Bulk Phase Change Materials Research Articles

Micro-encapsulated phase change material suspensions for heat and mass transfer: A thermo-physical characterization

Despite the growing popularity of phase change materials (PCM) and suspensions of micro-encapsulated phase change materials (mPCM) in both industrial and scientific applications, their properties characterization remains partial and mainly limited to thermal properties. The characterization of such suspension is a crucial aspect for comprehending their behavior and optimizing their performance. This paper is dedicated to a wide characterization of two suspensions containing micro-encapsulated phase change material. In pursuit of a thorough understanding, we also extend our characterization efforts to the bulk PCM encapsulated within the particles.Our primary focus is on determining the materials thermal properties such as latent heat and specific heat capacity using differential scanning calorimetry. Different cooling and heating rates have been employed in our measurements. Regarding our experiments, the bulk phase change material is predominantly identified as octadecane. Results obtained from calorimetry are then compared between the bulk PCM and the mPCM suspensions. By comparing the magnitudes of latent heat obtained for suspensions with the bulk material, we can accurately determine the mass fraction of PCM within each suspension. Noteworthy, during the solidification process an additional latent heat peak is observed only for the encapsulated PCM.The density of the materials is also measured. The phase change of PCM included in the capsules can be observed in densimetry: within the studied temperature range (283.15–306.15 K), the most significant density variations occur during intervals associated with phase change transitions. In ranges where the PCM, included in the capsules, is in a single-phase state (solid/liquid), we provide linear laws that account for density variations with temperature.Finally, our characterization work focuses on the rheological properties of the materials. While the bulk PCM in liquid phase exhibits a Newtonian viscosity, mPCM suspensions present a non-Newtonian viscosity. Both shear-thinning and shear-thickening behaviors are observed depending on the suspension particle volume fraction ϕ of mPCM. This paper concludes with a full comparison of our characterization results with models and correlations proposed in the literature.

Multiscale Porous Architecture Consisting of Graphene Aerogels and Metastructures Enabling Robust Thermal and Mechanical Functionalities of Phase Change Materials

AbstractPassive thermal energy storage systems using phase change materials (PCMs) are promising for resolving temporal‐spatial overheating issues from small‐ to large‐scale platforms, yet their poor shape stability due to solid–liquid transition incurs PCM leakage and weak resistance against mechanical disturbance, limiting practical applications. While foam‐stable templates for PCMs have shown complement, they reveal massive leakage and collapse of liquefied PCMs under external loads or impacts. Herein, a multiscale porous architecture consisting of graphene aerogels (GAs) and meta structures enabling robust thermal‐mechanical functionalities of PCMs (3D‐MPGA) toward sustainable phase change thermal energy storage composites is reported. 3D‐printed mechanical metamaterials employing octet‐truss cells provide supportive strength and directionally‐assisted leakage reduction, while GAs serve as porous templates with surface‐interfacial contacts, thereby fixing paraffin wax as PCM inside their nano/micropores. The 3D‐MPGA shows intrinsic thermal characteristics of bulk PCMs, and improved thermal‐mechanical‐chemical stability, confirmed by long‐term heating‐cooling cycle tests over 10 h. Moreover, it exhibits highly reinforced strength (200–5000%) within a low density across ambient and melting temperatures, and maintains original shapes in the liquefied PCMs without severe leakage, against external loads. This work inspires rational strategies for advancing robust thermal‐mechanical functionalities for PCM‐based thermal energy storage systems.

Investigating temperature change rate and pore confinement effect on thermal properties of phase change materials for de-icing and low-temperature applications in cementitious composites

Incorporation techniques of phase change materials (PCM) in cementitious composites have a significant influence on thermal properties. This study investigated the thermal behavior of low-temperature PCM when subjected to varying temperature change rates and pore confinement inside the porous network of lightweight aggregates (LWA) and encapsulation using melamine-formaldehyde-based polymer. Three categories of thermal energy storage (TES) specimens were prepared: (i) Bulk PCM (i.e., liquid PCM), (ii) micro-encapsulated PCM (MPCM), and (iii) four different LWAs infused with PCM (PCM-LWA). The thermal properties of small-scale individual TES specimens were analyzed using a low-temperature differential scanning calorimeter (LT-DSC) to evaluate the effect of ramp rates. Dynamic vapor sorption (DVS) analysis was utilized to characterize the pore structure of LWAs. LT-DSC results show that undercooling of the PCM significantly increases with the rise in ramp rate for all the specimens; temperature change rate affects the nucleation and crystallization growth process during the phase transition. Pore structure characterization of LWAs indicates that the majority of the pores (> 92 %) were larger than 17.3 nm (i.e., macropores). Confined liquid properties are subjected to modification due to interaction with the confining surfaces, as explained by Gibbs-Thomson theory. PCM incorporated in the LWA porous network experienced variable degree of supercooling during phase transition; magnitude of confinement pressure is dependent on the pore diameter, structure, and tortuosity. Experimental evidence suggested that PCM-LWA will exhibit gradual expulsion of enthalpy of fusion over a larger temperature range (i.e., ∼−5 °C to 4.28 °C) in comparison to MPCM (i.e., ∼4.28 °C).

