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Thermal performance of sodium acetate trihydrate based composite phase change material for thermal energy storage
Applied Thermal Engineering 143 (2018) 172–181
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Research Paper
Thermal performance of sodium acetate trihydrate based composite phase change material for thermal energy storage
T
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Liang Zhao , Yuming Xing, Xin Liu, Yegang Luo School of Aeronautic Science and Engineering, Beihang University, Beijing 100083, PR China
H I GH L IG H T S
compatibility of sodium acetate trihydrate was tested. • The sodium acetate trihydrate was modified with five kinds of nucleating agents to reduce the supercooling. • The heat storage units based on the copper foam/ modified sodium acetate trihydrate were proposed. • The • The heat charging and discharging performance of heat storage unit was investigated.
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal energy storage Sodium acetate trihydrate Copper foam Compatibility Supercooling Heat storage performance
Sodium acetate trihydrate (SAT) as a phase change material (PCM) suffers from large supercooling, unclear compatibilities and low thermal conductivity. In this paper, the compatibility of SAT was tested for 270 days with aluminum alloy and copper. The corrosion phenomenon was evaluated by the scanning electron microscope (SEM) images and EDS (energy dispersive spectrum) analyses, which indicated that the corrosion effect could be neglected. Then, SAT modified with the additives of 2 wt% disodium hydrogen phosphate dodecahydrate (DHPD) and 2 wt% carboxyl methyl cellulose (CMC) showed the best performance in reducing supercooling. Finally, a laboratory-scale experiment was conducted to investigate the heat-charging and discharging performance of the heat storage units based on a copper foam/SAT composite PCM. The findings indicated that the heat-charging rate was based mainly on the heat power level, and the composite PCM with fewer thermal conductivity enhancers showed better heat storage performance. The heat discharging process revealed that the heat storage units still had more supercooling than the modified SAT. Based on the results obtained, the copper foam/SAT composite PCM appears to be a promising heat storage material, while the supercooling still needs to be considered in application.
1. Introduction With the progressive intensification of energy shortages and environmental pollution [1–3], it becomes more urgent to improve energy efficiency, energy saving and emission reduction. Thermal energy storage is an important component of an energy application system, which could ease the mismatch in time and location between energy demands and supply [1,4–7]. In general, thermal energy storage can be divided into three groups: sensible energy storage, chemical energy storage and latent thermal energy storage. Compared with sensible energy storage and chemical energy storage, latent thermal energy storage is a more effective energy storage method because of the high storage energy density, suitable phase change temperature, stable chemical properties and low cost, and latent thermal energy storage has been widely used in
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the field of solar energy storage [8], building energy management systems [9,10], waste heat recovery systems [11,12] and thermal management systems [13,14]. The several kinds of phase change materials (PCMs) could be divided into three groups: the organic PCMs, the inorganic PCMs and the eutectic PCMs [15,16]. Many organic PCMs such as paraffin, sugar alcohols and fatty acids always have the drawbacks of relatively high cost, low thermal energy storage density and flammability, which are non-negligible and constrain the practical application of organic PCMs. Comparatively, the inorganic PCMs, especially hydrated salts, whose melting point ranges from a few degrees Celsius to over 100 °C always possess superior properties such as higher thermal energy storage density, better thermal conductivity, nonflammability and lower cost, which indicate that hydrated salts can be widely applied under the working conditions of human activities,
Corresponding author. E-mail address: [email protected] (L. Zhao).
https://doi.org/10.1016/j.applthermaleng.2018.07.094 Received 7 April 2018; Received in revised form 14 July 2018; Accepted 17 July 2018 Available online 18 July 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.
