Solar Thermal Energy Storage Applications Research Articles

Second Generation Zwitterionic Aza‐Diarylethene: Photoreversible CN Bond Formation, Three‐State Photoswitching, Thermal Energy Release, and Facile Photoinitiation of Polymerization

Diarylethenes are a well‐studied and optimized class of photoswitches with a wide range of applications, including data storage, smart materials, or photocontrolled catalysis and biological processes. Most recently, aza‐diarylethenes have been developed in which carbon‐carbon bond connections are replaced by carbon‐nitrogen connections. This structural elaboration opens an entire new structure and property space expanding versatility and applicability of diarylethenes. In this work, we present the second generation of zwitterionic aza‐diarylethenes, which finally allows for fully reversible photoswitching and precise control over all three switching states. High‐yielding photoswitching between the neutral open form and a zwitterionic Z isomer is achieved with two different wavelengths of light. The third zwitterionic E isomeric state can be reached up to 87% upon irradiation with a third wavelength. Its high energy content of >10 kcal/mol can be released thermally by deliberate solvent change as trigger mechanism, rendering aza‐diarylethenes into interesting candidates for molecular solar thermal energy storage (MOST) applications. The third state further serves as locking state, allowing to toggle light‐responsiveness reversibly between thermally labile and thermally stable switching. Further, irradiation of the zwitterionic states leads to highly efficient photopolymerization of methyl acrylate (MA), directly harnessing the unleashed chemical reactivity of aza‐diarylethene in materials applications.

Preparation and thermal stability research of oxalic acid dihydrate-glutaric acid/PAMPS phase change gel for solar thermal energy utilization

Thermal energy storage technology based on phase change materials (PCMs) can address the temporal and spatial mismatches in solar thermal energy conversion, thereby enhancing solar energy utilization efficiency. However, the liquid flow and poor thermal reliability of PCMs limit their large-scale application in solar thermal systems. In this study, we employ a simple “one-pot method” to prepare a form-stable and thermally reliable oxalic acid dihydrate-glutaric acid/poly 2-Acrylamido-2-methyl-1-propanesulfonic acid (OAD-GA/PAMPS) phase change gel. The OAD-GA/PAMPS phase change gel melts at 64.5 °C with a phase change enthalpy of 193.6 kJ/kg, the tensile strength is 7.707 MPa, and the compressive strength is 16.940 MPa. By incorporating 5 wt% BN particles, the thermal conductivity of OAD-GA/PAMPS phase change gel reaches 0.63 W/(m·K). After 200 heating-cooling cycles, the phase change temperature of the OAD-GA/PAMPS phase change gel remains nearly unchanged, and the phase change enthalpy decreases by only 5.7 %. The photo-thermal conversion efficiency of the OAD-GA/PAMPS phase change gel is 82.7 %. The high thermal reliability and photo-thermal conversion efficiency of the OAD-GA/PAMPS phase change gel makes it suitable for solar thermal energy storage applications.

Flexible Polyolefin Elastomer/Paraffin Wax/Alumina/Graphene Nanoplatelets Phase Change Materials with Enhanced Thermal Conductivity and Mechanical Performance for Solar Conversion and Thermal Energy Storage Applications.

In this study, electrically insulating polyolefin elastomer (POE)-based phase change materials (PCMs) comprising alumina (Al2O3) and graphene nanoplatelets (GNPs) are prepared using a conventional injection moulding technique, which exhibits promising applications for solar energy storage due to the reduced interfacial thermal resistance, excellent stability, and proficient photo-thermal conversion efficiency. A synergistic interplay between Al2O3 and GNPs is observed, which facilitates the establishment of thermally conductive pathways within the POE/paraffin wax (POE/PW) matrix. The in-plane thermal conductivity of POE/PW/GNPs 5 wt%/Al2O3 40 wt% composite reaches as high as 1.82 W m-1K-1, marking a remarkable increase of ≈269.5% when compared with that of its unfilled POE/PW counterpart. The composite exhibits exceptional heat dissipation capabilities, which is critical for thermal management applications in electronics. Moreover, POE/PW/GNPs/Al2O3 composites demonstrate outstanding electrical insulation, enhanced mechanical performance, and efficient solar energy conversion and transportation. Under 80 mW cm-2 NIR light irradiation, the temperature of the POE/PW/GNPs 5 wt%/Al2O3 40 wt% composite reaches approximately 65 °C, a notable 20 °C improvement when compared with the POE/PW blend. The pragmatic and uncomplicated preparation method, coupled with the stellar performance of the composites, opens a promising avenue and broader possibility for developing flexible PCMs for solar conversion and thermal storage applications.

