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KCl – expanded graphite composite phase change material
Applied Energy 115 (2014) 265–271
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Thermal property measurement and heat storage analysis of LiNO3/KCl – expanded graphite composite phase change material Zhaowen Huang, Xuenong Gao ⇑, Tao Xu, Yutang Fang, Zhengguo Zhang Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
h i g h l i g h t s A novel composite of LiNO3/KCl in expanded graphite PCMs was prepared. Thermal properties and heat storage performance of the composite were studied. The composite showed excellent abilities in energy storage and heat transfer. The composite had a potential use in solar thermal energy applications.
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Article history: Received 20 August 2013 Received in revised form 24 October 2013 Accepted 4 November 2013 Available online 28 November 2013 Keywords: Eutectic salt Phase change material Thermal energy storage Thermal conductivity
a b s t r a c t A LiNO3/KCl-expanded graphite (EG) composite phase change material (PCM) was prepared for solar thermal energy storage application at high temperature (200 °C). In such composite material, eutectic system LiNO3/KCl is characterized by high phase change latent heat and EG serves as the heat transfer promoter. Investigations by means of differential scanning calorimetry (DSC), hot disk analyzer and heat storage performance tests in a latent thermal energy storage (LTES) unit were devoted to the thermal property measurement and heat storage performance analysis of the LiNO3/KCl–EG composite. Experimental results revealed that the melting temperature of the composite material was close to that of the eutectic LiNO3/KCl, and the phase change latent heat ranging from 142.41 to 178.10 J/g was dependent on its mass fraction of EG. The thermal conductivities of the composites were 1.85–7.56 times higher compared with the eutectic LiNO3/KCl, and the conductivity value varied with the EG mass content and the apparent density of the composite. In addition, the heat transfer in the composite material during the heat storage process was enhanced through the thermal conductivity improvement, while the heat storage duration was affected by the phase change latent heat and the apparent density of the composite material. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Thermal energy storage is one of crucial technologies in the development of energy utilization. Especially for solar thermal energy application systems, where energy source depends on intermittent and instable solar radiation, it has been recognized as an indispensable element to address the time-dependent limitation of solar energy. Among the methods to store thermal energy, latent thermal energy storage (LTES) based on phase change material (PCM) is the most efficient one, since it can provide large energy storage capacity for a given volume and stable operating temperature during heat storage process [1–3]. Over the past few decades, studies have been conducted to develop PCMs with high performance for different LTES systems [1,4]. The property that defines ⇑ Corresponding author. Tel.: +86 2087113596; fax: +86 2087113870. E-mail address: [email protected] (X. Gao). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.11.019
the suitability of PCMs for desired applications firstly is phase change temperature. For instance, paraffin waxes with low melting point are better suited for thermal management in buildings [5,6]. In areas where solar thermal energy consumption focuses on high temperature, PCMs with phase change temperature higher than 120 °C has become an object of prime investigations in recent years. Their great potential to improve energy-efficiency and to achieve energy-saving is attractive especially for solar power plants, solar cooking settings, solar steam generation devices and solar heating/cooling systems. In temperature range higher than 120 °C, a great interest is focused on inorganic eutectic salts as PCMs because of their low investment costs compared with metal alloys and wide range of melting temperatures to accommodate different applications [4]. Binary and ternary eutectics on the basis of nitrates are frequently applied for energy storage purpose in temperature interval of 120– 300 °C. For example, eutectic system KNO3/NaNO3, which exhibits
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negligible undercooling, chemical stability and no phase segregation, is the popular solar energy storage medium [7–9]. Ternary system KNO3–NaNO2–NaNO3 commonly used as heat transfer liquid is also a promising alternative as high-temperature PCMs [10]. LiNO3–NaNO3–KNO3 ternary system developed by Wang et al. [11,12], demonstrating excellent reliability during thermal cycles, is considered as a potential candidate for use in solar energy storage systems. Unfortunately, these eutectic nitrates suffer a comparatively low latent heat of phase transition (106 J/g for KNO3/NaNO3 and 80 J/g for KNO3–NaNO2–NaNO3), which leads to a reduction in energy storage capacity per volume of a LTES tank. In addition, eutectic salts are characterized by poor thermal conductivity, being below 1 W/m K in the state of powder. This drawback will suppress their heat storage/retrieval rates and consequently block their widespread practical application [13,14]. After a careful survey and selection among inorganic salts as PCMs, eutectic salts based on the combination of nitrates and chlorides generated our interest. Their almost constant phase change temperature and high latent heat of phase transition (>200 J/g) were found to be available in high-temperature LTES systems typically operating at about 200 °C. Thermal–physical properties of a binary system on the basis of LiNO3 and KCl have been investigated by Gasanaliev et al. [15] and the results suggest many desirable characteristics—including good thermal stability and high latent heat of melting—that make it an appropriate high-temperature PCM. As for the problem relating to the low thermal conductivity, which is also the common disadvantage of most inorganic salts, a great amount of works have been devoted to PCMs’ thermal conductivity enhancement [16]. Techniques as insertion of fins, introduction of metal fillers or carbon materials and impregnation of PCMs into highly conductive porous structures are attempted to be used in LTES systems [17–19]. However, the configuration of fin tubes and addition of metal materials will bring significant weight and cost increase. Besides, corrosion issues will appear if they are employed to inorganic salts particularly at a high working temperature. Carbon materials such as carbon fibers, nano-graphite particles and expanded graphite (EG) are known for high thermal conductivity, low density, thermo-chemical stability and compatibility with most PCMs, which make them ideal choices as heat conductivity enhancers. According to the literatures, investigations on heat conductivity improvement by dispersing carbon fibers [20,21] and nano-graphite particles [22] in PCMs are mainly focused on low temperature, and EG seems to be the one successfully used to improve thermal behaviors of high-temperature salts. Do Couto Aktay et al. [13] prepared multi-component PCMs composed of eutectic nitrates