Thermochemical Heat Storage Materials Research Articles

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.

Investigation on the Initial Stage of the Dehydration Process in Mg(OH)2 by Density Functional Theory Calculations.

Mg(OH)2/MgO has been attracting considerable interest as a viable candidate for thermochemical heat storage materials, particularly within the temperature range of 200-400 °C. Nonetheless, the typical dehydration temperature of Mg(OH)2, which occurs within the 300-400 °C range, needs to be reduced to enhance its effectiveness in various applications for thermal energy storage. While several studies have shown that heterospecies doping can lower the dehydration temperature, the fundamental mechanism underlying this effect still remains unclear. Here, we employed density functional theory calculations to elucidate the dehydration mechanism of Mg(OH)2, with a particular focus on the initial stage of the dehydration that determines the temperature beginning the reaction. Our findings indicate that the formation of water molecules on the (001) surface is critical in the early stages of the dehydration. This discovery provides a comprehensive explanation for the role of dopants (Na, Li, or LiCl) in reducing the dehydration temperature by decreasing the formation energy of paired H and OH defects and the migration barrier of H on the surface. The present study will significantly advance the development of novel dopants for Mg(OH)2, facilitating a lower dehydration temperature and, thereby, increasing its suitability for heat storage applications.

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.

Preparation and parameter optimization of thermochemical heat storage materials with high cyclic stability

Thermochemical heat storage technology is a highly promising one, with high heat storage density and the ability to store heat across seasons. Thermochemical heat storage materials are mainly solid powders, which usually have a low heat transfer coefficient, a wide particle size distribution and poor mobility. Hence, their application in fixed-bed as well as fluidized-bed reactors is difficult. In this paper, core shell structure (CSS) pellets were prepared with silicon carbide as shell and Ca(OH)2 as core. The pellets were of uniform size, stable in shape and have good cyclic stability. The experimental validation and simulation of the heat storage properties and mechanical strength of the prepared pellets were carried out. The experimental results show that after 25 cycles of heat storage and release, the heat storage density of the pellet decreases within 20% and no cracking or crushing occurs. The simulation results show that the shell structure parameters affect both the heat storage performance and the mechanical strength of the pellets. In this paper, the shell structure parameters are optimized through simulations, which provide a reference for the preparation of CSS pellets.

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.

Agglomeration inhibition mechanism of SiO2 in the Ca(OH)2/CaO thermochemical heat storage process: A reactive molecular dynamics study

As a promising thermochemical heat storage material, the Ca(OH)2/CaO can be agglomerated after repeated heat storage cycles, which will affect its performance severely. Experimental evidence indicated that the dopant of SiO2 particles alleviated the agglomeration phenomenon of Ca(OH)2/CaO. However, the mechanisms have not been clarified in theory, including the agglomeration of Ca(OH)2/CaO and the agglomeration inhibition of SiO2. Investigating these mechanisms is important for the broad application of Ca(OH)2/CaO thermochemical heat storage technology. In this paper, reactive molecular dynamics is employed to simulate and quantify the agglomeration of Ca(OH)2 and the inhibition by SiO2 during the heat storage process. Then, the agglomeration phenomena under the two processes are quantitatively described. The mechanisms are analyzed by using the parameters of kinetics and thermodynamics. From the phenomenological analysis, smaller size changes, larger mass center distances, and smaller atomic displacements indicate that SiO2 doping reduces the tendency of nanoparticles to agglomerate into a larger cluster. From the kinetic perspective, the diffusion of pure Ca(OH)2 clusters conforms to viscous flow, while the diffusion of the SiO2-doped clusters is close to surface diffusion. The dopant of SiO2 increases the activation energy of the diffusion process, which would increase the atomic mobility resistance and thus inhibit the agglomeration. From the thermodynamic perspective, cluster agglomeration is mainly caused by the work done by van der Waals forces, which leads to a decrease in potential energy, especially surface potential energy. By doping SiO2, the interatomic van der Waals forces are increased, which lowers the stable potential energy surface of the clusters, thereby obtaining a more stable structure to inhibit agglomeration.