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For the hydrated salts, the compatibility of the thermal conductivity additives and the encapsulation materials with the PCM is a key issue to ensure the long-term applications of PCM-based thermal energy storage systems. The corrosion resistance of commercial metals in contact with several hydrated salts with the melting point ranging from 32 to 58 °C was tested by Cabeza et al. [34,35] with a duration up to 70 days. They found that the SAT had a slight corrosion effect on the copper and had almost no effect on the aluminum. Moreno et al. [36] experimentally evaluated the corrosion rate of two metals and two metal alloys that were in contact with different hydrated salt PCMs by weighing the mass change. Five kinds of container materials were immersed by Browne et al. in five different PCMs for a period of 722 days to investigate the compatibility of PCM with various materials [37]. The results showed that the steel was the most suitable container materials for all the PCMs. Considering the above-mentioned papers, various methods have been proposed for reducing the degree of supercooling, diminishing the phase separation, improving thermal conductivity and investigating the compatibility of SAT. The thermal conductivity of the solid SAT ranged from 0.17 W/mK to 0.7 W/mK [27,32,38] at different temperatures, and the thermal properties of the SAT composite varied significantly with different additives at different mass fraction [20,32,39]. Most related papers either reported the thermal properties of the composite SAT or focused on the thermal energy storage performance. Few research studies were systematically related to the modification of the SAT and its thermal energy storage performance. The compatibility of SAT with metals was tested with the immersion corrosion method and evaluated by weighing the mass changes. Little information about the analysis of the microphenomenon of corrosion was reported in detail. Consequently, it is valuable to systematically investigate the thermal property modification of the SAT and the thermal energy storage performance of the composite SAT-based PCM, and the compatibility of the SAT with commonly used metals. In the present study, the compatibility of SAT was investigated first. Second, the SAT was composited with different additives (disodium hydrogen phosphate dodecahydrate (DHPD), sodium carbonate decahydrate (SCD), sodium silicate nonahydrate (SCN), borax decahydrate (BDH), and quart sand (QS)) to modify the supercooling, and the carboxymethyl cellulose (CMC) was used as a thickening agent. Finally, two kinds of copper foam/hydrated salt composite PCMs were prepared by using the copper foams as supporting matrixes and the modified SAT as PCM, and the thermal energy storage performance of the composite PCMs was evaluated by funding a small-scale experimental test unit.
domestic instruments and equipment. Sodium acetate trihydrate (CH3COONa·3H2O, SAT) with the melting point of ∼58 °C is a representative of hydrate salt PCMs, which could easily be integrated with space heating and domestic hot water preparation [17], solar heating systems [18], and radiant floor heating systems [19]. However, SAT always suffers from serious supercooling problems and low thermal conductivity during the thermal energy storage process as most hydrated salts are [20]. Aiming at overcoming the drawbacks of SAT, extensive efforts have been conducted. Jin et al. [21] performed an experiment involving the cooling processes of a partially melted SAT. The results showed that the degree of supercooling of partially melted SAT increased with the elevation of the heating temperature. Zhou and Xiang [22] experimentally investigated the stable supercooling characteristics of an SAT mixture within three types of thermal storage units, indicating that the inner surface roughness, cooling rate, salt-water mass ratio and carboxyl methyl cellulose could all affect the stable supercooling of the SAT. Several kinds of nucleating agents and thickeners were selected by Mao et al. [23] to improve the phase-change energy storage properties of SAT. The experimental results indicated that adding disodium hydrogen phosphate dodecahydrate can reduce the degree of supercooling. Cabeza et al. [24] conducted an experiment to find the effective thickeners for SAT and discovered that SAT could be thickened successfully with starch and bentonite, with the mixtures showing a melting point similar to the SAT, and the enthalpy would decrease with the type and the amount of changing of the thickeners. Fashandi and Leung [25] found that adding a bio-derived chitin nanowhisker was an effective way to suppress the supercooling of SAT. Different mass fractions of silver nanoparticles were experimentally studied by Ramirez et al. [26] to reduce the supercooling of SAT. The heat transfer enhancement is another key issue for the SAT applied as a PCM for thermal energy storage. Some thermal conductivity enhancers such as nanoparticles, metal foams and graphite materials can not only reduce the degree of supercooling but could also effectively improve the thermal conductivity of SAT. Cui et