High Thermal Capacity Research Articles

Lamellar-structured phase change composites based on biomass-derived carbonaceous sheets and sodium acetate trihydrate for high-efficient solar photothermal energy harvest

Inorganic salt hydrates have a great potential for solar photothermal energy storage due to their high energy-storage density and low cost. However, there are some major drawbacks such as high supercooling degree, shape/form instability, phase segregation, and poor long-term durability found in salt hydrates. These greatly limit their practical applications. Herein, we designed and fabricated a type of eco-friendly and cost-efficient phase-change system based on sodium acetate trihydrate (SAT) as an inorganic phase change material and biomass-derived carbonaceous sheets (BDCSs) as a supporting material for high-efficient solar photothermal energy storage and shape stabilization. Derived from a biomass waste of watermelon rind, the BDCSs show some unique characteristics such as a stacking lamellar structure and a compact surface with some corrugations and ripples as well as the presence of abundant oxygen-rich groups. These can lead to a significant enhancement in photothermal conversion efficiency of solar energy when used as a supporting material for a SAT-based phase-change composite system. This type of composite system not only presents a high thermal energy capacity of 212.2 J/g and a very low supercooling degree of 0.9 °C, but also has a high photothermal conversion efficiency of up to 80.18% for solar energy. Moreover, the composite system exhibits high thermal conductance, good shape stability without phase segregation, and excellent thermal cycle stability for long-term solar photothermal energy storage and release. This study provides a simple and facile methodology for the development of bio-based phase-change composites for high-efficient solar energy utilization. • Biomass-derived carbonaceous sheets (BDCSs) were obtained from watermelon rind. • BDCSs exhibit a stacking lamellar structure with rich oxygen groups on their surface. • BDCSs/sodium acetate trihydrate composites were fabricated for solar photothermal energy harvest. • This type of phase-change composite system has high photothermal conversion efficiency. • This composite system reveals enhanced thermal conductance and reduced supercooling.

Heat transfer enhancement in latent heat thermal energy storage using copper foams with varying porosity

Latent Heat Thermal Energy Storage (LHTES) is a promising solution to alleviate the supply-demand mismatch in the field of energy utilization. LHTES relies on high-quality phase change materials (PCMs) for high thermal capacity and narrow temperature variation. As an important class of PCMs, paraffin shows advantages of safe, non-corrosive, low-cost, but has limited applications because of relatively low thermal conductivity. The heat transfer performance of LHTES could be remarkably improved by embedding metal foams into PCMs. In the present work, water, paraffin (RT54HC) and copper foam were used to study the heat transfer enhancement, which acted as heat transfer fluid (HTF), PCM and heat transfer enhancer, respectively. The non-thermal-equilibrium energy model was applied in the numerical investigation. The liquid fraction profile was compared between pure PCM and composite PCM. The melting process, interface evolution and the average heat flux density under copper foams with different porosity changing modes were investigated. Results indicated that the incorporation of copper foams into PCM significantly increased the heat transfer performance of LHTES. The full melting time was reduced by 73.7% compared to that of pure PCM. The average heat flux density was improved from 96.38 W to 330.16 W. The optimal method was that dividing the copper foam into upper and lower parts, where the porosity increased separately along with the positive x-direction and positive y-direction. Compared with fixed porosity, the full melting time and average heat flux density were reduced by 6.78% and increased by 4.26%, respectively.

