High Temperature Heat Storage Research Articles

Preparation and erosion resistance of SiC-based ceramic-PCM sensible-latent heat composite heat storage materials

Alloy phase change materials (PCM) are key materials in latent heat storage systems for solar thermal power generation, however, their application is limited by the high corrosion and oxidation problems of alloys. To reduce the erosion problem of alloy PCM, SiC-based ceramic encapsulated flake graphite (EG) modified Al-Cu28-Si6 alloy was used to successfully prepare composite PCM with high compatibility and oxidation resistance. Adopting static erosion test method, the composite PCM was subjected to 50 thermal cycles (25∼1000 °C holding time of 10 h was recorded as 1 cycle). The results showed that SiC-based ceramics have excellent corrosion resistance to composite PCM with 10 wt.% EG added, which due to the excellent reticulation and encapsulation of EG. Based on the Young-Laplace equation and the contact angle, the erosion resistance mechanism of silicon carbide ceramic alloys has been revealed. SiC-based ceramic-PCM sensible heat-latent heat composite heat storage material was expected to be used for high-temperature heat storage in solar thermal power generation.

Optimization of synthesis parameters for polyamide 610: Strategic tailoring for superior latent heat performance

The study systematically optimizes the synthesis of polyamide 610 with the aim of improving latent heat and melting temperature to meet the demanding requirements of high-temperature heat storage applications using Response Surface Methodology (RSM). The results clarify the critical impact of reaction times, washing solvents, and molar ratios on these vital thermal characteristics. Samples exhibiting exceptional latent heat capacities at various mole ratios were further analyzed using Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), Fourier Transform Infrared Spectroscopy (FTIR), and X-ray Diffraction (XRD). The TGA assessments were instrumental in determining the thermal degradation behaviors of the materials. At the same time, The DSC analysis was used for the determination of thermal properties of the materials, FTIR provided detailed insights into their molecular structures, and XRD elucidated the crystalline structure of the materials. The findings reveal that a specific mole ratio is essential for achieving superior thermal properties, indicative of the polymer's resilience and structural stability. It was evident that samples with a balanced molar ratio (1:1) and specific reaction time (60 min) showed the best latent heat, surpassing 80 J g-1, and melting temperatures up to 210 °C. A notable aspect of this research is the use of Sebacic Acid instead of the conventional Sebacoyl Chloride for synthesizing polyamide 610. The choice of Sebacic Acid was made due to its lower toxicity and the fact that its byproducts are easier to remove than those from sebacoyl chloride, which produces hydrochloric acid as a byproduct. The presence of HCL can lead to side reactions and negatively impact the reaction process. By using Sebacic Acid, we reduce the risk of such contaminants, thereby enhancing the thermal stability and performance of polyamide 610. This greener approach not only aligns with sustainable practices but also has significant implications for the performance of the polymer. In addition, the integration of RSM with comprehensive characterization techniques has been pivotal in elucidating the critical influence of synthesis parameters on material properties. This enhanced understanding holds the potential for significant contributions to the development of advanced materials for latent heat storage applications, combining sustainability with high performance.

How much hydrogen could we need in New Zealand? Understanding the diverse hydrogen applications and their regional mapping

ABSTRACT Green hydrogen could become a key solution for the decarbonisation of diverse sectors in the energy transition. Beyond supplying the current demand for ammonia, methanol, and steel, new applications like aviation fuels, high-temperature heat and seasonal storage are promising. While initial efforts have been made to understand the future of hydrogen in New Zealand, there has been no comprehensive quantification of hydrogen demands. In this study, using systems analysis, we quantify the potential demand for 20 hydrogen applications in the country, including their spatial allocation to each of the 16 regions. We sort these applications using the so-called Clean Hydrogen Ladder concept. The associated database is open source. The results show that New Zealand's total demand for green hydrogen would be around 2.8 Mt if all technically feasible applications switched to hydrogen. Prioritising the demand for applications in rungs A and B of the Clean Hydrogen Ladder concept, we estimate approximately 0.24 and 0.77 metric tons, respectively. The findings of this research contribute to the development of a hydrogen strategy and serve as inputs for optimal planning of integrated energy pathways for New Zealand.

