High-performance Thermal Energy Storage Research Articles

Novel protic ionic liquids-based phase change materials for high performance thermal energy storage systems

Phase change materials (PCMs) are an important class of innovative materials that considerably contribute to the effective use and conservation of solar energy and wasted heat in thermal energy storage systems (TES). The performance of TES can be improved by using environmentally friendly PCMs called ionic liquids (ILs) based on ethanolamines and fatty acids. The 2-hydroxyethylammonium, bis(2-hydroxyethyl)ammonium, and tris(2-hydroxyethyl)ammonium palmitate ILs, which function is in the temperature range of 30–100 °C and provide a safe and affordable capacity, are introduced in this study for the first time as PCMs. PCMs' chemical composition and microstructure were examined using fourier transformation infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM), respectively. DSC was used to evaluate the ILs' latent heat of fusion and specific heat capacity, while TGA was used to establish their thermal stability. Finally, a home-made device with a PCMs (synthesized ILs) container cell and a commercial thermoelectric generator device to record the real-time voltage (V) was used to convert thermal energy into electrical energy.

Waste polyvinyl chloride derived latent thermal energy storage composites for waste heat recovery via packed bed system

Latent thermal energy storage (TES) techniques utilizing phase change materials (PCMs) provide a promising way to mitigate the discrepancy between the intermittent nature of solar energy supply and consistent demand for human society. However, the development is hindered by the slow charging/discharging rates of the low thermal conductivity PCMs. Meanwhile, the high value-added utilization of waste plastic is receiving increasing attention as an environmentally sustainable strategy. In this regard, waste polyvinyl chloride (PVC) is recovered as SiC skeletons to obtain green phase change composites (PCCs) for waste heat recovery. After being infiltrated with paraffin wax (OP44E), proposed PCCs demonstrate a high retention ratio of latent heat up to 91.1% and a thermal conductivity of 2.4 W·m−1·K−1, which is benefitted from limited expansion and impurity remove process, respectively. With the elevated thermal conductivity and spectral absorptance, proposed PCCs exhibit superior solar-thermal energy storage rates. Furthermore, numerical simulation of the thermal charging/discharging performance and flow characteristics is carried out for the packed bed TES system. The results reveal a reduction in charging and discharging time by 21% and 41.1% for proposed PCCs, respectively. This work introduces a novel strategy for recycling waste heat via high-performance TES and repurposing waste plastics simultaneously.

Preparation of composite microencapsulated phase change material based on phosphogypsum for passive building applications

High-performance thermal energy storage technology based on phase change material (PCM) plays an important role in reducing the building energy consumption and realizing efficient energy utilization. However, the drawbacks such as liquid phase leakage, poor thermal conductivity, and low cycle stability, significantly reduces its service life and limits its further application. In this paper, a new microencapsulated phase change material (MPCM) containing graphite-SiO2 shell and eicosane-octadecane (E-O) core was prepared by the sol–gel method. A high thermal conductivity shell was successfully constructed inside the MPCM by double-layer encapsulation technology to accommodate the poor thermal conductivity of E-O. The prepared MPCM had core–shell structures and spherical morphologies, with great enhancement in the thermal conductivity. When the mass ratio of E-O/SiO2 is 2:1, the encapsulation ratio reaches 71.2%, and the corresponding phase change enthalpy of the MPCM is 133.2 kJ/kg. The MPCM is further added to phosphogypsum (PG) which is a by-product of the wet-process production of phosphoric acid, to form composite MPCM. The compressive strength of the composite material is still as high as 5.8 MPa with 15% MPCM despite the negative effect on strength by MPCM. The PG-MPCM showed a good thermal storage capacity and compatibility, furthermore, the thermal stability was maintained also after 2000 thermal cycles. Therefore, the composite PG-MPCM developed in this work is capable of energy storage to reduce the building energy consumption and provides great potential in the PG resource utilization for enhanced environmental protection.

High-performance thermal energy storage and thermal management via starch-derived porous ceramics-based phase change devices

Low thermal conductivity and leakage of phase change materials (PCMs) have severely limited their applications in thermal energy storage and thermal management of electronic devices. Here, we propose starch-derived porous SiC ceramics to achieve high thermal conductivity and prevent leakage of PCMs simultaneously. Porous SiC ceramics with high thermal conductivity of 30 W/m-K at high porosity of 80% are obtained, benefiting from directional pore structures and dense grains enabled via facile directional freeze-drying of starch combined with liquid silicon infiltration technology. Thermal conductivity and thermal energy storage density of SiC/paraffin composite PCMs (CPCMs) attenuated only slightly by 2.75% and 2.80% after 500 repeated heating-cooling cycles, respectively, confirming their longevity and good stability. The phase change enthalpy achieves 331.56 J/g with high thermal conductivity of 24.27 W/m-K maintained by replacing paraffin with LiOH-LiF eutectics. When applying into transient cooling of high-power chips, the chip temperature is 10 ℃ lower if replacing traditional copper by porous SiC/paraffin CPCMs as a cooling medium. Our work demonstrates a promising route to realize efficient thermal energy storage and thermal management of high-power electronics via starch-derived porous ceramics-based phase change devices.

