Granular Phase Change Material Research Articles

Numerical analysis of thermal performance of breathing air cooler based on granular phase change material

In this paper, we propose to use granular phase change material (PCM) as a heat accumulating material in a thermal protective respirator also called a heat exchanger to cool the breathing air. Such device is needed to protect human respiratory system from thermal injuries at indoor fires. Using the original solver for OpenFOAM software developed and validated previously, the ability of the commercially available granular PCM in the respirator to cool hot air for breathing during the time required for the human evacuation has been shown numerically: mass of 130 g is enough to cool the air from 177 °C to a temperature of less than 50 °C within 10 min. This result gives hope that such cooling element can be combined with “conventional” gas-and-smoke protective element, resulting in the creation of respirators for the protection of a person from both hot air and toxic combustion products. A parametric study of the performance of the thermal protective respirator is carried out in the paper: effect of PCM melting temperature, time variation of the outlet air flow rate and partial closure of the outlet from the cooling element have been analyzed. It has been shown that an increase in the PCM melting temperature while keeping the other parameters unchanged worsens the thermal performance of the breathing air cooler. The cooling efficiency is also worsened when the cooling element is narrowed or its outlet is partially closed, as this leads to an increase in the maximum temperature of the inhaled air. It has been shown that the time variation of the inhaled air flow rate, which is used in the calculations of the thermal protective respirator, must be close to the parameters of real human breathing, and the use of the assumption of constant air flow rate is unacceptable, since this can significantly affect the output characteristics of the respirator.

Addition of granular phase change materials (PCMs) and graphene to a cement-based mortar to improve its thermal performances

The addition of phase change materials (PCMs) into building materials represents a valid strategy towards the improvement of their thermal performance and therefore of the indoor comfort. However, when paraffin PCMs are used, the thermal conductivity of the mixture inevitably decreases due to the PCM low thermal conductivity. The research here presented focused on the addition of granular paraffin PCM into a cement-based mortar to improve its thermal storage capacity, and of hydrophobic graphene to counterbalance the reduction in thermal conductivity. Experimental tests at laboratory scale and numerical simulations were carried out to compare the behavior of two equivalent radiant floor samples in terms of temperatures and heat fluxes. Lower temperature fluctuations and delayed minimum temperatures were observed in the mortar with PCM and graphene, if compared to the Reference one. Eventually, in order to investigate the possibility of using the new mortar in case of building refurbishment, a floor with the enhanced mortar lowered 1 cm was simulated and then compared to the reference one. Results showed that the surface of the lowered floor reached the same maximum temperature with the system on and up to 3 K higher with the system off.

A novel computational model and OpenFOAM solver for simulating thermal energy storages based on granular phase change materials: Advantages and applicability

In this paper, we consider hydrodynamic and heat transfer processes in thermal energy storages based on granular or encapsulated phase change materials. Such heat storages can be applied in renewable and conventional energy, safety systems, civil building, and so on. A computational model, which is based on the assumption of interpenetrating interacting continua, has been developed to simulate the processes of phase transitions, heat transfer and fluid flow in the packed bed latent heat storages. To implement this model, we chose OpenFOAM – the leading free, open source software for computational fluid dynamics and heat transfer. Solver porousHeatTransferFoam, which is based on the SIMPLE algorithm, and additional libraries have been developed in the OpenFOAM framework. The solver and the libraries allow one to model granular materials with complex enthalpy-temperature diagrams during phase transitions, thus expanding the applicability of OpenFOAM. The advantage of the solver is high computational speed – its calculation time varies from a minute to a few minutes for 1D simulations and from a few minutes to a few hours for 2D simulations depending on the mesh sizes. Comparison between numerical results obtained and known experimental data has shown that the model based on the assumption of interpenetrating interacting continua adequately describes the heat transfer processes in the packed bed latent heat storages not only for the large ratio of the storage characteristic size to the particle diameter, but also for the small one (about 7–8). In contrast to many existing computational models, which applicability is usually limited to rather narrow class of the processes, our model covers a variety of hydrodynamic and heat transfer problems in the thermal energy storages based on granular phase change materials. This computational model and OpenFOAM solver developed allow one to simulate incompressible and compressible (liquid or gaseous) flows through the packed bed latent heat storages under various boundary conditions, with isothermal and non-isothermal phase transitions, with various properties of granular materials and coolant, in wide ranges of flow velocities, operating temperatures and particle sizes.

