Low-temperature Heat Storage Research Articles

Effects of firebricks for industrial process heat on the cost of matching all-sector energy demand with 100% wind-water-solar supply in 149 countries.

Refractory bricks are bricks that can withstand high temperatures without damage to their structures. They have been used to insulate kilns, furnaces, and other hot enclosures for thousands of years. Firebricks are refractory bricks that can, with one composition, store heat, and with another, insulate the firebricks that store the heat. Because firebricks are made from common materials, the cost per kilowatt-hour-thermal of a firebrick storage system is less than one-tenth the cost per kilowatt-hour-electricity of a battery system. It has thus been hypothesized that using excess renewable electricity to produce and store industrial process heat in firebricks can provide a low-cost source of continuous heat for industry. Here, it is hypothesized further that, upon a transition to 100% clean, renewable energy worldwide, using firebricks to store industrial process heat can reduce electricity generator, electricity storage, and low-temperature heat storage needs, thereby reducing overall energy cost. Both hypotheses are tested across 149 countries combined into 29 world regions. Results suggest, relative to a base case with no firebricks, using firebricks may reduce, among all 149 countries, 2050 battery capacity by ∼14.5%, annual hydrogen production for grid electricity by ∼31%, underground low-temperature heat storage capacity by ∼27.3%; onshore wind nameplate capacity by ∼1.2%, land needs by ∼0.4%, and overall annual energy cost by ∼1.8%. In sum, the use of firebricks for storing industrial process heat appears to be a remarkable tool in reducing the cost of transitioning to clean, renewable energy across all energy sectors.

Preparation and properties of activated carbon-based Na3PO4 composites for low-temperature thermochemical heat storage

With the advantages of low heat storage temperature, high heat storage density, and a low price, Na3PO4 has been proposed as a thermochemical heat storage candidate material for use in solar low-temperature heat storage engineering applications. However, the application of pure Na3PO4 is limited due to its propensity for agglomeration. In this study, activated carbon-based Na3PO4 (R-AC@Na3PO4) composites with mass fractions of 30 %, 50 %, and 80 % were synthesized by melt impregnation method using activated carbon as the porous matrix. The morphology and structure of the composites were characterized, and their heat storage performances were investigated. The hydration experiments were carried out in a constant temperature and humidity chamber and dehydration experiments were carried out in thermogravimetric analysis and differential scanning calorimetry simultaneously. The results showed that at 30 °C, 60 % relative humidity, the hydration equilibrium time for the composites were only 30 %–40 % of the pure material. Meanwhile, at a mass fraction of 80 %, material's volumetric heat storage can reach 1793 J·cm−3 without agglomeration, and the performance was stable after 20 cyclic tests. This study provides a reference for the research and development of low-temperature thermochemical heat storage materials and offers more possibilities for R-AC@Na3PO4 composites in future engineering applications.

The State of Agricultural Development and Industrial Prosperity Pathway and Strategy Design in the Context of Data-Driven and Rural Revitalization

Current advances in materials science are providing a huge variety of novel adsorbents that could be used for the transformation and storage of low-temperature heat, such as the heat generated by renewable energy sources.Achieving industrial prosperity is a key step in building an all-round well-off society and realizing agricultural modernization. In the process of industrial prosperity, there are also many problems: firstly, the long-term traditional farming in China has led to the information blockage and ideological constraint of most farmers in China, so that they are slow to accept new things and new ideas; secondly, due to the "one long and one short" characteristic of long growth cycle and short storage cycle of agricultural products, it is difficult for rural transportation and deep processing of agricultural products. Secondly, because of the "long and short" nature of the long growth cycle and short storage cycle of agricultural products, the demand for rural transportation and deep processing of agricultural products is high. This study is based on the problems related to sustainable agricultural development in traditional agricultural areas. The main purpose of this study is to clarify the sustainable development status of agricultural systems under different agricultural development paths in traditional agricultural areas, to establish a methodological system for studying regional agricultural development paths, to explore the optimal path for sustainable regional agricultural development, and orderly management of sustainable agricultural development in traditional agricultural areas, so as to promote the sustainable development of traditional agricultural areas with economic prosperity, social stability and ecological health. The experimental results prove that the optimized SGA algorithm has carried out good system dynamics modeling for the typical traditional agricultural area - the problem of choosing the agricultural development path, and the comprehensive evaluation of the simulation results.

