Composite Thermal Energy Storage Research Articles

Preparation and characteristics of microencapsulated stearic acid as composite thermal energy storage material in buildings

Microencapsulated stearic acid (SA) with silicon dioxide (SiO2) shell as composite thermal energy storage material was prepared using sol–gel methods. In the composite thermal energy storage material, the stearic acid was used as the core material that is the latent heat storage phase change material (PCM), and the silicon dioxide acted as the shell material which prevented the leakage of the melted stearic acid. Fourier transformation infrared spectroscope (FT-IR) and scanning electronic microscope (SEM) were used to determine chemical structure and microstructure of microencapsulated stearic acid with silicon dioxide shell. The thermal properties and thermal stability were investigated by the differential scanning calorimeter (DSC) and thermogravimetric analyzer (TGA). The SEM results showed that the stearic acid was encapsulated in the shell of the silicon dioxide shell. The DSC results indicated that the microencapsulated stearic acid with the silicon dioxide shell as composite thermal energy storage material solidifies at 52.6°C with a latent heat of 162.0kJ/kg and melts at 53.5°C with a latent heat of 171.0kJ/kg when the encapsulation ratio of the SA is 90.7%. The TGA results presented that the silicon dioxide shells can improve the thermal stability of the microencapsulated stearic acid as composite thermal energy storage material.

Phase change nanocomposites with tunable melting temperature and thermal energy storage density

Size-dependent melting decouples melting temperature from chemical composition and provides a new design variable for phase change material applications. To demonstrate this potential, we create nanocomposites that exhibit stable and tunable melting temperatures through numerous melt-freeze cycles. These composites consist of a monodisperse ensemble of Bi nanoparticles (NPs) embedded in a polyimide (PI) resin matrix. The Bi NPs operate as the phase change component whereas the PI resin matrix prevents nanoparticle coalescence during melt-freeze cycles. We tune melting temperature and enthalpy of fusion in these composites by varying the NP diameter. Adjusting the NP volume fraction also controls the composite's thermal energy storage density. Hence it is possible to leverage size effects to tune phase change temperature and energy density in phase change materials.

Highly Conductive Porous Graphene/Ceramic Composites for Heat Transfer and Thermal Energy Storage

AbstractA novel architecture of 3D graphene growth on porous Al2O3 ceramics is proposed for thermal management using ambient pressure chemical vapor deposition. The formation mechanism of graphene is attributed to the carbothermic reduction occurring at the Al2O3 surface to initialize the nucleation and growth of graphene. The graphene films are coated on insulating anodic aluminum oxide (AAO) templates and porous Al2O3 ceramic substrates. The graphene coated AAO possesses one‐dimensional isolated graphene tubes, which can act as the media for directional thermal transport. The graphene/Al2O3 composite (G‐Al2O3) contains an interconnected macroporous graphene framework with an extremely low sheet electrical resistance down to 0.11 Ω sq−1 and thermal conductivity with 8.28 W m−1 K−1. The G‐Al2O3 provides enormous conductive pathways for electronic and heat transfer, suitable for application as heat sinks. Such a porous composite is also attractive as a highly thermally conductive reservoir to hold phase change materials (stearic acid) for thermal energy storage. This work displays the great potential of CVD direct growth of graphene on dielectric porous substrates for thermal conduction and electronic applications.

Experimental Investigation of PCM-LWA Composite for Building Wallboard

In this paper, a experimental investigation of application of room wallboard containing Phase-Change-Material / Lightweight-Aggregate composite (PCM-LWA) was carried out. A PCM-LWA composite thermal energy storage material was prepared by absorbing the paraffin. In such a composite, the paraffin serves as a latent heat storage material and the porous lightweight aggregate acts as the supporting material. Mixture of cement paste and epoxy were used to seal the PCM-LWA surface. Through comparing temperature curves of a general room model and the model using wallboards containing PCM-LWA, it can be known that the highest temperature of the room model using the wallboards containing PCM can be controlled in a narrow range. Therefore, room model assembled with the walls containing phase change material can effectively reduce the highest indoor temperature and the range of reduced temperature is about 2~4°C.

