Passive Temperature Moderation Using Multi-Transformation Phase Change Materials (MTPCM) in Tropical Desert Climate

Transcription

1 Passive Temperature Moderation Using Multi-Transformation Phase Change Materials (MTPCM) in Tropical Desert Climate Ravindra Kumar*, Rohitash Kumar, Manoj Kumar Mishra, Brij Bala Tak, Pramod Kumar Sharma, and P.K. Khatri Defence Laboratory Jodhpur ( Rajasthan ) India 3411 ABSTRACT: In present study, a new phase change material(pcm), based on binary system of heavy fatty acids have been developed to act as passive temperature moderator in hot tropical climates in non air conditioned buildings, structures, vehicles, public transport system etc. Differential calorimetric studies reveal presence of major phase transformation peak in temperature range C with latent heat J/g and trace of second phase at 38-4 C. The new material overcome the problem of poor solidification during hot night shown by commercially available hydrated salt based PCMs. The material filled in Aluminum/ HDPE panel has been used as an internal lining, for passive temperature moderation in a real size prototype cabin at Jodhpur (Rajasthan) in India. It has performed well during whole summer by maintaining passive melting (during day) - solidification (during night) cycle and restricting inside temperature below 4 C. Keywords: Passive cooling, Phase change material, MTPCM, Latent heat. 1. INTRODUCTION: Phase change materials are being utilized world over for various applications like solar & waste heat storage 1-7, electronic cooling 8, transient thermal management during time varying work loads on electronics 9 etc. PCM have also been utilized for energy saving in buildings in cold countries Off late PCM has been utilized by Mozheveiov et. al. 1 for temperature moderation of a real size room using calcium chloride hexa hydrate as PCM. They have demonstrated that temperature moderation in -3 C and a 4 hour melting (during day) - solidification (during night) cycle is possible with Calcium chloride hexa hydrate as PCM in desert conditions of Israel. In tropical regions like Thar Desert in India, very severe heat condition prevails in the month of May and June. The ambient temperature remains in the range of 46-5 C for 5-6 hrs of the day ( 1: to 18: Hrs ). The temperature of surfaces directly exposed to sun rises to 7-7 C. Temperature inside confined spaces like non air conditioned buildings, structures and public transport systems may rise to above 5 C. In such conditions, temperature moderation using phase change materials is very attractive preposition to save electric power required for cooling. Hydrated salt based PCM tried by Mozheveiov et. al. 1 is not suitable for hotter regions as they do not solidify by ambient cooling during night. In present study, special PCM have been developed by making solid solution of binary and ternary systems of heavy fatty acids. Out of various systems studied, the binary system of Palmitic lauric acid shows presence of multiple numbers of hypo/hyper eutectic tunable phases. * Author for communication E Mail : rediffmail.com 1

