Thermophysical properties and applications of nano-enhanced PCMs: An update review

Highlights

• PCM is a solution to reduce energy consumption and greenhouse gas emissions.

• Evaluating the techniques used for the addition of nanoparticles to PCMs.

• Discussing on effects of nanoparticles on the thermophysical properties of PCMs.

• Examining the applications of nano-PCMs.

• Phase change rate increases with the addition of nanoparticles.

Abstract

Energy conservation management using latent heat storage (LHS) technique is nowadays employed as a solution to reduce energy consumption and greenhouse gas emissions. Phase change materials (PCMs) are the main candidate for LHS method. The main focus of this study is to evaluate the techniques used for the addition of nanoparticles to PCMs. The present paper is divided into three parts. The first part summarizes PCMs and nanoparticles. In the second part, the effects of nanoparticles on the most important thermophysical properties of PCMs are discussed. In the third part, the applications of nano-PCMs (NPCMs) in the fields such as thermal energy storage (TES), thermal control unit (TCU), photovoltaic thermal (PVT), solar still (SS) and building are examined. In general, all studies show that the phase change rate increases with the addition of nanoparticles. This means that the amount of energy stored/released during the phase change process is improved.

Introduction

Energy consumption management has always been one of the concerns of various researchers [1], [2], [3], [4], [5] because fossil fuel energy is non-renewable and the use of fossil fuels also causes environmental problems. The use of renewable energies can be substituted as a way to reduce fossil fuel consumption. Thus, scientists are looking for ways to make more energy from renewable sources. Another way to manage energy consumption is to use thermal energy storage systems [6], [7]. Using energy storage can prevent the transfer of energy to the environment, etc. [8], [9]. In other words, energy can be stored for future applications. Thermal energy storage is classified into three groups: sensible energy storage, latent energy storage and chemical energy storage [10] (Fig. 1). Each of the three methods mentioned above has some advantages and limitations for use in energy storage. This study investigates the latent energy storage.

Among the methods that fall into the latent energy storage group, the technique of using PCMs has unique capabilities [11], [12], [13], [14], [15]. PCMs are capable of storing or releasing large amounts of energy while their temperature is kept constant or slightly changed [16] (Fig. 2). Fig. 2 compares the amount of energy stored in the PCM of RT-27 with water (one of the most commonly used materials for energy storage). As we know, the behavior of the material is such that its temperature increases by receiving the heat. But if heat is given to PCMs at a phase change temperature, they can store a great deal of energy without change in their temperature. Therefore, the most important property of PCMs is the ability to store thermal energy.

PCMs have the ability to store a lot of energy. The high storage potential of PCMs is activated when their phase is changed. In other words, if these materials do not undergo phase change, they do not have a particular ability to store energy in comparison with the rest of the materials. Unfortunately, the thermal conductivity of PCMs is very low and comparable to the best insulators in this respect. Low thermal conductivity reduces the energy storage rate, i.e., energy is stored or released with fewer rates. For example, consider two PCMs A and B that have the same properties except their thermal conductivity. Now, if the two PCMs are used to store energy, the PCM with higher thermal conductivity has lower charging time. The same is true for energy release. The PCM with higher thermal conductivity has lower discharging time. It is important to note that the energy stored or released from the PCM may change (not necessarily) by changing the thermal conductivity. However, an increase of thermal conductivity has been desirable in many cases. Nanoparticles have a very high surface-to-volume ratio, which makes they have specific properties. For example, the thermal conductivity of MWCNT and graphene nanoparticles is about 3000 W/m.K and 5300 W/m.K, respectively [17], [18], while the thermal conductivity of PCMs is very low, for example that of paraffin is about 0.2 W/m.K [19]. Therefore, it is expected that by adding nanoparticles with high thermal conductivity into PCMs, their thermal conductivity increases. Studies have demonstrated that NPCM has better phase-change and storage parameters than PCM and can therefore be replaced in energy storage applications.

In this paper, the effects of nanoparticles on the thermophysical properties of PCM as well as the phase change process are thoroughly investigated. The study is divided into three sections. The first section briefly refers to the PCMs. In the second section, the effects of nanoparticles on the most important thermophysical properties of PCM are discussed. At the end of this section, the role of nanoparticles in improving the phase change process is examined. In the third section, the applications of NPCMs in the fields such as TES, TCU, PVT, solar still and building are discussed. Each section attempts to use new and up-to-date articles, and is additionally categorized in tables for ease of review.