Thermal energy storage（TES） systems use solar energy despite its irregular availability and day-night temperature difference.Current work reports the thermal characterizations of solar salt-based phase change composites in the presence of graphene nanoplatelets（GNP）.Solar salt（60:40 of NaNO 3 :KNO 3 ） possessing phase transition temperature and melting enthalpy of 221.01℃ and 134.58 kJ/kg is proposed as a phase change material（PCM） for high-temperature solar-based energy storage applications.Thermal conductivity must be improved to make them suitable for widespread applications and to close the gap between the system needs where they are employed.GNP is added at weight concentrations of 0.1%,0.3%,and 0.5% with solar salt using the ball milling method to boost its thermal conductivity.Morphological studies indicated the formation of a uniform surface of GNP on solar salt.FTIR spectrum peaks identified the physical interaction between salt and GNP.Thermal characterization of the composites,such as thermal conductivity,DSC and TGA was carried out for the samples earlier and later 300 thermal cycles.0.5% of GNP has improved the thermal conductivity of salt by 129.67% and after thermal cycling,the enhancement reduced to 125.21% indicating that thermal cycling has a minor impact on thermal conductivity.Phase change temperature decreased by around 2.32% in the presence of0.5% GNP and the latent heat reduced by 4.34% after thermal cycling.TGA thermograms depicted the composites initiated the weight loss at around 550℃ after which it was rapid.After thermal cycling,the weight loss initiated at ～40℃ lower compared to pure salt,which was found to be a minor change.Thermal characterization of solar salt and GNP-based solar salt composites revealed that the composites can be used for enhanced heat transfer in high-temperature solar-based heat transfer and energy storage applications.
Thermal energy storage technology has the advantages of high energy storage density,small size,and high energy conversion efficiency.It is one of the key energy storage technologies and an important way to achieve carbon peaking and carbon neutrality goals.Phase change materials （PCMs） offer this opportunity to store high amount of energy and release it in the form of latent heat when needed,and therefore have the advantage of being capable to alleviate the intrinsic intermittency of renewable power production,especially in wind and solar farms.
The chloride salts have great potential used as high-temperature thermal energy storage（TES） medium for the concentrated solar power system.In this study,LiCl,KCl and CaCl 2 were selected as energy storage materials in order to further broaden the working temperature of ternary chloride salt and improve its energy storage density.The new high-temperature energy storage ternary chloride composed of LiCl,KCl,and CaCl 2 was developed based on the phase diagram generated by FactSage.Three components of LiCl-KCl-CaCl 2 with the mass ratio of 37.85%-53.38%-8.77%,30.90%-13.82%-55.28% and 1.78%-18.61%-79.61% were developed,of which the corresponding melting temperature（Tm） 340.93,433.57 and 626.85℃,respectively,was obtained either.Considering that Tm of the third group of salts was too high,only the first two groups of salts were tested.DSC test showed that the actual melting point was only 0.46% and 1.64% different from the melting point predicted by Factsage.The thermal properties of the two ternary chloride salts were also compared.The solid and liquid-specific heat of ternary salts was determined by DSC using sapphire as the standard reference.The vapor pressure and decomposition temperature of ternary chloride salts were investigated.The results showed that the vapor pressure of salt 1 was almost constant below 650℃ by FactSage.Meanwhile,the TG results showed that the upper working temperature of salt 1 was 650℃ under the air atmosphere.In addition,the ternary chloride salts after short-term cycling still exhibited excellent thermal properties,which revealed that these good thermal properties make them have broad application prospects in high-temperature thermal energy storage systems.