Preparation and heat storage characteristics of high-temperature phase change macrocapsules of chloride eutectic salt

Chloride salts, as high-temperature phase change materials (PCMs), have advantages in terms of high application temperature and high latent heat, but the intrinsic disadvantages such as high corrosiveness and easy leakage have seriously limited their development. Employing the NaCl–KCl binary salt as PCM, a double-shell cladding strategy with an inner layer of expanded graphite and an outer layer of ceramic was proposed, thereby, the chloride phase change macrocapsule with good thermal storage performance was successfully prepared. The double-shell capsules exhibited better leakage resistance and cycling stability. On this basis, the preparation process of double-shell eutectic salt macrocapsules was simplified by using an in-situ melting method, employing the mixed NaCl and KCl as raw materials directly that were transferred to eutectic binary salt inside the capsules during sintering. It is also proposed to improve the heat storage density of the capsules by increasing the capsule size and isostatically pressing the capsule. The NaK-20-200MPa sample indicated the largest heat storage density in the temperature range from 600 °C to 700 °C, with a mass heat storage density of 190.38 J/g, which was 2.85 % higher than that of the NaK-20-0MPa capsules and 36.99 % higher than that of the NaK-5-0MPa capsules. Additionally, the volume heat storage density of the NaK-20-200MPa capsules was 339.19 J/cm3, which was 14.69 % higher than that of the NaK-20-0MPa capsules and 21.52 % higher than that of the NaK-5-0MPa capsules. The good thermal performance and low cost of chloride PCM could ensure the encapsulated salt for use in high-temperature heat storage.

Performance assessment of single-sided operation in a high-temperature latent heat storage system under fault conditions during charging and discharging

As the high-temperature latent heat storage (LHS) market continues to expand, ensuring its safe and stable operation is paramount for facilitating its rapid development. In specific operational scenarios and instances of failures, large-scale integrated LHS systems may operate with either single or partial sets of heat exchange tubes. However, a notable research gap exists in the comprehensive exploration of the impact of partial operational conditions on the thermal performance of LHS systems. Consequently, this study conducted experiments involving both- and single-sided charging and discharging using a cylindrical high-temperature LHS system equipped with two sets of heat exchange tubes. To comprehend the ramifications of single-sided operation on temperature distribution across both active and inactive sides, as well as its influence on charging/discharging power and accumulated stored/released energy on the active side. The analysis of phase change material (PCM) temperature distribution revealed that single-sided charging significantly influenced the inactive side, while single-sided discharging exhibited a comparatively minor impact on the inactive side. As PCM melted during charging, temperature disparities diminished due to natural convection, while non-uniform temperature persisted in unmelted PCM. Examination of the PCM liquidus indicated that the metal heat exchange tube enhanced heat transfer on the inactive side, resulting in substantial PCM melting during charging. However, the heat transfer effect of metal heat exchange tubes during discharging was insignificant, leading to a considerable amount of liquid PCM on the inactive side after discharging. The total heat storage for both-sided charging was 1002.71 MJ, whereas, for single-sided charging using two tubes, it was 1074.42 MJ and 1000.60 MJ, respectively. Correspondingly, during discharging, the average powers were 682.74 MJ, 543.62 MJ, and 538.11 MJ, respectively. The cycle efficiency for the both-sided operation was 68.09 %, while those for single-sided operations of two tubes were 50.6 % and 53.78 %, respectively. The reduced cycle efficiencies of single-sided operations were attributed to prolonged charging and discharging durations and the presence of more liquid PCM on the inactive side after discharging.

