内通流体套管式潜热蓄热器充放热过程的实验分析

通过建立与工业实际相似的加肋同心套管式潜热蓄热器模拟实验台，对潜热蓄热器内通流体时的充热、放热过程进行了实验研究。实验得出了流体的出口温度、充热量和放热量随时间的变化规律。     

Review on heat transfer analysis in thermal energy storage using latent heat storage systems and phase change materials

Thermal energy storage (TES) is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used later for heating and cooling applications and for power generation. TES has recently attracted increasing interest to thermal applications such as space and water heating, waste heat utilisation, cooling, and air conditioning. Phase change materials (PCMs) used for the storage of thermal energy as latent heat are special types of advanced materials that substantially contribute to the efficient use and conservation of waste heat and solar energy. This paper provides a comprehensive review on the development of latent heat storage (LHS) systems focused on heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy, and the formulation of the phase change problem. The main categories of PCMs are classified and briefly described, and heat transfer enhancement technologies, namely dispersion of low‐density materials, use of porous materials, metal matrices and encapsulation, incorporation of extended surfaces and fins, utilisation of heat pipes, cascaded storage, and direct heat transfer techniques, are also discussed in detail. Additionally, a two‐dimensional heat transfer simulation model of an LHS system is developed using the control volume technique to solve the phase change problem. Furthermore, a three‐dimensional numerical simulation model of an LHS is built to investigate the quasi‐steady state and transient heat transfer in PCMs. Finally, several future research directions are provided.     

Thermal performance enhancement of melting and solidification process of phase‐change material in triplex tube heat exchanger using longitudinal fins

Efficient application of intermittent renewable energy sources, like solar, waste heat recovery, and so forth, depends on a large extent on the thermal energy storage methods. Latent heat energy storage with the use of phase‐change material (PCM) is the most promising one because it stores large energy in the form of latent heat at a constant temperature. The current study investigates melting and solidification of PCM in the triplex tube heat exchanger (TTHX) numerically. The two‐dimensional numerical model has been developed using Ansys Fluent 16.2, which considers the effects of conduction as well as natural convection. To overcome the limitation imposed by the poor thermal conductivity of PCM, use of fins is the better solution. In the current study, longitudinal fins are used for better performance of TTHX, which increases heat‐transfer area between PCM and heat‐transfer fluid. The effects of location of fins, that is, internal, external, and combined internal‐external fins, are observed. All three configurations improve melting as well as solidification process. During the melting process, internal and combined internal‐external fins are equally efficient, in which maximum 59% to 60% reduction in melting time is achieved. For solidification, internal‐external fins combination gives maximum 58% reduction in solidification time.     

Numerical simulation of a multi-layer latent heat thermal energy storage system

A computational model for the prediction of the thermal behaviour of a compact multi-layer latent heat storage unit is presented. The model is based on the conservation equations of energy for the phase change material (PCM) and the heat transfer fluid (HTF). Electrical heat sources embedded inside the PCM are used for heat storage (melting) while the flow of an HTF is employed for heat recovery (solidification). Parametric studies are performed to assess the effect of various design parameters and operating conditions on the thermal behaviour of the unit. Results indicate that the average output heat load during the recovery period is strongly dependent on the minimum operating temperature, on the thermal diffusivity of the liquid phase, on the thickness of the PCM layer and on the HTF inlet mass flowrate and temperature. It is, on the other hand, nearly independent of the wall thermal diffusivity and thickness and of the maximum operating temperature. Correlations are proposed for the total energy stored and the output heat load as a function of the design parameters and the operating conditions. © 1998 John Wiley & Sons, Ltd.     

潜热储热系统强化传热研究进展

近年来,潜热储热系统在太阳能和工业废能的利用中发挥着极其重要的作用,因此用于潜热储热的相变材料受到普遍关注.文章对国内外潜热储热系统众多强化传热技术进行了综述与讨论.     

