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.

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.

Comparison of Thermal Performance between Annular Fins and Longitudinal Fins in Latent Heat Storage Unit

As a simple and effective method of heat transfer enhancement,fins are widely used in latent heat storage systems.However,the choice of annular fins and longitudinal fins has always been controversial.In this paper,the melting process of phase change material(PCM)in annular fins and longitudinal fins latent heat storage units with the same volume is numerically simulated.To ensure the same thermal penetration,three-dimensional spaces occupied with fins are specially controlled to be the same.Combined with finned structures,the effects of natural convection(NC),placement mode and heat transfer fluid(HTF)inlet direction on the melting process are studied.The results show that the melting time in annular finned structure is always 10%less than that in longitudinal finned structure,which demonstrates the superior of the annular fins in the latent heat storage unit.The melting time is the shortest in vertical unit with annular fins and HTF inlet at the bottom.Additionally,the correlation formulas of the liquid fraction are proposed in the vertical unit with HTF inlet at the bottom.

Temperature-Dependent Rheological Behaviors of Binary Eutectic Mixtures of Sugar Alcohols for Latent Heat Storage:A Comparative Study with Pure Sugar Alcohols

The temperature-dependent rheological behaviors of five selected binary eutectic mixture sugar alcohols,with great potential for latent heat storage in the range of 353.15 K to 523.15 K,were investigated.It was found that the rheological behaviors of the mixture sugar alcohols depend on those of the pure compounds as well as their molar ratios.The two mixtures of xylitol(75 mol%)+erythritol and erythritol(84 mol%)+d-mannitol behave like pseudoplastic fluids with typical non-Newtonian shear-thinning behaviors,as indicated by the power law index of 0.99(<1).The mixture of d-mannitol(70 mol%)+d-dulcitol is a nonlinear Bingham fluid,exhibiting a slight yield stress(0.001 Pa to 0.01 Pa)at low shear rates.The rest two mixtures containing the cyclic-structured inositol behave like Herschel-Bulkley fluids.The infinite shear viscosities of the eutectic mixtures over the entire temperature range appear to be higher than those of their respective pure compounds,except for inositol.The mixture of xylitol(75 mol%)+erythritol at its melting point shows higher dynamic viscosity of about 0.546 Pa·s than the values of about 0.396 Pa·s and 0.035 Pa·s for xylitol and erythritol,respectively.In addition,the activation energies of viscous flow of the mixtures,as determined by fitting the dynamic viscosity-temperature curves using the Arrhenius model,also exhibit higher values than those of their pure compounds.The activation energy of viscous flow of the mixture xylitol(75 mol%)+erythritol was determined to be about 92400 J/mol in the supercooled liquid state,while the supercooled liquid xylitol and erythritol have much lower values of 83500 J/mol and 51900 J/mol,respectively.Both the increased dynamic viscosities and activation energies of viscous flow can result in deteriorated crystallization performance during latent heat retrieval.

High Thermal Conductivity of Carboxyl-rich Carbon/Polyethylene Glycol Composites for Enhanced Photothermal Conversion and Latent Heat Storage

Polyethylene glycol(PEG)as phase change material(PCM)and carboxyl-rich carbon(RHTC)with different carboxyl content was introduced to prepare RHTC/PEG composites.The composites have high thermal conductivity,photothermal conversion ability,and improved the phase change enthalpy of the system.The microstructure and thermal properties of the samples were analyzed by various characterization methods.RHTC promoted the benign growth of PEG crystallinity(110.0%),influenced the phase change enthalpy of PEG,and improved the latent heat storage capacity of composites.The non-leakage loading of PEG in the composites can be as high as 97 wt%,and the actual phase change enthalpy is 10.7 J/g higher than that of pure PEG,10%higher than the theoretical value.Owing to the heat storage bridge formed between PEG and RHTC,the thermal conductivity of the composites was up to 0.51 W/mK,which was 99.4%higher than that of PEG.Besides,the introduction of environmentally friendly and low-cost RHTC gives the composites excellent photothermal conversion capability,and its photothermal conversion energy storage efficiency is as high as 90.1%.These results prove that the prepared composites are promising potential candidates in the field of thermal storage and thermal management systems.

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.

Research of Thermal Energy Storage Technology in the Solar Thermodynamic Power

Recently, although renewable energy has a great development, primary source is still thermal power generation, which uses fossil fuel as the energy source. Supply and demand of fossil fuel are essential for social and economy development. However, development pattern that excessively relies on the natural source is impossible to provide a sustainable development way for us. As a result, we should combine renewable energy with new energy technology as the aim of economy. It means that it is urgent to exploit new energy. Meanwhile, the ratio of energy waste cannot be ignored. How to decrease energy waste is also significant. Construction sector costs a lot of energy, which is mainly used for heating and refrigeration. In the new energy generation technology, thermal energy can be transformed to electricity with combination of BIPV and thermal energy storage technology. Photovoltaic generation has a great progress in the building construction. As a result, the thermal energy storage technology becomes the key link in the production chain. In this paper, feasibility of applying phase-change material (PCM) in the thermal energy storage will be analyzed. And analysis results are provided with a relative mathematical model.

Microstructure and Thermophysical Properties of Mg-Ga Alloys

The microstructure and thermal characteristics of Mg-36%Ga,Mg-43%Ga,and Mg-45%Ga(wt%) alloys were investigated.The experimental results show that the microstructure of Mg-36%Ga alloy is mainly composed of primary a-Mg phase and a-Mg+Mg_(5)Ga_(2) eutectic phase,the microstructure of Mg-43%Ga alloy is mainly composed of a-Mg+Mg_(5)Ga_(2) eutectic phase,and the microstructure of Mg-45%Ga alloy is mainly composed of primary Mg_(5)Ga_(2) phase and a-Mg+Mg_(5)Ga_(2) eutectic phase.The melting enthalpies of Mg-36%Ga,Mg-43%Ga,and Mg-45%Ga are 146.41,171.90,and 113.90 J/g,with the phase change temperature of 422.57,422.70,and 422.90 ℃,respectively.Mg-43%Ga alloy contains the highest melting enthalpy because of the highest content of a-Mg+Mg_(5)Ga_(2) eutectic phase.In addition,the thermal expansion of the three alloys increases with increasing temperature,while the thermal diffusivity and thermal conductivity decreases with increasing content of Ga.

Evaluation and Optimization of the Thermal Storage Performance of a Triplex-Tube Thermal Energy Storage System with V-Shaped Fins

Adding fins to a shell-and-tube phase change thermal storage is a simple and effective way to enhance the performance of the phase change heat storage unit,and the proper arrangement of the fins is essential to enhance the performance of the storage unit.To enhance the performance of the triplex-tube thermal storage unit,a novel V-shaped fin structure is presented in this paper.And the heat storage performance of the thermal storage system is studied by numerical simulation.Firstly,the performance of the triplex-tube thermal energy storage unit with different arrangements of V-shaped fins is investigated by a two-dimensional model and compared with the use of the traditional rectangular fin structure,and the optimal fin arrangement is derived.The results show that the V-shaped fins with the optimal arrangement can decrease the time for the PCM melting in the heat storage unit by 31.92%compared to the conventional rectangular fins.On this basis,the influence of fin angle and thickness on the heat storage unit was studied.Then,a three-dimensional model of the thermal storage unit was established.And the effect of the flow parameters(inlet temperature,inlet flow rate)of the heat transfer fluid(HTF)on its performance was discussed in detail.Finally,the stored energy analysis of the whole thermal storage unit is carried out.