Microencapsulated sodium nitrate with titanium-dioxide shell for high-temperature and high-density latent heat storage

In this study, a molten salt, i.e., sodium nitrate (NaNO3), that melts at 308 °C is microencapsulated with titanium dioxide (TiO2) shells via water-limited surfactant-free sol-gel processes for high-temperature and high-density thermal energy storage. The effects of the catalyst and precursor concentrations are examined to determine the optimal conditions for the two materials to achieve microcapsules with outstanding thermal performance and thermal reliability. The morphological characteristics of the core–shell structures of the microcapsule are investigated using scanning electron microscopy, whereas its chemical composition is analyzed using Fourier transform infrared spectroscopy to corroborate the synthesis of robust TiO2 shells. Calcination heat-treatment is performed at 300 °C to induce a transition from amorphous to anatase such that the shell robustness is enhanced and the thermal reliability of the NaNO3@TiO2 microcapsule is secured. The thermal performance of the NaNO3@TiO2 microcapsule is determined via differential scanning calorimetry between 220 and 360 °C. The encapsulation ratio is calculated to be 72.7–69.0 %. Finally, the thermal reliability of the NaNO3@TiO2 microcapsule is examined using DSC for 50 repeated heating and cooling cycles.

Encapsulation effectiveness and thermal energy storage performance of aluminum-graphite composite phase change materials subjected to oxide coating

The utilization of metals as phase change materials (PCMs) in high-temperature latent heat storage technology holds promising prospects, especially when integrated with concentrated solar power (CSP) systems, as it enables higher working temperatures for CSP and enhances power generation cycle efficiency. However, the practical application of metal-based phase change processes faces challenges such as liquid leakage and high-temperature corrosion, which impede their widespread implementation. In this study, aluminum (Al) is employed as the PCM, while expanded graphite (EG) is used as the matrix material. Composite PCMs with embedded structures were synthesized using powder metallurgy techniques. Notably, an oxidation pre-treatment process was employed to create an oxide layer on the surface of Al particles, thereby improving the encapsulation performance and thermal properties of the composite PCM. By increasing the oxidation pre-treatment temperature to 1100°C, the maximum encapsulation ratio was enhanced from 50 wt% to 80 wt%, and the latent heat energy storage density increased from 101.0 J/g to 188.2 J/g. Moreover, the composite PCM subjected to oxidation pre-treatment at temperatures of 670°C and above maintained a stable structure, chemical composition, and energy storage density after 50 thermal cycles. Within the temperature range of 600–700°C, the total energy storage density of the composite PCM reached 284.5 J/g. These results indicate that the oxidation pre-treatment is a promising technology in high-temperature energy storage technology.

Microstructure and properties of forsterite-zirconia composite ceramics for solar thermal storage

Solar thermal power generation is an important direction of energy utilization, and thermal storage materials are the key to ensure the continuous use of energy. In this paper, forsterite - zirconia composite ceramics were prepared by adding different contents of 3Y–ZrO2 and their physical properties, phase composition, microstructure thermal shock resistance and thermal physical properties were studied. The results showed that the water absorption, bulk density and bending strength of sample A3 sintered at 1600 °C (59.10 wt% fused magnesia, 40.90 wt% quartz and 30 wt% 3Y–ZrO2) were 0.35 %, 3.27 g/cm3 and 83.66 MPa, respectively. The crystal phase of sample A3 was forsterite and c-ZrO2. Mg2+ and Y3+ replace Zr4+, formed oxygen vacancy in the anion position, distort the coordination layer and release some interlayer stress to promote the stability of c-ZrO2. In the sintering process, the zirconia solid solution existed in the form of second phase, which reduced the growth rate of periclase, facilitated the pore discharge in the sample and promotes sintering. The thermal conductivity of forsterite - zirconia composite ceramics at room temperature was 5.52 W/(m·K). At 1000 °C, the specific heat capacity and heat storage density were 1.09 J/(g·K) and 1062.36 kJ/kg, respectively. The forsterite - zirconia composite ceramic prepared in this study was suitable for high temperature environment, its heat storage capacity was excellent, and it was suitable for high temperature heat storage system.

Experimental demonstration of a dispatchable power-to-power high temperature latent heat storage system

The development of electricity storage solutions is crucial to support the integration of variable renewable electricity sources in electricity systems. This study experimentally characterizes a state-of-the-art full-scale electrical thermal energy storage (ETES) system in outdoor conditions. The system integrates a 600 kWhth high-temperature latent heat storage module and a 13 kWe Stirling engine. The heat storage module uses an 88Al12Si metallic alloy as the phase change material to store electricity converted to thermal energy by a resistive heater in charging mode. The Stirling engine uses hydrogen as the working fluid for re-electrification (discharging). The system uses liquid sodium as the heat transfer fluid between the latent heat storage module and both the charging (electrical heater) and discharging (engine) sub-systems. Over the characterization period, the ETES prototype produced stable electricity at an average rate of 10.5 ± 1 kW for 13 consecutive hours daily with overall and power block first-law efficiencies of 23 % and 25 %, respectively. The effective storage utilization ratio varied between 0.58 and 0.94 depending on the discharge parameters. The response time was <5 s for output power regulation. The results highlight the potential of this latent heat thermal energy storage system to satisfy long-duration electricity storage applications and therefore contribute to the stabilization of electricity grids.