Polyethylene glycol phase change material embedded in a hierarchical porous carbon with superior thermal storage capacity and excellent stability

Hierarchical porous materials are recommended to trade off the mismatch between high loading and efficient crystallization in pore-based composite phase change materials (PCMs), coupling the functions of expanded pores (mesopores and macropores) along with maintained micropores. Hierarchical porous carbon (HPC) was successfully synthesized from metal organic framework MOF-5 with a large specific surface area (1345 m2/g) and high pore volume (2.69 cm3/g). The adsorption capacity of HPC for low temperature PCMs, polyethylene glycol (PEG) and stearic acid (SA) reaches over 90 wt%. The introduction of HPC has very little impact on the crystallinity of the PCMs, as a result, the composites possess similar high thermal storage capacity to pure PCM. The as-prepared composites efficiently perform heat store and release with reasonable reliability. Moreover, the supercooling of PCM was strongly suppressed due to the large surface area of HPC. Molecular dynamics (MD) simulation confirms that the smaller pores enable a stronger force of the carbon skeleton on the PCM, which ensures effective anchoring of the PCM. Simultaneously, those larger pores provide enough space for storage of PCM, with a reduced negative effect on its crystallization. After the compounding, the phonon vibration matching between guest and host is strengthened, which is beneficial to the transfer of energy thus receives an enhanced thermal conductivity. Our research demonstrates the great potential of using hierarchical porous skeleton to immobilize phase change materials for practical thermal storage.

Phase change dispersions: A literature review on their thermo-rheological performance for cooling applications

Phase change dispersions are two-phase fluids that consist of a phase change material dispersed in a continuous phase and stabilised with the aid of surfactants. Due to the high thermal storage capacity, on accounts of the latent heat of phase change of the dispersed phase change material, phase change dispersions present as prospective heat transfer fluids in cooling applications. However, to implement phase change dispersions into cooling systems, for the design of heat exchanger and pipe geometries, detailed fundamental knowledge of the heat transfer and rheological behaviour needs to be understood. Alongside this, limitations encountered such as stability and supercooling need to be addressed. Within this work, a detailed review of the types of dispersion found in literature, their thermophysical, heat transfer and rheological properties are discussed. Current and past methods of improving formulations and overcoming the aforementioned problems are presented. Furthermore, potential figures of merit to evaluate the efficiency of phase change dispersion utilisation are presented and discussed. An outlook to the existing research gap is given.

Influence of thermal and optical criteria of spectral fluid filters for hybrid concentrated photovoltaic/thermal systems

Recently, applying spectral beam splitting technologies into concentrated photovoltaic/thermal (CPV/T) systems have received unprecedented attention owing to their abilities to take full advantage of solar broadband. Such hybridized systems require liquid absorptive-filters capable of enhancing the combined conversion efficiency achievable by controlling their ‘thermal’ and ‘optical’ characteristics. Although numerous studies have strived to develop the optical performance of thermal fluids for peaking the spectral window of the PV, maximizing their heat-transfer capabilities have not gained widespread attention. Therefore, this study aims to fill the gap and provides the appropriate thermal-optical selective criteria of liquid spectral-filters under various solar concentrations (CRs) using a precisive 3D optical-thermal coupled numerical model. The results revealed that using liquid filters with high thermal conductivity (k) and heat capacity (Cp) jointly has the potential to improve the total solar energy conversion in all CRs. Whereas low k and Cp thermal fluids have the preference under low CRs and vice versa in high CRs from the exergy viewpoint. Besides, the presence of high thermal absorption filters above the undesired PV-wavebands promotes co-conversion efficiency cross all CRs, while the high permeability filters over the spectral window of PVs are recommended from the exergy viewpoint. Since several liquid filters can meet various user needs (whether it is thermal or electrical outputs), the ideality of optical characteristics of used fluid-filters primarily depends on the desired energy form, so researchers should carefully select the appropriate thermal fluid for the desired purpose.

Assessing the use of copper slags as thermal energy storage material for packed-bed systems

Thermocline tanks using packed-bed of rocks have become feasible candidates for improving the performance of Concentrated Solar Power plants, enabling high operating temperatures and reduced capital costs when industrial byproducts are employed as filler materials and low-cost working fluids, being competitive against molten salts thermal storage systems. The present work assesses the potential of using copper slags in packed-bed systems as filler material. Through a thermal characterization, it is demonstrated that copper slags show similar properties to other slags proposed in the literature for thermal storage medium and better thermal capacity (1.4–1.5 J/(gK)). A heat transfer model was developed to predict the cyclic behavior of a packed-bed storage using copper slags and employed in a parametric analysis to assess the impact of storage dimensions on 1st and 2nd law efficiencies for different storage materials, allowing to identify several design considerations depending on tank’s volume. The main findings indicate that the high thermal capacity of copper slags favors the development of a steeper thermocline, keeping a low rate of exergy loss at storage’s outlet, and also higher energy density stored of 138 kWh/m3 against 129 kWh/m3 of other byproducts under similar storage dimensions.