and EG by various elaboration methods including infiltration, warm and cold compression, and obtained at least 5 times increase in effective thermal conductivity. Influences of different manufacturing processes on heat conductivity enhancement were also revealed. Lopez et al. [7,8] developed KNO3/NaNO3-graphite composites through simple uni-axial and isostatic compression of salt powders and expanded natural graphite particles. Test results of these composites with graphite amounts between 15 and 20 wt% presented that their thermal conductivity were close to 20 W/m K. Thermal characteristics of the salt/EG composites composed of different nitrates and various mass fraction of EG were experimentally and numerically studied by Xiao et al. [14], and the results were taken as references to design and model the LTES systems. Although eutectic system LiNO3/KCl has been mentioned previously and EG is widely used to enhance heat conduction process, little information about the utilizations of eutectic LiNO3/KCl in LTES systems and the thermal conductivity improvement of eutectic LiNO3/KCl with the assistance of EG has been reported. Hence, the development and study of a new LiNO3/KCl–EG composite
PCM is valuable for the promotion of solar thermal energy utilization. In the present work, a kind of composite materials with eutectic system LiNO3/KCl as PCM and EG as heat transfer promoter was prepared. The thermal properties of the LiNO3/KCl–EG composite were studied by measuring the melting temperature, latent heat of solid–liquid phase change and thermal conductivity. Moreover, the thermal energy storage performances of the composites with different EG mass contents and various apparent densities in a LTES unit were experimentally tested and compared. 2. Experimental description 2.1. Preparation of materials Raw expandable graphite with an expandable rate of 300 ml/g (mesh 50, from Qingdao Graphite Co. Ltd., China) was subjected to irradiation treatment in a domestic microwave oven (Midea Inc., China) with an overall power of 800 W for 10–20 s to yield EG. The solid–liquid PCM selected in this work was the eutectic system LiNO3/KCl. It was obtain by mixing corresponding amounts of lithium nitrate (>99 % pure) and potassium chloride (>99 % pure) with a mass ratio of 1:1. The thermal–physical properties of this eutectic salt are given in Table 1. Then the eutectic LiNO3/KCl powder was kept at 120 °C for 24 h to discharge the absorbed water, followed by being heated to 200 °C in order to be liquefied. The new LiNO3/KCl–EG composite was prepared by physical mixing of the liquid salt and EG with a roll mixer to ensure the pores in EG were occupied by the liquid salt. Then the composite material particles were allowed to cool down until the PCM was solidified. The microstructures of EG particles and the LiNO3/KCl–EG composite observed by using a scanning electron microscope (SEM, S3700N, Hitachi Inc., JNP) are shown in Fig. 1. The magnified images of EG demonstrates a worm-like structure with large volume and rich pores. After the PCM has been mixed with the EG particles, it can be observed that eutectic salt crystals were distributed in the porous network of EG and wrapped by micron-size layers owning to the high capillary force [23,24]. To explore the variations of the phase change latent heat and thermal conductivity of this composite material as the mass fraction of EG increasing, a series of the LiNO3/KCl–EG composites with EG mass fraction of 10, 15, 20, 25 and 30 % were prepared. Preliminary experiments indicated that when EG mass fraction was less than 10 %, PCMs could not be fully impregnated into EG, and the composite materials would suffer salt leakage after a few melting/solidification cycles. Thus, the composites with mass fraction less than 10 % were not taken into account in this work. 2.2. Phase change characterization The influences on phase change characteristics of the eutectic system LiNO3/KCl, including the melting temperature and phase change latent heat, after the addition of EG were investigated by differential scanning calorimeter (DSC, Q20, TA Instrument Inc., USA). The DSC tests were performed under the protection of nitrogen from 140 to 190 °C at a heating speed of 10 °C/min. The
Table 1 Thermo-physical properties of the eutectic system LiNO3/KCl. Thermo-physical properties
Eutectic system LiNO3/KCl
Melting temperature (°C) Temperature of the melting peak (°C) Phase change latent heat (J/g) Specific heat (J/g °C) Density (g/cm3) Thermal conductivity (W/m K)
165.60 170.72 201.70 1.10 (s); 1.87 (l) 2.01 1.749; 0.3315 (powder)
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Fig. 1. SEM images: (a–b) EG particles; (c–d) the LiNO3/KCl–EG composite.
accuracy of the calorimeter was within 1 % and the measurement error of temperature was within 0.01 °C. Prior to DSC test, each sample was dried at 120 °C for 24 h to minimize the error caused by the absorbed water. The quantity of each sample was in the range of 5–20 mg. The latent heat of melting for the composite material was obtained by calculating the area of the endothermic peak in a heat flow vs. time curve plotted by DSC. The melting temperature derived from the extrapolated onset point in a heat flow vs. temperature curve also could be obtained in the same DSC test. Three DSC experiments were carried out with each sample, and the mean values of the melting temperature and latent heat were reported. 2.3. Thermal conductivity measurement Hot disk thermal constant analyzer (TPS2500, Hot Disk Inc., Sweden) was applied in this study to measure the thermal conductivities of the composite materials with a type 8563 probe acting as both heat source and sensor. The probe used in these tests consisted of a thin nickel foil embedded between two thin layers of kapton polyimide protective films. A transient plane source method, in which two samples (disk-shaped) of the test materials with a similar thickness were required to be placed in contact with the probe and heated at constant power for a setting scanning time, was selected for these measurements. The type of the probe, heating power and scanning time for these tests were chosen based upon the diameter, thickness and range of thermal conductivity of each sample. The uncertainty of the measurements was within ±2 %.
with the help of a temperature control system with an accuracy of ±1 °C. The LTES unit consisted of a cylindrical stainless-steel container with an inner diameter of 50 mm, a wall thickness of 0.5 mm and a length of 80 mm. Two K-type thermocouples were placed in the test unit to monitor the temperature variations of each sample within a measurement error of ±0.1 °C. Locations of the thermocouples are also presented in Fig. 2. The two test points both were spaced at a same depth, being 32 mm away from bottom of the container, to make sure the effects of input heat flux from top and bottom of the container on them were the same. Initially, four-fifth of the LTES unit volume was filled with the prepared composites at 120 °C, and the remaining space was left to accommodate the thermal expanding of PCMs. During the heat storage process, the thermostatic air bath box was kept at constant temperature 200 °C. A data logger (Agilent34970A, USA) was used to collect the voltage signal generated by the thermocouples and convert it to digital data.
