Phase Change Material Volume Fraction Research Articles

Heat transfer characteristics of a hybrid thermal energy storage tank with Phase Change Materials (PCMs) during indirect charging using isothermal coil heat exchanger

Charging of modular thermal energy storage tanks containing water with submerged Phase Change Materials (PCMs) using a constant temperature coil heat exchanger was numerically investigated. Under appropriate operating conditions, the energy density of this hybrid system can be significantly increased (two to five times) relative to a system containing water only (sensible energy storage system). However, due to the low thermal conductivity of PCMs, the geometry and configuration of the PCM encapsulation requires optimization to meet typical charging and discharging cycles. In the current study rectangular PCM modules in a modular storage tank (∼200L) were studied to determine the effect of PCM volume fraction and spacing between the modules on the heat transfer characteristics as well as the charging rate of the storage system. Two-dimensional Computational Fluid Dynamics (CFD) simulations using ANSYS CFX 14.0 showed an 85% increase of the charge rate of the system by increasing the PCM volume fraction from 0.025 to 0.15. The charge rate was not affected when the PCM volume fraction was increased beyond 0.15. For a fixed coil heat exchanger design, further increase of the PCM volume fraction caused the thermal resistance on the PCM side to be significantly lower than that on the coil side. As a result, further increase of the charge rate necessitates increasing the surface area of the heat exchanger. Simulations showed that the gap spacing between the PCM modules had a negligible impact on the heat transfer characteristics for the studied cases, where the gap spacing was larger than the thermal boundary layer thickness.

Experimental passive electronics cooling: Parametric investigation of pin-fin geometries and efficient phase change materials

This experimental investigation focuses on the optimization of passive cooling system using extruded finned surfaces with phase change materials (PCMs) as the thermal conductivity enhancers (TCEs). The study develops comparison between fins of circular and square cross-sectional area, made of aluminium. Further classification is done in configuration in terms of staggered and inline arrays. The volume fraction of fins is kept constant at 9% of total volume of heat sink. The purpose is to single out the better arrangement with and without PCM. Six PCMs of varying phase change temperature and heat capacities, namely Paraffin wax, RT-54, RT-44, RT-35HC, SP-31 and n-eicosane are selected for thermal conductivity enhancement. The volume fraction of PCM is also constant at 90% of the heat sink volume, giving a 10% volume for expansion after melting. Moreover, power levels are used in a range of 4–8W with an increment of 1W. The analysis was carried out on graphical trends produced and explanations were given accordingly. The most effective PCMs were also discussed considering their enhancement time, enhancement ratios and other material properties. Finally, the results were justified by the scientific knowledge and found in compliance with the work of famous researchers as well.

Novel form-stable phase change material composite for high-efficiency room temperature control

Phase change material (PCM) driven thermal energy storage (TES) systems are showing promise for cooling energy saving in residential and office buildings. State-of-the-art TES for interior items such as tiles and furnishings require the material matrix to transfer heat rapidly between the adjacent PCM and the indoors. TES developers offer a large number of PCM composites, in which, however, the high thermal conductivity counterbalances the PCM volume capacity and material form-stability above the PCM melting point. Herein, we report a novel aluminium matrix with open ultrathin (mainly, below 150nm) pores and the developed surface area (~7m2 g-1) that solves the dilemma. The porous (22–46vol%) aluminium matrix results from the compacted aluminium nanoflakes reactively (1min at 873K in N2) sintered with the aid of oxidation of the nanoflake protector, stearic acid. The preparation method leads to partial transformation of the pores and an increase of the specific surface area. In the PCM fraction range, the prepared composite has the thermal conductivity in the range of 10–60Wm-1K-1. The effects of the PCM volume fraction on the thermal conductivity and specific heat capacity are described. As an example, we have considered an affordable organic PCM such as coconut oil that can readily cling to the Al2O3-filmed surface of the material pores through the hydroxyl functionalities. With the use of vacuum infiltration, heated (373 K) coconut oil deeply penetrates the matrix pores. The PCM composite has shown the excellent form stability in terms of the coconut oil mass loss (below 1.5wt%) at an elevated temperature of 318 K for over 1000h.Applied internally, this kind of TES composite can rapidly and efficiently transfer a considerable amount of heat energy within a large building area and regulate the indoor temperature.