Study on physical property regulation and high-temperature heat storage properties of Al–Si composite phase change materials prepared by one-step pressing

In this paper, we investigate a metal composite prepared using a one-step pressing method, where Al–Si alloy acts as the phase change materials (PCMs) and Al2O3/AlN serve as the structural support materials. The alloy formation and PCM composite preparation are synthesized in a single step. The study aims to characterize the microstructure, thermodynamic, and thermal properties of the composite material. To assess the heat storage properties of the prepared composite PCMs at temperatures higher than 600 °C, a high-temperature test system is established using a high-frequency induction heater. Additionally, we explore how to improve the issue of liquid leakage under high-temperature conditions. The results demonstrate that Al2O3 and AlN (ceramic materials) exhibit excellent chemical compatibility with Al–Si composite PCMs. After sintering, the ceramic materials form a strong bond with Al–Si, effectively retaining the liquid phase of Al–Si alloy within the composite material. Consequently, the Al–Si composite PCMs do not exhibit high-temperature liquid leakage under elevated conditions. Both Al2O3/Al–Si and AlN/Al–Si composites possess favorable high temperature heat storage properties. The phase change enthalpy of Al2O3/Al–Si composite reaches 400 kJ/kg, with a thermal conductivity of 36 W/(m·K). Similarly, the AlN/Al–Si composite achieves a phase change enthalpy of 404 kJ/kg and a thermal conductivity of 43 W/(m·K). Both composites fulfill the requirements for high-temperature heat storage applications.

Macroencapsulation of sodium chloride by a double layer strategy for high temperature phase change thermal storage over 800 °C

To solve the key problems of high corrosiveness, easy leakage and low thermal conductivity of sodium chloride as phase change material (PCM) for high-temperature heat storage, this study reports a double-shell coating method to prepare centimeter-sized macrocapsules. The capsules are made of powder raw materials with the help of organic adhesives such as polyvinyl pyrrolidone and carboxymethyl cellulose for shape formation, in which the inner expanded graphite (EG) layer and the outer Al2O3 layer are coated on the salt core spheres and finally are sintered formed at high temperatures. The double-shell capsules can prevent salt from leakage because the inner EG layer is repellent to liquid salt, in contrast, the single Al2O3 shell is penetrated by liquid salt due to their wettability. After 100 cyclic tests, the double-shell capsules have good stability and high durability for long-term use. In order to improve the heat storage density of the capsules, the method of isostatically pressing the cores to increase the compactness is studied. At 200 MPa, the capsules in size of 20@2@2 show a high heat storage capacity of 1024.54 J/cm3 and 551.56 J/g from 600 to 900 °C. The double-shell encapsulated salt PCM can be widely used in high-temperature heat storage over 800 °C in solar thermal power plants and industrial waste heat recovery.

Thermal characteristics of a small-scale medium- and high-temperature latent heat storage system at different inlet flow rates and their influencing factors

This study investigates the impact of heat transfer fluids (HTFs) operational parameters on latent heat storage (LHS) systems, focusing on medium and high-temperature storage. A small-scale, visualized setup with a compact shell-and-tube heat exchanger, featuring helical fins, was constructed to enhance heat exchange. Dense thermocouples and flow meters recorded temperature and flow data of the phase-change material (solar salt) and HTF (heat transfer oil) during charging and discharging cycles, enabling detailed thermal analysis.Results show that higher HTF flow rates significantly reduce cycle time. Increasing the flow rate from 15 to 35 L/min decreased the discharging time from 176.5 to 146.4 min, a 17 % reduction, and improved energy injection/extraction efficiency. Lower rates, however, stabilize energy output and slightly increase cycle efficiency (from 51.58 % at 15 L/min to 52.45 % at 35 L/min). Additionally, the study identified a correlation between storage temperatures and flow rates: higher temperatures boost the efficiency of high flow rates in reducing melting time, while lower temperatures enhance the positive effects of low flow rates on circulation efficiency. These findings provide essential guidance for parameter selection in LHS system engineering applications.

