水平管壳式相变蓄热装置的强化传热研究

基于焓法模型对水平管壳式相变蓄热装置热性能的增强进行研究,首先分析蓄热过程中传统管壳式装置内材料的传热及流动机理;然后引入椭圆元素并对比椭圆内管及外壳的强化传热效果;最后对热源温度、相变材料导热系数及初始温度对装置热性能的作用规律进行探讨。结果显示,椭圆外壳的强化传热效果优于内管,同等条件下,长短轴之比为2的椭圆外壳可使蓄热时间缩短53.5%。热源温度升高,椭圆外壳的强化传热效果进一步增强,相变材料的导热系数及初始温度对装置热性能的影响较小。     

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.     

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%。     

Exergy analysis of two phase change materials storage system for solar thermal power with finite-time thermodynamics

A mathematical model for the overall exergetic efficiency of two phase change materials named PCM1 and PCM2 storage system with a concentrating collector for solar thermal power based on finite-time thermodynamics is developed. The model takes into consideration the effects of melting temperatures and number of heat transfer unit of PCM1 and PCM2 on the overall exergetic efficiency. The analysis is based on a lumped model for the PCMs which assumes that a PCM is a thermal reservoir with a constant temperature of its melting point and a distributed model for the air which assumes that the temperature of the air varies in its flow path. The results show that the overall exergetic efficiency can be improved by 19.0-53.8% using two PCMs compared with a single PCM. It is found that melting temperatures of PCM1 and PCM2 have different influences on the overall exergetic efficiency, and the overall exergetic efficiency decreases with increasing the melting temperature of PCM1, increases with increasing the melting temperature of PCM2. It is also found that for PCM1, increasing its number of heat transfer unit can increase the overall exergetic efficiency, however, for PCM2, only when the melting temperature of PCM1 is less than 1150 K and the melting temperature of PCM2 is more than 750 K, increasing the number of heat transfer unit of PCM2 can increase the overall exergetic efficiency. Considering actual application of solar thermal power, we suggest that the optimum melting temperature range of PCM1 is 1000-1150 K and that of PCM2 is 750-900 K. The present analysis provides theoretical guidance for applications of two PCMs storage system for solar thermal power.     

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。     

Numerical analysis and parameters optimization of shell-and-tube heat storage unit using three phase change materials

In this paper, a mathematical model of shell-and-tube latent heat thermal energy storage (LHTES) unit of two-dimension of three phase change materials (PCMs) named PCM1, PCM2 and PCM3 with different high melting temperatures (983 K, 823 K and 670 K, respectively) and heat transfer fluid (HTF: air) with flowing resistance and viscous dissipation based on the enthalpy method has been developed. Instantaneous solid–liquid interface positions and liquid fractions of PCMs as well as the effects of inlet temperatures of the air and lengths of the shell-and-tube LHTES unit on melting times of PCMs were numerically analyzed. The results show that melting rates of PCM3 are the fastest and that of PCM1 are the slowest both x, r directions. It is also found that the melting times of PCM1, PCM2 and PCM3 decrease with increase in inlet temperatures of the air. Moreover, with increase in inlet temperatures of the air, decreasing degree of their melting times are different, decreasing degree of the melting time of PCM1 is the biggest and that of PCM3 is the smallest. Considering actual application of solar thermal power, we suggest that the optimum lengths are L₁ = 250 mm, L₂ = 400 mm, L₃ = 550 mm (L = 1200 mm) which corresponds to the same melting times of PCM1, PCM2 and PCM3 are about 3230 s and inlet temperature of the air is about 1200 K. The present analysis provides theoretical guidance for designing optimization of the shell-and-tube LHTES unit with three PCMs for solar thermal power.     

Experimental research on thermal characteristics of PCM thermal energy storage units

To explore thermal management integration in electric vehicles (EVs), a phase change materials (PCMs) thermal energy storage unit using flat tubes and corrugated fins is designed. The investigation focuses on the thermal characteristics of the PCM unit, such as the temperature variation, heat capacity, and heat transfer time, etc. Meanwhile, the heat storage and release process will be influenced by different inlet temperature, liquid flow rate, melting point of the PCM, and the combination order of the units. Under the same inlet temperature and flow rate condition, the PCM unit with higher melting point enters the latent heat storage stage slowly and enters the phase change melting release stage quickly. Furthermore, the heat storage and release rates increase with increasing liquid flow rates, but the effects are diminishing in the middle and later periods. The multiple PCM units with different melting temperatures are cascaded to help recycle low-grade heat energy with different temperature classes and exhibit well heat storage and release rates.     

