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Cold thermal energy storage with lauryl alcohol and cetyl alcohol eutectic mixture: Thermophysical studies and experimental investigation
Journal of Energy Storage 27 (2020) 101060
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Cold thermal energy storage with lauryl alcohol and cetyl alcohol eutectic mixture: Thermophysical studies and experimental investigation Nadiya Philipa, G. Raam Dheepb, A. Sreekumara, a b
T
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Department of Green Energy Technology, Madanjeet School of Green Energy Technologies, Pondicherry University (A Central University), Puducherry 605014, India Power Electronics School of Electrical and Electronics Engineering, Lovely Professional University, Phagwara 144411, Punjab, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase change material Thermal energy storage Thermal stability Binary eutectic
The surging demand for thermal energy storage (TES) directs considerable attention towards the development of phase change materials (PCMs). This study has the objective to develop a eutectic PCM for effective cold thermal energy storage. The work involves preparation, thermophysical property analysis and experimental studies of a novel binary eutectic mixture. The technique of Differential Scanning Calorimetry is used in determination of thermophysical properties of the prepared eutectic. Results show that the 80:20 eutectic composition of lauryl alcohol and cetyl alcohol with melting temperature of 20.01 °C and latent heat of 191.63 J g−1 is fit for use in cold thermal energy storage. The material also proves to have good thermal conductivity. Thermal stability and reliability studies which are carried out using accelerated thermal cycling tests and thermo gravimetric analysis provide positive reports for the new mixture. An experimental investigation on the effectiveness of the identified PCM is carried out by using it in a cold storage chamber. Experimental studies report a solid improvement in cold energy containment of the PCM incorporated storage chamber resulting in considerable energy savings.
1. Introduction The escalating need for energy compel novel ways for its generation and storage. Abundant energy sources such as solar energy to be efficaciously made use of, demands an inevitable vital component: an efficient and economical heat storage system. TES gains importance when heat demand and production do not match. In TES, storage media is heated during a charging period and stored heat is released when required, which facilitates its application in buildings and in solar applications such as concentrated solar power plants, solar water heaters, air heating systems, etc. [1–3]. Of the different available TES systems, latent energy storage systems (based on PCMs) with solid to liquid phase transition are preferred namely due to their typical high energy storage density and heat storage at a constant phase transition temperature. Various aspects of TES system has been reported in the literature [3–6] based on its classifications, viability, accelerated thermal cycle tests, encapsulation, etc. Naturally available organic phase change materials (PCM) such as methyl esters, fatty acids and alcohols have fixed phase transition temperatures and thereby limited applications. The melting temperature of these materials maybe higher or lower than the temperature which an application requires. By forming eutectic mixtures, phase change materials can be developed in the desired temperature range of ⁎
application. Thermal degradation and latent heat of fusion are other parameters which fall within required range of the desired application [6,7]. Studies incorporating PCMs in cooling applications show overall improvement in the system performance [8–11]. A limited number of potential PCMs are available for use in building thermal comfort application, which has directed the attention towards the development of eutectic PCMs. Yajun Lv et al. investigated a eutectic mixture of lauric acid and stearic acid (75.5:24.5) to obtain a melting temperature of 34.16 °C and a latent heat of fusion of 167.3 kJ kg−1 for use in building applications [12]. The performance of a eutectic mixture prepared from methyl palmitate and lauric acid (60:40) with a melting temperature of 25.6 °C and a latent heat of fusion of 205.4 kJ kg−1 was evaluated for use as a potential phase change material by Saeed et al. [13] Thermal analysis for the same was carried out by DSC and important thermophysical properties were determined. A eutectic mixture of lauric acid and myristic acid (66:34) was investigated by Sedat et al. and the eutectic was experimentally found to possess a melting temperature of 34.2 °C and latent heat of fusion around 166.8 kJ kg−1, ideal for low temperature solar energy storage applications [14]. A capric acid and cetyl alcohol (70:30) binary eutectic phase change material with melting temperature of 22.89 °C and freezing temperature of 11.97 °C (with latent heats of melting and freezing being 144.92 and 145.85 kJ kg−1) was developed for cold
Corresponding author. E-mail address: [email protected] (A. Sreekumar).
https://doi.org/10.1016/j.est.2019.101060 Received 17 September 2019; Received in revised form 25 October 2019; Accepted 1 November 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
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among the existing eutectics that has a phase transition temperature of 20.01 °C. On confirmation of the thermophysical properties of the identified PCM as according to the desired characteristics, subsequently the phase change material is tested for application in cold energy storage. The conducted experimental study aims to analyze the improvement of cold energy storage system by the addition of phase change material into the cold chamber of an existing refrigeration system. As no prior reports exist on application studies of lauryl alcohol and cetyl alcohol combination to prove its potentiality for thermal energy storage, the present work is novel. Advantages such as non-toxicity, availability and affordability of the eutectic PCM constituents present the applicability of the present work. Therefore, the study is a pioneer report on identification and experimental analysis of the novel PCM in building applications.
storage by Veerakumar and Sreekumar [15]. Thermal stability and corrosion analysis tests were also carried out. Acetanilide/benzoic acid PCM (30:70) having melting point of 75.56 °C and latent heat of fusion of 193.56 kJ kg−1 was identified by Khushboo Purohit for thermal energy storage in a solar water heating system [16]. The thermal performance of a new eutectic PCM comprising caprylic acid/1-dodecanol (70:30) with a melting temperature of 6.52 °C and a latent heat of fusion of 171.06 kJ kg−1 was investigated by Zuo and found suitable for application in cold storage [17]. The phase diagram and thermophysical properties of erythritol-urea eutectic mixture (54.9:45.1) was studied by G. Diarce for use as a phase change material [18]. The eutectic has a melting temperature of 81.1 °C and a latent melting enthalpy of 248 kJ kg−1. A low temperature PCM with good thermal stability and reliability based on stearic acid and hexanamide (46.2:53.8) with a melting temperature of 331.15 K and a latent heat of 176.62 J g−1 was studied by Guixiang Ma for thermal energy storage [19]. Thermal properties of a binary system of lauric acid and 1-tetradecanol (40:60) with melting point of 24.33 °C and latent heat of 161.45 kJ kg−1 were investigated for cold thermal energy storage by Zuo [20] Lauric, myristic and palmitic fatty acids were found to have eutectic combinations with 1-dodecanol (29:71, 17:83 and 10:90) [21]. Melting points and latent heat of fusions of the eutectics were found as 17, 18.43, 20.08 °C and 175.3, 180.8, 191 kJ kg−1, respectively. The aim of conducting repetitive thermal cycles is to ensure the stability and reliability of PCM over long term usage. An experimental analysis was carried out on the thermal stability of capric acid with lauric acid, palmitic acid and myristic acid respectively of which capric and myristic acid combination was shown to have the best thermal stability over a period of 2000 thermal cycles [22]. Experimental investigations had been carried out on the thermal stability of a paraffin mixture to about 10,000 cycles [23]. Heat of fusion degeneration was found as 9.1% while the melting point was not much affected. A review on thermal cycling tests conducted on various PCMs were reviewed by Rathod et al. [24] The detailed literature review on eutectic phase change materials shows that, numerous research activities are being focussed on identification and analysis on thermo-physical properties of novel eutectic phase change materials for heating and cooling applications. However, it has been noticed that only a limited number of literatures are based on development of PCMs with phase transition temperatures in the range 19–29 °C, suitable for building cooling applications. Accordingly, this study has been focussed to identify a new eutectic PCM, befitting human comfort in buildings. Table 1 shows summary of the reviewed eutectic PCMs with their phase transitions temperature. The major contribution of this research lies in identification of a binary eutectic combination (lauryl alcohol and cetyl alcohol) that has been unexplored till date. The eutectic composition of the new combination is determined to be 80:20. As highlighted in the summary presented in Table 1, the identified eutectic is the only combination
2. Materials and methods 2.1. Materials Lauryl alcohol (C12H26O) is a colourless liquid obtained by reduction of fatty acids of coconut oil. Cetyl alcohol (C16H34O) is a fatty alcohol often in the form of white solid. Lauryl alcohol (98% pure) and cetyl alcohol (99% pure) were purchased from LOBA Chemie, India. A summary of properties of the two PCMs used are presented in Table 2. 2.2. Preparation of lauryl alcohol-cetyl alcohol (LC) eutectic For thermal property studies, the eutectic point of the mixture was to be determined experimentally. The pure components were considered to be mixed in the mass ratio 0–100% at 10% intervals. The individual components were weighed using an analytical balance (SHIMADZU AW320). Each mixture was heated to a temperature of 80 °C and stirred properly. It was then subjected to probe sonication (PROBE SONICATOR, Model No. PRO-250, Make: LABMAN) for a period of 5 min. Then mixtures were cooled to room temperature and stowed. 2.3.Characterization. of the binary mixture 2.3.1. Differential scanning calorimetry (DSC) DSC thermal analysis was used in the determination of the melting point and the enthalpy of (i) pure materials (ii) binary eutectic. Samples around the mass composition of the theoretical eutectic point were considered for analysis. A DSC 6000- PerkinElmer was used in performing DSC analysis by the principle of heat flux. 10 mg of the sample is taken for analysis. Heating and cooling is done at a rate of 10 °C min−1. Samples were subjected to heating and cooling between 0 and 100 °C. Thermal properties of the mixture as its latent heat and phase transition temperature were determined.