The design of thermal conductivity enhancers (TCE) is quite critical to overcoming the disadvantage of the poor thermal conductivity of phase change materials (PCM). The main contribution of this study is firstly to discuss how to actively enhance natural convection of the melted PCM in cellular structure by the fin formed in the structure under the condition of the same metal mass, apart from simultaneously improving heat conduction, which can boost the heat transfer performance. Also, a tailored hybrid fin-lattice structure (HFS) as TCE is designed and fabricated by additive manufacturing (AM). A two-equation numerical method is applied to study the heat transfer of the PCM, and its feasibility is validated with the experimental data. The numerical results indicate that enhanced natural convection and improved heat conduction can be obtained simultaneously when a well-designed fin is embedded into a lattice structure. The enhanced natural convection results in the improved melting rate and the decreased wall temperature; e.g., the complete melting time and the wall temperature are reduced by 11.6% and 19.7%, respectively, because of the fin for metal aluminum. Moreover, the parameters of HFS including the porosity, pore density, and fin dimension have a great impact on the heat transfer. The enhancement effect of the fin for HFS on the melting rate of the PCM increases as the thermal conductivity of the base material decreases. For example, when the fin is introduced into the lattice structure, the complete melting time is reduced by 24.1% for metal titanium. In summary, this study enables us to obtain a good understanding of the mechanism of the heat transfer and provides necessary experimental data for the structural design of HFS fabricated by AM.
Heat storage properties of phase change materials (PCMs) are essential characteristics that perform a key role in thermal heat energy storage systems. The thermal properties of PCMs can be improved by developing metal foam/PCM composites. The addition of metal foam in PCMs has a significant effect on the thermal characteristics of PCMs. In this paper, the heat storage properties of two different metal foam/PCM composites were experimentally examined. The behavior of paraffin in metal foam (copper and iron-nickel)/paraffin composites concerning pure paraffin at a constant heat flux of 1000 W/m 2 in three directions simultaneously ( x , y , and z ) was studied. Paraffin was infiltrated into copper and iron-nickel foams to develop composite materials which resulted in enhancing the thermal conductivity of the paraffin. A comparative analysis is made on the heat storage properties of paraffin in copper and iron-nickel foams/paraffin composites. Inner temperature distribution during the phase transition process is experimentally evaluated. This comparison indicates that temperature uniformity in copper foam/paraffin composite is better than in iron-nickel foam/paraffin composite and pure paraffin at the same heat flux. Experimental results show that at heat flux of 1000 W/m 2 , the heat storage time for copper foam/paraffin composite is 20.63% of that of iron-nickel foam/paraffin composite.
Present work investigates the heat transfer and melting behaviour of phase change material (PCM) in six enclosures (enclosure-1 to 6) filled with paraffin wax. Proposed enclosures are equipped with distinct arrangements of the fins while keeping the fin’s surface area equal in each case. Comparative analysis has been presented to recognize the suitable fin arrangements that facilitate improved heat transfer and melting rate of the PCM. Left wall of the enclosure is maintained isothermal for the temperature values 335 K, 350 K and 365 K. Dimensionless length of the enclosure including fins is ranging between 0 and 1. Results have been illustrated through the estimation of important performance parameters such as energy absorbing capacity, melting rate, enhancement ratio, and Nusselt number. It has been found that melting time (to melt 100% of the PCM) is 60.5% less in enclosure-2 (with two fins of equal length) as compared to the enclosure-1, having no fins. Keeping the fin surface area equal, if the longer fin is placed below the shorter fin (enclosure-3), melting time is further decreased by 14.1% as compared to enclosure-2. However, among all the configurations, enclosure-6 with wire-mesh fin structure exhibits minimum melting time which is 68.4% less as compared to the enclosure-1. Based on the findings, it may be concluded that fins are the main driving agent in the enclosure to transfer the heat from heated wall to the PCM. Proper design and positioning of the fins improve the heat transfer rate followed by melting of the PCM in the entire area of the enclosure. Evolution of the favourable vortices and natural convection current in the enclosure accelerate the melting phenomenon and help to reduce charging time.
