1. Introduction
Thermal energy could be stored in two different ways: sensible and latent. Energy storage using sensible heat uses a solid or liquid medium without a phase change as an energy carrier, while latent heat energy storage (LHTES) uses the phase change enthalpy during melting or solidification to store energy. LHTES provides higher heat storage capacity with a lower weight and volume of the material than the sensible type. Using phase change materials (PCMs) for storing latent heat is the top-rated on the list of developed, applied, and investigated energy storage methods
[1][2]. In PCMs, supplying or extracting energy is performed via a slight temperature change during the phase change, and the phase change occurs in an intermediate temperature range
[3]. Another application of PCMs is thermal management and temperature control
[4], for example, the thermal protection of electronic devices against pulsed power loads
[5] or battery temperature management
[6]. The worst thing about PCMs blocking their way to becoming a reliable method of energy storage is their low thermal conductivity.
PCM performance improvement may be briefed as the thermal conductivity enhancement of the PCM material. Therefore, many studies have been established to find methods to reach more significant thermal conductivities of PCMs. These studies also have led to collateral effects, such as lower volume changes during the phase change, more uniform temperature distributions, shorter charging times, and more. Here, some effective methods for PCM improvements are summarized.
2. Porous Media as Composite PCMs
A reliable scheme to improve PCM thermal conductivity is using highly porous structures as catalysts with high thermal conductivities. In general, implementing porous media and composite PCMs could be two different methods of PCM enhancement. However, all known composite PCMs are usually porous media. Therefore, here, these two methods are combined and defined as one single coupled method.
Since this text concentrates on lattice Boltzmann method applications in PCM studies, there are investigations in the simulation of porous media-filled cavities of PCMs. Wu et al.
[7] studied the solid-liquid phase change problem considering convection heat transfer in porous media. Deploying a generalized model of the LB based on the Darcy-Brinkman-Forchheimer model considers the existence of the porous medium. The aim of the study and other correlated studies
[8][9] was to illustrate the agreement between this model and previous analytical and numerical models. Another type of study in this scope has focused on the geometrical characteristics of the porous medium used to enhance the energy storage of PCMs. A recent study on this topic was from Mabrouk et al.
[10], which investigated the porosity effects on this realm.
2.1. Metal Foam Composite PCMs
A key process in the provision of composite PCMs is impregnating PCMs into porous metals
[4]. High thermal conductivity and the specific surface ratios of metal foams are the main reasons why they have attracted attention
[11]. Metal foams significantly affect the phase change heat transfer of melting and solidification
[12]. They shorten the charging time and construct a more uniform temperature distribution
[13][14][15][16]. Adding metal substances to PCMs would lead to greater thermal conductivity
[17]. Most of the disquisitions in this area have focused on foam geometry. Some has followed the LBM method, e.g., Gaedtke et al.
[18] and Mabrouk et al.
[10]. Others such as Sai Dinesh et al.
[19][20] and Parida et al.
[21] who investigated the porosity and pore size of metal foams implemented other schemes that simulate the flow through metal foams. They concluded that even for the same porosity value, a smaller pore size results in faster melting and a higher heat transfer rate. Furthermore, for lower porosities, the melting time is reduced due to the higher thermal conductivity (see
Figure 1).
Figure 1. The melting front for different porosities of metal foams after 163 s performed by Parida et al.
[21]; (
a) 50% porosity, (
b) 60% porosity, and (
c) 70% porosity.
In a work conducted by Hu et al.
[22], the pore density was explained. Low porosity and high pore density composite PCMs melt faster than high porosity and low pore density ones. As this method has the most visible impact on the melting process of PCMs, there are various experimental investigations in this area. For instance, Mohamed Moussa and Karkri
[23] conducted a study on the effects of different heat condition types in metal foam-enhanced PCMs. The thermal behavior of a PCM filled with a low porosity metal foam was the target of an experimental study by Hu et al.
[24], and more recently, the geometrical characteristics of a metal foam were investigated in a set of experiments which were led by Iasiello et al.
[25]. A major concern of the studies in this area is whether the solution domain is three or two-dimensional. In three-dimensional domains, one could thoroughly study the effect of metal foam geometry as it has various conduction paths from behind and in front of the 2D section. However, in 2D studies, these conduction paths would not be declared. This phenomenon that would distinguish the conduction paths is critical in some applications to be observed, such as photovoltaic panels’ energy storage enhancement by adding metal foam-embedded PCMs, which was the main concern of a recent study by Li et al.
[26]. Some problems addressed were energy storage tanks enhanced by PCMs impregnated with metal foams
[27]. Despite the latter applications, which require 3D simulation studies, there are multiple applications that implement metal matrixes (not necessarily foams) that do not have complex geometries (such as open-cell irregular foams) and could be simplified in 2D studies without significantly affecting the results. Shirbani et al.
[28] addressed these applications and implemented the LBM to study the effects of the geometrical characteristics of a solid matrix on the melting process of a PCM. Their main goal was to reach the best pore arrangement in cylindrical-shaped pores in a solid matrix along with the best pore diameter to reach the most enhanced performance of a PCM during melting. They concluded that a bigger mean pore diameter would help the natural convection, and then the arrangement would not affect the melting time. However, for a small pore mean diameter, the arrangement and conduction path thickness would be important.