Preparation of Phase-Change-Material/Lightweight Aggregate Composite as an Energy Storage Material

In this paper, a phase-change-material/lightweight aggregate (PCM-LWA) composite thermal energy storage material was prepared by absorbing the lauryl alcohol, which is one kind of phase change materials, into porous lightweight aggregates (LWAs) that have an excellent absorbability. In such a composite, the lauryl alcohol serves as a latent heat storage material and the porous lightweight aggregate acts as the supporting material. In order to prevent the melted lauryl alcohol leak from the porous LWAs, surface seal processing for the PCM-LWA was necessary. In this research, pure cement paste and polymer modified cement paste were used to seal the PCM-LWA surface. Through comparison between the differential scanning calorimetry (DSC) tests for lauryl alcohol and PCM-LWA, it can be known that the solid-liquid phase change temperature of the composite PCM-LWA was slightly higher than that of the lauryl alcohol, and latent heat of the PCM-LWA was smaller than that of the pure PCM.

Preparation and characterization of fatty acid ester/building material composites for thermal energy storage in buildings

In this study, fatty acid ester/building material composites as novel form-stable phase change materials (PCMs) were prepared by absorbing liquid fatty acid esters into porous network of conventional building materials. In the composite erythritol tetrapalmitate (ETP) and erythritol tetrastearate (ETS) esters were used as PCM for thermal energy storage, and cement and gypsum used as supporting material. The composite PCMs were characterized using scanning electron microscope (SEM) and Fourier transformation infrared (FT-IR) analysis technique. The SEM results showed that the esters were confined in the porous network of the cement and gypsum. Thermal properties and thermal stabilities of the composite PCMs were determined by using differential scanning calorimetry (DSC) and thermal gravimetric (TG) analysis techniques. DSC results showed that the melting temperatures and the latent heats of the prepared composite PCMs were in range of 21.6–32.3°C and 35.9–43.3J/g, respectively. TG analysis indicated that the composite PCMs had good thermal stability. The thermal cycling test including 1000 heating and cooling process was conducted to determine the thermal reliability of the composite PCMs and the test results revealed that the composite PCMs have good thermal reliability and chemical stability.

Analysis and Design of a Paraffin/Graphite Composite PCM Integrated in a Thermal Storage Unit

The addition of latent heat storage systems in solar thermal applications has several benefits including volume reduction in storage tanks and maintaining the temperature range of the thermal storage. A phase change material (PCM) provides high energy storage density at a constant temperature corresponding to its phase transition temperature. In this paper, a high temperature PCM (melting temperature of 80°C) made of a composite of paraffin and graphite was tested to determine its thermal properties. Tests were conducted with a differential scanning calorimeter and allowed the determination of the melting and solidification characteristics, latent heat, specific heat at melting and solidification, and thermal conductivity of the composite. The results of the study showed an increase in thermal conductivity by a factor of 4 when the mass fraction of the graphite in the composite was increased to 16.5%. The specific heat of the composite PCM (CPCM) decreased as the thermal conductivity increased, while the latent heat remained the same as the PCM component. In addition, the phase transition temperature was not influenced by the addition of expanded graphite. To explore the feasibility of the CPCM for practical applications, a numerical solution of the phase change transition of a small cylinder was derived. Finally, a numerical simulation and the experimental results for a known volume of CPCM indicated a reduction in solidification time by a factor of 6. The numerical analysis was further explored to indicate the optimum operating Biot number for maximum efficiency of the composite PCM thermal energy storage.

Calorimetric studies of thermochemical heat storage materials based on mixtures of MgSO 4 and MgCl 2

Attapulgite granulate impregnated with mixtures of MgSO 4 and MgCl 2 hydrates was investigated for suitability as a composite thermal energy storage material. These mixtures were chosen because of the very different deliquescence relative humidities of the salts. The thermochemical heat storage of these materials was characterized by measurements of isothermal heat of sorption and thermogravimetry (TG). The salt mixtures showed a different sorption behavior compared to pure MgSO 4. The reduction of the deliquescence relative humidity of the mixture by the partial substitution of MgSO 4 by MgCl 2 increases the capacity of condensation and therefore the released heat. The energy density of the composite heat storage material containing a salt mixture of 20 wt% MgSO 4 and 80 wt% MgCl 2 was 1590 kJ/kg measured by calorimetry (at 30 °C/85% RH) with a desorption temperature of 130 °C.