Dodecyl alcohol (DDA) is a promising solid-liquid phase change material (PCM) due to its favorable latent heat storage (LHS) characteristics. However, the leakage issue of PCM in a melted state during the heating period and low thermal conductivity restricts its utilization potential in thermal energy storage (TES) practices. Within the same context, the present work aims to overcome the leakage issue and improve the thermal conductivity of the DDA. With this in mind, a novel leak-proof layered double hydroxide (LDH)/DDA composite PCM is proposed through a solution-based impregnation method. The leak-proof impregnation ratio of the DDA impregnated within the cavities of the synthesized Al/Fe-LDH was determined to be 60%. Detailed morphological, physicochemical, and thermal properties of the fabricated composite were studied by scanning electron microscopy (SEM), Fourier transforms infrared (FTIR) spectroscopy, X-ray diffraction (XRD) spectroscopy, differential scanning calorimetry (DSC), thermalgravimetric analysis (TGA), and thermal cycling study. The results show that the LDH/DDA composite has a suitable phase change temperature (about 20°C) for passive solar thermal management of building envelopes. This composite PCM showed high LHS enthalpy (about 136 J/g), good thermal stability, and cycling LHS reliability. It also showed nearly 152% higher thermal conductivity compared to that of pure DDA, ultimately reducing the melting and solidification time of the pure DDA by 44.9% and 45.5%, respectively.
Energy efficiency issues are being focused on the growing concern of global warming and environmental pollution. The high-temperature heat pipe (HTHP) is an effective and environmental-friendly heat transfer device employed in many industries, including solar power generation, high-temperature flue gas waste heat recovery, industrial furnaces, nuclear industries, and aviation. As a critical factor in HTHPs, thermal performance is mainly introduced in the entire paper. To date, most reviews have been published concerning one or several application scenarios. However, to the best of authors’ knowledge, it is hard to find a review discussing how to improve the thermal performance of HTHPs comprehensively. First, the impact on the performance of three main components of HTHPs over the past 30 years is introduced: the working fluid, the HTHP structure, and the wick structure. Herein, it is a considerable review of the optimal operating conditions for each direction, and we expect this paper contribute to improving the thermal performance of HTHPs. Then, current numerical simulations and theoretical research on the heat transfer limit of HTHPs are recommended. The significant hypotheses used in numerical simulations and the present theoretical studies are compiled here. Finally, some potential future directions and tentative suggestions for HTHP research are put forward.
The application of thermal diodes, which allow heat to flow more readily in one direction than the other, is an important way to reduce energy consumption in buildings and enhance the battery heat dissipation of electric vehicles. Depending on various factors including the specific design, materials used, and operating conditions, the convective thermal diode can exhibit the best thermal rectification effect in intended applications compared to the other thermal diodes. In this study, a novel convective thermal diode with a wick was proposed based on the phase change heat transfer mechanism. This design takes advantage of both capillary forces provided by the wick and gravity to achieve enhanced unidirectional heat transfer performance for the designed convective thermal diode. The effect of the filling liquid ratio on the thermal performance of the thermal diode was experimentally investigated, which was in good agreement with the theoretical analysis. The research findings showed that with an optimal liquid filling ratio of 140%, the thermal diode with a wick can achieve a better thermal rectification ratio when subjected to a lower heating power, and the maximum thermal rectification ratio of 21.76 was experimentally achieved when the heating power of the thermal diode was 40 W.