3. Results and discussion 3.1. DSC analysis of the LiNO3/KCl–EG composites Fig. 3 shows the typical DSC thermograms of the selected LiNO3/ KCl–EG composites with different EG mass fractions, and the
2.4. Thermal energy storage performance evaluation The thermal performances of the LiNO3/KCl–EG composites during thermal energy storage process were investigated in order to evaluate the effects of EG on heat transfer behavior. The schematic diagram of the experimental system shown in Fig. 2 mainly comprised a thermostatic air bath box and a LTES unit. The temperature of the air bath was automatically controlled and measured
Fig. 2. Schematic diagram of the experimental set-up (a) PC, (b) data logger, (c) thermostatic air bath box, (d) temperature control system, (e) sample and (f) thermocouples: (1) r = 0 mm, h = 32 mm and (2) r = 15 mm, h = 32 mm.
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corresponding data collected from the DSC tests are summarized in Table 2. The DSC analysis of the eutectic system LiNO3/KCl (sample with EG mass fraction of 0 %) was introduced as a reference to investigate the thermal property changes after the addition of EG. Obviously, the composites all exhibit similar curve shapes with that of the eutectic LiNO3/KCl, indicating that it was the eutectic salt that acted the role of the latent thermal energy storage during the melting process. However, the phase change characteristics of the eutectic LiNO3/KCl were affected by adding EG. As listed in Table 2, the melting temperatures of the composites are approximately the same as that of the eutectic salt, whereas the melting peak temperature slightly shifts from 170.72 °C (Table 1) for the eutectic salt to around 168 °C for the composites. This temperature discrepancy is also depicted in Fig. 3. It is obvious that for the composites, the angles on melting point are shaper and the endothermic peaks are narrower than that of the eutectic LiNO3/KCl. This phenomenon could be ascribed to the porous EG network in the composites that provided heat conduction path in the eutectic salt and consequently accelerated the phase change speed of the composites [25,26]. The phase change latent heats of the LiNO3/KCl–EG composites are in the range from 142.41 to 178.10 J/g (Table 2) according to the DSC tests. Because EG did not undergo a phase transition process and it was the eutectic salt that accounted for the latent thermal heat, the latent heat decreases with an increase of the mass fraction of EG. The relation can be expressed as Eq. (1):
DHcomposite ¼ ð1 w%ÞDHsalt
ð1Þ
where DHcomposite represents the calculated phase change latent heat of the LiNO3/KCl–EG composite, DHsalt is the latent heat of the eutectic system LiNO3/KCl and w % is the mass fraction of EG. The experimental phase change latent heats of the composites are compared with their calculated values in Table 2, revealing that the experimental values show an agreement with the calculated values with relative errors less than 3 %. 3.2. Thermal conductivity measurement of the LiNO3/KCl–EG composites Since the LiNO3/KCl–EG composite particles pack together loosely under natural conditions and thermal conductivities of porous materials are significantly influenced by their densities, the apparent density of the composite material was taken into account in this thermal conductivity measurement. For LTES systems, the density of PCMs cannot be too low considering of the heat storage capacity for a given volume and the investment cost of the whole system [18]. The apparent density of the LiNO3/KCl–EG composite
Fig. 3. DSC curves of the LiNO3/KCl–EG composites with different mass fractions of EG.
also cannot be too high because severe salt leakage will appear if the void space within the porous EG is not enough for salt volume expansion [8,9]. In the experiments, the apparent densities of all samples were lower than 1.50 g/cm3 to make sure that the salt leakage could not be observed after the composites were maintained at 200 °C for 5 h. The results of the thermal conductivity measurements are shown in Fig. 4. Fig. 4(a) presents the thermal conductivities of the LiNO3/ KCl–EG composites as a function of the EG mass fraction for a same apparent density. Clearly, the thermal conductivity is remarkably improved with an increase of EG mass content. For example, while the apparent density is 1.01 g/cm3, the thermal conductivity of the eutectic LiNO3/KCl (1.749 W/m K) can be improved by 1.85 times using 10 % EG in the composite material, and it can be further improved by 6.65 times when 30 % EG is added into the composite material. This improvement was caused by the addition of EG which was acting as a type of thermal conductive matrix with a much higher thermal conductivity than the eutectic salt. Eq. (2) is derived from the plot in Fig. 4(a) that shows an almost linear relationship between the thermal conductivity of the LiNO3/ KCl–EG composite (y) and mass fraction of EG (x).
y ¼ 0:4224x þ 0:7595 ðR2 ¼ 0:99964Þ
ð2Þ
For investigating the effect of the different apparent densities on the thermal conductivity of the composite material, the composites with EG mass fraction of 20 % were taken as example. Fig. 4(b) indicates a trend that the thermal conductivity increases obviously with the rising apparent density. It can be noticed that while the EG mass fraction is 20 %, the thermal conductivity for 0.75 g/cm3 is about 5.12 W/m K, and the value for 1.42 g/cm3 is 14.98 W/m K which is almost 2 times higher. This trend could be attributed to the reduction of void space within the composite material and extension of contact surface area for the composite particles. Data in Fig. 4(b) is fitted by an ‘‘S’’-shape curve. The relation is expressed as Eq. (3):
y ¼ 16:6845
12:5050 R2 ¼0:98932 7:0556 x 1 þ 1:0904
ð3Þ
3.3. Thermal energy storage performance in a LTES unit For the research on the thermal energy storage performance of the LiNO3/KCl–EG composite, the EG mass content and apparent density of the composite material were considered as two important factors. A comparatively low apparent density ranging from 0.3 to 0.5 g/cm3 was applied in these tests for simplification of packing samples into the container and observation convenience. Fig. 5(a) shows the temperature profiles of the studied composites with different EG mass fractions during the heat storage process when the apparent density is 0.4 g/cm3. It can be observed that all test points exhibit a three-step behavior during the melting, which is similar to that occurred in different types of LTES systems with various PCMs [18,26–28]. At the first step when the temperature climbed sharply until it reached the melting point, heat conduction inside the solid phase dominated the heat transfer. After that, the liquid phase of the eutectic LiNO3/KCl began to emerge. Since the melting region was confined within the smallsize porous skeleton of EG and natural convection inside the liquid phase was severely suppressed, heat conduction was still the vital factor that driven the heat transfer speed. Previous works on NaNO3/EG composite [18], AC/EG composite [26] and paraffin/EG composite [27] have proved the heat conduction-dominated mechanism.
