Heat Storage Density Research Articles

Flexible core–shell structured Al-Cu alloy phase change materials for heat management

The development of core–shell structured phase change materials (PCMs) with tunable melting points has been attracted much attention for thermal storage of solar energy, and a flexible shell is crucial for their practical applications. Herein, we prepared microencapsulated Al-Cu alloy PCMs (MEPCMs) with a flexible shell by a deposition-oxidizing calcination method. The Cu nanoparticles are chemically deposited on the surface of Al powders by a rapid replacement reaction, which further promotes the formation of Al-Cu alloy microspheres and the oxidation of Al to form Al2O3 shell during oxidation calcination. The prepared MEPCMs show excellent thermal cyclic performance and tunable melting point (549–592 °C). The presence of a thin film between the Al2O3 shell and Al-Cu alloy core may act as a buffer to accommodate volume change during melting-freezing process, obtaining a flexible core–shell structure. The heat storage density of Al-Cu alloy MEPCMs decreases by only 0.09% after 100 melting-freezing cycles, indicating high thermal cycling stability. In addition, the microencapsulated Al-Cu alloys present high latent heat (208.6–222.3 J·g−1) and relatively high thermal conductivity (∼1.8 W·(m·K)−1), which makes it attractive for application in high-temperature thermal storage systems.

In situ generation of carbon-doped modified KCl–LiCl high-efficiency thermal storage materials with 3D network structure

Molten salt phase change thermal storage materials are essential in efficient thermal storage for high-temperature concentrated solar power (CSP). Modified KCl–LiCl eutectic molten salt thermal storage materials with stable structure and thermal properties were formed by direct high-temperature carbonization using xylitol as the carbon precursor by the molten salt template method. The molten salt is encapsulated in a carbon matrix with a three-dimensional entangled network structure, and the rich pore structure binds the molten salt to the carbon material. The interconnected network channels provide pathways for heat transfer from the molten salt and improve the heat transfer capability of the modified molten salt. The results showed that the best performance enhancement of the modified molten salt was achieved when the xylitol content was 4%. The thermal conductivity was 2.412 W/m/K, which was improved by 59.8%, and the specific heat was increased from 0.45 J/g/K to 0.75 J/g/K, an increase of 66.7%, and the latent heat of phase change reaches 212.0 J/g, which still possess a high heat storage density. Due to the connection channels of the network structure, the modified molten salt exhibits better stability and leak resistance. In the self-assembled test setup, the modified molten salt still has good radiative heat transfer performance under the concentrated radiative heat flux, and the maximum temperature difference is only 34.4 °C. Modified molten salts have great potential in high-temperature solar storage systems, providing new ideas for efficiently utilizing solar energy, reducing resource waste, and promoting sustainable development.

Thermodynamic analysis of photovoltaic/thermal heat pump based on phase change slurry

To explore the feasibility and relative advantages of phase change slurry (PCS) as operating working medium, a solar hybrid vapor compression heat pump based on phase change slurry (PCS-PV/T-VCHP) is constructed by directly combining PCS as operating fluid with evaporator. Furthermore, performance of PCS-PV/T-VCHP under different initial conditions is studied and demonstrated by simulation method. PCS as operating medium has higher PV/PT and total efficiencies, which respectively reach 15.46%/83.17% and 98.63%. PCS has multiple advantages for VCHP applications due to high heat storage density and stable heat release. Under given initial conditions, PV/PT efficiencies are 15.15%–15.77%, 67.63%–84.03% respectively, and the maximum solar efficiency are 98.81%. The minimum and maximum values of VCHP coefficient of performance (COP) reach 2.23 and 5.82 respectively, and total COP is distributed in the range of 3.02 and 23.71. The maximum electrical exergy efficiency and thermal exergy efficiency are 15.77% and 7.57% respectively, and total exergy efficiency ranges from 16.61% to 23.25%. PCS-PV/T-VCHP shows good heating performance in typical cities, COP reaches 1.99–5.87 in selected cities. The feasibility of using PCS as operating fluid to supply heat has been confirmed, and PCS-PV/T-VCHP has significant performance advantages, low emissions and good economy.