Microencapsulation phase change material slurry (MEPCMS) becomes a potential working fluid for cooling high energy density miniaturized components, thanks to the latent heat absorption of particles in the heat transfer process. In this work, the Discrete Phase Model (DPM) based on the Euler-Lagrangian method is used to numerically investigate the convective heat transfer characteristics of MEPCMS flowing through a rectangular minichannel with constant heat flux. The results show that particles of MEPCMS are mainly subjected to drag force during the flow. Even so, they can migrate from the high-temperature region to the low-temperature region driven by the thermophoretic force, affecting the particle distribution and phase change process. Moreover, the Nux of the MEPCMS fluctuates due to particle phase change with varying specific heat capacities. Specifically, the value increases first, then decreases, and eventually increases again until it approaches the fully developed value of the pure base fluid as the particles gradually melt. Furthermore, the heat transfer performance of the MEPCMS is influenced by the combination of fluid inlet temperature (Tin), fluid inlet velocity (v), and mass concentration (cm) of MEPCM particles. The result shows that the maximum reduction of the maximum bottom wall temperature difference (ΔTw) is 23.98
The heat storage performance of latent heat storage systems is not good due to the poor thermal conductivity of phase change materials. In this paper, a new type of pointer-shaped fins combining rectangular and triangular fins has been employed to numerically simulate the melting process in the heat storage tank, and the fin geometry parameter effects on heat storage performance have been studied. The results indicate that compared with the bare tube and the rectangular finned tank, the melting time of the phase change material in the pointer-shaped finned tank is reduced by 64.2% and 15.1%, respectively. The closer the tip of the triangular fin is to the hot wall, the better the heat transfer efficiency. The optimal height of the triangular fin is about 8 mm. Increasing the number of fins from 4 to 6 and from 6 to 8 reduces the melting time by 16.0% and 16.7% respectively. However, increasing the number of fins from 8 to 10 only reduces the melting time by 8.4%. When the fin dimensionless length is increased from 0.3 to 0.5 and from 0.5 to 0.7, the melting time is shortened by 17.5% and 13.0%. But the melting time is only reduced by 2.9% when the dimensionless fin length is increased from 0.7 to 0.9. For optimising the design of the thermal storage system, the results can provide a reference value.
This paper studied the thermal physical properties of foundation materials in the molten salt tank of thermal energy storage system after molten salt leakage by Transient plane source experiment and X-ray computed microtomography simulation methods.The microstructure,thermal properties and pressure resistance with different particle diameters were addressed.The measured heat conductivities from Transient plane source experiment for three cases are 0.49 W/（m·K）,0.48 W/（m·K）,and 0.51 W/（m·K）,and the porosity is 30.1%,30.7%,and 31.2% respectively.The heat conductivity simulating results of three cases are 0.471 W/（m·K）,0.482W/（m·K）,and 0.513 W/（m·K）.The ratio of difference between the results of simulation and Transient plane source measurement is as low as 1.2%,verifying the reliability of experimental and simulation results to a certain degree.Compared with the heat conductivity of 0.097-0.129 W/（m·K） and porosity of 71.6%-78.9% without leaking salt,the porosity is reduced by more than 50% while the heat conductivity increased by 4 to 5 times after molten salt leakage.This significant increase in heat conductivity has a great impact on security operation,structure design,and modeling of the tank foundation for solar power plants.
In conventional parabolic trough collectors（PTCs）,sunlight is concentrated at the bottom of the absorber tube,resulting in a significant circumferential temperature gradient across the absorber tube,heat loss and thermal deformation,which affects the safety and thermal performance of PTCs.In this study,a new receiver with homogenizer and spiral（RHS） is proposed,achieving the optical and thermal synergy to ameliorate the thermal deformation of the absorber tube and enhance thermal efficiency.A plane structure homogenizer is designed to improve uniformity of the concentrated solar flux of absorber tube through second reflection.In combination with the spiral,it improves the optical-thermal efficiency of the PTC by enhancing heat exchange between the fluid and the backlight side of the absorber tube.The performance of the collector is numerically studied by building a three-dimensional coupled light-thermal-structure model.The results show that the thermal deformation of the RHS is reduced by more than 96% and the optical-thermal efficiency is improved by 1.2%-0.63% compared with conventional receivers（CRs） under the same inlet temperature conditions.The proposed receiver is validated to be effective in reducing thermal deformation and improving optical-thermal efficiency.
