Thermal energy storage： Challenges and the role of particle technology

Thermal energy is at the heart of the whole energy chain providing a main linkage between the primary and secondary energy sources. Thermal energy storage （TES） has a pivotal role to play in the energy chain and hence in future low carbon economy. However, a competitive TES technology requires a number of scientific and technological challenges to be addressed including TES materials, TES components and devices, and integration of TES devices with energy networks and associated dynamic optimization. This paper provides a perspective of TES technology with a focus on TES materials challenges using molten salts based phase change materials for medium and high temperature applications. Two key challenges for the molten salt based TES materials are chemical incompatibility and low thermal conductivity. The use of composite materials provides an avenue to meeting the challenges. Such composite materials consist of a phase change material, a structural supporting material, and a thermal conductivity enhancement material. The properties of the supporting material could determine the dispersion of the thermal con- ductivity enhancement material in the salt. A right combination of the salt, the structural supporting material, and the thermal conductivity enhancement material could give a hierarchical structure that is able to encapsulate the molten salt and give a substantial enhancement in the thermal conductivity. Understanding of the structure-property relationships for the composite is essential for the formulation design and fabrication of the composite materials. Linking materials properties to the system level performance is recommended as a key future direction of research.     

Thermal energy storage: Challenges and the role of particle technology

Thermal energy is at the heart of the whole energy chain providing a main linkage between the primary and secondary energy sources. Thermal energy storage (TES) has a pivotal role to play in the energy chain and hence in future low carbon economy. However, a competitive TES technology requires a number of scientific and technological challenges to be addressed including TES materials, TES components and devices, and integration of TES devices with energy networks and associated dynamic optimization. This paper provides a perspective of TES technology with a focus on TES materials challenges using molten salts based phase change materials for medium and high temperature applications. Two key challenges for the molten salt based TES materials are chemical incompatibility and low thermal conductivity. The use of composite materials provides an avenue to meeting the challenges. Such composite materials consist of a phase change material, a structural supporting material, and a thermal conductivity enhancement material. The properties of the supporting material could determine the dispersion of the thermal conductivity enhancement material in the salt. A right combination of the salt, the structural supporting material, and the thermal conductivity enhancement material could give a hierarchical structure that is able to encapsulate the molten salt and give a substantial enhancement in the thermal conductivity. Understanding of the structure–property relationships for the composite is essential for the formulation design and fabrication of the composite materials. Linking materials properties to the system level performance is recommended as a key future direction of research.     

Multi-walled carbon nanotubes added to Na2CO3/MgO composites for thermal energy storage

Na2CO3/MgO composites with added multi-walled carbon nanotubes （MWCNTs） were prepared and tested as phase change materials （PCMs） for thermal energy storage. Na2CO3/MgO composite PCMs were prepared and their chemical compatibility and thermal stability were studied. MWCNTs introduced with Na2CO3/MgO composite PCMs were also investigated and scanning electron microscopy （SEM） characterization was used to demonstrate the uniform dispersion of MWCNTs in Na2CO3/MgO composite PCMs. The composites with added MWCNTs still display good thermal stability with mass losses lower than 5%. Introducing MWCNTs into composite Na2CO3/MgO PCMs by material formation/calcination signifi.cantly enhances the thermal conductivity of the composite PCMs. The thermal conductivity of the composite PCMs was found to increase with an increase in the weight fraction of the added MWCNTs and an increase in the testing temperature. This study may present a promising way to prepare high temperature phase change materials with superior properties such as improved thermal stability.     

Experimental study of enhanced heat transfer by addition of CuO nanoparticle

An energy storage system has been designed to study the thermal characteristics of paraffin wax with an embedded nano size copper oxide (CuO) particle. This paper presents studies conducted on phase transition times, heat fraction as well as heat transfer characteristics of paraffin wax as phase change material (PCM) embedded with CuO nanoparticles. 40?nm mean size CuO particles of 2, 5 and 10% by weight were dispersed in PCM for this study. Experiments were performed on a heat exchanger with 1.5–10?l/min of heat transfer fluid (HTF) flow. Time-based variations of the temperature distributions are revealed from the results of observations of melting and solidification curves. The results strongly suggested that the thermal conductivity enhances 6, 6.7 and 7.8% in liquid state and in dynamic viscosity it enhances by 5, 14 and 30% with increasing mass fraction of the CNEPs. The thermal conductivity ratio of the composites can be augmented by a factor up to 1.3. The heat transfer coefficient during solidification increased about 78% for the maximum flow rate. The analysis of experimental results reveals that the addition of copper oxide nanoparticles to the paraffin wax enhances both the conduction and natural convection very effectively in composites and in paraffin wax. The paraffin wax-based composites have great potential for energy storage applications like industrial waste heat recovery, solar thermal applications and solar based dynamic space power generation with optimal fraction of copper oxide nanoparticles.     

