Enhanced thermal performance and study the influence of sub cooling on activated carbon dispersed eutectic PCM for cold storage applications

Accepted Manuscript Title: Enhanced thermal performance and study the influence of sub cooling on activated carbon dispersed eutectic PCM for cold storage applications Authors: S. Imran Hussain, R. Dinesh, A. Ameelia Roseline, S. Dhivya, S. Kalaiselvam PII: DOI: Reference:

S0378-7788(17)30052-X http://dx.doi.org/doi:10.1016/j.enbuild.2017.03.011 ENB 7436

To appear in:

ENB

Received date: Revised date: Accepted date:

11-1-2017 2-3-2017 3-3-2017

Please cite this article as: S.Imran Hussain, R.Dinesh, A.Ameelia Roseline, S.Dhivya, S.Kalaiselvam, Enhanced thermal performance and study the influence of sub cooling on activated carbon dispersed eutectic PCM for cold storage applications, Energy and Buildings http://dx.doi.org/10.1016/j.enbuild.2017.03.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced thermal performance and study the influence of sub cooling on activated carbon dispersed eutectic PCM for cold storage applications

S. Imran Hussain1, R. Dinesh1, A Ameelia Roseline2, S.Dhivya1, S. Kalaiselvam1 *

1

Department of Applied Science and Technology, AC Tech Campus, Anna University,

Chennai 600 025, India. 2

Department of Electronics and Communication Engineering, Panimalar Engineering College,

Chennai 600 123, India.

*

Corresponding Author: [email protected]

Telephone No. +91 44 22359220

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Graphical abstract

Graphical abstract of activated carbon (AC) enhanced eutectic PCM

Highlights: 1. 2. 3. 4.

Capric and Oleic Acid/Activated Carbon were prepared for cold storage application. Thermal conductivity has been enhanced upto 55% in comparison to base fluid. Degree of sub cooling has been completely eliminated for 0.1 wt% of AC. Latent heat capacity decrement was 4.6% than the pure eutectic PCM.

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Abstract This article investigates the thermo-physical property enhancement of eutectic (oleic and capric acid) based latent thermal energy storage materials for cold storage applications. Highly porous activated carbon (AC) nanosheets were synthesized, which act as nucleators to enhance thermo-physical properties. FT-IR results confirm that the interaction of AC was only physical and no exo/endothermic reactions. Thermo-physical property enhancement was depicted based on the results obtained from the freezing/solidification, DSC, and thermal diffusivity analysis. The heat transfer rate linearly increases for 0.02 wt% to 0.1 wt% of nano dispersed PCM and the maximum time saving was 54% with respect to pure eutectic. Thermal conductivity of the pure PCM were 0.194 W/mK (liquid), 0.201 W/mK (solid) and has been improved by 55% for 0.1 wt% nano dispersed PCM. Linear decrement in the sub-cooling was found to vary between 30% to 63% up to 0.08 wt% of AC, high nucleation rate eradicated sub-cooling from the PCM at 0.1 wt% of AC in base fluid. Slender crystallinity decrement of 4.6% was observed. In addition to thermo-physical properties, theoretical prediction of the PCM properties has also been discussed. The thermo-physical properties of the AC charged eutectics render them elegant candidates for cold storage applications.

Keywords: Eutectic PCM; Activated Carbon; Sub cooling; Crystallinity; Latent heat.

