Thermally Conductive Magnetic Composite Phase Change Materials for Anisotropic Photo/Magnetic-to-Thermal Energy Conversion

The distinctive thermal energy storage properties of phase change materials (PCMs) are critical for solving energy issues. However, their inherently low thermal conductivity and limited energy conversion capability impede their applications in advanced thermal energy harvesting and storage systems. Herein, we developed magnetic composite PCMs with enhanced thermal conductivity for anisotropic photothermal and magnetic-to-thermal energy conversions. The hierarchically interconnected ferroferric oxide-coated boron nitride/poly(vinyl alcohol) (BN@Fe3O4/PVA) porous scaffolds were constructed by a unidirectional freeze-casting method to enhance the directional heat transfer capability of the composite PCMs with a through-plane thermal conductivity of 1.84 W m-1 K-1 at a BN@Fe3O4 loading of 25.4 wt %. The superparamagnetic Fe3O4 nanoparticles endow the composite PCMs with unique solar absorption and magnetic response properties, and the energy conversion efficiency can be regulated by controlling the orientation of the synthesized magnetic particles in the composite PCMs. As a consequence, the resulting composite PCMs exhibit superior photo/magnetic-to-thermal energy conversion efficiency along the direction of orientation of magnetic particles. These novel findings provide an instructive guide to yield composite PCMs for efficient energy conversion.

Preparation and Characteristics of Na2HPO4·12H2O-K2HPO4·3H2O/SiO2 Composite Phase Change Materials for Thermal Energy Storage

In this paper, a series of eutectic hydrated salts was obtained by mixing Na₂HPO₄·12H₂O (DHPD) with K₂HPO₄·3H₂O (DHPT) in different proportions. With the increase in the content of DHPT, the phase transition temperature and melting enthalpy of eutectic hydrated salts decreased gradually. Moreover, the addition of appropriate deionized water improved the thermal properties of eutectic hydrated salts. Colloidal silicon dioxide (SiO₂) was selected as the support carrier to adsorb eutectic hydrated salts, and the maximum content of eutectic hydrated salts in composite PCMs was 70%. When the content of the nucleating agent (Na₂SiO₃·9H₂O) was 5%, the supercooling degree of composite PCMs was reduced to the minimum of 1.2 °C. The SEM and FT-IR test results showed that SiO₂ and eutectic hydrated salts were successfully combined, and no new substances were formed. When the content of DHPT was 3%, the phase transition temperature and melting enthalpy of composite PCMs were 26.5 °C and 145.3 J/g, respectively. The results of thermogravimetric analysis and heating-cooling cycling test proved that composite PCMs had good thermal reliability and stability. The application performance of composite PCMs in prefabricated temporary houses was investigated numerically. The results indicated that PCM panels greatly increased the Grade I thermal comfort hours and reduced energy consumption. Overall, the composite PCM has great development potential building energy conservation.

Preparation of SA-PA-LA/EG/CF CPCM and Its Application in Battery Thermal Management

To improve the heat dissipation efficiency of batteries, the eutectic mass ratios of each component in the ternary low-melting phase change material (PCM), consisting of stearic acid (SA), palmitic acid (PA), and lauric acid (LA), was explored in this study. Subsequently, based on the principle of high thermal conductivity and low leakage, SA-PA-LA/expanded graphite (EG)/carbon fiber (CF) composite phase change material (CPCM) was prepared. A novel double-layer CPCM, with different melting points, was designed for the battery-temperature control test. Lastly, the thermal management performance of non-CPCM, single-layer CPCM, and double-layer CPCM was compared via multi-condition charge and discharge experiments. When the mass ratio of SA to PA is close to 8:2, better eutectic state is achieved, whereas the eutectic mass ratio of the components of SA-PA-LA in ternary PCM is 29.6:7.4:63. SA-PA-LA/EG/CF CPCM formed by physical adsorption has better mechanical properties, thermal stability, and faster heat storage and heat release rate than PCM. When the CF content in SA-PA-LA/EG/CF CPCM is 5%, and the mass ratio of SA-PA-LA to EG is 91:9, the resulting SA-PA-LA/EG/CF CPCM has lower leakage rate and better thermal conductivity. The temperature control effect of single-layer paraffin wax (PW)/EG/CF CPCM is evident when compared to the no-CPCM condition. However, the double-layer CPCM (PW/EG/CF and SA-PA-LA/EG/CF CPCM) can further reduce the temperature rise of the battery, effectively control the temperature and temperature difference, and primarily maintain the battery in a lower temperature range during usage. After adding an aluminum honeycomb to the double-layer CPCM, the double-layer CPCM exhibited better thermal conductivity and mechanical properties. Moreover, the structure showed better battery temperature control performance, while meeting the temperature control requirements during the charging and discharging cycles of the battery.

Methods to Characterize Effective Thermal Conductivity, Diffusivity and Thermal Response in Different Classes of Composite Phase Change Materials

The phase change materials (PCMs) used in devices for thermal energy storage (TES) and management are often characterized by low thermal conductivity, a limit for their applicability. Composite PCMs (C-PCM), which combine active phase (proper PCM) with a passive phase with high conductivity and melting temperature have thus been proposed. The paper deals with the effect of length-scale on thermal characterization methods of C-PCM. The first part of the work includes a review of techniques proposed in the scientific literature. Up to now, special focus has been given to effective thermal conductivity and diffusivity at room or low temperature, at which both phases are solid. Conventional equipment has been used, neglecting length-scale effect in cases of coarse porous structures. An experimental set-up developed to characterize the thermal response of course porous C-PCMs also during active phase transition at high temperature is then presented. The setup, including high temperature-heat flux sensors and thermocouples to be located within samples, has been applied to evaluate the thermal response of some of the above C-PCMs. Experimental test results match Finite Elements (FE) simulations well, once a proper lattice model has been selected for the porous passive phase. FE simulations can then be used to estimate temperature difference between active and passive phase that prevents considering the C-PCM as a homogeneous material, to describe it by effective thermo-physical properties. In the engineering field, under these conditions, the design steps for TES systems design cannot be simplified by considering C-PCMs as homogeneous materials in FE codes.

