A review of experimental/computational studies to enhance the thermal conductivity of phase change materials (PCM) that were conducted over many decades is presented. Thermal management of ...electronics for aeronautics and space exploration appears to be the original intended application, with later extension to storage of thermal energy for solar thermal applications. The present review will focus on studies that concern with positioning of fixed, stationary high conductivity inserts/structures. Copper, aluminum, nickel, stainless steel and carbon fiber in various forms (fins, honeycomb, wool, brush, etc.) were generally utilized as the materials of the thermal conductivity promoters. The reviewed research studies covered a variety of PCM, operating conditions, heat exchange and thermal energy storage arrangements. The energy storage vessels included isolated thermal storage units (rectangular boxes, cylindrical and annular tubes and spheres) and containers that transferred heat to a moving fluid medium passing through it. A few studies have focused on the marked role of flow regimes that are formed due to the presence of thermally unstable fluid layers that in turn give rise to greater convective mixing and thus expedited melting of PCM. In general, it can be stated that due to utilization of fixed high conductivity inserts/structures, the conducting pathways linking the hot and cold ends must be minimized.
A review of studies focused on enhancing the thermal conductivity of phase change materials (PCM) for thermal energy storage upon introduction of nanostructures is presented. These emerging materials ...have only been studied since 2005 and represent a clear departure from previous/existing practices of utilizing fixed, stationary high-conductivity inserts/structures into PCM. Carbon-based nanostructures (nanofibers, nanoplatelets and graphene flakes), carbon nanotubes, both metallic (Ag, Al, C/Cu and Cu) and metal oxide (Al2O3, CuO, MgO and TiO2) nanoparticles and silver nanowires have been explored as the materials of the thermal conductivity promoters. Emphasis of the work so far has been placed on the dependence of the enhanced thermal conductivity on mass fraction of the nanostructures and temperature for both liquid and solid phases, however issues related to modifications of the degree of supercooling, melting temperature, viscosity, heat of fusion, etc. are also reported. In general, carbon-based nanostructures and carbon nanotubes exhibit far greater enhancement of thermal conductivity in comparison to metallic/metal oxide nanoparticles due to the high aspect-ratio of these nanofillers. Utilizing a figure of merit for the observed thermal conductivity enhancement, the majority of 340+ measured data points in both liquid and solid phases are summarized.
Effective thermal conductivity of composites of graphite foam infiltrated with phase change materials (PCM) was investigated numerically and experimentally. Graphite foam, as a highly-conductive, ...highly-porous structure, is an excellent candidate for infiltrating PCM into its pores and forming thermal energy storage composites with improved effective thermal conductivity. For numerical simulation, the graphite structure was modeled as a three-dimensional body-centered cube arrangement of uniform spherical pores, saturated with PCM thus forming a cubic representative elementary volume (REV). Thermal analysis of the developed REV was conducted for unidirectional heat transfer and the total heat flux was determined, which leads to the effective thermal conductivity evaluation. For experimental verification, a sample of graphite foam was infiltrated with PCM. The effective thermal conductivity was evaluated using the direct method of measuring temperature within the sample under fixed heat flux in unidirectional heat transfer. The results indicate a noticeable improvement in the effective thermal conductivity of composites compared to the PCM. Our numerical and experimental results are in agreement and are also consistent with reported experimental results on graphite foam. Moreover, the role of natural convection within the pores is found to be negligible.
Molecular dynamics simulations were utilized to investigate the relationship between the structure of paraffins in solid and liquid states and its thermal conductivity. We observe that upon ...crystallization, a nanocrystalline paraffin structure develops and the value of thermal conductivity doubles, in agreement with experimental data. The introduction of carbon nanotubes or graphene layers leads to liquid ordering and associated thermal conductivity enhancement. More prominently, carbon nanofillers provide a template for directed crystallization and lead to even greater thermal conductivity increases. Our results indicate that introducing carbon nanotubes and graphene into long-chain paraffins leads to a considerable enhancement in thermal conductivity, not only due to the presence of a conductive filler, but also due to the filler-induced alignment of paraffin molecules.