al. [27] reported that adding Nano-Cu could not only reduce the degree of supercooling of SAT but also increase the thermal conductivity of the composite SAT by nearly 20% compared to the pure SAT. Johansen et al. [28] investigated the effects on the thermal conductivity and supercooling stability of SAT of adding graphite powder. The results showed that the graphite powder was a promising thermal conductivity enhancer. A series of expanded graphite-based composite PCMs using an SAT-urea non-eutectic mixture was prepared and tested by Fu et al. [19]. The test results showed that the composite PCMs with the mass fraction of 8% had the thermal conductivity of 2.076 W/mK and the degree of supercooling of 1.54 °C. Mao et al. [29] prepared a novel composite phase-change material based on SAT by adding a certain proportion of expanded graphite, which showed a relatively small degree of supercooling and larger thermal conductivity. They [30] also proposed a series of SAT-disodium hydrogen phosphate dodecahydratecarboxyl methyl cellulose/expanded graphite composite PCMs. They concluded that the composite PCM with 3 wt% expanded graphite had the most suitable thermal properties and that the degree of supercooling and the thermal conductivity were 2 °C and 1.37 W/mK, respectively. Shin et al. [31] proposed a series of SAT composites containing different mass fractions of expanded graphite and carboxymethyl cellulose. The measured results manifested that expanded graphite could effectively improve the thermal conductivity and diminish the supercooling effect of SAT. Dannemand et al. [32] prepared a series of SAT-based composite PCMs and measured their thermal conductivity. The results revealed that the composite PCM with SAT, 1% xanthan rubber and 5% graphite flakes had the highest thermal conductivity of 1.1 W/m K. Li et al. [33] studied the supercooling and thermal conductivity of the SAT composited with copper foam, which showed smaller degree of supercooling and larger thermal conductivity than the pure SAT.
2. Materials and instruments SAT, DHPD, SCD, SCN, BHD, QS and CMC (analytical reagent grade, purity > 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Copper foams with the pore size of 20 PPI (PPI means the numbers of pores in per inch) and the porosity of 0.98 and 0.88 were offered by Zhuoer Technology Co. Ltd. (Changchun, China). The apparatus used for the experiments included a differential scanning calorimeter (DSC, 214 Polyma, Netzsch, Germany), an energy dispersive spectroscope (EDS, Hitachi Inc., Japan), scanning electron microscope (SEM, Hitachi S4800, Hitachi Inc., Japan), several PT100 temperature sensors (precision ± 0.1 °C), DC power supply module by Beisina Tchnologies (SNK-22H06, 0-220 V/0-6 A), data acquisition module (ADVANTECH ADAM-4015), and a high-low temperature test chamber (BPH-120B, Shanghai Everone Precision Instruments Co., Ltd., China). 3. Experimental work 3.1. Preparation of SAT composites SAT was selected as the phase change material for thermal energy storage. However, the pure SAT has a considerably large degree of 173
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supercooling. The SAT composites containing different additives (DHPD, SCD, SCN, BHD, QS and CMC) were prepared by applying a simple blending method. First, the SAT and the additives were weighed with the selected contents of nucleating agents and CMC and were then well mixed in the porcelain crucible. Second, the mixture was poured into a sealed glass tube and placed into a thermostatic water bath and heated at the temperature of 75 °C until the mixture was completely melted. Finally, the mixture was cooled and solidified as the sample for a supercooling experiment. By repeating the above procedures, several kinds of modified SATs could be prepared. The copper foam/modified SAT composite PCM was prepared by using a vacuum impregnation method in which the modified SAT composite was utilized as the PCM, and the copper foam was applied as thermal conductivity enhancer. The preparation procedure contained three steps with vacuum assistance to increase the filling rate. First, the modified SAT composite in the stainless-steel container was heated at the temperature of 75 °C in the constant temperature water bath until the modified SAT composite was completely melted. Second, the prepared copper foam with the dimensions of 100 mm × 100 mm × 21 mm was completely immersed in the liquidmodified SAT composite. The vacuum pump connected to the container started working to maintain the pressure in the container below 50 Pa. To fill the porous space of the copper foam with the liquid-modified SAT composite completely. This filling process lasted for 2 h with the liquid-modified SAT composite maintained at the temperature of 75 °C. Finally, the sample in the stainless-steel container was cooled to the temperature of 25 °C in the thermostatic water bath until the modified SAT composite was entirely in solid phase. Then, the vacuum pump was switched off, the copper foam filled with modified SAT composite was brought out, and the surplus modified SAT composite on the sample was cleared away. The copper foam enhanced composite PCM was obtained.
