Improving Heat Transfer in Parabolic Trough Solar Collectors by Magnetic Nanofluids

A parabolic trough solar collector(PTSC)converts solar radiation into thermal energy.However,low thermal efficiency of PTSC poses a hindrance to the deployment of solar thermal power plants.Thermal performance of PTSC is enhanced in this study by incorporating magnetic nanoparticles into the working fluid.The circular receiver pipe,with dimensions of 66 mm diameter,2 mm thickness,and 24 m length,is exposed to uniform temperature and velocity conditions.The working fluid,Therminol-66,is supplemented with Fe3O4 magnetic nanoparticles at concentrations ranging from 1%to 4%.The findings demonstrate that the inclusion of nanoparticles increases the convective heat transfer coefficient(HTC)of the PTSC,with higher nanoparticle volume fractions leading to greater heat transfer but increased pressure drop.The thermal enhancement factor(TEF)of the PTSC is positively affected by the volume fraction of nanoparticles,both with and without a magnetic field.Notably,the scenario with a 4%nanoparticle volume fraction and a magnetic field strength of 250 G exhibits the highest TEF,indicating superior thermal performance.These findings offer potential avenues for improving the efficiency of PTSCs in solar thermal plants by introducing magnetic nanoparticles into the working fluid.

Heat transfer and parametric studies of an encapsulated phase change material based cool thermal energy storage system

This work investigates the transient behaviour of a phase change material based cool thermal energy storage (CTES) system comprised of a cylindrical storage tank filled with encapsulated phase change materials (PCMs) in spherical container integrated with an ethylene glycol chiller plant. A simulation program was developed to evaluate the temperature histories of the heat transfer fluid (HTF) and the phase change material at any axial location during the charging period. The results of the model were validated by comparison with experimental results of temperature profiles of HTF and PCM. The model was also used to investigate the effect of porosity, Stanton number, Stefan number and Peclet number on CTES system performance. The results showed that increase in porosity contributes to a higher rate of energy storage. However, for a given geometry and heat transfer coefficient, the mass of PCM charged in the unit decreases as the increase in porosity. The St number as well as the Ste number is also influential in the performance of the unit. The model is a convenient and more suitable method to determine the heat transfer characteristics of CTES system. The results reported are much useful for designing CTES system.

Effect of Radial Porosity Oscillation on the Thermal Performance of Packed Bed Latent Heat Storage

Owing to its high heat storage capacity and fast heat transfer rate,packed bed latent heat storage(LHS)is considered as a promising method to store thermal energy.In a packed bed,the wall effect can impact the packing arrangement of phase change material(PCM)capsules,inducing radial porosity oscillation.In this study,an actual-arrangement-based three-dimensional packed bed LHS model was built to consider the radial porosity oscillation.Its fluid flow and heat transfer were analyzed.With different cylindrical sub-surfaces intercepted along the radial direction in the packed bed,the corresponding relationships between the arrangement of capsules and porosity oscillation were identified.The oscillating distribution of radial porosity led to a non-uniform distribution of heat transfer fluid(HTF)velocity.As a result,radial temperature distributions and liquid fraction distributions of PCMs were further affected.The effects of different dimensionless parameters(e.g.,tube-to-capsule diameter ratio,Reynolds number,and Stefan number)on the radial characteristics of HTF and PCMs were discussed.The results showed that different diameter ratios correspond to different radial porosity distributions.Further,with an increase in diameter ratio,HTF velocity varies significantly in the near wall region while the non-uniformity of HTF velocity in the center region will decrease.The Reynolds and Stefan numbers slightly impact the relative velocity distribution of the HTF-while higher Reynolds numbers can lead to a proportional improvement of velocity,an increase in Stefan number can promote heat storage of the packed bed LHS system.