High-performance phase change material capsule by Janus particle

A new method is proposed to fabricate high-performance phase change material capsules with high thermal storage capacity and increased thermal conductivity. The capsules are achieved by free radical polymerization against the example paraffin/water emulsions stabilized with Janus particles (JPs). It is key that the highly conductive Ag-contained JP adopt parallel lying at the emulsion interface, which is responsible for increasing the interfacial thermal conductivity. Thermal storage capacity can be retained over 90% with no paraffin leakage, which is stable after many cycles of heating and cooling. The high-performance phase change material capsules are promising in effective thermal management in the form of either powder or slurry.

Dynamic heat transfer analysis of a direct-expansion CO2 downhole heat exchanger

Abstract Compared with conventional second-loop ground-coupled heat pump, direct-expansion ground-coupled heat pump can achieve higher energy efficiency. To maximize the heat exchange rate of direct-expansion heat exchanger, a novel direct-expansion downhole heat exchanger is proposed in this study, using supercritical pressure CO2 as the refrigerant. The heat exchanger is placed inside a closed water well so that heat transfer can be enhanced by natural convection. A high accuracy three-dimensional transient thermal resistance and capacity model is developed and validated by a field experiment. The measured average effective unit heat flux is significantly improved from a previous study by 168.8%. According to the simulation result, due to the drastic change of pseudocritical CO2 properties, an improper inlet pressure or U-pipe diameter can lead to a 34.8% reduction in the heat exchange rate. Moreover, the natural convection will aggravate the thermal short-circuit effect between U-pipe branches; however, its positive effect of enhancing heat transfer is more significant. Adding insulation to the upper part of the upward pipe can balance a weaker thermal short-circuit effect with a smaller total heat transfer resistance. With this design, the unit heat flux can be increased by 49.0% from that of conventional direct-expansion heat exchanger.

Development of Renewable Biomass-Derived Carbonaceous Aerogel/Mannitol Phase-Change Composites for High Thermal-Energy-Release Efficiency and Shape Stabilization

As a type of promising phase-change materials, sugar alcohols have a great potential for thermal energy storage due to their high energy-storage density. However, the intermolecular hydrogen bonding of sugar alcohols results in high supercooling and low thermal-energy-release efficiency. In this paper, we proposed a strategy for fabricating biomass-derived carbonaceous aerogel (BDCA)/mannitol phase-change composites by a simple vacuum impregnation method to depress supercooling and enhance the thermal-energy-release efficiency. The BDCA derived from a biomass waste, watermelon rind, exhibits unique features like light weight, abundant porosity, and rich oxygen groups in nature and can be used as an ideal supporting material for the fabrication of shape-stabilized phase-change composites. The BDCA/mannitol phase-change composites developed by this work not only possess a high thermal energy capacity of over 280 J/g but also exhibit reduced supercooling, improved thermal conductivity, good thermal stability, superior shape stability, and excellent thermal-cycle stability. More importantly, the BDCA/mannitol phase-change composites present extremely high thermal-energy-release efficiency of over 95% at the BDCA loading higher than 9 wt %. This study offers a promising methodology for the development of sugar-alcohol-based phase-change composites with a high thermal-energy-storage density and high energy-release efficiency for medium-temperature thermal-energy-storage applications.