2 One composition in this system has been tailored to absorb maximum heat during day time by melting near human body temperature (35-38 C). The kinetics of phase reversal has been accelerated by introducing traces of high temperature phase to ensure completion of PCM solidification in 6-8 hrs of ambient cooling during hot desert night in the temperature range of 3-35 C. The material, when used as internal lining in a prototype real size cabin has provided moderation of temperature by 1-15 C. The material has worked well through out the summer season by maintaining melting (during day) - solidification (during night) cycles. Present paper discusses details of the study.. MATERIALS AND METHODS: Chemicals of Loba / Merck make (AR grade) have been used in the study to prepare various compositions of PCM by melting and solution making. Thermal properties namely phase change temperature and latent heat associated with each transformation were studied using differential scanning calorimeter make M/s Water USA, model: Q1. Phase reversal kinetics and cyclic stability was studied in programmable environmental chamber make M/s CM equipment Bangalore. Optimized composition of PCM was filled in aluminum panel of size 7.5 cm width X 4. cm thickness and varied lengths. A prototype cabin of inner dimensions of 4cm X 18 cm X 1 cm was fabricated using medium density fiber board (MDF) as outer walls and MTPCM filled aluminum panel as internal lining. The quantity of PCM required was decided based on total heat flow during day through 1/ thick MDF panel. Complete PCM solidification by ambient cooling during night was ensured by maintaining natural air convection from three windows provided in the cabin. The temperature profiles of outer and inner surface of cabin was monitored through out the summer season using probe type digital / IR thermometer. 3. RESULTS & DISCUSSION: Automatic phase reversal ( solidification ) of PCM by ambient cooling during night is an essential condition for PCM application for passive temperature moderation of buildings, vehicles and structures. In tropical regions, this has to be completed in 6-8 hrs (1:-8: Hrs.) during early morning when ambient temperature is in the range 3-35 C. Use of low melting point PCM like hexa hydrated calcium chloride (M.P.: 9-3 C) as used by Mozheveiov et. al. 15, although desirable from human comfort criteria, is ruled out in such a harsh climatic condition of tropical regions. Next best option, although not so comfortable is to use a PCM with phase change temperature near to human body temperature (36-37 C). One such material, Zinc nitrate hexa hydrate having melting point in the temperature range C was tried in the present study. However, the material was not solidifying during night due to requirement of high degree of under cooling for solidification (phase reversal temperature < 5 C). Addition of % Boric acid as extrinsic heterogeneous nucleating agent, although, improves the solidification temperature to 3 C, but still not able to ensure complete phase reversal during night. Separation of nucleating agents after few cycles due to density difference, hygroscopic nature and incongruent melting- solidification behavior were other factors making hydrated salts unsuitable for passive cooling in tropical desert. Seeing these limitations of hydrated salts, fatty acids were studied for their suitability in hot desert climate. Out of various binary/ternary systems studied, lauric acid, - Palmetic acid binary system was found promising. During differential scanning analysis the system shows, depending on composition, presence of multi transformation peaks, 1 to 4 in numbers (Fig.1), in 34-6 C temperature range. The number and position of the peaks can be

3 tailored to some extent by controlling composition to alter heat absorption and phase reversal behavior for different applications. Sample: pala8 Size:.7 mg Comment: heating cooling rate 5 deg/min 6 FIG.1. CURVE OF LAURIC - PALMITIC BINARY SYSTEM SHOWING 3.47 C File: C:\TA\Data\\laupametic\pala8.3 Sample: pala9 Size: 3.53 mg Comment: heating cooling rate 5 deg/min TUNABLE Run Date: MULTIPLE 16-Dec-8 16:7 TRANSFORMATIONS Instrument: Q1 V9.7 Build 91 6 File: C:\TA\Data\\laupametic\pala9.3 Run Date: 18-Dec-8 14:36 Instrument: Q1 V9.7 Build C Heat Flow (W/g) C 15.6J/g 31.7 C 15.4J/g Heat Flow (W /g) C 16.4J/g 34.8 C 153.4J/g C Sample: pala5 File: C:\TA\Data\\laupametic\pala5.4 Size:.3 mg -6-4 Run Date: 1-Dec-8 6 1:34 8 Comment: heating cooling rate 5 deg/min Instrument: Q1 V9.7 Build C Universal V4.E TA Instruments C 4.33 C Sample: laric 5%, palmitic75% File: C:\TA\Data\\exp data\lapasevfiv.1 Size: mg Method: heating - cooling method 4 Run Date: 1-Jan :59 8 Instrument: Q1 V9.7 Build 91 Universal V4.E TA Instruments C C Heat Flow (W /g) C C 184.6J/g 183.8J/g Heat Flow (m W ) C 185.J/g C 18.1J/g C C 4.6 C C C C Universal V4.E TA Instruments 54.6 C 39.5 C Universal V4.3A TA Instruments 3

4 In present study the material was tailored in such a way (composition is under Indian patent, application no. 58/DEL/7) that it absorbs heat near human body temperature in temperature range C. The material also has minor phase change peak at 36.9 C (Fig ). During phase reversal initial nuclei of solid start forming at comparatively higher temperature of 35.9C. These nuclei facilitate further solidification. The overall kinetics of solidification is thus accelerated and gets completed in 6-8 hrs in ambient temperature range of 3-35 C available during night. The experiment in controlled temperature chamber confirms complete phase reversal in 6-8 hrs. Unlike in hydrated salt system, the second phase in new material is intrinsic in nature and hence no separation of phases takes place. This improves the material stability. No significant deterioration in thermal properties has been observed even up to 5 cycles of melting and solidification.. Sample: pala67 Size: 3.58 mg FIG. Comment: heating cooling rate.5 deg/min CURVE OF DESERT TUNED MTPCM-37 File: C:\TA\Data\\laupametic\pala67.4 Run Date: 4-Dec-8 11:3 Instrument: Q1 V9.7 Build C 34.9 C.4 Heat Flow (W/g) C C 154.9J/g C 156.8J/g 35.9 C Start of Phase reversal at 35.9 C 4.87 C 39.9 C Traces of high temp. phase 36.9 C Universal V4.E TA Instruments 4