Experimental and numerical investigation of discharging process of direct contact thermal energy storage for use in conventional air-conditioning systems

Direct contact thermal energy storage (TES) for use in conventional air-conditioning systems is proposed to reduce the operational energy demand. Thermal performance of a novel kind of phase change material (PCM) prepared for use in conventional air-conditioning systems with the proposed direct contact TES tank, is evaluated. A 3-dimensional (3D) numerical model is built using ANSYS FLUENT to investigate dynamic characteristics of the discharging process of TES system. The model is validated by comparing the numerical with the experimental results. The effects of heat transfer fluid (HTF) flow rate, HTF inlet temperature, liquid PCM volume fraction, complete discharging time, discharging capacity of the tank, and temperature distribution in direct contact TES tank are investigated. The results indicate that increasing the HTF flow rate speeds up the discharging capacity and the discharging time of the direct contact TES system reduces. When the flow rate increases from 0.503m3/h to 0.936m3/h, the melting PCM increases from 63.33vol.% to 70.74vol.% within 600s. The discharging capacity increases with HTF inlet temperature; however, the whole discharging capacity does not change obviously by changing HTF inlet temperature.

Thermal performance of phase change material (PCM) based pin-finned heat sinks for electronics devices: Effect of pin thickness and PCM volume fraction

The present experimental investigation focuses on the passive cooling of electronic devices by using phase change material (PCM) based pin-fin heat sinks to increase reliability, to ensure sufficiently lower temperature, to stretch the operating duration and to improve the functionality of installed features. Paraffin wax is used as a PCM and filled in heat sinks made of aluminum. As the thermal conductivity of PCM is very low, aluminum square fins are used as thermal conductivity enhancer (TCE). A volume fraction of TCE is kept constant at 9% and the uniform heat flux is applied to finned and un-finned heat sinks. An un-finned heat sink is used for base line comparison. Fin thicknesses of TCE of 1mm,2mm, and 3mm with square cross sectional area are investigated with a constant height of 20mm. Volume fractions of PCM are varied as 0.00, 0.33, 0.66 and 1.00 for each heat sink to determine the thermal performance. The present study reports thermal performance at various heat fluxes to enhance the operating time for different set point temperatures (SPTs) and to compare the latent heat phase duration for various heat sinks tested. The results reveal that maximum thermal performance in operating time is achieved for 2mm thick pin-fin heat sink filled with PCM volumetric fraction of 1.00.

The influence of microencapsulated phase change material (PCM) characteristics on the microstructure and strength of cementitious composites: Experiments and finite element simulations

Phase Change Materials (PCMs) incorporated into cementitious systems have been well-studied with respect to energy efficiency of building envelopes. New applications of PCMs in infrastructural concrete, e.g., for mitigating early-age cracking and freeze-and-thaw induced damage, have been proposed. Hence this paper develops a detailed understanding of the characteristics of cementitious systems containing two different microencapsulated PCMs. The PCMs are evaluated using thermal analysis, vibrational (FTIR) spectroscopy, and electron microscopy, and their dispersion in cement pastes is quantified using X-ray Computed Microtomography (μCT). The influences of PCMs on cement hydration and pore structure are evaluated. The compressive strength of mortars containing PCMs is noted to be strongly dependent on the encapsulation properties. Finite element simulations carried out on cementitious microstructures are used to assess the influence of interface properties and inter-inclusion interactions. The outcomes provide insights on methods to tailor the component phase properties and PCM volume fraction so as to achieve desirable performance.

Investigation of the thermal performance of phase change material/mini-channel coupled battery thermal management system

In order to extend the cycle life of power battery pack within electric vehicle, a phase change material (PCM)/mini-channel coupled power battery thermal management (BTM) system, as well as the three-dimensional battery thermal model, was designed in this paper. The effect of various influencing factors, especially mass flow rate of water, phase change temperature and thermal conductivity of PCM, were investigated numerically. The results showed that the liquid volume fraction of PCM was greatly influenced by the thermal conductivity and the phase change temperature of PCM. The increasing number of channels results in a decrease of the maximum temperature (TMax) and maximum temperature difference (ΔT) of battery packs. The optimal phase change temperature and thermal conductivity of PCM were 308.15K and 0.6Wm−1K−1 respectively when the number of channel was eight and the mass flow rate was 8×10−4kgs−1. Moreover, a maximum temperature of 320.6K was predicted for the PCM/mini-channel coupled BTM system, while a maximum temperature of 335.4K was predicted for the PCM-based BTM system. Additionally, the PCM/mini-channel coupled BTM system presented more effective thermal performance and the research will be a clear indicator for the design of the PCM/liquid coupled BTM system.

