Heat Storage Applications Research Articles

Room Temperature Metal Hydrides for Stationary and Heat Storage Applications: A Review

Hydrogen has been long known to provide a solution toward clean energy systems. With this notion, many efforts have been made to find new ways of storing hydrogen. As a result, decades of studies has led to a wide range of hydrides that can store hydrogen in a solid form. Applications of these solid-state hydrides are well-suited to stationary applications. However, the main challenge arises in making the selection of the Metal Hydrides (MH) that are best suited to meet application requirements. Herein, we discuss the current state-of-art in controlling the properties of room temperature (RT) hydrides suitable for stationary application and their long term behavior in addition to initial activation, their limitations and emerging trends to design better storage materials. The hydrogen storage properties and synthesis methods to alter the properties of these MH are discussed including the emerging approach of high-entropy alloys. In addition, the integration of intermetallic hydrides in vessels, their operation with fuel cells and their use as thermal storage is reviewed.

Encapsulation of Salt Hydrates by Polymer Coatings for Low-Temperature Heat Storage Applications

Efficient and cheap storage of energy from renewable resources presents a key technology to facilitate the ongoing energy transition. Storing heat in thermochemical materials (TCMs), such as salt hydrates, provides a promising concept to meet this demand. TCMs can capture heat reversibly and loss-free by relying on equilibrium hydration reactions of the salts. Persistent bottlenecks in the full-scale application of this technology are the low mechanical resilience of salt grains and their tendency to coagulate or dissolve when in contact with water vapor. To overcome this, the salt grains can be encapsulated by a stabilizing polymer coating. Ideal coatings combine high water vapor permeability with reversible deformability to minimize the resistance for water transport and to accommodate the volumetric changes of the TCM during repetitive (de)hydration, respectively. Here, a systematic study into the applicability of commercially available polymers as coating materials is presented. Mechanical analysis and wet-cup experiments on freestanding polymer films revealed that cellulose-based coatings successfully combine permeability and ductility and meet the engineering demands for domestic TCM-based heat storage applications. The validity of using freestanding films as model system was confirmed by encapsulating granular TCMs in ethyl and hydroxyl propyl cellulose using fluidized bed coating. The permeability was retained and an enhanced structural integrity of the TCM grains during (de)hydration cycles was observed.

A study of the natural convection of water-AA7075 nanoliquids in low-porosity cylindrical annuli using a local thermal non-equilibrium model

Natural convection in nanoliquid-saturated porous cylindrical annuli due to uniform heat and mass influxes from the solid cylinder and effluxes from the outer hollow cylinder is investigated analytically. The Darcy model and the modified version of the Buongiorno two-phase model are used, and local thermal non-equilibrium between the phases is assumed. A nanoliquid-saturated porous medium made up of glass balls with a dilute concentration of AA7075 alloy nanoparticles well-dispersed in water is considered. Out of three types of annuli considered, shallow annuli provide the best heat transport and tall annuli show the worst performance. The presence of a dilute concentration of nanoparticles significantly enhances the heat transport in the system. Of nine nanoparticle shapes considered, lamina-shaped nanoparticles enhance heat transport the most. Heat transport is enhanced in the case of heat-and-mass-driven convection compared to the case of purely heat-driven convection. The results for a rectangular enclosure are obtained as a particular case of the present study. Two asymptotic routes that take us to the results of thermal equilibrium are shown. The vanishing limit of the concentration Rayleigh number yields the result for a single-phase model. Results for the base-liquid-saturated porous medium form a limiting case of the present study. We conclude that a shallow cylindrical annulus saturated with water-AA7075 lamina-shaped alloy nanoparticles is best suited for heat transfer due to its high effective thermal conductivity in comparison with that of other shaped nanoparticles and a tall rectangular enclosure saturated by water is best suited for heat storage applications.

Tuning Mg(OH)2 Structural, Physical, and Morphological Characteristics for Its Optimal Behavior in a Thermochemical Heat-Storage Application.