NaI as suitable alkali halide hydrate for domestic thermochemical heat storage

The limits and potentials of alkali (Li - Cs) halides (F - I) as salt hydrate-based thermochemical heat storage materials are studied. Most alkali halides are found unsuitable, and only the hydrates of sodium iodide (NaI) and sodium bromide (NaBr) were investigated experimentally. We confirmed that both have high energy densities of 1.5 GJ/m3 and 1.6 GJ/m3 respectively, based on the crystal structures of the dihydrate phases. NaI demonstrated full cyclic stability at water vapor pressures of 12 and 14.5 mbar, but NaBr did not show rehydration at these conditions. Therefore, NaBr was deemed unsuitable for domestic applications. For the NaI – H2O system, we have measured the p,T equilibrium line between the hydration states and constructed p,T and T,x – phase diagrams. This showed that the hydration temperature is 42 °C for 12 mbar water vapor pressure which is enough for domestic space heating. Additionally, the phase diagrams revealed an incongruent melting point where the NaI·2H2O is in equilibrium with the anhydrate and a saturated aqueous NaI solution. This melting can overlap with dehydration and thereby hinders the cyclability, since in the melting process part of the salt is dissolved which blocks the pores formed during dehydration. Therefore, the performance of NaI can be improved by ensuring dehydration is completed below the incongruent melting point of 68 °C. We also found that pre-cycling can prevent deliquescence which is expected to occur at 25 °C and 12 mbar. Overall, NaI could be a candidate for low temperature heat storage.

Microstructural changes in thermochemical heat storage material over cycles: Insights from micro-X-ray computed tomography

This paper studies the effect of successive (de)hydration cycles on the structure of potassium carbonate K2CO3·1.5H2O grains for low-temperature heat storage applications. Such structural changes are caused by exposure of the salt to water vapor or removal of water from it, accompanied by successive swelling and shrinkage. Understanding the material's internal structure is key to predicting its behaviour and optimizing its design. However, due to the simultaneous and persistent occurrence of structural changes and transport mechanisms throughout the process, gaining a complete understanding of the phenomenon can be challenging.Unlike conventional experimental approaches and two-dimensional imaging techniques used for porosity assessment, our study showcases the qualitative and quantitative alterations in the porosity and microstructure of potassium carbonate. This analysis is achieved by using Micro-X-ray computed tomography (Micro-CT). The study focuses on the impact of cycling on grain microstructure, investigating pore volume distribution, radial variation of pore sizes, and density of individual grains. It was noted that the porosity increased from 6.4 % to 19.7 % after seven cycles. Initially, we observed a greater number of pores in the core of the uncycled salt grain. However, after cycling, we noticed a more even distribution, with a higher number of pores in the outer region of the grain, which caused a radial change in porosity. Lastly, this research provides the intrinsic and apparent densities of both non-cycled and cycled potassium carbonate specimens.Micro-CT is a good tool for a better understanding of changes in thermochemical material at a structural level. Calculation of porosity provided a pathway to calculate apparent and intrinsic density. The demonstrated method can be used for a wide range of salt hydrates, enhancing the scope and applicability of this study in the field of low-temperature heat storage applications. Additionally, it gives the measuring parameter needed to calculate energy density and change in volume during the reaction.

