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sodium acetate trihydrate composite phase change materials
Renewable Energy 99 (2016) 1029e1037
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Experimental studies on the supercooling and melting/freezing characteristics of nano-copper/sodium acetate trihydrate composite phase change materials Wenlong Cui, Yanping Yuan*, Liangliang Sun, Xiaoling Cao, Xiaojiao Yang School of Mechanical Engineering, Southwest Jiaotong University, 610031, Chengdu, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2016 Received in revised form 17 July 2016 Accepted 1 August 2016 Available online 8 August 2016
This paper reports that Nano-copper (Nano-Cu), which possesses high thermal and electrical conductivity, as an additive, can improve the supercooling properties of sodium acetate trihydrate (CH3COONa$3H2O, SAT) and enhance its thermal conductivity. To investigate the effect of Nano-Cu content on the degree of supercooling of SAT, composite phase change materials containing SAT, Nano-Cu (0.4%, 0.5%, 0.6%, 0.7% and 0.8%), CMC (thickening agent) and sodium dodecyl sulfonate (C12H25NaO3S, dispersant) were prepared. Melting-freezing experiments involving the composite materials indicated that the rate of heat transfer increased by nearly 20%. When an optimal amount of Nano-Cu (i.e., 0.5%) was added to SAT, the degree of supercooling was reduced to approximately 0.5 C. Compared to the use of inorganic salt hydrates as nucleating agents, Nano-Cu is significantly advantageous in reducing the degree of supercooling of SAT. The maximum improvement in supercooling was observed when the meltingfreezing experiment was conducted at an initial temperature of 70 C. The thermal conductivity of the reported composite phasechange materialsis approximately 20% higher than that of pure SAT. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Sodium acetate trihydrate Nano-copper Supercooling Cooling curve Heat transfer characteristics Thermal conductivity
1. Introduction In recent years, phasechange materials for thermal energy storage have been studied and applied widely because of their enhanced energy efficiency and environmental friendliness. Several low-to-medium temperature phasechange materials for thermal energy storage have been explored extensively [1]. CH3COONa$3H2O, Na2SO4$10H2O and CaCl2$6H2O are well known phasechange materials that can be used effectively at ambient temperatures suitable for the working conditions of instruments, equipment and human activities [2e4]. Indeed, these materials have been utilized in household thermal insulation, solar energy storage, peak load shifting of power systems, energy conservation for air conditioning units and buildings, as well as in the recycling of industrial and domestic waste heat [5,6]. Sodium acetate trihydrate (CH3COONa$3H2O, SAT) is a representative inorganic salt phase change material with a melting point of 58 C and a relatively high latent heat of fusion [7]. It can be used for heat storage at low temperatures because it is non-toxic, widely
* Corresponding author. E-mail address: [email protected] (Y. Yuan). http://dx.doi.org/10.1016/j.renene.2016.08.001 0960-1481/© 2016 Elsevier Ltd. All rights reserved.
available, and inexpensive. However, SAT displays a relatively large degree of supercooling. Naumann et al. [8] have shown that SAT at molten state does not crystalize even when the temperature reaches 40 C, which greatly affects its heat storage and release performance. Consequently, a number of research groups have focused on finding solutions to this issue, and many nucleating agents have been identified to improve the supercooling of SAT. Supercooling is named the phenomenon when a phase change material in liquid state cools down below its freezing point without solidifying, and leaving it in a metastable state where the latent heat of fusion is not released [9]. At most time, people think supercooling is undesirable, because it prevents the heat of fusion from being released in time [10]. In the past decades, researchers made a lot of studies on how to prevent undesirable supercooling, they even think the supercooling should be down to 0 C ideally [11,12]. In fact, long term thermal energy storage can be made by letting the melted salt hydrate remain in supercooled state. If the thermal energy is needed, some measures can be carried out to make PCMs crystallize, so that the latent heat of fusion will be released [13]. Similar to other salt hydrate phase change materials, the supercooling ofSAT is caused by poor nucleating properties [14]. According to the theory of phase transition kinetics, crystal growth
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is initiated from a crystal nucleus, which can be in the form of bumps on the container wall, impureparticles in the liquid phase change material, unmelted crystal grains, crystal particles of nucleating agents or the nuclei that are formed by the liquid phase change material itself. Except for the crystal grains of the material itself, which will result in a homogeneous nucleation, all of the other types of nuclei will result in non-homogeneous nucleation [1]. The surface of the nucleating agents has a large influence on crystal growth because it can increase the surface free energy and promote nucleation. Therefore, the effect of nucleation is highly dependent on the affinity of the nucleating agents for the crystals [15,16]. It is known that several effective nucleating agents, including Na4P2O7$10H2O, Na2HPO4$12H2O, Na2B4O7$10H2O and Na2SiO3$9H2O, influence nucleation differently because of their distinct affinities for the resulting crystals. Some studies suggest that optimal nucleation can be induced when the difference in lattice constants between the crystal and the nucleating agent is 15% [17,18]. At present, however, the screening of nucleating agents remains largely empirical, with inconsistent results being reported by different research groups. Wada [19] and Naumann [8] have reported that the degree of supercooling of a CH3COONa$3H2O hybrid thermal energy storage system can be reduced to under 5 C upon the addition of 1% and 0.5%Na4P2O7$10H2O, respectively. Notably, the performance of the hybrid systems did not diminish after repeated heating-cooling processes. Xu et al. showed that the degree of supercooling was maintained under 3 C after 160 heating-cooling cycles using 30 g of SAT as the base energy storage material with 1% Na2HPO4$12H2O as the nucleating agent and 3%carboxymethyl cellulose as the thickening agent [20]. Luisatested a formula identical to that ofXu's and was able to reduce the supercooling to under 5 C without the addition of a thickening agent [21]. Additionally, Li et al. added 5% Na2HPO4$12H2O and 3% gelatin to their hybrid thermal energy storage material and found that the phase transition temperature stabilized at 55 C, with a degree of supercooling of approximately 4 C [22]. Further, Xu added 0.5%, 1%, 2%, 4% and 6% of the nucleating agent Na2B4O7$10H2O (borax) to 50 g of SAT and observed that the degree of supercooling was reduced to under 1 C with increasing amounts of borax [23]. Wang et al. studied the effect of adding 0.5%, 1%, 1.5%, 2%, 3% and 4% Na2SiO3$9H2O to10 g of SAT on supercooling. The results indicated that Na2SiO3$9H2O has no direct effect on the degree of supercooling, which remained at approximately 5 C [24]. In contrast, Lang et al. demonstrated that SAT with 0.5% Na2SiO3$9H2O exhibited the highest inhibition of supercooling. Moreover, when 1% and 1.5% Na2SiO3$9H2O were added during the experiment, the hybrid system remained in the liquid state even when the temperature decreased to 48 C [25]. In recent years, nano technology has been applied to improve the supercooling characteristics of phase change materials, and favorable results have been obtained using nanomaterials as the nucleating agents for SAT [26,27]. Lu et al.studied the effects of the nanomaterials AlN, Si3N4, SiO2, ZrB2, BC4and SiB6 on the supercooling behavior of. In this case, it was demonstrated that AlN, Si3N4, SiO2and ZrB2 have very good effects on nucleating properties, with 5% AlN or Si3N4 able to reduce supercooling to under 1 C [28]. Hu et al. used 5% AlN nanoparticles as the nucleating agent and 4% carboxymethyl cellulose (CMC) as the thickening agent and found that the supercooling of SAT can be inhibited quite well; indeed, the degree of supercooling was approximately 1 C even after 50 cycles [29]. Generally, most PCMs (SAT included) get low thermal conductivity, which affects the rate of heat release and storage [30]. The most regular way to enhance the thermal conduction of PCMs is mixing additives with high thermal conductivity to make PCM composites. Dannemand et al made a composite with SAT,