1. Introduction The hunger for electrical energy on a daily basis is reflected in terms of miserable air quality and environmental degradation, which reminds us the dependence on non-renewable energy sources like thermal power plants, nuclear power plants etc. Though the electrical energy needs for the development of countries are unavoidable, alternative ways have to be identified without sacrificing human comfort. Researchers have found that the building sector emits 30% of greenhouse gases for their energy demands [1, 2]. For the past several decades, many countries have started to work on alternative renewable energy sources like wind, tidal, hydro, solar etc to

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meet their commercial energy demands. Focus is also on storing the energy retrieved from these sources. Latent thermal storage materials have a renowned name called phase change materials (PCM) and also have greater potential to store energy [ 3,4] hence attracted several researchers. PCMs having transformation temperature less than 30 °C are being used as energy savers in buildings alongside with Heating Ventilation and Air-Conditioning (HVAC) systems [5-8] towards energy conservation. Fatty acid based PCMs have certain good properties like chemical stability, thermal reliability, self-nucleation, reduced sub-cooling effect, less volume expansion during phase changes in comparison to salt hydrates, alkanes [9-10]. Despite using commercially available PCMs, two or more PCMs can be combined to form a eutectic mixture which can melt and freeze at similar temperatures respectively rather than the phase transition occurring at two different temperatures. Several works have been carried out to investigate the thermo-physical properties of fatty acid based eutectic PCMs and it has been found that there is some decrease in thermal properties of eutectics, compared to pure fatty acids [11-13]. Reinforcement of high surface energy materials as a nucleating agent helps to overcome inefficient thermal properties of pure PCMs. For example, Jianwu Zhang et al [14] have investigated capric-palmitic acid based eutectic PCMs with expanded pearlite composites and found that the latent thermal energy was reduced from 137 J/g to 88 J/g. Hadi Fauzi [15] et al have studied the use of acid based surfactant namely sodium laurate and have observed an increase in thermal conductivity by 4.44% and a reduction in sub-cooling to 0.43 °C on myristicpalmitic eutectics. There are different ways to improve thermal properties of thermal energy storage systems, namely mixtures of PCMs [16], microencapsulation of PCMs [17], and another important type is to mix nanomaterials in PCMs [18-20]. Graphene, graphene oxide, carbon nanotubes mixed with PCM materials are used now a days because of their high surface area, mechanical strength and good thermal properties [21-23]. Activated Carbon (AC) also has properties similar to such carbonaceous materials. Cannabis sativa (hemp) which grows naturally without any fertilizers at different climates, can be used for many applications including building materials, supercapacitors, medicine, cloth materials, fuel and oil [24-25]. The hemp fiber and stem biomass carbonized and activated by chemical method, can be used for energy storage applications.

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The present work focuses on the formation of new eutectic PCM with thermally credible two organic PCMs namely oleic acid and capric acid with a latent heat of 139 J/g and 155 J/g whose transformation was at 4 °C and 30.5 °C respectively. Both the PCMs exist in two different states, oleic acid in liquid state and capric acid in solid state under ambient conditions. Equal amount of the samples were mixed together with reference to the eutectic curves for low thermal energy storage applications. The heat transfer characterization was taken and sub-cooling degrees was identified from the heat transfer curve. AC was prepared via hydrothermal route followed by chemical activation using KOH. AC was added in different weight percentages to eliminate sub-cooling and to increase thermal conductivity in the eutectic mixture which can be attributed to its properties like enlarged surface area and high porosity. Thermal property increment was determined using the differential scanning calorimetry (DSC) analysis, laser flash analyzer. 2. Experimental 2.1. Materials Chemicals used, such as sulphuric acid (H2SO4), hydrochloric acid (HCl), potassium hydroxide (KOH), oleic acid (C18H36O2), and capric acid(C10H20O2) were purchased from Merck, India with analytical grade purity and were used without further purification. Hemp fibers were collected from agricultural land in Tamilnadu, India.