Figure of merit for the thermal performance of cementitious composites containing phase change materials

This paper presents a novel method to quantitatively characterize the thermal performance of composite materials containing phase change materials (PCM) based on a figure of merit we termed the energy indicator. The method features (i) commonly used specimen geometry, (ii) straightforward experimental implementation, and (iii) sensitivity to relevant design parameters including PCM volume fraction, enthalpy of phase change, composite effective thermal conductivity, and specimen dimensions. The experimental method and the concept of energy indicator were demonstrated on PCM-mortar composites using various volume fractions of two commercial microencapsulated PCMs. This was supported by transient two-dimensional heat transfer simulations. The energy indicator was shown to increase linearly with increasing microencapsulated PCM volume fraction and latent heat of fusion and quadratically with the specimen radius. This figure of merit can be used to rapidly screen and select microencapsulated PCM composite materials for energy efficient buildings or crack-resistant concretes.

Annual energy analysis of concrete containing phase change materials for building envelopes

This paper examines the annual energy and cost savings potential of adding microencapsulated phase change material to the exterior concrete walls of an average-sized single family home in California climate zones 3 (San Francisco, CA) and 9 (Los Angeles, CA). The annual energy and cost savings were larger for South- and West-facing walls than for other walls. They were also the largest when the phase change temperature was near the desired indoor temperature. The addition of microencapsulated phase change material to the building walls reduced the cooling load in summer substantially more than the heating load in winter. This was attributed to the cold winter temperatures resulting in nearly unidirectional heat flux on many days. The annual cooling load reduction in an average-sized single family home in San Francisco and in Los Angeles ranged from 85% to 100% and from 53% to 82%, respectively, for phase change material volume fraction ranging from 0.1 to 0.3. The corresponding annual electricity cost savings ranged from $36 to $42 in San Francisco and from $94 to $143 in Los Angeles. From an energy standpoint, the best climate for using building materials containing uniformly distributed microencapsulated phase change material would have outdoor temperature oscillations centered around the desired indoor temperature for the entire year.

Numerical modeling of heat transfer in open-cell micro-foam with phase change material

The heat transfer behavior of micro-foam impregnated with phase change material (PCM) was investigated. Both microscopic and macroscopic modeling approaches were considered. An open-cell PCM/micro-foam structure under constant heat flux was modeled using an ideal micro-foam structure based on a cubic unit cell with spherical micro-pores arranged in a BCC lattice. Results from direct numerical simulations (DNS) which take into account the intricate foam geometry were compared with those from one- and two-temperature volume-averaged simulations. The enthalpy-porosity method was used for phase change and conjugated heat transfer was applied at the PCM/micro-foam interface. The DNS simulations provided insight into the complicated heat transfer and a three-dimensional PCM melting front and temperature distribution was observed. Along with serving as a benchmark for the less computationally expensive volume average methods, the DNS results were also used to extract thermo-physical parameters such as effective thermal conductivity and interstitial heat transfer coefficients. These coefficients were subsequently used in the volume averaged simulations. The choice of the effective thermal conductivity of the PCM/micro-foam structure was found to be crucial in matching the temperature profile and liquid PCM volume fraction results from volume averaged simulations and DNS. Effective thermal conductivity values based on the nonlinear Progelhoff model with power factor derived from DNS were found to provide an improvement over the more widely used value based on the arithmetic mean.