Table 1 Summary of reviewed eutectic PCMs. Eutectic PCM
Composition
Melting temperature [°C]
References
Lauric acid-Stearic Acid Methyl palmitate-Lauric acid Lauric acid-Myristic acid Capric acid-Cetyl alcohol Acetanilide-Benzoic acid Caprylic acid-1-dodecanol Erythritol-Urea Stearic acid-Hexanamide Lauric acid-1-tetradecanol Palmitic acid-1-tetradecanol Lauric acid-1-tetradecanol Myristic acid-1-tetradecanol Lauryl alcohol-Cetyl alcohol
75.5:24.5 60:40 66:34 70:30 30:70 70:30 54.9:45.1 46.2:53.8 40:60 10:90 29:71 17:83 80:20
34.16 25.60 34.20 22.89 75.56 6.52 81.10 58.15 24.33 20.08 17.00 18.43 20.01
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [21] [21] Present study
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Table 2 Thermophysical properties of lauryl alcohol and cetyl alcohol [15,25]. Property
Lauryl alcohol
Cetyl alcohol
IUPAC name Molecular weight [g mol−1] Melting point [°C] Latent heat [g mol−1]
1-Dodecanol 186.34 25.83 215.83
1-Hexadecanol 242.45 52.14 207.39
2.3.2. Thermal conductivity test Thermal conductivity studies of the eutectic mixtures were done by transient line heat source method (MODEL: KD2 Pro). The KD2 Pro analyser is a combination of a microcontroller and a KS-1 needle sensor. The needle sensor which is used has a sufficient length to diameter ratio (60:1.27) to serve as an infinitely long and thin heating source. The transient method works by recording the temperature variations (every 1 s) in response to a consistent heating pulse and a successive cooling pulse, applied for 30 s each, on the sensor. Sensor is positioned vertically in the glass container which holds the sample (50 ml) to be tested. Thermal properties of the tested liquid mixture are the determinants of its temperature response. Recorded temperature is processed by the 16 bit microcontroller and results are displayed. Conductivity measurements were taken for 10 °C, 20 °C, 30 °C and 40 °C.
Fig. 1. (a) Cold chamber test setup (b) Inner view of cold chamber.
0.74 kg h−1). The following studies were conducted on the cold storage chamber (shown in Fig. 1), to analyze the effectiveness of the newly identified eutectic PCM in cold energy storage applications:
• Without PCM, the discharge performance of the system was studied under natural and forced convection modes. • With different PCM loads, the discharge performance of the system was studied under a constant discharge mass flow rate
2.3.3. Thermogravimetric analysis (TGA) TGA test (TGA 4000-PerkinElmer) was used to analyze the thermal stability of the lauryl-cetyl alcohol eutectic. TGA is based on analysing the deviation in weight of the sample with respect to time, in a controlled environment. A sample of weight 10 mg is kept on a pan that is supported by a precision balance (0.01% precision). The pan carrying the sample, is placed inside a furnace and subjected to progressive heating from 25 °C to 300 °C at 10 °C min−1 scan rate. Inert nitrogen atmosphere is used so that the sample shows reaction only during decomposition temperature. By examining the TGA curve, decomposition temperature can be determined from the region in the plot where a drastic drop in weight of sample occurs.
3. Results and discussion 3.1. Pure components Figs. 2 and 3 present the endotherms and exotherms obtained on DSC analysis of pure lauryl alcohol and pure cetyl alcohol. Endotherm corresponding to melting is obtained on heating the sample while the exotherm corresponding to freezing is obtained while cooling the sample. DSC analysis for pure lauryl alcohol reveals melting and freezing temperatures to be 25.91 °C and 14.56 °C respectively with enthalpy changes during the processes as 217.49 and 220.40 J g−1. The smaller peak near the freezing point of the mixture indicates phase transformation (solid to solid). In case of cetyl alcohol, one endotherm is observed on heating and two exotherms are observed on cooling the sample. Cetyl alcohol as observed from the DSC curve shows a melting point at 51.50 °C and the freezing temperatures of 43.97 and 38.09 °C respectively. Two exotherms are seen on cooling curve. First peak
2.3.4. Accelerated thermal cycling test Accelerated thermal cycling was used to study the thermal stability of the eutectic mixture. 10 g of the eutectic mixture is sealed in a test tube and was subjected to repetitive heating and cooling cycles (using water as the heat transfer fluid). To ensure melting and solidification of the PCM during heating and cooling, the heat transfer fluid was subjected to temperature in the range 5–60 °C. Thermal cycling was done for a total of 1000 cycles. After 500 and 1000 cycles, samples each weighing 2 g were taken out for DSC thermal analysis. 2.4. Experimental charge-discharge studies on LC binary eutectic The new binary eutectic, was tested for use as a phase change material for application in cold energy storage. The study aimed to analyze the increase in time of free coldness in the chamber of an existing refrigeration system by the addition of phase change material. The rectangular chamber in which experiment was conducted had a 0.25 TR capacity with inner dimensions 60 cm × 40 cm. Cylindrical metallic containers were chosen for the purpose of storing the PCM in the refrigeration chamber. The container used had a radius of 2.25 cm and length of 28 cm. Considering 80% of each cylinder to be filled with PCM, the total mass of PCM required to fill 10 containers was calculated approximately as 3 kg (components lauryl alcohol and cetyl alcohol are taken in the eutectic ratio of 80:20). Lauryl alcohol (2.4 kg) was added to cetyl alcohol (0.6 kg); heated, stirred and ultrasonated for the eutectic composition preparation. The prepared eutectic was filled into 10 cylinders for testing in the cold storage chamber. Discharging was carried out by natural and forced convection modes (mass flow rate of
Fig. 2. DSC curve of lauryl alcohol. 3
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Fig. 3. DSC curve of cetyl alcohol.
depicts liquid to rotator phase transition at 43.97 °C and second peak shows rotator phase transition to a crystalline structure at 38.09 °C [26,27]. 3.2. Lauryl alcohol-cetyl alcohol binary mixture
Fig. 4. DSC heating curves of lauryl alcohol-cetyl alcohol binary mixtures of different mass compositions.
The molar ratios of the individual compounds in the mixtures for the eutectic composition are theoretically estimated by Schröder–van Laar equation given as Eq. (1).
ln xk =
−ΔHkm ⎛ 1 1 − k ⎞⎟ ⎜ R ⎝T Tm ⎠
Table 3 Peak temperatures of lauryl alcohol-cetyl alcohol binary mixtures.
(1)
in which xk is the molar ratio of k-component, ΔHkm is the melting enthalpy (J mol−1), T and Tkm are the melting temperature values corresponding to pure component and mixture (K) and R is the universal gas constant (8.31 J mol−1 K−1). The eutectic phase transition temperature of the combination of lauryl alcohol and cetyl alcohol, in the mass ratio 85:15, was calculated to be 23.50 °C (by Eq. (1)) and is lower than melting points of both lauryl alcohol and cetyl alcohol. Thermograms obtained by DSC analysis are shown in the Fig. 4. Every thermogram shows two endotherm peaks. First peak occurs at an almost constant temperature (around 20 °C) for every composition considered. This depicts an invariant phase transition point called the eutectic point [15]. Second peak's temperature varies with the mass composition of the pure components becoming larger with the increasing percentage of one of the components and its peak temperature reaches melting temperature of the pure component. The eutectic temperature of 20.01 °C agrees with the theoretically calculated temperature. The peak temperatures obtained by DSC analysis of different mixture compositions are given in Table 3. The DSC curve corresponding to the lauryl alcohol-cetyl alcohol eutectic mixture (80:20) is shown in Fig. 5. The melting temperature and freezing temperature of the eutectic are 20.01 °C and 12.04 °C respectively with corresponding latent heats being 191.63 J g−1 and 189.51 J g−1. Supercooling refers to solidification of a liquid below its freezing point. It is largely dependent on factors as cooling rate as well as size, homogeneity and purity of sample. The extent to which a sample is supercooled is the difference between its freezing temperature and the actual temperature of initial crystallization. From the DSC curve (Fig. 5), degree of supercooling of the lauryl alcohol-cetyl alcohol eutectic is only around 2.2 °C. However, if a material has its degree of supercooling as high as 5 °C or more it is unsuitable in potential
Lauryl alcohol
Cetyl alcohol
Second peak [°C]
First peak [°C]
0 50 60 70 80 90 100
100 50 40 30 20 10 0
51.50 37.07 32.71 20.86 20.01 22.53 25.91
– 20.64 21.71 – – – –
Fig. 5. DSC curve of lauryl alcohol-cetyl alcohol eutectic mixture.