Numerical Analysis of the Influence of Side Wall Shape on the Efficiency of Thermal Energy Storages Based on Granular Phase Change Materials

Numerical Analysis of the Influence of Side Wall Shape on the Efficiency of Thermal Energy Storages Based on Granular Phase Change Materials

Alternative experimental characterization of phase change material plasterboard using two-step temperature ramping technique

The increasing energy demand for heating and cooling is leading to the development of new strategies for reducing energy consumption and greenhouse gas emissions. Phase change materials applied on the building envelope can act as a heat modulation passive cooling strategy, reducing the energy requirement. An essential aspect for an effective application of PCM is the knowledge of its thermo-physical properties, namely heat conductivity, specific heat and latent heat. Differential scanning calorimetry (DSC) is one of the most used techniques, but the characteristics of the sample necessary for the test make it unsuitable for all those inhomogeneous or composite materials for which a small sample may not be representative of their properties. Alternative and unconventional tests have been developed to overcome this limitation and estimate experimentally one or more thermal properties of a selected material. In the research presented here, a new low-cost experimental set up was developed for the experimental estimation of the thermal conductivity, the specific heat and the latent heat of a plaster containing granular phase change materials through both steady-state as well as unsteady-state tests. The estimated values were then used in the numerical modelling of the experimental set up. A good agreement in thermal response between the experimental and the simulated values validates the proposed experimental estimation method.

Numerical model for studying axisymmetric thermal energy storages based on granular phase change materials with gaseous heat transfer fluid

A mathematical model and novel numerical method are proposed for calculating flows of gaseous heat transfer fluid in axisymmetric thermal energy storages (TES) based on granular phase change materials (PCM). The developed computational technique is based on finite difference method and uses a combination of explicit and implicit schemes. Using the resolution tests, the detailed convergence analysis of the numerical method is carried out. The efficiency of the developed model is demonstrated by simulating the processes of heating, energy storage and cooling of the TES taking into account the lateral heat losses at various values of heat loss intensity and PCM heat conductivity. A comparison of the solutions obtained by 1D and 2D models is carried out, and the applicability of commonly used 1D approach for simulating processes in the axisymmetric TES with lateral heat losses is analyzed. It is shown that the ability of the 1D model to predict the averaged parameters decreases not only with increasing heat transfer coefficient, but also with decreasing PCM heat conductivity. The use of the 1D approach at large lateral heat losses does not provide guaranteed lower or upper bound for the sought parameters.

Granular PCM-Enhanced Plaster for Historical Buildings: Experimental Tests and Numerical Studies

The construction sector is among the major players responsible for global energy consumption and therefore related emissions, both because of the constantly increasing indoor air quality standard which requires increasingly higher energy demands as well as the great share of historical buildings which are now obsolete and are not up to date with current regulations. Phase change materials (PCMs) applied on the building envelope represent a feasible possibility to improve the performance of existing buildings, also the historical ones, increasing their thermal inertia without violating any legal restriction or causing further alterations to the structure. More specifically, focus of this research was on the addition of a granular paraffin PCM into a lime-based plaster. Experimental tests at lab scale and numerical simulations with COMSOL Multiphysics were carried out to characterize the plasters realized, namely one reference lime-based plaster and one with incorporated 10% by mass of granular PCM (named REFp and PCMp, respectively). The behavior of these plasters applied on the exterior side of a wall was then simulated and compared in terms of temperatures and heat fluxes. However, considering that the estimated thermal conductivity of the reference lime-based plaster was lower than the values found in literature, the simulations were carried out considering an additional plaster, namely a lime-based plaster (renamed LITp), whose properties were found in literature and considered quite representative of a consistent share of existing historical buildings. Great improvements were observed from the application of PCM into the plaster, with reductions of the incoming energy between 9% and 18%.