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Z. Huang et al. / Applied Energy 115 (2014) 265–271 Table 2 Phase change properties of the LiNO3/KCl–EG composites with different EG mass contents. Mass fraction of EG (%)
Melting temperature (°C)
Temperature of the melting peak (°C)
10 15 20 25 30
165.55 165.87 165.57 165.55 165.58
169.09 169.70 168.57 168.15 168.38
Fig. 4. The thermal conductivity variation of the LiNO3/KCl–EG composite: (a) with the mass fraction of EG and (b) with the apparent density of the composite material.
During the thermal energy storage period, the LTES unit was heated continuously from its surrounded walls and heat was transferred from its outer cylindrical surface to inner, thus the temperature at test point 1 and 2 increases at a different rate (see Fig. 5(a)). As shown in Fig. 5(b), the composite material with EG mass fraction of 30 % has obtained a slight range of temperature difference between the two test points (DT = T2 T1), whereas the range is extended when the EG mass content decreases. The maximum temperature differences (DTmax) for EG mass fractions of 10, 20 and 30 % are determined as 8.16, 4.83 and 3.01 °C, suggesting that with higher mass content of EG, the heat transfer in the composite material would be further enhanced. This phenomenon resulted from the improvement of the thermal conductivity with the assistance of increasing EG mass fraction. Moreover, based on the heat conduction-dominated mechanism, the thermal energy storage process could be considered as an unsteady heat conduction process, thus the time for samples to reach temperature equilibrium was positively related to PCMs’ energy storage density. As shown in Fig. 5(a), the heat storage durations the LiNO3/KCl–EG composites with EG mass fraction of 10, 20 and 30 % are determined as 3480, 2810 and 2460 s at the central position of the LTES unit (test point 1) under the same test conditions. It is clear that
Phase change latent heat Calculated value (J/g)
Experimental value (J/g)
Relative errors (%)
183.33 173.145 162.96 152.77 142.59
178.10 170.69 164.50 150.96 142.41
2.85 1.42 0.95 1.19 0.13
Fig. 5. Thermal energy storage performances of the LiNO3/KCl–EG composites with various mass fractions of EG: (a) temperature profile of the composites and (b) temperature differences inside the LTES unit.
with higher mass content of EG, the composite material can finish the heat storage period in a shorter time. The result was caused by the reduction of the phase change latent heat of the composite material via increasing the EG mass fraction, which consequently leaded to a lower energy storage density. Fig. 6(a) shows the temperature profiles of the LiNO3/KCl–EG composites with different apparent densities, when the mass fraction of EG is 20 %. During the heat storage period, it takes 2610, 2810 and 3160 s for the samples with apparent densities of 0.3, 0.4 and 0.5 g/cm3 to reach the thermal balance, respectively. Since the energy storage density of the LTES unit increased with the increasing apparent density of the sample, the composite material with a higher apparent density would take a longer time to complete the heat storage period. Fig. 6(b) demonstrates the curves of temperature difference DT inside the test unit while using the LiNO3/KCl–EG composites with various apparent densities. It is observed that DTmax for the apparent densities of 0.3, 0.4 and 0.5 g/cm3 are 10.38, 4.83 and 2.89 °C, respectively. By comparing the temperature difference ranges, the increase of the apparent density would be desirable for
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rial, which mainly depended on the energy storage density, was affected by its phase change latent heat and apparent density. Consequently, the thermal energy storage duration was shorten when the LiNO3/KCl–EG composite with higher EG mass fraction was applied in the LTES unit, whereas it was extended with the apparent density increased.
Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 20976056).
References
Fig. 6. Thermal energy storage performances of the LiNO3/KCl–EG composites with different apparent densities: (a) temperature profile of the composites and (b) temperature differences inside the LTES unit.
increasing the heat transfer ability of the composite material during the heat storage process. Similar to the phenomenon in Fig. 5(b), the reason was the improvement of the thermal conductivity by increasing the apparent density. 4. Conclusions In this study, a new kind of composite PCMs was prepared and investigated for thermal energy storage application at high temperature (200 °C) by using eutectic system LiNO3/KCl whose latent heat of phase transition was high and EG which was a heat transfer enhancer. The results revealed that the LiNO3/KCl–EG composite could be suggested as a potential candidate to storage thermal energy in solar energy utilizations. The conclusions were drawn as follows. (1) The presence of EG within the PCM induced no significant change in the phase change temperature of the eutectic LiNO3/KCl, but a decrease in phase change latent heat of the composite material from 178.10 to 142.41 J/g depending on the mass content of EG. (2) The addition of EG into the eutectic LiNO3/KCl resulted in a remarkable improvement in the thermal conductivity. According to the measurements, the thermal conductivities of the studied composites ranged from 5 to 15 W/m K, which was not only related to the mass ratio of EG but also associated with the apparent density of the composite material. (3) During the thermal energy storage process, the utilization of the LiNO3/KCl–EG composite with higher apparent density and EG mass fraction could further enhance the heat transfer through the thermal conductivity improvement. Moreover, the thermal energy storage duration of the composite mate-
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Thermal property measurement and heat storage analysis of LiNO3/KCl – expanded graphite composite phase change material Zhaowen Huang, Xuenong Gao ⇑, Tao Xu, Yutang Fang, Zhengguo Zhang Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
h i g h l i g h t s A novel composite of LiNO3/KCl in expanded graphite PCMs was prepared. Thermal properties and heat storage performance of the composite were studied. The composite showed excellent abilities in energy storage and heat transfer. The composite had a potential use in solar thermal energy applications.