A Low-Temperature Heat Output Photoactive Material-Based High-Performance Thermal Energy Storage Closed System.

Designing and synthesizing photothermal conversion materials with better storage capacity, long-term stability as well as low temperature energy output capability is still a huge challenge in the area of photothermal storage. In this work, we report a brand new photothermal conversion material obtained by attaching trifluoromethylated azobenzene (AzoF) to reduced graphene oxide (rGO). AzoF-rGO exhibits outstanding heat storage density and power density up to 386.1 kJ·kg−1 and 890.6 W·kg−1, respectively, with a long half-life (87.7 h) because of the H-bonds based on high attachment density. AzoF-rGO also exhibits excellent cycling stability and is equipped with low-temperature energy output capability, which achieves the reversible cycle of photothermal conversion within a closed system. This novel AzoF-rGO complex, which on the one hand exhibits remarkable energy storage performance as well as excellent storage life span, and on the other hand is equipped with the ability to release heat at low temperatures, shows broad prospects in the practical application of actual photothermal storage.

Biomass-based shape-stabilized phase change materials from artificially cultured ship-shaped diatom frustules with high enthalpy for thermal energy storage

The high adsorption capacity of the phase change mediums in porous supports is a key requirement for the shape-stabilized phase change materials (ss-PCMs) with high latent heat. Here, ship-shaped diatom (Pennales) frustule-based composite ss-PCMs with high polyethylene glycol (PEG) absorption capacity and high phase change enthalpy was prepared by a solution-assisted vacuum impregnation method for high-performance thermal energy storage. To improve the diatom frustules’ specific surface area and form a multi-level pore structure, the effects of calcination temperature on the microstructure of diatom frustules were studied. It was found that diatom frustules calcined at 400 °C (400CDF) had a relatively high specific surface area (~155.9 m2/g) with a well-maintained skeleton, which was a suitable PEG supporter. The devised PEG/400CDF composites with 72.7% loading of PEG4000 that had a latent heat value of 128.9 J/g for melting and 136.7 J/g for freezing, and the relative enthalpy efficiency reached up to 97.7%. The composite ss-PCMs exhibited thermal and chemical stability even after 200 thermal cycles. The current work demonstrated that ss-PCMs from biomass-based artificially cultured diatoms could slow the spread of heat by absorbing thermal energy. Moreover, the phase change mechanisms of the PEG/CDF composites under the nanoconfinement in the diatom frustules framework were also explored to explain the obtained high adsorption capacity.

Solvent-Assisted Nanochannel Encapsulation of a Natural Phase Change Material in Polystyrene Hollow Fibers for High-Performance Thermal Energy Storage

This work demonstrates a green method for the encapsulation of a natural phase change material (PCM), lauric acid (LA), in polystyrene (PS) hollow fibers through a solvent-assisted diffusion proces...

Nucleation promoting effect of fat shell on phase change material particles dispersed in an emulsion for thermal energy storage medium

Abstract Phase change material emulsions (PCMEs) can be prepared in which a paraffin type phase change material (PCM) is dispersed as small particles in water by the action of a surfactant. Using the latent heat of PCM particles in the PCME, the PCME can be used as an efficient heat storage and transport medium. PCM particles in the PCME are stabilized by the action of a surfactant, but they irreversibly coalesce over time. When the PCM particles change from liquid to solid, they can also show large supercooling because of their small size. These problems must be overcome for PCMEs to be used as efficient thermal media. In this study, PCM particles are emulsified with a high-melting-point fat containing a hydrophobic group consisting of single bonds, in an attempt to overcome these problems. The high-melting-point fat exists as fat shell around PCM. The stability and supercooling characteristics of the PCM particles are investigated. The amount of fat affects the stability of the emulsion. Adding 1.0–5.0 wt% fat is the optimum amount for stabilizing the emulsion. The fat promotes the nucleation of the PCM particles, and the degree of supercooling is suppressed to 3.0 K. Furthermore, the latent heat retained by the emulsion emulsified with fat is about 10% larger than the value obtained by multiplying the latent heat of the PCM by the content of the PCM. These features indicate that PCME prepared with high-melting point fat is suitable as high performance thermal energy storage medium for thermal energy storage and transport system.