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Results of the 0th cycle can be used as reference for comparing with thermophysical characteristics after 500 and 1000 cycles [15]. The lauryl-cetyl eutectic showed an increase in melting point from the initial 20.01 to 21.44 °C after a total of 1000 complete thermal cycles along with decrease in latent heats of fusion from 191.63 at the eutectic point to 163.36 J g−1 after 1000 cycles. Considering the cooling curves, the freezing temperature reduced from an initial 12.04 °C to 11.10 °C after 1000 cycles with latent heats diminishing from 189.51 to 184.10 J g−1 by the end of 1000th cycle. The relative percentage difference (RPD) of melting points and latent heats of fusion after 500th and 1000th cycle can be calculated using Eq. (2).
RPD =
Xn, i − X0, i × 100% X0, i
(2)
in which i is the eutectic PCM property that is considered (melting point or latent heat of fusion) and Xn,i and X0,i are corresponding values after nth cycle and 0th cycle, respectively. The calculated melting point and latent heat of fusion RPDs are presented in Table 4. Negative RPD values for melting point and latent heat of fusion denotes a decrease in the respective values when compared to the uncycled eutectic. Upon completion of 1000 thermal cycles, there was not much disparity in melting point and latent heat when compared to the uncycled sample. Hence, the new eutectic having found to possess good thermal stability for more than a 1000 cycles, promises the performance of a good thermal energy storage material.
Fig. 6. Phase diagram of lauryl alcohol-cetyl alcohol binary system.
building comfort applications which require systems with long life. This is because a high degree of supercooling leads to reduced performance of the PCM thermal energy storage system due to: requirement of a large operating temperature range and chances of phase separation. Supercooling and phase separation causes thermal cycling degradation which eventually limits the life of eutectic. In cases of materials with high supercooling, techniques as addition of nucleating and thickening agents and adoption of macro encapsulation over microencapsulation are adopted to reduce it [1,9]. Peak temperatures of the endotherms obtained experimentally are plotted against different mass fractions of lauryl alcohol to obtain the phase diagram. From the phase diagram, it can be observed that the melting point of the binary mixtures first decreases and then increases with the mole fraction of lauryl alcohol. From the phase diagram of the lauryl alcohol- cetyl alcohol binary system given in Fig. 6, the temperature at the lowest point of the graph in the lauryl-cetyl alcohol is 20.01 °C and the corresponding mass fraction of lauryl alcohol is 0.8. The enthalpy of fusion at the eutectic point is 191.632 kJ kg−1. The new eutectic PCM has a suitable phase transition temperature and a high latent heat of fusion thus proving suitable for consideration in cold thermal energy storage.
3.4. Thermal conductivity Higher the material thermal conductivity, higher will be rate of transfer of heat. Thermal conductivity of the developed binary phase change material till temperature for application is shown in Fig. 8. Thermal conductivity of the organic liquids are generally low and within the range 0.1 to 0.2 W m−1 K−1 [28]. The new eutectic also shows thermal conductivity in this range. There are no significant variations in their thermal conductivities with respect to temperature within the range under study. 3.5. TGA From TGA analysis, mass loss with respect to increase in temperature can be found. Results (shown in Fig. 9) point out that, the new binary eutectic of lauryl-cetyl alcohol starts decomposition around 169.69 °C. Probable thermal fluctuations and any resulting temperature rise will not affect the new PCM stability as, the decomposition temperature is confirmed to be several times higher than its melting point (20.01 °C). This confirms the thermal stability of PCM for application in cold thermal energy storage.
3.3. Accelerated thermal cycling Melting point and latent heats of the eutectics were measured after 500 and 1000 cycles by DSC analysis and results are shown in Fig. 7.
Fig. 7. DSC curve of lauryl alcohol-cetyl alcohol eutectic mixture after (a) 0 cycle (b) 500 cycles (c) 1000 cycles. 5
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Table 4 Variation in melting point and latent heat of the eutectic after thermal cycling. Cycles
Melting point [°C]
RPD of melting point [%]
Latent heat of fusion (LHF) [J g−1]
RPD of LHF [%]
0 500 1000
20.01 21.39 21.44
– 6.90 7.15
191.63 174.55 163.36
– −8.91 −14.75
Fig. 8. Thermal conductivity of the lauryl-cetyl alcohol eutectic at different temperatures.
Fig. 9. TGA curve of lauryl alcohol-cetyl alcohol eutectic mixture. Fig. 10. Charging characteristics (a) without PCM (b) with 3 kg PCM.
3.6. Performance analysis of experimental setup using LC PCM the bottommost portion being the coldest. Prior to the cooling process, temperature inside the chamber was monitored, which is close to ambient temperature. The initial phase of cooling shows a steep decline in chamber temperatures followed by a gradual decline till temperature is about 14 °C. The temperature of the chamber without PCM was brought down to 18 °C in 60 min (1 h). Experiment with 3 kg of PCM took around 210 min (nearly 3.50 h) to reach the phase transition temperature. Extra time was imparted for the addition of cold energy to the PCM material, which is available for retrieval afterwards. Charging process with new lauryl alcohol-cetyl alcohol eutectic PCM involved cooling/charging the PCM kept in cylindrical containers
3.6.1. Performance comparison with and without the use of new eutectic PCM 3.6.1.1. Charging characteristics. Charging the experimental setup without PCM involves cooling the chamber up to a temperature of around 18 °C. The charging characteristics of the experimental setup with cold storage are presented in Fig. 10. The variation of chamber temperatures at two different locations in the chamber is recorded using temperature sensors placed 1 cm and 15 cm from the bottom of the chamber. The average temperature difference between the two monitored locations was between 1 and 4 °C, 6
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Fig. 11. Discharge characteristics of chamber without PCM under (a) Natural convection (b) Forced convection (0.74 kg h−1).
Fig. 12. Discharge characteristics of chamber with 3 kg PCM under (a) Natural convection (b) Forced convection (0.74 kg h−1).
inside the chamber. Through the process of charging, the PCM stores energy in the form of latent heat. As evident from the graph, the cooling of the chamber leads to cooling of the PCM stored. The PCM temperature variations can be reasoned as sensible heat storage till about the first 70 min, as latent heat storage for the next 140 min and further again as sensible heat storage. Sensible heat storage is expressed by the falling portion in the graph where the temperature reduces with respect to time. Latent heat storage region is depicted by the almost constant temperature region in the graph. In spite of the fall in chamber temperature in this region, the PCM temperature is found to remain constant. This is evident from the drop in difference between the chamber and PCM temperatures (70–210 min duration in Fig. 10(b)). Further abatement in temperature signifies sensible heat storage in the material.
chamber till chamber temperature reached ambient temperature. Discharging was studied separately by natural and forced convection modes (at mass flow rate of 0.74 kg h−1). In case where no PCM was used, chamber reached ambient temperature in 118 min by natural convection mode. In case of forced convection, ambient temperature was attained at 101 min. Forced convection requires lesser time for discharge owing to its greater heat transfer rate. Higher the mass flow rate, greater is the rate of discharge. 3.6.1.3. Discharging characteristics with 3 kg PCM. The sole purpose of a phase change material is to store energy for later use, thus, reducing the energy demand of the system. Discharge characteristics of the lauryl alcohol-cetyl alcohol binary eutectic under different convection modes are shown in Fig. 12. In this experiment, with the embodiment of PCM (3 kg) in the cooling chamber, compared to the case without PCM, higher is the time required for discharging to take place. Table 5 summarises the results of experiment carried out in the cold storage chamber before and after the incorporation of PCM. Taking into account the disparate discharge modes carried out in the cold chamber (with and without the usage of PCM), it can be observed that using the
3.6.1.2. Discharging characteristics without PCM. The discharge characteristics of the cold chamber without PCM are depicted in Fig. 11. Experiments are conducted under both natural and forced convection modes of discharge. Discharging process was carried out by letting out the cold air in the 7
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Table 5 Summary of discharge characteristics of chamber without PCM and with 3 kg PCM under different modes of convection. Mode of convection
Without PCM Cooling Discharge time [min] time [min]
WITH 3 kg PCM Charging Discharge time [min] time [min]
Natural Forced (0.74 kg h−1)
60
210
118 101
416 402
new eutectic, the time taken for discharge is on an average 3.7 times higher than in the case where no PCM is used. This is advantageous, as, more the time it takes for the chamber temperature to reach the ambient temperature, for a longer duration the cooling effect will be retained in the refrigerator. Creditably, the newly identified lauryl alcohol-cetyl alcohol PCM used for study, by virtue of its latent heat of 191.63 kJ kg−1 permits satisfactory energy storage per unit mass thus increasing the time span of cold energy storage to more than 400 min. In the case where PCM was not incorporated in the chamber, the system required frequent charging and discharging. For domestic and industrial applications low frequency charge-dicharge cycles are suited which makes PCM incorporation in cold thermal energy storage desirable. This means that higher will be the effectiveness of a cold storage system with improvement in energy efficiency when the novel eutectic is integrated. 3.6.2. Performance analysis of different loads of new eutectic PCM in the experimental setup In this set of studies it was monitored how the charge-discharge characteristics of PCM varied with different load conditions at a constant mass flow rate. The experiment is conducted for different PCM loads of 1 kg, 2 kg and 3 kg. All three loads are subjected to similar discharge condition of forced convection with a discharge mass flow rate of 0.74 kg h−1. The temperature-time relation for the charging and discharging of the eutectic PCM is shown in Figs. 13 and 14 respectively. 3.6.2.1. Charging characteristics with different PCM loads. The difference in time taken for charging the different PCM loads is an important measure in determining the charging characteristics. For this purpose the temperature-time graphs presented in Fig. 13 can be made use of. Taking into consideration the difference in charging times from the case where no PCM has been used (Fig. 12(a)) to the cases where uniformly increasing loads have been taken (shown in Figs. 13 and 12(b)), it can be seen that the increase in quantity of PCM demands a higher time for charging/energy storage. While the cold chamber required nearly 60 min to cool without any PCM, the charging time increased to nearly 175 min, 190 min and 210 min for 1 kg, 2 kg and 3 kg loads respectively. With uniformly increasing PCM loads the time needed to charge increases almost in a linear pattern. Each of the charging graphs show a sensible heat storage period followed by latent heat storage period and the further continuation of sensible heat storage again. The chamber and PCM temperatures show similar decreasing trends except for the latent heat region where, despite the chamber temperature reduction, the PCM temperature remains constant. Ambient temperatures are also depicted for the purpose of reference.