Thermal performance investigation of finned latent heat storage of shell-and-tube,shell-and-nozzle,and shell-and-reducer models

Three models of latent heat storage with circular fins were studied numerically and experimentally in this paper. The models were shell-and-tube, shell-and-nozzle, and shell-and-reducer. These models were investigated for two different inlets of heat transfer fluid (HTF), from the bottom and top of the models, so the number of studied cases was six. The results of the comparison between the cases showed that the different HTF inlet with a fixed mass flow rate greatly affects the completion time of the melting process; the bottom inlet of HTF accelerates the melting compared to the top inlet because it enhances the role of natural convection. Compared with shell-and-tube with bottom HTF inlet, the shell-and-nozzle with bottom inlet reduces the melting time by 11.2%, while the shell-and-reducer with bottom inlet delays the melting by 24%. The results of the top HTF inlet cases showed that shell-and-nozzle delays the melting by 16% compared with shell-and-tube, while the melting is not completed in shell-and-reducer. Shell-and-nozzle with a bottom HTF inlet shows the shortest melting time and the best thermal performance among all the other cases due to the geometric design of the model. On comparing the numerical and experimental results, good agreement was found between them.     

Channel formation and visualization of melting and crystallization behaviors in direct-contact latent heat storage systems

Thermal storage systems are an essential component for increasing the share of renewable energies in residential heating and for the valorization of waste heat. A key challenge for the widespread application of thermal storage systems is their limited power-to-capacity ratio. One potential solution for this challenge is represented by direct-contact latent heat storage systems, in which a phase change material (PCM) is in direct contact with an immiscible heat transfer fluid (HTF). To demonstrate the applicability of the direct-contact concept for domestic hot water production, a PCM with a phase change temperature of 59°C is chosen. To enable cost-efficient implementation of the storage system, a eutectic mixture of two salt hydrates, magnesium chloride hexahydrate and magnesium nitrate hexahydrate, is chosen as the PCM. One key aspect for the direct-contact concept is that, during discharge, the HTF channels in the PCM do not become clogged during the solidification of the PCM. In this study, the formation and topology of the channels in direct-contact systems under an optimized flow condition are investigated via visual observation and X-ray computed tomography. The elucidation of the channel structure provides information on the melting and crystallization behaviors of the PCM, which are shown schematically.     

壳管式相变蓄热器传热效率研究

设计了一套定量测试不同工况下壳管式相变蓄热器传热效率装置。采用壳管式相变储热,石蜡填充入壳管间,管内通入冷、热载流体,模拟吸热放热过程。测试发现:相同入口条件下,单位时间传热量随入口水温增加呈线性增加;管内载流体流量加大有助于提高传热水平,15~60 L/h流量内单位时间传热量增速随流量增加放缓;不同材质传热管单位时间传热量变化并不明显,表明管道热阻在相变蓄热器总热阻中所占份额较小;相同工况下的蓄热过程,热载流体由下向上流动传热形式明显优于由上向下管排形式;尝试在封装相变材料中添加金属网状结构,强化相变材料内部热传导速率,对比发现相同工况下相变材料中添加金属网状结构,可提高10%~15%左右传热量。     

Numerical simulations on performance enhancement of a cross-flow latent thermal energy storage heat exchanger

基于列管式换热器具有传热面积大、结构紧凑、操作弹性大等优点,使其在相变储能领域具有广阔的应用前景。本文建立一种新型列管式相变蓄热器模型,在不考虑自然对流的情况下,利用Fluent软件对相变蓄热器进行二维储热过程的数值模拟。本文主要研究斯蒂芬数、雷诺数、列管排列方式、肋片数以及相变材料的导热系数对熔化过程的影响,并对熔化过程中固液分界面的移动规律进行了分析。模拟结果表明,内肋片强化换热效果明显,特别是对应用低导热系数相变材料[导热系数小于1 W/(m·K)]的列管式蓄热器,相对于无肋片结构,加入肋片（N_fn=2）可缩短熔化时间52.6%。     

Analysis of a latent heat cold storage unit

This paper presents a conduction based model for solving the phase change heat transfer problem around a vertical cylinder submersed in the phase change medium. The energy equation is coupled to the flow problem by an energy balance. The system of equations is solved numerically by using the average control volume technique and the ADI approach. The results show the effects of the variation of the Biot number, Stefan number, the inlet fluid temperature and the ratio of the outer to the inner tube radius on the solidified mass fraction, NTU, effectiveness and the time for complete solidification. © 1997 by John Wiley & Sons, Ltd.     