Latent heat storage composites composed of Al‐Si microencapsulated phase change material and alumina matrix

AbstractLatent heat storage (LHS) using phase change materials (PCMs) is expected for application to heat utilization at high‐temperature because it can provide a heat source of high density and constant temperature. Even among PCMs, metals/alloys are promising for high‐temperature operation. However, metals and alloys PCM are concerned about corrosion reactions with structural materials. As a result, micro‐encapsulated PCMs (MEPCMs) have been created in which the surface of alloy PCM is encased in a stable oxide coating. Since the surface of MEPCM is an oxide, the creation of structures with the function of LHS in a variety of shapes by combining MEPCM with sintering aids is also possible. In this study, composite PCMs were prepared by combining Al‐25 mass% Si MEPCM with fine α‐Al2O3 particles as a sintering aid. Consequently, PCM composites containing 70‐90 vol.% of the MEPCM and heat‐treated at 1200°C or 1300°C were successfully fabricated. In particular, a high heat storage density of 0.47 GJ m−3 was obtained under conditions containing 90 vol.% of the MEPCM and a heat treatment temperature of 1200°C, which is 1.6‐fold higher than that of existing sensible heat storage materials. Additionally, the composite PCM retained its shape and the latent heat capacity even after 300 cycles of cyclic testing. Thus, the high heat storage density and high durability of the composite PCM are expected to further promote the expansion of high‐temperature heat utilization in future studies.

Ca/Co-based composites with improved cyclic stability and optical absorption for advanced thermochemical energy storage systems

Thermochemical energy storage (TCES) is a promising technology to overcome solar intermittency and volatility. However, weak solar absorption, poor cyclic stability for calcium carbonates, and cost issues for metal oxides hinder the applicability of these materials for thermochemical energy storage. Herein, an advanced, affordable, and effective TCES system is proposed, and the corresponding calcium carbonate-metal oxide synergistic material is designed with improved solar absorption and cyclic stability. The optimum molar ratio is Ca:Co:Mn:Mg = 100:20:7.5:5 for the synergistic material fabricated by the modified sol–gel method, where Ca element plays the primary role in heat storage, Co element serves as the primary optical performance enhancer and secondary heat storage component, and Mn/Mg elements act as the secondary performance modifiers. The solar absorptivity is boosted to ∼79.3%, and the heat storage density is 1295 kJ/kg after 30 high-temperature cycles, which are 6 times and 2.2 times higher than those of commercial calcium carbonate, respectively. Homogeneous mixing of modifiers at the microscale and stable, well-developed pore structure are critical factors for cyclic stability enhancement. The change of electronic structure on the Fermi level caused by Co/Mn ion doping is the key to optical absorption improvement, and the promoting amplitude exhibits wavelength dependence. This work expands the limited list of currently available TCES materials and the comprehensive description of optical properties offers guidance for future implementation of TCES materials with high solar absorption in specific wavelengths.

Flexible and leakage-proof phase change composite for microwave attenuation and thermal management

Miniaturization of high-speed electronics requires multi-functional materials to address electromagnetic interference (EMI) and overheating issues at one time. Phase change material (PCM) could be an ideal solution for thermal management via thermal energy storage to avoid bulky heat sink. Herein, we propose a flexible PCM composite sheets SPP, consisting of the single wall carbon nanotube (SWCNT) and the phase change material-polyethylene glycol (PEG) in a Polydimethylsiloxane (PDMS) matrix. Due to the double-percolation microstructure constructed by SWCNT and PEG, the SPP could exhibit over −20 dB microwave attenuation with great tunability of working frequency throughout the entire 2–18 GHz at a fixed SWCNT filling ratio of only 0.2 vol%. Meanwhile, 70 vol% of PEG can be loaded without compromising the mechanical robustness and great anti-leakage performance, resulting in a latent heat storage density of over 110 J/g. With such superior capability of thermal energy storage, it's demonstrated that the SPP composite sheets could act as cooling patches for electronic components to reduce the working temperature and also stabilize the temperature fluctuations. Based on the above, it is believed that the SPP could shed light on materials solutions of electromagnetic compatibility and thermal mitigation for next-generation ultra-compact electronic systems in a wide frequency range.