thickening agent and 5% graphite flakes, the thermal conductivity up to 1.1 W/K [31]. Li et al. made an experiment about heat storage and heat release performance of the composite phase change material which uses SAT as host material. 10% expanded graphite can be dispersed evenly in the composite phase change material, the thermal conductivity of the composite is significantly improved double [32]. Some nanomaterials (e.g. Nano-Cu, Nano-Al, etc) produced in the size range between 1 and 100 nm, with different surface morphologies, can be also used for the enhancement of the thermal storage properties of PCMs [33]. Zhang et al. proposed a new kind of PCM (0.4% nano-copperþ 99.6% erythritol), enhancing thermal conductivity up to 3.3 times compared with pure erythritol [34]. Previous studies have mostly concentrated on improving the degree of supercooling of SAT using inorganic salt hydrates. At most time, Nano-Cu is only used to enhance the thermal property of PCMs. However, no reports focused on the use of Nano-Cu for improving the performance of both supercooling and heat transfer. In the present study, Nano-Cu was selected as the nucleating agent based on its high stability and thermal conductivity in an effort to improve the supercooling and heat transfer properties of SAT. Several composite phasechange materials were prepared and tested at different initial temperatures (i.e., 60 C, 70 C, 75 Cand 80 C) to examine their heat transfer characteristics during the melting-freezing process. The thermal conductivity of the samples was also measured to study the overall effect of Nano-Cu on the supercooling and heat transfer properties. 2. Materials and instruments Sodium acetate trihydrate (CH3COONa$3H2O, purity 99.5%), sodium dodecyl sulfonate (C12H25NaO3S, purity99.5%) and carboxymethyl cellulose (CMC, RnOCH2COONa, purity 99.5%) were manufactured by Tianjin Kemiou Chemical Reagent Co., Ltd. NanoCu (particle size 10e30 nm, purity99.9%) was provided by Aladdin Chemical Reagent Co. Ltd. The apparatus used for the experiments includes an electronic balance(precision ±0.1 C), type T thermocouple (precision ±0.1 C), Agilent 34980A Multifunction Switch/Measure Unit (digital precision ±0.001 C), Differential Scanning Calorimetry (DSC, Q20, Wurst, USA, for the measurement of phase change temperature and latent heat),temperature controlled water bath, magnetic stirrer, Netzsch LFA 457 MicroFlash® (for the measurement of thermal conductivities, measurement range: 0.1e2000 W/(m$K), temperature range: 125e1100 C), cylindrical metal unit, and rubber plug. 3. Experimental work 3.1. Preparation of the Nano-Cu/CH3COONa·3H2O phasechange composite (1) Suitable amounts of SAT, Nano-Cu, and C12H25NaO3S were measured so that the mass fraction of Nano-Cu was0.4%, 0.5%, 0.6%, 0.7%or 0.8%,the content of CMC (as thickening agent) was 3 wt% and the mass fraction of C12H25NaO3S(as dispersant) was 0.25%.Recently, there are two ways to prepare nano fluid with high thermal property and noprecipitation. One is in the presence of dispersant, nano-metal is dispersed in the liquid medium by magnetically stirring, and the nano fluid prepared by this way achieves good behaviors in suspension and stability [35]. Another one is vapor deposition which makes the nano particles suspend in nano fluid evenly [36]. In this paper, we choose the first one, because it is efficient, easy and cost-effective.
W. Cui et al. / Renewable Energy 99 (2016) 1029e1037
(2) Acylindrical metal unit containing SATand CMCwas placed in a 70 C water bath. After completion of the phase change, Nano-Cu was slowly added to the liquid SAT and the mixture was magnetically stirred. C12H25NaO3S was then added, and the mixture was stirred for an additional 15 min at 70 C to ensure complete suspension and dispersion of Nano-Cu throughout the mixture. (3) Once stirring was completed, the unit was placed in an air bath at room temperature for cooling. Upon crystallization, a Nano-Cu/CH3COONa$3H2O phasechange composite was obtained, with the material being termed a “mass fraction Cu nanoparticle” (e.g.,0.4% Cu nanoparticles). The samples of pure SAT used in the experiment and 0.5% Cu nanoparticlesare shown in Fig. 2. By contrast, Nano-Cu and SAT blend well visually.
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3.3. Measurement of phase change temperature and latent heat First, a certain quantity (5e10 mg) of composite phasechange material (or sample) was taken into crucibles. Second, putting the crucibles on DSC. Scans were made at 2 C/min and nitrogen gas at 50 ml/min was used. At last, database were read and figured. 3.4. Determination of the thermal conductivity A suitable amount of composite phasechange material (or sample) was pressed into a 10 mmcylindrical disk under 10MPaof pressure. The density of the disk material was calculated based on its thickness and mass as measured by a vernier caliper (mm) and electronic balance (g). The thermal conductivity was then determined using a laser flash thermal conductivity instrument. The principle used to calculate the thermal conductivity is shown in Equation (1):
l ¼ r$a$cp
(1)
where
3.2. Melting-freezing experiments (1) The units containing the composite phasechange materials (or samples) were placed and subsequently heated in a temperature controlled water bath keeping at 70 C, and the temperature was chosen by most researchers [20,28,29]. The thermocouple was fixed at the center of the unit to monitor the temperature change during the melting process. The instrument was set to measure every 5 s. At the end of the phasechange process, the temperature of the unit was set to its highest setting for an additional 15 min before the heating process was stopped. (2) The unit containing the melted sample was removed from the water bath and placed in a airat the ambient temperature for freezing until the exothermic crystallization of the sample was completed. Data collection continued until the sample reached room temperature. The temperature-time relationship was plotted to obtain the cooling curve of the sample. (3) The melting-freezing experiments were carried out for 50 cycles to verify the effect of stability and nucleation. The experimental apparatus is shown in Fig. 1.
l - thermal conductivity, W=ðm,KÞ; r - density, g=cm3 ;
a - thermal diffusion coefficient, mm2 =s; cp - specific heat at constant pressure, J=ðkg,KÞ
3.5. Error analysis The errors in the experiment root in the error of purity of chemicals, the accuracy and errors of the instruments, the homogenization deviation and the errors of the indirect measurement. First, the SAT and Nano-Cu used are not 100% inpurity, which may have an effect the phase change temperature and latent heat of the samples. However, these errors can't be forecast, and they are classified into system errors [37]. At the same time, these materials (SAT, Nano-Cu, C12H25NaO3S and CMC) were mixed to the composites, which would bring the homogenization deviation. In industrial production, the calculation of standard deviation requires 20e30 samples, so it is not considered here [37]. Second, the results are unique and unrepeatable for every temperature data, so multi-
Fig. 1. Schematic diagram of the experimental apparatus.