Research on the low-temperature waste heat storage performance of a novel microencapsulated phase change material with a double-shell encapsulation of hexadecylamine

Phase change materials (PCMs) have the advantages of high energy storage density and the temperature can be kept stable when the phase change process occurs, so they can be widely applied to the recovery of industrial waste heat. In this paper, hexadecylamine (HDA) was selected as PCMs, polystyrene (PS) and calcium carbonate (CaCO3) as wall materials, and PS/CaCO3/HDA microencapsulated phase change materials (MEPCMs) were prepared by ultrasonic emulsification. Then the prepared PCMs were characterized and their properties were tested by using transmission electron microscopy, scanning electron microscopy, X-ray diffraction analyzer, X-ray photoelectron spectrometer, Fourier infrared spectrometer, thermogravimetry, differential scanning calorimetry and other characterization methods. The results showed that PS/CaCO3/HDA MEPCMs with microcapsule structure were successfully prepared with a diameter of about 1.5 μm. The thermal conductivity of PS/ CaCO3/HDA MEPCMs was 0.491 W/(m·K), which was 3.41 times of the thermal conductivity of pure HDA. The highest latent heat of PS/CaCO3/HDA MEPCMs was 33.96 kJ/kg, and no leakage problems occurred. The above results indicate that PS/CaCO3/HDA MEPCMs have good stability and high thermal conductivity, and they have good thermal storage performance at low temperatures.

Double Carbon Networks Reinforce the Thermal Storage and Thermal Transfer Properties of 1-Octadecanol Phase Change Materials.

Using thermal storage materials with excellent thermal properties in the energy utilization system enables efficient use of renewable energy sources. Organic phase change materials (PCMs) have the advantages of high heat storage density, no corrosion, and low cost, but low thermal conductivity and insufficient heat transfer capacity have always been the bottlenecks in their application. In this paper, melamine foam@ reduction graphene oxide (MF@rGO) and carbon foam@ reduction graphene oxide (CF@rGO) composite foams with double carbon networks were prepared by self-assembly method and further employed in 1-octadecinal (OD) PCMs. The microstructure, chemical composition, phase change behavior, thermal conductivity, and photothermal conversion performance of MF@rGO/OD and CF@rGO/OD were studied in detail using SEM, FTIR, Raman DSC, and LFA. The melting and solidification enthalpies of CF@rGO/OD composite PCMs were 208.3 J/g and 191.4 J/g, respectively, its thermal conductivity increased to 1.54 W/m·K, which is 6.42 times that of pure OD. The porous structure and high thermal conductivity of the double carbon network substantially enhance the efficiency of energy storage and release in composite PCMs. CF@rGO/OD composite PCMs have excellent heat storage performance and heat transfer capacity, and a wide range of application prospects in the fields of low-temperature solar heat storage, precision instrument temperature control, and intelligent buildings.

High-power-density packed-bed thermal energy storage using form-stable expanded graphite-based phase change composite

Thermal energy storage is highlighted as a crucial strategy for energy saving and utilization, in which domain, latent heat storage using phase change materials has gained great potential for efficient heat storage and thermal management applications. A strategy for developing high energy-storage-density and power-density latent heat storage units, through the compression-induced assembly of expanded graphite based stearic acid composites and the macro encapsulation method by using polyethylene shells, is demonstrated. The fabricated composite shows a satisfactory phase change enthalpy of 161.24 ± 0.5 J g−1, and enhances thermal conductivity to 13.4 ± 0.8 W m−1 K−1. The resulting heat storage unit also exhibits form-stable, leakage-proof, good homogeneity, and high-power-density behaviors. A 0.462 kWh proof-of-concept prototype of the packed-bed latent-heat-storage system by using 492 heat storage units has demonstrated its feasibility in fast heat charging/discharging operations. The outlet air temperature in the discharging process can maintain above 30 °C for over 1.74 h with a heat storage utilization efficiency of 90.3 ± 6.1% and an effective discharging efficiency of 93.5 ± 9.4%, under a volumetric flow rate of 30 m3 h−1 and heat storage temperature of 27–86 °C. The maximum and average power density, and effective energy density are obtained as 20.7 ± 1.6 kW m−3, 14.2 ± 0.9 kW m−3, 24.8 ± 2.5 kWh m−3, respectively, with a discharging threshold temperature of 30 °C. This high-power-density apparatus using form-stable heat storage units has realized hourly rapid heat charging-discharging processes, showing its prospective potential of low-temperature heat storage and thermal management.