Numerical Simulation of Two-Dimensional Gas Flows through Granular Phase Change Materials

A mathematical model and a numerical method for studying two-dimensional plane gas flows through thermal accumulators based on granular or capsular phase change materials (PCMs) are proposed. The considered objects are modeled as porous media with phase transitions occurring in the condensed component. The study is based on methods of heterogeneous continuum mechanics without detailing the processes inside individual particles. The proposed numerical method combines explicit and implicit finite-difference schemes. The method is described in detail, and its convergence is experimentally analyzed. The heating of smoothly narrowing and smoothly expanding thermal accumulators consisting of granular PCM is studied. It is shown that heating in such objects near the inclined walls occurs more slowly than in their central part even in the absence of heat transfer through the sidewalls.

Effect of side walls shape on charging and discharging performance of thermal energy storages based on granular phase change materials

Abstract In this work, thermal energy storages based on granular phase change material with gaseous heat transfer fluid are studied to determine their optimal shape. The shape of the energy storage unit affects the gas dynamics in the object, and this affects the processes of heat transfer, heat accumulation and heat recovery. A novel numerical model is developed and used for investigating the gas-dynamic and thermophysical processes in packed bed thermal energy storages. Using numerical experiments, the effect of narrowing and expansion of side walls on charging and discharging processes of the energy storages is studied for two types of boundary conditions: the known input mass flow rate of the gas and the known gas pressure at the open boundaries of the object. To determine the most optimal shape of the thermal energy storage, several efficiency criteria are used. It is shown that preference of the storage shape at both charging and discharging processes depends on the choice of efficiency criterion and specific conditions of the process such as boundary conditions, phase change temperature of granular material, and so on. Narrowing or expanding thermal energy storages have advantages in rare cases; the storages with straight walls are often most preferable.

Numerical analysis of the influence of side wall shape on the efficiency of thermal energy storages based on granular phase change materials

This work is devoted to a numerical study of processes in thermal energy storages based on granular phase change materials. The influence of the side walls shape on the efficiency of such heat accumulators is studied when the plane-parallel flows of gas heat transfer fluid take place. The shape of the energy storage affects the heat transfer fluid flow in the object, and this affects the heat transfer, heat accumulation and heat recovery. Using the novel numerical model, the influence of narrowing and expansion of the side walls on the charging and discharging processes of thermal energy storages with rectangular cross sections are studied under two types of boundary conditions: the known mass flow rate of gas at the object inlet and the known gas pressure drop at its open borders for different phase change temperatures of the phase change material .Different efficiency criteria are used to estimate the preferred shape of a heat storage. For the charging process, the preference criteria considered are the maximum instantaneous storage efficiency, the maximum cumulative storage efficiency, and the minimum time to fully charge the device. For the discharge process, the considered preference criteria are the maximum energy recovery efficiency, the maximum total utilization ratio, and the maximum time to maintain the temperature of the heat transfer fluid at the outlet not lower than the desired value. It is shown that the preferred shape of the energy storage depends on the choice of the efficiency criterion and specific process conditions such as boundary conditions, phase transition temperature, etc. Narrowing and expanding thermal energy storages have an advantage in rare cases, and storages with straight walls are often most preferable.

On two-dimensional gas flows through granular phase change material

A novel numerical model is proposed for modelling two-dimensional planar gas flows in granular phase change material (PCM). The mentioned model consists of system of equations based on the classical methods of mechanics of heterogeneous media which is solved by original finite-difference algorithm. Effect of heat transfer with ambient medium on mushy zone propagation in the bed of granular PCM is investigated by means of numerical experiment. It is shown that phase transition speed can decrease significantly near the side walls of the bed in relation to the central region which provides curvature of the mushy zone boundaries and nonuniform melting of the PCM in the bed. Kind of curvature of these boundaries can change in dependence of heat conductivity coefficient of the PCM.

