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1 Applied Mechanics and Materials Vols (2011) pp Online available since 2011/Jul/27 at (2011) Trans Tech Publications, Switzerland doi: / Studies on magnesium chloride hexahydrate as phase change materials Yuting LI 1, Dongjun YAN 1,3, Yafei GUO 1,3, Shiqiang Wang 1 and Tianlong DENG 1,2,3,a 1. Tianjin Key Lab of Mar Resources & Chem., Tianjin Univ. of Sci. & Tech., Tianjin, , China 2. CAS Key Lab of Salt Lakes Res. & Chem., Qinghai Inst of Salt Lakes at CAS, , China 3. Coll. of Mater. Chem. and Chem. Eng., Chengdu Univ. of Technol., Chengdu, , China a Corresponding author, tldeng@tust.edu.cn Keywords: Phase change material, Magnesium chloride hexahydrate, Supercooling, Phase segregation. Abstract. Magnesium chloride hexahydrate (MgCl 2 6H 2 O), which has a melting temperature of K and latent heat of kj/kg, is one of the cheapest materials used as the thermal energy storage material. Like most hydrated salts, MgCl 2 6H 2 O also has the problems of supercooling and phase segregation to limit its application. In this paper, some new progresses on magnesium chloride hexahydrate as phase change materials are summarized, and the new trend in the future is pointed out. Introduction At present, energy shortage is faced around the world with the rapid economic growth. A number of studies are focused on the high effective regenerated green energy. Hence, the development of novel materials which can store and release energy becomes an important means of energy conservation. As the most typical representative, phase change materials (PCMs) have variety of interesting advantages as energy storage materials, such as large heat fusion, stable temperature and properties, abundance reserves [1]. Materials as PCMs on hydrated salts, paraffin waxes, fatty acids and eutectics of organic and non-organic compounds were studied during the last 40 years. Hydrated salts become a type of attractive PCMs due to their high volumetric storage density, relatively high thermal conductivity and moderate costs [2]. Magnesium chloride hexahydrate (MgCl 2 6H 2 O), which is one of the common hydrated salts, also can be used for thermal energy storage. Due to magnesium-containing salt lakes are widely distributed in the area of Qaidam Basin, Qinghai-Xizang(Tibet) Plateau, China, there are abundant of bischofite resources (MgCl 2 6H 2 O). For restriction of technology, it is not effectively used and even formed the situation called as harmful magnesium [3]. Therefore, the exploitation of MgCl 2 6H 2 O as PCM is a new approach to effectively use those magnesium resources, i.e. it can be combined with inorganic or organic materials to obtain a new kind of green PCMs to store thermal energy. Choice of magnesium chloride hexahydrate composition to be perspective PCM Magnesium chloride hexahydrate as PCM. MgCl 2 6H 2 O as a hydrated salt is an attractive material for using in thermal energy storage due to its high volumetric energy density, relatively high thermal conductivity, low cost, no-corrosive and non-toxic. Choi et al. [4] presented MgCl 2 6H 2 O as a medium temperature PCMs. Essen et al. [5] investigated four salt hydrates including MgSO 4 7H 2 O, Al 2 (SO 4 ) 3 18H 2 O, CaCl 2 2H 2 O and MgCl 2 6H 2 O on their potential for compact seasonal heat storage in built environment, and the reaction temperatures of which can be reached by a medium temperature solar collector, as the results showed that MgCl 2 6H 2 O is the most promising salt hydrate of the four selected salts. Based on the above research, Zondag et al. [6] who used TGA equipment to measure MgCl 2 6H 2 O samples in order to verify the performance of MgCl 2 6H 2 O, and the results showed that the dehydration of hexahydrate started at about 323 K and the formation of monohydrate started at about 393 K. Those results indicated that the dehydration temperature of MgCl 2 6H 2 O occurs in a All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: /08/11,09:55:17)

2 Applied Mechanics and Materials Vols range of solar thermal collectors, and it also showed that the dehydration of MgCl 2 6H 2 O and subsequent hydration at 12 mbar vapor pressure give the reactor bed a good temperature rise of 20 K. This is a promising result for the future use of MgCl 2 6H 2 O for seasonal domestic heat storage for space heating and tap water heating. Sebaii et al. [7] investigated MgCl 2 6H 2 O to be used as storage media inside solar cookers for five hundred cycles. The results showed that the melting temperature and latent heat fusion were changed with repeating thermal cycling. Although the supercooling was not observed under the condition of their experiment which may due to vibrations, the phase segregation problem of MgCl 2 6H 2 O existed during its thermal cycling. In a word, MgCl 2 6H 2 O was not recommended as a storage material inside solar cooker for cooling indoors due to the instability during thermal cycling, so a