Polymeric stabilization of salt hydrates for thermochemical energy storage

Non-stabilized thermochemical materials impose several limitations on their use. These include swelling/shrinkage, cracking, and agglomeration over cycles. In addition, the deliquescence transition cannot be used and is even considered an unwanted side effect. In this work several salt hydrates for low temperature heat storage (K2CO3, CaCl2 and LiCl) are stabilized within a highly porous mm-sized polymer matrix. The composites containing wetting salt solutions are shown to be stable towards deliquescence. Three different composites were cycled. A K2CO3-polymer composite was cycled for 50 hydration/dehydration cycles and found to be kinetically and mechanically stable over all cycles, with swelling at higher cycle numbers. A LiCl-polymer composite was cycled for 40 cycles after which the composite became unstable. The composite containing CaCl2 was found to be kinetically and mechanically stable for 15 cycles. Composites with energy densities up to 2.4 GJ·m-3 and a peak power output of 325 W·kg-1 were fabricated which is equal or higher compared to previously reported systems. All composites have power outputs which are sustained at higher levels throughout the full discharge cycle. This work opens new pathways to stabilize salt hydrates as well-defined mm-sized particles exhibiting cyclic stability, while maintaining a high energy density and power output.

Preparation and low temperature heat storage properties of 1-hexadecylamine/3D graphene aerogel composite phase change materials

Preparation and low temperature heat storage properties of 1-hexadecylamine/3D graphene aerogel composite phase change materials

MgCl2-MXene based nanohybrid composite for efficient thermochemical heat storage application

Thermochemical heat storage is a practical approach for overcoming the challenge in solar energy applications. Herein, we synthesize a novel MgCl2/MXene (Ti3C2Tx) composite sorbent by impregnation method for low-temperature thermochemical heat storage application. Various techniques, including Thermogravimetric coupled with Differential Scanning Calorimetric (TG-DSC), X-Ray, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were applied to characterize the mechanical and thermal properties of MgCl2/MXene composites. The result showed that Ti3C2Tx possess a larger specific surface area and the hydrophilic groups allow maximum MgCl2 adsorption without structural distortion. The TG-DSC studies showed that the fourth DSC peak of MgCl2 (181.33 °C) is shifted in composites MX/Mg 15 % (159.63 °C) and MX/Mg 10 %, (141.67 °C) toward lower dehydration temperature. The cyclicability study demonstrated that in the 10 consecutive rehydration cycles, MgCl2 shows 45 % decrease in the enthalpy, while Mg/MX 10 % and Mg/MX 15 % composites record 4 % and 10 % decrease, respectively. Water uptake kinetics highlighted that at 85 % RH, the highest water sorption was recorded for Mg/MX 15 % where it absorbs 2.21 g/g mass. The energy storage density result reveals that the maximum loading rate of MgCl2·6H2O reaches 97.6 wt% for Mg/MX 15 % showed the highest enthalpy 2227 J/g at 85 % RH. Therefore, all the results indicate that MXene-based salt composites play an important role in thermochemical heat storage application, leading to a potential candidate for long-term heat storage.

Multicomponent numerical model for heat pump control with low-temperature heat storage: A benchmark in the conditions of Central Europe