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Article history: Received 20 August 2013 Received in revised form 24 October 2013 Accepted 4 November 2013 Available online 28 November 2013 Keywords: Eutectic salt Phase change material Thermal energy storage Thermal conductivity
a b s t r a c t A LiNO3/KCl-expanded graphite (EG) composite phase change material (PCM) was prepared for solar thermal energy storage application at high temperature (200 °C). In such composite material, eutectic system LiNO3/KCl is characterized by high phase change latent heat and EG serves as the heat transfer promoter. Investigations by means of differential scanning calorimetry (DSC), hot disk analyzer and heat storage performance tests in a latent thermal energy storage (LTES) unit were devoted to the thermal property measurement and heat storage performance analysis of the LiNO3/KCl–EG composite. Experimental results revealed that the melting temperature of the composite material was close to that of the eutectic LiNO3/KCl, and the phase change latent heat ranging from 142.41 to 178.10 J/g was dependent on its mass fraction of EG. The thermal conductivities of the composites were 1.85–7.56 times higher compared with the eutectic LiNO3/KCl, and the conductivity value varied with the EG mass content and the apparent density of the composite. In addition, the heat transfer in the composite material during the heat storage process was enhanced through the thermal conductivity improvement, while the heat storage duration was affected by the phase change latent heat and the apparent density of the composite material. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Thermal energy storage is one of crucial technologies in the development of energy utilization. Especially for solar thermal energy application systems, where energy source depends on intermittent and instable solar radiation, it has been recognized as an indispensable element to address the time-dependent limitation of solar energy. Among the methods to store thermal energy, latent thermal energy storage (LTES) based on phase change material (PCM) is the most efficient one, since it can provide large energy storage capacity for a given volume and stable operating temperature during heat storage process [1–3]. Over the past few decades, studies have been conducted to develop PCMs with high performance for different LTES systems [1,4]. The property that defines ⇑ Corresponding author. Tel.: +86 2087113596; fax: +86 2087113870. E-mail address: [email protected] (X. Gao). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.11.019
the suitability of PCMs for desired applications firstly is phase change temperature. For instance, paraffin waxes with low melting point are better suited for thermal management in buildings [5,6]. In areas where solar thermal energy consumption focuses on high temperature, PCMs with phase change temperature higher than 120 °C has become an object of prime investigations in recent years. Their great potential to improve energy-efficiency and to achieve energy-saving is attractive especially for solar power plants, solar cooking settings, solar steam generation devices and solar heating/cooling systems. In temperature range higher than 120 °C, a great interest is focused on inorganic eutectic salts as PCMs because of their low investment costs compared with metal alloys and wide range of melting temperatures to accommodate different applications [4]. Binary and ternary eutectics on the basis of nitrates are frequently applied for energy storage purpose in temperature interval of 120– 300 °C. For example, eutectic system KNO3/NaNO3, which exhibits
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negligible undercooling, chemical stability and no phase segregation, is the popular solar energy storage medium [7–9]. Ternary system KNO3–NaNO2–NaNO3 commonly used as heat transfer liquid is also a promising alternative as high-temperature PCMs [10]. LiNO3–NaNO3–KNO3 ternary system developed by Wang et al. [11,12], demonstrating excellent reliability during thermal cycles, is considered as a potential candidate for use in solar energy storage systems. Unfortunately, these eutectic nitrates suffer a comparatively low latent heat of phase transition (106 J/g for KNO3/NaNO3 and 80 J/g for KNO3–NaNO2–NaNO3), which leads to a reduction in energy storage capacity per volume of a LTES tank. In addition, eutectic salts are characterized by poor thermal conductivity, being below 1 W/m K in the state of powder. This drawback will suppress their heat storage/retrieval rates and consequently block their widespread practical application [13,14]. After a careful survey and selection among inorganic salts as PCMs, eutectic salts based on the combination of nitrates and chlorides generated our interest. Their almost constant phase change temperature and high latent heat of phase transition (>200 J/g) were found to be available in high-temperature LTES systems typically operating at about 200 °C. Thermal–physical properties of a binary system on the basis of LiNO3 and KCl have been investigated by Gasanaliev et al. [15] and the results suggest many desirable characteristics—including good thermal stability and high latent heat of melting—that make it an appropriate high-temperature PCM. As for the problem relating to the low thermal conductivity, which is also the common disadvantage of most inorganic salts, a great amount of works have been devoted to PCMs’ thermal conductivity enhancement [16]. Techniques as insertion of fins, introduction of metal fillers or carbon materials and impregnation of PCMs into highly conductive porous structures are attempted to be used in LTES systems [17–19]. However, the configuration of fin tubes and addition of metal materials will bring significant weight and cost increase. Besides, corrosion issues will appear if they are employed to inorganic salts particularly at a high working temperature. Carbon materials such as carbon fibers, nano-graphite particles and expanded graphite (EG) are known for high thermal conductivity, low density, thermo-chemical stability and compatibility with most PCMs, which make them ideal choices as heat conductivity enhancers. According to the literatures, investigations on heat conductivity improvement by dispersing carbon fibers [20,21] and nano-graphite particles [22] in PCMs are mainly focused on low temperature, and EG seems to be the one successfully used to improve thermal behaviors of high-temperature salts. Do Couto Aktay et al. [13] prepared multi-component PCMs composed of eutectic nitrates and EG by various elaboration methods including infiltration, warm and cold compression, and obtained at least 5 times increase in effective thermal conductivity. Influences of different manufacturing processes on heat conductivity enhancement were also revealed. Lopez et al. [7,8] developed KNO3/NaNO3-graphite composites through simple uni-axial and isostatic compression of salt powders and expanded natural graphite particles. Test results of these composites with graphite amounts between 15 and 20 wt% presented that their thermal conductivity were close to 20 W/m K. Thermal characteristics of the salt/EG composites composed of different nitrates and various mass fraction of EG were experimentally and numerically studied by Xiao et al. [14], and the results were taken as references to design and model the LTES systems. Although eutectic system LiNO3/KCl has been mentioned previously and EG is widely used to enhance heat conduction process, little information about the utilizations of eutectic LiNO3/KCl in LTES systems and the thermal conductivity improvement of eutectic LiNO3/KCl with the assistance of EG has been reported. Hence, the development and study of a new LiNO3/KCl–EG composite