5 The optimized composition of MTPCM was filled in rectangular aluminum pipes/ HDPE container and used as internal lining in a prototype cabin. Temperature profile of cabin (sun exposed outer roof and inside) on a typical summer day is shown in fig.3. Maximum inside temperature of the cabin remains below 4 C through out the day while roof temperature exposed to sun goes to more than 7 C. In similar conditions, the inside temperature of another cabin with out PCM lining goes to more than 5 o C. The door and windows of the cabin was kept open during night to facilitate automatic phase reversal by natural air flow. The experiment was repeated through out summer (one week data given in table 1) and the performance in terms of holding inside maximum temperature below 4 C was maintained. TABLE.1 PERFORMANCE OF PCM COOLED CABIN OVER A WEEK Date Time Solar induced surface Temperature on Out side roof ( C ) Temperature(maximum) in center of shelter ( C) : : : : : : : : TEMPERATURE( OC) TIME OF DAY(:Hrs.) Ambient Temp. Solar Induced Temp. Temp. in a confined space MTPCM moderated Temp. FIG. 3 TEMPERATURE PROFILE OF CABIN HAVING PCM INNER LINING 5

6 4. CONCLUSIONS : In tropical regions, during summer, temperature inside confined spaces in non A/C buildings and public transport systems rises to intolerable levels of 5-55 C. Passive temperature moderation using PCM is attractive to save electrical power. However, conventional PCM like calcium chloride hexa hydrate can not be utilized in extreme hot climates due to poor solidification by ambient cooling in night. In present study we have developed a new PCM based on organic alloying in binary system of fatty acids. The material has been tailored in such a manner that it starts absorbing environmental heat in day time when temperature exceeds human body temperature. Phase reversal kinetic of new material has been enhanced by introducing traces of high temperature phase as intrinsic nuclei to facilitate further solidification of major phase. PCM filled aluminum panels, when used as internal lining, does not allow internal temperature of a prototype cabin to rise above 4 C through out the extreme summer. The new PCM was completing melting (day) solidification (night) cycle with diurnal cycle of hot desert. The material can be helpful in various applications involving passive temperature moderation in non air conditioned buildings, power saving in cooling and providing cooling backups during power failures. 5. ACKNOWLEDGEMENT: Authors are thankful to Dr N.K.Jain Director Defence laboratory Jodhpur for consistent guidance and encouragement in carrying out this work. 6. REFERENCES: 1. J.O C. Young; Phase change materials as energy storage media; Sun world Vol. 5(6), pp (198). A. Abhat; Low temperature latent heat thermal energy storage; Solar energy, vol. No. 3, pp (1983) 3. D.W. Hawes, D.Feldman and D. Banu; Latent heat storage in building materials; Energy and Buildings, Vol., PP (1993) 4. Fath.H.E.S.; Assessment of solar thermal energy storage technologies; Renewable Energy, 14 pp 35-4, ( 1998 ). 5. Mehmet Esen; Thermal performance of a solar aided latent heat store used for space heating by heat pump; Solar energy, vol. 69, no 1 pp 15-5, () 6. Akiyama, Tomodhiro; Yagi, Jun- Ichiro; Encapsulation of phase change materials for storage of high temperature waste heat; High temperature materials and processes, 19, 19-, (). 7. Katsunori Nagano et. al.; An experimental study of thermal characteristics of phase change materials for effective utilization of urban waste heat; IEA, ECES, IA Annex 17, Advanced thermal energy storage through phase change materials and chemical reactions feasibility studies and demonstration projects 3 rd workshop, Tokyo Japan 1- October. 6

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