This paper presents a computational parametric study on increasing the Seasonal Coefficient of Performance for residential heat pumps, utilizing an enhanced computational model and comparing it to previous research. The model computes a system that consists of a heat pump, low-temperature heat storage, heat exchanger and circulation pump, and a control unit which allows the heat pump to choose either low-temperature heat storage or ambient air as the favorable heat source. A parametric study is conducted to obtain results from a numerical model describing in detail the heat storage, surrounding soil, and the behavior of a water tank with and without temperature stratification and with two different geometries. These are combined into three different systems. Each system uses an algorithm for heat pump control that combines equithermal regulation and deferred heat storage discharging based on long-term temperature trends. Python is used to transform the numerical model into a computational model, and the assessment of heat pump operation is made based on meteorological data from the years 2012–2021 recorded in the city of Brno, Czech Republic, Central Europe. Previous studies have suggested that this combination for heat pump control and a more detailed numerical model of the system should result in a SCOP increase. In this study, heat losses of an actual occupied building with floor area of 140 m2 without considering heat radiation are used. The results show that stratification cannot be omitted for numerical modelling, and that the geometry of the storage influences how the storage should be designed. For a wide and shallow storage, it is better to insulate the system and increase the SCOP from 3.8 to 4.65, but non-insulated deep and narrow storage can increase the from 3.8 to 4.95. Flow rate is also an important parameter with a local maximum between 0.01 and 0.02 kg/s for every cubic meter of the storage. When a system consisting of parts readily available on the market is used at optimal conditions, the resulting increase can be from 11.8% up to as high as 25.7% with low-temperature heat storage ranging from 9 to 25 cubic meters.

The Impact of Baffle Geometry in the PCM Heat Storage Unit on the Charging Process with High and Low Water Streams

This paper presents the effect of baffle geometry on the charging process of a low-temperature heat storage unit. Four different geometry variants were considered for this purpose. Each of them was simulated and the results were compared. The following parameters were selected as comparison criteria: the charging time of the heat storage unit, the change in the liquid and solid fractions of the phase change material, and the change in its temperature over time. The analysis showed that, independent from the heat transfer fluid velocity, the use of baffles did not significantly affect the charging time. Furthermore, the application of baffles of all studied types did not bring an essential decrease in charging time. It was found that the optimal solution was to use the simplest construction. Tuning of the HTF flow by the use of baffles is applicable to shell and tube heat exchangers; however, it adds no significant effects in the case of heat storage units of the proposed design. The abovementioned effect has been explained by the heat flux analysis, which shows that the heat transfer in the PCM is radically less intense, when comparing to the working fluid. Therefore, it is expected that enhancing the heat transfer between HTF and PCM material is possible by modifying the PCM–side design.

Thermal Performance Analysis on the Seasonal Heat Storage by Deep Borehole Heat Exchanger with the Extended Finite Line Source Model

Deep borehole heat exchanger is promising and competitive for seasonal heat storage in the limited space underground with great efficiency. However, seasonal heat storage performance of the essentially deep borehole heat exchanger reaching kilometers underground was seldom studied. In addition, previous research rarely achieved comprehensive assessment for its thermal performance due to seasonal heat storage. Insight into the complicated heat transfer characteristics during the whole process of prior charging and subsequent discharging of deep borehole heat exchanger is in urgent need to be clarified. To this end, an extended finite line source model is proposed to investigate thermal performance of the deep borehole heat exchanger during charging and discharging stages. It is developed with modifications of classical finite line source model to consider the spatio-temporally non-uniform distribution of heat flux density and anisotropic thermal conductivity of deep rock. In general, simulation results demonstrate that thermal performance of the deep borehole heat exchanger deteriorates rapidly both during charging and discharging stages, making it impossible to sustain long-term efficient operation. Specifically, it was discovered that low temperature heat storage utilized only upper section of the borehole as effective heat storage section, and enhancement for heat extraction potential during the heating season was not significant. While high temperature heat storage by deep borehole heat exchanger could only enhance the heat extraction potential for 30 to 40 days in the initial stage of heating. Throughout the discharging, maximum thermal performance enhancement up to 11.27 times was achieved and the heat storage efficiency was evaluated at 2.86 based on average heat exchange rate. The findings of this study are intended to provide a guidance for decisionmakers to determine the most suitable seasonal heat storage strategy for the deep borehole heat exchanger and facilitate the application in engineering practice.