2.2. Composite PCMs with Carbon Materials
Carbon nanomaterials, expanded graphite (EG), and graphene-based materials are examples of carbon materials that have been extensively studied and used as thermal conductivity-enhancing materials
[4]. Carbon substances are suitable to formulate a solution with barely corrosive PCMs, such as salt hydrates. The fabrication processes of carbon/graphite foam composite PCMs
[29][30][31][32] and graphene aerogel
[33] are the same as metal foams, called the vacuum impregnation process. More applicable studies have been conducted recently, such as Polytetrafluoroethylene (PTFE)-based PCMs
[34] and gypsum-based PCMs
[35], which both use carbon fibers as the enhancement agent for the PCM.
3. Nanoparticle-Enhanced PCMs
A typical disadvantage of PCMs is their low thermal conductivity. Khodadadi et al.
[36][37] were among the first researchers who came up with the idea of dispersing high thermal conductivity nanoparticles into PCMs. According to the results of their study, the application diversity of PCMs could be increased when the thermal conductivity of the PCM is improved.
Recent studies were conducted to report LBM implementation on the simulation of nanoparticle-enhanced phase change material (NEPCM) heat transfer. Jourabian et al.
[38] studied the LBM application in simulating ice melting enhanced with Cu nanoparticles. They presented a good concurrence between their numerical results and those of previous numerical and experimental methods. They also declared that adding nanoparticles would hasten the melting process. Then, Feng et al.
[39] conducted a study on the same material with a novel LB method and different boundary conditions. Their results approved the previously mentioned work, and they concluded that by increasing the nanoparticles’ volume fraction, the stored energy and molten fraction would increase, in line with faster development in the temperature field and melting interface.
Ibrahem et al.
[40], in their work, could add one more characteristic change in PCMs when improved by adding nanoparticles. They explained that the presence of nanoparticles enhanced the movement of the phase change front, summarized in
Figure 2. Ren et al.
[41] studied the melting process of nanoparticle and metal foam-enhanced PCMs in a heat pipe. They concluded that increasing the porosity of metal foams or increasing the nanoparticle volume fraction would speed up the melting process of nanoparticle PCMs. Moreover, they mentioned that the immersed boundary enthalpy-based lattice Boltzmann method would be a suitable method for a numerical study on this matter. Recent studies addressed the complex geometries and combinations of various PCM enhancement methods. For instance, in the work published by Jourabian et al.
[42], the geometry was an inclined elliptical annulus, and the NEPCM was combined with porous media and a metal matrix. They concluded that for inclined geometries, it is better to use the solid matrix in PCMs to have stable stratification. Recently Peng and Sadaghiani
[43] studied the effect of alumina nanoparticle-enhanced PCMs around a cylindrical battery in a rectangular finned domain. Adding a slope to the geometry of the later problem, meaning the study of an inclined finned domain consisting of hybrid nanoparticle-enhanced PCMs, is what Laouer et al.
[44] studied. They reported the effect of the number of fins and their optimum length ratios. Adding fins to a domain-containing PCM is another way of increasing the heat transfer coefficient and is reviewed in the next section.
Figure 2. Interface positions in the Ibrahem et al.
[40] study. (
a) Pure water, (
b) nano-PCM No.1, and (
c) nano-PCM No.2.
4. Effect of Adding Fins
A common, straightforward, and applicable way to improve the thermal conductivity of PCMs is through the use of extended surfaces called fins. However, this has some drawbacks, such as increased system weight
[4]. Here, the main goal is to review LBM-based studies on the effects of fins on the phase change behavior of PCMs.
Talati and Taghilou
[45][46] investigated the LBM application on solidification in a finned container. In their study, they defined the optimum aspect ratio of the container to maximize the solidification time, which was 0.5. Also, they concluded that changing the fin’s material would not significantly impact the results. As a complementary study to their previous work, they conducted a study on the melting of a PCM in the same conditions
[47]. As an additional result, they found that moving a solid body would accelerate the melting process.
As a novel study, Ren and Chan
[48] implemented a multi-relaxation time (MRT)-LBM to simulate the heat transfer of finned PCMs. Their first major result was the development of the method that they implemented. The secondary results illustrated that by increasing the number of fins, the melting process was completed faster (see
Figure 3). Furthermore, the melting rate grew when the length of the inner fins expanded, despite a limit. They found, however, that fewer long fins were more effective than more short fins.
Figure 3. Comparison of melting fronts for a cavity (
a) without fins and (
b) with fins from the study published by Ren and Chan
[48].
A phase change material coupled with conduction fins was also simulated using the MRT-LBM for transient melting heat transferrin, a work published by Gao et al.
[47]. Their novelty was the coupling of the MRT-LBM with the representative element volume scale (REV-scale). Their results indicated the same conclusion as that of Ren and Chan
[48], as previously addressed. Recent studies corresponding to adding fins in geometries to enhance the energy storage of PCMs are also about a combination of the enhancement methods, such as the NEPCM. Regardless of the simulation method, Chu et al.
[49] incorporated the NEPCMs by using a finned tank to enhance the energy storage of PCMs. The focus of their work was energy loss in various cases of the layouts of fins. As a recent study of the combined enhancement methods of PCMs, Yang et al.
[50] decided to investigate the effect of adding a metal foam to a finned shell and tube consisting of a PCM and concluded that this action decreased the nonuniformity of solidification and temperature distribution; the authors also mentioned that this hybrid method would lead to a 90.5% reduction in the total energy released compared to a pure PCM unit.
This entry is adapted from the peer-reviewed paper 10.3390/en16031059