Study on Miscibility, Thermomechanical Behavior, and Thermoregulation Performance of Paraffin Wax/Bituminous Blends for Solar Thermal Energy Storage Applications.

The goal of this work was to study the miscibility, thermal stability, thermomechanical properties, and temperature regulation performance of paraffin wax/bitumen blends for their potential use in solar thermal energy storage applications. Results indicated that these blends present a suitable thermal stability, and their thermomechanical properties are strongly dependent on composition, developed microstructure, and temperature. Among all paraffin wax concentrations studied, the blend containing 40 wt % paraffin wax displays enhanced binder elastic properties together with lower thermal susceptibility compared to base bitumen. In addition, this binder also presents improved thermal properties (thermal conductivity and specific heat capacity) and still maintains a high crystallinity, thereby retaining a large enough latent heat to be used for thermal energy storage. Thus, results from the temperature regulation test, which was conducted by subjecting the sample to simulated solar irradiation at a constant radiant flux density, provide a higher latent heat thermoregulation index value than other microencapsulated phase change materials systems. Therefore, it can be stated that paraffin wax/bitumen blends are promising base materials to formulate form-stable products for thermal energy storage applications for thermoregulation purposes.

MXene-based eutectic salt hydrate phase change material for efficient thermal features, corrosion resistance & photo-thermal energy conversion

Salt hydrate phase change materials (PCMs) possess low thermal conductivity, degree of supercooling, lack of corrosion resistance and poor photo thermal conversion ability. To overcome these challenges, a two dimensional, chemically stable, optically & thermally efficient MXene@eutectic salt hydrate composite PCM is developed for effective harnessing of solar energy and thermal energy storage (TES) applications. Low temperature eutectic salt hydrate PCM with phase transition temperature of 27.6 °C and 216 J/g melting enthalpy is developed using sodium sulphate decahydrate (SSD) and sodium phosphate dibasic dodecahydrate (SPDD) based on Schrader equation. Advanced 2D layered material, MXene (Ti3C2) is synthesised from MAX phase (Ti3AlC2). Furthermore, MXene based PCM nanocomposite is developed with binary eutectic PCM of SSD/SPDD at different weight fractions through two step technique. Developed nanocomposite PCM exhibits better chemical stability, high optical absorbance of (1.1), low transmissibility (10.2 %), reliable thermal conductivity (0.621 W/m⋅K), improved melting enthalpy (161.2 J/g) and considerable cycle stability. The optical absorbance and thermal conductivity demonstrated a superior photo-thermal conversion capability and thermal networks for the developed MXene based composite sample at 0.9 wt% of MXene. The above properties are experimentally inferred by evaluating the developed MXene@ eutectic salt hydrate and base eutectic PCM under the solar simulator. The interfacial attraction between the two dimensional MXene nano flakes and the developed eutectic PCM ensures a promising ability to trap solar energy effectively for thermal regulation in a sustainable direction. Subsequently, the effect of MXene with SSD/SPDD in regard to the issue of corrosion is explored for copper and aluminium metal surface.

MXene/rGO grafted sponge with an integrated hydrophobic structure towards light-driven phase change composites

While phase change materials (PCMs) have great potential for use in solar energy storage, they suffer from a lack of shape stability and energy conversion ability. In this study, proper amination of melamine sponge (MS) was designed to construct an integrated MXene and reduced graphene oxide (rGO) structure. The MXene/rGO layer is sufficiently robust to endure the capillary pressure caused by solvent evaporation during the airdrying process. In addition, the reduction of GO using oleylamine (OA) contributes to the protection of MXene from oxidation by preventing the surface of MXene nanosheets from being exposed to oxygen and moisture. The as-designed MXene/rGO sponges have been shown to effectively enhance the thermophysical and photo absorption properties of paraffin wax (PW) in the composite PCM. The composite with the highest amount of MXene/rGO maintained 93.3% of the latent heat of pure PW. The photothermal storage efficiency can reach as high as 93.0% at an MXene content of around 1%. A thermal conductivity enhancement of 66.9% can be achieved compared to the pure MS/PW composite. Therefore, this study presents a new approach for designing of high-performance phase change composites for waste-heat recovery and solar thermal energy storage applications.

