查看更多>>摘要：Solar energy as a renewable energy has sufficient development potential in energy supply applications, with the help of heat storage equipment that deals with its intermittence problem. To further improve melting/solidification efficiency, a novel energy storage tank filled by phase change materials with graded metal foams is proposed. Three gradient structures (positive gradient, non-gradient, and negative gradient) in porosity or pore density are designed. Three pieces of metal foam with the fixed porosity of 0.94 but varying pore densities of 15, 45, and 75 pore per inch is packed to form gradient in pore density design. For gradient in porosity, three selected porosities of 0.90, 0.94, and 0.98 are employed. A test bench for the phase interface visualization is set up and experiment on melting/solidification evolution and temperature are carried out. Achieving satisfactory agreement with experiments, numerical models are employed to explore the thermal features for phase change materials embedded in various kinds of graded metal foams during melting and solidification procedures. Results demonstrate that gradient design in pore density does not affect the melting and solidification procedures, while graded porosity helps notably the melting and solidification processes. Design on graded porosity reduces the complete melting time by 21.1% compared with the non-gradient structure. The global phase change process can be strengthened only if the melting is strengthened. The optimal gradient in porosity reduces the overall cycle of melting and solidification time by 10.9%, compared to the uniform filling pattern.
查看更多>>摘要：Overcharging occurs owing to the malfunction of charge control and inappropriate battery management system. Overcharging mechanisms, aging mechanisms, and the influence of aging on overcharging are studied in this work. The results indicate that normal charging, lithium plating, electrolyte oxidation and decomposition, excessive electrolyte decomposition, and solid electrolyte interface decomposition and regeneration occur with charging. Much heat is produced in the reactions between lithium plating and the electrolyte, which is the main reason for thermal runaway during overcharging. The primary aging mechanisms of a battery cycled at 40 degrees C and 10 degrees C are solid electrolyte interface growth and lithium plating, respectively, where the former increases and the latter decreases the safety of the lithium-ion battery during overcharging. The trigger for thermal runaway during overcharging changes from a local, micro-level internal short circuit to solid electrolyte interface decomposition and regeneration when the state of health is less than 70 % cycled at 40 degrees C. If the electrolyte is depleted and the temperature is less than that promoting cathode decomposition, thermal runaway does not occur during overcharging.
查看更多>>摘要：Due to the increasing use of high-temperature processes in mobile applications, the demand for compact and efficient high-temperature heat exchangers continues to rise. However, with increasingly reduced installation space, the parasitic effect of axial heat conduction in the wall material of the heat exchanger is also gaining considerable influence. In this manuscript, the influence of axial heat conduction in the wall and fin material on the thermal performance of an additively manufactured high temperature heat exchanger is to be examined. For this purpose, an analytical calculation model as well as the analytical solution method is presented and successfully validated by means of experimentally recorded measurement data of an additively manufactured heat exchanger, which was tested at temperatures up to 750 degrees C. Subsequently, the calculation programme is used to determine the influence of the axial heat conduction on the effectiveness as well as on the NTU value of an additively manufactured Plate-Fin heat exchanger. The results show a drop in the NTU value of up to 50% at balanced flow compared to the case without heat conduction, which reduces the effectiveness and thus the performance of the heat exchanger by up to 5.5%.
查看更多>>摘要：Increasing building energy consumption rates is leading to significant surcharges for both the peak grid loads and the peak building energy consumption costs. The power load mismatches between user demands and energy supplies can be effectively reduced by ice storage systems. In order to solve the problem of heat transfer attenuation in the cold storage process. This study used a two-dimensional axisymmetric transient model of an annular finned tube ice storage unit to investigate the solidification characteristics and heat transfer rates with water as the storage medium and ethylene glycol as the heat transfer medium. The model was validated against experimental data. The calculations then investigated the effect of the annular fin height and fin spacing on the phase change and the solidification front velocity. The results show that increasing the fin height and decreasing the fin spacing both increase the ice storage solidification rate and improve the cold storage capacity. At a fin height of 50 mm, its solid phase fraction was 4.96 times larger than that of the bare tube. The cold storage capacity with the 50 mm high fins is more than 3.68 times that without fins at 480 min. The 4 mm fin spacing has a 26.3% greater cold storage capacity than the 12 mm fin spacing.