Fig. 2. Schematic diagram of the supercooling experimental apparatus.
bubbles, precipitates, surface changes, and a pitting process. The EDS mapping of the tube inner surface was provided to further investigate the corrosion phenomenon. 3.3. The supercooling experiment The step cooling curve method and DSC were adopted in the supercooling experiment. The cylindrical metal tubes that were made of stainless steel containing the composite PCMs (approximately 10 g) were placed in the high-low temperature test chamber for SAT melting and solidification by changing the temperature of the chamber as shown in Fig. 2. The PT100 temperature sensors (precision ± 0.1 °C) were installed at the center of the tubes to monitor the temperature variation during the experiment.
3.2. The compatibility experiment In this study, the compatibility experiment contained four steps: (1) the preparation of samples, (2) the corrosion test of copper and aluminum alloy, (3) sample clean-up, and (4) corrosion evaluation. Before the experiment, two aluminum alloy test tubes and two copper test tubes were designed for containing the SAT as shown in Fig. 1(a). Because pure SAT has considerably large supercooling that could be in the liquid phase even at room temperature, the four test tubes filled with liquid SAT (approximately 10 g) were maintained in the high-low temperature test chamber at the temperature of 70 °C for 240 h, and then maintained at room temperature for 270 days as the corrosion experiment. During the corrosion experiment, the tubes were sealed with nuts to avoid the contact with environmental agents. After the test, the SAT in the tubes was removed, and the tubes were cleaned up and cut into small pieces for corrosion evaluation as shown in Fig. 1(b). The corrosion evaluation was conducted by applying the SEM to seek for
3.4. Experimental heat storage unit Two small-scale heat storage units applying the copper foam/SAT composite PCMs were prepared as presented in Fig. 3(b). The copper foam (20 PPI, the porosity of 98% and 88%) was cut to the size of 100 mm × 100 mm × 21 mm, as shown in Fig. 3(a). A simple experiment was founded to investigate the thermal performance of copper foam/SAT composite-based heat storage unit. The heat storage was put into the high-low temperature test chamber, heated by a film heater at the bottom, and cooled by natural convection in the chamber. The schematic diagram of the experiment is shown in Fig. 4. A 100 × 100 mm2 film heater was applied as the mimicry of heat generation, which was pasted on the bottom of the heat storage unit. The power required by the heater was provided by a DC power supply module. As the key parameter being measured in this investigation,
Fig. 1. Compatibility test samples: (a) Four tubes, (b) The test samples. 174
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Fig. 3. Photographs of the copper foam (a) and heat storage unit (b).
nine PT100 temperature sensors that had a very high sensitivity of ± 0.1 °C were pasted onto the heater as shown in Fig. 5. The data acquisition module was connected to a computer and recorded the temperature at intervals of 2 s. The transient temperature variations were the average value of the nine PT100 temperature sensors.
4. Results and discussion 4.1. The compatibility of SAT As the SAT shows alkalinity, it may cause corrosion of the container material and metal foam that affects the thermal performance and the reliability of the heat storage unit for long-term application. Aluminum alloy and copper are the most common container and thermal enhancement materials. During the storage time, the SAT was stored in the aluminum alloy test tubes and copper test tubes in the liquid phase. After the experiment, the tubes were cut into the samples as shown in Fig. 1(b) to scan the inner surface of the tubes to investigate whether corrosive phenomena appeared. For comparison, the samples with the same original metal materials and same machined pattern were regarded as the samples before the compatibility experiment and analyzed, which had never been contact with SAT. The inner surface of the aluminum alloy tubes showed no color variation. The color of the inner surface of the copper tubes turned into the gray and black. Further investigating the corrosion effect, the SEM images of samples before the compatibility experiment and after the compatibility experiment were taken as shown in Figs. 6 and 7. No obvious corrosive phenomena such as bubbles, precipitates, surface changes, and pitting processes appear on the surface of the aluminum
Fig. 5. Distribution of PT100 temperature sensors.