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Fig. 2. Two samples used in the experiment.
measurement average methods can't be used to get the final experimental data here [38]. Temperature is measured by Agilent34980A multifunctional data acquisition instrument, and the data is transmitted by thermocouples. The digital display accuracy of the data acquisition instrument is 0.0010 C, the temperature range is-50-500 C, and the accuracy is±0.1 C. Thus, the error of temperature in the experiment is limited to 0.1 C. Third, thermal conductivity was measured by indirect measurement, so the weighting error (±0.001 C), the error of volume measurement (±0.0005 cm3), and the error of the instrument (±0.001 J/(g·K)). According to the theory of propagation of error for the product function [39], the final error is about ±0.002 W/(m·K). 4. Result and discussion 4.1. The effect of Nano-Cu content on the heating process of CH3COONa·3H2O during melting The experiment for investigating the melting process of SAT was conducted using equal amounts of composite phase change material containing 0.4e0.8 wt% Nano-Cu and pure SAT in a temperature controlled water bath. The results are shown in Fig. 3. At the beginning of the heating stage, the composite phasechange materials consistently attained the same temperature before the pure SAT did. Here, it was found that a higher Nano-Cu content leads to a steeper slope of the melting curve. Calculations involving the time points corresponding to identical temperature points indicated that the rate of the temperature rise increases by over 20% through the addition of Nano-Cu. The time required for the phasechange of all of the composite samples was significantly shorter than that of pure SAT. These data suggest that the addition of Cu positively influenced the efficiency of the phasechange heat-storage of SAT. Copper is a good conductor of heat flow, and it remains suspended without morphological changes during the melting stage [40]. Because the thermal conductivity of metals is far greater than that of liquids, the addition of Nano-Cu can facilitate the heat storage properties of SAT in a relatively short period of time. Indeed, the present data confirm that by adding 0.4e0.8 wt% Nano-Cu, the heat transfer rate can be increased, and the heat storage properties can be improved, too. 4.2. The effect of the Nano-Cu content on the supercooling of CH3COONa·3H2O Melting-freezing experiments were performed with Nano-Cu/
CH3COONa$3H2O composite phase change materials containing 0.4%, 0.5%, 0.6%, 0.7%and 0.8% Nano-Cu. The temperature changes during the freezing process were recorded and plotted to obtain the cooling curves. The results are shown in Fig. 4. During the experiment, every data can be get every 5 s by the Multifunction Switch and output in excel. To present the result more clearly, so some core statistics are calculated and list in Table 1. Based on the definition of supercooling, the degree is calculated by the difference between the maximum crystallization temperature and the minimum one [9]. According to the results, at ambient temperature, the supercooling of SAT can be as high as several dozen degrees Celsius. The degree of SAT supercooling initially decreases before gradually increasing as the Nano-Cu content increases. The 0.5% Cu nanoparticles exhibited the minimum degree of supercooling of approximately 0.5 C. The degree of supercooling gradually increased to 3.527 C for 0.8%Cu nanoparticles. Because the density of Nano-Cu is greater than that of the CH3COONa solution, it is likely that any excess copper powder will aggregate and deposit on the bottom of the reaction vessel, which reduces the amount of Nano-Cu available to act as an impurity to facilitate nucleation. Moreover, the aggregation and deposition of Nano-Cu would lead to inferior heat release properties of the sample, prohibit crystal nucleation and result in an increased supercooling effect. After 50 melting-freezing cycles, the degree of supercooling of the 0.5% Cu nanoparticles was still very low (as is show in Fig. 5), but up to 1.01 C. Because the melting point of copper is much higher than that of solid SAT, it is highly stable under the heating conditions used in the thermostatic water bath [41]. Clearly, this attribute is advantageous compared to conventional inorganic nucleating agents, which often suffer from supercooling and phase separation problems that could cause instability upon repeated use. As a result, the addition of Nano-Cu allows its smooth crystallization, which demonstrates that Nano-Cu is an effective nucleating agent [42]. The composite with CH3COONa$3H2O, CMC, C12H25NaO3S and 0.5% Nano-Cu almost gets low supercooling. The phase change temperature and latent heat were measured by DSC, which are shown in Figs. 6 and 8. According to Fig. 6, the pure SAT shows a high ability (the latent heat is 242.4 J/g) and a suitable temperature (58.53 C) for thermal energy storage. According to Figs. 6 and 7, Nano- Cu and other additives have no effect on the phase change temperature of SAT. Because the quality of thermal energy storage materials per unit mass decreases, the latent heat of composite is 234.5 J/g which is below to pure SAT, i.e., 242.4 J/g. There is 96.25%wt SAT in the composite, ideally, the latent heat of the PCMs should be proportional to their mass [6]. However,
W. Cui et al. / Renewable Energy 99 (2016) 1029e1037
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Fig. 3. The melting rate curves of different samples.
the result above shows the latent heat of the composite is 96.6% of the pure SAT. As a result, after adding the additives, there is almost no influence on the energy storage capability of SAT. The phase change temperature and latent heat of composites after 50meltingfreezing cycles were also measured as shown in Fig. 8. Due to the thickening agent, CMC, phase separation can be prevented and enhance the lifetime of PCMs. The phase change temperature is 57.88 C which is almost the same as the first melting, and the latent heat is 231.4 J/g. It is demonstrated that the composite achieve high stability. Generally, during the phase change process, the cumulative energy values stored in the PCMs can be calculated by Equation (2):
Q ¼ f $m$H
(2)
When the temperature is higher than the phase change temperature, the energy should be calculated by Equation (3):
Q ¼ m$H þ cp mðt tm Þ
(3)
where cp - specific heat at constant pressure,J=ðkg,KÞ; t - temperature, C; tm - phase change temperature, C; The latent heat values could be changed and decreased/ increased from the single phase to the other state, the latent heat energy associated with a phase change is important for efficient and inexpensive energy storage devices.
where 4.3. Comparisons to other nucleating agents Q - cumulative energy, J; f - melting rate, %; m - quantity of PCMs,g; H - latent heat,J/g;
As mentioned in Section 4.2, Nano-Cu can be as effective nucleating agent of SAT and Nano-Cu is much more stable than conventional hydrated salts nucleating agents. The effect of Nano-
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Fig. 6. DSC melting curve of pure CH3COONa$3H2O.
Fig. 4. The effect of Nano-Cu content on the supercooling of CH3COONa$3H2O.
Cu on nucleation was compared to those of inorganic nucleating agents as follows: First, the 0.5% Cu nanoparticles was prepared. At same time, three additional samples with other nucleating agents and SAT were prepared. The nucleating agent Na4P2O7$10H2O, Na2SiO3$9H2O and Na2HPO4$12H2O with the optimal ratios of 0.5%, 0.5%and 1%, respectively, were employed [8,20,25]. Then four samples were then tested simultaneously in the melting-freezing experiments at an initial temperature of 70 C. The results are shown in Table 2 and Fig. 9. As seen, the nucleation effect of NanoCu is superior to those of the other inorganic salt hydrates at the optimized ratios. Detailed analysis of the cooling curves reveals that the samples quickly release heat in the first stage of the re-
Table 1 The effect of Nano-Cu on the crystallization of CH3COONa$3H2O. Nano-Cu content
Maximum crystallization temperature/ C
Minimum crystallization temperature/ C
Degree of supercooling/ C
Time of heat release/min
0.4% 0.5% 0.6% 0.7% 0.8%
54.982 56.541 56.355 55.701 55.748
53.425 55.944 55.388 54.421 52.227
1.557 0.597 0.967 1.242 3.527
35.8 49.4 42.5 54.4 46.6
Fig. 5. Freezing curve ofthe composites containing 0.5% Nano-Cu (after 50meltingfreezing cycles).
Fig. 7. DSC melting curve ofthe 0.5% Cu nanoparticles (first melting).