Study on performance of a packed bed latent heat thermal energy storage unit integrated with solar water heating system

In thermal systems such as solar thermal and waste heat recovery systems, the available energy supply does not usually coincide in time with the process demand. Hence some form of thermal energy storage (TES) is necessary for the most effective utilization of the energy source. This study deals with the experimental evaluation of thermal performance of a packed bed latent heat TES unit integrated with solar flat plate collector. The TES unit contains paraffin as phase change material (PCM) filled in spherical capsules, which are packed in an insulated cylindrical storage tank. The water used as heat transfer fluid (HTF) to transfer heat from the solar collector to the storage tank also acts as sensible heat storage material. Charging experiments were carried out at varying inlet fluid temperatures to examine the effects of porosity and HTF flow rate on the storage unit performance. The performance parameters such as instantaneous heat stored, cumulative heat stored, charging rate and system efficiency are studied. Discharging experiments were carried out by both continuous and batchwise processes to recover the stored heat, and the results are presented.

Numerical Assessment on Fin Design Parameters Employed for Augmentation of Natural Convection and Fluid Flow in a Horizontal Latent Heat Thermal Energy Storage Unit

The present work focus on the thermal performance of a horizontal concentric heat exchanger, which is numerically investigated to evaluate the heat transfer enhancement process by adding fins with different configurations. As a part of this investigation, the melting process is simulated from the onset of phase change to the offset involving physics of natural convection in PCM fluid pool. The investigation is carried out by ANSYS Fluent code, which is an efficient numerical analysis tool for investigating fluid flow and convective heat transfer phenomena during PCM melting process. The attention is mainly focused on the extension of contact area between the PCM body and cylindrical capsule to enhance heat transfer rates to PCM bodies during the melting process by employing longitudinal fins in the enclosed capsule. Two commercial PCMs: RT50 and C58, are introduced in a 2D cylindrical pipe with their thermo-physical properties as input for modelling. The selected modelling approach is validated against experimental result with respect to the total enthalpy changes that qualify our model to run in the proceeding calculation. It is ensured that an isothermal boundary condition (373 K) is applied to the inner pipe throughout the series of simulation cases and the corresponding Rayleigh number (Ra) ranges from 104 - 105 and Prandtl number (Pr) 0.05 - 0.07. Finally, parametric study is carried out to evaluate the effect of length, thickness and number of longitudinal fins on the thermal performance of PCM-LHTES (Latent Heat Thermal Energy Storage) system associated with the physics of natural convection process during PCM melting.

A Study on Thermal Performance of Palladium as Material for Passive Heat Transfer Enhancement Devices in Thermal and Electronics Systems

In this work,the thermal behavior of fin made of palladium material under the influences of thermal radiation and internal heat generation is investigated.The thermal model for the extended surface made of palladium as the fin material is first developed and solved numerically using finite difference method.The influences of the thermal model parameters on the heat transfer behaviour of the extended surface are investigated.The results show that the rate of heat transfer through the fin and the thermal efficiency of the fin increase as the thermal conductivity of the fin material increases.This shows that fin is more efficient and effective for a larger value of thermal conductivity.However,the thermal conductivity of the fin with palladium material is low and constant at the value of approximately 75 W/mK in a wider temperature range of-100℃and 227℃.Also,it is shown that the thermal efficiencies of potential materials(except for stainless steel and brass)for fins decrease as the fin temperatures increase.This is because the thermal conductivities of most of the materials used for fins decreases as temperature increases.However,keeping other fin properties and the external conditions constant,the thermal efficiency of the palladium is constant as the temperature of the fin increases within the temperature range of-100℃and 227℃.And outside the given range of temperature,the thermal conductivity of the material increases which increases the efficiency of the fin.The study will assist in the selection of proper material for the fin and in the design of passive heat enhancement devices under different applications and conditions.