Numerical Model of Time-Dependent Gas Flows Through Bed of Granular Phase Change Material

A novel mathematical model and original numerical method for investigating time-dependent gas flows through a bed of granular phase change material (PCM) are proposed and described in detail. Such material is modeled as a porous medium, and continua mechanics method are used for constructing the mathematical model. The numerical method is based on a combination of explicit and implicit finite-difference schemes. Comparison of calculation results with known experimental data demonstrates a very good coincidence. The results of the study can be applied in modeling the thermal energy storage with granular PCM in advanced adiabatic compressed air energy storage and other heat storage devices.

Influence of gas compressibility on gas flow through bed of granular phase change material

One-dimensional time-dependent gas flow through a bed of granular phase change material (PCM) is considered. Such process takes place in thermal energy storage (TES) based on granulated or encapsulated PCM. Using a novel numerical model, the influence of gas compressibility on considered processes is investigated. The study is carried out in a wide range of problem parameters for both known gas pressure at the open boundaries of the bed and known gas flow rate at the object inlet. It is shown that for some problem parameters the neglect of the gas compressibility can lead to large errors in the solution of the problem, and it can cause significant inaccuracies in calculating the full heating time and the full melting time of the granular PCM in TES. Moreover, the neglect of the gas compressibility cannot give guaranteed upper or lower bound for the real time of the process. So, for some problem parameters the gas compressibility must be taken into account to model the TES with granulated or encapsulated PCM more adequately and accurately.

Modeling of Gas Flow through a Granular Bed of a Thermal Storage Phase-Change Material

A mathematical model and a numerical method for description of time-dependent gas flows through granular thermal storage phase-change materials (PCMs) have been proposed. Such granular PCMs are modeled as porous solid–solid phase-change media using the methods of heterogeneous continuum mechanics without detailing the processes inside individual particles. The calculation results are compared with the experimental measurements to show their good agreement with each other.

Characterization of granular phase change materials for thermal energy storage applications in fluidized beds

This work investigates commercially available granular phase change materials (PCMs) with different transition temperatures for the use of thermal-energy storage systems in fluidized beds. The hydrodynamic characteristics of granular PCMs were tested in cylindrical-3D and planar-2D fluidized beds. The density, particle size distribution and angle of repose were measured for various PCM materials. Further attrition studies were conducted with changes in particle surface from abrasion, which were characterized using a Scanning Electron Microscope (SEM). The results indicate that some materials with smaller particle size and thinner supporting structure can lose the paraffin during the fluidization process, when paraffin is in a liquid state. As a consequence, the particles agglomerate, and the bed defluidizes. For all of the tested materials, only GR50 (with a transition temperature of 50°C) properly fluidizes when the paraffin is in the liquid state and has shown to endure >75h of continuous operation and 15 melting-solidification cycles in a fluidized bed. Additional differential scanning calorimetry (DSC) measurements of the cycled particles did not show a decrease in energy storage capacity of the granular PCM, which corroborates that there is no loss of material after >75h of fluidization.

Development of granular expanded perlite/paraffin phase change material composites and prevention of leakage

In this study, granular phase change material (PCM) composites were developed by absorbing paraffin into the pores of expanded perlite particles with two grades of particle size. Because of the different particle sizes and pore structures, the absorption mechanisms of the expanded perlite particles were found to differ. A significant amount of paraffin leakage was found when the PCM composites were applied directly into the cement mixture. To prevent such leakage, a new method by using hydrophobic silane was investigated as surface modification for the PCM composites. The method was then compared with nanosilica deposition method. Although both methods prevented leakage effectively, cement composites incorporating silane-modified PCM composites had lower compressive strength than those incorporating nanosilica modified PCM composites. Thermal performances of expanded perlite/paraffin composites were compared with that of microencapsulated PCM and the results showed expanded perlite/paraffin composites were superior under certain conditions.