W. Cui et al. / Renewable Energy 99 (2016) 1029e1037
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Fig. 8. DSC melting curve ofthe 0.5% Cu nanoparticles (after 50melting-freezing cycles).
action because of the large temperature difference between the high and low temperature heat sources (i.e., the liquid sample and the air bath). Latent heat is released in the second stage of the process during the crystallization of the sample. Heat is gradually released in the third stage of the process as the sample temperature gradually returns to room temperature. Notably, the time of latent heat transfer for the 0.5% Cu nanoparticles was shorter than that for the other samples during the second stage of the process, which indicates that the Nano-Cu with the smaller particle size acted as an effective agent and facilitated the rapid nucleation of SAT [29]. Additionally, we note that the slopes of the cooling curve for the sample containing Nano-Cu are steeper than those for the other samples in both the first and third stages of the process. The 0.5% Cu nanoparticles gets only 0.596 C of supercooling which is significantly less than other sample. Moreover, Nano-Cu can help to increase the rate of heat release of SAT, so Nano-Cu is an excellent nucleating agent. 4.4. The effect of initial temperature on the Nano-Cu/ CH3COONa·3H2O systems The literature reports that 80 C is the maximum working temperature for SAT [16]. Because the composites and units were different from some researchers' study, the effect of the initial temperature (the temperature in water bath) on the Nano-Cu/ CH3COONa$3H2O composite phasechange material was studied. Initial temperatures of 65 C, 70 C, 75 Cand 80 C were selected, and four 0.5% Cu nanoparticles samples were placed into four thermostatic water baths at these different initial temperatures to evaluate their subsequent melting-freezing behaviors. The results are shown in Table 3 and Fig. 10. It is evident that the composite phasechange material displays the lowest degree of supercooling at an initial temperature of 70 C.
Fig. 9. The effect CH3COONa$3H2O.
of
different
nucleating
agents
on
the
supercooling
of
Further, the rate of latent heat release was also the highest at this temperature. It is hypothesized that liquid SATcan undergo precipitation with an increase in the initial temperature [43]. Therefore, the degree of supercooling increases significantly at temperatures above 70 C. When the initial temperature is 65 C, which is close to the phase transition temperature of pure SAT, and it may cut the rate of SAT melting during the heating progress [16]. This observation also reveals that an initial temperature of 70 C is conducive to the activation of Nano-Cu. Although slightly affected by the initial temperature, the results clearly demonstrate that Nano-Cu can be used as a relatively stable nucleating agent to improve the supercooling behavior of SAT. As a result, 70 C is a perfect initial temperature for the composites to work. 4.5. Determination of the thermal conductivity Compressed disks of pure SAT and 0.5% Cu nanoparticles were prepared and tested for thermal conductivity. The results are
Table 2 The effect of different nucleating agents on the crystallization of CH3COONa$3H2O. Nucleating agents
Maximum temperature during crystallization/ Minimum temperature during crystallization/ Degree of supercooling/ Duration of heat release/ C C C min
0.5% Nano-Cu 0.5% Na4P2O7$10H2O 0.5% Na2SiO3$9H2O 1%Na2HPO4$12H2O
56.508 56.542
55.929 54.761
0.569 1.782
52 67
55.881 56.602
52.544 54.737
3.337 1.865
54 66
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Table 3 The effect of the initial temperature on the crystallization of CH3COONa$3H2O/Nano-Cu. Initial temperature/ Maximum temperature during crystallization/ Minimum temperature during crystallization/ Degree of supercooling/ Duration of heat release/ C C C C min
65 70 75 80
56.582 56.516 56.511 56.453
54.947 55.983 55.067 54.483
1.635 0.533 1.444 1.97
61.3 53 55 58.6
supercooling and thermal conductivity properties. The results indicate that Nano-Cu not only is an excellent nucleating agent but also effectively enhances the thermal conductivity of the composite materials.
Fig. 10. The effect of initial temperature on the supercooling of the composite systems.
Table 4 Results of the thermal conductivity experiments. Test temperature( C)
20 30 40
Thermal conductivity(W/(m$K)) 0.5% nanoparticles
100% CH3COONa$3H2O
Ratio of increase
1.155 0.936 0.750
0.921 0.777 0.634
25.1% 20.5% 18.3%
C
shown in Table 4. Because 50 is closed to the phase change temperature, there may be solid-liquid two-phase in the container. As a result, the thermal conductivity was measured when the samples were in solid phase. As seen, the thermal conductivity of the composite SAT phase change system containing Nano-Cu increased by nearly 20% compared to the pure material. This observation confirms that the addition of Nano-Cu, which in itself possesses high thermal and electrical conductivity, is an effective method for enhancing the thermal conductivity of the composite material. Moreover, this result is consistent with the results discussed previously, in which a significant increase in the rate of temperature rise was noted during the heat release stage. 5. Conclusion Several composite phasechange materials were prepared using Nano-Cu, which in itself possesses high thermal and electrical conductivity, CMC was as thickening agent and sodium dodecyl sulfonate (C12H25NaO3S) was as dispersing agent. The meltingfreezing and heat transfer characteristics of the composite phasechange materials were investigated at different initial temperatures (i.e., 60 C, 70 C, 75 Cand 80 C), and the thermal conductivities of the composites were determined in an effort to evaluate the overall effect of Nano-Cu on improving both the
(1) Compared to several conventional nucleating agents, NanoCu possesses a greater stability and higher thermal conductivity and heat transfer rate. The thermal conductivity of the composite material is increased by over 20% compared to pure SAT through the addition of 0.5% Nano-Cu. Concomitantly, the rate of heat transfer also increases by nearly 20%. (2) The degree of supercooling of 0.5% Cu nanoparticles can be controlled to approximately 0.5 C following heating at 70 C and subsequent cooling to room temperature. Compared to several conventional nucleating agents, Nano-Cu can reduce the supercooling of SAT to approximately 0.5 C and increase the rate of heat storage and release. (3) Nano-Cu has slight effect on the phase change temperature of SAT. However, after adding Nano-Cu and other additives, the latent heat of the composite (234.5 J/g) is close to the pure SAT's (242.4 J/g). After 50 melting-freezing cycles, the degree of supercooling of the composites containing 0.5% Nano-Cu is still very low (about 1 C) and the latent heat (231.4 J/g)does not drop much compared to the first melting, which indicates the composite achieves a good behavior of stability. Acknowledgment The work is supported by theYouth Science and Technology Innovation Team of Sichuan Province of Building Environment and Energy Efficiency (No: 2015TD0015). References [1] Laura Solomon, AliF. Elmozughi, Alparslan Oztekin, Sudhakar Neti, Effect of internal void placement on the heat transfer performancedEncapsulated phase change material for energy storage, J. Renew. Energy 78 (2015) 438e447. [2] Jun Wang, Enshen Long, Wen Qin, Long Xu, Ultrathin envelope thermal performance improvement of prefabhouse by integrating with phase change material, J. Energy Build. 67 (2013) 210e216. Aguiar, Fernando Pacheco-Torgal, Effect of temperature on [3] Sandra Cunha, Jose mortars with incorporation of phase change materials, J. Constr. Build. Mater. 98 (2015) 89e101. [4] Jie Jia, W.L. Lee, Experimental investigations on using phase change material for performance improvement of storage-enhanced heat recovery room airconditioner, J. Energy 93 (2015) 1394e1403. [5] Nan Zhang, Yanping Yuan, Yanxia Du, Xiaoling Cao, Yaguang Yuan, Preparation and properties of palmitic-stearic acid eutectic mixture/expanded graphite composite as phase change material for energy storage, J. Energy 78 (2014) 950e956. [6] Zhang Nan, Yuan Yanping, Yuan Yaguang, Cao Xiaoling, Yang Xiaojiao, Effect of carbon nanotubes on the thermal behavior of palmitic-stearic acid eutectic mixtures as phase change materials for energy storage, J. Sol. Energy 110 (2014) 64e70. [7] Belen Zalba, Jose M. Marin, Luisa F. Cabeza, Harald Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, J. Appl. Therm. Eng. 23 (2003) 251e283. [8] R. Naumann, T. Fanghanel, H.H. Emons, Thermoanalytical investigation of sodium acetate trihydrate for application storage material, J. Therm. Analysis