Composite phase change material based on Al alloy with durability of over 10,000 cycles for high-temperature heat utilization

High-temperature latent heat storage (LHS) technology using alloy-based phase change materials (PCMs) is promising for high-temperature heat applications owing to its high heat storage capacity at a constant temperature. Microencapsulated PCMs (MEPCMs) coated with stable ceramics can help prevent corrosive reactions observed in alloy PCMs during heat storage. Because MEPCMs are considered as ceramic particles, a composite PCM can be prepared by compositing them with a sintering aid. In this study, composite PCMs consisting of MEPCM with Al–12 mass% Si alloy (melting point: 577 °C) particles with diameters of 31.7 or 68.2 μm, and α-Al2O3 as the sintering aid were prepared. The MEPCM used in the preparation of the composite PCM included alumina hydroxide and various alumina polymorphs as coatings. Pellet-like compacts were successfully produced by mixing each MEPCM with α-Al2O3. After heat treatment, the composite PCMs retained their shape. In particular, the latent heat capacities of the composite PCMs made from small and large MEPCMs with α-Al2O3 coatings were 171 and 231 J g−1, respectively. Moreover, the composite PCMs retained their shape and more than 80 % of their latent heat capacity after 10,000 and 1000 cycles. The developed composite PCMs, with their high latent heat capacity and durability, can be employed for high-temperature heat applications.

Enhanced radiation heat transfer performance of Alumina-Spinel composite sphere with hollow structure for rapidly ultra-high temperature thermal storage/release process

Heat storage technologies for high temperatures have recently been gaining increasing attention. For heat storage applications at high temperatures, such as industrial regenerative burner systems, the impact of radiant heat transfer on the efficiency of rapid heat storage and release cannot be ignored. Hence, the development of heat storage materials with high infrared radiation absorption is of great significance for energy saving and CO2 emission reduction. Consequently, this study innovatively proposes an advanced composite heat storage material, composited by alumina and a Fe-Co-Mn-Al spinel, with the aim of improving absorptivity. A medium-entropy approach is employed to improve IR absorptivity of alumina. Through the strategic adding of various transition metal oxides, the radiative absorptivity of the alumina material in the 600–1200 °C range increased from an initial 0.2 to an impressive 0.8, provided that doping additives remain below 10 wt%. The high-temperature heat storage material balances cost, heat recovery efficiency and thermal stability. In addition, the heat storage material is prepared as a hollow sphere to further enhance the heat storage density of rapid heat storage/release and reduce cost. Multi-scale numerical simulations are carried out to reveal the heat transfer enhancement mechanisms of modified materials and hollowing. radiative modification can significantly increase the heat storage and release capacity, especially at ultra-high temperatures above 1000 °C. Experimental evaluations using an actual regenerative burner further confirmed the heat recovery performance of this novel heat storage sphere. Comparing with conventional regenerative burner system, heating power remarkably increased by 24.6 %, and the heat storage density of packed bed improved by 16.7 %. This study promises to significantly improve the efficiency of rapid heat recovery at high temperature.

Improving the Heat Transfer Performance of the Tower Molten Salt Solar Receiver With the Novel Folded Flow Tubes

Abstract The molten salt has been widely used in concentrated solar power generation as an effective high-temperature heat transfer and heat storage working fluid. However, due to the concentrating characteristic of the tower receiver, the solar flux distribution of the molten salt receiver is extremely non-uniform, and thus the circumferential non-uniform heat flux has a prominent effect on the heat transfer performance and reliability of the traditional solar molten salt receiver tube (TRT). In this contribution, in order to solve above problems, we propose some novel folded flow tubes (NFTs), which add a partition in the tube and seal the top with end cap so that the inflow and outflow of the fluid can only proceed from the same cross section. Then, we apply the binary nitrate (solar salt) as a heat transfer fluid, which is a mixture of 60% sodium nitrate and 40% potassium nitrate. First, we analyze some effects such as flow parameters, structure, and heat flux loading direction on the convective heat transfer performance of the NFTs. The results show that the circumferential temperature difference of NFTs is about 17–92 K lower than that of TRT, and the molten salt temperature distribution is more uniform accordingly. Moreover, the heat transfer coefficient is increased about 88.37–122.85%, which can provide a guidance for the structural optimization of practical solar molten salt receivers to improve the heat transfer performance and reliability.