Development of Self-Passivating, High-Strength Ferritic Alloys for Concentrating Solar Power (CSP) and Thermal Energy Storage (TES) Applications

Concentrating solar power (CSP) and thermal energy storage (TES) based on molten salts still lacks economic feasibility, with the material investment costs being a major drawback. Ferritic stainless steels are a comparatively cheap class of materials that could significantly contribute to cost reductions. The addition of aluminum to ferritic steel can result in self-passivation by forming a compact Al2O3 top layer, which exhibits significantly higher corrosion resistance to solar salt compared to the Cr2O3 surface layers typically formed on expensive structural alloys for CSP and TES, such as austenitic stainless steels and Ni-base super alloys. However, to date, no ferritic stainless steel combining Al2O3 formation and sufficient structural strength is available. For this reason, cyclic salt corrosion tests under flowing synthetic air were carried out on seven Laves phase-forming, ferritic model alloys (17Cr2-14Al0.6-1Nb2.6-4W0.25Si), using “solar salt” (60 wt. % NaNO3 and 40 wt. % KNO3). The Al content was varied to investigate the influence on the precipitation of the mechanically strengthening Laves phase, as well as the impact on the formation of the Al-oxide top layer. The W and Nb contents of the alloys were increased to examine their influence on the precipitation of the Laves phase. The salt corrosion experiments demonstrated that simultaneous self-passivation against a molten salt attack and mechanical strengthening by precipitation of fine Laves phase particles is possible in novel ferritic HiperFerSCR (salt corrosion-resistant) steel. Microstructural examination unveiled the formation of a compact, continuous Al2O3 layer on the surface of the model alloys with Al contents of 5 wt. % and higher. Furthermore, a stable distribution of fine, strengthening Laves phase precipitates was achieved in the metal matrix, resulting in a combination of molten salt corrosion resistance and potentially high mechanical strength by a combination of solid solution and precipitation strengthening. These results show that high-strength ferritic alloys are suitable for use in CSP applications.

Comprehensive thermal energy storage analysis of ceramic foam-enhanced molten salt in a shell-and-tube unit

Ceramic foam can be used to enhance the energy storage efficiency of molten salt for high-temperature solar thermal applications. However, its performance in a shell-and-tube unit is not fully understood. In this study, the energy storage performance of ceramic foam-enhanced molten salt in a shell-and-tube unit is investigated. The effects of ceramic foam configurations such as the filling height, porosity and outer diameter are studied. The results show that when the ceramic foam reaches the inner tube, the enhancement performance is remarkable, while it is insignificant in the case of below the inner tube. Although the total stored energy of the full foam-filled case is decreased by 13.6% compared to the no foam-filled case, the energy storage rate is improved by 54.9%. The variation of energy storage rate with porosity is generally regular. Ceramic foam with a small outer diameter has great enhancement performance; as the outer diameter increases, the difference in the enhancement gets less remarkable. The three factors are compared through a normalised effective porosity and the ceramic foam with varying outer diameters shows the best performance including the shortest melting time, the largest total stored energy and the highest energy storage rate. It is attributed that the ceramic foam surrounds the tube completely and concentrates around the tube, so its performance of conducting the heat of the tube is the best. The corrosion resistance of ceramic foams and the cost advantage make it a suitable thermal enhancer for molten salt in high-temperature solar thermal energy storage applications.

Aza-bicyclooctadiene/tetracyclooctane couples as promising photoswitches for molecular solar thermal energy storage applications

The position of N in the aza-BOD/TCO photoswitching systems govern the thermochemical and photophysical properties for molecular solar thermal energy storage application.

Visible light activated energy storage in solid-state Azo-BF2 switches.