Dehydration performance of a novel solid solution library of mixed Tutton salts as thermochemical heat storage materials

To improve energy efficiency and reduce greenhouse gas emissions, energy storage technologies are of paramount importance. Thermochemical energy storage (TCES) materials offer high energy storage densities, and systems based on the dehydration of common salt hydrates like MgSO4·7H2O have been extensively investigated, though frequent problems such as agglomeration and unfavorable kinetics highlight the need for further material development. We consulted our VIENNA TCES-database and became aware of the Tutton salts, A2M(XO4)2·6H2O, (X = S or Se) which have also been previously investigated as TCES materials, although the effects of cation substitution on reactivity had remained poorly characterized. Herein, we report the synthesis and characterization of 41 mixed Tutton salts with the composition K2Zn1-xMx (SO4)2·6H2O (M = Mg, Co, Ni, Cu). Two trends were readily apparent: increasing the amount of nickel leads to a predictable increase in the dehydration onset temperature while preserving the simple form of the single dehydration step observed, while the use of copper leads to a predictable decrease in dehydration onset.

A systematic deliquescent additive selection approach for enhancement of reaction kinetics of thermochemical heat storage materials

A series of 16 salt mixtures consisting of potassium carbonate (K2CO3) as the base salt and a secondary salt hydrate in a 20 : 1 mol ratio of anhydrous K2CO3 to anhydrous additive were investigated as potential thermochemical heat storage (TCHS) composites. Those materials were evaluated based on their (de)hydration temperatures and reaction kinetics to find a suitable secondary salt mixture with enhanced phase change behaviour. The improved performance is expected to come from the enhancement of ionic mobility due to the deliquescence of the secondary salt. Based on the screening, we found that deliquescent and highly soluble salts are prone to reacting with K2CO3, forming new compounds that do not impact the behaviour of the base salt. Therefore, the most promising additives were salts that share a common ion with K2CO3 (KF, Cs2CO3) or salts that can react with K2CO3 forming a highly deliquescent salt (CsF, Cs2SO4). Based on the findings, we were able to design a selection procedure that can be applied to other salt hydrates considered for TCHS applications that suffer from poor kinetics and large reaction hysteresis.

A study on vermiculite-based salt mixture composite materials for low-grade thermochemical adsorption heat storage

High-performance renewable energy technologies are desired to meet the enormous demand during the clean energy transition. Thermal energy storage can help balance the mismatch between renewable energy supplies and end-user's demands. Thermochemical adsorption heat storage (TAHS) has attracted widespread attention for its ability to efficiently utilise low-grade renewables and waste heat. Composite adsorbent materials have been gaining increased research interest, which combine hygroscopic salts and host matrix via impregnating salts in the matrix. This paper reviews recent progress in composite materials for TAHS and provides material characterisation analysis on different vermiculite-based composites. The composites use vermiculite as the host matrix with impregnation of different binary and ternary salt mixtures (i.e., MgSO4–CaCl2, MgCl2–LiNO3, MgSO4–LiCl and MgSO4–LiNO3–MgCl2). Vermiculite impregnated with a binary mixture of MgSO4–CaCl2 demonstrated a high energy storage density of 1213 kJ/kg, with fast desorption kinetics in the temperature range of low-grade heat. It shows good suitability for domestic TAHS applications, particularly for space heating, with stable cyclic performance over 20 charging-discharging cycles, maintaining approximately 91.3% of its initial energy storage density. The findings of this study contribute to the growing body of research on composite materials and demonstrate the potential of vermiculite-based composites impregnated with binary salt mixtures for low-grade TAHS.

Development and characterisation of an alginate and expanded graphite based composite for thermochemical heat storage

Thermochemical heat storage is one of the most attractive technologies to store heat from solar thermal energy or waste heat from industrial processes for its high energy density and long-term storage capability. This research presents a novel expanded graphite/alginate polymer matrix encapsulated with hydrated salts as highly efficient thermochemical heat storage materials. Through the simple synthesis method, the composite material can be sized and shaped to fit multiple applications, and be easily scaled where needed. Through the reversible hydration and dehydration reaction, the incorporated CaCl2 salt can store and release heat. Thermal energy from solar thermal generators or low grade waste heat sources (< 200 °C) is appropriate for the dehydration of CaCl2. A salt loading value of 84% has been achieved with visible porosity maintained. Static heat is used to study the charge reaction, whereas a flow of humid air through a packed bed is used to study the discharge reaction where temperature uplifts between 10–14 °C were observed. A vermiculite/CaCl2 composite is used as a comparison in both reactions. Additionally, bulk density, surface porosity, surface area, moisture sorption and thermal conductivity are considered. The results show that the novel composite materials developed in this study can achieve better packing density and comparable energy density comparing to the conventional vermiculite/CaCl2 composite, but with higher thermal conductivity leading to enhanced energy efficiency.Graphical

Nanoengineering of MgSO4 nanohybrid on MXene substrate for efficient thermochemical heat storage material