Shape-stabilized 1-hexadecanol/g-C3N4/Cu composite phase change material with high thermal conductivity for efficient photo-thermal conversion

Phase change materials (PCM) have received much attention for thermal energy storage and release in the form of latent heat. 1-hexadecanol (HD), as a representative of organic phase change materials, has the advantages of high thermal storage density, almost constant operating temperature and stable thermochemical properties, which shows outstanding potential in the field of phase change energy storage. Nevertheless, single HD as thermal storage material inevitably faces challenges in terms of liquid phase leakage, low thermal conductivity, and poor photo-thermal conversion. To further improve the practical application of HD, we demonstrate a convenient encapsulation and modification strategy of composite phase change materials (CPCMs) with low liquid leakage and high thermal conductivity. Hence, a novel shape-stabilized Cu-doped HD/ graphitic carbon nitride (g-C3N4) CPCM was developed for efficient photo-thermal storage. By the melt blending method, HD is infiltrated into tightly stacked and aligned nano-porous g-C3N4, and the carbon-based lattice inlay achieves the encapsulation of HD to solve the leakage problem during the solid–liquid phase transition. Then, metallic Cu powder is introduced to further strengthen the thermal conductivity of the materials. With this encapsulation and modification strategy, the obtained 5 wt% Cu-doped HD/g-C3N4 composites (CPCM3) exhibits superior performance in both thermal and optical energy storage. This composite has appropriate phase change temperature and extremely high heat storage density, with melting temperature and latent heat of 48.88 °C and 249.63 J/g. The corresponding solidification temperature and latent heat is 48.52 °C and 242.22 J/g, respectively. Its thermal conductivity is enhanced to 0.5307 W/m·K, which is 59.8% higher than pure HD. The photo-thermal conversion efficiency is up to 81.6% for rapid solar energy harvesting and delivery. In particular, after 100 times thermal cycling tests and thermogravimetric analysis, this special CPCM can maintain good thermal reliability and thermal stability. In conclusion, the composite phase change materials obtained by the synergistic modification strategy of nano-porous g-C3N4 and metallic Cu significantly improved the thermochemical properties of HD. This work provides a practical and efficient synthetic strategy for the preparation of CPCMs, which is expected to present a promising future in the field of solar conversion and storage.

Thermodynamic assessment of binary chloride salt material for heat transfer and storage applications in CSP system

Molten chloride salt is recognized as a promising heat transfer fluid and thermal energy storage material (TES) in concentrating solar power (CSP) in recent years, and its phase diagram as key data for material design still need further research. In this work, the phase equilibrium characteristics of solid-liquid equilibrium and vapor-liquid equilibrium for binary chloride salt system (Na/K–Ca/Mg) are investigated respectively. Based on Wilson model, the activity coefficient and non-ideal behavior of liquid salts are predicted effectively, and then the solid-liquid phase diagrams of binary salts are calculated, and the predicted eutectic temperature and component agree well with experimental data. Vapor-liquid phase diagrams of binary salts are also calculated, and azeotropic temperature can only be found in the MgCl2 based mixed salts. As the system pressure rises, the azeotropic temperature increases, while azeotropic component slightly changes. According to the variation of vapor pressure for eutectic salts, the correlations for vapor pressure with temperature and composition are developed. Thermal stability of eutectic salts is experimentally studied by Thermo-gravimetric analyzer (TGA) and operating temperature ranges of these mixed salts are revealed. It can be found that the mass loss rate at different temperature is directly proportional to the vapor pressure. In general, present binary chloride salts show wide operating temperature range and high heat storage density, and eutectic NaCl–MgCl2 with advantage of heat storage density and azeotropic point existence is recommended as TES in the CSP plant.