PCM is valuable for the promotion of solar thermal energy utilization. In the present work, a kind of composite materials with eutectic system LiNO3/KCl as PCM and EG as heat transfer promoter was prepared. The thermal properties of the LiNO3/KCl–EG composite were studied by measuring the melting temperature, latent heat of solid–liquid phase change and thermal conductivity. Moreover, the thermal energy storage performances of the composites with different EG mass contents and various apparent densities in a LTES unit were experimentally tested and compared. 2. Experimental description 2.1. Preparation of materials Raw expandable graphite with an expandable rate of 300 ml/g (mesh 50, from Qingdao Graphite Co. Ltd., China) was subjected to irradiation treatment in a domestic microwave oven (Midea Inc., China) with an overall power of 800 W for 10–20 s to yield EG. The solid–liquid PCM selected in this work was the eutectic system LiNO3/KCl. It was obtain by mixing corresponding amounts of lithium nitrate (>99 % pure) and potassium chloride (>99 % pure) with a mass ratio of 1:1. The thermal–physical properties of this eutectic salt are given in Table 1. Then the eutectic LiNO3/KCl powder was kept at 120 °C for 24 h to discharge the absorbed water, followed by being heated to 200 °C in order to be liquefied. The new LiNO3/KCl–EG composite was prepared by physical mixing of the liquid salt and EG with a roll mixer to ensure the pores in EG were occupied by the liquid salt. Then the composite material particles were allowed to cool down until the PCM was solidified. The microstructures of EG particles and the LiNO3/KCl–EG composite observed by using a scanning electron microscope (SEM, S3700N, Hitachi Inc., JNP) are shown in Fig. 1. The magnified images of EG demonstrates a worm-like structure with large volume and rich pores. After the PCM has been mixed with the EG particles, it can be observed that eutectic salt crystals were distributed in the porous network of EG and wrapped by micron-size layers owning to the high capillary force [23,24]. To explore the variations of the phase change latent heat and thermal conductivity of this composite material as the mass fraction of EG increasing, a series of the LiNO3/KCl–EG composites with EG mass fraction of 10, 15, 20, 25 and 30 % were prepared. Preliminary experiments indicated that when EG mass fraction was less than 10 %, PCMs could not be fully impregnated into EG, and the composite materials would suffer salt leakage after a few melting/solidification cycles. Thus, the composites with mass fraction less than 10 % were not taken into account in this work. 2.2. Phase change characterization The influences on phase change characteristics of the eutectic system LiNO3/KCl, including the melting temperature and phase change latent heat, after the addition of EG were investigated by differential scanning calorimeter (DSC, Q20, TA Instrument Inc., USA). The DSC tests were performed under the protection of nitrogen from 140 to 190 °C at a heating speed of 10 °C/min. The
Table 1 Thermo-physical properties of the eutectic system LiNO3/KCl. Thermo-physical properties
Eutectic system LiNO3/KCl
Melting temperature (°C) Temperature of the melting peak (°C) Phase change latent heat (J/g) Specific heat (J/g °C) Density (g/cm3) Thermal conductivity (W/m K)
165.60 170.72 201.70 1.10 (s); 1.87 (l) 2.01 1.749; 0.3315 (powder)
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Fig. 1. SEM images: (a–b) EG particles; (c–d) the LiNO3/KCl–EG composite.
accuracy of the calorimeter was within 1 % and the measurement error of temperature was within 0.01 °C. Prior to DSC test, each sample was dried at 120 °C for 24 h to minimize the error caused by the absorbed water. The quantity of each sample was in the range of 5–20 mg. The latent heat of melting for the composite material was obtained by calculating the area of the endothermic peak in a heat flow vs. time curve plotted by DSC. The melting temperature derived from the extrapolated onset point in a heat flow vs. temperature curve also could be obtained in the same DSC test. Three DSC experiments were carried out with each sample, and the mean values of the melting temperature and latent heat were reported. 2.3. Thermal conductivity measurement Hot disk thermal constant analyzer (TPS2500, Hot Disk Inc., Sweden) was applied in this study to measure the thermal conductivities of the composite materials with a type 8563 probe acting as both heat source and sensor. The probe used in these tests consisted of a thin nickel foil embedded between two thin layers of kapton polyimide protective films. A transient plane source method, in which two samples (disk-shaped) of the test materials with a similar thickness were required to be placed in contact with the probe and heated at constant power for a setting scanning time, was selected for these measurements. The type of the probe, heating power and scanning time for these tests were chosen based upon the diameter, thickness and range of thermal conductivity of each sample. The uncertainty of the measurements was within ±2 %.
with the help of a temperature control system with an accuracy of ±1 °C. The LTES unit consisted of a cylindrical stainless-steel container with an inner diameter of 50 mm, a wall thickness of 0.5 mm and a length of 80 mm. Two K-type thermocouples were placed in the test unit to monitor the temperature variations of each sample within a measurement error of ±0.1 °C. Locations of the thermocouples are also presented in Fig. 2. The two test points both were spaced at a same depth, being 32 mm away from bottom of the container, to make sure the effects of input heat flux from top and bottom of the container on them were the same. Initially, four-fifth of the LTES unit volume was filled with the prepared composites at 120 °C, and the remaining space was left to accommodate the thermal expanding of PCMs. During the heat storage process, the thermostatic air bath box was kept at constant temperature 200 °C. A data logger (Agilent34970A, USA) was used to collect the voltage signal generated by the thermocouples and convert it to digital data.