Modeling and experiments of energy storage in a packed bed with PCM

This work presents a numerical and experimental study of the transient response of a packed bed filled with a granular phase change material (PCM). The proposed numerical model accounts for the progressive evolution of the enthalpy with temperature during the phase change rather than using a constant phase change temperature. This temperature-dependent enthalpy is included in the model as an apparent specific heat that is dependent on temperature according to the measurements obtained by differential scanning calorimetry (DSC). The model also includes the energy stored in the wall, which has been shown to have a non-negligible effect in several experimental facilities.The equations presented are non-dimensionalized, which results in the same differential equation system regardless of whether a granular PCM or sensible heat storage material is used. In this manner, the same numerical method can be used in cases with or without a granular PCM. Numerical and experimental results are obtained for a conventional granular material (sand) and two commercial granular PCMs with different phase change temperatures. The numerical and experimental heating results exhibit good agreement, and the energy stored in the wall of the bed represents between 8 and 16% of the energy stored in the granular material.

Experimental study of fixed and fluidized beds of PCM with an internal heat exchanger

This work presents the results of experiments performed in a thermal energy storage tank filled with particles, which was heated using hot air and then discharged with a cold water stream circulating inside a heat exchanger immersed in the bed. Both fixed- and fluidized-bed configurations were studied. The materials used were sand, which is a material commonly employed in sensible heat storage, and a granular phase change material (PCM), in which the phase change occurs over the temperature range of 40–50°C. Three different heat exchangers with different helical coil geometries were tested by measuring the temperatures in the bed and at the water inlet and outlet. Higher heat transfer coefficients between the bed and the water flow and higher heat exchanger effectiveness were observed for the heat exchanger with the greatest distance between coils, as it allows better contact between the bed particles and the heat exchanger surface when the particles are fluidized.

Modeling the Heat Transfer Coefficient Between a Surface and Fixed and Fluidized Beds With Phase Change Material

The objective of this work is to model the heat transfer coefficient between an immersed surface and fixed and bubbling fluidized beds of granular phase change material (PCM). The model consists of a two-region model with two different voidages in which steady and transient conduction problems are solved for the fixed and fluidized bed cases, respectively. The model is validated with experimental data obtained under fixed and fluidized conditions for sand, a common material used in fixed and fluidized beds for sensible heat storage, and for a granular PCM with a phase change temperature of approximately 50 °C. The superficial gas velocity is varied to quantify its influence on the convective heat transfer coefficient for both the materials. The model proposed for the PCM properly predicts the experimental results, except for high flow rates, which cause the contact times between the surface and particles to be very small and lead the model to overpredict the results.

Study on the thermal storage performance of a gravity-assisted heat-pipe thermal storage unit with granular high-temperature phase-change materials

A novel GAHP-TSU (gravity-assisted heat-pipe thermal storage unit) is presented, in which the composite granular solid–liquid PCM (phase-change material) is piled up as the porous-medium layer. The composite PCM, which melts over a temperature range of 200–250 °C, is composed of a mixed metal salt and inorganic bentonite. Naphthalene is presented as the phase-change heat-transfer medium of the GAHP-TSU. The mechanism of charging/discharging modes is similar to that of the gravity-assisted heat pipe, which is superior to the traditional direct-contact or indirect-contact heat storage unit. Heat transfer between the PCM and the heat resource or cold resource in this device occurs in the form of a cyclic phase change of the heat-transfer medium, which directly occurs on the surface of the PCM and has an extremely high heat-transfer coefficient. In this paper, a series of experiments are carried out first to investigate the properties of the new heat storage unit. Then, a series of theoretical analyses, such as heating time, power distribution, temperature difference between the inside and outside of the granular PCM, are presented. The results show that the whole system has an excellent heat-storage/heat-release performance.