Melting enhancement in triplex-tube latent heat energy storage system using nanoparticles-metal foam combination
Introduction
Technologies for storing energy are of a great practical importance due to their potential for correcting the mismatch between energy supply and energy demand particularly for intermittent energy sources like solar and wind. The three main methods used for storing energy in TES systems are sensible, latent, and thermochemical. The latent method is more attractive than others due to its relatively high storage density and nearly isothermal storage performance. The phase change materials (PCMs) used as storage media in latent TES systems, can store 5–14 times more energy than sensible storage materials with the same volume [1]. However, most PCMs suffer from undesirable property of relatively low thermal conductivity which strongly suppresses energy charging/discharging rates and makes the system response time too long to meet the requirements. A way to overcome this issue is through modifications in the PCM container structure such as insertion of fins [2], application of heat pipes [3], and/or metal foams [4], [5], [6], [7], [8], [9], [10], [11]. One of the more recent ways of improving the thermal conductivity is through dispersion of highly conductive nanoparticles that have nominal sizes ranging from 1 to 100 nm [12], [13], [14], [15].
Among the various enhancement techniques available for PCM-based TES applications, metal foams have been shown to be one of the most efficient solutions in terms of the storage improvement [16]. For example, Zhao et al. [5] reported that phase-change rate can be increased by up to 10 times with inclusion of metal foams. Metal foams are porous metallic structures with small openings called pores or voids. They are mainly characterized by two parameters: porosity (ε) and pore density (ω). Porosity is the ratio of the void volume to the total volume occupied by the foam and the void space. Pore density is the number of pores per linear inch (PPI). The undesirable characteristic of metal foams is they critically reduce the available PCM volume and consequently cause less overall storage capacity. So, only metal foams with high porosity (⩾90%) are recommended for use in energy storage applications. The high porosity makes the metal foam light in weight, large in void passages, and enhanced in interstitial heat transfer to the PCM due to the formation of thin boundary layers. The heat transfer, as will be proposed later on in this study, can be further enhanced by incorporation of highly-conductive nanoparticles. Studies such as [13], [14] have shown that successful dispersion of nanoparticles can enable the PCM to achieve higher thermal conductivity and exhibit better thermal storage performance.
One of the pioneering studies on improving the functionality of phase change materials (PCM) through dispersion of nanoparticles is by Khodadadi and Hosseinizadeh [12]. The study showed that phase-change heat transfer enhancement by nanoparticles is promising for utilization in thermal energy storage systems. Wu et al. [13] experimentally investigated the melting/solidification characteristics of copper/paraffin as nanoparticle-enhanced phase change material. The results revealed that the thermal conductivity of paraffin can be enhanced by the use of copper nanoparticles. The same authors [15] presented numerical simulation studies for melting of copper/paraffin nanocomposites. Sciacovelli et al. [17] studied the thermal behavior of the latent TES unit charged with nano-enhanced PCM. A melting time reduction of 15% was reported by doping nano-enhanced PCM. Arasu and Mujumdar [14] investigated the melting of paraffin wax dispersed with alumina nanoparticles in a square cavity heated either from below or from the vertical side. The study pointed out that the melting rate was higher when the cavity is heated from the side than when heated from below. Lin and Al-Kayiem [18] showed that dispersing copper nanoparticles in paraffin wax not only enhances its thermal conductivity but also improves its thermal stability and reduces its supercooling effect during the discharge phase. Mahdi and Nsofor [19] showed that dispersing alumina nanoparticles of (3–8% by volume) can reduce the solidification time of paraffin RT82 up to 20% in a triplex-tube TES system. Moreover, changing the charging temperature of the HTF from 65 °C to 70 °C does not significantly affect the time savings due to nanoparticles dispersion. Myers et al. [20] dispersed (2% by volume) CuO nanoparticles in three nitrate salts: sodium nitrate (NaNO3), potassium nitrate (KNO3), and the KNO3–NaNO3 eutectic. Results showed no chemical degradation under thermal cycling but showed significant improvement in thermal performance of the nano-enhanced salts relative to the pure salts.
Due to the high thermal conductivity, large area-to-volume ratio, and strong mixing capability, porous metal foams (e.g. copper and aluminum) are considered to be one of the most promising heat transfer enhancement materials [21], [22]. Lafdi et al. [4] ran experiments to study the effects of porosity and pore size of aluminum foam on melting evolution of paraffin wax as a PCM and found that steady-state temperature can be reached faster in both higher porosity foams and bigger pore sizes. Zhao et al. [5] studied the effects of embedding metal (copper) foam on thermal performance of solid-liquid phase change process of RT58 as a PCM. Results revealed that better phase-change rate can be achieved depending on the metal foam structure and material. Cui [7] showed through an experimental investigation that metal foams embedded in PCMs can enhance heat transfer rate, increase the melting rate and shorten the charging period. Li et al. [8] reported that uniformity of temperature distribution inside paraffin-saturated in copper foams can be augmented either by decreasing pore density to enhance natural convection or by decreasing porosity to improve the effective thermal conductivity. Sundarram and Li [9] numerically studied the performance of PCM infiltrated microcellular metal foams under high heat generation and low cooling conditions. The results indicate that a smaller pore size leads to a longer time period and that effective thermal conductivity could be doubled by reducing the pore size from 100 μm to 25 μm. Mancin et al. [10] conducted experimental investigations on copper foam embedded in three different melting-point paraffin waxes. The results indicated that foam matrix can act as a heat spreader in the PCM, which allows a more uniform temperature distribution inside the PCM and helps in earlier ending of the phase change process. Atal et al. [11] investigated the phase change process of paraffin wax saturated in aluminum foam. The results showed that the use of conducting matrix with a PCM can significantly reduce the charging/discharging period. Hossain et al. [23] numerically investigated the thermal performance of a rectangular TES system filled with a porous medium (aluminum foam) saturated by a nanoPCM (C₆H12 + CuO nanoparticle). The results showed that the PCM melting rate is faster with the combination of (porous medium + nanoPCM) compared to the cases of nanoPCM only or porous medium only. A similar study by Tasnim et al. [24] was conducted to deal with effects of heating at the side instead of at the bottom on the overall heat transfer process. The results show that both conduction and convection heat transfer are degraded by the presence of nanoparticles.