We present here a group of Azo-BF2 photoswitches that store and release energy in response to visible light irradiation. Unmodified Azo-BF2 switches have a planar structure with a large π-conjugation system, which hinders E-Z isomerization when in a compacted state. To address this challenge, we modified the switches with one or two aliphatic groups, which altered the intermolecular interactions and arrangement of the photochromes in the solid state. The derivative with two substituents exhibited a non-planar configuration that provided particularly large conformational freedom, allowing for efficient isomerization in the solid phase. Our discovery highlights the potential of using double aliphatic functionalization as a promising approach to facilitate solid-state switching of large aromatic photoswitches. This finding opens up new possibilities for exploring various photoswitch candidates for molecular solar thermal energy storage applications.

Bismuth-mesoporous silica-based phase change materials for thermal energy storage

Effective heat storage at temperatures above 200 °C enables large scale, concentrated solar thermal energy storage or heating applications. Metals can be used for latent heat storage, as they provide high volumetric energy densities at low cost. The volume change during transition limits their stability, but it can be overcome through encapsulation or impregnation into porous matrices. We report the first study on Bismuth-based phase change materials using mesoporous silica matrices or silica shells. High metal fractions (50–70% wt.) were obtained. The metal phase form sub-micron sized domains, with good dispersion inside the silica matrix. Samples obtained by encapsulation show low enthalpy and reliability. High enthalpy values (22–32 Jg−1), comparable to that of Bi particles were obtained for samples containing mesoporous silica. These composites exhibit good thermal reliability and shape-stability above the metal melting point, in contrast to Bi particles, which show molten metal leakage. Nanoconfinement of the metal phase decreases its melting point by 1–3 °C and its heat of fusion by less than 1%. The good heat storage capacity of composites containing 70% wt. Bi could be explained by a reduction in metal oxidation, caused by the presence of the mesoporous silica matrix.

Investigation of thermal performance and chemical stability of graphene enhanced phase change material for thermal energy storage

Phase change materials (PCMs) have received widespread thermal energy storage (TES) and release properties due to their unique characteristics. However, the PCMs suffer from poor thermal conductivity, resulting in the least thermal performance and heat transfer characteristics. This research focused on enhancing the heat transfer and storage characteristics by developing an organic paraffin wax composite by dispersing highly conductive graphene powder using a two-step technique. The results show that the developed nano enhanced PCM significantly improves the thermal conductivity by 72.2% at 0.6 wt% of graphene powder. Furthermore, the Fourier transform infrared spectrum shows there is no additional peak observed, means physically and chemically stable, and the reduced light transmission capability was enhanced by 32.0% than pure PCM. Due to its extreme characteristics, the developed PCM is an outstanding material for medium temperature solar thermal energy storage applications.

Effect of surfactant on functionalized multi-walled carbon nano tubes enhanced salt hydrate phase change material

Phase change materials (PCMs) are effective thermal energy storage materials; however, their low thermal conductivity nature tends to affect heat storage performance. Salt hydrate being inexpensive, incombustible and ensuring high phase change enthalpy, are highly attractive for energy storage. The potential of multi-walled carbon nanotubes (MWCNTs) in improving the thermophysical properties of salt hydrate PCMs makes it a hotspot of current research. Therefore, in this research article, MWCNTs and functionalized multi-walled carbon nanotubes (FMWCNTs) nanoparticles were dispersed with inorganic salt hydrate at different concentrations (0.3, 0.5, and 1.0 wt%), in the presence and absence of surfactant. The role of surfactant with salt hydrate PCM has been discussed extensively. The results obtained have ensured an enhancement in melting enthalpy of prepared composites by 4.92 %, and 28.5 % for 0.5 wt% MWCNT dispersed PCM (SHM0.5), and 0.5 wt% FMWCNT dispersed PCM (SHF0.5), respectively. Furthermore, the maximum thermal conductivity was enhanced by 50.0 % and 84.78 % for 0.5 wt% MWCNT dispersed PCM with surfactant (SHMS0.5), and SHF0.5 respectively, compared to salt hydrate PCM. From the improvement in thermal conductivity, light absorptance, thermal stability, latent heat, and chemical stability, it is evident that the prepared nanocomposite is a potential candidate for solar thermal energy storage applications.