Influence of applied in-plane strain on transverse thermal conductivity of 0°/90° and plain weave ceramic matrix composites

A computationally economic finite-element-based stress analysis model, developed previously by the authors, has been extended to predict the thermal behaviour of ceramic matrix composites with strain-induced damage. The finite element analysis utilises a solid element to represent a homogenised orthotropic medium of a heterogeneous uni-directional tow. The non-linear multi-axial strain dependent thermal behaviour has been discretised by multi-linear curves, which have been implemented by a user defined subroutine, USDFLD, in the commercial finite element package, ABAQUS. The model has been used to study the performance of two CMC composites: a SiC (Nicalon) fibre-calcium aluminosilicate (CAS) matrix, 0°/90° cross-ply laminate Nicalon-CAS; and, carbon fibre-dual carbon-SiC matrix (C/C-SiC), plain weave laminate DLR-XT. The global through-thickness thermal conductivity degradation with composite uni-axial strain has been predicted. Comparisons have been made between the predictions and experimental data for both materials, and good agreement has been achieved. For the Nicalon-CAS 0°/90° cross-ply the dominant mechanism of thermal conductivity degradation is combined fibre failure and associated wake debonding; and, for the DLR-XT plain weave the same mechanisms act in combination with out-of-plane shear failure.     

Meshless method for nonlinear heat conduction analysis of nano-composites

Carbon nanotubes (CNTs) may become ideal reinforcing materials for high-performance nano-composites due their exceptional properties. Still, much work is needed to be done before the potentials of CNT based composites can be fully realized. The evaluation of effective material properties of nano-composites is one of many difficult tasks. Simulations using continuum mechanics approach can play a significant role in the analysis of these composites. In the present work, nonlinear heat conduction analysis of CNT based composites has been carried out using continuum mechanics approach. Element free Galerkin method has been applied as a numerical tool. Thermal conductivities of nanotube and polymer matrix are assumed to vary quadratically with temperature. Picard and quasi-linearization schemes have been utilized to obtain the solution of a system of nonlinear equations. Cylindrical representative volume element has been used to evaluate the thermal properties of nano-composites. Present simulations show that the temperature dependent matrix thermal conductivity has a significant effect on the equivalent thermal conductivity of the composite, whereas temperature dependent nanotube thermal conductivity has a small effect on the equivalent thermal conductivity of the composite. The results obtained by Picard method have been found almost similar with those obtained by quasi-linearization approach.     

陶瓷刀具切削区温度场的计算机模拟

在切削金属过程中所消耗的能量几乎９０％以上都转化为热，致使工件，切屑和刀具的温度都上升，其中刀具的温升与切削机理及切削参数密切相关，并且直接影响刀具磨损及其全用寿命。为了研究陶瓷刀具切削温度分布对其磨损规律和机理的影响，根据传热理论建立了数学模型，用计算机模拟编制出陶瓷刀具切削区温度场计算的专业软件，可以得出不同陶瓷料刀具在不同切削条件下切步同工件材料过程中的温度分布曲线图，而且实际测量值与模拟值     

Thermal fracture of a functionally graded/homogeneous bimaterial with system of cracks

The thermal fracture of a bimaterial consisting of a homogeneous material and a functionally graded material (FGM) with a system of internal cracks and an interface crack is investigated. The bimaterial is subjected to a heat flux. The thermal properties of FGM are assumed to be continues functions of the thickness coordinate, while the elastic properties are constants. The method of the solution is based on the singular integral equations. For a special case where the interface crack is much larger than the internal cracks in the FGM the asymptotic analytical solution of the problem is obtained as series in a small parameter (the ratio between sizes of the internal and interface crack) and the thermal stress intensity factors (TSIFs) are derived as functions of geometry of the problem and material characteristics. A parametric analysis of the effects of the location and orientation of the cracks and of the inhomogeneity parameter of FGM’s thermal conductivity on the TSIFs is performed. The results are applicable to such kinds FGMs as ceramic/ceramic FGMs, e.g., TiC/SiC, MoSi₂/Al₂O₃ and MoSi₂/SiC, and also some ceramic/metal FGMs.     

Effect of dispersion method on thermal conductivity and stability of nanofluid

Preparing a stable nanofluid with high thermal conductivity is of a great concern. In order to find an optimum dispersion method to achieve a better performance, five different carbon nanotube (CNT) structures, namely SWNTs (single wall CNT), DWNTs (double wall CNT), FWNTs (few wall CNT) and two different MWNTs (multiwall nanotubes) were synthesized to prepare nanofluids with three different dispersion methods namely functionalization, SDS/ultrasonic probe and SDS/ultrasonic bath. The experiments reveal that the best stability and thermal conductivity are associated with the functionalized nanofluids. Specifically, for the times after 50 h, the functionalized profiles begin to level off due to their higher stability, while the other two paths continue their declining trend.     