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Experimental studies on the supercooling and melting/freezing characteristics of nano-copper/sodium acetate trihydrate composite phase change materials Wenlong Cui, Yanping Yuan*, Liangliang Sun, Xiaoling Cao, Xiaojiao Yang School of Mechanical Engineering, Southwest Jiaotong University, 610031, Chengdu, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2016 Received in revised form 17 July 2016 Accepted 1 August 2016 Available online 8 August 2016
This paper reports that Nano-copper (Nano-Cu), which possesses high thermal and electrical conductivity, as an additive, can improve the supercooling properties of sodium acetate trihydrate (CH3COONa$3H2O, SAT) and enhance its thermal conductivity. To investigate the effect of Nano-Cu content on the degree of supercooling of SAT, composite phase change materials containing SAT, Nano-Cu (0.4%, 0.5%, 0.6%, 0.7% and 0.8%), CMC (thickening agent) and sodium dodecyl sulfonate (C12H25NaO3S, dispersant) were prepared. Melting-freezing experiments involving the composite materials indicated that the rate of heat transfer increased by nearly 20%. When an optimal amount of Nano-Cu (i.e., 0.5%) was added to SAT, the degree of supercooling was reduced to approximately 0.5 C. Compared to the use of inorganic salt hydrates as nucleating agents, Nano-Cu is significantly advantageous in reducing the degree of supercooling of SAT. The maximum improvement in supercooling was observed when the meltingfreezing experiment was conducted at an initial temperature of 70 C. The thermal conductivity of the reported composite phasechange materialsis approximately 20% higher than that of pure SAT. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Sodium acetate trihydrate Nano-copper Supercooling Cooling curve Heat transfer characteristics Thermal conductivity
1. Introduction In recent years, phasechange materials for thermal energy storage have been studied and applied widely because of their enhanced energy efficiency and environmental friendliness. Several low-to-medium temperature phasechange materials for thermal energy storage have been explored extensively [1]. CH3COONa$3H2O, Na2SO4$10H2O and CaCl2$6H2O are well known phasechange materials that can be used effectively at ambient temperatures suitable for the working conditions of instruments, equipment and human activities [2e4]. Indeed, these materials have been utilized in household thermal insulation, solar energy storage, peak load shifting of power systems, energy conservation for air conditioning units and buildings, as well as in the recycling of industrial and domestic waste heat [5,6]. Sodium acetate trihydrate (CH3COONa$3H2O, SAT) is a representative inorganic salt phase change material with a melting point of 58 C and a relatively high latent heat of fusion [7]. It can be used for heat storage at low temperatures because it is non-toxic, widely
* Corresponding author. E-mail address: [email protected] (Y. Yuan). http://dx.doi.org/10.1016/j.renene.2016.08.001 0960-1481/© 2016 Elsevier Ltd. All rights reserved.
available, and inexpensive. However, SAT displays a relatively large degree of supercooling. Naumann et al. [8] have shown that SAT at molten state does not crystalize even when the temperature reaches 40 C, which greatly affects its heat storage and release performance. Consequently, a number of research groups have focused on finding solutions to this issue, and many nucleating agents have been identified to improve the supercooling of SAT. Supercooling is named the phenomenon when a phase change material in liquid state cools down below its freezing point without solidifying, and leaving it in a metastable state where the latent heat of fusion is not released [9]. At most time, people think supercooling is undesirable, because it prevents the heat of fusion from being released in time [10]. In the past decades, researchers made a lot of studies on how to prevent undesirable supercooling, they even think the supercooling should be down to 0 C ideally [11,12]. In fact, long term thermal energy storage can be made by letting the melted salt hydrate remain in supercooled state. If the thermal energy is needed, some measures can be carried out to make PCMs crystallize, so that the latent heat of fusion will be released [13]. Similar to other salt hydrate phase change materials, the supercooling ofSAT is caused by poor nucleating properties [14]. According to the theory of phase transition kinetics, crystal growth
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is initiated from a crystal nucleus, which can be in the form of bumps on the container wall, impureparticles in the liquid phase change material, unmelted crystal grains, crystal particles of nucleating agents or the nuclei that are formed by the liquid phase change material itself. Except for the crystal grains of the material itself, which will result in a homogeneous nucleation, all of the other types of nuclei will result in non-homogeneous nucleation [1]. The surface of the nucleating agents has a large influence on crystal growth because it can increase the surface free energy and promote nucleation. Therefore, the effect of nucleation is highly dependent on the affinity of the nucleating agents for the crystals [15,16]. It is known that several effective nucleating agents, including Na4P2O7$10H2O, Na2HPO4$12H2O, Na2B4O7$10H2O and Na2SiO3$9H2O, influence nucleation differently because of their distinct affinities for the resulting crystals. Some studies suggest that optimal nucleation can be induced when the difference in lattice constants between the crystal and the nucleating agent is 15% [17,18]. At present, however, the screening of nucleating agents remains largely empirical, with inconsistent results being reported by different research groups. Wada [19] and Naumann [8] have reported that the degree of supercooling of a CH3COONa$3H2O hybrid thermal energy storage system can be reduced to under 5 C upon the addition of 1% and 0.5%Na4P2O7$10H2O, respectively. Notably, the performance of the hybrid systems did not diminish after repeated heating-cooling processes. Xu et al. showed that the degree of supercooling was maintained under 3 C after 160 heating-cooling cycles using 30 g of SAT as the base energy storage material with 1% Na2HPO4$12H2O as the nucleating agent and 3%carboxymethyl cellulose as the thickening agent [20]. Luisatested a formula identical to that ofXu's and was able to reduce the supercooling to under 5 C without the addition of a thickening agent [21]. Additionally, Li et al. added 5% Na2HPO4$12H2O and 3% gelatin to their hybrid thermal energy storage material and found that the phase transition temperature stabilized at 55 C, with a degree of supercooling of approximately 4 C [22]. Further, Xu added 0.5%, 1%, 2%, 4% and 6% of the nucleating agent Na2B4O7$10H2O (borax) to 50 g of SAT and observed that the degree of supercooling was reduced to under 1 C with increasing amounts of borax [23]. Wang et al. studied the effect of adding 0.5%, 1%, 1.5%, 2%, 3% and 4% Na2SiO3$9H2O to10 g of SAT on supercooling. The results indicated that Na2SiO3$9H2O has no direct effect on the degree of supercooling, which remained at approximately 5 C [24]. In contrast, Lang et al. demonstrated that SAT with 0.5% Na2SiO3$9H2O exhibited the highest inhibition of supercooling. Moreover, when 1% and 1.5% Na2SiO3$9H2O were added during the experiment, the hybrid system remained in the liquid state even when the temperature decreased to 48 C [25]. In recent years, nano technology has been applied to improve the supercooling characteristics of phase change materials, and favorable results have been obtained using nanomaterials as the nucleating agents for SAT [26,27]. Lu et al.studied the effects of the nanomaterials AlN, Si3N4, SiO2, ZrB2, BC4and SiB6 on the supercooling behavior of. In this case, it was demonstrated that AlN, Si3N4, SiO2and ZrB2 have very good effects on nucleating properties, with 5% AlN or Si3N4 able to reduce supercooling to under 1 C [28]. Hu et al. used 5% AlN nanoparticles as the nucleating agent and 4% carboxymethyl cellulose (CMC) as the thickening agent and found that the supercooling of SAT can be inhibited quite well; indeed, the degree of supercooling was approximately 1 C even after 50 cycles [29]. Generally, most PCMs (SAT included) get low thermal conductivity, which affects the rate of heat release and storage [30]. The most regular way to enhance the thermal conduction of PCMs is mixing additives with high thermal conductivity to make PCM composites. Dannemand et al made a composite with SAT,