2.2. Preparation of activated carbon nanosheets Hemp fibers were collected from cannabis sativa, which is the precursor to obtain carbon via hydrothermal route, subsequently activated by chemical method. Hemp fiber 4.5 g was added to 75 ml of diluted sulfuric acid in a 100 ml stainless steel autoclave. It was then closed tightly and placed in muffle furnace. The temperature of the furnace was slowly increased to 200 °C at a heating rate of 5 °C/min and maintained at this temperature for 24 h. After a day the autoclave was cooled down to room temperature and the product (biochar) was collected through filtration, washed several times with double distilled (DD) water till neutral pH and dried overnight in an oven at 70 °C. Chemical activation was carried out with potassium hydroxide and biochar in the

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ratio of 1:1. The mixture was thoroughly ground in an agate mortar then heated at 800 °C for an hour at a heating rate of 5 °C/min in nitrogen atmosphere. The resultant sample was washed with diluted HCl and DD water several times till pH = 7. Finally the filtered product, activated carbon was dried in an oven at 70 °C for 12 h. 2.3. Preparation of nano (AC) dispersed eutectic PCM Binary fatty acid mixtures were formed in confined environment maintained at around 50 °C. From the eutectic curve, the proportion of the oleic and capric acid that suits for low thermal energy storage application i.e operating temperature = 10 °C were prepared. Capric acid whose phase change temperature is 30 °C was taken in a container and heated it in a round bottom flash heater. The melted capric acid was kept in a confined environment. Pure oleic acid of 50 ml quantity was taken in a container and kept in a sonicator whose bath temperature is maintained at 50 °C. Pure melted capric acid of 50 ml in quantity was added drop wise to the oleic acid. Though the low thermal energy storage applications are taken for the study, PCM mixtures are in liquid state at normal ambient temperature of 28 °C, still taken for low thermal energy storage application studies. Sonication helps in fine mixing of molecules without any agglomeration, sonication is continued for 3 to 4 h. The prepared activated carbon is a nanosheets so the weight percentage of the nanosheets are started from 0.02 wt% with an increment of 0.02 wt% up to 0.1 wt% corresponding to the amount of eutectics taken. The weights nanosheets are added to the eutectics while the eutectics were in the sonicator for the fine dispersion of nanosheets throughout the eutectics. 2.4. Prediction of eutectic point and latent heat of the PCM mixtures. Phase change temperature for binary fatty acids follow Van’t hoofs relation [12].The eutectic point of the pure eutectics is predicted using the Schroder’s equation. ln 𝑋 =

∆𝑓𝑢𝑠 ℎ 𝑅

(𝑇

1

𝑓𝑢𝑠

1

− ) 𝑇

(1)

X is the mole fraction of the major component at the temperature T, R the gas constant 8.314 J/mol K, ∆fush the molar enthalpy of fusion, Tfus is the melting temperature of the pure component. From the above equation and Fig.1, the eutectic point mole fraction of oleic and capric mixture is found to be 70:30.

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Correlation for predicting the latent thermal energy of the pure eutectic mixture is [26], 𝐻𝑚 = 𝑇𝑚 ∑𝑛𝑖=1

𝑋𝑖 𝐻𝑖 𝑇𝑖

(2)

Where Xi, Hi, Ti are mole fraction, enthalpy, phase change temperature of pure PCM. Hm, Tm are enthalpy and phase change temperature of an eutectic PCM. The enthalpy temperature curve of the eutectic PCM is shown in Fig.2, which gives the enthalpy of the mixture for various proportions. The enthalpy curve for all proportions of the eutectic mixture were predicted and the latent heat capacity of all the mixtures lies below the latent heat capacity of pure PCM. 2.5. Experimental set-up Phase change behavior of the AC dispersed eutectic PCMs during melting and freezing was experimentally investigated using the well-insulated constant temperature bath set-up. The constant temperature bath set-up encompasses of refrigeration unit and high sensitive heater for attaining the set point temperature in the bath which is maintained at the desired value with the assistance of control unit coupled to the cooling and heating units. The control unit can maintain the set temperature up to ± 1.5 degrees. The heat transfer fluid was isopropanol for cooling process and water bath for heating process which can go up to -90 °C on cooling and 100 °C on heating without any phase change inside the bath. Nano eutectic PCM was encapsulated inside the balls and the t-type thermo-couples were fitted to the center of the PCM balls to monitor the temperature variation of the PCM. During freezing process, the constant temperature bath was filled with pure isopropanol and the temperature of the isopropanol bath was maintained at -5 °C and the PCM balls fitted with thermo-couples were submerged fully into the bath. Outer surface of the PCM balls was maintained at constant temperature of -5 °C and uniform distribution of the temperature throughout the bath was obtained with the help of a stirrer. This was left undisturbed till the PCM temperature reached 0 °C because the predicted phase change temperature of the eutectic PCMs was 10 °C which was found using the eutectic curves. During the melting process, the PCM balls fitted with thermo-couples were submerged fully in the bath maintained at 30 °C, and was left undisturbed till the PCM reaches 30 °C. This has been repeated for all the mixtures of activated carbon to eutectic. The temperature profile data of the nano enhanced eutectic PCMs were saved and retrieved.