•Thermal conductivity of eicosane-based phase change materials was enhanced by suspending silver nanoparticles.•Three different solidification routes, namely ice-water bath, room temperature and oven ...solidification were tested.•The oven solidification route samples exhibited the highest values while the ice-water samples showed the least increase.•For loadings greater than 2wt%, thermal conductivity varied non-monotonically with weight fraction of nanoparticles.•A decrease in the latent heat and the melting point of the samples was observed as the additives loading increased.
Thermal conductivity of eicosane-based phase change materials was enhanced by suspending highly-conductive silver nanoparticles. Three batches of solid eicosane-silver samples with mass fractions (0, 1, 2, 3.5, 5, 6.5, 8 and 10wt%) of nanoparticles were obtained under three different solidification routes: ice-water bath, room temperature and oven solidification. The transient plane source technique was used to measure the thermal conductivity at different temperatures starting at 10°C and ending close to the melting point of each sample. Results showed an increase in the value of thermal conductivity as the temperature increased, and when close to melting point, a sharp rise in thermal conductivity was observed. Also, the slowly-prepared oven solidification route samples exhibited the highest thermal conductivity values while the ice-water samples that were prepared quickly showed the least increase. For samples with an additive loading greater than 2wt%, a non-monotonic relationship was obtained between the thermal conductivity and the weight fraction of Ag nanoparticles, regardless of the method of solidification. In addition to thermal conductivity measurement, the latent heat of fusion of the samples was investigated, utilizing differential scanning calorimetry. Results exhibited a decrease in the latent heat and the melting point of the samples as the additives loading increased due to the decrease in the number of molecules of eicosane in the samples.
An experimental and computational investigation directed at understanding the role of buoyancy-driven convection during constrained melting of phase change materials (PCM) inside a spherical capsule ...is reported. The computations are based on an iterative, finite-volume numerical procedure that incorporates a single-domain enthalpy formulation for simulation of the phase change phenomenon. A Darcy’s Law-type porous media treatment links the effect of phase change on convection. Paraffin wax
n-octadecane was constrained during melting inside a transparent glass sphere through the use of thermocouples installed inside the sphere. The melting phase front and melting fraction of the PCM are analyzed and compared with numerical solution obtained from the CFD code Fluent. Following a short period of symmetric melting due to prominence of diffusion, expedited phase change in the top region of the sphere and a wavy surface at the bottom of the PCM are observed. The computational predictions point to the strong thermal stratification in the upper half of the sphere that results from rising of the molten liquid along the inner surface of the sphere thus displacing the colder fluid. The waviness and excessive melting of the bottom of the PCM is shown to be underestimated by the experimental observation. This discrepancy is linked to the use of a support structure to hold the sphere. Measured temperature data and computational results near the bottom indicate the establishment of an unstable fluid layer that promotes chaotic fluctuations and is responsible for waviness of the bottom of the PCM. On the other hand, the comparison between the measured and computed temperatures in the top half of the sphere show the stable nature of the molten liquid layer. The computational results start to deviate from the thermocouple readings as one moves lower from the top of the sphere. This delay in predicting the melting instant is linked to the thermal stratification within the “constant temperature bath” that encloses the capsule.
Melting of nano-enhanced phase change materials (NePCM) inside an annular cavity formed between two circular cylinders is investigated experimentally and numerically. The inner cylindrical tube is ...subjected to a constant heat flux, while the outer shell is thermally insulated. The phase change material (PCM) used is n-octadecane that is dispersed with CuO nanoparticles as thermal conductivity enhancer (TCE). The experiments involved recording of temperatures at different radial and angular locations inside the test cell, thus allowing for tracking the progress of the melting front under various rates of heat flux and multiple nanoparticle concentrations. The finite element approach is applied to solve the simultaneous governing equations computationally. The computational model is validated and the results showed a good agreement with previous related work. Furthermore, the agreement between the experimental and simulated results is reasonable. The effects of the nanoparticle concentration and the amount of applied heat flux (Rayleigh number) on the melting process are examined. The characteristics of the melting process are described by temperature of NePCM, progress of the shape of the solid–liquid interface, melting rate, melt fraction and charging time. The experimental and numerical results reveal that there is an improvement in melting characteristics with the emulsion of more nanoparticles in PCM (intensifying the effective thermal conductivity) and raising the wall heat flux and the corresponding Rayleigh number (augmenting the role of natural convection). This enhancement in the melting process can be indicated by increasing the melting rate which leads to acceleration of the melting time. In early stages, melting is driven by conduction heat transfer identified by concentric isotherm patterns, high melting rate and lack of fluctuations in temperature data. As time progresses, natural convection will develop and increase the melting rate in the top half of the annulus with high rate of temperature fluctuations and thermal instabilities. On the other hand, melting at the bottom region is characterized by diffusion and thermal stability in liquid melt where no temperature oscillations are recorded. The melt fraction, melting rate and rate of enhancement in charging time are improved with the increasing of the Rayleigh number. Additionally, the rate of acceleration in melting is comparably high for low nanoparticle concentration and it will degrade as the amount of nano-additives increases as the augmentation in viscosity, agglomeration and precipitation may outweigh the enhancement in thermal conductivity. The simulated findings exhibit that the subcooling has a negative effect on the melt fraction, solid–liquid interface progress and time required to complete melting. This effect is insignificant at higher supplied heat flux or higher values of Rayleigh number. Finally, the impact of eccentricity by lowering the center of the inside heated tube is also evaluated. The predicted results show that the eccentric mode has higher melt fraction in comparison with the concentric arrangement.