Thermochemical materials (TCM) are among the most promising systems to store high energy density for long-term energy storage. To be eligible as candidates, the materials have to fit many criteria such as complete reversibility of the reaction and cycling stability, high availability of the material at low cost, environmentally friendliness, and non-toxicity. Among the most promising TCM, the Mg(OH)2/MgO system appears worthy of attention for its properties in line with those required. In the last few decades, research focused its attention on the optimization of attractive hydroxide performance to achieve a better thermochemical response, however, often negatively affecting its energy density per unit of volume and therefore compromising its applicability on an industrial scale. In this study, pure Mg(OH)2 was developed using different synthesis procedures. Reverse deposition precipitation and deposition precipitation methods were used to obtain the investigated samples. By adding a cationic surfactant (cetyl trimethylammonium bromide), deposition precipitation Mg(OH)2 (CTAB-DP-MH) or changing the precipitating precursor (N-DP-MH), the structural, physical and morphological characteristics were tuned, and the results were compared with a commercial Mg(OH)2 sample. We identified a correlation between the TCM properties and the thermochemical behavior. In such a context, it was demonstrated that both CTAB-DP-MH and N-DP-MH improved the thermochemical performances of the storage medium concerning conversion (64 wt.% and 74 wt.% respectively) and stored and released heat (887 and 1041 ). In particular, using the innovative technique not yet investigated for thermal energy storage (TES) materials, with NaOH as precipitating precursor, N-DP-MH reached the highest stored and released heat capacity per volume unit, ~684 MJ/m3.

Scalable Production of EP/CaCl2@C Multistage Core–Shell Sorbent for Solar-Driven Sorption Heat Storage Application

The sorption heat storage based on composites with salt hydrates enables the efficient utilization and trans-seasonal storage of solar energy. Nevertheless, the limitation of heat and mass transfer of salt hydrates has restricted the implementation of this technology. Here, a novel strategy is proposed to synthesize a multistage core–shell sorbent, where salt hydrates are dispersed in the macropores of the expanded perlite (EP) before being coated with a carbon layer outside the EP/Ca particles. The abundant macropores in the matrix are beneficial to the high salt loading and efficient mass transfer. Moreover, the carbon shell significantly enhances the light-to-heat conversion performance of the sorbent, and the heat can be directly transferred from the shell to the internal salt hydrates for desorption, thus realizing the absorption, conversion, and storage of solar energy in the multistage core–shell sorbent. The sorption capacity of EP/Ca@C-0.25 is determined to be 1.22 g-H₂O/g-sorbent under the conditions of 20 °C and RH 80%, and 84% of the water can be desorbed after irradiation under the 1 kW/m² simulated sunlight for 2 h, with a heat storage density of 1698 J/g-sorbent in the heat storage process. Additionally, we also studied the performance of EP/Ca@C-0.25 in a solar-driven thermal energy storage system. In summer, the sorbent is placed outdoors and desorbed under sunlight within 2 h to achieve heat storage. In winter, the sorbent is placed in a fixed bed for adsorption, and the heat released during adsorption increases the air temperature by 5 °C for 10 h. Typically, the multistage core–shell sorbent demonstrates a potential sorbent for large-scale use in solar-driven thermal energy storage for practical purposes.

Efficient utilization of interparticle mesopores in aluminosilicate towards thermal energy storage