Thermodynamic design and optimization of pumped thermal electricity storage systems using supercritical carbon dioxide as the working fluid

Pumped thermal electricity storage systems are a potential approach to large-scale energy storage, and supercritical carbon dioxide (SCO2) is a promising working fluid. Therefore, this study designed a SCO2 pumped thermal electricity storage system based on the reversible Brayton cycle and clarified the characteristics and restrictions of using SCO2 as the working fluid. Then, a series of improved variants that included recuperation, the elimination of medium–low temperature heat storage, and recompression were developed in a targeted manner. Roundtrip efficiency and energy density were defined as the performance indicators of different configurations, and multi-parameters were optimized by using the Find minimum of constrained nonlinear multivariable function algorithm. This study focused on the influence of the heat transfer matching of SCO2 and storage medium on system performance. Results showed that the improved configuration using recompression had the highest roundtrip efficiency of 69.38%, which was 33.48%-pts and 14.11%-pts higher than the roundtrip efficiencies of the basic configuration without recuperation and the configuration with simple recuperation, respectively. Exergy analysis was carried out to quantify the exergy losses of components further. The maximum portion of exergy was destroyed in the heat exchangers between SCO2 and the storage medium in the nonrecuperative system, and in the recuperators in the recuperative systems. Therefore, improving the heat transfer matching of SCO2 with the storage medium and recuperator performance is the key to increasing efficiency. Finally, different high temperature heat storage materials for the SCO2 pumped thermal electricity system were compared, and suggestions for the selection of storage medium were provided.

Deviceful LiCl salt hydrate confinement into a macroporous silicone foam for low-temperature heat storage application

LiCl is a well-investigated salt hydrate for low-temperature thermal energy storage due to its high energy storage density (∼1250 Wh/kg), which, however, suffers from deliquescence. Its hygroscopicity induces relevant issues during hydration/dehydration cycles due to water vapor mass transfer, swelling, and agglomeration. Finding a proper semipermeable container that retains the salt and allows free water vapor flow would enhance the dehydration/hydration kinetics and conversion. In this study, a new promising macro-porous LiCl filled composite foams was evaluated for storing low-temperature heat below 100 °C. Composite foams at varying salt hydrate content were prepared (0–70% wt.). The optimal formulation was addressed by coupling morphological, absorption, and energy storage density performances. The morphological analysis evidenced a relationship between foam microstructure and salt hydrate content. Hydration/dehydration measurements indicate that the composite foam allows the water vapor diffusion thanks to its interconnected microporous structure, preventing mass diffusion issues. An energy storage density of up to 665 kWh/m 3 was estimated. Furthermore, the relatively homogeneous dispersion of the salt hydrate filler in the matrix facilitates the hydration/dehydration process, leading to a notably less pronounced hysteresis area. Based on sorption, manufacturing, service, and handling feasibility, these results indicate this material as a potentially effective option for this application. • Assessment of a LiCl salt hydrate confinement into a macroporous silicone foam semipermeable container. • Morphological evaluation of salt hydrate composite foams at varying filler content. • Experimental characterization of sorption composite foams for thermal energy storage applications. • Correlation between energy storage capability and macro-micro-structure of the composite was pointed out.

Experimental design of 1-dodecanol@methylated melamine-formaldehyde microencapsulated composite phase change material and the application in energy storage field

The purpose of this study is to explore a new clean and environment protecting composite microcapsule material with 1-dodecanol (DD) as the core and modified methylated melamine-formaldehyde (MMF) as the shell, to meet the needs of low temperature heat or energy storage (0–40 °C). The microencapsulated phase change material (MPCM) base with a mass ratio of 14:3 was synthesized by in-situ emulsion polymerization, then two kinds of additives 10 wt% boron nitride (BN) and 5 wt% BN mixed with 5 wt% carbon nanotubes (CNTs) were applied to solve the defects of easy leakage and low thermal conductivity, while the samples were defined and shorted as BPCM and BCPCM, respectively. The experimental results showed that the prepared BCPCM presented good storage stability in the temperature range of 0–40 °C, which was due to its good thermal conductivity of 0.584 W/m·K, latent heat of 163.8 J/g, and strong supramolecular interaction between 1-DD and MMF. Through charging and discharging tests for batteries under low temperature, BCPCM indicated excellent thermal management performance, which proposed its application potential in the cryogenic storage environments and battery thermal management system (BTMS) field.