Spark plasma sintering and optical characterization of lunar regolith simulant

To satisfy the essential needs, including energy requirements, for human and robotic space explorations on planetary objects like Moon, Mars and asteroids, the proper exploitation of resources available in-situ represents a crucial issue. Along this line, the present work investigates the potential of a sintered lunar regolith simulant (JSC-1A) for possible solar energy harvesting and thermal energy storage applications. Regolith simulant powders are first consolidated by Spark Plasma Sintering (SPS) at 700 and 900 °C to produce bulk samples with different relative densities, i.e. 86 and 98%, respectively, and surface porosities. Negligible changes from the compositional point of view are induced by SPS at 700 °C, whereas a decrease of the original glassy phase content is observed when operating at 900 °C. The optical properties of sintered samples and pristine regolith powders are compared, considering the spectral absorptance/emittance, the integrated solar absorptance and the integrated thermal emittance estimated in a temperature range representative for the ISRU application, i.e. from 100 to 1300 K. We found that sintering changes the optical properties of regolith in a process-dependent way, with an increased solar absorptance and thermal emittance shown by sintered pellets with respect to pristine powders.

Photon Energy Storage in Strained Cyclic Hydrazones: Emerging Molecular Solar Thermal Energy Storage Compounds

The generally small Gibbs free energy difference between the Z and E isomers of hydrazone photoswitches has so far precluded their use in photon energy storing applications. Here, we report on a series of cyclic and acyclic hydrazones, which possess varied degrees of ring strain and, hence, stability of E isomers. The photoinduced isomerization and concurrent phase transition of the cyclic hydrazones from a crystalline to a liquid phase result in the storage of a large quantity of energy, comparable to that of azobenzene derivatives. We demonstrate that the macrocyclic photochrome design in combination with phase transition is a promising strategy for molecular solar thermal energy storage applications.

One-step construction of novel phase change composites supported by a biomass/MXene gel network for efficient thermal energy storage

Phase change composites with excellent light-to-thermal capacity can enhance the utilization efficiency of solar energy and reduce the energy crisis immensely. Current methods for constructing phase change composites include aerogel-based vacuum impregnation, melt blending, and other complex multi-steps methods, hindering their large-scale practical application in solar thermal storage. Here, we report an ingenious one-step strategy to construct sodium alginate (SA)/MXene-wrapped polyethylene glycol (PEG) crosslinked with calcium ions (Ca 2+ ) to form a gel network, resulting in shape-stable biomass/MXene phase change composites (SMPCCs). The SMPCCs exhibit excellent solar thermal conversion capability and thermal reliability that the temperature will elevate significantly to 75.4 °C in 780 s and storage solar energy as latent heat during illumination. Furthermore, the neglectable decline of thermal energy storage property for the SMPCCs is less than 1% and 2.9% even after 100 cooling-heating and 20 light-thermal cycles, respectively. Thus, this work proposes a simple and effective approach for fabricating biomass-based phase change composites with good shape stability and excellent thermal energy storage performance, which have great potential for solar thermal energy storage applications. • A novel phase change composites (PCCs) supported by a biomass/MXene gel network was developed via a one-step strategy. • The PCCs show high relative enthalpy efficiency, superior thermal reliability, and light-thermal cycling stability. • The PCCs have excellent thermal energy storage and conversion performance.

Novel urea‐based compounds as solid‐solid phase change materials: 1,3‐bisstearoylurea and 1,1,3,3‐tetrastearoylurea for thermal energy storage applications

1,3-Bisstearoylurea and 1,1,3,3-tetrastearoylurea were produced as solid-solid phase change materials (SS-PCMs) for potential solar thermal energy storage (TES) applications via condensation of urea with the respective amount of carboxyl chloride (stearoyl chloride). Both 1,3-bisstearoylurea and 1,1,3,3-tetrastearoylurea were characterized by Fourier-transform infrared (FT-IR), 1H nuclear magnetic resonance, differential scanning calorimetry (DSC), and thermogravimetric (TG) analysis techniques. 1,3-Bisstearoylurea and 1,1,3,3-tetrastearoylurea had considerable melting/crystallization enthalpy. The reason for crystallization was structural symmetry and flexibility of long alkyl groups there. Di and tetra alkyl groups made it possible to discuss the structure-property relationship for organic PCMs. Besides, the compounds were characterized through DSC and FT-IR spectroscopy before and after thermal cycling to determine their thermal reliability. Solid-solid phase change enthalpies of them for melting and crystallization were found 95.86 and −87.6 Jg−1 for 1,3-bisstearoylurea, respectively and 27.7 and −25.8 Jg−1 for 1,1,3.3-tetrastearoylurea, respectively. Thermal conductivity was studied to increase by simply expanded graphite addition. The thermal endurance limits of 1,3-bisstearoylurea and 1,1,3,3-tetrastearoylurea compounds were studied using TG analysis.