Macro encapsulated Cu-based phase change material for high temperature heat storage with characteristic of self-sealing and high durability

The objective of this research is to develop encapsulated phase change materials with applications exceeding 1000 °C that can be applied to concentrated solar power systems and industrial waste heat recovery. In this study, a macro-encapsulation phase change storage material with a copper-based core and an alumina-based shell was developed by slip-casting method and filling mixture of copper beads and Cu-40 %Al powder. The capsule was able to achieve self-sealing by local oxidation reaction at the entrance without leakage. After aging tests at 1100 °C for up to 1000 h, no leakage or damage was found and mass increase of the encapsulate phase change material was only 4.5 %, confirming the superior durability and oxidation resistance of the encapsulated phase change material. The heat storage density of the copper-based phase change material was evaluated as high as 147 J/g. The heat storage capacity did not decrease after 1000 h and ten cycles of aging. The mechanism of its high durability was explored by morphological observations and Raman spectroscopy. In the original material, a small amount of aluminum was oxidized with priority to alumina which significantly enhanced the durability of the capsules. This macro-encapsulated phase change material exhibits excellent thermal storage performance, as well as self-sealing and durability properties, which are of great significance for improving the efficiency of thermal energy recovery above 1000 °C.

Kinetics and Structural Optimization of Cobalt-Oxide Honeycomb Structures Based on Thermochemical Heat Storage

Thermochemical heat storage is an important solar-heat-storage technology with a high temperature and high energy density, which has attracted increasing attention and research in recent years. The mono-metallic redox pair Co3O4/CoO realizes heat storage and exothermic process through a reversible redox reaction. Its basic principle is to store energy by heat absorption through a reduction reaction during high-irradiation hours (high temperature) and then release heat through an exothermic-oxidation reaction during low-irradiation hours (low temperature). This paper presents the design of a cobalt-oxide honeycomb structure, which is extruded from pure Co3O4, a porous media with a high heat-storage density and a high conversion rate. Based on the experimental data, a three-dimensional axisymmetric multi-physics numerical model was developed to simulate the flow, heat transfer, mass transfer, and chemical reaction in the thermochemical heat-storage reactor. Unlike the previous treatment approach of equating chemical reactions with surface reactions, the model in this paper considers the consumption and generation of solids and the diffusion and transfer of oxygen in the porous medium during the reaction process, which brings the simulation results closer to the real values. Finally, the influence of the physical parameters of the honeycomb-structured body on the storage and exothermic process is explored in a wide range. The simulation results show that the physical-parameter settings and structural design of the cobalt-oxide honeycomb structure used in this paper are reasonable, and are conducive to improving its charging/discharging performance.

Thermal energy storage properties of carbon nanotubes/sodium acetate trihydrate/sodium monohydrogen phosphate dodecahydrate composite phase-change materials as promising heat storage materials

A novel hydrated salt composite phase change material was prepared. Firstly, the phase transition process of sodium acetate trihydrate (SAT)/ sodium monohydrogen phosphate dodecahydrate (DSP) eutectic salt was studied. And the optimization effect of hydrophilic carbon nanotubes (HCNTs) on the thermal properties of eutectic salt was analyzed. The results showed that the cycle stability of the eutectic salt was the best when the mass ratio of SAT/DSP was 9:1 and the degree of supercooling was 2.5 ℃. With increasing HCNTs mass fraction, the thermal conductivity of the composite showed a linear increase, reaching the highest value of 1.29 W/(m K), which was 182.89% higher than that without HCNTs. The heat storage density exhibited a linear decrease, with a maximum decrease of 5.59%. Finally, the cyclic stability of the HCNTs/SAT/DSP composites was tested, and the results showed that after 200 solid–liquid cycles, each group of materials showed good cycle stability for phase change heat storage system. In this study, the problems of supercooling and phase separation of sodium acetate trihydrate were solved. And its thermal conductivity was significantly strengthened, the results of this study provide a reference for the collaborative adjustment of phase-change materials and thermal storage system components.