Porous confinement, a method to prepare form-stable composite phase change materials (PCMs) by impregnating PCMs into the pores of porous scaffold materials, is proven to be an extraordinarily effective mean to address the leakage problem of PCMs during the phase transition process. For the energy storage capacities of composite PCMs are closely associated with the pore structure of scaffold materials, the research on the correlation between the structure of mesoporous scaffold materials and its loaded capability phase change materials is conducive to enriching the research system of heat storage materials. Herein, a novel form-stable composite PCM was fabricated successfully by integrating polyethylene glycol (PEG) into the mesopores of nano-aluminosilicate aggregates (n-ASA). The structure and the thermal properties of the samples were characterized. The results indicated that n-ASA had a self-sustaining microstructure by the aggregation of individual granules, and a very high mesopore content of 98.3 % and mesopore volume of 1.16 cm3/g. The maximum mass fraction of PEG loaded in the composite can reach up to 77.5 % and the melting latent heat of n-ASA/PEG were 133.8 J/g and 115.0 J/g, respectively. An increasement of 42.4 % in thermal conductivity of PEG was achieved after loading it into n-ASA with interconnected pores due to the provision of a faster heat transfer channel. Moreover, the thermal characteristics of n-ASA/PEG had no distinct change after 200 thermal cycles, indicating its prominent thermal reliability and stability. It is convinced that the mesopores of n-ASA are actually preferable over the micro- and macro-pores for holding PEG, and the synthesized n-ASA/PEG composite would be a prospective and potential candidate for low-temperature heat storage applications.

Heat storage characteristics and application analysis of heat source tower in soil thermal balance of ground source heat pump

In order to solve the problem of soil thermal imbalance of ground source heat pump (GSHP), a new method of using heat source tower (HST) to absorb heat from air and store heat to soil through borehole heat exchange (BHE) in summer was proposed. The finite element iteration calculation of heat and mass transfer of HST was realized by MATLAB. The results show that the heat storage capacity increases obviously with the increasing of air temperature and relative humidity and the decreasing of soil temperature. Increasing the number of BHE and reducing the water flow rate of each BHE are also beneficial to heat storage. Among the total heat storage, the ratio of air latent heat even reaches 70%. Under the average meteorological parameters in July of Changchun and the water flow rate of 5 kg/s in HST, the heat storage demand of a 1000 m2 building can be completed in 34 days and the coefficient of performance (COP) of the HST system is up to 27.54. Heating the soil out of season by HST is a high efficiency and low cost method to solve the problem of soil thermal imbalance of GSHP.

Reversible Dual-Stimuli-Responsive Chromic Transparent Wood Biocomposites for Smart Window Applications.

Transparent wood (TW)-based composites are of significant interest for smart window applications. In this research, we demonstrate a facile dual-stimuli-responsive chromic TW where optical properties are reversibly controlled in response to changes in temperature and UV-radiation. For this functionality, bleached wood was impregnated with solvent-free thiol and ene monomers containing chromic components, consisting of a mixture of thermo- and photoresponsive chromophores, and was then UV-polymerized. Independent optical properties of individual chromic components were retained in the compositional mixture. This allowed to enhance the absolute optical transmission to 4 times above the phase change temperature. At the same time, the transmission at 550 nm could be reduced 11−77%, on exposure to UV by changing the concentration of chromic components. Chromic components were localized inside the lumen of the wood structure, and durable reversible optical properties were demonstrated by multiple cycling testing. In addition, the chromic TW composites showed reversible energy absorption capabilities for heat storage applications and demonstrated an enhancement of 64% in the tensile modulus as compared to a native thiol–ene polymer. This study elucidates the polymerization process and effect of chromic components distribution and composition on the material’s performance and perspectives toward the development of smart photoresponsive windows with energy storage capabilities.

A Review of Composite Phase Change Materials Based on Porous Silica Nanomaterials for Latent Heat Storage Applications.