Design and Development of a PCM-Based Two-Phase Heat Exchanger Manufactured Additively for Spacecraft Thermal Management Systems

For spacecraft thermal management systems, it is crucial to diminish the overall mass of on-board thermal storage system and minimize the temperature fluctuations when the environmental temperature changes drastically. Since there is no atmosphere in outer space, heat can only be rejected to space using radiation (e.g., radiators). The heat sink conditions, and the heating power subjected to be rejected vary continuously at the orbiting stage of the spacecraft. Without thermal storage capability, the radiator is required to be large enough to release the highest power at the hottest of the heat sink. By engaging and integrating phase-change materials (PCMs) into a passive two-phase heat exchanger, the radiator can be designed and sized for the average rather than the maximum power. To meet the NASA needs, the prototype developed and fabricated in the present study is a novel two-phase heat exchanger that integrates PCMs within a vapor chamber through multiple thin drawers loaded with bulk PCM. The entire solid structure of the prototype is manufactured additively using Maraging Steel with the Direct Laser Metal Sintering (DLMS) technique. The chosen working fluid is Methanol to undertake two-phase heat transfer via saturated vapor from the heated surface of evaporator to the condenser surface, and the condensate then returns back to the heated region and keeps recirculated using the capillary pumping pressure performed by the wick structure without any reliance on the gravitational magnitudes. Depending on the heat sink temperature and PCM melting temperature, the developed heat exchanger can either function as a thermal capacitor or a two-phase heat exchanger with thermal storage capability, that is, two modes of operation to be engaged within the spacecraft's launching/landing stages or orbiting stage, respectively.In the present study, the geometry and number of the PCM-loaded drawers enclosed inside the heat exchanger's casing are optimized based on the mass ratio (the ratio of PCM mass to the total heat exchanger mass), additive manufacturing (AM) constraints, and total thermal resistances imposed on the system for either of the operation modes. The AM printed prototype with its all components are also represented along with technical drawings and specifications. Furthermore, the support structure model generated for metal printing of the proposed heat exchanger will be exposed and discussed. The wick structure design procedure is also presented and its main performance characteristics (e.g., effective pore size, effective thermal conductivity, porosity, and permeability) are reported to analytically examine the capillary pumping limit for the proposed heat exchanger.

Multi-Scale Numerical Modelling for Predicting Thermo-Physical Properties of Phase-Change Nanocomposites for Cooling Energy Storage

One major limitation of phase-change materials (PCM) for thermal energy storage comes from their poor thermal conductivity hindering heat transfer process and power density. Nanocomposites PCMs, where highly conductive nanofillers are dispersed into PCM matrices, have been exploited in the past decades as novel latent heat storage materials with enhanced thermal conductivity. A computational model based on continuum simulations capable to link microscopic characteristics of nanofillers and the bulk PCM with the macroscopic effective thermal conductivity of the resulting nanocomposite is the aim of this work. After preliminary mean-field simulations investigating the impact of the nanofiller aspect ratio on the thermal conductivity of the nanocomposite, finite element simulations at reduced aspect ratios have been performed with corrected thermal conductivity values of the filler, to take into account the thermal interface resistances between fillers and matrix. Finally, the thermal conductivity at the actual aspect ratios has been extrapolated by the results obtained at reduced aspect ratios thus saving computational time and meshing efforts. This method has been validated through comparison against previous literature evidence and new experimental characterizations of nanocomposite PCMs.