Fig. 13. Charging characteristics for (a) 1 kg PCM (b) 2 kg PCM.
of PCM took 343 min, 2 kg of PCM took around 372 min, 3 kg of PCM took around 402 min, respectively for discharge at 0.74 kg h−1 mass flow rate. As the quantity of PCM used in the cold storage tank increases, more will be the time for which cold energy will be retained in it. The difference in time observed for even 1 kg PCM used, is nearly 4 h more than in the case where no PCM is used. Higher the quantity of PCM used, greater the cold energy that can be stored. This is evident from two factors. Firstly, the time required for PCM to charge increases with the increase in the quantity of PCM used in the cold storage chamber (more energy will be stored). Secondly, the time required for the PCM to discharge the stored cold energy increases with increase in PCM load (larger amount of cold energy to be released).
4. Conclusion
3.6.2.2. Discharging characteristics with different PCM loads. Discharge characteristics of the eutectic showing variations of chamber temperature and PCM temperature are presented in Fig. 14. Forced convection mode of discharge removes the cold energy stored faster when compared to natural convection. As the mass flow rate of discharge air is increased, the time taken by chamber and PCM to reach ambient temperature is reduced (faster discharge). While 1 kg
Thermal energy storage gains relevance when heat demand and production do not match. Cold energy storage using phase change materials is one of the highly recommended measures in the ongoing circumstance of gaining importance of energy saving and conservation. Realizing the significance of identifying newer thermal energy storage, 8
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Authors acknowledge Department of Science and Technology (DST), Government of India, for funding the project through its scheme called ‘Materials for Energy Storage’. References [1] A.B. Lingayat, Y.R. Suple, Review on phase change material as thermal energy storage medium : materials, application, Int. J. Eng. Res. Appl. 3 (2013) 916–921. [2] C. Veerakumar, A. Sreekumar, Phase change material based cold thermal energy storage: materials, techniques and applications - A review, Int. J. Refrig. (2016), https://doi.org/10.1016/j.ijrefrig.2015.12.005. [3] D.N. Nkwetta, F. Haghighat, Thermal energy storage with phase change material - A state-of-the art review, Sustain. Cities Soc. (2014), https://doi.org/10.1016/j.scs. 2013.05.007. [4] S.D. Sharma, K. Sagara, Latent heat storage materials and systems: a review, Int. J. Green Energy (2005), https://doi.org/10.1081/ge-200051299. [5] E. Oró, A. de Gracia, A. Castell, M.M. Farid, L.F. Cabeza, Review on phase change materials (PCMs) for cold thermal energy storage applications, Appl. Energy (2012), https://doi.org/10.1016/j.apenergy.2012.03.058. [6] K. Pielichowska, K. Pielichowski, Phase change materials for thermal energy storage, Prog. Mater. Sci. (2014), https://doi.org/10.1016/j.pmatsci.2014.03.005. [7] L.G. Socaciu, Thermal energy storage with phase change material, Leonardo electron, J. Pract. Technol. (2012). [8] A. Caron-Soupart, J.F. Fourmigué, P. Marty, R. Couturier, Performance analysis of thermal energy storage systems using phase change material, Appl. Therm. Eng. 98 (2016) 1286–1296, https://doi.org/10.1016/j.applthermaleng.2016.01.016. [9] Y. Pan, X. Zhao, X. Zhao, Application of the phase change material in building energy efficiency, Adv. Mater. Res. (2013) 168–171 doi:10.4028/www.scientific.net/AMR.684.168. [10] O. Ghahramani Zarajabad, R. Ahmadi, Numerical investigation of different PCM volume on cold thermal energy storage system, J. Energy Storage. (2018), https:// doi.org/10.1016/j.est.2018.04.013. [11] E. Oró, C. Barreneche, M.M. Farid, L.F. Cabeza, Experimental study on the selection of phase change materials for low temperature applications, Renew. Energy (2013), https://doi.org/10.1016/j.renene.2013.01.043. [12] Y. Lv, W. Jin, Y. Yang, H. Zhang, X. Zhang, F. Ding, W. Zhou, Experimental thermal storage research of organic binary phase change materials in building environment, Int. J. Green Energy (2017), https://doi.org/10.1080/15435075.2017.1339042. [13] R.M. Saeed, J.P. Schlegel, C. Castano, R. Sawafta, V. Kuturu, Preparation and thermal performance of methyl palmitate and lauric acid eutectic mixture as phase change material (PCM), J. Energy Storage. (2017), https://doi.org/10.1016/j.est. 2017.08.005. [14] S. Keleş, K. Kaygusuz, A. Sari, Lauric and myristic acids eutectic mixture as phase change material for low-temperature heating applications, Int. J. Energy Res. 29 (2005) 857–870, https://doi.org/10.1002/er.1111. [15] C. Veerakumar, A. Sreekumar, Preparation, thermophysical studies, and corrosion analysis of a stable capric acid/cetyl alcohol binary eutectic phase change material for cold thermal energy storage, Energy Technol. (2018), https://doi.org/10.1002/ ente.201700540. [16] K. Purohit, V.V.S. Murty, R.C. Dixit, A. Sharma, Development of an acetanilide/ benzoic acid eutectic phase change material based thermal energy storage unit for a passive water heating system, Bull. Mater. Sci. (2019), https://doi.org/10.1007/ s12034-019-1731-6. [17] J. Zuo, W. Li, L. Weng, Thermal performance of caprylic acid/1-dodecanol eutectic mixture as phase change material (PCM), Energy Build 43 (2011) 207–210, https:// doi.org/10.1016/j.enbuild.2010.09.008. [18] G. Diarce, L. Quant, J.M.Sala Campos-Celador, A. García-Romero, Determination of the phase diagram and main thermophysical properties of the erythritol–urea eutectic mixture for its use as a phase change material, Sol. Energy Mater. Sol. Cells (2016), https://doi.org/10.1016/j.solmat.2016.08.016. [19] G. Ma, J. Sun, Y. Zhang, Y. Jing, Y. Jia, A novel low-temperature phase change material based on stearic acid and hexanamide eutectic mixture for thermal energy storage, Chem. Phys. Lett. (2019), https://doi.org/10.1016/j.cplett.2018.11.003. [20] J. Zuo, W. Li, L. Weng, Thermal properties of lauric acid/1-tetradecanol binary system for energy storage, Appl. Therm. Eng. (2011), https://doi.org/10.1016/j. applthermaleng.2011.01.008. [21] R. Kumar, S. Vyas, A. Dixit, Fatty acids/1-dodecanol binary eutectic phase change materials for low temperature solar thermal applications: design, development and thermal analysis, Sol. Energy (2017), https://doi.org/10.1016/j.solener.2017.07. 082. [22] S. Mengjie, W.M. Pun, D. Swapnil, P. Dongmei, M. Ning, Thermal stability of organic binary PCMs for energy storage, Energy Procedia (2017), https://doi.org/10. 1016/j.egypro.2017.12.459.
Fig. 14. Discharge characteristics: Forced convection at 0.74 kg h−1for (a) 1 kg PCM (b) 2 kg PCM.
a new organic eutectic phase change material which can act as latent heat storage material was developed. The eutectic PCMs used in this work had not been subjected to previous studies. Characterization of the novel PCM was done using DSC, TGA, thermal reliability and thermal conductivity analysis. Results conclude that the new PCMs possess suitable melting points and latent heats, have good thermal for long term usage. Eutectic PCM was tested in a cold chamber for performance analysis. As is evident from experimental observations, even with 1 kg of the PCM, the chamber retains its coldness for extra 4 h. Also, the time required for PCM to charge and discharge increases with the increase in the quantity of PCM used in the cold storage chamber. Usage of PCM also facilitated low frequency charge-discharge cycles. An economical quantity of PCM would ensure the effectiveness of new eutectic PCM as a cold energy storage material. The newly identified eutectic is thus proposed to be appropriate for thermal energy storage in human comfort range, making it suitable for diverse applications.