Diurnal thermal analysis of microencapsulated PCM-concrete composite walls

This paper examines the benefits of adding microencapsulated phase change material (PCM) to concrete used in building envelopes to reduce energy consumption and costs. First, it establishes that the time-dependent thermal behavior of microencapsulated PCM-concrete composite walls can be accurately predicted by an equivalent homogeneous wall with appropriate effective thermal properties. The results demonstrate that adding microencapsulated PCM to concrete resulted in a reduction and a time-shift in the maximum heat flux through the composite wall subjected to diurnal sinusoidal outdoor temperature and solar radiation heat flux. The effects of the PCM volume fraction, latent heat of fusion, phase change temperature and temperature window, and outdoor temperature were evaluated. Several design rules were established including (i) increasing the PCM volume fraction and/or enthalpy of phase change increased the energy flux reduction and the time delay, (ii) the energy flux reduction was maximized when the PCM phase change temperature was close to the desired indoor temperature, (iii) the optimum phase change temperature to maximize the time delay increased with increasing average outdoor temperature, (iv) in extremely hot or cold climates, the thermal load could be delayed even though the reduction in daily energy flux was small, and (v) the choice of phase change temperature window had little effect on the energy flux reduction and on the time delay. This analysis can serve as a framework to design PCM composite walls in various climates and seasons and to take advantage of time of use electricity pricing.

Thermal performance of a PCM heat sink under different heat loads: An experimental study

This paper reports the results of an experimental study to quantify the effect of heat transfer performance of a phase change material (PCM) based plate fin heat sink matrix under constant and intermittent heat loads. n-Eicosane with a melting temperature of 36.5 °C is used as the phase change material. Such heat sinks, in general require heat transfer enhancement techniques, because of the low thermal conductivity associated with PCMs. A plate fin matrix made of aluminum is used in the present study to augment the heat transfer. The performance of the heat sink matrix with and without PCM is compared. Additionally, a heat sink filled with PCM, but without any fin is used for baseline comparisons. A series of experiments was conducted at a constant power level of 5–10 W (corresponding to a flux of 2–4 kW/m2) in steps of 1 W. Extensive experimental investigations were carried out at varying power levels by applying the heat flux intermittently by keeping the input heat constant. A step heat input is also applied for a fixed duration at various usage modes usually encountered in the thermal management of portable electronic devices. Parametric studies have been conducted by changing the PCM volume fraction, which is defined as the ratio of the volume of the PCM to the difference between the total empty volume of the heat sink and the volume occupied by the fins and the effect of these on the thermal performance of the plate fin heat sink matrix for both melting and solidification are elucidated in this study.

Thermal optimization of PCM based pin fin heat sinks: An experimental study

This paper reports the results of an experimental investigation carried out to characterize the thermal performance of different configurations of phase change material (PCM) based pin fin heat sinks. Paraffin wax and n-eicosane are used as the PCMs. Aluminium is used to make the heat sinks and the volume fraction is varied by changing the number of pin fins. Baseline comparisons are done with a heat sink filled with PCM, without any fin. The effect of PCM volume fraction on the heat transfer performance is also studied. The results showed that there exists sufficient scope to optimize the thermal design of the heat sink. An Artificial Neural Network – Genetic Algorithm hybrid algorithm is then developed to determine the optimum configuration of the pin fin heat sink that maximizes the operating time for the n-eicosane based heat sink. The resulting optima was found to be valid even for the paraffin wax based PCM.

Utilization of phase change materials and rubber particles to improve thermal and mechanical properties of mortar

Phase change materials (PCMs) and rubber particles from recycled tires were used in portland cement mortar with a potential application as insulation mortar. Series of tests were designed and conducted including compressive strength, flexural strength, thermal conductivity, drying shrinkage, and bond strength tests at 7 days and 28 days of age. Different types of additives were also used in the mix designs. The experimental results showed that mechanical properties of rubberized-mortar can be improved by adjusting the size of rubber particles and the amount of additives. With the same volume fraction of PCM and rubbers, the mechanical properties of PCM-modified mortar are either the same or better than that of rubberized-mortar. In terms of thermal properties, the rubber particles can reduce thermal conductivity of the mortar significantly, while the PCM can improve heat capacity of the mortar.

A parametric study of phase change material (PCM)-based heat sinks

A numerical study is conducted to evaluate the thermal characteristics of a PCM-based heat sink which can be potentially used for cooling of mobile electronic devices such as personal digital assistants (PDAs) and notebooks. The heat sink consists of a conventional, extruded aluminum sink embedded with appropriate PCMs. Some important parameters, such as PCM volume fraction, temperature difference, aspect ratio, and PCM properties, were studied to investigate their effects on the thermal performance of the hybrid cooling system.