alloy tube. On the surface of the copper tube, the granulated particles were uniformly distributed, reflecting that the SAT had a corrosive effect on copper. To investigate the corrosive phenomena on the inner surface of the tubes digitally, the main compositions of the original metal materials of the tubes and the inner surface of the metal tubes after the experiment were analyzed by an EDS as presented in Fig. 8. The detailed data, listed in Table 1, showed that after the experiment,
Fig. 4. Diagrammatic sketch of the experimental system. 175
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Fig. 6. SEM photograph of the inner surface of the aluminum alloy tube: (a) before the experiment, (b) after the experiment.
and the thin corrosion film could not be observed from the cross section of the copper sample as shown in Fig. 7(c), which demonstrated that the corrosion did not infiltrate into the copper material. The copper oxide layer was also reported to increase the corrosion resistance of the copper [41,42]. Therefore, the SAT has a limited corrosive effect on aluminum alloy and copper, which could be used for container materials and thermal conductivity enhancement materials for long-term use. And due to the much larger thermal conductivity of the copper, the copper foam was utilized as the thermal conductivity enhancer, the aluminum alloy was used as container material.
the main composition of the inner surface of the aluminum alloy tube in weight ratio were magnesium 1.12%, aluminum 70.49% and oxygen 28.39%, and the main composition of the inner surface of the copper tube in weight ratio were copper 62.44%, zinc 4.7% and oxygen 32.86% after the experiment. Compared with the original metal materials, the element of the oxide increased significantly in weight. The original copper sample had a little element of oxide which was induced mainly by the unclear surface of the sample. The composition of magnesium, aluminum and copper all appeared as aluminum oxide, magnesium oxide and copper oxide, which indicated that the SAT had a corrosive effect on both aluminum alloy and copper. However, synthesizing the phenomenon in the SEM images, we can conclude that the corrosive effect on aluminum alloy induced by SAT could be neglected, and it is well known that the aluminum oxide is a stable oxidizing material that could prevent further corrosion [40]. While the corrosive phenomenon on copper induced by the SAT seems to be more serious, a thin corrosion film appears. However, the corrosion rate was very slow,
4.2. The effect of different nucleating agents on the supercooling of SAT For most of the supercooling experiments, the SAT composites were heated in the constant temperature water and cooled under the natural convection conditions [23,24,29]. In this study, “Scientific method” was applied for the formulation test, five kinds of nucleating agents
Fig. 7. SEM photographs of the inner surface of the copper tube; (a) before the experiment, (b) after the experiment, (c) SEM photograph of the cross section of the copper tube. 176
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Fig. 8. EDS photographs of the inner surface of tubes; (a) the aluminum alloy tube before the experiment, (b) the aluminum alloy tube after the experiment, (c) the copper tube before the experiment, (d) the copper tube after the experiment.