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[26] H. Balance, Aspects of nanotechnology polymorphism, crystallinity and hydrophilic-lipophilic balance (HLB) of cetearyl alcohol and cetyl alcohol as raw materials for solid lipid nanoparticles (SLN), 1 (2018) 52–60. [27] T.S. Awad, E.S. Johnson, A. Bureiko, U. Olsson, Colloidal structure and physical properties of gel networks containing anionic surfactant and fatty alcohol mixture, J. Dispers. Sci. Technol. (2011), https://doi.org/10.1080/01932691.2010.488134. [28] G. Latini, M. Pacetti, The thermal conductivity of liquids — a critical survey, Therm. Conduct 15 (1978), https://doi.org/10.1007/978-1-4615-9083-5_31.
[23] L. Zhang, J. Dong, Experimental study on the thermal stability of a paraffin mixture with up to 10,000 thermal cycles, Therm. Sci. Eng. Prog. (2017), https://doi.org/ 10.1016/j.tsep.2017.02.005. [24] M.K. Rathod, J. Banerjee, Thermal stability of phase change materials used in latent heat energy storage systems: a review, Renew. Sustain. Energy Rev. (2013), https:// doi.org/10.1016/j.rser.2012.10.022. [25] V. Chinnasamy, S. Appukuttan, A real-time experimental investigation of building integrated thermal energy storage with air-conditioning system for indoor temperature regulation, Energy Storage (2019), https://doi.org/10.1002/est2.43.
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Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
Cold thermal energy storage with lauryl alcohol and cetyl alcohol eutectic mixture: Thermophysical studies and experimental investigation Nadiya Philipa, G. Raam Dheepb, A. Sreekumara, a b
T
⁎
Department of Green Energy Technology, Madanjeet School of Green Energy Technologies, Pondicherry University (A Central University), Puducherry 605014, India Power Electronics School of Electrical and Electronics Engineering, Lovely Professional University, Phagwara 144411, Punjab, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Phase change material Thermal energy storage Thermal stability Binary eutectic
The surging demand for thermal energy storage (TES) directs considerable attention towards the development of phase change materials (PCMs). This study has the objective to develop a eutectic PCM for effective cold thermal energy storage. The work involves preparation, thermophysical property analysis and experimental studies of a novel binary eutectic mixture. The technique of Differential Scanning Calorimetry is used in determination of thermophysical properties of the prepared eutectic. Results show that the 80:20 eutectic composition of lauryl alcohol and cetyl alcohol with melting temperature of 20.01 °C and latent heat of 191.63 J g−1 is fit for use in cold thermal energy storage. The material also proves to have good thermal conductivity. Thermal stability and reliability studies which are carried out using accelerated thermal cycling tests and thermo gravimetric analysis provide positive reports for the new mixture. An experimental investigation on the effectiveness of the identified PCM is carried out by using it in a cold storage chamber. Experimental studies report a solid improvement in cold energy containment of the PCM incorporated storage chamber resulting in considerable energy savings.
1. Introduction The escalating need for energy compel novel ways for its generation and storage. Abundant energy sources such as solar energy to be efficaciously made use of, demands an inevitable vital component: an efficient and economical heat storage system. TES gains importance when heat demand and production do not match. In TES, storage media is heated during a charging period and stored heat is released when required, which facilitates its application in buildings and in solar applications such as concentrated solar power plants, solar water heaters, air heating systems, etc. [1–3]. Of the different available TES systems, latent energy storage systems (based on PCMs) with solid to liquid phase transition are preferred namely due to their typical high energy storage density and heat storage at a constant phase transition temperature. Various aspects of TES system has been reported in the literature [3–6] based on its classifications, viability, accelerated thermal cycle tests, encapsulation, etc. Naturally available organic phase change materials (PCM) such as methyl esters, fatty acids and alcohols have fixed phase transition temperatures and thereby limited applications. The melting temperature of these materials maybe higher or lower than the temperature which an application requires. By forming eutectic mixtures, phase change materials can be developed in the desired temperature range of ⁎
application. Thermal degradation and latent heat of fusion are other parameters which fall within required range of the desired application [6,7]. Studies incorporating PCMs in cooling applications show overall improvement in the system performance [8–11]. A limited number of potential PCMs are available for use in building thermal comfort application, which has directed the attention towards the development of eutectic PCMs. Yajun Lv et al. investigated a eutectic mixture of lauric acid and stearic acid (75.5:24.5) to obtain a melting temperature of 34.16 °C and a latent heat of fusion of 167.3 kJ kg−1 for use in building applications [12]. The performance of a eutectic mixture prepared from methyl palmitate and lauric acid (60:40) with a melting temperature of 25.6 °C and a latent heat of fusion of 205.4 kJ kg−1 was evaluated for use as a potential phase change material by Saeed et al. [13] Thermal analysis for the same was carried out by DSC and important thermophysical properties were determined. A eutectic mixture of lauric acid and myristic acid (66:34) was investigated by Sedat et al. and the eutectic was experimentally found to possess a melting temperature of 34.2 °C and latent heat of fusion around 166.8 kJ kg−1, ideal for low temperature solar energy storage applications [14]. A capric acid and cetyl alcohol (70:30) binary eutectic phase change material with melting temperature of 22.89 °C and freezing temperature of 11.97 °C (with latent heats of melting and freezing being 144.92 and 145.85 kJ kg−1) was developed for cold
Corresponding author. E-mail address: [email protected] (A. Sreekumar).
https://doi.org/10.1016/j.est.2019.101060 Received 17 September 2019; Received in revised form 25 October 2019; Accepted 1 November 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
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among the existing eutectics that has a phase transition temperature of 20.01 °C. On confirmation of the thermophysical properties of the identified PCM as according to the desired characteristics, subsequently the phase change material is tested for application in cold energy storage. The conducted experimental study aims to analyze the improvement of cold energy storage system by the addition of phase change material into the cold chamber of an existing refrigeration system. As no prior reports exist on application studies of lauryl alcohol and cetyl alcohol combination to prove its potentiality for thermal energy storage, the present work is novel. Advantages such as non-toxicity, availability and affordability of the eutectic PCM constituents present the applicability of the present work. Therefore, the study is a pioneer report on identification and experimental analysis of the novel PCM in building applications.
storage by Veerakumar and Sreekumar [15]. Thermal stability and corrosion analysis tests were also carried out. Acetanilide/benzoic acid PCM (30:70) having melting point of 75.56 °C and latent heat of fusion of 193.56 kJ kg−1 was identified by Khushboo Purohit for thermal energy storage in a solar water heating system [16]. The thermal performance of a new eutectic PCM comprising caprylic acid/1-dodecanol (70:30) with a melting temperature of 6.52 °C and a latent heat of fusion of 171.06 kJ kg−1 was investigated by Zuo and found suitable for application in cold storage [17]. The phase diagram and thermophysical properties of erythritol-urea eutectic mixture (54.9:45.1) was studied by G. Diarce for use as a phase change material [18]. The eutectic has a melting temperature of 81.1 °C and a latent melting enthalpy of 248 kJ kg−1. A low temperature PCM with good thermal stability and reliability based on stearic acid and hexanamide (46.2:53.8) with a melting temperature of 331.15 K and a latent heat of 176.62 J g−1 was studied by Guixiang Ma for thermal energy storage [19]. Thermal properties of a binary system of lauric acid and 1-tetradecanol (40:60) with melting point of 24.33 °C and latent heat of 161.45 kJ kg−1 were investigated for cold thermal energy storage by Zuo [20] Lauric, myristic and palmitic fatty acids were found to have eutectic combinations with 1-dodecanol (29:71, 17:83 and 10:90) [21]. Melting points and latent heat of fusions of the eutectics were found as 17, 18.43, 20.08 °C and 175.3, 180.8, 191 kJ kg−1, respectively. The aim of conducting repetitive thermal cycles is to ensure the stability and reliability of PCM over long term usage. An experimental analysis was carried out on the thermal stability of capric acid with lauric acid, palmitic acid and myristic acid respectively of which capric and myristic acid combination was shown to have the best thermal stability over a period of 2000 thermal cycles [22]. Experimental investigations had been carried out on the thermal stability of a paraffin mixture to about 10,000 cycles [23]. Heat of fusion degeneration was found as 9.1% while the melting point was not much affected. A review on thermal cycling tests conducted on various PCMs were reviewed by Rathod et al. [24] The detailed literature review on eutectic phase change materials shows that, numerous research activities are being focussed on identification and analysis on thermo-physical properties of novel eutectic phase change materials for heating and cooling applications. However, it has been noticed that only a limited number of literatures are based on development of PCMs with phase transition temperatures in the range 19–29 °C, suitable for building cooling applications. Accordingly, this study has been focussed to identify a new eutectic PCM, befitting human comfort in buildings. Table 1 shows summary of the reviewed eutectic PCMs with their phase transitions temperature. The major contribution of this research lies in identification of a binary eutectic combination (lauryl alcohol and cetyl alcohol) that has been unexplored till date. The eutectic composition of the new combination is determined to be 80:20. As highlighted in the summary presented in Table 1, the identified eutectic is the only combination
2. Materials and methods 2.1. Materials Lauryl alcohol (C12H26O) is a colourless liquid obtained by reduction of fatty acids of coconut oil. Cetyl alcohol (C16H34O) is a fatty alcohol often in the form of white solid. Lauryl alcohol (98% pure) and cetyl alcohol (99% pure) were purchased from LOBA Chemie, India. A summary of properties of the two PCMs used are presented in Table 2. 2.2. Preparation of lauryl alcohol-cetyl alcohol (LC) eutectic For thermal property studies, the eutectic point of the mixture was to be determined experimentally. The pure components were considered to be mixed in the mass ratio 0–100% at 10% intervals. The individual components were weighed using an analytical balance (SHIMADZU AW320). Each mixture was heated to a temperature of 80 °C and stirred properly. It was then subjected to probe sonication (PROBE SONICATOR, Model No. PRO-250, Make: LABMAN) for a period of 5 min. Then mixtures were cooled to room temperature and stowed. 2.3.Characterization. of the binary mixture 2.3.1. Differential scanning calorimetry (DSC) DSC thermal analysis was used in the determination of the melting point and the enthalpy of (i) pure materials (ii) binary eutectic. Samples around the mass composition of the theoretical eutectic point were considered for analysis. A DSC 6000- PerkinElmer was used in performing DSC analysis by the principle of heat flux. 10 mg of the sample is taken for analysis. Heating and cooling is done at a rate of 10 °C min−1. Samples were subjected to heating and cooling between 0 and 100 °C. Thermal properties of the mixture as its latent heat and phase transition temperature were determined.