This study is to utilize the heat-absorbing and releasing capabilities of phase change materials (PCM) to regulate the surface temperature fluctuations of batteries during charging and discharging. The goal is to keep the battery within the optimal operating temperature range. The impact of PCM thickness and phase change temperature on battery temperature is investigated by encircling a cylindrical battery with a PCM ring. To improve the thermal conductivity of PCM, expanded graphite (EG) is added to make a composite phase change material (CPCM), and the effects of various EG mass ratios on battery surface temperature and CPCM utilization level are investigated. The findings indicate that increasing PCM thickness effectively extends temperature control time, but its impact is limited. The difference in phase change temperature of PCM controls the battery temperature in different temperature ranges. Lower phase change temperatures are unsuitable for controlling battery temperature in high temperature environments. The addition of EG enhances the thermal conductivity of PCM, leading to further control of battery temperature. The results show that the addition of 6% (mass ratio) EG to CPCM extends the effective temperature control time by 11 min and improves by 28% compared to a single PCM. The CPCM utilization is also more satisfactory and achieved a balance between heat storage and thermal conductivity in a battery thermal management system (BTMS) based on PCM.
This study evaluates the effectiveness of phase change materials (PCMs) inside a storage tank of warm water for solar water heating (SWH) system through the theoretical simulation based on the experimental model of S. Canbazoglu et al. The model is explained by five fundamental equations for the calculation of various parameters like the effectiveness of PCMs, the mass of hot water, total heat content, and duration of charging. This study simulated eleven PCMs to analyze their effectiveness like Sodium hydrogen phosphate dodecahydrate (SHPD), OM 37, N-Eicosane (NE), Lauric acid (LA), Paraffin wax (PW), OM 48, Paraffin wax C20-33 (PW-C20-33), Sodium acetate trihydrate (SAT), Palmitic acid (PA), Myristic acid (MA), and Stearic acid (SA). Among all PCMs, the SHPD has found the highest value of effectiveness factor of 3.27. So, it is the most recommended PCM for the storage tank of the SWH system. The study also includes the melt fraction analysis of all enumerated PCMs corresponding to container materials of stainless steel, glass, aluminum mixed, tin, aluminum, and copper. This melt fraction analysis is performed by making a coding program in the FORTRAN programming language. Through the analysis, copper container material is found to have high melting rate for all PCMs so it is superior to other container materials.
The specific heat capacity of working fluid is an important influence factor on heat transfer characteristic of the pulsating heat pipe（PHP）.Due to the relatively large specific heat capacity of micro encapsulated phase change material（MEPCM） suspension,a heat transfer performance experimental facility of the PHP was established.The heat transfer characteristic with MEPCM suspension of different mass concentrations（0.5% and 1.0%） and ultra-pure water were compared experimentally.It was found that when the PHP uses MEPCM suspension as its working fluid,operating stability is impoverished under lower heating power and the operating stability is better under higher heating power.At the inclination angle of 90°,the temperature at heating side decreases compared to ultra-pure water and the temperature at heating side decreases with the raising of MEPCM suspension mass concentration.The heat transfer characteristic of the PHP is positively correlated with the inclination angle and the 90° is optimum.The unfavorable effect of the inclination angle decreases with heating power increasing.When the inclination angle is 90°,the PHP with MEPCM suspension at 1.0% of mass concentration has the lowest thermal transfer resistance and followed by ultra-pure water and MEPCM suspension at 0.5% of mass concentration has the highest thermal transfer resistance.When the inclination angles are 60° and30°,the effect of gravity on the flow direction is reduced to 86.6% and 50% of that on the inclination angle of 90°,respectively,and the promoting effect of gravity on the working fluid is further weakened as the inclination angle further decreases.Due to the high viscosity of MEPCM suspension,the PHP with ultra-pure water has the lowest heat transfer resistance.When the inclination angles is 60°,the thermal resistance with MEPCM suspension at0.5% of the mass concentration is lower than that at 1.0% at the heating power below 230 W.The thermal resistance of MEPCM suspension tends to be similar for heating power of 230-250 W.At the heating power above 270 W,the thermal resistance with MEPCM suspension at 1.0% of the mass concentration is lower than that at 0.5%.
