforated partition plate was installed in the PCM region in the heat storage vessel. PCM coated oil droplets were broken by the perforated partition plate, preventing the solidified height of the PCM from increasing. The solidification and melting processes were repeated using metal fiber. It was found that installing the metal fiber was more effective than installing the perforated partition plate to prevent the flow out problem of the PCM.

Thermal energy storage inside the chamber with a brick wall using the phase change process of paraffinic materials:A numerical simulation

Phase change materials are one of the potential resources to replace fossil fuels in regards of supplying the energy of buildings.Basically,these materials absorb or release heat energy with the help of their latent heat.Phase change materials have low thermal conductivity and this makes it possible to use the physical properties of these materials in the tropical regions where the solar radiation is more direct and concentrated over a smaller area.In this theoretical work,an attempt has been made to study the melting process of these materials by applying constant heat flux and temperature.It was found that by increasing the thickness of phase change materials’layers,due to the melting,more thermal energy is stored.Simultaneously it reduces the penetration of excessive heat into the chamber,so that by increasing the thickness of paraffin materials up to 20 mm,the rate of temperature reduction reaches more than 18%.It was also recognized that increasing the values of constant input heat flux increases buoyancy effects.Increasing the Stefan number from 0.1 to 0.3,increases the temperature by 6%.

Research on the Phase Change Solar Energy Fresh Air Thermal Storage System

In this article, a new kind of solar fresh air system is designed in order to realize the improvement of thermal efficiency by the integrated application of the PCMs and heat pipe technology. Under the adequate sunshine condition, the fresh air is directly delivered into the indoor environment after being heated by the solar collector. When the sun radiation is reduced, the heated air temperature can not satisfy the need of supply of air temperature.The main heat source is changed to phase change heat storage equipment instead of solar energy. The system adopt heat pipe for a high-efficiency and isothermal heat transfer which recover the shortcomings of PCMs such as： low coefficient of thermal conductivity and poor thermal efficiency. This article establishes the physical model of phase change solar energy fresh air thermal storage system and creates the mathematical model of its unsteady heat transfer to simulate and analyse the operation process by using Fluent software. The results of the study show that, compared to normal fresh air system, the phase change solar energy fresh air thermal storage system has a significant improvement in energy saving and indoor comfort level and will play an important role in the energy sustainable development.

Impact of Fin Arrangement on Heat Transfer and Melting Characteristics of Phase Change Material

Present work investigates the heat transfer and melting behaviour of phase change material(PCM) in six enclosures(enclosure-1 to 6) filled with paraffin wax.Proposed enclosures are equipped with distinct arrangements of the fins while keeping the fin's surface area equal in each case.Comparative analysis has been presented to recognize the suitable fin arrangements that facilitate improved heat transfer and melting rate of the PCM.Left wall of the enclosure is maintained isothermal for the temperature values 335 K,350 K and 365 K.Dimensionless length of the enclosure including fins is ranging between 0 and 1.Results have been illustrated through the estimation of important performance parameters such as energy absorbing capacity,melting rate,enhancement ratio,and Nusselt number.It has been found that melting time(to melt 100% of the PCM) is 60.5%less in enclosure-2(with two fins of equal length) as compared to the enclosure-1,having no fins.Keeping the fin surface area equal,if the longer fin is placed below the shorter fin(enclosure-3),melting time is further decreased by 14.1% as compared to enclosure-2.However,among all the configurations,enclosure-6 with wire-mesh fin structure exhibits minimum melting time which is 68.4% less as compared to the enclosure-1.Based on the findings,it may be concluded that fins are the main driving agent in the enclosure to transfer the heat from heated wall to the PCM.Proper design and positioning of the fins improve the heat transfer rate followed by melting of the PCM in the entire area of the enclosure.Evolution of the favourable vortices and natural convection current in the enclosure accelerate the melting phenomenon and help to reduce charging time.

Operating performance of novel reverse-cycle defrosting method based on thermal energy storage for air source heat pump

To solve the fundamental problem of insufficient heat available during defrosting while ensuring the efficient and safe system operation for air-source heat pumps (ASHPs). A novel reverse-cycle defrosting (NRCD) method based on thermal energy storage to eliminate frost off the outdoor coil surface was developed. Comparative experiments using both the stand reverse cycle defrosting (SRCD) method and the NRCD method were carried out on an experimental ASHP unit with a nominal 2.5 kW heating capacity. The results indicate that during defrosting operation, using the NRCD method improves discharge and suction pressures by 0.24 MPa and 0.19 MPa, respectively, shortens defrosting duration by 60%, and reduces the defrosting energy consumption by 48.1% in the experimental environment, compared with those by the use of SRCD method. Therefore, using the NRCD method can shorten the defrosting duration, improve the indoor thermal comfort, and reduce the defrosting energy consumption in defrosting.