A comprehensive review on microchannel heat sinks for electronics cooling

The heat generation of electronic devices is increasing dramatically,which causes a serious bottleneck in the thermal management of electronics,and overheating will result in performance deterioration and even device damage.With the development of micro-machining technologies,the microchannel heat sink(MCHS)has become one of the best ways to remove the considerable amount of heat generated by high-power electronics.It has the advantages of large specific surface area,small size,coolant saving and high heat transfer coefficient.This paper comprehensively takes an overview of the research progress in MCHSs and generalizes the hotspots and bottlenecks of this area.The heat transfer mechanisms and performances of different channel structures,coolants,channel materials and some other influencing factors are reviewed.Additionally,this paper classifies the heat transfer enhancement technology and reviews the related studies on both the single-phase and phase-change flow and heat transfer.The comprehensive review is expected to provide a theoretical reference and technical guidance for further research and application of MCHSs in the future.

Experimental Study of Heat Transfer Enhancement and Friction Loss Induced by Inserted Rotor-assembled Strand （II） Lubricant Oil 22

The paper described experimental investigation of heat transfer and single-phase pressure drop through tubes with different rotor-assembled strands inserted in the Reynolds number range of 800-9000 with lubricant as working fluid. In the experiment, fixed mounts were employed to eliminate the entrance effect. The experimental results showed that the employment of fixed mounts led to a visible bias of friction factor in the laminar regime while it could not affect the Nusselt numbers significantly. Experiment for the tube inserted with rotors-assembled strand showed remarkable improvement for heat transfer with the Nusselt number increased by 200%-225% in the laminar regime and 125%-160% in the transitional regime. Meanwhile, the friction factor increased inevitably by 200%-300% within the same range of Reynolds number. The comparison of different rotor-assembled strands in-serted tubes and plain tube showed that the heat transfer benefited from the increase of the diameter of rotor-assembled strand with the same lead and the decrease of the lead of rotor-assembled strand, so does the friction factor. Based on experimental data and thorough multivariant linear normal regression method, the correlations of average Nusselt number and friction factor are established.

Experimental Investigation on Prototype Latent Heat Thermal Battery Charging and Discharging Function Integrated with Solar Collector

This paper reports the performance investigation of a newly developed Latent Heat Thermal Battery(LHTB)integrated with a solar collector as the main source of heat.The LHTB is a new solution in the field of thermal storage and developed based on the battery concept in terms of recharge ability,portability and usability as a standalone device.It is fabricated based on the thermal battery storage concept and consists of a plate-fin and tube heat exchanger located inside the battery casing and paraffin wax which is used as a latent heat storage material.Solar thermal energy is absorbed by solar collector and transferred to the LHTB using water as Heat Transfer Fluid(HTF).Charging experiments have been conducted with a HTF at three different temperatures of 68°C,88°C and 108°C and three different flow rates of 30,60 and 120 l/h.It is followed by discharging experiments on fully charged LHTB at three different temperatures of 68°C,88°C and 108°C using HTF at three different flow rates of 30,60 and 120 l/h.It is found that both higher HTF inlet temperature and flow rate have a positive impact on stored thermal energy.However,charging efficiency was decreased by increasing the HTF flow rate.The highest charging efficiency of 29%was achieved using HTF of 108°C at a flow rate of 30 l/h.Most of paraffin melted in this case,while part of the paraffin remained solid in other experiments.On the other hand,the results from discharging experiments revealed that both recovered thermal energy and recovery efficiency increased by either increasing the LHTB temperature or HTF flow rate.Highest recovered thermal energy of 5,825 KJ at 35%recovery efficiency achieved at LHTB of 108°C using 120 l/h of HTF.

Numerical Study on Heat Transfer Characteristics of Heated/Cooled Rods Having a Composite Board in between: Effect of Thermal Vias

By placing a sample between a heated and a cooled rod, a thermal conductivity of the sample can be evaluated easily with the assumption of a one-dimensional heat flow. However, a three-dimensional constriction/spreading heat flow may occur inside the rods when the sample is a composite having different thermal conductivities. In order to investigate the thermal resistance due to the constriction/spreading heat flow, the three-dimensional numerical analyses were conducted on the heat transfer characteristics of the rods. In the present analyses, a polymer-based composite board having thermal vias was sandwiched between the rods. From the numerical results, it was confirmed that the constriction/spreading resistance of the rods was strongly affected by the thermal conductivity of the rods as well as the number and size of the thermal vias. A simple equation was also proposed to evaluate the constriction/spreading resistance of the rods. Fairly good agreements were obtained between the numerical results and the calculated ones by the simple equation. Moreover, the discussion was also made on an effective thermal conductivity of the composite board evaluated with the heated and the cooled rod.