4‐Temperatures Approach: Testing Thermochemical Heat Storage Materials Under Application Conditions

AbstractThermochemical heat storage offers the possibility to achieve high thermal energy storage capacities compared to sensible and latent heat storage. To quantify this advantage, thermochemical storage systems have to be evaluated under application conditions. The proposed 4‐temperatures approach delivers a first step toward a realistic testing of thermochemical working pairs. As an example, the approach is applied to testing the working pair zeolite/water under the conditions of the application of seasonal storage of solar thermal heat for domestic heating purposes.

Performance of a flat grooved heat pipe with a localized heat load

Heat pipes are phase change heat transfer devices used in wide range of heat transport applications due to their high thermal transport capacities with low temperature differences. Heat pipes are especially preferred for electronic cooling applications and aerospace avionics to satisfy high heat transfer rate requirements. In this study, heat transfer and phase change mechanisms of working fluid are investigated and modeled using a 3-D thermal resistance network for multichannel flat grooved heat pipes. First, heat transfer and fluid flow are modeled in half of a single grooved structure due to symmetry, and is subjected to uniform heat flux. Radius of meniscus curvature and temperature distribution along the groove are calculated. Results are compared with experiments in the literature and show good agreement. The validated heat transfer and fluid flow models are extended to a multichannel model to observe performance of grooved heat pipes with localized heat sources, not covering the entire width, a vital feature for realistic simulation of operational devices. Predictions of the temperature distribution along the multichannel of the heat pipe are provided and the effect of the distribution of heat sources on the heat pipe is discussed.

A heat-pulse method for detecting ice formation on surfaces

Early detection of ice formation on surfaces is a crucial requirement in many applications such as wind turbines, aeroplane’s wings, and radar domes. This paper presents the concept of a heat-pulse ice detection system. Its principle consists of recording the temperature evolution and assessing the relaxation time after the surfaces are subject to a heat excitation generated by an electrical pulse. The sensor comprises thin graphene films embedded in pre-preg layers of aeronautical grade as well as thermocouples to record the temperatures profiles at given locations. The criterion of ice detection exploits the high thermal capacity of ice compared to the air to capture the formation of icy spots. Numerical simulations and comparison with experimental investigations have been carried out to validate the principle and criteria of the ice detection system.

Numerical investigation of the solar activated carbon/methanol adsorption refrigeration system in Tehran’s climate

Solar adsorption refrigeration systems are used to extract heat using solar radiation based on the adsorption phenomena. In these systems, the temperature of the solar collector plays an important role in the efficiency of a system. In this study, two different models, including the lumped and the distributed ones, are investigated to predict the temperature distribution in a specific solar collector. The operating conditions are the same for both cases. Moreover, for the solar radiation as one of the boundary conditions, the data for Tehran solar irradiation is used. The results of the temperature analysis show that the distributed model predicts a less maximum collector temperature than the lumped model which clearly results in a lower system performance. In addition, it can be concluded that because of using steel as a main material for the collector and its high thermal capacitance, it takes almost 3 days for the system to reach the periodic operating conditions.

Evaluation of Energy Efficiency in Thermally Improved Residential Buildings, with a Weather Controlled Central Heating System. A Case Study in Poland

Optimization of energy consumption and related energy efficiency can be realized in various ways, both through measures to reduce heat losses through building partitions and the introduction of modern systems of regulation and management of heat distribution. In order to achieve the best possible results, these actions should be interlinked, especially in older buildings that have undergone thermomodernization. Therefore, the aim of the study was to evaluate actions aimed at improving energy efficiency of buildings made in prefabricated technology. These buildings were thermomodernized and then the weather-controlled central heating system was installed. The study assessed whether the application of the change of the method of central heating regulation from the traditional one, taking into account only the change of external temperature to the weather-controlled one, will contribute to the increase of energy efficiency of buildings. The research was carried out in the existing residential buildings, for which data on the actual energy consumption was collected and elaborated and includes periods before modernization, after thermomodernization and the period after the introduction of the central heating system with weather control. The collected data cover an eighteen-year period of buildings’ use. The obtained results indicate that in Polish conditions the introduction of weather-controlled regulation system in buildings made in prefabricated technology (made of large slab) allows to achieve energy savings in the range of 16–23%, it may be related to their high thermal capacity resulting from the use of concrete elements in the building envelope.