Study on performance of a packed bed latent heat thermal energy storage unit integrated with solar water heating system

In thermal systems such as solar thermal and waste heat recovery systems, the available energy supply does not usually coincide in time with the process demand. Hence some form of thermal energy storage (TES) is necessary for the most effective utilization of the energy source. This study deals with the experimental evaluation of thermal performance of a packed bed latent heat TES unit integrated with solar flat plate collector. The TES unit contains paraffin as phase change material (PCM) filled in spherical capsules, which are packed in an insulated cylindrical storage tank. The water used as heat transfer fluid (HTF) to transfer heat from the solar collector to the storage tank also acts as sensible heat storage material. Charging experiments were carried out at varying inlet fluid temperatures to examine the effects of porosity and HTF flow rate on the storage unit performance. The performance parameters such as instantaneous heat stored, cumulative heat stored, charging rate and system efficiency are studied. Discharging experiments were carried out by both continuous and batchwise processes to recover the stored heat, and the results are presented.

Numerical Assessment on Fin Design Parameters Employed for Augmentation of Natural Convection and Fluid Flow in a Horizontal Latent Heat Thermal Energy Storage Unit

The present work focus on the thermal performance of a horizontal concentric heat exchanger, which is numerically investigated to evaluate the heat transfer enhancement process by adding fins with different configurations. As a part of this investigation, the melting process is simulated from the onset of phase change to the offset involving physics of natural convection in PCM fluid pool. The investigation is carried out by ANSYS Fluent code, which is an efficient numerical analysis tool for investigating fluid flow and convective heat transfer phenomena during PCM melting process. The attention is mainly focused on the extension of contact area between the PCM body and cylindrical capsule to enhance heat transfer rates to PCM bodies during the melting process by employing longitudinal fins in the enclosed capsule. Two commercial PCMs: RT50 and C58, are introduced in a 2D cylindrical pipe with their thermo-physical properties as input for modelling. The selected modelling approach is validated against experimental result with respect to the total enthalpy changes that qualify our model to run in the proceeding calculation. It is ensured that an isothermal boundary condition (373 K) is applied to the inner pipe throughout the series of simulation cases and the corresponding Rayleigh number (Ra) ranges from 104 - 105 and Prandtl number (Pr) 0.05 - 0.07. Finally, parametric study is carried out to evaluate the effect of length, thickness and number of longitudinal fins on the thermal performance of PCM-LHTES (Latent Heat Thermal Energy Storage) system associated with the physics of natural convection process during PCM melting.

Heat Storage/Heat Release of Phase-Change Filling Body with Casing Heat Exchanger for Extracting Geothermal Energy

Arranging heat exchanger in filling body to extract geothermal energy is an effective way to alleviate the problems of high ground pressure and high ground temperature in deep resource exploitation.Filling body with casing heat exchanger was acted as research object,encapsulating phase change materials(PCMs)in annular space.During heat storage and heat release process,the effects of different PCMs on temperature distribution,phase-change process and heat transfer performance were studied.The result indicates:During heat storage process,the temperature increases rapidly and the melting process is accelerated for the position closer surrounding rock.CaCl_(2)·6H_(2)O/EG can make filling body complete heat storage process in the shortest time because of its good thermal diffusivity.The heat storage capacity of PCMs backfill is significantly higher than that of ordinary backfill;it increases by 36.6%-67.3%at heat storage of 10 h.During heat release process,the closer to the heat exchange tube,the greater the temperature drop in filling body.The maximum value of heat release rate and heat release capacity is in CaCl_(2)·6H_(2)O/EG backfill,it can release 116.4%more heat than RT35backfill after heat release of 12 h,the maximum value of effectiveness and its heat transfer rate also is in CaCl_(2)·6H_(2)O/EG backfill.This paper provides the basic data for the selection of PCMs in phase-change thermal storage filling body.