查看更多>>摘要：Liquid metals are promising heat transfer fluids since they remain liquid in a wide temperature range and can transfer heat efficiently due to their high thermal conductivity. A first-of-its-kind lab-scale thermal energy storage system with filler material and with lead-bismuth as heat transfer fluid is currently tested at the Karlsruhe Institute of Technology, while a 100-kWh storage system is under construction. This numerical study aims to analyse the influence of the filler parameters on the system's efficiency when the fluid used is a liquid metal. The filler should store part of the thermal energy, be efficiently discharged during the cyclic process and buffer the degradation of the thermocline during standby. For each of these purposes different particle diameters and values of some thermophysical properties of the filler, such as thermal conductivity, specific heat capacity and density, may be advantageous. Their influence on the thermocline extension is numerically investigated using a one-dimensional concentric dispersion model.The results of the parameter study show that for an efficient discharge process in a liquid metal dual-media storage, a small filler particle size is beneficial (d < 10 mm for the reference case chosen in this work). In contrast, the standby phase is favoured by larger diameters, here an order of 10-20 mm. Furthermore, a high thermal conductivity of the filler material improves the discharge performance, due to the enhanced heat transfer, but leads to an accelerated growth of the thermocline during standby. For this case, the optimum value is 5-10 W/mK. Moreover, using a filler material with a high volumetric heat capacity leads to the best overall performance.A full factorial analysis shows that the filler diameter has the strongest effect on the discharge behaviour, while, during standby, the volumetric heat capacity has the largest influence.
查看更多>>摘要：This work studies the heat exchange process of a latent heat thermal energy storage (LHTES) system equipped with a compact finned tubes heat exchanger (HE) as this is one of the most important aspects of the storage system, the capacity for effectively delivering its stored energy. This work fills in a literature gap for 3D, transient heat transport fluid (HTF) flow models concerning storage systems with phase change materials (PCMs) with fins and nanoparticles allowing for an evaluation on the quality of heat delivered by the system. Numerical simulations, for full turbulent conditions of the HTF flow, were developed to access the influence of the fin pitch and the PCM thermal properties in the performance of the energy discharge process. Samples of commercial paraffin-wax A53 doped with graphene based nanoplatelets were tested and characterised. Different types of nanoplatelets were employed in the range of 0.5% to 6% weight. Measured data of the thermal conductivity, specific heat and fusion latent heat are presented. The simulations were developed for three fin pitch values 5 , 10 and 20 mm and for 1%wt and 6%wt nanoparticles loads. The effect of fins and combination of fins and nanoparticles in the outlet temperature and liquid fraction distribution inside the LHTES unit during the discharge process in a 3D full scale model was analysed. The system performance was evaluated based of off the outlet temperature of HTF to ascertain both the quantity and quality of the heat provided. The results show that the PCM thermal conductivity is significantly enhanced by the addition of graphene nanoparticles with a high aspect ratio. The addition of only 1%wt doubled the solid phase PCM thermal conductivity and for a 6%wt load the thermal conductivity increased by a factor of 3.5. Meanwhile, specific and latent heat values of the samples are relatively unaffected. The numerical results further show that applying thin fins is an effective approach to enhance LHTES systems discharge performance. Increasing the fin number significantly enhances the heat transfer rate and the HTF discharge temperature during solidification and has a positive impact in the useful discharge heat capacity, providing better quality heat. Combining fins and nanoparticles improves the discharge process, nevertheless the role of nanoparticles becomes secondary as the fins number increases. The results demonstrate that standardised compact finned heat exchangers ubiquitously used in the HVAC industry can successfully overcome the low thermal conductivity of common PCMs without compromising the useful heat discharge capacity or resorting to nanoparticles decreasing the discharge time between 60 and 77% with adequate fin number.