Table 1 Summary of reviewed eutectic PCMs. Eutectic PCM
Composition
Melting temperature [°C]
References
Lauric acid-Stearic Acid Methyl palmitate-Lauric acid Lauric acid-Myristic acid Capric acid-Cetyl alcohol Acetanilide-Benzoic acid Caprylic acid-1-dodecanol Erythritol-Urea Stearic acid-Hexanamide Lauric acid-1-tetradecanol Palmitic acid-1-tetradecanol Lauric acid-1-tetradecanol Myristic acid-1-tetradecanol Lauryl alcohol-Cetyl alcohol
75.5:24.5 60:40 66:34 70:30 30:70 70:30 54.9:45.1 46.2:53.8 40:60 10:90 29:71 17:83 80:20
34.16 25.60 34.20 22.89 75.56 6.52 81.10 58.15 24.33 20.08 17.00 18.43 20.01
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [21] [21] Present study
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Table 2 Thermophysical properties of lauryl alcohol and cetyl alcohol [15,25]. Property
Lauryl alcohol
Cetyl alcohol
IUPAC name Molecular weight [g mol−1] Melting point [°C] Latent heat [g mol−1]
1-Dodecanol 186.34 25.83 215.83
1-Hexadecanol 242.45 52.14 207.39
2.3.2. Thermal conductivity test Thermal conductivity studies of the eutectic mixtures were done by transient line heat source method (MODEL: KD2 Pro). The KD2 Pro analyser is a combination of a microcontroller and a KS-1 needle sensor. The needle sensor which is used has a sufficient length to diameter ratio (60:1.27) to serve as an infinitely long and thin heating source. The transient method works by recording the temperature variations (every 1 s) in response to a consistent heating pulse and a successive cooling pulse, applied for 30 s each, on the sensor. Sensor is positioned vertically in the glass container which holds the sample (50 ml) to be tested. Thermal properties of the tested liquid mixture are the determinants of its temperature response. Recorded temperature is processed by the 16 bit microcontroller and results are displayed. Conductivity measurements were taken for 10 °C, 20 °C, 30 °C and 40 °C.
Fig. 1. (a) Cold chamber test setup (b) Inner view of cold chamber.
0.74 kg h−1). The following studies were conducted on the cold storage chamber (shown in Fig. 1), to analyze the effectiveness of the newly identified eutectic PCM in cold energy storage applications:
• Without PCM, the discharge performance of the system was studied under natural and forced convection modes. • With different PCM loads, the discharge performance of the system was studied under a constant discharge mass flow rate
2.3.3. Thermogravimetric analysis (TGA) TGA test (TGA 4000-PerkinElmer) was used to analyze the thermal stability of the lauryl-cetyl alcohol eutectic. TGA is based on analysing the deviation in weight of the sample with respect to time, in a controlled environment. A sample of weight 10 mg is kept on a pan that is supported by a precision balance (0.01% precision). The pan carrying the sample, is placed inside a furnace and subjected to progressive heating from 25 °C to 300 °C at 10 °C min−1 scan rate. Inert nitrogen atmosphere is used so that the sample shows reaction only during decomposition temperature. By examining the TGA curve, decomposition temperature can be determined from the region in the plot where a drastic drop in weight of sample occurs.
3. Results and discussion 3.1. Pure components Figs. 2 and 3 present the endotherms and exotherms obtained on DSC analysis of pure lauryl alcohol and pure cetyl alcohol. Endotherm corresponding to melting is obtained on heating the sample while the exotherm corresponding to freezing is obtained while cooling the sample. DSC analysis for pure lauryl alcohol reveals melting and freezing temperatures to be 25.91 °C and 14.56 °C respectively with enthalpy changes during the processes as 217.49 and 220.40 J g−1. The smaller peak near the freezing point of the mixture indicates phase transformation (solid to solid). In case of cetyl alcohol, one endotherm is observed on heating and two exotherms are observed on cooling the sample. Cetyl alcohol as observed from the DSC curve shows a melting point at 51.50 °C and the freezing temperatures of 43.97 and 38.09 °C respectively. Two exotherms are seen on cooling curve. First peak
2.3.4. Accelerated thermal cycling test Accelerated thermal cycling was used to study the thermal stability of the eutectic mixture. 10 g of the eutectic mixture is sealed in a test tube and was subjected to repetitive heating and cooling cycles (using water as the heat transfer fluid). To ensure melting and solidification of the PCM during heating and cooling, the heat transfer fluid was subjected to temperature in the range 5–60 °C. Thermal cycling was done for a total of 1000 cycles. After 500 and 1000 cycles, samples each weighing 2 g were taken out for DSC thermal analysis. 2.4. Experimental charge-discharge studies on LC binary eutectic The new binary eutectic, was tested for use as a phase change material for application in cold energy storage. The study aimed to analyze the increase in time of free coldness in the chamber of an existing refrigeration system by the addition of phase change material. The rectangular chamber in which experiment was conducted had a 0.25 TR capacity with inner dimensions 60 cm × 40 cm. Cylindrical metallic containers were chosen for the purpose of storing the PCM in the refrigeration chamber. The container used had a radius of 2.25 cm and length of 28 cm. Considering 80% of each cylinder to be filled with PCM, the total mass of PCM required to fill 10 containers was calculated approximately as 3 kg (components lauryl alcohol and cetyl alcohol are taken in the eutectic ratio of 80:20). Lauryl alcohol (2.4 kg) was added to cetyl alcohol (0.6 kg); heated, stirred and ultrasonated for the eutectic composition preparation. The prepared eutectic was filled into 10 cylinders for testing in the cold storage chamber. Discharging was carried out by natural and forced convection modes (mass flow rate of
Fig. 2. DSC curve of lauryl alcohol. 3
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Fig. 3. DSC curve of cetyl alcohol.
depicts liquid to rotator phase transition at 43.97 °C and second peak shows rotator phase transition to a crystalline structure at 38.09 °C [26,27]. 3.2. Lauryl alcohol-cetyl alcohol binary mixture
Fig. 4. DSC heating curves of lauryl alcohol-cetyl alcohol binary mixtures of different mass compositions.
The molar ratios of the individual compounds in the mixtures for the eutectic composition are theoretically estimated by Schröder–van Laar equation given as Eq. (1).
ln xk =
−ΔHkm ⎛ 1 1 − k ⎞⎟ ⎜ R ⎝T Tm ⎠
Table 3 Peak temperatures of lauryl alcohol-cetyl alcohol binary mixtures.
(1)
in which xk is the molar ratio of k-component, ΔHkm is the melting enthalpy (J mol−1), T and Tkm are the melting temperature values corresponding to pure component and mixture (K) and R is the universal gas constant (8.31 J mol−1 K−1). The eutectic phase transition temperature of the combination of lauryl alcohol and cetyl alcohol, in the mass ratio 85:15, was calculated to be 23.50 °C (by Eq. (1)) and is lower than melting points of both lauryl alcohol and cetyl alcohol. Thermograms obtained by DSC analysis are shown in the Fig. 4. Every thermogram shows two endotherm peaks. First peak occurs at an almost constant temperature (around 20 °C) for every composition considered. This depicts an invariant phase transition point called the eutectic point [15]. Second peak's temperature varies with the mass composition of the pure components becoming larger with the increasing percentage of one of the components and its peak temperature reaches melting temperature of the pure component. The eutectic temperature of 20.01 °C agrees with the theoretically calculated temperature. The peak temperatures obtained by DSC analysis of different mixture compositions are given in Table 3. The DSC curve corresponding to the lauryl alcohol-cetyl alcohol eutectic mixture (80:20) is shown in Fig. 5. The melting temperature and freezing temperature of the eutectic are 20.01 °C and 12.04 °C respectively with corresponding latent heats being 191.63 J g−1 and 189.51 J g−1. Supercooling refers to solidification of a liquid below its freezing point. It is largely dependent on factors as cooling rate as well as size, homogeneity and purity of sample. The extent to which a sample is supercooled is the difference between its freezing temperature and the actual temperature of initial crystallization. From the DSC curve (Fig. 5), degree of supercooling of the lauryl alcohol-cetyl alcohol eutectic is only around 2.2 °C. However, if a material has its degree of supercooling as high as 5 °C or more it is unsuitable in potential
Lauryl alcohol
Cetyl alcohol
Second peak [°C]
First peak [°C]
0 50 60 70 80 90 100
100 50 40 30 20 10 0
51.50 37.07 32.71 20.86 20.01 22.53 25.91
– 20.64 21.71 – – – –
Fig. 5. DSC curve of lauryl alcohol-cetyl alcohol eutectic mixture.