Table 1 The EDS analysis results of the surface of samples before and after the compatibility experiment. Tube
Before
After
Element
Weight %
Element
Weight %
Aluminum alloy tube
Al Mg O
98.41 1.59 –
Al Mg O
70.49 1.12 28.39
Copper tube
Cu Zn O
92.13 7.38 0.49
Cu Zn O
62.44 4.7 32.86
were selected as proposed in some former research [20,23,29,33], and the SAT composites were heated and cooled in the high-low temperature test chamber by regulating the temperature of the chamber to address the degree of supercooling. To search the best ratio of the SATbased composite and reduce the experimental workload, the different mass ratios of the nucleating agent were added into the pure SAT, and 2 wt% CMC was applied as a thickener [29] introduced to investigate the degree of supercooling. The step cooling temperature variations of pure SAT and three kinds of SAT-based composites containing 8 wt% BDH, 8 wt% QS and 8 wt% SCD were compared as shown in Fig. 9. It shows that the nucleating agent QS and BDH could almost not reduce the degree of supercooling of SAT, and the supercooling degree of SAT could be improved to a limited extent by SCD, which was approximately 13 °C. Fig. 10 shows the step cooling temperature curves of the SAT composited with different mass ratios of SCD, which shows that the degree varied with the mass ratio of SCD in the composite. The SAT composite containing 4 wt% SCD exhibited the minimum degree of supercooling of approximately 6.85 °C. The temperature variations presented in Fig. 11 indicate that when the mass ratio of SAT to SCN was 100:6, the minimum degree of supercooling of the SAT-based composites was approximately 4.83 °C. The SAT-based composites containing 2 wt% and 4 wt% showed a slightly
Fig. 9. Cooling curves of pure SAT and SAT composites.
larger degree of supercooling of 6.44 °C and 6.02 °C, respectively. As shown in Fig. 12, it is clear that when the mass ratio of DHPD was 2%, the degree of supercooling of the SAT-based composite was relatively small, approximately 4.6 °C. After the supercooling analysis, among these five kinds of nucleating agents, the DHPD with the mass ratio of 2% could solve the problem of supercooling more efficiently. Then, the latent heat of the SAT with 2 wt% CMC and 2 wt% DHPD was determined by applying a DSC thermal analysis at a heating rate of 3 °C/min under a N2 atmosphere as shown in Fig. 13. Compared with the latent heat of the pure SAT of 275.8 J/g, the SAT composite had a smaller value of 253.6 J/g. While the phase temperature varied a little, the value for the pure SAT was 57.85 °C, and it was 57.76 °C for the SAT composite containing 2 wt % DHPD and 2 wt% CMC.
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Fig. 13. DSC curves of pure SAT and modified SAT. Fig. 10. Cooling curves of SAT composites with different mass ratios of SCD.
foam. Recall that the heat-charging process was preceded by heat flux, and the heat discharging process was driven by natural convection in the high-low temperature test chamber.
4.3.1. The heat-charging process The PCM-based heat storage units have a thermal energy storage mechanism that includes three steps: pre-heating, phase change process and super-heating. In the first step, heat is absorbed by the specific heat of the solid PCM to raise its temperature to its melting point. In the second step, the PCM begins to melt at an approximately constant temperature until the PCM is fully in the liquid phase, during which time heat is absorbed by the latent heat of the PCM. In the third step, the PCM is completely in the liquid phase, and heat is absorbed by the specific heat of the liquid PCM. Unlike heating with temperature and convection conditions, the temperature of the PCMs would continually increase with further heat absorption. In general, most of the heat is absorbed by the latent heat of the PCM during the phase change process. In the first section, the heat-charging process was experimentally investigated under different working conditions. To identify the influence of natural convection on the charging process, the comparative experiment was conducted. The units were wrapped in glass wool with a thickness of 2 cm for the insulation conditions, and the units were put into the chamber by controlling the temperature at 20 °C to realize the natural convection condition with a temperature of 20 °C. An average temperature reading of the temperature sensors on the bottom of the unit was used to record the time-temperature response. When the units were wrapped by the glass wool, the heat generated by heater could be almost completely absorbed by the unit. The heat-charging rate could be reflected from the temperature variation due to the isothermal operating characteristics of the PCM. The time-temperature history for two heat storage units with insulation and with the ambient temperature of 20 °C is shown in Fig. 14 for the power level of 20 W. The heat-charging process could be divided into three steps, as mentioned before. The unit enhanced by the copper foam of 88% porosity and insulated with glass wool is used as an example. The temperatures rose quickly as the heat power was loaded. The discrepancy between the temperature change curves for the four cases is small in the pre-heating and phase change processes as marked in Fig. 14. This discrepancy is induced by two reasons: one reason is that the heat power is relatively small, and another reason is that the container material of aluminum alloy also has a much higher thermal conductivity, which demonstrates that they all had the ability to transfer and dissipate the heat as soon as possible at the power level of 40 W, indicating that both units had a similar heat-charging rate. As the heat-charging processes, the third step, superheating starts at different
Fig. 11. Cooling curves of SAT composites with different mass ratios of SCN.
