Potential applications of phase change materials in concrete technology

In internal curing, pre-wetted lightweight aggregates (LWA) serve as internal reservoirs to supply the extra water needed by the cementitious and pozzolanic components of the concrete during their hydration processes. Due to their porous nature and reasonably high absorption capacity, the LWA can also be filled with other materials, such as phase change materials (PCMs). In this paper, three potential applications of PCM-filled LWA in concrete technology are presented. In addition to the previously explored application of increasing the energy storage capacity of concrete in residential and commercial construction by using a PCM with a transition temperature near room temperature, applications for higher and lower temperature PCMs also exist. In the former case, a PCM can be used to reduce the temperature rise (and subsequent rate of temperature decrease) of a large concrete section during (semi)adiabatic curing, to minimize thermal cracking, etc. In the latter case, a PCM can perhaps reduce the number or intensity of freeze/thaw cycles experienced by a bridge deck or other concrete exposed to a winter environment. In this paper, these latter two applications are preliminarily explored from both experimental and modeling viewpoints.     

Heat transfer study of submicro-encapsulated PCM plate for food packaging application

In recent studies, encapsulated phase change materials (PCM) were developed as novel materials for food packaging because of their improved thermal insulation capacity. The PCMs (often liquid in room temperature) are encapsulated in a shell material so as they can be practically handled. In this work, the thermal behaviour of an encapsulated PCM material (Rubitherm RT5 encapsulated in polycaprolactone PCL) with two different PCM mass fractions was studied. The model was validated by experimental cooling and heating processes, under controlled air temperature conditions. The numerical result demonstrated a better thermal buffering capacity of the encapsulated PCM material compared to a standard one (cardboard).     

Finite element cohesive fracture modeling of airport pavements at low temperatures

Low temperature cracking induced by seasonal and daily thermal cyclic loads is one of the main critical distresses in asphalt pavements. The safety of aircraft departure and landing becomes a crucial issue in runways when thermal cracks occur in airport pavements. The low-temperature fracture behavior of airport pavements was investigated using a bilinear cohesive zone model (CZM) implemented in the finite element method (FEM). Nonlinear temperature gradients of pavement structures were estimated based on national weather data and an integrated climate prediction model. Experimental tests were conducted to obtain the numerical model inputs such as viscoelastic and fracture properties of asphalt concrete using creep compliance tests, indirect tensile strength tests (IDT), and disk-shaped compact tension (DC(T)) tests. The finite element pavement fracture models could successfully predict the progressive crack behavior of asphalt pavements under the critical temperature and heavy aircraft gear loading conditions.     

Integrating phase change materials into concrete through microencapsulation using cenospheres

Phase change materials (PCMs) can enhance the building energy efficiency through thermal energy storage and thermal regulation. Microencapsulated PCMs (MEPCMs) provide a better utilization of PCMs with building materials. This study proposes a novel method to encapsulate PCMs into cenospheres which are hollow fly ash particles generated in coal burning power plants with size ranging from a few micrometers to hundreds of micrometers. The shell of the cenosphere inherently has some small pores which are sealed by a thin layer of glass-crystalline film. By removing this film through chemical etching, these holes can be exposed, providing paths for PCMs moving into the internal void of cenospheres. A thin layer of silica is coated on the PCM loaded cenospheres to prevent the possible leakage of liquid PCMs. The produced PCM microcapsules are referred to as CenoPCM, which can be directly added into traditional construction and building materials such as concrete to produce thermally active concrete. Prototype thermally active cement mortar integrated with the produced CenoPCM capsules have also been manufactured and characterized for its mechanical and microstructural properties. The characterizations showed that there was only minor reduction in strength and the mortar remained strong enough for building application. From this work, it is found that the produced CenoPCM capsules have great potential to be added into construction materials for reducing energy consumptions in buildings.     

On the feasibility of using phase change materials (PCMs) to mitigate thermal cracking in cementitious materials

Temperature changes driven by hydration reactions and environmental loading are a leading cause of thermal cracking in restrained concrete elements. This work describes preliminary investigations on the use of microencapsulated phase change materials (PCMs) as a means to mitigate such thermal cracking. Special attention is paid to quantify aspects of: heat absorption and release, the development of unrestrained/restrained thermal stresses and strains and the mechanical properties including: compressive strength, elastic modulus and fracture behavior. First, PCMs incorporated in cementitious systems absorb and release heat, which scales as a function of their dosage and enthalpy of phase change. Second, for restrained and unrestrained conditions and for equal temperature change, the thermal deformation and stresses developed are noted to be similar to a plain cement system independent of the PCM dosage. However, PCM additions are noted to reduce the rate of deformation and stress development so long as the phase transition is active. Third, while the presence of PCMs does depress the compressive strength and elastic modulus (in increasing proportion with dosage), the fracture toughness is impacted to a lesser degree. While of a preliminary nature, the studies highlight a novel means of exploiting phase transitions to control thermal stress evolutions in restrained elements.     

