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•Developed a facile method to device a tri-component SiO2/TiO2/PDA shell.•An articulated system that promotes heating through the light scattering effect.•Effectively promoted ...localized heating effect due to wider light absorption.•The fabricated ES-T-PDA exhibits ultraviolet and visible-light photoactivity.•Provides a new shell design for effective solar-thermal conversion of PCM.
To improve the solar-thermal conversion efficiency of encapsulated phase change materials (PCM), a novel hierarchical SiO2/TiO2/polydopamine (ES-T-PDA) composite shell was designed and synthesized through interlayer arrangement on an encapsulated PCM. The corresponding structures, chemical compositions, crystallinity, and thermal induction capacity of the ES-T-PDA nano/microcapsule shell was extensively investigated. The articulated system attained phase-change enthalpies around 125.92 J/g, with a thermal heating increase of ~2.16 ± 0.34 °C due to stimulated light-driven localized heating for efficient solar energy storage. This study confirmed that the combination of SiO2, TiO2 and PDA layered shell was a practical way not only to enhance the structural stability, durability and solar photocatalytic activity but also to promote localized surface photothermal effect due to light scattering effect and visible light absorption. By coupling these two features, the structural design developed in this work displays a considerable prospect for straightforward solar energy employment in various PCM solar-thermal conversion applications such as temperature management of greenhouses, solar water heating systems, solar cookers, and solar-thermal electricity generating systems.
•The R&D status and the technique development trends of USTES are introduced.•The basic research and the most pressing problems of USTES for future work are studied.•The policy system including ...pre-planning and technical policy in China are put forward.•The task and mission of USTES in the future are summed up.
Underground seasonal thermal energy storage (USTES) facilitates the efficient utilization of renewable energy sources and energy conservation. USTES can effectively solve the mismatching characteristics of renewable energy heating system in terms of time, space and strength, which can transfer the renewable energy heating from the summer or transition seasons to the winter, and overcome the instability and low efficiency of the short-term thermal storage system. This research describes the technical classification and operating principle of USTES, reviews the state-of-art of USTES worldwide, and also predicts the development trends of the technology. It reveals that USTES has significant economic, social and environmental benefits. However large heat loss and low solar fraction are still the common challenges for large-scale applications. More work should be carried out on fundamental research including overall design and parameter matching optimization, heat loss mechanism, heat and moisture transfer properties. Moreover, the characteristic and performance of unsteady and transient heat transfer in complex underground environment, and their control strategies of the USTES have been also the most pressing problems. In addition, the supporting policy should be strengthened in planning formulation, legislative support, operation supervision, standard formulation and incentive measures to guide an orderly, healthy and sustainable development of USTES.
The use of a latent heat storage system using phase change materials (PCMs) is an effective way of storing thermal energy and has the advantages of high-energy storage density and the isothermal ...nature of the storage process. PCMs have been widely used in latent heat thermal-storage systems for heat pumps, solar engineering, and spacecraft thermal control applications. The uses of PCMs for heating and cooling applications for buildings have been investigated within the past decade. There are large numbers of PCMs that melt and solidify at a wide range of temperatures, making them attractive in a number of applications. This paper also summarizes the investigation and analysis of the available thermal energy storage systems incorporating PCMs for use in different applications.
Latent heat thermal energy storage (LHTES) uses phase change materials (PCMs) to store and release heat, and can effectively address the mismatch between energy supply and demand. However, it suffers ...from low thermal conductivity and the leakage problem. One of the solutions is integrating porous supports and PCMs to fabricate shape-stabilized phase change materials (ss-PCMs). The phase change heat transfer in porous ss-PCMs is of fundamental importance for determining thermal-fluidic behaviours and evaluating LHTES system performance. This paper reviews the recent experimental and numerical investigations on phase change heat transfer in porous ss-PCMs. Materials, methods, apparatuses and significant outcomes are included in the section of experimental studies and it is found that paraffin and metal foam are the most used PCM and porous support respectively in the current researches. Numerical advances are reviewed from the aspect of different simulation methods. Compared to representative elementary volume (REV)-scale simulation, the pore-scale simulation can provide extra flow and heat transfer characteristics in pores, exhibiting great potential for the simulation of mesoporous, microporous and hierarchical porous materials. Moreover, there exists a research gap between phase change heat transfer and material preparation. Finally, this review outlooks the future research topics of phase change heat transfer in porous ss-PCMs.
•The recent advances investigations in phase change heat transfer in porous ss-PCMs are reviewed.•Paraffin and metal foams are the mostly used PCM and porous support respectively in the experimental studies.•The pore-scale simulation can provide extra flow and heat transfer characteristics in pores.•There exists a research gap between phase change heat transfer and material preparation.