Enhancing Heat Transfer and Energy Storage Performance of Shell-and-Tube Latent Heat Thermal Energy Storage Unit with Unequal-Length Fins

Previous studies in literatures adequately emphasized that inserting fins into phase change material is among the most promising techniques to augment thermal performance of shell-and-tube latent heat thermal energy storage unit.In this study,the novel unequal-length fins are designed from the perspective of synergistic benefits of heat transfer and energy storage performance,and the effects of arrangement,number and total length of unequal-length fins are numerically investigated.Results show that utilization of fins with ascending length,when short and long fins are located in the inlet and outlet of heat transfer fluid respectively,can further promote the heat transfer and energy storage performance compared with equal length fins,and a maximum 6.17%and 0.43%increment of heat transfer performance and stored energy is achieved in full melting time,respectively.The number of unequal-length fins plays a major role in the energy storage,and 18.95%and 0.91%improvement of heat transfer performance and stored energy is realized when equipped with 2 unequal-length fins.A 21.17%improvement of the heat transfer performance is obtained when the total length of unequal-length fins is 18 mm.The present study is helpful to make further efforts to enhance heat transfer and energy storage of shell-and-tube latent heat thermal energy storage unit with unequal-length fins.

Natural Convection Melting in a Rectangular Heat Storage Tank of Carbon Nanotube Dispersed Latent Heat Storage Material

A dispersion system fluid can convect even if the dispersoid is a solid phase.Therefore,heat exchange performance can be improved while maintaining fluidity using a material with high thermal conductivity as the dispersoid.This study presents the melting performance evaluation results of a latent heat storage material with a carbon nanotube(CNT)dispersion system with high thermal conductivity,which enhances the thermal conductivity of the latent heat storage material and does not limit free convection.Increasing the thermal conductivity and enhancing the melting convection of the heat storage material result in increased latent heat storage speed.In this study,the thermal conductivity of the latent heat storage material was successfully increased by dispersing CNTs in the material.When 0.1%(in mass)of multi-wall CNT(MWCNT)was dispersed in a paraffin-based latent heat storage material,the shear stress increased by 1.5 times at a shear rate of 500 s^(-1),while taking into account the potential effects of convective inhibition.Therefore,a latent heat storage experiment was conducted in a rectangular heat storage tank using the CNT dispersion composition ratio as a parameter.A rectangular vessel with a heated vertical surface was used for the latent heat storage experiment.The melting speed was determined by comparing the amount of latent heat stored in a CNT-dispersed latent heat storage material and a single-phase latent heat storage material sample.The experimental results show that the time required for the latent heat storage material to completely melt in the heat storage tank was the shortest for the single-phase latent heat storage material sample.However,the fastest melting progress was observed for the sample with 0.02%(in mass)MWCNT content in the melting rate range of up to approximately 40%in the tank.The results indicate that this phenomenon is caused by the difference in the melting rates in the upper part of the tank.The generated data are useful for determining the shape and heat transfer surface arrangement of the latent heat storage tank.

Performance Characteristics of Geothermal Single Well for Building Heating

The single well geothermal heating(SWGH)technology has attracted extensive attention.To enhance heat extraction from SWGH,a mathematical model describing heat transfer is set up,and the key influence factor and heat transfer enhancement method are discussed by thermal resistance analysis.The numerical results show that the thermal resistance of rock is far greater than that of well wall and fluid.So,reducing rock thermal resistance is the most effective method for enhancing the heat extraction power.For geothermal well planning to drill:rock thermal resistance can be reduced by increasing well diameter and rock thermal conductivity;the temperature difference between liquid and rock can be raised by increasing well depth.For already existing geothermal well:an insulator with thermal conductivity of 0.2 W/(mK)is sufficient to preserve fluid enthalpy;a decrease in injection water temperature causes the increase of heat extraction power from geothermal well and heat output from heat pump simultaneously;increasing injection velocity causes the increase of pump power consumption and heat extraction power from geothermal well as well as net heat output between them.The entrepreneurs may refer to the above data in actual project.Furthermore,filling composite materials with high thermal conductivity into leakage formation is proposed in order to reduce the thermal resistance of rocks.