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2.6. Characterization methods of AC nanosheets The structural properties of the AC was analyzed by Bruker's D2 PHASER X-ray Diffraction (XRD) with 1-D LynxEye detector using Cu K radiation =0.1540 nm in the range 5° to 80° with a step size 0.02 at the speed of 0.5 °/min.The morphology of the prepared AC was observed using SU6600 Hitachi Field Emission Scanning Electron Microscopy (FESEM) and the EMAX Horiba Elemental Dispersive X-ray Spectroscopy confirms the presence of elements. Raman spectroscopy was recorded by confocal WiTech Raman spectrometer. The optical absorption spectra was recorded using double beam UV-vis spectrophotometer (UV-1800, Shimadzu) and the functional group analysis was done by Bruker ALPHA Fourier Transform Infrared Spectrometer (FT-IR) in the range of 500 - 4000 cm-1. 2.7. Heat transfer characterization techniques for pure and nano enhanced PCM The freezing and melting characterization of pure and nano reinforced PCMs were carried out in the constant temperature bath setup. The thermal energy storage capacity of the composite PCMs were tested using the DSC Q200 V23.10 Build 79 instrument cooled and heated at the rate of 5 °C/min which gives the exothermic and endothermic curve of the PCM. Thermal conductivity of the composite PCMs were tested using NETZH laser flash analyzer for both solid and liquid state of the composite PCM.

3. Results and discussion The reinforcement of AC towards thermo-physical property enhancement on base eutectic PCM responds well. Noteworthy enhancements in the energy saving time (Heat transfer enhancement), crystallinity behavior, thermal conductivity, thermal expansion behavior, chemical stability are presented below. 3.1. Morphological, crystallinity & chemical stability characterization for nano eutectic PCMs Fig.3.a.shows FE-SEM images of activated carbon derived from hemp biomass via hydrothermal route. Sulfuric acid mixed with double distilled water used as the solvent for the hydrothermal method causes the breakup of the fiber into smaller pieces. The 3-dimensional FE-