Grid scale electrical energy storage is considered facilitative for the increased deployment of renewable energy. Recent progress in the development of large scale thermal energy storage systems ...operated at medium and high temperatures has sparked the interest in the application of this technology as a storage sink for electricity. Life expectancies in the range of 20–30 years, low capacity-specific costs, a low environmental impact and flexibility regarding sites make thermo-mechanical energy storage a promising option for future bulk storage of electricity. A large number of concepts have been developed, which vary in storage efficiency, complexity and maturity. This paper provides an overview of the basic concepts for thermo-mechanical energy storage and describes various implementations and their characteristics. The utilization of waste heat, the combined delivery of heat and power during discharge and the integration of storage modules into power plants are described as additional options for some of these thermo-mechanical concepts.
•Enhancement methods of thermal conductivity for medium-high temperature PCMs.•Design and optimisation strategies for LTES heat exchangers.•LTES energy systems for heat recovery, storage and ...utilisation.•An in-depth summary of the medium-high temperature LTES materials.
Thermal energy storage (TES) technology is considered to have the greatest potential to balance the demand and supply overcoming the intermittency and fluctuation nature of real-world heat sources, making a more flexible, highly efficient and reliable thermal energy system. This article provides a comprehensive state-of-the-art review of latent thermal energy storage (LTES) technology with a particular focus on medium-high temperature phase change materials for heat recovery, storage and utilisation. This review aims to identify potential methods to design and optimise LTES heat exchangers for heat recovery and storage, bridging the knowledge gap between the present studies and future technological developments. The key focuses of current work can be described as follows: (1) Insight into moderate-high temperature phase change materials and thermal conductivity enhancement methods. (2) Various configurations of latent thermal energy storage heat exchangers and relevant heat transfer enhancement techniques (3) Applications of latent thermal energy storage heat exchangers with different thermal sources, including solar energy, industrial waste heat and engine waste heat, are discussed in detail.
•Repurposing a geothermal exploration well for BTES.•Comparison of local and global sensitivity analyses for BTES.•Development of a new metric for evaluating storage efficiency for ...BTES.•Determination of most influential parameters using statistical methods.•Comparison of extraction only DBHE operation to storage-extraction operation.
Borehole thermal energy storage (BTES) is an important technology to minimise greenhouse gas emissions by storing surplus heat from industrial processes, space cooling or even excess summertime renewable wind or solar energy. This paper investigates the efficiency of BTES via a single deep ex-geothermal exploration well in Newcastle, retrofitted using a coaxial deep borehole heat exchanger (DBHE) completion. Previously, few studies have investigated 1) the use of a single DBHE for thermal energy storage or 2) the retrofitting of an ex-geothermal exploration well; therefore, this study investigates deep BTES through numerical modelling on MATLAB by testing the impacts of 10 design parameters on operational performance of a DBHE using both global and local sensitivity analyses.
Under base-case conditions, a DBHE operating at a depth of 920 m could achieve a heat extraction rate in excess of c.54 kW recorded at the end of a 6 month (winter) heat production phase. When applying a 6 month (summer) thermal charge phase prior to extraction (recorded as 250 kW at the end of the charge period), the thermal yield recorded at the end of extraction was increased to a minimum of c.69 kW. In total, over an annual cycle, 1.23 GWh of heat was injected into the formation, and 0.46 GWh was extracted. Across all local sensitivity simulations, the average heat extraction rate was increased by 9.5–55.6 kW following a 6 month period of charge. The global sensitivity analysis demonstrated that thermal recovery was most influenced by parameters such as the undisturbed geothermal gradient, flow rate, inlet temperature during charge and inlet temperature during extraction. Most of these are operational parameters, indicating deep BTES systems can be optimised through careful engineering. The study concludes that single DBHEs have some capacity to store surplus heat. However, the additional heat yield during extraction is only a modest proportion of the heat reinjected to the formation during the charging phase (calculated as <20 % using the new storage efficiency metric proposed in this study). This approach is only likely to be viable where there is a large source of surplus heat with little alternative value, and where there is an existing deep borehole suitable for retrofitting. If these conditions do not exists, more conventional, shallower, multi-borehole arrays are likely to be more suitable for BTES.