Evaluation of Liquid Water Content in Thermal Efficient Heating Mechanism Using Water Vapor for Industrial Furnace

Thermal efficiency has improved by using high-temperature vapor produced by spraying water vapor along with flame from a burner. This study aims to apply high-temperature steam heating mechanism in a high-efficient industrial furnace and household gas range. Past studies in this laboratory show that the heat transfer is promoted due to the appropriate amount of water content in each convection, radiation heat transfer. Then, water vapor-added industrial metal melting furnace has been researched. However, the existing furnace was intended to evaluate only the effect of water vapor except measuring surrounding environment, for example temperature and humidity. In this study, the effect of surrounding environment to the furnace is examined, and possibility of heat transfer enhancement is estimated. As a result, surrounding experimental condition has little effect on the change of heating ability, while this experimental furnace shows gradual degradation of heating ability in every experimental trial. Then optimum amount of water supply to the apparatus was discussed. Too much water injection leads to more consumption of heat as latent heat of water in phase change, and exceeds the effect of water vapor in heat transfer. There is a possibility of suitable total water supply, despite that there is no significant change in gas usage in water injection case compared with no water injection.

Convective heat transfer enhancement of laminar flow of latent functionally thermal fluid in a circular tube with constant heat flux: internal heat source model and its application

This paper analyzes the convective heat transfer enhancement mechanism of latent heat functionally thermal fluid. By using the proposed internal heat source model, the influence of each factor affecting the heat transfer enhancement of laminar flow in a circular tube with constant heat flux is analyzed. The main influencing factors and the mechanisms of heat transfer enhancement are clarified, and the influences of the main factors on the heat transfer enhancement are quantitatively analyzed. A modified Nusselt number for internal flow is introduced to describe more effectively the degree of heat transfer enhancement for latent functionally thermal fluid.

Thermal energy storage inside the chamber with a brick wall using the phase change process of paraffinic materials:A numerical simulation

Phase change materials are one of the potential resources to replace fossil fuels in regards of supplying the energy of buildings.Basically,these materials absorb or release heat energy with the help of their latent heat.Phase change materials have low thermal conductivity and this makes it possible to use the physical properties of these materials in the tropical regions where the solar radiation is more direct and concentrated over a smaller area.In this theoretical work,an attempt has been made to study the melting process of these materials by applying constant heat flux and temperature.It was found that by increasing the thickness of phase change materials’layers,due to the melting,more thermal energy is stored.Simultaneously it reduces the penetration of excessive heat into the chamber,so that by increasing the thickness of paraffin materials up to 20 mm,the rate of temperature reduction reaches more than 18%.It was also recognized that increasing the values of constant input heat flux increases buoyancy effects.Increasing the Stefan number from 0.1 to 0.3,increases the temperature by 6%.

The regulation mechanism and heat transfer enhancement of composite mixed paraffin and copper foam phase change materials

Phase change materials(PCMs)have remarkable energy storage capacity and promising applications in the field of thermal control of electronic products.The problem of thermal property improvement and heat transfer of PCMs in metal-foam heatsinks is an important task for thermal management of electronic components.Mixed paraffin samples were prepared by mixing appropriate proportions of paraffin(mass)at various temperatures.Differential scanning calorimetry analysis revealed that the maximum enthalpy of 206.3 J/g is obtained by mixing 20%of 17°C liquid paraffin and 80%of 29℃ solid paraffin.Heating and cooling cycling tests revealed that mixed paraffin exhibits excellent thermal stability and that the regulation method marginally affects thermal stability.Moreover,composites were prepared by embedding PCM into a copper foam by melt impregnation.The thermal conductivity of the composites increased to 4.35 W/(m K),corresponding to 20 times its original value.In addition,density functional theory and experimental results were in good agreement,indicating that the regulation method is practical and effective.