2D MXene nanohybrid is a next-generationenergy-storage material. Here, we demonstrate a novel thermochemical heat storage material using magnesium sulfate heptahydrate/titanium carbide and MXene (MgSO4/Ti3C2Tx) with improved hydration/dehydration enthalpy, superior thermal conductivity, good cyclic ability, maximum water sorption performance, and larger thermal energy conversion. The TG-DSC fitted with a Wetsys flow humidity generator shows outstanding thermal performance and larger hydration for MgSO4/Ti3C2Tx (1963 J/g) and dehydration (1861 J/g) enthalpies with 5 % fluctuation as compared to pure MgSO4 (38 %). The cyclicability results showed that MgSO4/MXene has good stability of TCMs across several charging/discharging cycles (up to 20 cycles), confirming that the MXene matrix avoids hydrated salt agglomeration and displays an average of 12 % fluctuation. This illustrates that prepared composites potentially improve storage performance. The MgSO4/Ti3C2Tx nanohybrid exhibit a better radiation absorption and 45 % longer backup than the pristine MgSO4 salt. TEM study revealedthat maximum salt contents can be located in the interlayer gaps, predicted sheets thickness of the MgSO4/MXene layer to be 16 Åand the Ti3C2Tx layers to be 7.3 Å. Nanohybrid has stronger water sorption than MgSO4 due to MXene nanosheets larger interlayer gaps. The results conclude that the resultant MgSO4/MXene has excellent long-term thermal energy storage, photo-to-thermal conversion and larger water sorption performance. The study validates the effectiveness of nanohybrid material as a promising candidate for thermochemical heat storage applications and set to pave the way to explore other hybrid systems to improve the efficiency of thermochemical energy storage systems.

Thermal transport of charging/discharging for hydrogen storage in a metal hydride reactor coupled with thermochemical heat storage materials

Metal hydride is a promising alternative for hydrogen storage. A novel metal hydride reactor coupled with thermochemical heat storage material is proposed to reduce the energy loss. The hydrogen charging and discharging processes are numerically and analytically investigated in this paper. Hydrogen is stored in a compacted metal hydride disk, so the reaction characteristics under low permeability are studied. The dehydration kinetic equation has been re-established to determine the equilibrium temperature at a corresponding pressure. The reactor takes 139 min for the hydrogen charging process and takes 512.8 min for the hydrogen discharging process. The reaction characteristics are found to be affected by heat transfer and gas pressure at the permeability of 5.9×10-16m2. The analytical model is found to be only applicable to the prediction of reaction time with high permeability of hydrogen in metal hydride. The thermal management of thermochemical heat storage materials has been proven to be feasible. By increasing the hydration equilibrium temperature from 321.8 °C to 350 °C, the reaction time can be decreased to 43.1 %. By decreasing the dehydration equilibrium temperature from 300 °C to 270 °C, the reaction time can be decreased to 84.5 %. To enhance the heat transfer, meal foam is added to the thermochemical heat storage material. Adding metal foam can effectively shorten the reaction time, but will reduce the hydrogen production. A metal foam with a porosity of 0.96 will reduce the production of hydrogen by 3 %.

Integration of hydrogen storage and heat storage in thermochemical reactors enhanced with optimized topological structures: Charging process

The low density for hydrogen storage can be solved by metal hydrides, and the energy loss for hydrogen storage with metal hydrides can be recovered by the combination of metal hydrides (Mg/MgH2) with thermochemical heat storage materials (MgO/Mg(OH)2) under the different reaction temperatures. However, the poor heat conduction of thermochemical materials limits the heat transfer rate and the implementation of this technology. In this study, an accurate theoretical model of hydrogen charging process in the hydrogen storage reactor assisted with heat storage was developed to accommodate the operating pressure. A novel method to overcome the poor heat transfer problem was proposed by topology optimization of high thermal conductivity fins for matching the heat transfer and reaction rate in the hydrogen storage side with those in the heat storage side. Increasing the average temperature of MgO/Mg(OH)2 was considered for the formulation of the topology optimization problem, and a density-based approach was used. The influence of the conductivity of materials on the topology optimization was discussed. The outcomes displayed that higher thermal conductivity accommodated more heat transfer directions, resulting in a distinct branch structure. Furthermore, the obtained topologies were geometrically reconstructed and evaluated during the hydrogen charging process. Results revealed that the topology optimized geometry had outstanding heat transfer performance, with an obvious improvement in terms of hydrogen charging time over the reactor without fins. This work can provide the theoretical basis for thermochemical conversion processes of hydrogen storage and heat storage, the implementation procedure for topology optimization, and the design guidance of thermochemical reactors for hydrogen storage assisted with heat storage.