3. Results and discussion 3.1. DSC analysis of the LiNO3/KCl–EG composites Fig. 3 shows the typical DSC thermograms of the selected LiNO3/ KCl–EG composites with different EG mass fractions, and the
2.4. Thermal energy storage performance evaluation The thermal performances of the LiNO3/KCl–EG composites during thermal energy storage process were investigated in order to evaluate the effects of EG on heat transfer behavior. The schematic diagram of the experimental system shown in Fig. 2 mainly comprised a thermostatic air bath box and a LTES unit. The temperature of the air bath was automatically controlled and measured
Fig. 2. Schematic diagram of the experimental set-up (a) PC, (b) data logger, (c) thermostatic air bath box, (d) temperature control system, (e) sample and (f) thermocouples: (1) r = 0 mm, h = 32 mm and (2) r = 15 mm, h = 32 mm.
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corresponding data collected from the DSC tests are summarized in Table 2. The DSC analysis of the eutectic system LiNO3/KCl (sample with EG mass fraction of 0 %) was introduced as a reference to investigate the thermal property changes after the addition of EG. Obviously, the composites all exhibit similar curve shapes with that of the eutectic LiNO3/KCl, indicating that it was the eutectic salt that acted the role of the latent thermal energy storage during the melting process. However, the phase change characteristics of the eutectic LiNO3/KCl were affected by adding EG. As listed in Table 2, the melting temperatures of the composites are approximately the same as that of the eutectic salt, whereas the melting peak temperature slightly shifts from 170.72 °C (Table 1) for the eutectic salt to around 168 °C for the composites. This temperature discrepancy is also depicted in Fig. 3. It is obvious that for the composites, the angles on melting point are shaper and the endothermic peaks are narrower than that of the eutectic LiNO3/KCl. This phenomenon could be ascribed to the porous EG network in the composites that provided heat conduction path in the eutectic salt and consequently accelerated the phase change speed of the composites [25,26]. The phase change latent heats of the LiNO3/KCl–EG composites are in the range from 142.41 to 178.10 J/g (Table 2) according to the DSC tests. Because EG did not undergo a phase transition process and it was the eutectic salt that accounted for the latent thermal heat, the latent heat decreases with an increase of the mass fraction of EG. The relation can be expressed as Eq. (1):
DHcomposite ¼ ð1 w%ÞDHsalt
ð1Þ
where DHcomposite represents the calculated phase change latent heat of the LiNO3/KCl–EG composite, DHsalt is the latent heat of the eutectic system LiNO3/KCl and w % is the mass fraction of EG. The experimental phase change latent heats of the composites are compared with their calculated values in Table 2, revealing that the experimental values show an agreement with the calculated values with relative errors less than 3 %. 3.2. Thermal conductivity measurement of the LiNO3/KCl–EG composites Since the LiNO3/KCl–EG composite particles pack together loosely under natural conditions and thermal conductivities of porous materials are significantly influenced by their densities, the apparent density of the composite material was taken into account in this thermal conductivity measurement. For LTES systems, the density of PCMs cannot be too low considering of the heat storage capacity for a given volume and the investment cost of the whole system [18]. The apparent density of the LiNO3/KCl–EG composite
Fig. 3. DSC curves of the LiNO3/KCl–EG composites with different mass fractions of EG.
also cannot be too high because severe salt leakage will appear if the void space within the porous EG is not enough for salt volume expansion [8,9]. In the experiments, the apparent densities of all samples were lower than 1.50 g/cm3 to make sure that the salt leakage could not be observed after the composites were maintained at 200 °C for 5 h. The results of the thermal conductivity measurements are shown in Fig. 4. Fig. 4(a) presents the thermal conductivities of the LiNO3/ KCl–EG composites as a function of the EG mass fraction for a same apparent density. Clearly, the thermal conductivity is remarkably improved with an increase of EG mass content. For example, while the apparent density is 1.01 g/cm3, the thermal conductivity of the eutectic LiNO3/KCl (1.749 W/m K) can be improved by 1.85 times using 10 % EG in the composite material, and it can be further improved by 6.65 times when 30 % EG is added into the composite material. This improvement was caused by the addition of EG which was acting as a type of thermal conductive matrix with a much higher thermal conductivity than the eutectic salt. Eq. (2) is derived from the plot in Fig. 4(a) that shows an almost linear relationship between the thermal conductivity of the LiNO3/ KCl–EG composite (y) and mass fraction of EG (x).
y ¼ 0:4224x þ 0:7595 ðR2 ¼ 0:99964Þ
ð2Þ
For investigating the effect of the different apparent densities on the thermal conductivity of the composite material, the composites with EG mass fraction of 20 % were taken as example. Fig. 4(b) indicates a trend that the thermal conductivity increases obviously with the rising apparent density. It can be noticed that while the EG mass fraction is 20 %, the thermal conductivity for 0.75 g/cm3 is about 5.12 W/m K, and the value for 1.42 g/cm3 is 14.98 W/m K which is almost 2 times higher. This trend could be attributed to the reduction of void space within the composite material and extension of contact surface area for the composite particles. Data in Fig. 4(b) is fitted by an ‘‘S’’-shape curve. The relation is expressed as Eq. (3):
y ¼ 16:6845
12:5050 R2 ¼0:98932 7:0556 x 1 þ 1:0904
ð3Þ
3.3. Thermal energy storage performance in a LTES unit For the research on the thermal energy storage performance of the LiNO3/KCl–EG composite, the EG mass content and apparent density of the composite material were considered as two important factors. A comparatively low apparent density ranging from 0.3 to 0.5 g/cm3 was applied in these tests for simplification of packing samples into the container and observation convenience. Fig. 5(a) shows the temperature profiles of the studied composites with different EG mass fractions during the heat storage process when the apparent density is 0.4 g/cm3. It can be observed that all test points exhibit a three-step behavior during the melting, which is similar to that occurred in different types of LTES systems with various PCMs [18,26–28]. At the first step when the temperature climbed sharply until it reached the melting point, heat conduction inside the solid phase dominated the heat transfer. After that, the liquid phase of the eutectic LiNO3/KCl began to emerge. Since the melting region was confined within the smallsize porous skeleton of EG and natural convection inside the liquid phase was severely suppressed, heat conduction was still the vital factor that driven the heat transfer speed. Previous works on NaNO3/EG composite [18], AC/EG composite [26] and paraffin/EG composite [27] have proved the heat conduction-dominated mechanism.