Enhanced Phase Change Heat Storage of Layered Backfill Body under Different Boundary Conditions

In view of the high temperature problem faced by mining activities,the coordinated mining of ore deposit and geothermal energy is a solution in line with the concept of green mining.The layered backfill body with finned double-pipe heat exchanger continuously exchanges heat with the surrounding thermal environment,which plays an effective role in gathering geothermal energy.In this paper,the heat storage process of each layered backfill body under different boundary conditions is simulated by Fluent.The results show the heat storage characteristic of layered backfill body can be significantly improved by adding fins to the double-pipe heat exchanger.On the whole,the heat storage effect of bottom layer backfill body(BLBB)is the best.The total heat storage capacity of top layer backfill body(TLBB),middle layer backfill body(MLBB)and BLBB with the finned double-pipe heat exchanger is 666.3 kJ,662.2 kJ,1003.0 kJ;1639.0 kJ,1760.8 kJ,1911.2 kJ and 1731.1 kJ,1953.3 kJ,1962.8 kJ respectively at 1 h,8 h and 24 h.This study explores the law of heat storage of layered backfill body under different boundary conditions and also expands the idea for layered backfill body to efficiently accumulate geothermal energy.

Sr含量对Al-Si合金显微组织和热导率的影响

以纯铝、Al-20Si和Al-10Sr中间合金为原料,Sr为变质剂(含量为0.01%、0.02%、0.04%和0.06%,质量分数),制备了Al-7Si-xSr、Al-12Si-xSr和Al-20Si-xSr合金,研究了Sr含量对Al-Si合金相变储热材料显微组织及热导率的影响。利用Hot Disk热常数分析仪测量合金的热导率,通过扫描电镜观察及分析合金的显微组织。结果表明:在Al-Si合金中添加变质元素Sr会影响合金的热导率,Al-7Si-0.04Sr合金热导率较Al-7Si合金增加了73.47 W·m^(-1)·K^(-1),Al-20Si-0.04Sr合金的热导率较Al-20Si合金增加了24.09 W·m^(-1)·K^(-1),Al-12Si-0.04Sr合金的热导率较Al-12Si合金增加了17.79 W·m^(-1)·K^(-1)。铝硅合金热导率的增长主要与α(Al)、共晶硅和初晶硅的形貌有关。经过Sr变质之后,Al-7Si合金中共晶Si立体形貌均由片层状转变为珊瑚状,Al-12Si和Al-20Si合金中共晶Si立体形貌由片层状转变为枝条状;其中,Al-7Si合金中α(Al)尺寸明显减少、排列紧密,二次枝晶臂间距逐渐减小;Al-20Si合金中的初晶Si尺寸明显减小,其形貌由多角的大块状变为小块状;α(Al)形态的转变不仅能够为自由电子的传输提供快速通道,而且还会使得共晶Si的排列更加规则,减少自由电子发生散射的几率,对合金的热导率影响较大。共晶Si由片层状转变为珊瑚状或枝条状,增加电子的平均自由程,有利于电子的传输。Al-20Si合金的热导率与初晶Si的形态有着重要联系,大尺寸且形状完整的初晶Si会发生晶格振动,会提高合金的热导率。

废旧发泡混凝土定型相变材料制备及热性能研究

为规模化消纳城市固体废弃物,降低废旧发泡混凝土大量堆积对城市生态环境的破坏。提出以废旧发泡混凝土作为骨架材料,对其进行预烧结处理,以Na_(2)CO_(3)为相变材料制备定型相变材料。结果表明:废旧发泡混凝土可以负载质量分数为45%的Na_(2)CO_(3);差示扫描量热法(DSC)测得该定型相变材料的熔化潜热为126.7 J/g;X射线衍射法(XRD)和傅里叶变换红外法(FT-IR)表明,骨架材料和相变材料之间有良好的化学相容性;激光导热分析法(LFA)测得该定型相变材料的导热系数最大为0.24 W/(m·K)。

三套管式加肋相变蓄热单元的强化传热特性

相变蓄热技术在利用工业余热、太阳能等解决热能与用户之间的供需不平衡问题方面起着重要的作用,然而受相变材料自身物性的限制,蓄热熔化时仍存在蓄热速度低、熔化不均匀等问题。本文以三套管式相变蓄热单元为基本结构,利用FLUENT软件模拟研究了在考虑自然对流情况下添加纵向肋片的不同参数(肋片数量、长度、厚度、偏心距及肋片排布方式)对三套管式相变蓄热单元蓄热性能的影响。结果表明,三套管式蓄热单元内外双壁面共同加热,与普通套管式蓄热单元相比,换热面积增大,相变材料全部熔化需要的时间缩短;加装纵向直肋片的三套管蓄热单元的熔化速度进一步加快,相同数量肋片下,肋片的长度和厚度是影响蓄热单元蓄热量与平均蓄热速率的主要因素,肋片长度占相变材料区域径向长度的50%时,蓄热单元的熔化速度加快幅度明显且蓄热量与平均蓄热速率较高。通过优化设计,发现内管向下偏移距离e对相变材料的熔化过程作用显著,偏心距离较短时相变材料完全熔化所需时间最少,相比光滑管蓄热单元,相变材料的熔化速率提升了48.5%,偏移距离过长则会延长蓄热时间;通过加密蓄热单元底部外层肋片的排布,蓄热单元的熔化进程有不同程度的加快,总体蓄热速率较原肋片结构有所提高。