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。     

An experimental and numerical investigation of heat transfer during technical grade paraffin melting and solidification in a shell-and-tube latent thermal energy storage unit

The latent thermal energy storage system of the shell-and-tube type during charging and discharging has been analysed in this paper. An experimental and numerical investigation of transient forced convective heat transfer between the heat transfer fluid (HTF) with moderate Prandtl numbers and the tube wall, heat conduction through the wall and solid–liquid phase change of the phase change material (PCM), based on the enthalpy formulation, has been presented. A fully implicit two-dimensional control volume Fortran computer code, with algorithm for non-isothermal phase transition, has been developed for the solution of the corresponding mathematical model. The comparison between numerical predictions and experimental data shows good agreement for both paraffin non-isothermal melting and isothermal solidification. In order to provide guidelines for system performance and design optimisation, unsteady temperature distributions of the HTF, tube wall and the PCM have been obtained by a series of numerical calculations for various HTF working conditions and various geometric parameters, and the thermal behaviour of the latent thermal energy storage unit during charging and discharging has been simulated.     

High‐temperature latent heat storage technology to utilize exergy of solar heat and industrial exhaust heat

Latent heat storage (LHS) using phase change materials is quite attractive for utilization of the exergy of solar energy and industrial exhaust heat because of its high‐heat storage capacity, heat storage and supply at constant temperature, and repeatable utilization without degradation. In this article, general LHS technology is outlined, and then recent advances in the uses of LHS for high‐temperature applications (over 100 °C) are discussed, with respect to each type of phase change material (e.g., sugar alcohol, molten salt, and alloy). The prospects of future LHS systems are discussed from a principle of exergy recuperation. In addition, the technologies to minimize exergy loss in the future LHS system are discussed on the basis of the thermodynamic analysis by ‘thermodynamic compass’. Copyright © 2016 John Wiley & Sons, Ltd.     

肋参数对复合肋套管流动和换热的影响

同心复合套管式回热器是微型燃气轮机的重要部件—紧凑式回热器的一种类型。采用Fluent对有交错肋的同心套管中的对流与换热进行了数值模拟。结果表明：在给定的边界条件下,肋的一些参数,比如冷、热流体通道内的肋之间的夹角,肋片的导热系数和管长会对Nu数产生影响,但夹角的变化对摩擦系数几乎没影响。     

Experimental investigation and modeling of a new kind of latent heat storage system—indirect-contact galisol system

A new kind of latent heat storage system, namely, indirect-contact galisol (gaseous–liquid–solid) system, was investigated experimentally and theoretically. A physical model describing the performance of the system is proposed. Through experiments and by our model the performance characteristics of the system and its salient features are clarified. The limitations of the system operation in charging and discharging modes are formulated. There is good agreement between theoretical and experimental results. © 1999 Scripta Technica, Heat Trans Asian Res, 28(2): 115–128, 1999     

Recapitulation on latent heat hybrid buildings

In the existent paper, the performance of thermal storage hybrid buildings exploiting the latent heat of phase change materials (PCMs) for thermal refrigeration and heating of the contemporary period has been investigated. The conventional buildings consume a large amount of electricity, primarily for the heating and cooling applications. Electricity generation primarily relies on coal-based thermal power plants. The emissions from these establishments pose a serious threat to the environment. Moreover, conventional heating/cooling units rely on exorbitant energy cost. The usage of any kind of thermal storage system is an efficacious way of stockpiling thermal energy and utilizing it when needed. This paper gives a comprehensive overview of the available thermal storage units incorporating PCMs. The various segments of the buildings, viz, ceiling, window, wall, and floor have been analyzed in details. The results are quite promising in terms of load reduction and overall energy saving. Indoor surface temperature reduction of up to 7^oC has been achieved. The energy saving of up to 40% can be realized by employing PCM. A comprehensive list of the PCMs is also tried to build up for end users according to their temperature requirement.     