thickening agent and 5% graphite flakes, the thermal conductivity up to 1.1 W/K [31]. Li et al. made an experiment about heat storage and heat release performance of the composite phase change material which uses SAT as host material. 10% expanded graphite can be dispersed evenly in the composite phase change material, the thermal conductivity of the composite is significantly improved double [32]. Some nanomaterials (e.g. Nano-Cu, Nano-Al, etc) produced in the size range between 1 and 100 nm, with different surface morphologies, can be also used for the enhancement of the thermal storage properties of PCMs [33]. Zhang et al. proposed a new kind of PCM (0.4% nano-copperþ 99.6% erythritol), enhancing thermal conductivity up to 3.3 times compared with pure erythritol [34]. Previous studies have mostly concentrated on improving the degree of supercooling of SAT using inorganic salt hydrates. At most time, Nano-Cu is only used to enhance the thermal property of PCMs. However, no reports focused on the use of Nano-Cu for improving the performance of both supercooling and heat transfer. In the present study, Nano-Cu was selected as the nucleating agent based on its high stability and thermal conductivity in an effort to improve the supercooling and heat transfer properties of SAT. Several composite phasechange materials were prepared and tested at different initial temperatures (i.e., 60 C, 70 C, 75 Cand 80 C) to examine their heat transfer characteristics during the melting-freezing process. The thermal conductivity of the samples was also measured to study the overall effect of Nano-Cu on the supercooling and heat transfer properties. 2. Materials and instruments Sodium acetate trihydrate (CH3COONa$3H2O, purity 99.5%), sodium dodecyl sulfonate (C12H25NaO3S, purity99.5%) and carboxymethyl cellulose (CMC, RnOCH2COONa, purity 99.5%) were manufactured by Tianjin Kemiou Chemical Reagent Co., Ltd. NanoCu (particle size 10e30 nm, purity99.9%) was provided by Aladdin Chemical Reagent Co. Ltd. The apparatus used for the experiments includes an electronic balance(precision ±0.1 C), type T thermocouple (precision ±0.1 C), Agilent 34980A Multifunction Switch/Measure Unit (digital precision ±0.001 C), Differential Scanning Calorimetry (DSC, Q20, Wurst, USA, for the measurement of phase change temperature and latent heat),temperature controlled water bath, magnetic stirrer, Netzsch LFA 457 MicroFlash® (for the measurement of thermal conductivities, measurement range: 0.1e2000 W/(m$K), temperature range: 125e1100 C), cylindrical metal unit, and rubber plug. 3. Experimental work 3.1. Preparation of the Nano-Cu/CH3COONa·3H2O phasechange composite (1) Suitable amounts of SAT, Nano-Cu, and C12H25NaO3S were measured so that the mass fraction of Nano-Cu was0.4%, 0.5%, 0.6%, 0.7%or 0.8%,the content of CMC (as thickening agent) was 3 wt% and the mass fraction of C12H25NaO3S(as dispersant) was 0.25%.Recently, there are two ways to prepare nano fluid with high thermal property and noprecipitation. One is in the presence of dispersant, nano-metal is dispersed in the liquid medium by magnetically stirring, and the nano fluid prepared by this way achieves good behaviors in suspension and stability [35]. Another one is vapor deposition which makes the nano particles suspend in nano fluid evenly [36]. In this paper, we choose the first one, because it is efficient, easy and cost-effective.
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(2) Acylindrical metal unit containing SATand CMCwas placed in a 70 C water bath. After completion of the phase change, Nano-Cu was slowly added to the liquid SAT and the mixture was magnetically stirred. C12H25NaO3S was then added, and the mixture was stirred for an additional 15 min at 70 C to ensure complete suspension and dispersion of Nano-Cu throughout the mixture. (3) Once stirring was completed, the unit was placed in an air bath at room temperature for cooling. Upon crystallization, a Nano-Cu/CH3COONa$3H2O phasechange composite was obtained, with the material being termed a “mass fraction Cu nanoparticle” (e.g.,0.4% Cu nanoparticles). The samples of pure SAT used in the experiment and 0.5% Cu nanoparticlesare shown in Fig. 2. By contrast, Nano-Cu and SAT blend well visually.
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3.3. Measurement of phase change temperature and latent heat First, a certain quantity (5e10 mg) of composite phasechange material (or sample) was taken into crucibles. Second, putting the crucibles on DSC. Scans were made at 2 C/min and nitrogen gas at 50 ml/min was used. At last, database were read and figured. 3.4. Determination of the thermal conductivity A suitable amount of composite phasechange material (or sample) was pressed into a 10 mmcylindrical disk under 10MPaof pressure. The density of the disk material was calculated based on its thickness and mass as measured by a vernier caliper (mm) and electronic balance (g). The thermal conductivity was then determined using a laser flash thermal conductivity instrument. The principle used to calculate the thermal conductivity is shown in Equation (1):
l ¼ r$a$cp
(1)
where
3.2. Melting-freezing experiments (1) The units containing the composite phasechange materials (or samples) were placed and subsequently heated in a temperature controlled water bath keeping at 70 C, and the temperature was chosen by most researchers [20,28,29]. The thermocouple was fixed at the center of the unit to monitor the temperature change during the melting process. The instrument was set to measure every 5 s. At the end of the phasechange process, the temperature of the unit was set to its highest setting for an additional 15 min before the heating process was stopped. (2) The unit containing the melted sample was removed from the water bath and placed in a airat the ambient temperature for freezing until the exothermic crystallization of the sample was completed. Data collection continued until the sample reached room temperature. The temperature-time relationship was plotted to obtain the cooling curve of the sample. (3) The melting-freezing experiments were carried out for 50 cycles to verify the effect of stability and nucleation. The experimental apparatus is shown in Fig. 1.
l - thermal conductivity, W=ðm,KÞ; r - density, g=cm3 ;
a - thermal diffusion coefficient, mm2 =s; cp - specific heat at constant pressure, J=ðkg,KÞ
3.5. Error analysis The errors in the experiment root in the error of purity of chemicals, the accuracy and errors of the instruments, the homogenization deviation and the errors of the indirect measurement. First, the SAT and Nano-Cu used are not 100% inpurity, which may have an effect the phase change temperature and latent heat of the samples. However, these errors can't be forecast, and they are classified into system errors [37]. At the same time, these materials (SAT, Nano-Cu, C12H25NaO3S and CMC) were mixed to the composites, which would bring the homogenization deviation. In industrial production, the calculation of standard deviation requires 20e30 samples, so it is not considered here [37]. Second, the results are unique and unrepeatable for every temperature data, so multi-
Fig. 1. Schematic diagram of the experimental apparatus.