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SEM images shows large surface area with high macroporosity. In addition to that, the energy dispersive X-ray spectrum (EDX) shown in Fig.3.b. indicates the atomic composition of carbon particles to an extent of 100% suggesting the highly crystalline nature. Raman spectra confirms the structure of activated carbon nanosheets, shown in Fig.3.c. The results exhibit that the Dband is placed at 1345 cm-1 which is due to the defect or disorder while G-band is placed at 1600 cm-1 and is due to an in-plane vibrational mode. The intensity of the G-band is higher than that of D-band also ID/IG ratio is 0.91 which indicates higher conductivity. The UV visible absorption spectrum of activated carbon nanosheets is shown in Fig.3.d. with a maximum absorption peaks at 279 nm. X-ray diffraction pattern of activated carbon shown in Fig.3.e. The peaks at 2=23o, 44.2o corresponds to the (002) and (100) diffractions of activated carbon nanosheets which is similar to that of graphene nanosheets. The diffraction pattern can be indexed to a hexagonal crystal system with JCPDS No: 00-008-0415. 3.2. FT-IR analysis of OA, CA, eutectic mixture and AC in eutectic PCMs: To investigate the chemical interaction between oleic acid, capric acid and the as prepared eutectic mixture, the transmittance spectra of different weight percentage of AC dispersed eutectic mixtures were recorded and is shown in Fig.4.a. & 4.b. The peaks obtained at 2921-2922 and 2855-2858 cm-1 are attributed to CH2 asymmetric and symmetric stretching, peaks at 1707 and 1284 cm-1 corresponds to C=O and C-O stretching and peaks at 1454 and 933 cm-1 are assigned to the in plane and out of plane bending of O-H [27]. Small differences in the structure and constitutions of samples are identified by observing the finger print region in IR spectra. Fig. 4.b. shows differences at 1111, 722 and 525 cm-1 between oleic acid, capric acid and eutectic mixtures which evidences the change in spectra. 3.3. Phase transformation and degree of sub-cooling analysis Heat transfer characterization of the pure and nano enhanced eutectic PCM was carried out to find the solidification and melting rate variation of the PCMs in accordance to the nano particle dispersion. The solidification of the nano reinforced PCMs was tested in the constant temperature bath. The solidification starts whenever the temperature around the PCM balls go lower than the phase transformation temperature of the PCM. The heat energy of the liquid eutectic PCMs were transferred to the constant temperature isopropanol bath which is reflected

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back on the temperature profile of the PCM. The sensible temperature drop of the PCM indicates the heat transfer and the constant temperature denotes the heat storage capacity of the eutectic PCM. Incongruity was noted on the solidification curve with phase change temperature of 10 °C, shown in the Fig.5.and also the sub-cooling u-bend with sub cooling of 2 oC appears readily between the sensible temperature drop and the constant temperature phase change region. Addition of the highly porous activated carbon to suppress the sub-cooling effect in the eutectic PCM, also increases thermal conductivity which is discussed in forth coming sections. Highly dispersed nucleating agent namely AC has high surface energy because of the sheet like structure and its non-settling properties has linearly reduced the degree of sub-cooling for every proportion. The percentage of decrement in the degree of sub-cooling for 0.02, 0.04, 0.06, 0.08 wt% of the mixture was 30%, 37%, 45%, 63% in comparison to the pure eutectic PCM. But for the 0.1 wt% of the activated carbon mixture, perfect solidification curve was observed without any u-bends due to the nucleation effect offered, helps the uniform crystal formation to grow over them. The melting graph also shows same thermal behavior with increasing time saving with enhanced heat transfer properties as depicted in the Fig.6. The time taken for complete freezing and melting of the composite PCM has also been reduced and is shown in Fig.5&6. The percentage of decrement in time for 0.02, 0.04, 0.06, 0.08, 0.1 wt% of AC in eutectic PCM was 27%, 40%, 45%, 49%, 54% compared to the pure eutectic PCM.

3.4. Thermal storage capacity of eutectic PCM Nucleation rate of the PCMs are highly influenced by the thermal environment and the nuclei formation. The nuclei formation throughout the PCM will act as the core for crystalline growth during solidification. The DSC curve depicts the thermal energy storage capacity of the eutectic PCM and resistances offered by the nano particles in latent heat storage capacity. The mole fraction of the oleic to capric mixture used here was 37:63 and has been taken as the pure eutectic in this study. The addition of the AC to the pure PCMs helps to reduce the extra energy consumed while initial crystal formation in the PCM. The activated carbon added to the pure form of the PCM enhances the thermal conductivity which is reflected on the melting and freezing graph and have also been proved from thermal conductivity measurements, shown in the fig.8. Owing to the presence of the two PCMs in the mixture, the presence of secondary peak