Evaluation of hygrothermal performance of wood-derived biocomposite with biochar in response to climate change

Wood is a sustainable resource and building material. It provides an excellent response to climate change and has excellent insulation performance. However, structural defects may occur due to decay from moisture, resulting in poor dimensional stability. The rich organic substances contained in wood can lead to mold when the moisture content is consistently high, adversely affecting the health of occupants. Therefore, we attempted to compensate for the disadvantages of wood in regard to water stability while maintaining the high thermal insulation performance and carbon dioxide storage capacity, using biochar from thermally decomposed spruce under oxygen limiting conditions. A wood-derived biocomposite was prepared by mixing biochar and soft wood-based chips using the hot-press method, and the thermal conductivity, specific heat, water vapor resistance factor, moisture adsorption, and moisture desorption performances were analyzed. The thermal conductivity of WB10 with 10 wt% biochar content was 0.09301 W/mK. This is a 7.98% decrease from 0.10108 W/mK, the thermal conductivity of WB0 without biochar. The water vapor resistance factor tended to increase when the biochar ratio increased. As the proportion of biochar increased, the equilibrium moisture content in high relative humidity tended to decrease, and it was found that the moisture adsorption and desorption performances were affected by the ratio of the biochar. Therefore, wood-derived biocomposites using biochar can be used in environmentally friendly materials, with improved thermal insulation performance and water stability.

Therapeutic effects of antibiotics loaded cellulose nanofiber and \u03ba-carrageenan oligosaccharide composite hydrogels for periodontitis treatment

Periodontitis is an inflammatory disease that can lead to the periodontal pocket formation and tooth loss. This study was aimed to develop antimicrobials loaded hydrogels composed of cellulose nanofibers (CNF) and κ-carrageenan oligosaccharides (CO) nanoparticles for the treatment of periodontitis. Two antimicrobial agents such as surfactin and Herbmedotcin were selected as the therapeutic agents and the hydrogels were formulated based on the increasing concentration of surfactin. The proposed material has high thermal stability, controlled release, and water absorption capacity. This study was proceeded by investigating the in vitro antibacterial and anti-inflammatory properties of the hydrogels. This material has strong antibacterial activity against periodontal pathogens such as Streptococcus mutans, Porphyromonas gingivalis, Fusobacterium nucleatum, and Pseudomonas aeruginosa. Moreover, a significant increase in malondialdehyde (MDA) production and a decrease in biofilm formation and metabolic activity of the bacteria was observed in the presence of hydrogel. Besides, it reduced the reactive oxygen species (ROS) generation, transcription factor, and cytokines production in human gingival fibroblast cells (HGF) under inflammatory conditions. In conclusion, the hydrogels were successfully developed and proven to have antibacterial and anti-inflammatory properties for the treatment of periodontitis. Thus, it can be used as an excellent candidate for periodontitis treatment.

Thermal analysis & modelling of phase change material used in low temperature industrial wastewater applications

The use of residual heat can reduce the energy needs of manufacturing plants. Low temperature industrial wastewater (IRW) heat can be used in heating plants. IRW heat removal can be accomplished using a Thermal Energy Storage Device (TES), which uses Phase Transition Materials (PCM). Wastewater The analysis is carried out by CFDs in the ANSYS CFX software. The heat absorption capacity and the IRW temperature difference are determined using paraffin Nano-fluid and alumina. The modelling represents the alumina Nano-fluid has a higher thermal capacity than paraffin wax, which is very useful for pharmaceutical systems that use industrial waste water for heating in the workshop. Temperature curves for different timings for paraffin and alumina based Nano-fluids were constructed using ANSYS CFX and temperature profiles were evaluated. Heat absorption in PCM is reduced when the material comes out of the industrial discharge pipe. Industrial wastewater loses maximum heat in the aluminium oxide water Nano-fluid, which is obtained in the form of a drop in temperature from 343 K to 324 K.

