Thermal and rheological properties of microencapsulated phase change materials
Highlights
► We studied the thermal and rheological properties of a series of prepared MPCS. ► The MPCS was Newtonian fluid when mass fraction is less than 0.35. ► The viscosity is higher for bigger particle slurries.
Introduction
Phase change materials (PCMs) have long been used for thermal energy storage due to the large amount of heat absorption/release while undergoing phase changes, with only small temperature variations [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Organic and inorganic materials are two most common groups of PCMs [11]. Organic materials are further described as paraffin and non-paraffin. Most organic PCMs are non-corrosive and chemically stable, and have little or no sub-cooling. They are compatible with most building materials and have a high latent heat per unit weight and low vapour pressure. But they also have disadvantages in low thermal conductivities, high changes in volume on phase change and flammability. In contrast, inorganic materials (salt hydrate and metallic) have a high latent heat per unit volume and high thermal conductivities, and are non-flammable and low in cost in comparison to organic materials. However, they are corrosive to most metals and suffer from decomposition and sub-cooling, which can affect their phase change properties. Therefore, In order to overcome these problems, a new technique of utilizing microencapsulated phase change material (MPCMs) in thermal energy storage system has been developed. Microencapsulated PCMs provide a means to solve the supercooling problem and interfacial combination with the circumstance materials [12]. The main merits of microencapsulated phase change material (MPCM) over PCM are as follows: (1) increasing heat transfer area; (2) reducing PCMs reactivity toward the outside environment and controlling the changes in the storage material volume as phase change occurs. The use of microencapsulated phase change materials (MPCMs) is one of the most efficient ways of storing thermal energy and it has received a growing attention in the past decade [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. Since MPCM was developed, it had been mainly used in the textile [19], [28], [29], [30], [31] and building applications [32], [33], [34], [35], [36].
Due to the low thermal conductivity of PCMs [10], a new approach was proposed to improve the thermal performance of the thermal system (e.g. the secondary refrigeration and air conditioning loops). When the MPCM is dispersed into the carrier fluid, e.g. water, a kind of suspension named as microencapsulated phase change material slurry (MPCS) is formed [37]. Water is normally used as the carrier fluid since it has no obvious negative effect on fabricating MPCS and is cheap to get, although the carrier fluid should have a high thermal conductivity and a large specific heat capacity. In comparison of conventional phase change material slurries (PCS), better heat transfer performance can be achieved due to the relatively large surface area to volume of MPCM. Therefore, it can be used as both thermal energy storage and heat transfer media [38], [39], [40], [41], [42], [43]. The thermal and physical properties of MPCS are crucial for the MPCS system design, and they are very different from those of the MPCM materials and carrier fluids. These mainly include the thermal conductivity, viscosity and specific heat. The optimum design of thermal energy storage systems, which run with microencapsulated phase change material slurry, requires a good knowledge of flow and heat transfer characteristics of two-phase slurry involved in phase change, in order to reduce the capital cost, system size, and energy consumption [44].
The purpose of this study is to investigate the thermal and rheological properties of the MPCMs. In this paper, a series of MPCS were prepared for experimental test. The chemical structure, morphology, microstructure, diameter and its size distribution, thermal properties of the MPCMs and rheological properties of MPCS were obtained from experimental measurements.
Section snippets
Investigated materials
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DPNT06-0182 (Ciba Specialty Chemicals, UK), properties of microcapsule as given by the manufacture: it comprises 87.5% paraffin wax and 12.5% crosslinked acrylic polymer shell (core/shell ratio: 87.5:12.5).
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Micronal® DS 5008X (BASF, Germany), properties of microcapsule as given by the manufacture: it comprises a paraffin mixture and highly crosslinked polymethyl methacrylate (PMMA) shell; formaldehyde-free (core/shell ratio: 7:3).
Chemical composite analysis
The chemical structure was analyzed using PerkinElmer-Spectrum 100
Microencapsulated phase change material
The FTIR spectra of the microcapsules (DPNT06-0182, CIBA) are presented in Fig. 1. The alkyl C–H stretching vibrations are found around 2900 cm 1. C–H stretching peaks of crosslinked acrylic polymer are found around 1715 cm 1. The peak at 1466 cm 1 is due to C–H bending and the peaks at 1170 cm 1 can be assigned to the C–O stretching of the ester group. FTIR spectra show both characteristic peaks of crosslinked acrylic polymer and paraffin wax. Fig. 2 shows the FTIR spectra of the microcapsules (DS
Conclusions
Experimental investigations have been carried out to investigate two different kinds of microencapsulated phase change materials (MPCMs) in terms of their thermal and rheological properties. The results showed that most of the microcapsules were spherical shaped and had smooth surfaces, and the structure analysis demonstrated that MPCMs had been successfully fabricated by their manufacturers. Particle size distribution (CIBA sample: 10–100 μm; BASF sample: 1–20 μm) was quite satisfactory for
Acknowledgment
This work is supported by the UK Engineering and Physical Science Research Council (EPSRC grant number: EP/F061439/1), the National Natural Science Foundation of China (Grant No: 51071184). The authors are thankful to the Ciba Specialty Chemicals, UK, and the BASF, Germany, for supplying samples. The authors are also thankful to the Birmingham Science City: Energy Efficiency and Demand Project (Project Ref: SY/SP8008).
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