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Fig. 2. Two samples used in the experiment.
measurement average methods can't be used to get the final experimental data here [38]. Temperature is measured by Agilent34980A multifunctional data acquisition instrument, and the data is transmitted by thermocouples. The digital display accuracy of the data acquisition instrument is 0.0010 C, the temperature range is-50-500 C, and the accuracy is±0.1 C. Thus, the error of temperature in the experiment is limited to 0.1 C. Third, thermal conductivity was measured by indirect measurement, so the weighting error (±0.001 C), the error of volume measurement (±0.0005 cm3), and the error of the instrument (±0.001 J/(g·K)). According to the theory of propagation of error for the product function [39], the final error is about ±0.002 W/(m·K). 4. Result and discussion 4.1. The effect of Nano-Cu content on the heating process of CH3COONa·3H2O during melting The experiment for investigating the melting process of SAT was conducted using equal amounts of composite phase change material containing 0.4e0.8 wt% Nano-Cu and pure SAT in a temperature controlled water bath. The results are shown in Fig. 3. At the beginning of the heating stage, the composite phasechange materials consistently attained the same temperature before the pure SAT did. Here, it was found that a higher Nano-Cu content leads to a steeper slope of the melting curve. Calculations involving the time points corresponding to identical temperature points indicated that the rate of the temperature rise increases by over 20% through the addition of Nano-Cu. The time required for the phasechange of all of the composite samples was significantly shorter than that of pure SAT. These data suggest that the addition of Cu positively influenced the efficiency of the phasechange heat-storage of SAT. Copper is a good conductor of heat flow, and it remains suspended without morphological changes during the melting stage [40]. Because the thermal conductivity of metals is far greater than that of liquids, the addition of Nano-Cu can facilitate the heat storage properties of SAT in a relatively short period of time. Indeed, the present data confirm that by adding 0.4e0.8 wt% Nano-Cu, the heat transfer rate can be increased, and the heat storage properties can be improved, too. 4.2. The effect of the Nano-Cu content on the supercooling of CH3COONa·3H2O Melting-freezing experiments were performed with Nano-Cu/
CH3COONa$3H2O composite phase change materials containing 0.4%, 0.5%, 0.6%, 0.7%and 0.8% Nano-Cu. The temperature changes during the freezing process were recorded and plotted to obtain the cooling curves. The results are shown in Fig. 4. During the experiment, every data can be get every 5 s by the Multifunction Switch and output in excel. To present the result more clearly, so some core statistics are calculated and list in Table 1. Based on the definition of supercooling, the degree is calculated by the difference between the maximum crystallization temperature and the minimum one [9]. According to the results, at ambient temperature, the supercooling of SAT can be as high as several dozen degrees Celsius. The degree of SAT supercooling initially decreases before gradually increasing as the Nano-Cu content increases. The 0.5% Cu nanoparticles exhibited the minimum degree of supercooling of approximately 0.5 C. The degree of supercooling gradually increased to 3.527 C for 0.8%Cu nanoparticles. Because the density of Nano-Cu is greater than that of the CH3COONa solution, it is likely that any excess copper powder will aggregate and deposit on the bottom of the reaction vessel, which reduces the amount of Nano-Cu available to act as an impurity to facilitate nucleation. Moreover, the aggregation and deposition of Nano-Cu would lead to inferior heat release properties of the sample, prohibit crystal nucleation and result in an increased supercooling effect. After 50 melting-freezing cycles, the degree of supercooling of the 0.5% Cu nanoparticles was still very low (as is show in Fig. 5), but up to 1.01 C. Because the melting point of copper is much higher than that of solid SAT, it is highly stable under the heating conditions used in the thermostatic water bath [41]. Clearly, this attribute is advantageous compared to conventional inorganic nucleating agents, which often suffer from supercooling and phase separation problems that could cause instability upon repeated use. As a result, the addition of Nano-Cu allows its smooth crystallization, which demonstrates that Nano-Cu is an effective nucleating agent [42]. The composite with CH3COONa$3H2O, CMC, C12H25NaO3S and 0.5% Nano-Cu almost gets low supercooling. The phase change temperature and latent heat were measured by DSC, which are shown in Figs. 6 and 8. According to Fig. 6, the pure SAT shows a high ability (the latent heat is 242.4 J/g) and a suitable temperature (58.53 C) for thermal energy storage. According to Figs. 6 and 7, Nano- Cu and other additives have no effect on the phase change temperature of SAT. Because the quality of thermal energy storage materials per unit mass decreases, the latent heat of composite is 234.5 J/g which is below to pure SAT, i.e., 242.4 J/g. There is 96.25%wt SAT in the composite, ideally, the latent heat of the PCMs should be proportional to their mass [6]. However,
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Fig. 3. The melting rate curves of different samples.
the result above shows the latent heat of the composite is 96.6% of the pure SAT. As a result, after adding the additives, there is almost no influence on the energy storage capability of SAT. The phase change temperature and latent heat of composites after 50meltingfreezing cycles were also measured as shown in Fig. 8. Due to the thickening agent, CMC, phase separation can be prevented and enhance the lifetime of PCMs. The phase change temperature is 57.88 C which is almost the same as the first melting, and the latent heat is 231.4 J/g. It is demonstrated that the composite achieve high stability. Generally, during the phase change process, the cumulative energy values stored in the PCMs can be calculated by Equation (2):
Q ¼ f $m$H
(2)
When the temperature is higher than the phase change temperature, the energy should be calculated by Equation (3):
Q ¼ m$H þ cp mðt tm Þ
(3)
where cp - specific heat at constant pressure,J=ðkg,KÞ; t - temperature, C; tm - phase change temperature, C; The latent heat values could be changed and decreased/ increased from the single phase to the other state, the latent heat energy associated with a phase change is important for efficient and inexpensive energy storage devices.
where 4.3. Comparisons to other nucleating agents Q - cumulative energy, J; f - melting rate, %; m - quantity of PCMs,g; H - latent heat,J/g;
As mentioned in Section 4.2, Nano-Cu can be as effective nucleating agent of SAT and Nano-Cu is much more stable than conventional hydrated salts nucleating agents. The effect of Nano-
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W. Cui et al. / Renewable Energy 99 (2016) 1029e1037
Fig. 6. DSC melting curve of pure CH3COONa$3H2O.
Fig. 4. The effect of Nano-Cu content on the supercooling of CH3COONa$3H2O.
Cu on nucleation was compared to those of inorganic nucleating agents as follows: First, the 0.5% Cu nanoparticles was prepared. At same time, three additional samples with other nucleating agents and SAT were prepared. The nucleating agent Na4P2O7$10H2O, Na2SiO3$9H2O and Na2HPO4$12H2O with the optimal ratios of 0.5%, 0.5%and 1%, respectively, were employed [8,20,25]. Then four samples were then tested simultaneously in the melting-freezing experiments at an initial temperature of 70 C. The results are shown in Table 2 and Fig. 9. As seen, the nucleation effect of NanoCu is superior to those of the other inorganic salt hydrates at the optimized ratios. Detailed analysis of the cooling curves reveals that the samples quickly release heat in the first stage of the re-
Table 1 The effect of Nano-Cu on the crystallization of CH3COONa$3H2O. Nano-Cu content
Maximum crystallization temperature/ C
Minimum crystallization temperature/ C
Degree of supercooling/ C
Time of heat release/min
0.4% 0.5% 0.6% 0.7% 0.8%
54.982 56.541 56.355 55.701 55.748
53.425 55.944 55.388 54.421 52.227
1.557 0.597 0.967 1.242 3.527
35.8 49.4 42.5 54.4 46.6
Fig. 5. Freezing curve ofthe composites containing 0.5% Nano-Cu (after 50meltingfreezing cycles).
Fig. 7. DSC melting curve ofthe 0.5% Cu nanoparticles (first melting).