A case study on diurnal and seasonal variation in pavement temperature

An empirical numerical model was established for analysing the temperature variation of pavements in Xi'an area of China. This model can consider the influence of solar radiation, atmospheric temperature and humidity. It was verified by comparing the calculated and measured temperature of the natural ground surface. The effect of seasonal variations of temperature on asphalt and concrete pavement surfaces were then calculated. The temperature distribution of both asphalt and concrete pavements in January and July was investigated using the model. For each type of pavement, four groups of different pavement materials were considered to investigate the influence of thermal parameters for pavement materials on the temperature distribution. Furthermore, the diurnal variation of pavement temperature was analysed and discussed based on the normal climate characteristics in Xi'an. The results of the analysis showed that the diurnal variation of pavement temperature is very significant and must be considered in the design of a pavement.     

相变储能材料的研究进展与应用

相变储能技术对于能源的开发和合理利用具有重要意义，在太阳能利用、工业余热回收等方面有着显著的优点。综述了相变材料的研究进展，讨论了固—液相变、固—固相变储能材料的特性及其应用。固—液相变材料一般可分为无机和有机两种类型，其中无机类储能材料主要为无机盐水合物，它具有较大的溶解热和导热系数，但易出现“过冷”和“相分离”现象；有机类储能材料虽然避免了上述缺陷，但其导热性较差、溶解热较低。固—固相变材料种类较少，其中以多元醇应用最为广泛。探讨了这方面研究的发展方向，展望了储能技术市场化应用的前景。     

相变材料在夏热冬冷地区建筑围护结构中应用的性能研究

相变材料(Phase change material,PCM)在建筑围护结构中的应用是一种改善室内热环境和居住舒适度的有效方法。为探索相变材料在夏热冬冷地区应用的节能潜力,对比研究了不同相变材料应用方法的节能效果,以能源使用强度(Energy use intensity,EUI)评价不同应用方法的建筑性能,旨在找出最优的应用方法。所用的相变材料以高密度聚乙烯球封装,并嵌入在XPS保温板中,形成XPSPCM板。研究结果表明:在制冷季,将XPSPCM板安装在建筑物墙体内表面时的EUI比其装在建筑物墙体外表面时降低了0.27~0.66kWh/m2,采暖季的降低幅度为0.68~0.88kWh/m2。综合考虑全年工况时,当XPSPCM板安装至建筑物墙体靠近内表面时EUI值最小。以EUI雷达图对比XPSPCM板集中于建筑不同朝向的应用效果,结果显示对于熔点为25℃的相变材料,相变材料集中布置于西向外墙时时建筑能耗最低。     

Preparation and Thermodynamic Properties of Camphene/Stearic Acid Composites as Phase-Change Materials in Buildings

In this study, a series of shape-stabilized phase-change materials (PCMs) of camphene/stearic acid (CS) were prepared and their thermal properties were measured by differential scanning calorimetry. The results indicated that the mixture consisting of 60 mass% camphene and 40 mass% stearic acid is the most favorable as a PCM, in terms of the phase-change temperature and latent heat. Thereafter, the CS was absorbed in fly ash, pyroclastic, barite, and marble powder, which acts as a supporting material, to prepare four kinds of composite-based PCMs. DSC, FT-IR, and scanning electron microscopy measurements were made to investigate the structures and properties of the PCMs. DSC results showed that the latent heats of melting and freezing of the composite PCMs were sharply decreased. Morphology and structural characterization revealed that, in form-stable PCMs, the dispersion of the supporting materials in the camphene/stearic acid matrix is homogeneous and there is no chemical interaction between the CS and composites. The composite PCMs showed excellent thermal stabilities and reliabilities, when their phase-change temperatures were concerned. These indicate that the prepared composite-based PCMs are suitable for thermal energy storage because of their applicable temperature range, thermal reliability, and chemical stability.     

Incorporating phase change materials in concrete pavement to melt snow and ice

This paper discusses the use of phase change materials (PCM) in concrete pavement as a method to store energy which can be used as a heat source during cooling events to melt ice/snow. The experimental program includes: (1) use of low-temperature differential scanning calorimetry to evaluate thermal properties of PCM, and (2) use of large-scale concrete slabs containing PCM to evaluate the ability of the PCM concrete to melt snow on the surface of the concrete pavement. The temperature in the concrete slabs and the snow melting rate were monitored as quantitative measurements of the efficiency of the PCM in the concrete. In addition, time-lapse images were taken. Two approaches were used to incorporate PCM in concrete: placing the PCM in lightweight aggregate (LWA) which was then mixed into the concrete, and placing the PCM in embedded metal pipes embedded in the slab during concrete casting. In this study, paraffin oil was use as a PCM that is effective in releasing heat near the freezing temperature of PCM when the PCM undergoes a phase transformation from liquid to solid. The heat released during the phase transformation can melt ice and snow on the concrete pavement surface. The results indicate that incorporating PCM in concrete pavement is not only feasible, but also practical.     