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Results of the 0th cycle can be used as reference for comparing with thermophysical characteristics after 500 and 1000 cycles [15]. The lauryl-cetyl eutectic showed an increase in melting point from the initial 20.01 to 21.44 °C after a total of 1000 complete thermal cycles along with decrease in latent heats of fusion from 191.63 at the eutectic point to 163.36 J g−1 after 1000 cycles. Considering the cooling curves, the freezing temperature reduced from an initial 12.04 °C to 11.10 °C after 1000 cycles with latent heats diminishing from 189.51 to 184.10 J g−1 by the end of 1000th cycle. The relative percentage difference (RPD) of melting points and latent heats of fusion after 500th and 1000th cycle can be calculated using Eq. (2).
RPD =
Xn, i − X0, i × 100% X0, i
(2)
in which i is the eutectic PCM property that is considered (melting point or latent heat of fusion) and Xn,i and X0,i are corresponding values after nth cycle and 0th cycle, respectively. The calculated melting point and latent heat of fusion RPDs are presented in Table 4. Negative RPD values for melting point and latent heat of fusion denotes a decrease in the respective values when compared to the uncycled eutectic. Upon completion of 1000 thermal cycles, there was not much disparity in melting point and latent heat when compared to the uncycled sample. Hence, the new eutectic having found to possess good thermal stability for more than a 1000 cycles, promises the performance of a good thermal energy storage material.
Fig. 6. Phase diagram of lauryl alcohol-cetyl alcohol binary system.
building comfort applications which require systems with long life. This is because a high degree of supercooling leads to reduced performance of the PCM thermal energy storage system due to: requirement of a large operating temperature range and chances of phase separation. Supercooling and phase separation causes thermal cycling degradation which eventually limits the life of eutectic. In cases of materials with high supercooling, techniques as addition of nucleating and thickening agents and adoption of macro encapsulation over microencapsulation are adopted to reduce it [1,9]. Peak temperatures of the endotherms obtained experimentally are plotted against different mass fractions of lauryl alcohol to obtain the phase diagram. From the phase diagram, it can be observed that the melting point of the binary mixtures first decreases and then increases with the mole fraction of lauryl alcohol. From the phase diagram of the lauryl alcohol- cetyl alcohol binary system given in Fig. 6, the temperature at the lowest point of the graph in the lauryl-cetyl alcohol is 20.01 °C and the corresponding mass fraction of lauryl alcohol is 0.8. The enthalpy of fusion at the eutectic point is 191.632 kJ kg−1. The new eutectic PCM has a suitable phase transition temperature and a high latent heat of fusion thus proving suitable for consideration in cold thermal energy storage.
3.4. Thermal conductivity Higher the material thermal conductivity, higher will be rate of transfer of heat. Thermal conductivity of the developed binary phase change material till temperature for application is shown in Fig. 8. Thermal conductivity of the organic liquids are generally low and within the range 0.1 to 0.2 W m−1 K−1 [28]. The new eutectic also shows thermal conductivity in this range. There are no significant variations in their thermal conductivities with respect to temperature within the range under study. 3.5. TGA From TGA analysis, mass loss with respect to increase in temperature can be found. Results (shown in Fig. 9) point out that, the new binary eutectic of lauryl-cetyl alcohol starts decomposition around 169.69 °C. Probable thermal fluctuations and any resulting temperature rise will not affect the new PCM stability as, the decomposition temperature is confirmed to be several times higher than its melting point (20.01 °C). This confirms the thermal stability of PCM for application in cold thermal energy storage.
3.3. Accelerated thermal cycling Melting point and latent heats of the eutectics were measured after 500 and 1000 cycles by DSC analysis and results are shown in Fig. 7.
Fig. 7. DSC curve of lauryl alcohol-cetyl alcohol eutectic mixture after (a) 0 cycle (b) 500 cycles (c) 1000 cycles. 5
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Table 4 Variation in melting point and latent heat of the eutectic after thermal cycling. Cycles
Melting point [°C]
RPD of melting point [%]
Latent heat of fusion (LHF) [J g−1]
RPD of LHF [%]
0 500 1000
20.01 21.39 21.44
– 6.90 7.15
191.63 174.55 163.36
– −8.91 −14.75
Fig. 8. Thermal conductivity of the lauryl-cetyl alcohol eutectic at different temperatures.
Fig. 9. TGA curve of lauryl alcohol-cetyl alcohol eutectic mixture. Fig. 10. Charging characteristics (a) without PCM (b) with 3 kg PCM.
3.6. Performance analysis of experimental setup using LC PCM the bottommost portion being the coldest. Prior to the cooling process, temperature inside the chamber was monitored, which is close to ambient temperature. The initial phase of cooling shows a steep decline in chamber temperatures followed by a gradual decline till temperature is about 14 °C. The temperature of the chamber without PCM was brought down to 18 °C in 60 min (1 h). Experiment with 3 kg of PCM took around 210 min (nearly 3.50 h) to reach the phase transition temperature. Extra time was imparted for the addition of cold energy to the PCM material, which is available for retrieval afterwards. Charging process with new lauryl alcohol-cetyl alcohol eutectic PCM involved cooling/charging the PCM kept in cylindrical containers
3.6.1. Performance comparison with and without the use of new eutectic PCM 3.6.1.1. Charging characteristics. Charging the experimental setup without PCM involves cooling the chamber up to a temperature of around 18 °C. The charging characteristics of the experimental setup with cold storage are presented in Fig. 10. The variation of chamber temperatures at two different locations in the chamber is recorded using temperature sensors placed 1 cm and 15 cm from the bottom of the chamber. The average temperature difference between the two monitored locations was between 1 and 4 °C, 6
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Fig. 11. Discharge characteristics of chamber without PCM under (a) Natural convection (b) Forced convection (0.74 kg h−1).
Fig. 12. Discharge characteristics of chamber with 3 kg PCM under (a) Natural convection (b) Forced convection (0.74 kg h−1).
inside the chamber. Through the process of charging, the PCM stores energy in the form of latent heat. As evident from the graph, the cooling of the chamber leads to cooling of the PCM stored. The PCM temperature variations can be reasoned as sensible heat storage till about the first 70 min, as latent heat storage for the next 140 min and further again as sensible heat storage. Sensible heat storage is expressed by the falling portion in the graph where the temperature reduces with respect to time. Latent heat storage region is depicted by the almost constant temperature region in the graph. In spite of the fall in chamber temperature in this region, the PCM temperature is found to remain constant. This is evident from the drop in difference between the chamber and PCM temperatures (70–210 min duration in Fig. 10(b)). Further abatement in temperature signifies sensible heat storage in the material.