查看更多>>摘要：Energy consumption to achieve thermal comfort in buildings is one of the significant challenges in the construction industry. Hence, applying thermal energy storage (TES) systems, such as phase change material (PCM), is increasingly being considered as a promising solution. However, the low thermal conductivity of PCM has a negative effect on the thermal cycle and, consequently, the efficiency of the TES system. This study aims to develop a novel form-stable PCM cement composite and investigates the effects of two thermal conductive fillers (TCFs), graphite powder and nano titanium dioxide (nTiO(2)), on the thermal performance of PCM cement composites. For this purpose, the TCFs at different mass fractions (1 and 3 wt%) were dispersed in the PCM using sonication and impregnated into expanded glass aggregates (EGA). The microstructure studies revealed that TCF was well dispersed in the PCM and PCM-TCF was successfully impregnated into the EGA. The results of thermogravimetric analysis (TGA) and Fourier Transform Infrared Spectroscopy (FT-IR) demonstrated that there was no chemical reaction between PCM and TCFs, and the PCM-TCF were thermally stable in the operating temperature ranges. The differential scanning calorimetry (DSC) analysis showed that the latent heat capacity of PCM-TCF remained reasonable with a maximum deduction of 9.6% and supercooling temperature enhancement up to 3.4 degrees C. The thermal behaviour analysis obtained from infrared thermography (IRT) imaging showed a 50% improvement in the heat transfer rate of the PCM composite. Moreover, a room model experiment revealed that integrating TCFs into PCM significantly enhanced the performance of EGA/PCM cement mortar. This is evidenced by a reduction in peak indoor temperature of up to 2.5 degrees C compared to the control sample.
Sefidan, Ali M.Sellier, MathieuHewett, James N.Abdollahi, Ayoub...
16页
查看更多>>摘要：Spray drying is a commonly used method to rapidly produce dry powder from liquids or slurries. In the dairy industry, this method is preferred for producing milk powder to prolong shelf life as well as to make the product denser for transportation. In the current work, an experiment accompanied by a numerical model is employed to study the drying kinetics of an industrial-scale skim milk droplet. A comprehensive four-stage evaporation model was validated against the experimental results and was then used to study the temperature and solute distribution profiles within the wet-core of the drying droplet. The results showed that the heat diffuses within the wet-core sufficiently quickly (as indicated by the Lewis number of 25.53), such that the uniform wet-core temperature assumption remains valid for this case study despite Bi-ht = 0.281. The uniform solute concentration assumption, however, does not appear to be an appropriate assumption (Bi-mt = 77), especially for the later drying stages where the droplet becomes further concentrated and mass diffuses at slower rates. Furthermore, when the milk droplet dries faster under a higher evaporation rate, the solute had insufficient time to redistribute from the surface towards the centre, resulting in an uneven solute concentration profile. For the high evaporation rate presented in this study, a solute mass fraction of approximately 0.5 was calculated in the droplet centre, compared with 0.82 for a lower evaporation rate. Therefore, for obtaining a uniformly distributed solute in the final dried powder, a lower rate of evaporation is recommended.
查看更多>>摘要：In the thermal control design of spacecraft and the study of its infrared radiation characteristics, the influence of external heat flux must be considered emphatically. In order to solve the complex space occlusion problem of China Space Station, the centroid ontology coordinate system is first defined as the reference coordinate system in this paper. Then, the space quadrant and intelligent occlusion calculation methods are proposed. The former is mainly to solve the shielding of the external heat flux of spacecraft body. The latter is to solve the shielding of the external heat flux of large external hangings. The results show that the maximum occlusion time is up to 6.5 min and the maximum occlusion difference is about 0.7 in the same quadrant at different solar incident angles. Moreover, if the shielding of external hangings is not considered, the time is more than 15 min when the external heat flux difference exceeds 4000 W in the sunlight area, and the maximum difference is about 6000 W. Meaningfully, the space quadrant and intelligent occlusion calculation methods proposed in this paper can effectively solve most of the occlusion problems and is suitable for various spacecraft.
查看更多>>摘要：Thermal conductivity is a fundamental parameter in every battery pack model. It allows for the calculation of internal temperature gradients which affect cell safety and cell degradation. The accuracy of the measurement for thermal conductivity is directly proportional to the accuracy of any thermal calculation. Currently the battery industry uses archaic methods for measuring this property which have errors up to 50 %. This includes the constituent material approach, the Searle's bar method, laser/Xeon flash and the transient plane source method. In this paper we detail three novel methods for measuring both the thermal conductivity and the thermal diffusivity to within 5.6 %. These have been specifically designed for bodies like lithium-ion batteries which are encased in a thermally conductive material. The novelty in these methods comes from maintaining a symmetrical thermal boundary condition about the middle of the cell. By using symmetric boundary conditions, the thermal pathway around the cell casing can be significantly reduced, leading to improved measurement accuracy. These novel methods represent the future for thermal characterisation of lithium-ion batteries. Continuing to use flawed measurement methods will only diminish the performance of battery packs and slow the rate of decarbonisation in the transport sector.
NSTL
Elsevier