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Journal ArticleDOI: 10.1016/J.APPLTHERMALENG.2020.116485

Parameter analysis and fast prediction of the optimum eccentricity for a latent heat thermal energy storage unit with phase change material enhanced by porous medium

05 Mar 2021-Applied Thermal Engineering (Pergamon)-Vol. 186, pp 116485
Abstract: The melting performance of porous-medium-enhanced phase change material (PCM) in a horizontal shell-and-tube latent-heat thermal energy storage unit (LHTESU) with eccentric structure is studied through a fixed-grid numerical method. The effects of ratio of outer and inner diameter, thermal conductivity of porous medium, porosity and heating temperature on the recommended eccentricity of LHTESU are discussed. Correlations for fast prediction of the optimum eccentricity are built. Results show that the mixed use of porous medium and eccentric structure can save as much as 43.1% of the total melting time compared with the pure use of porous medium. There exists an optimum eccentricity to get the shortest total melting time. The optimum eccentricity increases with the increase of ratio of outer and inner diameter, porosity and heating temperature, and decreases with the increase of thermal conductivity. For fast prediction of the optimum eccentricity and saved melting time, a modified Rayleigh number is proposed to reflect the effect of porous medium on natural convection, especially the flow resistance caused by porous medium. The effects of the four parameters on the optimum eccentricity can be integrated by the relationship between the optimum eccentricity and the modified Rayleigh number. Correlations of The optimum eccentricity and saved melting time are obtained respectively for convenient calculation and evaluation of the feasibility of eccentricity.

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Topics: Eccentricity (behavior) (59%), Porous medium (55%), Phase-change material (51%) ... show more

9 results found

Journal ArticleDOI: 10.1016/J.RENENE.2021.04.092
Zuo Hongyang1, Yuan Zhou1, Mingyang Wu1, Kuo Zeng1  +4 moreInstitutions (3)
01 Sep 2021-Renewable Energy
Abstract: A parallel combined sensible-latent heat storage unit with intermittent flow was developed. Heat transfer fluid (HTF) alternately flowed between two tubes for creating cyclic intermittency within a single tube to enhance the heat transfer performance. In this study, a two-dimensional transient model was established and validated with experimental data from the previous study. The effect of heat transfer fluid intermittency cycle length on the heat transfer performance was investigated and the change rule of the optimum intermittency length in different stages during the melting process was designed by a recursion method. It is demonstrated that the optimum intermittency cycle length in each stage plotted as a function of time or melting fraction has an initial non-linear regime followed by a linear regime. The critical point separating these two regimes is related to propagation of melting front. These correlations between heat transfer fluid intermittency and melting behavior provided guidance for enhancing heat transfer performance in the parallel combined sensible-latent heat storage unit. A multistage intermittent mode is recommended to further reduce full melting time by 8.73% compared with that under the continuous mode. This design can improve the charging/discharging power, thus providing better economical efficiency for concentrated solar power (CSP).

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Topics: Heat transfer (61%), Phase-change material (60%), Thermal energy storage (58%) ... show more

4 Citations

Journal ArticleDOI: 10.1016/J.APPLTHERMALENG.2021.116752
Hao Zhou1, Hao Zhou2, Li-ying Wei3, Qing-lin Cai2  +7 moreInstitutions (3)
Abstract: Previously, optimal eccentric parameters were considered to be greatly overall size- dependent, but the inherent factor haven’t been determined. In this study, we suggest that areas above and below inner tube may be the core factor, based on this, “eccentric space ratio” is proposed to explore the thermal behavior of phase change material in different radius ratio models. The results indicate the melting period can be shortened by more than 40% between 4:1 and 14:1, as is generally valid for different radius ratios. Additionally, the solidification can be shortened consistently by more than 45% between 1:4 and 1:14 in large radius ratios models which breaks the previous consensus, but still extended for small radius ratios models. Remarkably, optimal eccentric range in solidification is symmetrical with that in melting. With these numerical and mechanistic analyses, we find that areas above and below inner tube can significantly influence natural convection and temperature distribution, while, as a simple approximation, “eccentric space ratio” is well employed to analyze the area effect irrespective of diameters. Further, this analysis strategy provides a robust optimal eccentric parameter range for phase change heat exchangers and can be widely applied in rapid latent heat storage.