Design and performance analysis of deep peak shaving scheme for thermal power units based on high-temperature molten salt heat storage system

The transition to renewable energy production is imperative for achieving the low-carbon goal. However, the current lack of peak shaving capacity and poor flexibility of coal-fired units hinders the large-scale consumption of renewable energy. This study takes a 670 MW coal-fired unit as the research object and proposes eight design schemes for molten salt heat storage auxiliary peak shaving system. And through simulation calculations using Ebsilon software, the thermal performance, peak shaving capacity, environmental performance, and investment cost of each scheme were compared and analyzed. The results show that the molten salt heat storage auxiliary peak shaving system improves the flexibility of coal-fired units and can effectively regulate unit output; The combination of high-temperature molten salt and low-temperature molten salt heat storage effectively overcomes the problem of limited working temperature of a single type of molten salt, and can further improve the peak shaving capacity of coal-fired units, and the overall efficiency of operation is not low; Choosing LiNaK carbonates as high parameter molten salt and Hitec as low parameter molten salt has greatly expanded the operating range of the unit; Upgrading the combined molten salt solution with the existing low pressure cylinder zero output pipeline of the power plant can further improve the thermal performance, peak shaving performance, and environmental performance of the thermal power molten salt coupling system, with a peak shaving depth of up to 90.2 %; Combined with the zero output technology of low-pressure cylinder, when heat storage, the intermediate pressure cylinder exhaust steam extraction and electric heater heating molten salt. When releasing heat, use combined molten salt to heat bypass water supply. During heat release, a combination of molten salt is used to heat the bypass water supply. This scheme is the best flexible peak shaving transformation plan for the unit studied in this article, which can recover the initial investment within five years and meet the requirements of technical transformation difficulty.

A novel value-added utilization route for washed residue of secondary aluminum dross: Preparing corundum-spinel ceramic spheres for high-temperature heat storage

Secondary aluminum dross (SAD) is a kind of hazardous waste with high aluminum content produced in the aluminum industry. From another point, SAD is also considered as a valuable aluminum resource, which can be used as high-quality raw material for value-added products such as refractory materials. In this study, a novel route for preparing corundum-spinel heat storage spheres (HSS) using washed residue of secondary aluminum dross (WSAD) as the main raw material was proposed. FactSage 8.1 software was used for the detailed thermodynamic calculation, which provided accurate theoretical guidance for the actual synthesis experiments. The effects of roasting temperature and formula on the main phases, compressive strength, porosity, and bulk density of HSS were systematically analyzed. The strengthening mechanism of the liquid phase on the sintering strength was revealed. The results showed that the HSS with good properties could be obtained with 90 wt% WSAD, 10 wt% kaolin, and 1 wt% bentonite. The HSS mainly consisted of corundum and spinel phases. The synthetic temperature of corundum-spinel ceramic was reduced by more than 200 °C. This study provides a novel way for value-added utilization of SAD as well as realizing the deep detoxification and the removal of nitrogen of it.