Improved functionality of phase change materials (PCM) through dispersion of nan oparticles is reported. The resulting nanoparticle-enhanced phase change materials (NEPCM) exhibit enhanced thermal ...conductivity in comparison to the base material. Starting with steady state natural convection within a differentially-heated square cavity that contains a nanofluid (water plus copper nanoparticles), the nanofluid is allowed to undergo solidification. Partly due to increase of thermal conductivity and also lowering of the latent heat of fusion, higher heat release rate of the NEPCM in relation to the conventional PCM is observed. The predicted increase of the heat release rate of the NEPCM is a clear indicator of its great potential for diverse thermal energy storage applications.
•Reporting a negative temperature dependence on thermal interfacial conductance (TIC) of n-eicosane.•Proposing a thermally coupled-decoupled model for heat transfer across the van der Waals force ...bonded flexible molecular interfaces.•Elucidating the relation between the TIC and the properties of the interface.•Establishing a new paradigm to study interfacial heat transfer mechanisms in a diverse set of soft materials, such as paraffins and lipids.
Progress toward improving thermal transport properties of n-eicosane, a promising phase change material for thermal energy storage near human body temperature, is hindered by lack of a general theoretical framework to model thermal interfacial conductance (TIC) of the van der Waals (vdW) force bonded molecular interfaces, which have little compositional/structural mismatch. Combining molecular dynamics simulations at temperatures up to the melting point and a time-domain analysis of interfacial heat currents in individual molecules, we unveil that (1) the heat flux across a molecular vdW interface has an alternating coupling-decoupling pattern at the single molecule level due to thermal fluctuations, and (2) the net heat transfer during the thermally coupled periods is orders of magnitude larger than that during the thermally decoupled periods. Furthermore, we demonstrate that a vdW interface's TIC correlates almost linearly with two atomistic-scale coupling-decoupling parameters: αc and σc, that represent the duration and strength of the thermal coupling, respectively. This thermal coupling-decoupling mechanism proposes a new paradigm that explicitly accounts for the effects of structural flexibility and dynamical fluctuation on heat transfer across molecular interfaces, and will likely provide new insights into modeling thermal transport properties in a wide-ranging soft materials, including self-assembled monolayers, paraffins or lipids.
To elucidate the underlying mechanism of crystallization processes of n-alkane mixtures, molecular dynamics simulation is used to assess the effect of the morphology on thermal conductivity. Binary ...mixtures of n-eicosane and n-triacontane with different molecular number ratios are solidified following two approaches: the surface potential route and the free surface route. There exists a marked discrepancy of the orientation factor of the resulting solid n-alkane molecules following the imposed surface potential and the free surface routes corresponding to observed layered systems and random ordering of molecular chains, respectively. In addition, the thermal conductivities of the n-alkane mixtures are evaluated using non-equilibrium molecular dynamics. The results suggest that the orientation factor of the molecular system greatly impacts the thermal transport of n-alkane mixtures at the nanometer scale, with a negligible influence of the number ratio of n-triacontane. Moreover, the solid ratio plays a significant role on the thermal conductivity for low orientation factor systems by adjusting the number of thermal interfaces of the entire molecular system.
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