A comprehensive investigation and artificial neural network modeling of shape stabilized composite phase change material for solar thermal energy storage

Shape stabilized composite phase change materials (SSCPCM) are an attractive option for solar thermal energy storage applications. The objective of the present study is to understand and quantify the shape stability of a CPCM and its kinetics behavior. A Feed-Forward Backpropagation Neural Network (FFBPNN) algorithm is developed for the prediction of phase change temperature and heat flow of composite PCMs. Myristyl alcohol mixed with expanded graphite at various loadings (5%, 10%, and 15%) are used to study the shape stability of CPCM. The characterization of PCM and CPCM is done using Fourier-transform analysis, X-ray diffraction analysis, Scanning electron microscope, and Brunauer-Emmett-Teller (BET) analysis, Differential scanning calorimetry, and Thermogravimetric analysis. The thermophysical properties of PCM and CPCM are investigated using the C-Therm instrument. DSC experiments at different heating rates are performed to estimate the activation energy of the PCM and CPCM. The SSCPCM showed 11.82%, 11.70% higher activation energy than PCM as determined by Kissinger and Ozawa models. The activation energy of CPCM increases with the loading of expanded graphite. The charging and discharging time of SSCPCM is 29.8%, 23.3% lower than the PCM, respectively. The thermal conductivity of SSCPCM is 17.6 times higher than the PCM. Therefore, SSCPCM is an attractive solution to replace the conventional available PCMs, owing to no leakage problem and a considerable improvement in thermal conductivity. Artificial Neural Network (3–18–2) topology effectively predicts the phase change temperature and heat flow of shape stabilized composite PCMs with a coefficient of determination greater than 0.989, which is helpful in the design and selection of PCM for latent heat thermal energy storage systems.

A comprehensive review on phase change materials for heat storage applications: Development, characterization, thermal and chemical stability

Phase change materials (PCMs) utilized for thermal energy storage applications are verified to be a promising technology due to their larger benefits over other heat storage techniques. Apart from the advantageous thermophysical properties of PCM, the effective utilization of PCM depends on its life span. Moreover, PCMs which are utilized for different solar thermal energy storage applications are required longer thermal and chemical stability for the extended performance of a system. This review shows the in-depth details on thermal stability and reliability of different PCMs such as organic, inorganic, eutectics, and composites materials for heat storage applications. Different methods for measuring the thermophysical properties along with the classification of PCMs based on applications and temperature ranges have been discussed. This paper also covers the selection criteria and commercial viability of PCMs for different domestic and industrial applications. In addition to this, the effect of thermal cycle testing on the properties of different organic, inorganic, eutectic, and composite PCMs has been summarized. The present article can be highly useful for researchers and practice engineers in the areas related to thermal energy storage applications.

Experimental investigation of shellac wax as potential bio-phase change material for medium temperature solar thermal energy storage applications

Thermal performance of shellac wax as a novel bio-phase change material (BPCM) and Therminol®-55 as heat transfer fluid (HTF) in a vertical shell and tube latent heat thermal energy storage (LHTES) unit is analyzed experimentally. Operational parameters considered, namely HTF flow rate and inlet temperature in the range of 2–5 LPM and 100–120 °C, respectively. The comprehensive study of contours and plots reveals the impact of natural convection and the progress of the melting and solidification front in the charging and discharging process. As the HTF flow rate increases, the charging rate improves considerably, and a maximum reduction in melting time is obtained as 43.6% for 4 LPM. The maximum reduction in melting time and storage efficiency are 42.2% and 73.4%, respectively, at 120 °C and 4 LPM. However, the discharging process's increased flow rate has no significant effect on solidification and discharge efficiency, which attributes the dominant mode of heat transfer is conduction during the solidification. Shellac wax storage efficiency is comparable to existing paraffin wax, stearic acid and palmitic acid-based LHTES unit. In this regard, shellac wax can be a potential Bio-PCM for medium temperature range (60–80 °C) solar thermal applications such as domestic water heating and food drying.