Influence of the Fin Design on the Melting and Solidification Process of NANO3 in a Thermal Energy Storage System

The melting and solidification process of sodium nitrate, which is used as energy storage material, is studied in a vertical arranged energy storage device with two different bimetal finned tube designs （with and without additional lateral fins） for enhancing the heat transfer. The finned tube design consists of a plain steel tube while the material for the longitudinal （axial） fins is aluminum. The investigation analyses the influence of the lateral fins on the charging and discharging process. Three-dimensional transient numerical simulations are performed using the ANSYS Fluent 14.5 software. The results show that, every obstruction given by lateral fins reduces the melting and solidification velocity in direction to the outer shell.

Advances in thermal energy storage development at the German Aerospace Center(DLR)

Thermal energy storage(TES)is a key technology for renewable energy utilization and the improvement of the energy efficiency of heat processes.Sectors include industrial process heat and conventional and renewable power generation.TES systems correct the mismatch between supply and demand of thermal energy.In the medium to high temperature range(100~1000℃),only limited storage technology is commercially available and a strong effort is needed to develop a range of storage technologies which are efficient and economical for the very specific requirements of the different application sectors.At the DLR's Institute of Technical Thermodynamics,the complete spectrum of high temperature storage technologies,from various types of sensible over latent heat to thermochemical heat storages are being developed.Different concepts are proposed depending on the heat transfer fluid(synthetic oil,water/steam,molten salt,air)and the required temperature range.The aim is the development of cost effective,efficient and reliable thermal storage systems.Research focuses on characterization of storage materials,enhancement of internal heat transfer,design of innovative storage concepts and modelling of storage components and systems.Demonstration of the storage technology takes place from laboratory scale to field testing(5 kW^1 MW).The paper gives an overview on DLR's current developments.

Thermal Distribution Performance of NPCM: NaCl, NaNO₃and KNO₃in the Thermal Storage System

The experiment is studied on thermal distribution in the thermal energy storage system with non-phase change materials (NPCM): NaNO3, KNO3 and NaCl in the range of 25°C - 250°C. The cylindrical storage system was made of stainless steel with 25.6 cm-diameter and 26.8 cm-height that was contained of these NPCM. There was one pipe for heat transfer fluid (HTF) with 1.27 cm-diameter that manipulates in the storage tank and submerges to NPCM. The inner pipe was connected to the 2.27 cm-diameter outer HTF tube. The tube was further connected to the thermal pump, heater and load. The pump circulates the synthetic oil (Thermia oil) within the pipe for heat transferring purposes (charging and discharging). An electric heater is used as the heat source. The limitation of the charging oil temperature is maintained at 250°C with the flow rates in the range of 0.58 to 1.45 kg/s whereas the inlet temperature of the discharge oil is maintained at 25°C. Thermal performances of TES (thermal energy storage) such as charging and discharging times, radial thermal distribution, energy storage capacity and energy efficiency have been evaluated. The experimental results show that the radial thermal distribution of NaCl for TR inside, TR middle and TR outside was optimum of temperature down to NaNO3 and KNO3 respectively. Comparison of NPCMs with oil, flow rates for NaCl were charging and discharging heat transfer than KNO3 and NaNO3. The thermal stored NaCl ranged from 5712 - 5912 J;KNO3 ranged from 7350 - 7939 J and NaNO3 ranged from 6623 - 6930 J respectively. The thermal energy stored for experimental results got with along the KNO3, NaNO3 and NaCl respectively. The thermal energy efficiency of NaCl, KNO3 and NaNO3 was in the range 66% - 70%.