A novel hydrated salt-based phase change material for medium- and low-thermal energy storage

Magnesium nitrate hexahydrate (MNH) was considered as a promising medium- and low-temperature phase change material (PCM) owing to high latent heat, weak corrosion, and low price. While supercooling performance greatly restricted the application of such PCM. In this work, MNH-based composite PCM containing a novel nucleating agent was prepared. Supercooling behavior and thermophysical properties of composites were analyzed. Additionally, thermal performance of composites in a fin-plate heat storage unit was investigated. Results showed that 2 wt% magnesium sulfate heptahydrate (MSH) effectively suppressed supercooling performance and was chemically compatible with MNH. Composites containing 2 wt% MSH and 2 wt% sodium carboxymethyl cellulose (CMC) performed favorable thermal properties, including supercooling degree of 0.6 °C, phase change temperature of 89 °C, latent heat of 166.88 J/g, and heat storage density of 220.78 J/g. Effective working temperature of the composite should be no more than 140 °C. Magnesium sulfate tetrahydrate (MST) was actually the real component in MSH that inhibited the supercooling of MNH. Compared with paraffin, encapsulated MNH-based composite PCM with a 6720 kJ higher heat storage capacity possessed a much shorter charging/discharging time due to the higher thermal conductivity. The results might provide a foundation for the practical application of MNH.

A medium-temperature, tubeless heat exchanger based on alloy microencapsulated phase change material (MEPCM)/ceramic composites

High-performance heat storage materials and devices are urgently needed to meet the demand for efficient heat storage. Herein, a novel SnBi58 alloy microencapsulated phase change material (MEPCM)/ceramic composite with flow channels inside is fabricated for medium-temperature heat storage, which combines the characteristics of sensible heat storage (SHS) and latent heat storage (LHS). Then a tubeless heat exchanger based on the composite is constructed, and its heat storage performance is experimentally studied. The results show that the thermal conductivity of the composite-based heat exchanger is doubled (up to 3.09 W/(m·K)) and the heat storage density is increased by 30.4% to 559.1 MJ/m3 compared with the ceramic-based heat exchanger. Meanwhile, the heat storage performance of the heat exchanger can be improved with the increase of inlet temperature and mass flow rate, and it is more sensitive to the response of the inlet temperature. Furthermore, the average heat storage duty of the heat exchanger reaches 1183 W/(m3·°C), which is 3.56 times the mean value of other heat storage devices. Finally, test results confirm the high thermal reliability of the composite, which can meet the tubeless heat exchange requirements.

Advances in High-Temperature Molten Salt-Based Carbon Nanofluid Research

Molten salt is an excellent medium for heat transfer and storage. The unique microstructure of carbon nanomaterials leads to good mechanical stability, low density, high thermal conductivity, and high strength, etc. The addition of carbon nanomaterials to molten salt to form molten salt nanofluid can remarkably enhance the specific heat capacity and thermal conductivity of molten salt and reduce the molten salt viscosity, which is of great importance to increase the heat storage density and reduce the heat storage cost. Nevertheless, some challenges remain in the study of such nanofluids. The main challenge is the dispersion stability of carbon nanomaterials. Therefore, to improve research on carbon nanofluids, this paper summarizes the progress of carbon-based molten salt nanofluid research worldwide including the preparation methods of molten salt nanofluids, the improvement of heat transfer performance, and the improvement of heat storage performance. The effects of carbon nanoparticle concentration, size, and type on the heat transfer and storage performance of molten salt are derived, and the effects of nanoparticle shape on the heat transfer performance of molten salt are analyzed while more promising preparation methods for carbon-based molten salt nanofluids are proposed. In addition, the future problems that need to be solved for high-temperature molten salt-based carbon nanofluids are briefly discussed.