The complex flow phenomenon of rotating instability (RI) and its induced non-synchronous vibration (NSV) have become a significant challenge as they continuously increase aerodynamic load. This study aims to provide an understanding of the non-synchronous blade vibration phenomenon caused by the rotating instability of a transonic axial compressor rotor. In this case, blade vibrations and non-synchronous excitation are captured by strain gauges and unsteady wall pressure transducer sensors. Unsteady numerical simulations for a full-annulus configuration are used to obtain the non-synchronous flow excitation. The results show that the first-stage rotor blade exhibits an NSV close to the first bending mode; NSV is accompanied by a sharp increase in pressure pulsation; amplitude can reach 20%, and unsteady aerodynamic frequency will lock in a structural mode frequency when the blade vibrates in a large-amplitude motion. The predicted NSV frequency aligns well with the experimental results. The dominant mode of circumferential instability flow structure is approximately 47% of the number blades, and the cell size occupies 2-3 pitches in the circumferential direction. The full-annulus unsteady simulations demonstrate that the streamwise oscillation of the shedding and reattachment vortex structure is the main cause of NSV owing to the strong interaction between the tip leakage and separation vortices near the suction surface.
The steam ejector is a crucial component in the waste heat recovery system. Its performance determines the amount of recovered heat and system efficiency. However, poor ejector performance has always been the main bottleneck for system applications. Therefore, this study proposes an optimization methodology to improve the steam ejector’s performance by utilizing computational fluid dynamics (CFD) techniques, response surface methodology (RSM), and genetic algorithm (GA). Firstly, a homogeneous equilibrium model (HEM) was established to simulate the two-phase flow in the steam ejector. Then, the orthogonal test was presented to the screening of the key decision variables that have a significant impact on the entrainment ratio (ER). Next, the RSM was used to fit a response surface regression model (RSRM). Meanwhile, the effect of the interaction of geometric parameters on the performance of the steam ejector was revealed. Finally, GA was employed to solve the RSRM’s global optimal ER value. The results show that the RSRM exhibits a good fit for ER ( R 2 =0.997). After RSM and GA optimization, the maximum ejector efficiency is 27.94%, which is 48.38% higher than the initial ejector of 18.83%. Furthermore, the optimized ejector efficiency is increased by 46.4% on average under off-design conditions. Overall, the results reveal that the combination of CFD, RSM, and GA presents excellent reliability and feasibility in the optimization design of a two-phase steam ejector.
Thermal energy storage (TES) is of great importance in solving the mismatch between energy production and consumption. In this regard, choosing type of Phase Change Materials (PCMs) that are widely used to control heat in latent thermal energy storage systems, plays a vital role as a means of TES efficiency. However, this field suffers from lack of a comprehensive investigation on the impact of various PCMs in terms of exergy. To address this issue, in this study, in addition to indicating the melting temperature and latent heat of various PCMs, the exergy destruction and exergy efficiency of each material are estimated and compared with each other. Moreover, in the present work the impact of PCMs mass and ambient temperature on the exergy efficiency is evaluated. The results proved that higher latent heat does not necessarily lead to higher exergy efficiency. Furthermore, to obtain a suitable exergy efficiency, the specific heat capacity and melting temperature of the PCMs must also be considered. According to the results, LiF-CaF 2 (80.5 wt%:19.5 wt%) mixture led to better performance with satisfactory exergy efficiency (98.84%) and notably lower required mass compared to other PCMs. Additionally, the highest and lowest exergy destruction are belonged to GR25 and LiF-CaF 2 (80.5:19.5) mixture, respectively.
In this paper,a novel ternary eutectic salt Na 2 CO 3 -Li 2 CO 3 -LiF was designed and investigated for concentrated solar power（CSP）.The FactSage software was used to predict the composition and eutectic point of Na 2 CO 3 -Li 2 CO 3 -LiF.The microstructure,thermophysical properties,and thermal stability of eutectic salts were experimentally measured using various analytical methods.With a mass ratio of 57%:32%:11%,the eutectic salt exhibited excellent thermal storage properties with a fusion enthalpy of 413 J/g and a melting point of 426.8℃.The excellent thermal stability of the eutectic salt was reflected by a weight loss of only 0.8% at 600℃.