Uncertainty and sensitivity analysis of properties of phase change micro/nanoparticles for thermal protection during cryosurgery

Based on our previous study which tested the feasibility of protecting the healthy tissue around the cancerous tissue during cryosurgery by microencapsulated phase change materials (PCMs) with large latent heat and low thermal conductivity, uncertainties and sensitivities for thermal protection efficiency caused by the deviations of the PCM properties’ values, the PCMs concentration and the distance between the PCMs domain and the tumor domain were further investigated in this study. The preliminary results showed that the radius of the micro/nano PCM particle, the upper and lower phase transition temperatures of the PCM and the distance between the PCMs domain and the tumor domain should be accurately measured before performing thermal protection by PCMs during cryosurgery. Less than 20?% deviations of the heat capacities of solid and liquid PCM almost had no obvious influence on the thermal protection efficiency. The results obtained in this study will further help us to optimize the protection protocol by PCMs before performing cryosurgery.     

考虑水泥水化放热与太阳辐射的混凝土路面板温度场数值模拟

为了考察不同强度等级混凝土路面板不同季节、不同浇注时间的温度变化规律,建立了基于有限差分方法的综合考虑水化放热以及太阳辐射、大气温度变化等环境作用下路面板温度场数值计算模型。计算结果显示:混凝土路面板温度受太阳辐射和大气温度影响呈周期性变化,白天主要受太阳辐射影响,夜间主要受大气温降影响,浇注初期的水化放热亦对早期温升有贡献。混凝土路面板顶面与底面温度差以1d 为周期正负循环。其中温度变化幅度及上下面最大温差以夏季最大、秋冬次之、冬季最小。下午2 点浇注和采用较低强度的混凝土可抑制路面板的早期温升和温度梯度。     

The influences of soft and stiff inclusions on the mechanical properties of cementitious composites

The embedment of microencapsulated phase change materials (PCMs) is a promising means for improving the thermal inertia of concrete. However the addition of such soft microcapsules degrades the mechanical properties, i.e., the elastic moduli and compressive strength, of cement-based composites. This study experimentally quantifies the effects of stiff quartz inclusions and soft PCM microcapsules, individually, and when added together, on the mechanical properties of cementitious composites. In addition, a variety of effective medium approximations (EMAs) were evaluated for their ability to predict the experimentally measured composite effective moduli. The EMAs proposed by Hobbs and Garboczi and Berryman (G-B) reliably estimate experimental data. The experimental data and the EMAs were applied to develop a design rule for performance equivalence, such that the composite modulus of elasticity can be maintained equivalent to that of the cementitious paste matrix, in spite of the addition of soft PCM microcapsules.     

Measurements of Heat Capacity and Enthalpy of Phase Change Materials by Adiabatic Scanning Calorimetry

Phase change materials (PCMs) are substances exhibiting phase transitions with large latent heats that can be used as thermal storage materials with a large energy storage capacity in a relatively narrow temperature range. In many practical applications the solid–liquid phase change is used. For applications accurate knowledge of different thermal parameters has to be available. In particular, the temperature dependence of the enthalpy around the phase transition has to be known with good accuracy. Usually, the phase transitions of PCMs are investigated with differential scanning calorimetry (DSC) at fast dynamic scanning rates resulting in the effective heat capacity from which the (total) heat of transition can be determined. Here we present adiabatic scanning calorimetry (ASC) as an alternative approach to arrive simultaneously at the equilibrium enthalpy curve and at the heat capacity. The applicability of ASC is illustrated with measurements on paraffin-based PCMs and on a salt hydrate PCM.     

A comparative study of production control mechanisms using simulation-based multi-objective optimisation

There exist many studies conducted to compare the performance of different production control mechanisms (PCMs) in order to determine which one performs the best under different conditions. Nonetheless, most of these studies suffer from the problems that the PCMs are not compared with their optimal parameter settings in a truly multi-objective context. This paper describes how different PCMs can be compared under their optimal settings through generating the Pareto-optimal frontiers, in the form of optimal trade-off curves in the performance space, by applying evolutionary multi-objective optimisation to simulation models. This concept is illustrated with a bi-objective comparative study of the four most popular PCMs in the literature, namely Push, Kanban, CONWIP and DBR, on an unbalanced serial flow line in which both control parameters and buffer capacities are to be optimised. Additionally, it introduces the use of normalised hyper-volume as the quantitative metric and confidence-based significant dominance as the statistical analysis method to verify the differences of the PCMs in the performance space. While the results from this unbalanced flow line cannot be generalised, it indicates clearly that a PCM may be preferable in certain regions of the performance space, but not others, which supports the argument that PCM comparative studies have to be performed within a Pareto-based multi-objective context.     