A novel three-dimensional cross-arranged fin structure for performance enhancement of thermochemical heat storage

Thermochemical heat storage technology stores and releases heat through reversible reaction, which has the advantages of large heat storage density and long heat storage period. However, the poor heat transfer in reactor weakens the heat storage performance. In present paper, a novel three-dimensional cross-arranged fin structure is proposed to enhance the heat transfer process in a shell and tube reactor with Mg(OH)2 as thermochemical material. The comparative studies are performed for the three reactors with the novel fin, traditional axial fin under the same metal amount and without fin, respectively. It is found that the novel cross-arranged fin enhances heat transfer more efficiently and uniformly, thus it reduces heat storage time significantly. The charging time is reduced by 67.3 % and 25.3 % respectively, compared with no fin and axial fin reactors. After that, the effects of bed thermal conductivities, operating conditions and geometry parameters on heat storage performance of the novel fin reactor are analyzed to further verify the advantages of the new fin under wide working conditions. Moreover, the hydration process is also discussed. The cross-arranged fin reduces the discharging time by 18.9 % compared to the axial-fin reactor. In short, installing the cross-arranged fin is an effective way to improve the thermochemical heat storage performance due to its excellent heat transfer enhancement effect, and it is worth considering for the design and optimization of thermochemical heat storage reactor.

Cobalt phosphate-based composite with wall care putty as building envelope material for thermal energy storage

The four different thermal energy storage composites were synthesized by charging Co3(PO4)2·xH2O with wall care putty (Korean make) in aqueous media. These four different composites are abbreviated as A1, A3, A4 and A5 having composition 0.46 %, 1.00 %,1.63 % and 2.22 % of Co3(PO4)2·xH2O, respectively. Differential Scanning Calorimeter (DSC) was used to measure specific heat capacity (CP) for all above composites. Among all composites, the CP = 1.34 Jg-1K−1 is found maximum for composite A5 at 339 K in N2 atmosphere. The highest heat storage density is also observed, 80.95 ∼ 88.83 kJ kg−1, for composite A5 in temperature range 336 K to 342 K for storing heat up to ∼ 13 min. The optical band gap (Eg) is calculated using Tauc and Menth as well as Kubelka-Munk function for all composites and all found insulator. The calculated crystallite size of composite A3 is ∼ 400 nm.

Development of over 3000 cycles durable macro-encapsulated aluminum with cavity by a sacrificial layer method for high temperature latent heat storage

The high-temperature metallic phase change materials (PCMs) are attracting great attentions as alternatives to sensible heat storage materials in thermal energy storage (TES) systems. However, the high corrosivity, easy leakage and oxidation phenomena of metallic PCMs have limited their applications. This work reports a facile and novel sacrificial layer method with the purpose to create macro-encapsulating aluminum (Al) PCM by alumina (Al2O3) shell characterized by a ‘buffer’ inner cavity. The inner cavity could offer buffer space to accommodate the huge thermal volume expansion of Al core PCM during heating and melting, thereby preventing the breakage of the capsules and ensuring the long-term stability. A series of Al balls with diameters of 4–25 mm were firstly coated with paraffin sacrificial layer then by Al2O3 dough shell, which was finally conducted by the two-step heat treatment, in which a low-temperature stage aimed at removing the paraffin layer while the high-temperature stage aimed at the capsule sintering formation. The final macrocapsules with diameters of 6.5–31.5 mm were obtained, in which the capsules with Al core balls larger than 15 mm showed superior gravimetric and volumetric heat storage density because of their high core–shell ratio. For example, the 25 mm@0.75 mm@2.5 mm macrocapsules could present high heat storage density of 348 kJ/kg and 902 J/cm3 at the temperature range of 600–700 °C. The macrocapsules also showed a long-time thermal cycling stability, since that no damage or leakage was observed over 3000 times of melting and solidification under air atmosphere, indicating that they can be used over 8 years without any degradation of thermophysical properties. This demonstrates the potential application of the Al2O3 encapsulated Al macrocapsules with inner cavity in high-temperature TES systems.