Thermal response in thermal energy storage material around heat transfer tubes: effect of additives on heat transfer rates

The effects of carbon-fiber chips and carbon brushes as additives on the thermal conductivity enhancement of phase change materials (PCMs) using in latent heat thermal energy storage are investigated experimentally and numerically by considering the wall effect of the additives. The carbon-fiber chips are effective for improving the heat transfer rate in PCMs. However, the thermal resistance near the heat transfer surface is higher than that for the carbon brushes. As a result, the overall heat transfer rate for the fiber chips is lower than that for the carbon brushes. Consequently, the carbon brushes are superior to the fiber chips for the thermal conductivity enhancement under the present experimental conditions. The carbon brushes are moreover applied to the packed beds of particles to overcome their low thermal conductivity in chemical heat pump/storage. The carbon brushes essentially improve the heat transfer characteristics in the packed beds, though the thermal resistance is observed because the particles obstruct contact between the fibers and the heat transfer surfaces.     

Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material

This study aimed determination of proper amount of paraffin (n-docosane) absorbed into expanded graphite (EG) to obtain form-stable composite as phase change material (PCM), examination of the influence of EG addition on the thermal conductivity using transient hot-wire method and investigation of latent heat thermal energy storage (LHTES) characteristics of paraffin such as melting time, melting temperature and latent heat capacity using differential scanning calorimetry (DSC) technique. The paraffin/EG composites with the mass fraction of 2%, 4%, 7%, and 10% EG were prepared by absorbing liquid paraffin into the EG. The composite PCM with mass fraction of 10% EG was considered as form-stable allowing no leakage of melted paraffin during the solid–liquid phase change due to capillary and surface tension forces of EG. Thermal conductivity of the pure paraffin and the composite PCMs including 2, 4, 7 and 10 wt% EG were measured as 0.22, 0.40, 0.52, 0.68 and 0.82 W/m K, respectively. Melting time test showed that the increasing thermal conductivity of paraffin noticeably decreased its melting time. Furthermore, DSC analysis indicated that changes in the melting temperatures of the composite PCMs were not considerable, and their latent heat capacities were approximately equivalent to the values calculated based on the mass ratios of the paraffin in the composites. It was concluded that the composite PCM with the mass fraction of 10% EG was the most promising one for LHTES applications due to its form-stable property, direct usability without a need of extra storage container, high thermal conductivity, good melting temperature and satisfying latent heat storage capacity.     

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.     

Second law analysis of latent thermal storage for solar system

A theoretical model has been developed for analysis and optimization of the solar system using phase change material (PCM). The later consists of a solar air heating collector coupled with a cylindrical storage tank which contains spherical capsules filled with a PCM. Energy and exergy analyses are carried out to understand the behavior of the system using single PCM or multiple PCMs. Numerical results show that the performance of the latent thermal storage system can be ameliored by the judicious choice of the melting temperature of the PCM.     

Heat transfer intensification in horizontal shell and tube latent heat storage unit

Employment of latent heat storage unit (LHSU) utilizing phase change material (PCM) in a substantial scale is constrained by the poor thermal conductivity of PCMs. Future utilization of LHSU will therefore to a great extent rely on the heat transfer intensification techniques. Present research is on enhancement techniques in which heat transfer mechanism is altered without altering the mass of PCM and heat transfer surface area. The intensification mechanisms considered in the present research include imparting eccentricity to heat transfer fluid (HTF) pipe, imparting rotation to the LHSU and providing multi HTF tube. Numerical investigations are reported here towards comparative evaluation of the thermal characteristics associated with such intensification mechanisms for horizontal LHSU. In the present study stearic acid (melting point 55.7–56.6?°C) is used as PCM and water is used as HTF. Results infer that all the three mechanisms offer quicker melting rate. For the geometric configuration of LHSU considered in the present research, a reduction in melting time of 47.75% is evaluated for rotating LHSU. The rate of energy storage is higher for both eccentric and rotating LHSU. Solidification process is however not accelerated by such techniques. On the contrary, eccentric and multi HTF tube LHSU takes more time for solidification.     