Design and techno-economic analysis of a molten-salt driven energy conversion system for sustainable process heat supply

Storing off-peak cheap electricity from wind/solar farms for high-temperature heat storage in different mediums and using that for steam or high-temperature heat supply is a promising concept. A major issue for implementing high-temperature molten salt-based process heating systems is designing an appropriate heat exchanger system that can minimize the technical and operational risks and maximize the economic benefits. This study focuses on designing an optimal energy conversion unit based on this approach and conducts a techno-economic analysis of the developed system for a case study in Denmark to prove its proficiency. The system comprises a kettle boiler heat exchanger compatible with a high-temperature Sodium Hydroxide (NaOH) salt, and the case study is a cardboard factory demanding saturated steam at 10 bar, placed in the sustainable industrial business park GreenLab Skive. The results for the developed energy storage/conversion system with the given specifications and the salt are compared to an optimal design of the system when a conventional molten salt (solar salt mixture) is used. The results prove that the cost-effectiveness of the concept, in general, will be much dependent on the cost of charging electricity, and the system will only be promising if cheap off-grid electricity could be used during the whole charging process. It was found that the system with the benchmark salt NaOH for medium temperature use of ∼180 °C will be able to offer an LCOH of 67 €/MWh at an average cost of charging electricity of 63.6 €/MWh. The solar salt-cased energy conversion unit is less attractive economically but yet promising economically at cheap electricity charging rates.

Preparation and thermal properties of a novel ternary molten salt/expanded graphite thermal storage material

In thermal energy storage, the use of phase change materials (PCM) is a very efficient energy storage method. In the field of medium temperature thermal storage, nitrate PCM have always been a research hotspot, but their relatively high melting point and relatively low latent heat of phase change severely limit their application in thermal energy storage. In this paper, LiNO3 and NaCl with higher latent heat value were selected to improve the thermal properties of nitrate. A novel ternary mixed molten salt of NaNO3-LiNO3-NaCl with high latent heat was prepared and studied. NaNO3-LiNO3-NaCl/EG composite phase change material (CPCM) was prepared by selecting expanded graphite (EG) with a large thermal conductivity as the carrier adsorption material for molten salt. The thermophysical properties, thermal stability, and heat transfer ability of PCM and CPCM were tested. The composition and microstructure of the materials were also characterized. The experimental results showed that compared with pure NaNO3, the phase change temperature of NaNO3-LiNO3-NaCl was reduced by 45.3 °C, and the latent heat of phase change was increased by 59.4 J/g. NaNO3, LiNO3, NaCl and EG had good compatibility, without chemical reaction between them. From the microscopic morphology of the composite material, it can be seen that the PCM was uniformly dispersed into the porous structure of EG without agglomeration. The ternary new molten salt had higher thermal decomposition temperature and better thermal stability. When the addition ratio of EG was 20 %, the thermal conductivity of the ternary molten salt increased from 1.350 to 5.017 W/m·K. EG greatly improved the heat transfer capacity of PCM. And the PCM added with EG still had a high latent heat of phase change. The results show that NaNO3-LiNO3-NaCl/EG has broad application prospects in the field of medium and high temperature heat storage.

Heat transfer and thermal performance investigation on a visualized latent heat storage system in pilot-scale: Scalable medium-temperature thermal energy storage system

Large-capacity medium-temperature (100–300 °C) and high-temperature (>300 °C) latent heat storage (LHS) systems can enhance the reliability and performance of solar thermal utilization systems due to their characteristics of balancing the gap between energy supply and demand. In this study, a pilot-scale LHS system was developed and investigated, incorporating a mass of 270 kg of phase change material (PCM), specifically a commercial nitrate mixture. Moreover, the thermal energy storage unit featured a rectangular aperture at its center, covered by a quartz visualization window to facilitate the observation of the melting and solidification behavior of the PCM. The charging process lasted 111 min, with a mean average power, accumulated energy and efficiency of 11.90 kW, 79.33 MJ, and 77.67%, while those of the discharging process were 56.5 min, 12.99 kW, 44.11 MJ, and 71.41%, respectively. Furthermore, a comparative analysis was conducted concerning heat transfer performance and mechanisms during the charging and discharging processes. The heat transfer coefficients and average thermal resistances of the LHS system during charging and discharging were 613.08 W/(m2·K), 295.28 W/(m2·K), and 0.001984 (m2·K)/W, 0.005285 (m2·K)/W, respectively. The average thermal resistance observed during the charging process was notably lower than that observed during the discharging process, attributable to the variance in the predominant heat transfer mechanism. For a substantial portion of the charging process duration, heat transfer was dominantly governed by natural convection, whereas throughout a significant duration of the discharging process, heat transfer was primarily driven by thermal conduction.