Accelerating the reaction kinetics of K2CO3 through the addition of CsF in the view of thermochemical heat storage

Potassium carbonate (K2CO3) is a promising thermochemical heat storage material (TCM). However, it suffers from hysteresis between (de)hydration temperatures and poor reaction kinetics close to equilibrium conditions. Both aspects are caused by a nucleation barrier and low ionic mobility close to equilibrium. This study investigates the impact of caesium fluoride (CsF) incorporated through recrystallisation on the phase transitions. Thecomposition studies show that K2CO3and CsF react during synthesis, forming KF, which points to the formation of Cs2CO3. The secondary phases are not incorporated into the crystal structure but reside between the main phase's grain cracks due to capillary forces. Because the secondary phases are highly hygroscopic, they promote surface mobility by forming a liquid-like layer even at low water vapour pressures. As the effect of their presence, hydration kinetics are enhanced significantly in all investigated conditions, with the most pronounced impact when hydration of K2CO3is inherently inhibited. The benefits manifest themselvesthrough a faster reaction rate and shorter induction period.The dehydration is enhanced by the presence of the additive mainly far away from equilibrium conditions. Close to the equilibrium, the dehydration of the composite proceeds in an unusual 2-step manner, where the second step is much slower than the dehydration of pure K2CO3. The enhancement of dehydration kinetics is ascribed to the formation of defects during recrystallisation. The lowering of dehydration rates close to equilibrium is attributed to diffusion issues due to excess of a deliquescent phase present in the system.

Construction of biomass waste derived hierarchical porous biochar framework based lithium hydroxide composites for highly efficient and durable low temperature thermochemical heat storage

By virtue of its prominent properties of remarkable storage capability, achievable long-lasting storage period and extraordinary energy retention performance, salt hydrate-based thermochemical heat storage materials have become one of the most viable options for the effective use of renewable energy. Herein, a brand-new LiOH composite is demonstrated by adopting the strategy of compositing LiOH into a biomass waste-derived hierarchical porous biochar matrix and systematically characterized. The as-formed composites have exceptional hydration behavior while demonstrating superb storage capability up to 1865.7 kJ kg −1 attributed to the hierarchically porous framework with a large surface area. Furthermore, the LiOH composite with dramatically boosted thermal conductivity is able to achieve 91% energy retention after 15 cycles, elucidating its terrific cycle reliability as well as distinguished heat transfer properties. This research not only provides a valuable approach to synthesize the superb biochar materials and make the most of low-grade thermal energy, but may also inspire new ideas of turning waste into treasure to solve the predicament of renewable energy storage . • Biomass waste derived biochar framework is fabricated through a two-stage activation. • The biochar host matrix presents ultrahigh surface area with diversified porosity. • Li/BCM1(K4) exhibit remarkable storage capability and hydration performance. • Li/BCM1(K4)-40 shows excellent cycle reliability and heat transfer properties.

Development of covalent-organic frameworks derived hierarchical porous hollow carbon spheres based LiOH composites for thermochemical heat storage

Under the joint assistance of its excellent storage strength, accessible long storage lifespan, and high heat utilization efficiency, salt hydrate-based thermochemical heat storage (THS) materials give renewable energy an important outlet to alleviate the pressure of underutilization. Herein, an activated hollow spherical carbon (AHSC) with hierarchical porous architectures converted from covalent-organic frameworks (COFs) is constructed and utilized as the supporting matrix for LiOH·THS composite material for the first time. The obtained Li/AHSC3 composites have distinguished hydration performance while manifesting impressive storage ability up to 1916.4 kJ kg−1 with low operating temperature stemming from the collective effect of the void spherical framework, multimodal porosity, and high surface area of AHSC3. And the Li/AHSC3-40 composite with evidently progressed thermal conductivity is capable of realizing 94.5% heat preservation after twenty-five adsorption–desorption cycles, exhibiting its eminent cyclability and great heat transfer performance. This study not only brings new hope for overcoming the underutilization of low-grade heat but also may enlighten new ideas for enriching the application scenarios of COFs-derived carbonaceous materials.