further research is still needed in this field. Magnesium chloride hexahydrate and inorganic mixture as PCM. As the melting temperature of MgCl 2 6H 2 O is K, in principle, adding an appropriate hydrated salt would obtain ternary or multi-dimensional eutectic which has lower melting point than the original binary system. This idea provides a new way to use MgCl 2 6H 2 O for PCM [8]. Nagano et al. [9, 10] studied the thermal characteristics of a mixture with magnesium nitrate hexahydrate (Mg(NO 3 ) 2 6H 2 O) and MgCl 2 6H 2 O to modulate the melting point. Table 1 is the available enthalpy for mixtures with different ratios of magnesium chloride hexahydrate. In Table 1, it shows that the different ratios of MgCl 2 6H 2 O and Mg (NO 3 ) 2 6H 2 O to change the properties of Mg(NO 3 ) 2 6H 2 O. The experiment results showed that by adding up 20 % by mass of MgCl 2 6H 2 O into Mg(NO 3 ) 2 6H 2 O, the melting point can be reduced to about 333 K without reduce its latent heat and heat of fusion, and the durability experiments also showed that the mixtures melting point and heat of fusion were maintained after 1000 cycles of melting and solidifying. In addition, waste heat utilization, which produced by polymer electrolyte fuel cells (PEFC) with a temperature of K needed a PCM of high-density heat storage and a phase change temperature within this range. Therefore, the mixture of MgCl 2 6H 2 O and Mg(NO 3 ) 2 6H 2 O as PCM can be also used to recovery this kind of waste heat sources. Table 1 Available enthalpy for mixtures with different ratios of magnesium chloride hexahydrate Added MgCl 2 6H 2 O, Useful temperature range, T PCM [K] Available specific enthalpy, H PCM [kj/kg] Comparison with heat storage using water, ρ PCM H PCM /ρ w h w w, [% by mass] Combined CaCl 2 6H 2 O (66.6 %) with MgCl 2 6H 2 O (33.3 %), Heckenkamp et al. [11] measured that the melting temperature of the mixture was 298 K and the heat of fusion was 127 kj/kg. According to this result, the mixture can be used as low temperature PCM, and the mixture would be suitable for heat storage in home heating and cooling units. Magnesium chloride hexahydrate and organic mixture as PCM. Using organic phase change materials to mix with inorganic phase change materials, it is not only compensated the disadvantage of pure organic material of low latent heat thermal, but also made up the shortcoming of pure inorganic material of supercooling and high phase transition temperature. This idea is a valuable direction to develop PCMs [12]. Kubota et al. [13] studied that the mixture of MgCl 2 6H 2 O as an inorganic component and erythritol as an organic component can be used to heat domestic water. MgCl 2 6H 2 O with the melting temperature is around 390 K, and the latent heat is about 168 kj/kg. Its solid and liquid densities are comparable with those of erythritol while its melting temperature is just 2 K below of erythritol, whose melting temperature is 392 K. Hence, it may be reasonable to combine such two kinds of materials together. The characteristics of erythritol and MgCl 2 6H 2 O mixture and performed at

3 2600 Frontiers of Green Building, Materials and Civil Engineering various weight ratios of MgCl 2 6H 2 O were also demonstrated by Kubota, and the result showed the mixtures with MgCl 2 6H 2 O content between 10 % to 20 % have the melting temperature ranging from 353 to 373 K, which is applicable to latent heat storage for hot water supply, and the mixtures containing 20 % or less MgCl 2 6H 2 O is suitable for high-temperature heat storage application. The problems of supercooling and phase segregation Supercooling. Like most of the salt hydrates, MgCl 2 6H 2 O also has the problem of supercooling when used as thermal energy storage materials. Supercooling usually occurs when the temperature of the material drops below the melting point while the crystallized process is still delayed. This will affect the performance of the energy release even the storage energy can not be released if the problem is serious. It can be lessened by encourage suitable nucleating agents or by some mechanical vibration to start crystal growth in the storage medium [14, 15]. Ding et al. [16] studied the nucleation of salt hydrates and outlined how to select the nucleation catalyst which has the same crystal shape and the similar atoms order compared with the salt hydrates. Besides, their crystallographic date to be crystallized agree within 15 %. As mentioned above, Kubota et al. [13] suggested to combine MgCl 2 6H 2 O with erythritol, and found there was a problem that the mixture seen to have no effect on the promotion of nucleation. The mixture exhibit a large degree of supercooling about 47 K. Hence, they used ultrasound to continuous irradiation the mixture, the result showed that ultrasound was useful in the initiation of the nucleation on the melt surface and secondary nucleation by fragmentation of crystals in the melt. George [17] reported that