chamber till chamber temperature reached ambient temperature. Discharging was studied separately by natural and forced convection modes (at mass flow rate of 0.74 kg h−1). In case where no PCM was used, chamber reached ambient temperature in 118 min by natural convection mode. In case of forced convection, ambient temperature was attained at 101 min. Forced convection requires lesser time for discharge owing to its greater heat transfer rate. Higher the mass flow rate, greater is the rate of discharge. 3.6.1.3. Discharging characteristics with 3 kg PCM. The sole purpose of a phase change material is to store energy for later use, thus, reducing the energy demand of the system. Discharge characteristics of the lauryl alcohol-cetyl alcohol binary eutectic under different convection modes are shown in Fig. 12. In this experiment, with the embodiment of PCM (3 kg) in the cooling chamber, compared to the case without PCM, higher is the time required for discharging to take place. Table 5 summarises the results of experiment carried out in the cold storage chamber before and after the incorporation of PCM. Taking into account the disparate discharge modes carried out in the cold chamber (with and without the usage of PCM), it can be observed that using the
3.6.1.2. Discharging characteristics without PCM. The discharge characteristics of the cold chamber without PCM are depicted in Fig. 11. Experiments are conducted under both natural and forced convection modes of discharge. Discharging process was carried out by letting out the cold air in the 7
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Table 5 Summary of discharge characteristics of chamber without PCM and with 3 kg PCM under different modes of convection. Mode of convection
Without PCM Cooling Discharge time [min] time [min]
WITH 3 kg PCM Charging Discharge time [min] time [min]
Natural Forced (0.74 kg h−1)
60
210
118 101
416 402
new eutectic, the time taken for discharge is on an average 3.7 times higher than in the case where no PCM is used. This is advantageous, as, more the time it takes for the chamber temperature to reach the ambient temperature, for a longer duration the cooling effect will be retained in the refrigerator. Creditably, the newly identified lauryl alcohol-cetyl alcohol PCM used for study, by virtue of its latent heat of 191.63 kJ kg−1 permits satisfactory energy storage per unit mass thus increasing the time span of cold energy storage to more than 400 min. In the case where PCM was not incorporated in the chamber, the system required frequent charging and discharging. For domestic and industrial applications low frequency charge-dicharge cycles are suited which makes PCM incorporation in cold thermal energy storage desirable. This means that higher will be the effectiveness of a cold storage system with improvement in energy efficiency when the novel eutectic is integrated. 3.6.2. Performance analysis of different loads of new eutectic PCM in the experimental setup In this set of studies it was monitored how the charge-discharge characteristics of PCM varied with different load conditions at a constant mass flow rate. The experiment is conducted for different PCM loads of 1 kg, 2 kg and 3 kg. All three loads are subjected to similar discharge condition of forced convection with a discharge mass flow rate of 0.74 kg h−1. The temperature-time relation for the charging and discharging of the eutectic PCM is shown in Figs. 13 and 14 respectively. 3.6.2.1. Charging characteristics with different PCM loads. The difference in time taken for charging the different PCM loads is an important measure in determining the charging characteristics. For this purpose the temperature-time graphs presented in Fig. 13 can be made use of. Taking into consideration the difference in charging times from the case where no PCM has been used (Fig. 12(a)) to the cases where uniformly increasing loads have been taken (shown in Figs. 13 and 12(b)), it can be seen that the increase in quantity of PCM demands a higher time for charging/energy storage. While the cold chamber required nearly 60 min to cool without any PCM, the charging time increased to nearly 175 min, 190 min and 210 min for 1 kg, 2 kg and 3 kg loads respectively. With uniformly increasing PCM loads the time needed to charge increases almost in a linear pattern. Each of the charging graphs show a sensible heat storage period followed by latent heat storage period and the further continuation of sensible heat storage again. The chamber and PCM temperatures show similar decreasing trends except for the latent heat region where, despite the chamber temperature reduction, the PCM temperature remains constant. Ambient temperatures are also depicted for the purpose of reference.
Fig. 13. Charging characteristics for (a) 1 kg PCM (b) 2 kg PCM.
of PCM took 343 min, 2 kg of PCM took around 372 min, 3 kg of PCM took around 402 min, respectively for discharge at 0.74 kg h−1 mass flow rate. As the quantity of PCM used in the cold storage tank increases, more will be the time for which cold energy will be retained in it. The difference in time observed for even 1 kg PCM used, is nearly 4 h more than in the case where no PCM is used. Higher the quantity of PCM used, greater the cold energy that can be stored. This is evident from two factors. Firstly, the time required for PCM to charge increases with the increase in the quantity of PCM used in the cold storage chamber (more energy will be stored). Secondly, the time required for the PCM to discharge the stored cold energy increases with increase in PCM load (larger amount of cold energy to be released).
4. Conclusion
3.6.2.2. Discharging characteristics with different PCM loads. Discharge characteristics of the eutectic showing variations of chamber temperature and PCM temperature are presented in Fig. 14. Forced convection mode of discharge removes the cold energy stored faster when compared to natural convection. As the mass flow rate of discharge air is increased, the time taken by chamber and PCM to reach ambient temperature is reduced (faster discharge). While 1 kg
Thermal energy storage gains relevance when heat demand and production do not match. Cold energy storage using phase change materials is one of the highly recommended measures in the ongoing circumstance of gaining importance of energy saving and conservation. Realizing the significance of identifying newer thermal energy storage, 8
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Authors acknowledge Department of Science and Technology (DST), Government of India, for funding the project through its scheme called ‘Materials for Energy Storage’. References [1] A.B. Lingayat, Y.R. Suple, Review on phase change material as thermal energy storage medium : materials, application, Int. J. Eng. Res. Appl. 3 (2013) 916–921. [2] C. Veerakumar, A. Sreekumar, Phase change material based cold thermal energy storage: materials, techniques and applications - A review, Int. J. Refrig. (2016), https://doi.org/10.1016/j.ijrefrig.2015.12.005. [3] D.N. Nkwetta, F. Haghighat, Thermal energy storage with phase change material - A state-of-the art review, Sustain. Cities Soc. (2014), https://doi.org/10.1016/j.scs. 2013.05.007. [4] S.D. Sharma, K. 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(2013) 168–171 doi:10.4028/www.scientific.net/AMR.684.168. [10] O. Ghahramani Zarajabad, R. Ahmadi, Numerical investigation of different PCM volume on cold thermal energy storage system, J. Energy Storage. (2018), https:// doi.org/10.1016/j.est.2018.04.013. [11] E. Oró, C. Barreneche, M.M. Farid, L.F. Cabeza, Experimental study on the selection of phase change materials for low temperature applications, Renew. Energy (2013), https://doi.org/10.1016/j.renene.2013.01.043. [12] Y. Lv, W. Jin, Y. Yang, H. Zhang, X. Zhang, F. Ding, W. Zhou, Experimental thermal storage research of organic binary phase change materials in building environment, Int. J. Green Energy (2017), https://doi.org/10.1080/15435075.2017.1339042. [13] R.M. Saeed, J.P. Schlegel, C. Castano, R. Sawafta, V. Kuturu, Preparation and thermal performance of methyl palmitate and lauric acid eutectic mixture as phase change material (PCM), J. Energy Storage. (2017), https://doi.org/10.1016/j.est. 2017.08.005. [14] S. Keleş, K. Kaygusuz, A. Sari, Lauric and myristic acids eutectic mixture as phase change material for low-temperature heating applications, Int. J. Energy Res. 29 (2005) 857–870, https://doi.org/10.1002/er.1111. [15] C. Veerakumar, A. Sreekumar, Preparation, thermophysical studies, and corrosion analysis of a stable capric acid/cetyl alcohol binary eutectic phase change material for cold thermal energy storage, Energy Technol. (2018), https://doi.org/10.1002/ ente.201700540. [16] K. Purohit, V.V.S. Murty, R.C. Dixit, A. Sharma, Development of an acetanilide/ benzoic acid eutectic phase change material based thermal energy storage unit for a passive water heating system, Bull. Mater. Sci. (2019), https://doi.org/10.1007/ s12034-019-1731-6. [17] J. Zuo, W. Li, L. Weng, Thermal performance of caprylic acid/1-dodecanol eutectic mixture as phase change material (PCM), Energy Build 43 (2011) 207–210, https:// doi.org/10.1016/j.enbuild.2010.09.008. [18] G. Diarce, L. Quant, J.M.Sala Campos-Celador, A. García-Romero, Determination of the phase diagram and main thermophysical properties of the erythritol–urea eutectic mixture for its use as a phase change material, Sol. Energy Mater. Sol. Cells (2016), https://doi.org/10.1016/j.solmat.2016.08.016. [19] G. Ma, J. Sun, Y. Zhang, Y. Jing, Y. Jia, A novel low-temperature phase change material based on stearic acid and hexanamide eutectic mixture for thermal energy storage, Chem. Phys. Lett. (2019), https://doi.org/10.1016/j.cplett.2018.11.003. [20] J. Zuo, W. Li, L. Weng, Thermal properties of lauric acid/1-tetradecanol binary system for energy storage, Appl. Therm. Eng. (2011), https://doi.org/10.1016/j. applthermaleng.2011.01.008. [21] R. Kumar, S. Vyas, A. Dixit, Fatty acids/1-dodecanol binary eutectic phase change materials for low temperature solar thermal applications: design, development and thermal analysis, Sol. Energy (2017), https://doi.org/10.1016/j.solener.2017.07. 082. [22] S. Mengjie, W.M. Pun, D. Swapnil, P. Dongmei, M. 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Fig. 14. Discharge characteristics: Forced convection at 0.74 kg h−1for (a) 1 kg PCM (b) 2 kg PCM.
a new organic eutectic phase change material which can act as latent heat storage material was developed. The eutectic PCMs used in this work had not been subjected to previous studies. Characterization of the novel PCM was done using DSC, TGA, thermal reliability and thermal conductivity analysis. Results conclude that the new PCMs possess suitable melting points and latent heats, have good thermal for long term usage. Eutectic PCM was tested in a cold chamber for performance analysis. As is evident from experimental observations, even with 1 kg of the PCM, the chamber retains its coldness for extra 4 h. Also, the time required for PCM to charge and discharge increases with the increase in the quantity of PCM used in the cold storage chamber. Usage of PCM also facilitated low frequency charge-discharge cycles. An economical quantity of PCM would ensure the effectiveness of new eutectic PCM as a cold energy storage material. The newly identified eutectic is thus proposed to be appropriate for thermal energy storage in human comfort range, making it suitable for diverse applications.
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