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Topics: Radius (53%), Natural convection (50%), Annulus (firestop) (50%)

3 Citations

Journal ArticleDOI: 10.1016/J.EST.2021.102455
Abstract: Here we document the effect of heat transfer fluid (HTF) tube position and shell shape on the melting time and sensible energy requirement for melting a phase change material (PCM) in a multitube latent heat thermal energy storage (LHTES) application. Tube location and shell shape are essential as the shape of the melted region, i.e. similar to the boundary layer, affects convective heat transfer performance. HTF tube total area is fixed in all cases to have the same amount of PCM. In order to eliminate the effect of heat transfer surface area variation, results of two- and four-tube configurations were compared within themselves. Liquid fraction, sensible enthalpy content, and latent/sensible enthalpy ratio relative to time were documented for two and four HTF configurations in various shell shape and tube locations. Results show that eccentric two tubes with rectangular shell decreases melting time and sensible energy requirement from 67 min to 32 min and from 161.8 kJ/kg to 136.3 kJ/kg for 72.3% liquid fraction, respectively, in comparison to the concentric tubes with the circular shell. When the number of HTF tubes increases to four, then the required melting time and sensible energy decrease 80% and 3.8%, respectively, for PCM to melt completely as the concentric tubes and circular shell is replaced with eccentric tubes and rectangular shell. Results of liquid fraction variation relative to time show that S-curve of melting becomes steeper if PCM distribution is such that the intersection of melted regions is delayed. Therefore, melted PCM regions could be packed into a shell that minimizes melting time and required sensible energy. Even rectangular shell shape increases the heat transfer surface (increased heat loss rate) because melting time has decreased greatly, total energy lost to the ambient from the surfaces of shell decreases. Eccentricity slows down the solidification process but due to increased heat loss rate from the surface, rectangular shell enables faster solidification than circular shell shape. There is a trade off in between solidification time and heat loss energy for rectangular channels which can be optimized by selecting proper insulation thickness. Overall, the results show that without any thermal conductivity enhancement (TCE) method, melting performance and latent heat storage capability can be significantly enhanced as decreasing the sensible heat storage by fitting the melted PCM regions into a fixed space for the applications where charging speed is lot faster than discharging.

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Topics: Phase-change material (60%), Heat transfer (59%), Latent heat (57%) ... show more

2 Citations

Journal ArticleDOI: 10.1016/J.APPLTHERMALENG.2021.117360
Abstract: Latent heat storage (LHS) using phase change materials (PCMs) is a promising option for storing thermal energy. However, PCMs melting rate is low and non-uniform due to their low conductivity and local natural convection respectively, which limits the thermal energy storage efficiency. The combined full-fins and nanoparticles in gradient-concentration is proposed in this study to overcome this shortcoming. A vertical shell-tube LHS system is taken as the study objective. Two-dimensional transient numerical model considering natural convection was developed to analyze the performances of the storage system. The effects of half-fins, full-fins, and combination of both on the melting process were firstly studied. Results show that the design of full-fins inhibits the negative effect of natural convection at the last melting stage, which reduces the melting time by 5.4 and 4.1% compared to the case of half-fins at fins volume fraction (φf) of 0.5 and 1.0 vol%, respectively. Then, the finned case of φf = 1.0 vol% dispersed with 1.0 wt% multiple-walled carbon nanotubes (MWCNTs) was investigated and the effects of heat transfer fluid (HTF) inlet temperature and flow rate were explored. Results show that a 65.6% improvement on melting uniformity is obtained for combined full-fins and MWCNTs in the negative gradient-concentration compared to the combined half-fins and MWCNTs in uniform concentration. Besides, the complete melting time is reduced by 11.4%. The enhancement potential of combined full-fins and nanoparticles in the negative gradient-concentration first increases and then decreases with the increase of HTF inlet temperature, and it decreases with the increase of HTF flow rate.

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Topics: Phase-change material (58%), Natural convection (54%), Thermal energy (51%) ... show more

1 Citations


37 results found

Journal ArticleDOI: 10.1080/10407788808913615
A. D. Brent1, Vaughan R Voller1, K. J. Reid1Institutions (1)
Abstract: The melting of pure gallium in a rectangular cavity has been numerically investigated using the enthalpy-porosity approach for modeling combined convection-diffusion phase change. The major advantage of this technique is that it allows a fixed-grid solution of the coupled momentum and energy equations to be undertaken without resorting to variable transformations. In this work, a two-dimensional dynamic model is used and the influence of laminar natural-convection flow on the melting process is considered. Excellent agreement exists between the numerical predictions and experimental results available in the literature. The enthalpy-porosity approach has been found to converge rapidly, and is capable of producing accurate results for both the position and morphology of the melt front at different times with relatively modest computational requirements. These results may be taken to be a sound validation of this technique for modeling isothermal phase changes in metallurgical systems.

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Topics: Isothermal process (54%), Laminar flow (53%), Phase (matter) (51%) ... show more

1,110 Citations

Journal ArticleDOI: 10.1016/J.PECS.2013.02.001
Sarada Kuravi1, Jamie Trahan1, D. Yogi Goswami1, Muhammad M. Rahman1  +1 moreInstitutions (1)
Abstract: This paper presents a review of thermal energy storage system design methodologies and the factors to be considered at different hierarchical levels for concentrating solar power (CSP) plants. Thermal energy storage forms a key component of a power plant for improvement of its dispatchability. Though there have been many reviews of storage media, there are not many that focus on storage system design along with its integration into the power plant. This paper discusses the thermal energy storage system designs presented in the literature along with thermal and exergy efficiency analyses of various thermal energy storage systems integrated into the power plant. Economic aspects of these systems and the relevant publications in literature are also summarized in this effort.