Heat transfer enhancement for thermal energy storage using metal foams embedded within phase change materials (PCMs)

In this paper the experimental investigation on the solid/liquid phase change (melting and solidification) processes have been carried out. Paraffin wax RT58 is used as phase change material (PCM), in which metal foams are embedded to enhance the heat transfer. During the melting process, the test samples are electrically heated on the bottom surface with a constant heat flux. The PCM with metal foams has been heated from the solid state to the pure liquid phase. The temperature differences between the heated wall and PCM have been analysed to examine the effects of heat flux and metal foam structure (pore size and relative density). Compared to the results of the pure PCM sample, the effect of metal foam on solid/liquid phase change heat transfer is very significant, particularly at the solid zone of PCMs. When the PCM starts melting, natural convection can improve the heat transfer performance, thereby reducing the temperature difference between the wall and PCM. The addition of metal foam can increase the overall heat transfer rate by 3-10 times (depending on the metal foam structures and materials) during the melting process (two-phase zone) and the pure liquid zone. The tests for investigating the solidification process under different cooling conditions (e.g. natural convection and forced convection) have been carried out. The results show that the use of metal foams can make the sample solidified much faster than pure PCM samples, evidenced by the solidification time being reduced by more than half. In addition, a two-dimensional numerical analysis has been carried out for heat transfer enhancement in PCMs by using metal foams, and the prediction results agree reasonably well with the experimental data.     

Phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as PCM in a latent heat storage system

The phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as phase change material (PCM) during the melting and solidification processes were determined experimentally in a vertical two concentric pipes energy storage system. This study deals with three important subjects. First is determination of the eutectic composition ratio of the palmitic acid (PA) and stearic acid (SA) binary system and measurement of its thermophysical properties by differential scanning calorimetry (DSC). Second is establishment of the phase transition characteristics of the mixture, such as the total melting and solidification temperatures and times, the heat transfer modes in the melted and solidified PCM and the effect of Reynolds and Stefan numbers as initial heat transfer fluid (HTF) conditions on the phase transition behaviors. Third is calculation of the heat transfer coefficients between the outside wall of the HTF pipe and the PCM, the heat recovery rates and heat fractions during the phase change processes of the mixture and also discussion of the effect of the inlet HTF parameters on these characteristics. The DSC results showed that the PA–SA binary system in the mixture ratio of 64.2:35.8 wt% forms a eutectic, which melts at 52.3 °C and has a latent heat of 181.7 J g⁻¹, and thus, these properties make it a suitable PCM for passive solar space heating and domestic water heating applications with respect to climate conditions. The experimental results also indicated that the eutectic mixture of PA–SA encapsulated in the annulus of concentric double pipes has good phase change and heat transfer characteristics during the melting and solidification processes, and it is an attractive candidate as a potential PCM for heat storage in latent heat thermal energy storage systems.     

The effect of using two PCMs on the thermal regulation performance of BIPV systems

Building Integrated Photovoltaics (BIPVs) is one of the most promising applications for Photovoltaics (PVs). However, when the temperature in the BIPV increases, the conversion efficiency deteriorates. A PV/PCM system using Phase Change Materials (PCM) for BIPV thermal control has been experimentally and numerically studied previously. One of the main barriers for this application is how to improve the low thermal conductivity of the PCM in order to achieve a quick thermal dissipation response with longer thermal regulation in PVs. Although the metal fins inserted inside the PCM can improve the heat transfer, the thermal regulation period declines as the volume of the PCM is substituted by the metal mass of the PV/PCM system. A modified PV/PCM system integrated with two PCMs with different phase transient temperatures for improving the heat regulation needs to be investigated. The use of combinations of PCMs, each with a set of different phase transient temperatures, is expected to enhance the thermal regulation effect of the PV/PCM system and lengthen the thermal regulation time in PVs. In this study a developed PV/PCM numerical simulation model for single PCM application has been modified to predict the thermal performance of the multi-PCMs in a triangular cell in the PV/PCM system. A series of numerical simulations tests have been carried out in static state and realistic conditions in UK. The thermal regulation of the PV/PCM system with a different range of phase transient temperature PCMs has been discussed.     

Experimental Investigation of Heat Storage and Heat Transfer Rates during Melting of Nano-Enhanced Phase Change Materials (NePCM) in a Differentially-Heated Rectangular Cavity

In this work, an experimental study of melting heat transfer of nano-enhanced phase change materials(NePCM) in a differentially-heated rectangular cavity was performed. Two height-to-width aspect ratios of the cavity, i.e., 0.9 and 1.5, were investigated. The model Ne PCM samples were prepared by dispersing graphene nanoplatelets(GNP) into 1-tetradecanol, having a nominal melting point of 37℃, at loadings up to 3 wt.%. The viscosity was found to have a more than 10-fold increase at the highest loading of GNP. During the melting experiments, the wall superheat at the heating boundary was set to be 10℃ or 30℃. It was shown that with increasing the loading of GNP, both the heat storage and heat transfer rates during melting decelerate to some extent, at all geometrical and thermal configurations. This suggested that the use of NePCM in such cavity may not be able to enhance the heat storage rate due to the dramatic growth in viscosity, which deteriorates significantly natural convective heat transfer during melting to overweigh the enhanced heat conduction by only a decent increase in thermal conductivity. This also suggested that the numerically predicted melting accelerations and heat transfer enhancements, as a result of the increased thermal conductivity, in the literature are likely overestimated because the negative effects due to viscosity growth are underestimated.     