功能梯度材料结构热力耦合概率分析方法

基于自洽平均微观力学W-T(Wakashima-Tsukamoto)模型和拉丁超立方抽样,建立一种功能梯度平板热力耦合概率分析方法.以材料物理属性和空间分布的不确定性随机参数作为概率分析的输入,以热应力作为输出,基于本征正交分解POD(Proper Orthogonal Decomposition)对热应力的随机时间历...     

A length-dependent model for the thermomechanical response of ceramics

We present a length-dependent model for the thermomechanical response of ceramics through a concurrent multiscale scheme that accounts for: (i) the locally varying values of the sub-grain thermal conductivity tensor due to the interaction of phonons with microstructural features such as grain boundaries, and (ii) a continuum model of thermal stresses that explicitly resolves the polycrystalline structure of the material. At the sub-grain level, we compute the values of the thermal conductivity tensor using the Boltzmann transport equation under the relaxation time approximation. At the continuum level, the polycrystalline structure of the specimen is resolved explicitly by a finite element mesh and the texture of the polycrystal is assumed to be given. At this level, we adopt a Fourier model of heat conduction which utilizes values of thermal conductivity obtained at the lower scale. The mechanical response of the grains is modeled as elastic and anisotropic. The capabilities of the model are demonstrated through a series of examples, which highlight the potential of our approach for designing materials with improved thermomechanical response.     

材料高温力学性能理论表征方法研究进展

随着科学技术的迅猛发展,材料在高温领域的应用越来越广泛。然而高温下材料的力学性能和常温相比有很大差异,材料的高温力学性能研究和表征已成为当前的研究热点。论文文对材料在高温下力学行为理论表征方法研究的最新进展进行了总结和回顾。着重介绍了近年来高温陶瓷材料的断裂强度、金属材料的屈服强度、弹性模量与本构关系的温度相关性理论表征方法的研究进展。最后,总结已有研究工作的特点和不足之处,对材料高温力学性能理论表征方法的后续研究进行了展望,就进一步研究提供建议。     

考虑碳纳米管非均匀分布的复合材料电导率计算

碳纳米管作为导电相在机敏复合材料中广泛应用,但碳纳米管为团簇材料,在基体中很难均匀分散。本文考虑碳纳米管的非均匀分布特性,提出了计算碳纳米管复合材料电导率的数值方法。通过引入随机谐和函数,建立了碳纳米管体积分数的三维随机场模型。基于细观力学的有效介质理论、Mori-Tanaka方法和H-S界限理论,考虑碳纳米管之间的隧穿效应,发展了复合材料微小体积单元的电导率计算方法。在此基础上,构建了考虑碳纳米管非均匀分布的复合材料等效电导率三维有限元计算模型。数值分析结果与试验值能够很好吻合,表明这一方法可以准确计算碳纳米管复合材料的电导率。本文进一步分析了碳纳米管非均匀分布对复合材料电导率的影响。     

Heat transfer enhancement of PCM melting in 2D horizontal elliptical tube using metallic porous matrix

In this study, the melting process of ice as a phase-change material (PCM) saturated with a nickel–steel porous matrix inside a horizontal elliptical tube is investigated. Due to the low thermal conductivity of the PCM, it is motivated to augment the heat transfer performance of the system simultaneously by finding an optimum value of the aspect ratio and impregnating a metallic porous matrix into the base PCM. The lattice Boltzmann method with a double distribution function formulated based on the enthalpy method, is applied at the representative elementary volume scale under the local thermal equilibrium assumption between the PCM and porous matrix in the composite. While reducing or increasing the aspect ratio of the circular tubes leads to the expedited melting, the 90\(^{\circ }\) inclination of each elliptical tube in the case of the pure PCM melting does not affect the melting rate. With the reduction in the porosity, the effective thermal conductivity and melting rate in all tubes promoted. Although the natural convection is fully suppressed due to the significant flow blockage in the porous structure, the melting rates are generally increased in all cases.     

Composites graphite/sel pour le stockage d'énergie à haute température : étude des effets du graphite et de la microstructure des composites sur les propriétés de changement de phase des sels

Thermal energy storage at high temperature is an efficient way for energy saving in the industrial sector, as well as a key component for power generation based on renewable energy resources. Thermal energy storage technology based on phase change materials (mainly salts) has been identified to meet the requirements of investment costs and compactness. However, the low thermal conductivity of salts (1 W/m/K) could be a limiting factor concerning power. To overcome such a drawback, new materials combining salts with graphite have been developed. Nevertheless, it is important to verify that no degradation of the salts storage properties is induced by their thermal conductivity enhancement. In this Note, the effects of the graphite and the composites graphite/salt microstructure on the phase change properties of salts are analysed. To cite this article: J. Lopez et al., C. R. Mecanique 336 (2008).     