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Z. Huang et al. / Applied Energy 115 (2014) 265–271 Table 2 Phase change properties of the LiNO3/KCl–EG composites with different EG mass contents. Mass fraction of EG (%)
Melting temperature (°C)
Temperature of the melting peak (°C)
10 15 20 25 30
165.55 165.87 165.57 165.55 165.58
169.09 169.70 168.57 168.15 168.38
Fig. 4. The thermal conductivity variation of the LiNO3/KCl–EG composite: (a) with the mass fraction of EG and (b) with the apparent density of the composite material.
During the thermal energy storage period, the LTES unit was heated continuously from its surrounded walls and heat was transferred from its outer cylindrical surface to inner, thus the temperature at test point 1 and 2 increases at a different rate (see Fig. 5(a)). As shown in Fig. 5(b), the composite material with EG mass fraction of 30 % has obtained a slight range of temperature difference between the two test points (DT = T2 T1), whereas the range is extended when the EG mass content decreases. The maximum temperature differences (DTmax) for EG mass fractions of 10, 20 and 30 % are determined as 8.16, 4.83 and 3.01 °C, suggesting that with higher mass content of EG, the heat transfer in the composite material would be further enhanced. This phenomenon resulted from the improvement of the thermal conductivity with the assistance of increasing EG mass fraction. Moreover, based on the heat conduction-dominated mechanism, the thermal energy storage process could be considered as an unsteady heat conduction process, thus the time for samples to reach temperature equilibrium was positively related to PCMs’ energy storage density. As shown in Fig. 5(a), the heat storage durations the LiNO3/KCl–EG composites with EG mass fraction of 10, 20 and 30 % are determined as 3480, 2810 and 2460 s at the central position of the LTES unit (test point 1) under the same test conditions. It is clear that
Phase change latent heat Calculated value (J/g)
Experimental value (J/g)
Relative errors (%)
183.33 173.145 162.96 152.77 142.59
178.10 170.69 164.50 150.96 142.41
2.85 1.42 0.95 1.19 0.13
Fig. 5. Thermal energy storage performances of the LiNO3/KCl–EG composites with various mass fractions of EG: (a) temperature profile of the composites and (b) temperature differences inside the LTES unit.
with higher mass content of EG, the composite material can finish the heat storage period in a shorter time. The result was caused by the reduction of the phase change latent heat of the composite material via increasing the EG mass fraction, which consequently leaded to a lower energy storage density. Fig. 6(a) shows the temperature profiles of the LiNO3/KCl–EG composites with different apparent densities, when the mass fraction of EG is 20 %. During the heat storage period, it takes 2610, 2810 and 3160 s for the samples with apparent densities of 0.3, 0.4 and 0.5 g/cm3 to reach the thermal balance, respectively. Since the energy storage density of the LTES unit increased with the increasing apparent density of the sample, the composite material with a higher apparent density would take a longer time to complete the heat storage period. Fig. 6(b) demonstrates the curves of temperature difference DT inside the test unit while using the LiNO3/KCl–EG composites with various apparent densities. It is observed that DTmax for the apparent densities of 0.3, 0.4 and 0.5 g/cm3 are 10.38, 4.83 and 2.89 °C, respectively. By comparing the temperature difference ranges, the increase of the apparent density would be desirable for
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rial, which mainly depended on the energy storage density, was affected by its phase change latent heat and apparent density. Consequently, the thermal energy storage duration was shorten when the LiNO3/KCl–EG composite with higher EG mass fraction was applied in the LTES unit, whereas it was extended with the apparent density increased.
Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 20976056).
References
Fig. 6. Thermal energy storage performances of the LiNO3/KCl–EG composites with different apparent densities: (a) temperature profile of the composites and (b) temperature differences inside the LTES unit.
increasing the heat transfer ability of the composite material during the heat storage process. Similar to the phenomenon in Fig. 5(b), the reason was the improvement of the thermal conductivity by increasing the apparent density. 4. Conclusions In this study, a new kind of composite PCMs was prepared and investigated for thermal energy storage application at high temperature (200 °C) by using eutectic system LiNO3/KCl whose latent heat of phase transition was high and EG which was a heat transfer enhancer. The results revealed that the LiNO3/KCl–EG composite could be suggested as a potential candidate to storage thermal energy in solar energy utilizations. The conclusions were drawn as follows. (1) The presence of EG within the PCM induced no significant change in the phase change temperature of the eutectic LiNO3/KCl, but a decrease in phase change latent heat of the composite material from 178.10 to 142.41 J/g depending on the mass content of EG. (2) The addition of EG into the eutectic LiNO3/KCl resulted in a remarkable improvement in the thermal conductivity. According to the measurements, the thermal conductivities of the studied composites ranged from 5 to 15 W/m K, which was not only related to the mass ratio of EG but also associated with the apparent density of the composite material. (3) During the thermal energy storage process, the utilization of the LiNO3/KCl–EG composite with higher apparent density and EG mass fraction could further enhance the heat transfer through the thermal conductivity improvement. Moreover, the thermal energy storage duration of the composite mate-
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