Understanding molecular motion mechanism of phase change materials in mesoporous MCM-41

Abstract Although nanoporous shape-stabilized composite phase change materials (PCMs) could efficiently address the leakage issue of pure PCMs during the solid-liquid phase transition process, numerous experiments have verified that the thermophysical parameters of the nanoporous shape-stabilized composite PCMs are significantly different from those of macroscopic bulk PCMs due to the nanoconfinement effects. However, macroscopic experimental data cannot directly reflect the microscopic interaction mechanisms in composite PCMs. Herein, to systematically understand the molecular motion mechanisms of PCMs with different functional groups (octadecane, octadecylamine, octadecanol and stearic acid) confined in the mesoporous MCM-41, we utilize molecular dynamics to simulate the spatial distribution and motion states of surface adsorbed PCMs molecules. This study aims to systematically reveal the interaction mechanisms between mesoporous MCM-41 and surface adsorbed PCMs at the molecular and atomic levels, thus providing constructive theoretical guidance and reference data for the targeted design and preparation of high-performance MCM-41-based composite PCMs.

An intrinsically flexible phase change film for wearable thermal managements

Phase change materials (PCMs) involving significant amounts of latent heat absorbing and releasing at a constant transition temperature have been extensively utilized for thermal management of electronic devices. However, it is still a great challenge to apply thermal management for wearable devices using PCMs due to their solid rigidity and liquid leakage. Although considerable efforts have been dedicated to constructing flexible composite PCMs by physically blending bulk PCMs with different flexible polymers, design and fabrication of intrinsically flexible PCM films still remain unexplored. Herein, we report an intrinsically flexible PCM film with apparent solid-solid phase transition performance, outstanding self-support and shape-conformable property for wearable thermal management. Remarkably, the as-obtained PCM film behaves adjustable phase transition enthalpy and temperature in the region from about (5 to 60)°C, long-term cycling stability up to 1000 cycles, highly mechanical flexibility and remarkable shape tailorability and foldability. Notably, such flexible PCM films are easily integrated into wearable devices with a flexible graphene film as thermal source, revealing superior temperature control behaviors, together with unprecedented electro-thermal and photo-thermal energy conversion performance. The intrinsically flexible PCM film developed in this work is holding great potential for next-generation flexible thermal management electronics.

Physical models to evaluate the performance of impure phase change material dispersed in building materials

Abstract Currently, the introduction of phase change materials (PCM) in building materials is recognized as a popular technique for reducing interior temperature fluctuations and improving the energy efficiency of buildings. Conventionally, PCM in construction materials is different with bulk PCM as it incorporates impurities and acts as a binary mixture. For this reason, physical models to describe heat transfers inside such materials are highly requested. This paper presents four physical approaches that can be applied to examine heat transfer in impure PCM dispersed inside a building material. These models have been successfully verified and tested using experimental results from the literature. A significant hysteresis is observed during the phase transition as a result of thermal gradients within the composite material as well as the non-symmetric enthalpy profile as a function of temperature.

Thermal buffering effect of a packaging design with microencapsulated phase change material

Temperature fluctuations during storage and transportation are the most important factors affecting quality and shelf life of food products. Phase change materials (PCM) with their isothermal characteristics are used to control temperature in various thermal operations. In this study, octanoic acid as PCM candidate was used in a packaging material design for thermal control of a food product. The PCM candidate was microencapsulated in different shell materials in our laboratory. Among the synthesized microcapsules, microencapsulated PCM (mPCM) (ΔHm = 42.9 J/g) with styrene polymer as the shell material was selected based on its properties of being cost effective and compatibility with human health. Thermal buffering effect of PCM in bulk and microencapsulated forms was tested in a packaging design with special PCM pockets. Results showed that packages with mPCM and bulk PCM provided 8.8 and 6 hours of thermal buffering effect for 160 g of chocolate compared with the package without PCM (reference package).

Experimental investigation of thermal performance of microencapsulated PCM-contained wallboard by two measurement modes

Phase change materials (PCM) as a smart, feasible latent heat storage material has been utilized to reduce the energy demand and improve the thermal comfort of the buildings when incorporated into the building materials. Conventionally, the thermal behavior of the PCM mixed with the solid matrix (mortar or gypsum) is different with the bulk PCM. In this study, we designed an experimental apparatus to characterize the thermal performance of PCMs when they were utilized in their implementation state. Firstly, we investigated the effective thermal conductivity (λ) and the apparent specific heat (Cp) of the PCM-contained wallboard and the common gypsum wallboard, which were dependent on the temperature variation. Dynamic method was used to determine the phase change temperature, and the phase change process of the PCMs-wallboard was studied. Then, an “analogous step method” was proposed to determine the H(t) curves of the PCMs-wallboard, and the results were compared with the dynamic method. Finally, three methods of obtaining H(t) curves were compared and analyzed. From the obtained results, it can be concluded that when adopting the dynamic method appropriate heating rate (according to real application scenario) is necessary. When selecting the analogous step method, reducing the temperature interval can increase the accuracy of the H(t) even though it takes a lot of time. Anyway, both ways are more suitable for real wallboard test, comparing with the theoretical calculation method. The goal of our work is to investigate an appropriate method to evaluate the conductivities, apparent heat capacities, H(t) curves, and the phase change temperature of PCM-contained wallboards in their real application.