Bifunctional Microcapsules with n-Octadecane/Thyme Oil Core and Polyurea Shell for High-Efficiency Thermal Energy Storage and Antibiosis.

A new kind of bifunctional microcapsule containing a n-octadecane (OD) and thyme oil (TO) core based on polyurea shell designed for thermal energy storage and antibiosis was prepared successfully through interfacial polymerization. The scanning electron microscopic investigations reveal that the obtained composite microcapsules present the regular spherical morphology and the transmission electron microscopic observations confirm the clear core–shell structure. Morphological and chemical structure analyses prove the successful synthesis of bifunctional microcapsules. Thermogravimetric analysis indicates that the polyurea shell can protect the composite cores effectively. Differential scanning calorimetry examination shows that the bifunctional microcapsules can maintain high thermal storage capacity and the encapsulation efficiency of OD increases with the increase in TO. The supercooling crystallization can be notably suppressed by adding 7 wt.% of n-octadecanol. A study on the release behavior of TO from the bifunctional microcapsules reveals that the Higuchi kinetic model could better fit the TO release profile. The antibacterial results demonstrate that the bifunctional microcapsules can effectively inhibit the growth of Staphylococcus aureus and the inhibition rate can reach as high as 99.9% when the mass concentration of microcapsules is over 3 wt.%.

Temperature and pH dual-stimuli-responsive phase-change microcapsules for multipurpose applications in smart drug delivery

Temperature/pH dual-stimuli-responsive phase-change microcapsules (Dual-SR-MEPCM) were designed that can form the basis of smart drug delivery systems. They employed n-eicosane as a phase change material (PCM) microencapsulated in an acrylate-functionalized silica shell via emulsion-templated interfacial polycondensation. A poly(N-isopropylacrylamide-co-acrylate acid) functional layer was then fabricated on the surface of the silica shell through surfactant-assisted radical polymerization. The combination of phase-change microcapsules and stimuli-responsive material results in a system that synchronously implements thermal self-regulation by its PCM core together with controllable drug release through its polymeric functional layer. The resultant Dual-SR-MEPCM exhibits regularly spherical morphology, uniform particle-size distribution and well-defined core–shell microstructure, which indicates successful construction according to our design principle. This system has been further evaluated for multipurpose applications in thermal regulation and drug delivery with independent temperature and pH triggering. The Dual-SR-MEPCM not only shows a high thermal energy-storage capacity of over 160 J/g, but also good anti-leakage performance, thermal cycle stability and high-temperature shape stability. Our studies also reveal independent stimuli-responsive behaviors for temperature and pH when using bovine serum albumin and doxorubicin hydrochloride as model drugs. All of these features indicate that the Dual-SR-MEPCM designed in this study can serve as a promising candidate for smart drug delivery systems, with great potential for future multipurpose applications in advanced pharmaceutical areas.

Frequency response calculations of carbon nanotube based nanothermophones

Due to the extraordinary high thermal conductivity and low heat capacity, carbon nanotube (CNT) has shown a great potential as a thermo-acoustic (TA) material recently. It can be used to fabricate novel TA loudspeaker free of magnets and moving parts, a promising device that directly converts thermal energy into acoustic energy. In this paper, we have systemically analysed the frequency responses of the dot/wire/film TA emission from CNTs by using the equations we derived for acoustic field of TA emission from point source and arbitrary source. Formulas used for TA calculation from near-field to far-field can be the same. The dependence of sound pressure level (SPL)-frequcecy response of all kinds of CNT based nanothermophones on distance and angle are investigated in detail. It is found that TA sound pressure fluctuates in near-field, while increases linearly with logarithmic frequency in far-field. The amplitude of near-field fluctuation increases with increasing distance, but the frequency cycle of the fluctuation decreases and tends to be a constant depending on angle. These studies lay the foundation for further development of CNT based nanothermophones.