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was unavoidable. DSC characterization has confirmed the phase change behavior of the eutectic PCM and latent heat storage capacity of the pure PCM was found to be 60 j/g which starts to reduce for increasing mass percent of nano particles, depicted in Fig.7. The thermal energy stored in the secondary peak of the pure eutectic was negligible and was 8 j/g with increase the mass of AC in the eutectic PCM there is a starts reduction of the latent thermal energy of the eutectic PCM. Up to 5% decrement was observed for the various proportions, which directly projects the higher nucleation rate of the eutectic PCM. This decrease in the latent thermal energy of the eutectic PCM was also due to the physio-chemical changes. DSC test for the 0.1 wt% AC in eutectic pcm has been carried out after 200 thermal cycles, no considerable change in the latent thermal energy was observed. The DSC Table 1 infers that the crystallinity is getting reduced from 100% to 95.4% for pure PCM to 0.1 wt% nano enhanced PCM. No considerable decrement in latent thermal energy storage has been observed. The crystallinity was calculated using the correlation [28]. 𝐹𝑐 =

∆𝐻𝑐𝑜𝑚 ∆𝐻𝑝𝑢𝑟𝑒 ×β

× 100

(3)

∆HCom is the latent heat of the composite PCM, ∆HPure is the latent heat of pure PCM, β is the eutectic PCM over the AC.

3.5. Thermal conductivity analysis Nano particles dispersed to eutectic PCM act as a nucleating agent and increases the thermal conductivity and is evidenced from the change in melting and freezing time. The time taken by the pure eutectic PCM is around 550 seconds and for 0.1 wt% nano dispersed eutectic PCM it is 250 seconds, proves that temperature of the heat transfer fluid and heat transfer area remains constant. The only factor that depicts the heat transfer rate of PCM is thermal conductivity, which is indirectly reflected by linear decrement in the time taken for melting and freezing of PCM. The nucleation rate of the pure eutectic has been enhanced by AC. Laser flash analyzer also confirms the thermal conductivity enhancement of nano dispersed eutectic PCM, the observed improvement in the thermal conductivity of liquid eutectic PCM was 12%, 32%, 35%, 45%, 55% in comparison to the pure eutectics having thermal conductivity of 0.194 w/mk

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(liquid) and 0.201 w/mk (solid). The thermal conductivity for pure and nano dispersed PCM is shown in the Fig.8. Pure and all proportions of nano enhanced eutectic PCM has been tested for their volume expansion co-efficient by measuring the volume at two different temperature T1= 0 °C and T2 = 30 °C. The thermal expandability of the pure and nano enhanced PCM was predicted using the correlation [29]. α = [𝑉

(𝑉2 −𝑉1 )

1 ×(𝑇2 −𝑇1 )]

(4)

Where V1 & V2 are the volume at two different temperatures Thermal expandability of the PCMs are depicted in the Fig.9 and the non-settling behavior of the AC in base fluid due to the buoyancy force offered by the Sheet like structure of AC, shown in the Fig.10. Slight increment in thermal expandability was observed but the increment has been considerable while implementing it in the building energy saving applications. 3.6. Application The eutectic formed with completely eliminated sub-cooling is readily eligible for banana ripening cold storage application. As far as the implementation of thermal energy storage materials are necessary for energy saving purposes, materials with suitable phase change temperature are to be selected in accordance to the evaporator temperature and the cold storage temperature. The banana ripening plant requires around 16 to 21 oC for better ripening of bananas. So the grill temperature of the refrigeration plant was maintained between 4 to 6 oC & the phase change temperature of the PCM selected might be around 8 to 12 oC. The newly prepared eutectic PCM might be implemented for energy conservations. Schematic representation of the cold storage plant with thermal storage material with feedback system is depicted in the Fig.11. These systems have a capacity to reduce energy consumption and act as an alternative source of energy for banana ripening during electrical shut down.