Preparation and enhanced thermal performance of novel (solid to gel) form-stable eutectic PCM modified by nano-graphene platelets

This study presents the development of form-stable eutectic mixtures, modified with nanoscale structures for enhanced thermal performance. These additives may result in the next generation of phase change materials (PCMs) for thermal energy storage systems. An appropriate gelling or thickening agent (2-hydroxypropyl ether cellulose) is introduced so that the PCM will lose its fluidity, become form-stable, and the liquid leakage problem will be overcome. Nano-graphene platelets (NGPs) are added in order to enhance the thermal properties and overall heat transfer. Differential scanning calorimetry (DSC) was carried out for the thermal analysis of the PCMs. The paper experimentally studied in detail the enhanced thermo-physical properties required for stimulating and modelling the PCM in energy storage applications such as specific heat, thermal diffusivity, thermal conductivity, enthalpy, and density. The principle of the T-history method was applied using a parallel plate heating/cooling guarded plate apparatus to determine the true phase transition temperatures of bulk PCM. The supercooling of the enhanced shape stable mixture was found to be less than 0.1°C. The thermal reliability test indicated that the enhanced form-stable eutectic mixture had reliable thermal performance over a postulated lifetime of 80 years. As a result, the developed form stable PCM eutectic mixture is a promising material for thermal energy storage.

Novel CFD-based numerical schemes for conduction dominant encapsulated phase change materials (EPCM) with temperature hysteresis for thermal energy storage applications

Encapsulated phase change materials (EPCM) are the most common way to integrate with thermal systems for energy storage applications. Encapsulation greatly alters the thermal response of phase change materials (PCM) in terms of phase change temperatures and thermal hysteresis. Existing numerical schemes; however, can only simulate bulk PCM behavior and ignore the influence of encapsulation on the thermal response of EPCM. In this study, novel computational fluid dynamics (CFD)-based conduction dominant numerical schemes are developed for the first time to model the thermal response of EPCM and validated with the experimental DSC curve of the in-house fabricated EPCM capsules. The proposed heat source/sink scheme successfully predicts the heat-temperature responses and liquid volume fraction of EPCM with thermal hysteresis. It is recommended that the CFD-based conduction dominant heat source/sink scheme developed for EPCM in current study should be incorporated into energy simulation softwares for accurate performance predication when EPCM capsules are expected to be used in thermal energy storage systems and applications.

Novel microstructured polyol–polystyrene composites for seasonal heat storage

We propose a robust route to prepare novel supercooling microstructured phase change materials (PCMs) suitable for seasonal thermal energy storage (STES) or heat protection applications. Two supercooling polyols, erythritol and xylitol, are successfully prepared as novel microencapsulated PCM-polystyrene composites with polyol mass fractions of 62wt% and 67wt%, respectively, and average void diameter of ∼50μm. Thermal properties of the composites and bulk polyols are studied thoroughly with differential scanning calorimetry (DSC) and thermal conductivity analyzer. Significant differences in heat storage properties of microstructured and bulk PCM are observed. The heat release of microstructured erythritol is more controlled than that of bulk PCM, making the novel microengineered PCMs particularly interesting for STES. In the case of bulk PCM, the heat release may occur spontaneously due to crystallization by surface roughnesses or impurities, whereas these factors have only little impact on the crystallization of microstructured erythritol, making the novel composite more reliable for long-term heat storage purposes. In addition, microstructured polyol–polystyrene composites show anomalous enhancement in the specific heat as compared to bulk polyols. This enhancement may originate from strong polyol–surfactant interactions in the composites.

Natural convection in an internally finned phase change material heat sink for the thermal management of photovoltaics

Elevated operating temperatures reduce the solar to electrical conversion efficiency of building integrated photovoltaic devices (BIPV). Phase change materials (PCM) can be used to passively limit this temperature rise although their effectiveness is limited by their low thermal conductivities and crystallisation segregation during solidification. This paper presents an experimental evaluation of the effects of convection and crystalline segregation in a PCM as a function of efficiency of heat transfer within the finned PV/PCM system. The thermal performances of bulk PCM with crystallisation segregation for different internal fin arrangements are presented. It is noted that the addition of internal fins improves the temperature control of the PV in a PV/PCM system.