Preparation and Thermal Performance Enhancement of Low Temperature Eutectic Composite Phase Change Materials Based on Na₂SO₄·10H₂O

In this paper, a series of Na₂SO₄·10H₂O–KCl eutectic mixtures were prepared by adding different mass fractions of KCl (1 wt.%, 3 wt.%, 5 wt.%, or 7 wt.%) to Na₂SO₄·10H₂O. Polyacrylamide (PAM) was proposed as the thickener, sodium tetraborate decahydrate (STD) was proposed as the nucleating agent, and expanded graphite (EG) was proposed as the high thermal conductivity medium for Na₂SO₄·10H₂O–5 wt.% KCl eutectics. The results showed that in Na₂SO₄·10H₂O–5 wt.% KCl eutectics with 5 wt.% PAM and 5 wt.% STD, almost no phase separation occurred, and the degree of supercooling was reduced to 0.4 °C. The thermal performance of Na₂SO₄·10H₂O–5 wt.% KCl composite phase change materials (CPCMs) with varying contents of EG was explored. The results showed that EG could improve the thermal conductivity effectively and that the mass fraction of EG should be no more than 3%, otherwise the crystallization value and supercooling would deteriorate. The thermal reliability of the Na₂SO₄·10H₂O–5 wt.% KCl eutectic CPCMs containing 5 wt.% PAM, 5 wt.% STD, and 3 wt.% EG was investigated, mainly through the ambient temperature, thermal cycling test, and TGA analysis. The results demonstrated that these CPCMs showed perfect thermal reliability.

Polyethylene Glycol-Carbon Nanotubes/Expanded Vermiculite Form-Stable Composite Phase Change Materials: Simultaneously Enhanced Latent Heat and Heat Transfer

Polyethylene glycol (PEG)-carbon nanotubes (CNTs) with expanded vermiculite (EVM) form-stable composite phase change materials (PCE-CPCMs) were constructed via the efficient synergistic effect between EVM and CNTs. The resultant material demonstrated simultaneously enhanced latent heat and heat transfer. The unique EVM pore structure and CNTs surfaces contributed to the form stability of PCE-CPCMs. The adsorption capacity was 77.75–81.54 wt %. The latent heat of the PCE-CPCMs increased with increasing CNTs content due to the decreasing inhibition effect of EVM and the increasing adsorption capacity of PEG, which was 83.9 J/g during melting and 104.2 J/g during solidification for PCE7.09. The pore confinement and surface EVM interactions inhibited the heat storage capacity of the PCE-CPCMs. Moreover, the inhibition effect on the heat storage capacity of PCE-CPCMs during the melting process was stronger than during solidification due to the crystallization-promoting effect. The heat transfer of PCE-CPCMs was significantly enhanced by the CNTs filler (0.5148 W/(m·K) for PCE7.09) due to the decrease in interfacial thermal resistance and the formation of rapid thermally conductive pathways. Fourier transform infrared spectroscopy, thermogravimetric analysis, and thermal cycles test results confirmed that the PCE-CPCMs exhibited excellent chemical compatibility, thermal stability, and reliability.

Thermal Properties of Cement-Based Composites for Geothermal Energy Applications

Geothermal energy piles are a quite recent renewable energy technique where geothermal energy in the foundation of a building is used to transport and store geothermal energy. In this paper, a structural–functional integrated cement-based composite, which can be used for energy piles, was developed using expanded graphite and graphite nanoplatelet-based composite phase change materials (CPCMs). Its mechanical properties, thermal-regulatory performance, and heat of hydration were evaluated. Test results showed that the compressive strength of GNP-Paraffin cement-based composites at 28 days was more than 25 MPa. The flexural strength and density of thermal energy storage cement paste composite decreased with increases in the percentage of CPCM in the cement paste. The infrared thermal image analysis results showed superior thermal control capability of cement based materials with CPCMs. Hence, the carbon-based CPCMs are promising thermal energy storage materials and can be used to improve the durability of energy piles.

Design and Preparation of Carbon Based Composite Phase Change Material for Energy Piles

Energy piles-A fairly new renewable energy concept-Use a ground heat exchanger (GHE) in the foundation piles to supply heating and cooling loads to the supported building. Applying phase change materials (PCMs) to piles can help in maintaining a stable temperature within the piles and can then influence the axial load acting on the piles. In this study, two kinds of carbon-based composite PCMs (expanded graphite-based PCM and graphite nanoplatelet-based PCM) were prepared by vacuum impregnation for potential application in energy piles. Thereafter, a systematic study was performed and different characterization tests were carried out on two composite PCMs. The composite PCMs retained up to 93.1% of paraffin and were chemically compatible, thermally stable and reliable. The latent heat of the composite PCM was up to 152.8 J/g while the compressive strength of cement paste containing 10 wt % GNP-PCM was found to be 37 MPa. Hence, the developed composite PCM has potential for thermal energy storage applications.