Phase change materials (PCMs) can store thermal energy as latent heat through phase transitions. PCMs using the solid-liquid phase transition offer high 100–300 J g−1 enthalpy at constant temperature. However, pure compounds suffer from leakage, incongruent melting and crystallization, phase separation, and supercooling, which limit their heat storage capacity and reliability during multiple heating-cooling cycles. An appropriate approach to mitigating these drawbacks is the construction of composites as shape-stabilized phase change materials which retain their macroscopic solid shape even at temperatures above the melting point of the active heat storage compound. Shape-stabilized materials can be obtained by PCMs impregnation into porous matrices. Porous silica nanomaterials are promising matrices due to their high porosity and adsorption capacity, chemical and thermal stability and possibility of changing their structure through chemical synthesis. This review offers a first in-depth look at the various methods for obtaining composite PCMs using porous silica nanomaterials, their properties, and applications. The synthesis and properties of porous silica composites are presented based on the main classes of compounds which can act as heat storage materials (paraffins, fatty acids, polymers, small organic molecules, hydrated salts, molten salts and metals). The physico-chemical phenomena arising from the nanoconfinement of phase change materials into the silica pores are discussed from both theoretical and practical standpoints. The lessons learned so far in designing efficient composite PCMs using porous silica matrices are presented, as well as the future perspectives on improving the heat storage materials.

Experimental research on latent heat characteristics of binary mixed molten salt

At present, thermal storage is considered as one of the key technologies to alleviate the problem of instability and intermittence for renewable energy. Due to relatively high latent heat, latent heat storage by molten salt is considered to a potential technology. Because of the restriction of different thermophysical parameters, the application of latent heat storage by molten salt has many technical obstacles. So, it is an urgent work to find a better mixed molten salt formula with comprehensive thermophysical properties. In this paper, we selected halogen salts (NaCl, NaF) and alkali (NaOH) to prepare binary mixed molten salts. From the results, for the mixed molten salt of NaCl-NaOH, NaCl:NaOH=1:9 has a maximum latent heat value of 301.2J/g. The initial melting temperature is about 200°C. With the increase of the mass ratio of NaCl, the termination melting temperature increases firstly then decreases. For mixed molten salt of NaF-NaOH, NaF:NaOH=1:9 has a maximum latent heat value of 199.5J/g, the initial and termination melting temperatures are about 265°C and 380°C, repectively. The latent heat value of NaCl-NaOH and NaF-NaOH mixed molten salts decreased with the increase of the mass ratio of halogen salts.

Experimental Study on the Performance of Small Solar Updraft Tower in the Climate Region

Solar updraft tower (SUT) power plant is a unique system using solar energy, which consists of three main parts: the chimney, turbine, and collector. In tropical climate conditions, especially in Malaysia, the application of solar chimney can be deemed more competitive compared to other renewable energy systems. In this study, one prototype with a dimension of 3.5 m in diameter for the collector and 5 m height for the chimney, was built in Melaka, Malaysia. Vital parameters such as temperature field, pressure, humidity, and air movement were measured using data logger and stored using non-volatile memory. The most exciting finding was that the temperature and pressure difference was significantly seen during the experiment along the day. There was a significant surge for the humidity in the morning with an average of above 95% and the ambient temperature was approximately 35 °C due to low solar radiation. The humidity kept decreasing after sunrise due to the greenhouse effect produced in the solar collector. With the collection of these critical parameters, a solar chimney power plant was adequate to function and generate power from solar radiation in a country such as Malaysia, which is based in the equatorial region. These studies are limited as the analysis focused mainly for high radiation and did not investigate efficiency during low solar radiation. The addition of heat storage underneath the collector with current techniques is another direction of research for continuous system operation, particularly during low solar radiation. It is proposed that the synthesis of these factors should be extensively explored in future studies coupled with the application of heat storage.

Dynamic modelling of Adsorption systems: a comprehensive calibrateddataset for heat pump and storage applications

The growing efforts for the development of clean and efficient energy systems require the use of a multi-disciplinary approach and the integration of multiple generation appliances. Among the fields that can be considered enabling technologies, adsorption systems for air conditioning and thermal energy storages, are constantly increasing their maturity. However, for a proper design and integration of such systems, there is the need for a simulation framework that is reliable and computationally convenient. In the present paper, the implementation of a dynamic model for adsorption systems is presented, which includes different components (adsorber, phase changer, sorption materials) and is structured as a library. Modelica language and the commercial software Dymola® are used for the analysis. Data for different heat exchangers and working pairs are calibrated using experimental results and the calibrated model is subsequently used for the design of an adsorber based on a plate heat exchanger for thermal energy storage applications. The results proved that the model is fast and can reproduce experimental results with good accuracy, thus being a useful tool for the design and optimization of the different components of sorption systems.