Micromechanics-based thermoelastic model for functionally graded particulate materials with particle interactions

Thermoelastic behavior of functionally graded particulate materials is investigated with a micromechanical approach. Based on a special representative volume element constructed to represent the graded microstructure of a macroscopic material point, the relation between the averaged strains of the particle and matrix phases is derived with pair-wise particle interactions, and a set of governing equations for the thermoelastic behavior of functionally graded materials is presented. The effective coefficient of thermal expansion at a material point is solved through the overall averaged strain of two phases induced by temperature change under the stress-free condition, and is shown to exhibit a weak anisotropy due to the particle interactions within the graded microstructure. When the material gradient is eliminated, the proposed model predicts the effective coefficient of thermal expansion for uniform composites as expected. If the particle interactions are disregarded, the proposed model recovers the Kerner model. The proposed semi-analytical scheme is consistent and general, and can handle any thermal loading variation. As examples, the thermal stress distributions of graded thermal barrier coatings are solved for two types of thermal loading: uniform temperature change and steady-state heat conduction in the gradation direction.     

Analytical approximation for solidification processes in PCM storage with internal fins: imposed heat flux

Uses of thermal energy storage systems have expanded notably in recent decades. In thermal energy systems, internal heat transfer enhancement techniques such as fins are often used because of the low thermal conductivity of the phase change materials (PCMs). In this paper, solidification of a PCM is studied in a rectangular storage with horizontal internal plate fins and an imposed constant heat flux on the vertical walls. A simplified analytical solution is presented and its results are compared to those for a numerical approach based on an enthalpy method. The fraction of solidified PCM in storage is calculated with the derived analytical model which determines how much of the storage is solidified after a certain time. The results show that the analytical model satisfactorily estimates the solid–liquid interface and the temperature distribution for the fin, which are useful in the design of PCM-based thermal energy storage or cooling systems.     

Thermoelastic Stress Analysis of Structures Under Natural Vibrations

The paper focuses on stress analyses of structures subjected to excitation forces operating at resonant frequencies. The structures are analysed experimentally using the Thermoelastic Stress Analysis (TSA) technique. Experiments are carried out for fixed-free beams of different dimensions and materials, and also for a steel rectangular plate with clamped edges. These structures are excited by a shaker via a stinger. For materials with low thermal conductivity, the agreement between the theory, numerical results and experimental results is excellent. As the thermal conductivity of the material is increased, the correspondence is not as close. This is because of non-adiabatic behaviour. The implications of these results are discussed in detail in the paper and a means of deriving the severity of heat transfer is provided. Other factors that influence the TSA results from structures under natural loading are also discussed.     

相变温控导热增强的多孔金属梯度优化设计

高孔隙率金属多孔材料比表面积大、导热性能好且掺混能力强,是理想的相变换热导热增强体材料。增材制造能够精准制备几何高度复杂的微结构,为多孔金属任意梯度设计提供可能。为实现更高的相变换热性能,建立了多孔金属相变温控导热增强的梯度优化设计模型。该优化模型以孔隙率分布为设计变量,以多孔金属用量为约束,以关键位置的温度最低为设计目标,基于考虑相变过程的多孔介质两方程模型为分析方法,通过遗传算法对优化模型进行求解。通过与实验结果的对比,验证了分析方法的有效性。两个具体算例证实了梯度设计能够大幅度提高多孔金属介质导热增强的相变温控性能。     

Effects of the heat transfer fluid velocity on the storage characteristics of a cylindrical latent heat energy storage system: a numerical study

A numerical study of the effects of the thermal fluid velocity on the storage characteristics of a cylindrical latent heat energy storage system (LHESS) was conducted. Due to the low thermal conductivity of phase change materials (PCMs) used in LHESS, fins were added to the system to increase the rate of heat transfer and charging. Finite elements were used to implement the developed numerical method needed to study and solve for the phase change heat transfer (melting of PCM) encountered in a LHESS during charging. The effective heat capacity method was applied in order to account for the large amount of latent energy stored during melting of the PCM and the moving interface between the solid and liquid phases. The effects of the heat transfer fluid (HTF) velocity on the melting rate of the PCM were studied for configurations having between 0 and 18 fins. Results show that the overall heat transfer rate to the PCM increases with an increase in the HTF velocity. However, the effect of the HTF velocity was observed to be small in configurations having very few fins, owing to the large residual thermal resistance offered by the PCM. However, the effect of the HTF velocity becomes more pronounced with addition of fins; since the thermal resistance on the PCM side of the LHESS is significantly reduce by the large number of fins in the system.