This study aims to enhancement the thermal conductivity of RT35HC, as a commercial paraffin, by integrating boron carbide (B4C) nanoparticles for the first time, thereby producing B4C-nanoadditive ...nanocomposite PCMs. The B4C nanoparticles were reinforcement to RT35HC at mass fraction percentages (wt.%) of 0.5, 1, 1.5 and 2 by melting and physical mixing method. The structural and morphological characteristics of both pure and nanocomposite PCMs were examined using XRD, FT-IR, FE-SEM, and EDX. Thermal properties were investigated through DSC, TGA/DTA, and thermal conductivity measurements using the KD2-Pro device. The Gaussian process regression (GPR) model was used to analyze the Cp values in relation to temperature and additive ratio. Structural and morphological analysis results indicated a homogeneous distribution of nanoparticles within the PCM matrix, without any significant chemical or physical alterations. The introduction of B4C-nanoadditive did not markedly affect the melting and solidification temperatures. However, melting and solidification enthalpies decreased proportionally with increased nanoadditive ratios, with the greatest reductions being 7.44 % and 5.74 % at a 2 wt% nanoaddition rate, respectively. As the nanoadditive ratio increased, the thermal conductivity (k) and specific heat capacity (Cp) of RT35HC in both solid and liquid-phases enhanced significantly. Specifically, solid-phase (25 °C) k values increased by 67.51 % from 0.197 to 0.33, and liquid-phase (50 °C) k values by 15.29 % from 0.170 to 0.196. The highest Cp values in the solid and liquid-phases were measured as 3.01 and 2.49, respectively, in the nanocomposite with a high nanoadditive ratio. The GPR method yielded a success rate of 0.9015. Additionally, the nanocomposites exhibited enhanced thermal stability and higher thermal decomposition temperatures. Based on these characterizations, the fabricated B4C-nanoadditive nanocomposite PCMs show promise for application in TES and TM systems.
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•A novel PCM nanocomposite was produced adding B4C nanoparticles to RT35HC.•The incorporation of B4C nanoparticles into RT35HC notably enhanced its thermal conductivity and specific heat capacity.•The enhanced thermal decomposition temperature of the nanocomposite indicates its suitability for long-term use.•The enhanced thermal properties of the nanocomposites hold significant promise for TES and TM applications.
The corrosive effects of a molten nitrate mixture of 48% Ca(NO3)2+7% NaNO3+45% KNO3 were appraised at 390°C on two different types of stainless steels (AISI 304, 430), a low-Cr alloy steel (T22) and ...a carbon steel (A1).
The corrosion rates were determined using gravimetric tests, which measured the weight gain during 2000h to identify the corrosion products with scanning electron microscopy (SEM) and X-ray diffraction (XRD).
Studies on stainless steel revealed a better behaviour in corrosive environment, which mainly identifies the formation of (K,Na)CrO4 protective spinel.
Fe2O3 and Fe3O4 were the other important products that were found in the tests performed at 390°C; the formation of some stable compounds with salt impurities, such as carbonates and sulphates, was also observed.
•Hitec XL salt improve the corrosion resistance in storage materials.•Gravimetric study confirms the corrosion reduction in commercial steels.•Low wt% Cr addition improves the corrosion in carbon steels significantly.
•Hydrated salt PCMs have promising application prospects in thermal energy storage.•Research advances on form-stable hydrated salt PCMs are summarized and reviewed.•Effects on thermophysical ...properties of stabilized hydrated salt PCM are discussed.•Pointing out potential future research directions of form-stable hydrated salt PCM.
Thermal energy storage based on inorganic hydrated salt phase change materials (PCMs) has attracted considerable attention due to the apparent advantages of high energy storage density, non-toxic, cost effective, etc. It is expected to improve the utilization efficiency of renewable energy and mitigate the energy crisis. Nevertheless, there are still some limitations including phase segregation, supercooling, low thermal conductivity and leakage, etc. For these reasons, stabilizing of PCMs is being developed as promising technique to overcome the obstacles. Although many works have been reported concerning the preparation of form-stable hydrated salt PCMs, to date, these stabilizing techniques and their effect on the thermophysical properties of hydrated salt PCMs have never been systematically summarized. Hence, we are committed to review the works about stabilizing techniques for hydrated salt PCMs, present the analysis around the influence of stabilizing method on the thermophysical properties of form-stable hydrated salt PCMs. Three major types of stabilizing hydrated salt PCMs are found: adsorption and impregnation into the porous materials, core-shell encapsulation methods and organic three-dimensional network structures. Furthermore, the thermophysical properties such as phase change temperature, latent heat, thermal conductivity, supercooling and phase segregation as well as thermal cycling reliability are deeply discussed as well. In addition, the application areas of form-stable hydrated salt PCMs are presented from detailed investigations. Based on our best knowledge, this is the first profound review of stabilizing techniques for hydrated salt PCMs, and researcher will gain significant insight into the field for preparation and application of form-stable hydrated salt PCMs.
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