The influence of the type of loading and temperature on the modulus of elasticity of cement-bound mixes of milled bituminous concrete and crushed aggregates

Combinations in various proportions of milled bituminous concrete and crushed limestone aggregates were mixed in the laboratory with 5% by mass cement and 5% by mass water to simulate some of the materials that can be obtained when mix-in-place recycling with cement is used. The influence of the temperature and of the type of loading (different rates of stress application of ramp loading and different frequencies of sinusoidal loading) on the modulus of elasticity is examined. It is concluded that as the milled bituminous concrete content in the mix is increased, both the temperature and the rate of stress application increasingly influence the results, and this should be taken into consideration when specifying or designing pavements incorporating cement-bound materials of this type.     

Multiphysics design optimization model for structural walls incorporating phase-change materials

The development of energy-efficient building envelopes has been an ongoing effort in many countries owing to the pressing need to achieve energy independence. In this study numerical optimization techniques and finite element analysis provide the means to find a compromise point between adding phase-change materials (PCMs) to a concrete wall, the energy savings and the wall's structural capacity. The primary objective is to minimize the overall lifetime cost of a wall by understanding the implications of PCM layer thickness, material properties and position in the wall on the overall energy consumption. While it is difficult to manually configure a typical wall for the lowest total cost, the developed computational framework provides an automated tool for searching for the best design. The results show that successful designs can be obtained where material and energy costs can be minimized through a judicious combination of existing building materials with thermal energy storage materials.     

The effect of loading time on flexible pavement dynamic response: a finite element analysis

Dynamic response of asphalt concrete (AC) pavements under moving load is a key component for accurate prediction of flexible pavement performance. The time and temperature dependency of AC materials calls for utilizing advanced material characterization and mechanistic theories, such as viscoelasticity and stress/strain analysis. In layered elastic analysis, as implemented in the new Mechanistic-Empirical Pavement Design Guide (MEPDG), the time dependency is accounted for by calculating the loading times at different AC layer depths. In this study, the time effect on pavement response was evaluated by means of the concept of “pseudo temperature.” With the pavement temperature measured from instrumented thermocouples, the time and temperature dependency of AC materials was integrated into one single factor, termed “effective temperature.” Via this effective temperature, pavement responses under a transient load were predicted through finite element analysis. In the finite element model, viscoelastic behavior of AC materials was characterized through relaxation moduli, while the layers with unbound granular material were assumed to be in an elastic mode. The analysis was conducted for two different AC mixtures in a simplified flexible pavement structure at two different seasons. Finite element analysis results reveal that the loading time has a more pronounced impact on pavement response in the summer for both asphalt types. The results indicate that for reasonable prediction of dynamic response in flexible pavements, the effect of the depth-dependent loading time on pavement temperature should be considered.     

Improving performance of household refrigerators by incorporating phase change materials

Efforts to increase energy efficiency of refrigerators shall directly reduce energy consumption in residential buildings. Incorporating phase change materials (PCM) is a new approach to improve the performance of refrigerators. In this study, we have tested four different PCMs in two different refrigerator models. Compressor on/off time was optimized and better energy efficiency was achieved. Increasing condenser surface area by 20% enhanced the PCM effect. The use of only 0.95 kg of PCM has resulted in a 9.4% energy saving. Economic analyses show that using PCMs in household refrigerators is clearly a cost effective method that saves energy and reduces harmful emissions.     

Advanced Optical Programming of Individual Meta‐Atoms Beyond the Effective Medium Approach

Nanometer‐thick active metasurfaces (MSs) based on phase‐change materials (PCMs) enable compact photonic components, offering adjustable functionalities for the manipulation of light, such as polarization filtering, lensing, and beam steering. Commonly, they feature multiple operation states by switching the whole PCM fully between two states of drastically different optical properties. Intermediate states of the PCM are also exploited to obtain gradual resonance shifts, which are usually uniform over the whole MS and described by effective medium response. For programmable MSs, however, the ability to selectively address and switch the PCM in individual meta‐atoms is required. Here, simultaneous control of size, position, and crystallization depth of the switched phase‐change material (PCM) volume within each meta‐atom in a proof‐of‐principle MS consisting of a PCM‐covered Al–nanorod antenna array is demonstrated. By modifying optical properties locally, amplitude and light phase can be programmed at the meta‐atom scale. As this goes beyond previous effective medium concepts, it will enable small adaptive corrections to external aberrations and fabrication errors or multiple complex functionalities programmable on the same MS.