石膏基复合相变储能材料的研究进展

建筑能耗、工业能耗和交通能耗是能源消耗的主要方式。其中,建筑能耗约占能源消耗的40%,建筑能耗的持续上升会增加碳排放和加速化石能源的消耗,因此,如何提升建筑材料的保温节能性能逐渐成为建筑材料领域的研究热点。储热技术不仅可以降低建筑能耗,还可以减少环境污染。相变储能材料具有优异的储放热能力,是实现热能储存以及温度控制的重要技术手段,在建筑节能领域具有广阔的应用前景。该文主要综述了石膏基相变储能材料的研究进展;根据石膏基相变材料的不同,分析归纳了石膏基有机相变材料和石膏基复合相变材料;介绍了浸渍法、多孔材料吸附法、微胶囊法等制备石膏基复合相变储能材料的方法和机理及影响石膏基相变储能材料的因素;展望了石膏基相变储能材料的研究方向。

隧道热害环境下电力机车蓄冷式冷却塔散热性能研究

针对西部某铁路所处的高原隧道热害环境,提出一种电力机车蓄冷式冷却塔结构及其运行方案,通过对蓄冷及释冷过程进行仿真,分析换热器的冷却性能并对其结构进行优化。研究结果表明:当换热器蓄冷运行时,相对于壳体两侧无翅片结构,采用壳体空气侧加翅片和壳体双侧加翅片,相变蓄冷时间从113 min分别缩短至80 min和47 min;当换热器释冷运行时,乙二醇水溶液经相变蓄冷换热器冷却,传热管道添加间距为8 mm的翅片,有效冷却时间(冷却介质出口温度低于55℃)从无翅片结构的1.5 min延长至16 min,超过设计要求的14 min,有效解决了机车冷却塔的散热难题。

聚氨酯型固-固相变储能材料对沥青调温效果的影响研究

聚氨酯型固-固相变储能材料可以有效提高沥青的相变储能能力,从而在调节路面温度的同时,减小温度波动。基于上述背景,针对聚氨酯型固-固相变储能材料对沥青调温效果的影响展开研究。确定相变材料的分类标准,并分别研究其储能原理及具体应用情况,完成对聚氨酯型固-固相变储能材料特性的分析。在此基础上,定义周期性储热边界条件,根据沥青调温过程中内聚能密度的变化形式,推导相变储能材料的能量变化规律,从而确定聚氨酯型固-固相变储能材料对沥青调温效果的影响能力。

不同压缩比螺旋翅片对相变储能罐蓄热的影响机制

在传统螺旋翅片基础上提出3种不同压缩比的螺旋翅片结构。首先数值模拟不同压缩比的螺旋翅片储能罐相变材料的熔化过程;其次,讨论4个储能罐中温度场和固液界面的演变情况;再次,分析液相分数、平均温度和平均努塞尔数的变化趋势;最后,对比不同压缩比下相变材料的总蓄热量、熔化时间和平均蓄热速率。结果表明:设计压缩比合适的螺旋翅片可有效促进储能装置相变材料的熔化,减轻下部温度垂直分层现象,增强相变材料的自然对流,提升相变储能罐的蓄热性能;随着压缩比的增加,蓄热速率呈先增加后减小的趋势,而完全熔化时间则先减少后增加;与无压缩螺旋翅片的储能罐相比,当压缩比为3时,石蜡完全熔化时间缩短27.27%,平均蓄热速率提升33.33%,但当压缩比增加到7时,平均蓄热速率反而下降25%,完全熔化时间增加34.67%。

定形相变材料储热性能和强化传热研究进展

潜热蓄热技术被视为缓解能源供需矛盾的有效措施,其利用相变材料在相变过程中吸热/放热来实现能量的存储和释放,在建筑节能、温室控温、调温服装等领域具有极大的应用潜力。归纳了定形相变材料的种类和特点,对多孔基相变材料、微胶囊相变材料和聚合物基相变材料等制备技术及储热性能的研究进展进行了综述,分析了定形相变材料制备过程中存在的问题,介绍了定形相变材料的强化传热方法,最后讨论了今后研究工作的重点并展望了定形相变材料的发展前景。