W. Cui et al. / Renewable Energy 99 (2016) 1029e1037
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Fig. 8. DSC melting curve ofthe 0.5% Cu nanoparticles (after 50melting-freezing cycles).
action because of the large temperature difference between the high and low temperature heat sources (i.e., the liquid sample and the air bath). Latent heat is released in the second stage of the process during the crystallization of the sample. Heat is gradually released in the third stage of the process as the sample temperature gradually returns to room temperature. Notably, the time of latent heat transfer for the 0.5% Cu nanoparticles was shorter than that for the other samples during the second stage of the process, which indicates that the Nano-Cu with the smaller particle size acted as an effective agent and facilitated the rapid nucleation of SAT [29]. Additionally, we note that the slopes of the cooling curve for the sample containing Nano-Cu are steeper than those for the other samples in both the first and third stages of the process. The 0.5% Cu nanoparticles gets only 0.596 C of supercooling which is significantly less than other sample. Moreover, Nano-Cu can help to increase the rate of heat release of SAT, so Nano-Cu is an excellent nucleating agent. 4.4. The effect of initial temperature on the Nano-Cu/ CH3COONa·3H2O systems The literature reports that 80 C is the maximum working temperature for SAT [16]. Because the composites and units were different from some researchers' study, the effect of the initial temperature (the temperature in water bath) on the Nano-Cu/ CH3COONa$3H2O composite phasechange material was studied. Initial temperatures of 65 C, 70 C, 75 Cand 80 C were selected, and four 0.5% Cu nanoparticles samples were placed into four thermostatic water baths at these different initial temperatures to evaluate their subsequent melting-freezing behaviors. The results are shown in Table 3 and Fig. 10. It is evident that the composite phasechange material displays the lowest degree of supercooling at an initial temperature of 70 C.
Fig. 9. The effect CH3COONa$3H2O.
of
different
nucleating
agents
on
the
supercooling
of
Further, the rate of latent heat release was also the highest at this temperature. It is hypothesized that liquid SATcan undergo precipitation with an increase in the initial temperature [43]. Therefore, the degree of supercooling increases significantly at temperatures above 70 C. When the initial temperature is 65 C, which is close to the phase transition temperature of pure SAT, and it may cut the rate of SAT melting during the heating progress [16]. This observation also reveals that an initial temperature of 70 C is conducive to the activation of Nano-Cu. Although slightly affected by the initial temperature, the results clearly demonstrate that Nano-Cu can be used as a relatively stable nucleating agent to improve the supercooling behavior of SAT. As a result, 70 C is a perfect initial temperature for the composites to work. 4.5. Determination of the thermal conductivity Compressed disks of pure SAT and 0.5% Cu nanoparticles were prepared and tested for thermal conductivity. The results are
Table 2 The effect of different nucleating agents on the crystallization of CH3COONa$3H2O. Nucleating agents
Maximum temperature during crystallization/ Minimum temperature during crystallization/ Degree of supercooling/ Duration of heat release/ C C C min
0.5% Nano-Cu 0.5% Na4P2O7$10H2O 0.5% Na2SiO3$9H2O 1%Na2HPO4$12H2O
56.508 56.542
55.929 54.761
0.569 1.782
52 67
55.881 56.602
52.544 54.737
3.337 1.865
54 66
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W. Cui et al. / Renewable Energy 99 (2016) 1029e1037
Table 3 The effect of the initial temperature on the crystallization of CH3COONa$3H2O/Nano-Cu. Initial temperature/ Maximum temperature during crystallization/ Minimum temperature during crystallization/ Degree of supercooling/ Duration of heat release/ C C C C min
65 70 75 80
56.582 56.516 56.511 56.453
54.947 55.983 55.067 54.483
1.635 0.533 1.444 1.97
61.3 53 55 58.6
supercooling and thermal conductivity properties. The results indicate that Nano-Cu not only is an excellent nucleating agent but also effectively enhances the thermal conductivity of the composite materials.
Fig. 10. The effect of initial temperature on the supercooling of the composite systems.
Table 4 Results of the thermal conductivity experiments. Test temperature( C)
20 30 40
Thermal conductivity(W/(m$K)) 0.5% nanoparticles
100% CH3COONa$3H2O
Ratio of increase
1.155 0.936 0.750
0.921 0.777 0.634
25.1% 20.5% 18.3%
C
shown in Table 4. Because 50 is closed to the phase change temperature, there may be solid-liquid two-phase in the container. As a result, the thermal conductivity was measured when the samples were in solid phase. As seen, the thermal conductivity of the composite SAT phase change system containing Nano-Cu increased by nearly 20% compared to the pure material. This observation confirms that the addition of Nano-Cu, which in itself possesses high thermal and electrical conductivity, is an effective method for enhancing the thermal conductivity of the composite material. Moreover, this result is consistent with the results discussed previously, in which a significant increase in the rate of temperature rise was noted during the heat release stage. 5. Conclusion Several composite phasechange materials were prepared using Nano-Cu, which in itself possesses high thermal and electrical conductivity, CMC was as thickening agent and sodium dodecyl sulfonate (C12H25NaO3S) was as dispersing agent. The meltingfreezing and heat transfer characteristics of the composite phasechange materials were investigated at different initial temperatures (i.e., 60 C, 70 C, 75 Cand 80 C), and the thermal conductivities of the composites were determined in an effort to evaluate the overall effect of Nano-Cu on improving both the
(1) Compared to several conventional nucleating agents, NanoCu possesses a greater stability and higher thermal conductivity and heat transfer rate. The thermal conductivity of the composite material is increased by over 20% compared to pure SAT through the addition of 0.5% Nano-Cu. Concomitantly, the rate of heat transfer also increases by nearly 20%. (2) The degree of supercooling of 0.5% Cu nanoparticles can be controlled to approximately 0.5 C following heating at 70 C and subsequent cooling to room temperature. Compared to several conventional nucleating agents, Nano-Cu can reduce the supercooling of SAT to approximately 0.5 C and increase the rate of heat storage and release. (3) Nano-Cu has slight effect on the phase change temperature of SAT. However, after adding Nano-Cu and other additives, the latent heat of the composite (234.5 J/g) is close to the pure SAT's (242.4 J/g). After 50 melting-freezing cycles, the degree of supercooling of the composites containing 0.5% Nano-Cu is still very low (about 1 C) and the latent heat (231.4 J/g)does not drop much compared to the first melting, which indicates the composite achieves a good behavior of stability. Acknowledgment The work is supported by theYouth Science and Technology Innovation Team of Sichuan Province of Building Environment and Energy Efficiency (No: 2015TD0015). References [1] Laura Solomon, AliF. Elmozughi, Alparslan Oztekin, Sudhakar Neti, Effect of internal void placement on the heat transfer performancedEncapsulated phase change material for energy storage, J. Renew. Energy 78 (2015) 438e447. [2] Jun Wang, Enshen Long, Wen Qin, Long Xu, Ultrathin envelope thermal performance improvement of prefabhouse by integrating with phase change material, J. Energy Build. 67 (2013) 210e216. Aguiar, Fernando Pacheco-Torgal, Effect of temperature on [3] Sandra Cunha, Jose mortars with incorporation of phase change materials, J. Constr. Build. Mater. 98 (2015) 89e101. [4] Jie Jia, W.L. Lee, Experimental investigations on using phase change material for performance improvement of storage-enhanced heat recovery room airconditioner, J. Energy 93 (2015) 1394e1403. [5] Nan Zhang, Yanping Yuan, Yanxia Du, Xiaoling Cao, Yaguang Yuan, Preparation and properties of palmitic-stearic acid eutectic mixture/expanded graphite composite as phase change material for energy storage, J. Energy 78 (2014) 950e956. [6] Zhang Nan, Yuan Yanping, Yuan Yaguang, Cao Xiaoling, Yang Xiaojiao, Effect of carbon nanotubes on the thermal behavior of palmitic-stearic acid eutectic mixtures as phase change materials for energy storage, J. Sol. Energy 110 (2014) 64e70. [7] Belen Zalba, Jose M. Marin, Luisa F. Cabeza, Harald Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, J. Appl. Therm. Eng. 23 (2003) 251e283. [8] R. Naumann, T. Fanghanel, H.H. Emons, Thermoanalytical investigation of sodium acetate trihydrate for application storage material, J. Therm. Analysis
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