Nano-inclusion aided thermal conductivity enhancement in palmitic acid/di-methyl formamide phase change material for latent heat thermal energy storage

Highlights

• Enhanced thermal conductivity (TC) in PCM loaded with CBNP is reported.

• A TC enhancement of ∼ 67–153% is seen with CBNP loading.

• TC enhancement is due to low fractal dimension and volume filling capacity of CBNP.

• Phase contrast microscopy shows the role of aggregation on TC enhancement.

• Direct experimental evidence of cluster formation during freezing of PCM is provided.

Abstract

We report enhanced thermal conductivity (TC) in palmitic acid (PA)-di-methyl formamide (DMF) based organic phase change material (PCM) loaded with four different nano-inclusions, viz., carbon black nano powder (CBNP), multiwalled carbon nanotube, graphene nanoplatelets and α-Al₂O₃. The phase transition temperature is tuned from ∼ 61 to 31 ⁰C by varying the PA-to-DMF ratio. Significant TC enhancement (67–153 %) is achieved using the low density CBNP nano-inclusions which is beneficial for reducing the operational cost of PCMs. The fractal nature of the aggregates and volume filling capacity of CBNP leads to the formation of closely bound aggregates with reduced thermal barrier resistance that aided in efficient percolation heat transfer. Further, for the first time, we provide direct experimental evidence of microscale aggregation and cluster formation during liquid-solid phase transition in nano-inclusion loaded PCMs using optical phase contrast microscopy.

Introduction

Latent heat thermal energy storage (LTES) using organic phase change materials (PCM) has attracted considerable interest in the recent years, primarily from applications point of view due to its high energy storage density and lower temperature variation between energy storage and retrieval [[1], [2], [3], [4], [5], [6], [7]]. The use of PCM was first reported by Telkes and Raymond in 1949 [8], and thereafter, several types of PCMs have been proposed and experimentally demonstrated from applications point of view. Organic PCMs have found numerous applications in concentrated solar thermal plants, solar energy storage, domestic refrigeration, building thermal management, battery thermal management and textile industries [4,[9], [10], [11], [12], [13]]. Nevertheless, inherently low thermal conductivity of such PCMs remains as a hindrance for effective thermal energy storage [14,15]. Hence, numerous strategies have been adopted to improve the heat transfer properties of the PCMs [[16], [17], [18], [19]]. One of the most widely used method is dispersing high thermal conductivity nano-inclusion within the continuous medium of the PCM host matrix (nano-enhanced PCMs: NEPCM) that have shown reversible thermal conductivity enhancement during melting-freezing cycles [7,[20], [21], [22], [23], [24]]. The exact mechanism of nano-inclusion aided thermal conductivity enhancement is not well understood. It has been hypothesized that aggregation and cluster formation, during freezing, leads to the development of quasi-2D networks of percolating structures that facilitate enhanced heat transfer [2,11,22,25]. Various experimental and theoretical studies have shown that aggregation dynamics is the key to thermal conductivity enhancement in such systems [[26], [27], [28], [29], [30]]. However, there exist no direct experimental proof for cluster formation during liquid-solid phase transition of nano-inclusion loaded PCMs. Moreover, the majority of nano-inclusions used for dispersing in the PCM host matrices (like metallic: TiO₂, CuO, Al nanoparticles and carbon based: multi walled carbon nanotubes, graphene nanoplatelets, exfoliated graphite, graphite nano-flakes, etc.) are expensive and hence, to reduce the operational cost, there is a requirement of developing PCMs loaded with alternate nano-inclusions, which are cheaper.

The major objectives of the present study are twofold, viz. (1) to achieve significant thermal conductivity enhancement in a PCM using a comparatively cheaper carbon-based nano-inclusions with lower density and (2) to experimentally probe the aggregation dynamics and cluster formation, during liquid-solid phase transition of nano-inclusion loaded PCMs. In the present study, palmitic acid (PA) has been chosen as the phase change material, owing to its several beneficial properties such as, high latent heat of phase change, low supercooling, non-toxicity, non-corrosiveness, smaller volume changes during solid-liquid phase transition and repeatable thermal properties even after consecutive thermal cycles [20,31,32]. Additionally, palmitic acid is derived from raw vegetables and animal sources, which is environment friendly [20]. Earlier studies on palmitic acid based PCMs, loaded with various types of metallic and carbon based nano-inclusions have shown improved thermal stability, significant thermal conductivity enhancements and usability in practical applications [[33], [34], [35], [36], [37], [38]]. On the other hand, Lee et al. [39] proposed a PCM consisting of anhydrous and hydrated palmitic acid/camphene solid dispersions with high latent heat and specific heat.

However, the solid-liquid phase transition temperature of pure palmitic acid is ∼ 61–63 °C [20,33], which restricts is applications in various domestic applications. In the present study, we use di-methyl formamide (DMF) to tune the solid-liquid phase transition temperature of palmitic acid to ∼ 36 ^°C, which is ideal for various applications, like thermoregulation of buildings, thermal management of batteries and low energy solar thermal apparatus [40]. The chemical stability of the PA-DMF complex PCM is analyzed using density functional theory (DFT) and experimentally confirmed using Fourier transform infrared (FTIR) spectroscopy. The phase transition temperatures and latent heat values are estimated from the heat flow curves using differential scanning calorimetry (DSC). The phase transition temperatures are also verified from the temperature dependent variation of refractive index and using infrared thermography. In the present study, experiments are performed on PCMs loaded with varying concentrations of four different nano-inclusions, viz. carbon black nano powder (CBNP), multi walled carbon nanotubes (MWCNT), graphene nanoplatelets (GNP) and α-Al₂O₃. Significant thermal conductivity enhancement is achieved for the CBNP loaded PCMs, as compared to the results reported in literature for various other types of nano-inclusions. Further, phase contrast optical microscopy is used to probe solidification induced cluster formation and aggregation dynamics in real time, during freezing of the CBNP loaded PCMs, which showed the formation of percolating network of nano-inclusions that improved the heat transfer efficiency.