Experimental investigation of the performance of thermal energy storage with embedded phase change materials as wall insulator

Thermal energy storage (TES) has been widely adopted to bridge the mismatch between energy supply and demand. In general, thermal energy storage is segregated as sensible TES and latent TES whereby both have their own advantages and disadvantages. This study explores the possibility of utilizing the advantages of both sensible and latent TES by embedding phase change materials as a wall of the sensible TES. To investigate the performance of the proposed TES, an experimental set-up of TES with PCM embedded in the wall is developed. Temperature distribution within TES is measured and recorded during charging and discharging process. Several key parameters are evaluated to gauge their influence on the performance of the studied TES. The results indicate that the performance of TES with embedded PCM is superior that that without PCM, indicating the potential of the proposed design for high performance thermal energy storage application.

Structural Characterisation and Chemical Stability of Commercial Fibrous Carbons in Molten Lithium Salts.

The growing trend towards sustainable energy production, while intermittent, can meet all the criteria of energy demand through the use and development of high-performance thermal energy storage (TES). In this context, high-temperature hybrid TES systems, based upon the combination of fibrous carbon hosts and peritectic phase change materials (PCMs), are seen as promising solutions. One of the main conditions for the operational viability of hybrid TES is the chemical inertness between the components of the system. Thus, the chemical stability and compatibility of several commercial carbon felts (CFs) and molten lithium salts are discussed in the present study. Commercial CFs were characterised by elemental analysis, X-ray diffraction (XRD) and Raman spectroscopy before being tested in molten lithium salts: LiOH, LiBr, and the LiOH/LiBr peritectic mixture defined as our PCM of interest. The chemical stability was evaluated by gravimetry, gas adsorption and scanning electron microscopy (SEM). Among the studied CFs, the materials with the highest carbon purity and the most graphitic structure showed improved stability in contact with molten lithium salts, even under the most severe test conditions (750 °C). The application of the Arrhenius law allowed calculating the activation energy (in the range of 116 to 165 kJ mol−1), and estimating the potential stability of CFs at actual application temperatures. These results confirmed the applicability of CFs as porous hosts for stabilising peritectic PCMs based on molten lithium salts.

Gas-to-gas heat exchanger design for high performance thermal energy storage

The mathematical modelling and optimization of a gas-to-gas heat exchanger with a non-constant cross sectional area is presented. The design of the cross sectional area of the heat exchanger analyzed is based on an hexagonal mesh, which would be highly impractical to fabricate in a conventional way but could be built relatively easily through modern manufacturing techniques. The geometric configuration proposed allows attaining a high exergy efficiency and a significant cost reduction, measured in terms of volume per unit of exergy transfer. The relationship that exists between the overall exergy efficiency of the heat exchanger and its cost is thoroughly explained throughout the study.The results obtained from the modelling demonstrate the premise that it is possible to realize designs for heat exchangers that are highly exergy-efficient and very cheap, owing to the small volume of material required, if the constrains imposed by the limitations of traditional manufacturing methods are set aside. Furthermore, the study reveals a very important fact: the volume of material in a heat exchanger increases in quadratic proportion to its characteristic dimension, which implies that scaling up the geometry has a strong impact on its cost-effectiveness.

A novel phase-change cement composite for thermal energy storage: Fabrication, thermal and mechanical properties

Facing upon the increasingly severe energy crisis, one of the key issues for reducing the building energy consumption is to pursue high-performance thermal energy storage technologies based on phase-change materials. In this study, a novel cement composite incorporated with flaky graphite-doped microencapsulated phase-change materials (FGD-MPCMs) was developed. Various techniques, such as field emission-scanning electron microscopy (FE-SEM), optical microscopy (OM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to analyse the composite structure and thermal performances. The results indicate that the spherical microcapsules are well dispersed in the cement matrix. When combined within the cement, the thermal stability of the microcapsules was highly improved, and the inclusion of greater amounts of FGD-MPCMs further increased the latent heat of the composite. The mechanical properties of the cement composites were affected with the increase of FGD-MPCMs dosage and the porosity of the composites. In spite of this, the compressive strength and flexural strength of the cement composite with 30% FGD-MPCM could still reach to as high as 14.2MPa and 4.1MPa, respectively. Results from the infrared thermography and the model room test suggested that the composite filled with FGD-MPCMs is capable of reducing indoor temperature fluctuation and exhibits good potential for application in buildings to enhance energy savings and thermal comfort.

ARTIFICIAL DISPERSAL OF VOID PATTERNS IN UNIDIRECTIONAL FREEZING

Experiments were conducted to observe the effects of wire screens, wire wool mesh, and soluble gases on the solidification shrinkage void pattern for liquids frozen in constant-volume containers. Prospective applications of these techniques for creating artificial void dispersal are evaluated in light of the models for void initiation and growth developed in an accompanying article. All three approaches show promise of providing better management of solidification shrinkage voids when thermal energy is stored in phase-change materiats, especially under conditions where the voids are ordinarily concentrated to an unacceptable degree. We feel that these results will eventually help usher in a new generation of high-performance thermal energy storage systems due to better void control.