Role of metal foam in solidification performance for a latent heat storage unit

The current latent heat storage (LHS) units are usually poor in energy charging and discharging efficiency. Given this, a two dimensional (2D) numerical model of the energy discharging process is presented and comprehensively analyzed to predict the role of metal foam in the solidification performance of LHS units. In the model, the fractal geometry reconstructed by the fractal Brownian motion is utilized for the pore characterization of the metal foam. The proposed model is validated through a melting experiment in copper foams from the reference. The temperature dynamic response and the solidification front evolution in metal foam are analyzed and compared to those in a corresponding cavity. The roles of the fractal dimension and porosity in the solidification behaviors are quantitatively analyzed. The results report that the presence of metal foam enhances the solidification performance. For the main goal of maximizing the latent storage, the appropriate porosity of an LHS unit is dependent on the duration time for the heat discharging process in the real application of thermal energy storage. Even if the porosity is the same, the fractal dimension also affects the solidification performance. A decrease in the fractal dimension (lower degree of disorder for pore distribution) provides greater access to heat flow through the phase change material-foam composite and thus leads to improvement in the interstitial heat transfer, which in turn accelerates the rate of heat release. The fractal dimension is expected to be less than 1.5 for superior solidification performance.     

Cascaded latent heat storage for parabolic trough solar power plants

The current revival of solar thermal electricity generating systems (SEGS) unveils the still existing need of economic thermal energy storages (TES) for the temperature range from 250 °C to 500 °C. The TES-benchmark for parabolic trough power plants is the direct two tank storage, as it was used at the SEGS I plant near Barstow (USA). With the introduction of expensive synthetic heat transfer oil, capable to increase the operating temperature from former 300 °C up to 400 °C, the direct storage technology became uneconomical. Cascaded latent heat storages (CLHS) are one possible TES alternative, which are marked by a minimum of necessary storage material. The use of a cascade of multiple phase change materials (PCM) shall ensure the optimal utilization of the storage material.This paper reports experimental and numerical results from the investigation of cascaded latent heat storages with alkali nitrate salts like NaNO₃, KNO₃ and others more. The experiments were conducted with vertical shell and tube type heat exchanger devices under realistic operation parameters. The experimental results were used for a numerical model to simulate different CLHS configurations. Dymola/Modelica was used to conduct the simulation. The outcome of this work shows on the one hand, that the design of CLHS for this temperature range is more complex than for the temperature range up to 100 °C. And on the other hand, the low heat conductivity of available PCM is an obstacle which must be overcome to make full use of this promising storage technology.     

Analysis of solidification in a finite PCM storage with internal fins by employing heat balance integral method

Here, a simplified analytical model has been proposed to predict solid fraction, solid–liquid interface, solidification time, and temperature distribution during solidification of phase change material (PCM) in a two‐dimensional latent heat thermal energy storage system (LHTES) with horizontal internal plate fins. Host of boundary conditions such as imposed constant heat flux, end‐wall temperature, and convective air environment on the vertical walls are considered for the analysis. Heat balance integral method was used to obtain the solution. Present model yields closed‐form solution for temperature variation and solid fraction as a function of various modeling parameters. Also, solidification time of PCM, which is useful in optimum design of PCM‐based thermal energy storages, has been evaluated during the analysis. The solidification time was found to be reduced by 93% by reducing the aspect ratio from 8 to 0.125 for constant heat flux boundary condition. While, for constant wall temperature boundary condition, the solidification time reduces by 99% by changing the aspect ratio from 5 to 0.05. In case of convective air boundary surrounding, the solidification time is found to reduce by 88% by reducing the aspect ratio from 8 to 0.125. Based on the analytical solution, correlations have been proposed to predict solidification time in terms of aspect ratio and end‐wall boundary condition.     

Convective heat transfer and pressure drop of a tube with internal longitudinal fins

The fluid flows and heat transfer characteristics of a tube with internal longitudinal fins were investigated experimentally and numerically. The realizable turbulence model was adopted to simulate the problem, whose results indicate a good agreement with the measured data. Compared with the circular annulus tube, it was shown that the internal longitudinal finned tube provides excellent heat transfer performance greater than those of the circular annulus tube, with a great increase of simultaneous pressure drops. Furthermore, there exists a critical Reynolds number, about 1500, when the Reynolds number is less than the critical value, and the Nusselt number of an internal longitudinal finned tube will be smaller than that of a circular annulus tube in laminar flow. On the other hand, the transition Reynolds number for a tube with internal longitudinal fins from laminar to turbulent is greatly decreased due to the existence of internal longitudinal fins. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(2): 57–65, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20147