In other energy storage-related applications, some porous materials also find noteworthy uses as ion-separation membranes in Vanadium Redox Flow Batteries (VRFBs). These are promising energy storage technologies with advantages of system scalability, long cycling life, and high energy density [25]. They basically consist of two porous carbon electrodes and two circulating electrolytes separated by a membrane. The membrane (separator) is to prevent vanadium ions crossover between the electrodes during battery operation. An effective separation of protons from vanadium ions can be achieved by using porous membranes such as polybenzimidazole (PBI). Zhou et al. [25], [26], [27] showed that PBI membrane can act as a successful alternative to the commonly-used Nafion which has relatively high vanadium ions permeability that lessens columbic efficiency and promotes rapid capacity decay during long battery cycles.
This study aims to positively combine nanoparticles/metal foams as a compound heat transfer enhancement method to achieve improved PCM melting process in triplex-tube TES systems. With this combination, nanoparticles are expected to have good convection contribution (compared to applying foams alone) because they do not hinder much of the PCM melt movement as they could move with PCM particles during the phase-change process. Meanwhile, metal foams due to their high area-to-volume ratio will support better heat diffusion through the PCM (compared to using nanoparticles alone). In this study, high foam porosities (ε ⩾ 0.95) and low nanoparticle volume fractions (ϕ ⩽ 0.05) were selected noting that: (1) the decrease in foam porosity and/or increase in nanoparticle volume fraction will reduce the volume that can be occupied by the PCM, (2) segregation (separation of nanoparticles from PCM) is common with high nanoparticle concentrations [28], [29], (3) nanoparticle concentration significantly increases the viscosity of the PCM-melt which in turn affects convection contribution, and (4) high porosity of metal foams generates high flow-resistant forces and consequently affects the PCM-melt convection.
This study is unique because it contributes for the first time the results of the performance of a system that combines an improved PCM containment vessel (triplex-tube heat exchanger) that houses nano-enhanced PCM infiltrated metal foams. The possibility of applying both-sides heating approach at the annulus of the triplex tube heat exchanger that houses the PCM makes the phase transition to be shorter and the study did not neglect the extra natural convection contribution. This approach enables the PCM in the triplex-tube to have larger heat-exchange area compared to the common double-pipe heat exchanger. This leads to higher heat absorption/release and faster phase-change rate [30]. This study also evaluates how nanoparticles-metal foams combination can further enhance the phase-change process compared to the case of metal foams alone, or nanoparticles alone. Results of this study would be applicable to broad PCM-based applications such as thermal energy storage for solar applications [18], [30], [31], [32], energy efficient buildings [33], [34], and thermal management of electronic devices [35], [36], [37]. A two-temperature model based on enthalpy method was developed to numerically study the solid-liquid phase-change process in high porosity metallic foams (ε ⩾ 95%). In the model, natural convection of liquid paraffin, the non-Darcy effects, Brownian motion of nanoparticles, and the temperature-dependent physical properties were considered.
Section snippets
Problem statement and formulation
Fig. 1 illustrates the cross-section of the geometry of the porous-foam triplex-tube heat exchanger used in this study. The figure shows (a) the physical domain and (b) the computational domain. Due to symmetry in the θ-direction, only the right-half was considered in the computations. The triple tube heat exchanger can be used as a latent TES energy storage container for solar-powered liquid-desiccant air-conditioning systems [38]. It consists of three horizontally mounted concentric tubes,
Numerical procedure and validation
The predictions obtained from numerical simulation can be helpful in estimating the overall performance, especially in the pre-design stage when the optimization among different factors is of a major importance. In this study, numerical solutions of nanoPCM melting saturated in copper porous foam were obtained using the computational-fluid-dynamics code (FLUENT), utilizing the solidification/melting model in connection with non-thermal equilibrium porous-media model. This technique employs a
Results and discussion
Applying the mathematical model developed in the previous section, the numerical simulations were performed to study the combined effect of embedding porous copper foam and dispersing alumina nanoparticles on the thermo-fluidic performance of paraffin RT82 during melting in a triplex-tube heat storage system. It was assumed that the annulus between the inner tube and the middle tube that holds the porous nanoPCM is isothermally heated and is maintained at 363, 368 and 373 K with the hot HTF
Conclusions
This paper presents the results of studies carried out to investigate the behavior and heat transfer characteristics during melting of nanoPCM in a porous foam triplex-tube heat exchanger for thermal energy storage. This is useful for applications such as liquid-desiccant air-conditioning systems. The influence of embedding two different porosity foams with different volume fractions of alumina nanoparticles on the complete melting of paraffin (TR82) was studied numerically and validated
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