Multiphase-Change Materials for Energy Storage Application in Buildings

Experimental study has been carried out towards the development of a multiphase change material (MCM) by combining two fabricated microencapsulated phase change materials (MEPCM-oct and MEPCM-eic) samples. The study also covered the characterization of the developed samples for their thermal properties. For the purpose of validation the thermal properties of the developed MCM sample were compared with that of the core components. The developed MCM sample achieved a combined energy storage capacity of 186 kJ/kg with two melting temperatures of 23.4oC and 34.5oC. Even though the two melting temperatures of the MCM were slightly reduced by 0.18℃ and 0.48℃ in comparison with the MEPCM-oct and MEPCM-eic respectively, their mean energy storage capacities were in perfect agreement. The study has therefore shown that it is possible to overcome some of the scientific barriers towards the development of multiphase change materials but further investigations are needed to enhance overall thermal response.


I. Introduction
NERGY consumption in the building sector still represents the highest proportion of energy usage in most developed and developing countries. According to recent publication, the building sector is responsible for approximately 40% of final global energy consumption and CO2 emissions [1].
In the UK for instance, buildings account for about 43% of the country's total energy consumption with heating and hot water representing the bulk proportion [2][3]. Innovation in the building sector therefore represents a significant opportunity to help minimise energy consumption as well as reducing global greenhouse gas emissions targets. One promising and innovative technology that could make positive contribution and improve the thermal performance of existing and future building stock is through the use of phase change materials (PCMs). These materials such as alkenes (paraffin waxes) can be used for the passive/active storage and release of heat energy in buildings as they have relatively high energy storage capacities at constant temperatures. There are a number of past investigations which show that energy consumption in buildings could be reduced by integrating PCMs into building fabrics and ventilation systems [4][5][6][7][8][9][10]. For instance studies carried out by Darkwa [11] and Zhou et. al [12] showed that as much as 30% reduction in heating and cooling loads in buildings could be achieved with integrated PCM drywall systems. Other researchers [13][14] also demonstrated that PCM could be integrated into hot water storage system and used for shifting peak load from space conditioning and water heating systems.
However, there are some basic and applied research issues which need to be addressed before competitive scientific, technical and economic edge can be achieved. For instance commercially available microencapsulated PCMs can only function at fixed melting temperatures and unable to be tuned to other melting temperatures of say from 40 o C to 20 o C and vice versa. This means that in an area where both heating and cooling are required throughout the year, two separate PCMs would have to be applied to provide that dual function. This approach makes the materials thermally ineffective due to non-flexibility in their melting temperatures during energy storage/discharge process. The PCMs could also account for about 30% -45% of the finished product in terms of content and cost which have direct impact on the profitability of the product. Above all they possess relatively low thermal conductivities and do display poor thermal response factor when combined with other construction materials [15][16][17]. Even though other investigations [18][19][20][21][22][23] have uncovered different methods and techniques for enhancing and developing high conductivity composite microencapsulated phase change materials they still possess single phase transition E American Institute of Aeronautics and Astronautics temperatures. Other binary mixtures of PCMs have also been investigated but were unable to achieve the inherent functional and desirable variable temperature characteristics [24][25][26][27]. These limitations are barriers to the wider application of current PCMs. This study was therefore focused on developing a multiphase change material (MCM) capable of operating at two different melting temperatures.

II. Development of a Multiphase Change Material (MCM)
As summarised in Table 1, paraffins (n-octadecane and n-eicosane) were selected as the core materials for the fabrication of the MEPCMs. Melamine-formaldehyde solution was also selected and used as shell monomers and nano-silicon dioxide hydrosol as emulsifier. The other associated materials were ammonium chloride, sodium hydroxide and citric acid which were used as nucleating agents for modifying the pH values.  This enabled the pH value to be adjusted to 9 and to terminate the cross linking reaction process.
In order to produce the MCM, equal proportion of the manufactured microencapsulated PCM samples (noctadecane and n-eicosane) were mixed together with deionised water at a stirring speed of 200 rpm for 10 minutes to achieve a uniform particle size distribution. The mixture was finally washed and dried in an oven at a temperature of 60°C for 20 hours to obtain the MCM sample.

III. Results and Analysis
A. Energy storage and melting temperature By comparing these sets of results, it can be seen that even though the melting points of the MCM sample were slightly reduced by 0.18℃ and 0.48℃ respectively, its energy storage capacity was in perfect agreement with the mean value of the MEPCM-oct and MEPCM-eic.
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B. Thermogravimetric analysis (TGA)
Thermogravimetric analysis were conducted on the samples in order to examine the physical and thermal resilience of the samples in relation to their molecular structure.These tests were was carried out under nitrogen gas protection covering a heating range of 50 °C to 500 °C and at a heating rate of 10 °C /min. Fig. 3 represents the characteristic TG curves for the MEPCMs which show that the MEPCM-oct and MEPCM-eic achieved thermal stability up to 207.6°C and 211.9°C respectively. In Fig. 4, the MCM sample also displayed relatively good thermal stability behaviour up to about 197 ℃ which satisfies the environmental requirements for most applications in buildings.

C. Scanning Electron Microscopy (SEM) analysis
The SEM analysis was conducted in order to examine the external morphology (texture) and crystalline structure of the particles making up the samples. Figs. 5a and 5b are the results for the MEPCM samples. It can be seen that, most of the particles did achieve fairly good structural integrity of the shell materials after encapsulation without obvious sign of degradation. Similarly in Fig. 6, the structure and morphology of the MCM particles were good and physically well bonded. This shows that the MCM sample should be able to operate at the two specified temperatures for multiphase change processes.
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IV. Conclusion
The study has shown that some of the scientific and technical barriers associated with the development of multiphase change materials can be overcome. For instance thermal analysis of the test results clearly shows that it is possible to achieve multiple phase transitional temperatures with relatively good energy storage capacity and thermal stability.
The findings may be therefore summarised as follows: It is believed that the technology could significantly reduce the material content, cost, size and ultimately payback period of integrated MCM components since they should be able to perform multiple functions. There is however the need to develop it further to enhance its thermal conductivity.