Thermal properties of paraffin/graphite composite phase change materials in battery thermal management system

The thermal properties of paraffin/graphite composite phase change materials for power nickel metal hydride batteries were experimentally investigated. Two different modes for heat dissipation were designed in this experimental study: air cooling and cooling with phase change materials. Paraffin/graphite composite phase change thermal energy storage materials were prepared and tested by differential scanning calorimetry. It appeared that the battery thermal management system with phase change materials had better performance than air cooling, especially when the scale of paraffin/graphite composite material approximates 4:1.

Characterization of granular phase changing composites for thermal energy storage using the T-history method

The present article reports on the characterization of granular phase changing composites using the T-history method. Further modifications and improvements of the method are employed to handle granular materials undergoing phase change over a temperature range. The accuracy of the T-history method is shown to be limited by the assumption of temperature-independent specific heats and the difficulty of determining the limits of solid and liquid phases. The concept of enthalpy and its relationship with temperature has been employed in the analysis to overcome these difficulties. Enthalpy–temperature and apparent heat capacity curves similar to those obtained using DSC have been developed. These characteristic curves are necessary for accurate design, modeling, and optimization of latent heat thermal energy storage systems. Experiments have been also carried out to measure the transient temperature distribution inside a cylindrical packed bed using phase changing granulates. Analysis of temperature variation along the bed shows good agreement with the measured phase change characteristics. Copyright © 2009 John Wiley & Sons, Ltd.

06/01521 Study on paraffin/expanded graphite composite phase change thermal energy storage material: Zhang, Z. and Fang, X. Energy Conversion and Management, 2006, 47, (3), 303–310

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.

Study on paraffin/expanded graphite composite phase change thermal energy storage material

Abstract A paraffin/expanded graphite composite phase change thermal energy storage material was prepared by absorbing the paraffin into an expanded graphite that has an excellent absorbability. In such a composite, the paraffin serves as a latent heat storage material and the expanded graphite acts as the supporting material, which prevents leakage of the melted paraffin from its porous structure due to the capillary and surface tension forces. The inherent structure of the expanded graphite did not change in the composite material. The solid–liquid phase change temperature of the composite PCM was the same as that of the paraffin, and the latent heat of the paraffin/expanded graphite composite material was equivalent to the calculated value based on the mass ratio of the paraffin in the composite. The heat transfer rate of the paraffin/expanded graphite composite was obviously higher than that of the paraffin due to the combination with the expanded graphite that had a high thermal conductivity. The prepared paraffin/expanded graphite composite phase change material had a large thermal storage capacity and improved thermal conductivity and did not experience liquid leakage during its solid–liquid phase change.

Granular phase changing composites for thermal energy storage

Granular phase changing composites for thermal energy storage were made of granular porous materials and organic phase changing materials by means of vacuum impregnation method. Experimental studies on the vacuum impregnation method, phase changing behavior, chemical compatibility between porous materials and phase changing materials, and sealing performance of coating materials arrived in the following conclusions. Firstly, the vacuum impregnation method is effective in loading porous materials with phase changing materials; and its setup is simple, cheap and easy of scale-up. Secondly, organic phase changing materials (including fatty acids and their derivatives, and paraffin) and inorganic porous materials (including expanded clay, expanded fly ash and expanded perlite) are suitable raw materials for the phase changing composites with respect to chemical compatibility, large thermal energy storage density, and feasibility of large scale processing. Thirdly, thickened latex is the best choice of coating materials for the porous material granules, whose sealing performance is about 40-fold higher than that of normal cement paste and about sevenfold higher than that of the best polymer modified cement paste in this paper.

Polymeric phase change composites for thermal energy storage

AbstractThis article describes a group of thermal energy storage (TES) composites that combine TES and structural functionality. The composites are encapsulations of low melt temperature phase change materials (PCM) such as paraffin waxes in polymer matrices. Room temperature cured bisphenol‐A epoxy and styrene–ethylene–butylene–styrene (SEBS) polymers are chosen as matrix materials because of their excellent chemical and mechanical properties. The polymeric network structure in the composite encapsulates the PCMs, which transform from the solid to the liquid phase. The PCMs provide the energy storage function via the solid–liquid latent heat effect. The resulting composite exhibits dry‐phase transition in the sense that fluid motion of the PCM, when in the liquid phase, is inhibited by the structure of the polymer matrix. The polymer matrix is formulated to provide structural functionality. The latent heat, thermal conductivity and contact conductance, and structural moduli of composites having various PCM‐to‐matrix volume fractions are measured. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1240–1251, 2004