Sr(NH3)8Cl2-Expanded Natural Graphite composite for thermochemical heat storage applications studied by in-situ neutron imaging

This work presents an in-situ neutron imaging study of the ammonia sorption reaction in a SrCl2-Expanded Natural Graphite (ENG) composite loaded into a honeycomb-shaped stainless-steel scaffold and enclosed in a thermochemical heat storage (THS) reactor prototype. The performance of SrCl2/Sr(NH3)8Cl2-ENG under different pressures and temperatures was investigated and the spatio-temporal content of ammonia was calculated from neutron radiograms. Quantitative image analysis revealed the formation of Sr(NH3)8Cl2 upon ammonia uptake, while the desorption studies revealed a partial ammonia desorption resulting in the formation of the monoamine phase Sr(NH3)Cl2 via a stepwise release of ammonia. Neutron imaging also allowed the indirect evaluation of the stainless-steel honeycomb heat conductivity and showed that heat is transferred prevalently via the ENG matrix. Finally, neutron tomography of the reactor prototype was performed to ensure the stability of the THS system and composite material throughout the ammonia cycling experiments.

Investigation on compatibility and thermal reliability of phase change materials for low-temperature thermal energy storage

Two of the important aspects for the successful utilization of phase change materials (PCMs) for thermal energy storage systems are compatibility with container materials and stability. Therefore, the present study is focused on testing the corrosion resistance and surface characteristics of metals in contact with PCMs and thermal behavior of PCMs with heating/cooling cycles. The PCM selection is made by targeting low temperature (<100 °C) heat storage applications. The PCMs considered are paraffin wax, sodium acetate tri-hydrate, lauric acid, myristic acid, palmitic acid, and stearic acid. The metal specimens tested are aluminum, copper, and stainless steel because of their wide usage in thermal equipment. The tests are performed by the method of immersion corrosion test, and ASTM G1 standards are followed. The experiments are carried out at 80 °C and room temperature (30 °C) for the duration of 10, 30, and 60 days. Pertaining to thermal stability 1500 melting/freezing cycles are performed. Investigation has been carried out in terms of corrosion rate, SEM analysis of metal specimens, appearance of PCMs, and variation of thermophysical properties at 0th, 1000th, and 1500th thermal cycles. The most affected area of corrosion, including the dimension of pits, is presented, and comparison is made. Based on the corrosion experiments, recommendations are made for the metal–PCM pairs. Pure sodium acetate trihydrate is observed to suffer from phase segregation and supercooling. After 1500 thermal cycles, the variation in melting and freezing point temperatures for rest of the five PCMs are in the range of − 1.63 to 1.57 °C and − 4.01 to 2.66 °C. Whereas, reduction in latent heat of melting and freezing are in the range of 17.6–28.95% and 15.2–26.78%.

Preparation and study on thermophysical properties of inorganic salt hydrate as energy storage materials

In this paper, the preparation methods of new inorganic salt hydrate mixtures and their experimentally tested thermophysical properties are presented. Two inorganic salt hydrates mixed together at different weight proportions to study their thermodynamic properties for energy storage applications. Three samples having weight proportions of salt mixtures as 70:30, 80:20 and 90:10 are prepared and Differential Scanning Calorimetry (DSC) is used as a thermal analyzer to find the thermal stability of the samples. The DSC results of the salt hydrate are observed for determining the melting temperatures, latent heats and specific heats during melting and freezing process. The melting point, heat of fusion, and heat capacity of the salt hydrate samples are observed as 46.6 °C , 133.81 J/g and 8.75 J/gK for 70:30, 47.28 °C, 169.91 J/g and 15.95 J/gK for 80:20 and 47.26 °C, 173.96 J/g and 14.20 J/gK for 90:10. The time requirements for the completion of melting and freezing process of the salt mixture results are compared. Thermal and heat transfer characteristics of the newly prepared salt mixtures are experimentally investigated for the purpose of recommending it to use as thermal energy storage medium. The inexpensive inorganic salt hydrate mixtures, with greater enthalpy, higher heat storage capacity and good thermal reliability, shows great potential for heat storage applications in the low temperature solar thermal applications like water heaters, building space heating applications, domestic solar cookers and dryers.

