In the past decade the use of Phase Change Materials for latent heat storage has become widespread, this is mainly due to their multifaceted uses such as thermal energy storage, conditioning of buildings, off-peak power utilisation, thermal comfort in vehicles and computer cooling to name but a few. However, as the applications of phase change materials become more demanding and complex it has become evident that the existing latent storage compounds are limited in their ability to deal with these more demanding requirements. Glauber's salt, Soda ash, Sodium Acetate and paraffin wax are the most commonly used phase change materials. These compounds are fairly inexpensive, however, the packaging and processing necessary to get acceptable performance from them is complicated and costly. They do not offer a reliable pattern of releasing heat as the chemicals in these phase change materials separate and stratify when in their liquid state. These phase change materials have not always re-solidified properly. When temperatures drop, they did not completely solidify, reducing their capacity to store latent heat. These issues along with the desire to advance phase change material use has led to research into the use of new materials which will satisfy the requirements for high performance combined with longevity and financial viability. Phase change materials fall into one of three categories depending on their chemical composition (i) organic compounds (ii) inorganic compounds and (iii) inorganic eutectics or eutectic mixtures. It has been noted by Baetens, Jelle and Gustavsen [1] that research until recently has primarily been concerned with inorganic compounds, i.e. hydrated salts, which require support and containment, and cannot be directly incorporated into materials. One of the most significant questions asked was which container was the most suitable for each phase change material as cycling will cause degradation. There has more recently been research into the use of organic phase change materials and Hasnain [2] demonstrated the possibility of introducing organic compounds to porous building materials and in this way creating a direct gain storage structure. Organic phase change materials are generally chemically stable, are non toxic and do not cause corrosion, they also have a high latent heat of fusion. While most inorganic compounds used are paraffin, there are a wide range of other possible compounds that could benefit from research, these being fatty acids, esters, alcohols and glycols. While these compounds have great melting and freezing properties it was pointed out by Hasnain [2] that they are about three times more expensive than paraffin. Research by Kenar [3] into the characteristics of biobased oleochemical carbonates as potential phase change materials showed positive results. Oleochemical carbonates are organic based materials that are easily prepared through a carbonate interchange between C10-C18 fatty acids which are renewable and dimethyl or diethyl carbonate in the presence of a catalyst. These fatty acids that are easily obtained from renewable sources such as oils or fats as determined by Feldman et al.[4] Oleochemical carbonates have many uses such as fuel additive, cosmetic ingredient and as a lubricant, however, their use as a phase change material had never been examined. Carbonates ranging from 21-37 carbon atoms were evaluated to develop an extensive understanding of their solid to liquid transitions for use in thermal energy storage applications. Kenar determined that these oleochemical carbonates represented a novel phase change material chemical which was renewable and that could provide a potentially valuable biobased alternative to paraffin phase change materials that currently dominate the phase change material market. Kenar also envisioned that by changing the chain length of the carbonate along with the forming of eutectic mixtures of these compounds, then the temperature ranges over which these carbonates operate can be developed to suit the needs of a particular phase change material application. Further research by Rozanna et el [5] on the thermal characteristics of phase change material in gypsum board for building application looked at a selection of low melting organic materials to avoid some of the problems that are inherent in inorganic materials such as super cooling and segregation. The focus was mainly given to fatty acids which were incorporated into the gypsum by three different methods; encapsulation of the storage material within the board materials, direct incorporation and by immersion of the boards into molten phase change material. Results of testing showed that the latent heat was sufficient to be comparable to those of other phase change materials such as salt hydrates and polyalcohol’s, which are 100-250J.g-1 . The impregnated board had virtually no change to its thermal characteristics and the immersion did not change the physical characteristics of the boards. Rosanna et al recommended further study and recommended that a suitable insulator be attached to the phase change material gypsum board and that an accelerated thermal cycle test be performed to detect if there would be any thermal behavior change over long term use. Thermal cycling tests on inorganic phase change material compounds were also carried out by Shulka et al [6], these tests were performed by running up 1000 thermal cycles to determine their thermal stability. Shulka et al understood that the economic feasibility of employing a latent heat storage material in a system depended on the life of the storage material. They had also found that the thermo physical properties of commercially available phase change materials differed greatly than the qualities quoted for laboratory grade phase change materials.
Tests were carried out on the inorganic materials, Sodium Hydroxide, Di-sodium borate, Ferric nitrate and Barium hydroxide. Testing involved placing the phase change materials in an oven which was then heated and cooled cyclically. Time/Temperature graphs and differential scanning graphs were obtained for each cycle. Tests concluded that none of the inorganic phase change materials tested were suitable for use as latent heat storage and that there was a high deviation from their quoted properties. Further work was recommended which would include structure, chemical and molecular analysis before and after thermal cycling of inorganic phase change materials is needed to study the nature of their degradation and how to prevent it from occurring.
Development in the impregnating phase change materials into products has been recently reviewed, Nomura et al [7] studied the impregnation of a porous materials with phase change material, the phase change material selected were expanded perlite, diatom earth and gamma-alumina. Effects of vacuum impregnation treatment pore size of porous material, holding time and cyclic tests on the thermal properties i.e. latent heat and melting temperature were all examined. Results showed that the latent heat of the expanded perlite/erythritol composite prepared by vacuum impregnation was as much as 83% mass of pure erythritol. Results also showed that cyclic testing by heating and cooling the composite maintained 75% mass of the initial latent heat after the test was repeated four times.
From these findings we see that the principle behind phase change materials is indeed promising, excess energy at raised temperatures is stored by the phase change material and then returned at a certain temperature which results in an increased thermal mass within a narrow temperature range. While potentially high energy savings have been reported in literature it seems apparent that the current available and financially viable compounds do not seem to have reached an optimal point for wide spread applications. Further investigation is warranted to develop the manufacturing and financially viability of current and as of yet undiscovered suitable phase change material compounds.
[1] Baetens.R, Jelle.B,P, Gustavsen.A, Phase change materials for building applications: A state of the art review, Energy and Buildings 42 (2010) 1361-1368
[2] Hasnain, S.M, Review on sustainable thermal energy storage technologies, Part 1:heat storage materials and techniques, Energy Conversion and Management 39 (1998) 1127-1138
[3] Kenar.J, Latent heat characteristics of biobased oleochemical carbonates as potential phase change materials, Solar Energy Materials & Solar Cells 94 (2010) 1697-1703 Impact factor: 3.85
[4] Feldman.D, Banu.D, Hawes.D, Low chain esters of stearic acids as phase change materials for thermal storage in buildings, Solar Energy Materials & Solar Cells 36 (1995) 311-322 (Impact factor: 3.85)
[5] Rozanna.D, Salmiah.A, Chuah.T, Medran.R, Choong.S.Y, Saiari.M, A study of thermal characteristics of phase change materials in gypsum board for building applications, Journal of Palm Oil Research 17 (2005) 41-46
[6] Shulka.A, Buddhi.D, Sawhney.R.L, Thermal cycling test of few selected inorganic and organic phase change materials, Renewable Energy 33 (2008) 2606-2614 (Impact factor 2.226)
[7] Nomura.T, Okinaka.N, Akiyama.T, Impregnation of porous material with phase change material energy storage, Materials Chemistry and physics 115 (2009) 846-850 (Impact factor 2.015)