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4. Conclusion Eutectic PCM mixture for cold storage applications was obtained by combining pure oleic and capric acid. The phase change temperature of the pure eutectic PCM has been found to be 10 °C with 2 °C sub-cooling in it. The sheet like porous structure of the prepared AC was characterized by SEM analysis and FT-IR analysis and further dispersed into eutectic PCM. The following thermo-physical property variations in accordance to different proportion of activated carbon is given below 1. XRD results shows that the synthesized AC is crystalline in nature and the FT-IR results shows that the incorporation of AC into eutectic PCM is purely physical and no chemical changes occur, hence leading to long term chemical stability. 2.The time saving during thermal cycle for nano dispersed eutectic PCM was 27%, 40%, 45%, 49%, 54% for 0.02, 0.04, 0.06, 0.08, 0.1 wt% of AC in Eutectics in comparison to pure eutectic PCM was observed. 3. The thermal conductivity of liquid eutectic PCM was enhanced by 12%, 32%, 35%, 45%, 55% in comparison to the pure eutectics whose thermal conductivity values are 0.194 w/mk (liquid) and 0.201 w/mk (solid). 4. The decrement in degree of sub-cooling for 0.02, 0.04, 0.06, 0.08 wt% of the mixture was 30%, 37%, 45%, 63% as observed in comparison to pure eutectic PCM but 0.1 wt% AC in the eutectics completely eliminates the sub-cooling effect in the eutectic PCM. 5. The thermal expandability of the newly formed eutectic PCM was found to be around 3 × 10-3 °C-1 which makes the eutectic PCM practically implementable in low-temperature building energy storage applications. This paper reports the formation of new eutectic PCM for low temperature thermal energy storage applications. Thermo-physical properties of pure and nano dispersed eutectics were discussed in detail alongside with their theoretical correlations. Schematic representation of the PCM integrated cold storage plant has also been discussed.

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Acknowledgement The authors gratefully acknowledge DST, New Delhi for providing financial support to carry out this

research

work

under

DST-

CERI

and

PURSE

scheme

(

DST

File

No.

TMD/CERI/BEE/2016/038 (G). One of the authors, Mr. S. Imran Hussain is thankful to DST, New Delhi for the award of DST-PURSE fellowship.

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Fig. 1. Theoretical eutectic graph for binary mixture

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Fig. 2. Theoretical enthalpy graph for binary mixture

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Fig.3. a. Field emission scanning electron spectroscopy (FE-SEM) b. Energy dispersive X-ray (EDX) spectra c. Raman spectroscopy d. UV-Visible absorption spectroscopy e. X-ray diffraction (XRD) of as prepared activated carbon nanosheets

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Fig. 4.a. FT-IR images of pure and eutectic PCM with AC b. Enlarged FT-IR images of pure, eutectic PCM and 0.1 wt% AC eutectic PCM

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21

Fig. 5. Freezing graph for pure and nano enhanced eutectic PCMs

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Fig. 6. Melting graph for pure and nano enhanced eutectic PCMs

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Fig. 7. DSC analysis of pure and nano enhanced eutectic PCMs

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Fig.8. Thermal Conductivity for liquid/solid nano enhanced PCMs

25

26

Fig.9. Thermal Expandability of nano enhanced eutectic PCM

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Fig.10. Photographic view of well dispersed AC on eutectic PCM

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Fig.11. Schematic representation of eutectic PCM integrated cold storage plant

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Table 1. Phase change properties of pure and nano enhanced eutectic PCM. AC wt% on eutectic PCM

Onset Peak temperature ( o o temperature ( C) C)

Latent heat (J/g)

Crystallinity ( %)

Solidific ation

Melti ng

Solidifica tion

Meltin g

Pure eutectic

14.3

3.2

11.27

6.42

60

100

0.02

14.26

3.25

11.24

6.47

58.71

99.8

0.04

14.24

3.26

11.25

6.5

57.1

98.9

0.06

14.2

3.27

11.21

6.53

55.74

97.7

0.08

13.2

5

10.17

7.46

54.4

96.5

0.1

13.1

5.16

10.1

7.53

52.7

95.4

13.3

5.17

10.13

7.62

52.3

94.8

0.1 after 200 cycles

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