Toward new low-temperature thermochemical heat storage materials: Investigation of hydration/dehydration behaviors of MgSO4/Hydroxyapatite composite

A new two-component (composite) water sorbent MgSO 4 /Hydroxyapatite has been developed for sorption-based solar heat storage. The matrix of the composite is a hydroxyapatite (HAP) material with ordered structure, high surface area of 111.3 m 2 /g and mesopore dimensions centered at 45 nm. The composites, prepared by wet-impregnation of HAP with MgSO 4 , have lower specific surface area and similar mesopore dimensions as the matrix. The maximum water sorption capacity of HAP is 0.039 g/g, while the composite (20-MgSO 4 /HAP) possesses 3.7 times higher maximum water sorption capacity due to the presence of the salt in the matrix. The HAP composite containing 20% MgSO 4 achieved the highest heat of hydration 464 J/g. A long-term cycling (dehydration at 150 and hydration at 30 °C at a relative humidity of 60%) confirms a comparatively good stability of the composite. • MgSO 4 -hydroxiapatite composite materials were synthesized using impregnation method. • Physical characteristics of different composite materials were characterized. • Hydration kinetic and water uptake of different samples were investigated. • Composite salt of 20 wt% has a good stability over 20 hydration/dehydration cycles.

Exploration of Highly Porous Biochar Derived from Dead Leaves to Immobilize LiOH·H2O Nanoparticles for Efficient Thermochemical Heat Storage Applications

The inadequate utilization of low‐grade energy from industrial waste heat and solar energy calls for an efficient thermochemical heat storage material. The fabrication of a low‐cost but highly efficient composite comprising highly porous biochar and LiOH·H2O nanoparticles for heat storage applications is reported. The biochar is prepared by the KOH‐assisted pyrolysis of dead banyan leaves, and its microstructure and surface properties can be finely tuned by varying the KOH‐to‐biomass ratio. Such biochar can be mediated to possess a large specific surface area and total pore volume reaching as high as 1255.03 m2 g−1 and 3.65 cm3 g−1, respectively, superior to most previously reported biochar/biocarbon derived from different kinds of waste biomass. Besides, adequate functional groups and a hierarchical porous structure consisting of micropores and mesopores are demonstrated in the prepared biochar. The subsequent hydrothermal reaction with a short duration yields a biochar‐LiOH·H2O composite, where LiOH·H2O can be homogeneously immobilized with a nanoscale size within the porous biochar matrix. Thus, an enormous biochar/LiOH·H2O nanoparticles interface can be obtained, resulting in fast hydration reactions with water molecules. Consequently, a significant heat storage density of as high as 3089.6 kJ kg−1 is achieved within only 10 min, outstripping some recently reported thermochemical heat storage systems.

Enhancement of heat and mass transfer of potassium carbonate-based thermochemical materials for thermal energy storage

Salt hydrates are ideal for long-term thermochemical heat storage in a built environment, where K2CO3 is considered a promising thermochemical heat storage material. This class of materials continues to encounter certain technical bottlenecks in practical applications, and the enhancement of heat and mass transfer is a key challenge. Composite materials are a promising method to address this challenge because heat and mass transfer are coupled and material-dependent. In this study, a novel K2CO3-based composite material was synthesized from expanded graphite (EG), K2CO3, and octylphenol polyoxyethylene(10) ether (OP-10), and its structural information, reaction kinetic properties, thermodynamic properties, and cycling stability were determined. The results showed that the use of EG with OP-10 significantly enhanced mass and heat transfer. Compared to pure K2CO3, the thermal conductivities of the composites were increased by 9–15 times and their half-conversion time was shortened by approximately 3/4–4/5, indicating that their mass and heat transfer capacities were effectively improved. The mass and volume energy storage densities (ESDs) of K2CO3-based composites reached 429–563 kJ⋅kg−1 and 138–216 kWh⋅m−3, respectively, and these values depended on the content of K2CO3. In addition, the cycling stability results indicated that these K2CO3-based composites performed well and were suitable for medium-to-low-temperature thermochemical thermal storage applications.

Effects of Porous Graphene on LiOH Based Composite Materials for Low Temperature Thermochemical Heat Storage

Thermochemical heat storage material inorganic hydrate LiOH is selected as a promising candidate material for storing low-temperature heat energy because of its high energy density (1440 kJ/kg) and mild reaction process. However, the low hydration rate of LiOH limits the performance of low temperature thermochemical heat storage system as well as the thermal conductivity. In this study, porous-graphene/LiOH composite thermochemical heat storage materials with strong water sorption property and higher thermal conductivity were synthesized by hydrothermal process. The experimental results show that the hydration rate of the composites was greatly improved. The heat storage density of the composite materials was increased by 47% (from 661 kJ/kg to 974 kJ/kg). By combing the porous graphene, the thermal conductivity of composites with different contents were highly increased by 21.1% to 78.7%, but the increase of heat storage density is opposite to that of thermal conductivity. The development of high-performance materials for thermochemical heat storage should consider the relationship between the heat storage density and thermal conductivity of the material, and the thermal conductivity of the supporter needs to be further improved.