Fig. 12. Cooling curves of SAT composites with different mass ratios of DHPD.
4.3. Heat storage performance of the copper foam/SAT composite PCM For investigating the transient behavior of the copper foam/SAT composite PCM-based heat storage unit, the modified SAT composite (SAT + 2 wt% CMC + 2 wt% DHPD) was composited with copper 178
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Fig. 14. Heat-charging process of heat storage units with different boundary conditions.
Fig. 15. Heat-charging process of heat storage units with different ambient temperatures.
times for the four cases. The same unit needs more time to complete the phase change process when it is put in the ambient conditions with the temperature of 20 °C because natural convection could release heat during the heat-charging process. For the copper foam/modified SAT composite PCM, the quantity of the modified SAT in the copper foam per unit volume is directly related to the bulk porosity density (ε ). Thus, the estimated storable latent thermal energy could be calculated as shown in Eq. (2) [43]. The effective thermal conductivity of copper foam/modified SAT composite PCM could be calculated by the Bhattacharya model [44] as shown in Eq. (3), which has been verified by Li et al. [33].
chamber with different ambient temperatures. The temperature evolution of the units is shown in Fig. 15, which could also be divided into three steps. These steps are illustrated using the unit with ambient temperature of 40 °C and enhanced by copper foam of 88% porosity as an example. When the power level was increased to 60 W, although the ambient temperature had a difference of 20 °C, the temperature curves nearly overlap for the four cases during the pre-heating and phase change process, as shown in Fig. 15, indicating that both units have the similar heat-charging rate during the first two steps. The discrepancy of the temperature appears until the superheating region is reached, indicating that the ambient temperature change rarely has an effect on the temperature of the unit for the heat-charging. For the same unit, the phase change process seems to be a little longer at an ambient temperature of 20 °C because the weak natural convection could play a small role to release heat during the heat-charging process. The temperature at the onset phase change process is much higher than the melting point of the modified SAT because the thermal resistance during the heat-charging process would be more evident as the power level increases. A review of the above analysis shows that the natural convection has an influence on the charging time but rarely has an effect on the temperature of the unit during the charging rate. Unlike the temperature and convection boundary conditions, the effective thermal conductivity of the PCM is not the main factor to decide the heat-charging rate. The heat-charging rate is almost the same when the units are loaded with the same power level. In the second section, the heat-charging performance of two heat storage units with different heat levels (20 W, 30 W and 40 W) was assessed. The measured temperature variations during the heat-charging process are plotted in Fig. 16. The turning point of the start of melting appears earlier, and the heat-charging process would be reduced as the power level increases, which means the heat-charging rate is based mostly on the power level. The turning point of the start of melting has a higher temperature with higher power level, which reflects that the overheating will be more serious with a high level of power. The temperature curves for different units with the same power level nearly overlap during most of the heat-charging process, and they will depart from each other at the end stage of the heating process due to the different amounts of heat stored in the unit. The heat-charging rate is decided mainly by the power level. The SAT-based composite PCM with more thermal conductivity enhancer seldom reflects any advantages. However, the lost amount of latent heat is evident. As a result, the SAT-based composite PCM with the lower porosity of copper foam has much better heat-charging performance when the power level
Epcm = L pcm ρpcm
(1)
Eeff = Epcm ε = L pcm ρpcm ε
(2)
k eff = A [εk pcm + (1−ε ) k skeleton] +
1−A ε kpcm
+
1−ε k skeleton
(3)