Thermal performance analysis of ground source heat pump system for low-temperature waste heat recovery storage

This study investigated the thermal performance of a ground source heat pump (GSHP) system for low-temperature waste heat storage. Considering the large amount of low-temperature heat from industrial effluents wasted to the environment, this study intends to store this low-temperature waste heat in the ground. A GSHP system for low-temperature waste heat storage was established, which could simultaneously achieve heat storage and extraction. In this system, the borehole cluster comprises two parts: inner and outer layers of ground heat exchangers (GHEs). The inner layer was used for space heating as a common GHE, which extracts heat from the ground, whereas the outer layer connected to the industry was designed for injecting low-temperature waste heat to the ground seasonally. The heat fluxes of the GHEs for heat injection and extraction were set differently and were taken as two different strengths of heat sources in this study. A heat and moisture transfer model was established and experimentally validated. Subsequently, the waste heat storage and heat extraction processes are modelled numerically. The results show that the average soil temperature increases by at least 2.63 and 6% when the heat flux of the outer layer GHEs are 40 and 50 Km−1, respectively. It reveals that the injection of low temperature waste heat into the soil is not only beneficial to waste heat recovery, but can also alleviate soil thermal imbalance, providing significant support in heating dominated area, especially in cold regions. This study complements the thermal analysis between two different strength of heat sources, providing a reference for practical engineering design.

Investigation on pH/temperature-manipulated hydrothermally reduced graphene oxide aerogel impregnated with MgCl2 hydrates for low-temperature thermochemical heat storage

A novel reduced graphene oxide aerogel (rGOA)-MgCl2 composite sorbent is developed by impregnation for low-temperature thermochemical heat storage. Salt particles are dispersed on tunable rGOAs via pH values and hydrothermal temperatures. Parameters of salt particles including salt content, size, and crystallinity strongly depend on reduction degree of the rGOAs. High reduction degree of the rGOAs can improve salt content in composite sorbents and the highest salt content reaches up to 97.3 wt%. Moreover, lower reduction degree of the rGOAs is conducive to formation of salt particles with smaller size and lower crystallinity. Therefore, the composite sorbents with lower salt contents possess better sorption kinetics owing to weaker mass transfer barrier and the maximum water uptake is up to 1.16 g/g at 95% humidity under 25 °C. Dehydration temperature of the composite sorbents decreases to varying degrees compared with that of pure MgCl2⸱6H2O. Mass energy density (Qm) and volume energy density (Qv) of the composite sorbents increase with the rise of salt contents. The largest Qm of 2225.7 J/g is 216.5 J/g higher than that of MgCl2⸱6H2O and the largest Qv reaches up to 0.709 GJ/m3. Heat storage capacity of the composite sorbent after 5 dehydration-hydration cycles remains above 95% of the original. All the results indicate that the composite sorbent is a potential candidate for long-term heat storage.

An experimental study on the binary hydrated salt composite zeolite for improving thermochemical energy storage performance

Thermochemical energy storage is a promising approach in thermal energy storage because of its advantages in high heat storage density, low heat loss and long period stability. The hydrated salt is a commonly used material in low temperature heat storage. A thermochemical energy storage experiment is conducted based on the material of MgCl2 and CaCl2 binary hydrated salt composite zeolite. In the preparation of binary hydrated salt, it's found that the optimum concentration is 15 wt% and the better mass ratio of MgCl2 to CaCl2 is 1:1.5. The metal mesh net packed method is adopted for further improvement of thermal performance. The results of energy release process show that the binary hydrated salt composite zeolite can increase the temperature rise up to 45.8 °C at the air velocity of 0.18 m/s. It can achieve the highest energy storage density of 719 kJ/kg and thermal efficiency of 41.9% at the air velocity of 0.32 m/s. Also, the combination with metal mesh packed nets can further improve the temperature rise peak, energy storage density and the thermal efficiency to 52.7 °C, 918 kJ/kg and 46.1%, respectively. This study provides references for multicomponent composite material preparation and thermochemical reactor improvement.