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Topics: Power station (63%), Thermal energy storage (61%), Solar power (60%) ... show more

851 Citations

Journal ArticleDOI: 10.1115/1.1287793
V. V. Calmidi1, Roop L. Mahajan1Institutions (1)
Abstract: We report an experimental and numerical study of forced convection in high porosity (e∼0.89-0.97) metal foams. Experiments have been conducted with aluminum metal foams in a variety of porosities and pore densities using air as the fluid medium. Nusselt number data has been obtained as a function of the pore Reynolds number. In the numerical study, a semi-empirical volume-averaged form of the governing equations is used. The velocity profile is obtained by adapting an exact solution to the momentum equation. The energy transport is modeled without invoking the assumption of local thermal equilibrium. Models for the thermal dispersion conductivity, k d , and the interstitial heat transfer coefficient, h sf , are postulated based on physical arguments. The empirical constants in these models are determined by matching the numerical results with the experimental data obtained in this study as well as those in the open literature. Excellent agreement is achieved in the entire range of the parameters studied, indicating that the proposed treatment is sufficient to model forced convection in metal foams for most practical applications

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Topics: Forced convection (64%), Nusselt number (58%), Heat transfer coefficient (57%) ... show more

777 Citations

Open accessJournal ArticleDOI: 10.1016/J.SOLENER.2010.04.022
Changying Zhao1, W. Lu1, Yuan Tian1Institutions (1)
01 Aug 2010-Solar Energy
Abstract: In this paper the experimental investigation on the solid/liquid phase change (melting and solidification) processes have been carried out. Paraffin wax RT58 is used as phase change material (PCM), in which metal foams are embedded to enhance the heat transfer. During the melting process, the test samples are electrically heated on the bottom surface with a constant heat flux. The PCM with metal foams has been heated from the solid state to the pure liquid phase. The temperature differences between the heated wall and PCM have been analysed to examine the effects of heat flux and metal foam structure (pore size and relative density). Compared to the results of the pure PCM sample, the effect of metal foam on solid/liquid phase change heat transfer is very significant, particularly at the solid zone of PCMs. When the PCM starts melting, natural convection can improve the heat transfer performance, thereby reducing the temperature difference between the wall and PCM. The addition of metal foam can increase the overall heat transfer rate by 3-10 times (depending on the metal foam structures and materials) during the melting process (two-phase zone) and the pure liquid zone. The tests for investigating the solidification process under different cooling conditions (e.g. natural convection and forced convection) have been carried out. The results show that the use of metal foams can make the sample solidified much faster than pure PCM samples, evidenced by the solidification time being reduced by more than half. In addition, a two-dimensional numerical analysis has been carried out for heat transfer enhancement in PCMs by using metal foams, and the prediction results agree reasonably well with the experimental data.

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Topics: Phase-change material (60%), Heat transfer enhancement (60%), Metal foam (59%) ... show more

489 Citations

Journal ArticleDOI: 10.1016/J.APENERGY.2013.04.050
Xin Xiao1, Pengfei Zhang1, Ming Li2Institutions (2)
01 Dec 2013-Applied Energy
Abstract: The utilization of paraffin in the latent thermal energy storage (LTES) system for solar energy storage is hampered by its low thermal conductivity. Paraffin/nickel foam and paraffin/copper foam composite phase change materials (PCMs) were prepared using a vacuum impregnation method in the present study. The impregnation ratios which reflect the actual mass fraction of pure paraffin impregnated were studied comparatively for the impregnations with and without vacuum assistance. The surface porosity was obtained by employing the image processing approach. The thermal conductivities of the composite PCMs were measured by the transient plane heat source technique (TPS) as well as the steady state method, and the thermal behaviors were analyzed with a differential scanning calorimeter (DSC). It is found that the surface porosity obtained from image analysis was in the range of 90–94%, whereas the bulk porosity predicted by the mass fraction was about 97%. Compared with pure paraffin, the thermal conductivities of the composite PCMs were drastically enhanced, e.g., the thermal conductivity of the paraffin/nickel foam composite was nearly three times larger than that of pure paraffin. The presence of porous metal foam made the phase change temperatures shift slightly, e.g., the deviations of the peak melting temperatures of the paraffin/nickel foam composite and paraffin/copper foam composite with the pore size of 25PPI from those of pure paraffin were 0.55 °C and 0.40 °C, respectively.

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Topics: Metal foam (55%), Composite number (51%), Porosity (51%)

327 Citations