A Review on Heat Transfer Enhancement of Phase Change Materials Using Fin Tubes

Latent heat thermal energy storage (LHTES) has received more and more attention in the thermal energy storage field due to the large heat storage density and nearly constant temperature during phase change process. However, the low thermal conductivity of phase change material (PCM) leads to poor performance of the LHTES system. In this paper, the research about heat transfer enhancement of PCM using fin tubes is summarized. Different kinds of fins, such as rectangular fin, annular fin, spiral fin, etc., are discussed and compared based on the shape of the fins. It is found that the longitudinal rectangular fins have excellent heat transfer performance and are easy to manufacture. The effect of fins on heat transfer enhancement is closely related to the number of fins and its geometric parameters.

Experimental investigation on a strontium chloride/ammonia sorption heat storage system

• A thermochemical sorption heat storage system has been established. • The total highest heat storage density is 1871.03 kJ/kg SrCl 2 composite sorbent. • The total highest heat storage efficiency of the sorption system is 92.74%. Thermochemical sorption heat storage based on solid-gas interaction is considered as a cutting-edge technology for harvesting solar energy and industrial waste heat due to the significant advantages of high heat storage density, flexible working modes and long-term storage capacity with negligible heat loss. As a vital technology to enhance energy infrastructures, the thermochemical sorption heat storage has aroused extensive interest in the past decade. In this work, a solid-gas thermochemical sorption system was established and investigated for recovering the thermal energy. Working pair of strontium chloride/ammonia was selected, and strontium chloride composite sorbents were synthesized using the expanded graphite as matrix. The heat storage performances of strontium chloride/ammonia sorption system at the different temperatures were experimentally investigated. Under the condition of 92 °C charging temperature, 40 °C discharging temperature and -5 ℃ condensation/evaporation temperature, the highest sorption heat storage density of the sorption system obtained is up to 1630.78 kJ/kg SrCl 2 (or 1386.16 kJ/kg composite sorbent), and the total heat storage density can reach 2201.21 kJ/kg SrCl 2 (or 1871.03 kJ/kg composite sorbent). The total energy efficiency of the sorption system increases from 74.71% to 92.74% with the heat storage temperature rising from 68 ℃ to 92 ℃. Results indicated that strontium chloride and its composites are promising heat storage materials, and revealed great potentials of thermochemical sorption heat storage technology for rational and efficient utilization of thermal energy.

Multi-objective optimization for assisting the design of fixed-type packed bed reactors for chemical heat storage

• Multi-objective optimization applied on packed bed reactor for chemical heat storage. • Simultaneous maximization of heat storage density, rate and exergy efficiency. • Optimal design of thickness, reaction condition and fabrication of composite material. • High thermal contact conductance is recommended when utilizing enhanced composites. • Limitations on the packed bed’s thickness for achieving high exergy efficiency. The enhancement of heat transfer in packed bed reactors is essential for the practical application of chemical heat storage technology. This work presents the first attempt to utilize a multi-objective optimization algorithm for the design of packed bed reactors comprising heat-transfer-enhanced materials as an alternative to the insertion of metallic fins. A numerical model for the simulation of heat transfer with chemical reaction in a flat packed bed reactor comprising a composite of Ca(OH) 2 (reagent) and a silicon-impregnated silicon carbide foam (thermal conductivity enhancer) is developed. The genetic algorithm applied to the model aims to optimize four design parameters: the effective thermal conductivity of the packed bed material, its thickness, the thermal contact conductance, and the heat supply temperature. As a result, it determines the optimal trade-off between heat storage density, average heat storage rate and exergy efficiency. Under the assumptions of the model, the Pareto solution indicates that the optimal foam porosity is the interval 70-80%, while the optimal thickness is the interval 20-70 mm. The analysis suggests the necessity to improve the thermal contact conductance in packed bed reactors and, for achieving higher exergy efficiency, thinner (modular) packed beds result better performing than thicker ones.