Numerical investigation on latent heat storage enhancement using a gradient-concentration nanoparticles phase change material

Highlights

• PCM layered by the full-fins improves the melting and temperature uniformity.

• Dispersing nanoparticles has better enhancement effect on the layered PCM.

• Negative gradient-concentration nanoparticles outperforms other heat transfer techniques.

• HTF temperature has the strongest influence on negative gradient-concentration nanoparticles.

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 (φ_f) of 0.5 and 1.0 vol%, respectively. Then, the finned case of φ_f = 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.

Introduction

Thermal energy storage (TES) technology, which can reduce the imbalance between supply and demand of thermal energy and relieve the energy crisis, has received considerable attention in recent years. In general, thermal energy can be stored in the forms of sensible, latent and chemical reaction heats [1]. Latent heat storage (LHS) is more attractive than others since its working medium of phase change materials (PCMs) have a larger storage capacity, narrower heat-releasing temperature ranges and stronger stability [2], which has been used in various fields, such as solar collector, waste heat recovery, passive cooling of buildings, air condition [3], [4]. However, most of the PCMs have poor thermal conductivity, which leads to the low charging and discharging efficiency of the LHS system. Several methods have been proposed to enhance the heat transfer in LHS, such as using highly conductive fins [5], [6], nanoparticles [7], [8], metal foams [9], [10], heat pipes [11], [12] and combinations of them [13], [14].

Addition of fins increases the heat transfer area between HTF and PCMs and the thermal penetration into PCMs. Fins can be divided into longitudinal, circular/annular, plate, and pin ones. Joshi and Rathod [15] reported that the times required for Stearic acid to melt and solidify in a finned rectangular LHS system were decreased by 50.0 and 5.6% compared to the finless one, respectively. The present study of PCMs melting/solidification in a finned LHS system mainly focused on the effects of fins parameters (such as number [16], length [17], thickness [18], angle [1]), orientation [5], shape [19], arrangement [20], perforation [21] and material [22] on thermal performance. No study focused on the effect of full-length fins on PCM melting performance has been reported, although the full-fins is needed in some situations, such as the application of multiple-PCM [23], [24].

Nanoparticles-enhanced PCMs (NEPCMs), which could be mixtures of metals (e.g., Au, Cu), metal oxides (e.g., Al₂O₃, CuO), carbon-based nanomaterials (e.g., carbon nanotubes, graphene), carbides (SiC) or polymers, have higher thermal conductivities compared to the pure PCMs [25], [26]. Several studies have been performed on rectangular, cylindrical, annular and shell-tube type LHS systems using various NEPCMs [27], [28], [29]. The effects of nanoparticles type, size, concentration and shape on thermal performance were investigated [30], [31], [32]. Zarma et al. [31] explored the melting of paraffin wax dispersed with various nanoparticles in a rectangular container. They found that the complete melting times for paraffin wax dispersing with 5% Al₂O₃, CuO and SiO₂ nanoparticles were reduced by 20.5, 13.6 and 4.5%, respectively. Akhmetov [33] pointed out that the thermal conductivity of paraffin wax with 4 wt% Al₂O₃ was increased by 40.0% and the complete melting time was reduced by 21.5%. However, some studies found that the viscosities of PCMs will significantly increase with the increase of nanoparticles concentration [34], [35], [36], [37], which severely deteriorates the overall heat transfer. Arıcı et al. [36] reported the melting rate of PCM with 3.0 vol% Cu nanoparticles was lower than that with 1.0 vol% Cu nanoparticles. Therefore, it is necessary to design the concentration distribution of nanoparticles at a certain concentration to further improve the enhancement performance.

The non-uniformity of the melting process induced by local natural convection is another factor limiting the LHS efficiency. While natural convection accelerates PCM melting at the upper region, it delays the melting at the lower region. To overcome the defect, local fins [38], [39], gradient fins [18], [20], [40], local metal foam [41] and gradient metal foam [42], [43], [44], [45] have been proposed. Deng et al. [39] evaluated the melting performance of Lauric acid in an annular container with local double-fins. They found that the local fins saved 66.7% melting time compared with the uniform one. Pu et al. [20] numerically studied the charging process of a radially-finned vertical LHS system. They reported that shorter pitch of fins at the lower part provided more uniform melting compared with the uniform one and saved melting time. Singh et al. [40] investigated the charging process of the vertical shell-tube LHS system with various fins lengths. They found that continually shortening fins in the flow direction of HTF provided more uniform melting compared with the uniform fins and saved 16.0% melting time. Xu et al. [41] proposed a method to improve the melting by partially filling porous material. They reported that placing the porous material at the lower region improved the uniformity of the melting process. The gradient foam arranged in the radial direction was proposed by Wang et al. [45]. Experimental results demonstrated that the complete melting time for the case with negative-gradient copper foam was reduced by 37.6% compared to that with uniform one. However, the thermal performance of PCM with dispersing nanoparticles of different concentrations in different regions has not been studied yet.

In summary, the main shortcomings of the current literature including, (1) the effect of full-length fins configuration on the melting performance of finned LHS has not been studied even though it has already been used [23], [24]; (2) the melting performance of LHS system with nanoparticles of non-uniform concentration has not been studied. This present study was carried out to address these issues, which was done by exploring the melting performance of a vertical shell-tube LHS system with full-fins and with the combination of full-fins and nanoparticles in gradient concentration.