Heat transfer enhancement of fatty acids when used as PCMs in thermal energy storage

Phase change materials (PCM) used in latent heat storage systems usually have very low thermal conductivities. This is a major drawback in maintaining the required heat exchange rate between PCM and heat transfer fluid. This paper investigates the enhancement of the heat transfer between PCM and heat transfer fluid, using high thermal conductivity as additives like stainless steel pieces, copper pieces and graphite–PCM composite material. In the experiments, palmitic–lauric acid (80:20) (PL) and stearic–myristic acid (80:20) (SM) were used as PCMs. Test results show that heat transfer enhancement of copper pieces was better at 0.05 Ls^?1 flow rate compared to 0.025 Ls^?1. Using copper as an additive increased the heat transfer rate 1.7 times for melting and 3.8 times for freezing when flow rate was 0.050 Ls^?1. Decreasing the flow rate from 0.050 to 0.025 Ls^?1, increased the melting times 1.3 times and freezing times 1.8 times, decreasing heat transfer rates accordingly. The best result of heat transfer enhancement was observed for the PCM–graphite composite. However, changing the flow rate did not affect the heat transfer rate when graphite was used as additive. Copyright © 2007 John Wiley & Sons, Ltd.     

Thermal performance of a eutectic mixture of lauric and stearic acids as PCM encapsulated in the annulus of two concentric pipes

Thermal performance characteristics of a eutectic mixture of lauric and stearic acids as phase change material (PCM) during the melting and solidification processes were determined experimentally in a vertical two concentric pipe-energy storage system. This study deals with three important subjects: The first one is to determine the eutectic composition ratio of the lauric acid (LA) and stearic acid (SA) binary system, and to measure its thermophysical properties by DSC. The second one is to establish the thermal characteristics of the mixture such as total melting and solidification times, the heat transfer modes in melted and solidified PCM, and the effect of Reynolds and Stefan numbers as inlet heat transfer fluid (HTF) conditions on the phase transition behaviors. The final one includes the calculations of the heat transfer coefficients between the outside wall of the HTF pipe and the PCM, and heat fractions during the melting and solidification processes of the mixture, and also the discussion of the effect of inlet HTF parameters on these characteristics. The LA–SA binary system in the mixture ratio of 75.5:24.5 wt % forms a eutectic, which melts at 37°C and has a latent heat of 182.7 J g⁻¹, and, thus, these properties make it an attractive phase change material used for passive solar space heating applications such as building and greenhouse heating with respect to the climate conditions. The experimental results indicated that the mixture encapsulated in the annulus of two concentric pipes has good thermal and heat transfer characteristics during the melting and solidification processes, and it has potential for heat storage in passive solar space heating systems.     

组合式相变材料组分配比与储热性能研究

采用焓法对组合式相变材料(PCM)储热系统的相变过程进行了数值计算,分析了组合式相变材料中各个PCM组分质量分数的变化对系统储热性能的影响。结果表明,对于组合式相变材料储热系统,存在着最优组分配比,使得系统的储热性能达到最佳。     

Thermal Energy Storage Performance of Fatty Acids as a Phase Change Material

Thermal energy storage performance of fatty acids and a eutectic mixture as phase change materials (PCMs) has been investigated experimentally. The selected PCMs for this study were palmitic acid, myristic acid, stearic acid, and a mixture of stearic and myristic acids in eutectic combination ratio of 65.7 wt% myristic acid and 34.3 wt% stearic acid. The PCMs have a melting temperature range of 50.0°C to 61.20°C and a latent heat range of 162.0 J/g to 204.5 J/g. The inlet temperature and the mass flow rate of heat transfer fluid (HTF) were selected as experimental parameters to test the thermal energy storage performance of the PCMs. The transition times, temperature range, propagation of the solid-liquid interface, as well as heat flow rate characteristics of the employed cylindrical tube storage system were studied at varied experimental parameters. The experimental results show that the melting front moves to inward in the radial directions as well as in the axial directions from the top toward to the bottom of the PCM tube. It was observed that the convection heat transfer in the liquid phase plays an important role in the melting process. The changes in the studied HTF parameters have more effect on the melting processes than the solidification processes of the PCMs. The average heat storage efficiency calculated from data for all the PCMs is 51.5%, meaning that 48.5% of the heat actually was lost somewhere.