Assessing Cr incorporation in Mn2O3/Mn3O4 redox materials for thermochemical heat storage applications

Widening the use of renewable sources requires more efficient energy storage systems to overcome the inherent intermittence of solar energy. In this respect, thermal energy storage coupled to concentrated solar power represents an inexpensive technology to achieve that goal. In particular, the use of reversible thermochemical reactions is promising due to a higher energy storage density if compared with commercial sensible heat storage on molten salts. However, some of these systems that rely on gas-solid reactions can suffer a cycle-to-cycle loss of activity due to slow kinetics and materials degradation, which is detrimental for its potential future commercialization. In this work, we have assessed the incorporation of Cr cations in the redox couple Mn2O3/Mn3O4, as a way to stabilize the multi-cyclic activity over prolonged operation at high temperatures (650-1000 °C). Reduction has been studied with in situ XRD and kinetic analyses, which confirm that Cr incorporation shifts the reaction towards high temperatures. Long term redox cycling tests confirm that 5% Cr incorporation helps to stabilize the redox activity of Mn2O3/Mn3O4.

Exploring the Electronic Structure of New Doped Salt Hydrates, Mg1–xCaxCl2·nH2O, for Thermochemical Energy Storage

Chloride-based salt hydrates, MgCl2·nH2O and CaCl2·nH2O (n = 0,1,2,4,6), are promising materials for thermochemical heat storage systems due to their high sorption energy capacity. However, both salts have their own shortcoming characteristics within the operational temperature of the thermochemical heat storage applications. While the higher hydrates of CaCl2·nH2O (n = 4,6) have a low melting point, the lower hydrates of MgCl2·nH2O (n = 0,1,2) can form the highly toxic and corrosive HCl gas. Both shortcomings cap the individual use of these salts to a restricted range of the available hydrates. A combination of these two salts showed to have the potential to overcome these shortcomings. The present study focuses on finding stable configurations of potential superior salt hydrate combinations using the evolutionary algorithm USPEX as well as manual mutations of known pristine structures. The newly found structures are less stable than the pure salts, but stable enough to be combined. Extensive electronic density-derived tools, like the Density Derived Electronic and Chemical (DDEC6) bond orders and net atomic charges, as well as Bader topological analysis, are used to predict the HCl gas formation based on the chemical environment in the new metastable structures. We find that doping MgCl2·nH2O with calcium considerably reduces HCl formation compared to its pure form, caused by a combination of the stronger Ca-Cl interaction than Mg-Cl and a less polar H2O molecule in a calcium environment than in a magnesium environment. This provides the possibility to shift the p, T-equilibrium curve of HCl outside the thermal storage operational window.

Buoyancy-driven melting and solidification heat transfer analysis in encapsulated phase change materials