MgBr 2 6H 2 O addition as the nucleating agent to reduce the supercooling of MgCl 2 6H 2 O. Through the addition of different ratios of MgBr 2 6H 2 O, the result showed that: there was a very small decrease in melting point of the MgCl 2 6H 2 O when the ratios of % MgBr 2 6H 2 O was added; the supercooling was 1 K or less when 15 % MgBr 2 6H 2 O and 0.1 % Mg(OH) 2 were added. Other nucleating additives corresponding to BaO, BaCO 3, Ca(OH) 2, CaCO 3, Mg(OH) 2, MgO, MgCO 3, Na 3 AlF 6, and CaC 2 O 4 can also be preferred to suppress supercooling. George [18] studied the supercooling of MgCl 2 6H 2 O and Mg(NO 3 ) 2 6H 2 O mixture. The different kinds of additives with the same ratio to the constant ratio of MgCl 2 6H 2 O and Mg(NO 3 ) 2 6H 2 O mixture was shown in Table 2. The result showed the addition of MgO, Mg(OH) 2, NaBO 2 respectively can be used to suppress supercooling temperature of the mixture within 2 K. Composition 58.7%Mg(NO 3 ) 2 6H 2 O+ 41.3% MgCl 2 6H 2 O Table 2 Some selections of additives [18] Cycles Additive Average Material % by mass supercooling, K 5 None NaBO MgSO NiSO MgO Mg(OH) MgBr CaK 2 (SO 4 ) Phase segregation. Another problem of MgCl 2 6H 2 O as PCM is phase segregation to form a saturated aqueous phase and a solid phase. In this case, a lower hydrate or the anhydrous formed, which caused a separation of salt and saturated aqueous due to a result of density differences between the various phases. Many studies suggested to using some thickening agents to overcome the problem of phase segregation [19]. George [17] indicated that formation of MgCl 2 4H 2 O can occur during the melt-freeze cycle of MgCl 2 6H 2 O. If MgCl 2 6H 2 O is used as a thermal energy storage medium, any tetrahydrate formation would cause a segregation in the saturated aqueous and then collects at the

4 Applied Mechanics and Materials Vols bottom of the container, which result in significant loss of heat storage capacity. In order to avoid the segregation problem during melting of MgCl 2 6H 2 O, El-Sebaii et al. [7] suggested to use the extra water principle. However, it would reduce the storage density and MgCl 2 6H 2 O is not stable during its thermal cycling due to the phase segregation problem. Therefore, some more works still are needed in the future. Conclusions Along with the fast economic development of human society, the demanding of green energy storage technical is essential. MgCl 2 6H 2 O is harmless, uninflammable and easily available, which can be used as PCM or by combining with organic and inorganic materials to receive a new type of PCMs. It can be used for seasonal heat storage, waste heat recovery system, home heating and cooling and so on. During the applications of MgCl 2 6H 2 O as PCMs, some more works on how to effectively overcome the supercooling and phase segregation are still needed in the future. Due to the abundant recourses of MgCl 2 6H 2 O, the new valuable and potential function application for magnesium chloride hexahydrate as PCMs will be realized soon. Acknowledgment Financial support from the State Key Program of National Natural Science of China (Grant ), the Key Pillar Program of Tianjin Municipal Science and Technology, the Specialized Research Fund for the Doctoral Program of Chinese Higher Education (Grant ) and the Senior Professor Program in Tianjin Government for TUST is greatly acknowledged. References [1] Y. Dutil, D.R. Rousse, N.B. Salah, S. Lassue, L. Zalewski: Renew Sust Energy Rev Vol. 15 (2011), p [2] M.F. Demirbas: Energy Sources Vol. 1 (2006), p [3] Z.Q. Li, T.L. Deng, Y.F. Guo: Anhui Chem Ind Vol. 36 (2010), p [4] J.C. Choi, S.D. Kim: Energy Vol. 20 (1995), p [5] V.M. Essen, J.C. Gores, L.P.J. Bleijendaal, H.A. Zondag, R. Schuitema, M. Bakker, W.G.J. Helden: Energy Sustain July (2009), p [6] H.A. Zondag, V.M. van Essen, L.P.J. Bleijendaal, B.W.J. Kikkert: Berlin, Germany, [7] A.A. El-Sebaii, S. Al-Amir, F.M. Al-Marzouki, A.S. Faidah, A.A. Al-Ghamdi, S. Al-Heniti: Energy Convers Manage Vol. 50 (2009), p [8] D. Zeng, D. Lin: Eng Sci Vol. 7 (2005), p [9] K. Nagano, K. Ogawa, T. Mochida, K. Hayashi, H. Ogoshi: Appl Therm Eng Vol. 24 (2004), p [10] K. Nagano, K. Ogawa, T. Mochida, K. Hayashi, H. Ogoshi: Appl Therm Eng Vol. 24 (2004), p [11] J. Heckenkamp, H. Baumann: Sonderdruck aus Nachrichten Vol. 11 (1997), p [12] Y.S. Yu, Q.S. Jing, Y.Q. Sun: Chem Ind Eng Progress Vol. 29 (2010), p [13] M. Kubota, E.P. Ona, F. Watanabe, H. Matsude, H. Hidaka, H. Kakiuchi: J Chem Eng Jpn Vol. 40 (2007), p [14] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi: Ren Sust Energy Rev Vol. 13 (2009), p [15] P. Verma, V.S.K. Singal: Ren Sust Energy Rev Vol. 12 (2008), p [16] Y.M. Ding, L.C. Yan, J. H. Xue: Chin J Chem Phys Vol. 9 (1996), p [17] G.A. Lane, E. P. Patent B1 (1996). [18] G.A. Lane, H.E. Rossow: U. S. Patent 4,272,392. (1981). [19] A.Y. Chen, X.Y. Wang, X.Z. Gao: Mater Rev Vol. 17 (2003), p

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