Controlled melting and solidification in encapsulated phase change materials (PCMs) is of practical interest, for example, in latent heat storage applications. The choice of PCMs and the dynamic heat transfer characteristics during phase change - affecting the transient charging/discharging rates - are decisive for the energy and power density of the heat storage option. For example, highly conductive, high melting point (above 700 K) metal alloys have a potential advantage as a high energy and power density latent heat storage, compared to the widely used low conducting molten salt and paraffin wax. This advantage depends on the relative dominance of heat transfer modes that vary depending on the thermal properties of the PCM, shape of the encapsulation, and the load conditions, and must be quantified to warrant a fair comparison. We developed a 2D transient phase change model of encapsulated PCMs, accounting for phase change over a temperature range, volumetric expansion and contraction, and multi-mode heat transfer within the encapsulated PCM. The enthalpy-porosity method was used to model the phase change in a fixed grid. Validation was performed with literature data of low and high-conductivity PCM experiments (RT27 and lead). Two types of PCMs were subsequently investigated in detail: a high-conducting binary-eutectic alloy Al-12.6Si and a low-conducting commercially available RT27 paraffin wax (Rubitherm GmbH). The model was used to compare the phase change process and the strength of the various modes of heat transfer in the PCM filled cylindrical stainless-steel encapsulations in horizontal and vertical orientation with constant temperature walls. A full parameter study and non-dimensional analysis are presented for different PCMs. The results quantified the influence of boundary conditions, thermophysical properties and geometrical parameters on the phase change process and the contribution of the natural convection within the encapsulation. The non-dimensional analysis linked the melt fraction and heat transfer rates to a combination of the Fourier, Stefan, Rayleigh and Nusselt numbers. Fitting of the exponents of non-dimensional number groups to the heat transfer calculations allowed to provide general correlations for melting time, melt fraction, heat transfer rates, and characteristics of melting. Such correlations provide general understanding of the transient heat transfer in phase change media and provide engineering tools for designing, for example, a latent heat storage unit.

Graphite foam infiltration with mixed chloride salts as PCM for high-temperature latent heat storage applications

An improved hybrid vacuum and pressure-assisted infiltration technique was developed for the infiltration of phase change materials (PCM) into the graphite foam matrix. A single chamber with no moving part infiltration setup was designed and fabricated. Pelletized PCM was introduced to provide enough porosity for air removal using vacuum pumps with no need for transfer PCM after melting and infiltration. Simple design, the use of stainless steel, and the corrosion resistance feature of PCM provides a very cost-effective design and easily controllable process. Mixed alkali and alkaline chloride salts as PCM were infiltrated into the high porosity graphite foam to enhance the thermal conductivity of latent heat storage medium. Chloride based PCM with proven corrosion-resistant features, high energy storage density that enables storage and release of energy at nearly constant temperatures close to the melting temperature of 355 °C are used in this study, as an example. The composites of infiltrated graphite foam and chloride salt PCMs were studied using SEM, EDS microstructural analyses to determine the effectiveness of the infiltration process. An infiltration efficiency greater than 90% of the available porosity was achieved. The thermal conductivity of the infiltrated samples was analyzed using a laser Nano-flash thermal conductivity device. The thermal conductivity of the foam/PCM composite was shown to be larger by a factor greater than 40 times of pure chloride PCM (1–2 W/m-K). Low-cost infiltration with proven efficacy and repeatability of infiltrated PCM could be a breakthrough for 3rd generation of CSP plant applications with supercritical CO2 power cycles.

Thermal energy storage and thermal conductivity properties of fatty acid/fatty acid-grafted-CNTs and fatty acid/CNTs as novel composite phase change materials

In recent year, fatty acids (FAs) are heavily studied for heat storage applications and they have shown promising advantages over other organic phase change materials (PCMs). Among the FAs; capric, palmitic and stearic acids are the most studied PCMs. Several researchers have investigated these FAs and tried to improve their thermal properties, mainly by adding different high conducting fillers, such as graphite, metal foams, CNTs, graphene etc. In most cases, these fillers improved the thermal conductivity and heat transfer property but reduce the heat storage capacity considerably. These composites also lose the mixing uniformity during the charging and discharging process. To overcome these issues, selected FAs were grafted on the functionalized CNT surfaces and used as conductive fillers to prepare FA based composite PCMs. This process significantly contributed to prevent the drastic reduction of the overall heat storage capacity and also showed better dispersion in both solid and liquid state. Thermal cycling test showed the variations in the thermal energy storage values of all composite PCMs, however, within the tolerable grade and they had appreciable phase change stability and good chemical stability even after 2,000 cycles.