where ε is the porosity of the copper foam, ρpcm is the density of the modified SAT (kg m3 ), L pcm is the latent heat of the modified SAT (kJ kg ), Epcm is the latent heat stored by the modified SAT per unit volume (kJ m3 ), Eeff is the latent heat stored by the copper foam/ modified SAT composite PCM per unit volume (kJ m3), k eff is the thermal conductivity of the copper foam/modified SAT composite PCM (W mK ), k skeleton is the thermal conductivity of the copper foam (W mK ), k pcm is the thermal conductivity of the modified SAT (W mK ), A is the empirical value from experimental data and set as 0.35, which is proposed by Bhattacharya et al. [44] Eq. (3) indicates that due to the large difference between the thermal conductivity of SAT (k = 0.59 W/ mK as given in Ref. [33]) and the copper foam (k = 398 W mK ), the thermal conductivity of the copper foam/modified SAT composite PCM (k eff = 3.38 W mK for ε = 98%, k eff = 17.33 W mK for ε = 88% ) would increase with the decreasing porosity of the copper foam. Combining the equation of the latent heat stored by the copper foam/modified SAT composite PCM per unit volume with the experimental results, the conclusion could be reached that less time is needed to complete the phase process when PCM is enhanced by copper foam with a lower porosity under the same charging conditions because more thermal conductivity enhancer would decrease the energy stored in the unit, although the thermal conductivity is improved efficiently. To further investigate the influence of ambient temperature on the heat-charging process of the heat storage units, two units were tested with a larger power level of 60 W in the high-low temperature test 179
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beginning step of the cooling process, the temperature of the six cases dropped quickly as the sensible heat of the units was removed by natural convection. All the six cases had obvious supercooling during the heat-discharging process. The degree of supercooling was more than 6.76 °C, which is larger than the degree of supercooling of the modified SAT, mainly attributed to the different measurement modes. The degree of supercooling of the modified SAT was measured by inserting the temperature sensors into the PCM, while the degree of supercooling of the units was measured by pasting the temperature sensors on the bottom surface of the units. The copper foam with the PPI of 20 may have no effect on further reduction of the supercooling of the SAT. Moreover, the larger degree of supercooling of the heat storage unit should pay more attention to the actual application. In addition, Fig. 17 shows that a distinct temperature plateau appears, whereby the units seem to remain at the highly slow degression rate of temperature of approximately one hour, which implies that the phase change of the SAT composite was taking place from liquid phase to solid phase. The temperature during the phase change process was lower than the melting point of the modified SAT, as the lower ambient temperature and the lower temperature of the heat storage units show. This observation can be explained by the temperature being affected by the natural convection and the container material having a good ability for heat dissipation. When comparing two heat storage units at the different ambient temperatures, the heat storage unit enhanced by the copper foam with the porosity of 88% shows a higher temperature during the phase change process. Such a result indicates that the composite PCM with lower porosity copper foam has better heat transfer ability under convection conditions. However, the phase change process is a little shorter due to the loss of latent heat, as mentioned before.
Fig. 16. Heat-charging process of heat storage units with different power levels.
is fixed. 4.3.2. The heat-discharging process The heat-discharging performance is another main aspect of thermal energy storage. The thermal performance of the heat storage units applying copper foam/SAT composite PCM was investigated by putting the units in the high-low temperature test chamber, setting the temperature at different values. Before the heat-discharging experiment, the heat storage units were put into the chamber with the temperature of 75 °C for 4 h to ensure that the composite PCM in the units was all in the liquid phase and at the same temperature. Then, the temperature of the chamber was set at 20 °C, 15 °C and 5 °C to provide the cold source to activate the heat-discharging process. Fig. 17 displays the temperature variations of the two heat storage units during the heat-discharging process at different ambient temperatures. Unlike the heat-charging process, the heat-discharging process seems to be much longer because the units were cooled by weaker natural convection. The unit enhanced by copper foam of 88% porosity and cooled at the temperature of 20 °C was used as an example. The whole discharging process can be divided into three steps as marked in Fig. 17: pre-cooling (sensible heat storage of the liquid phase), phase change process (latent heat storage of phase change from liquid to solid) and supercooling (sensible heat storage of the solid phase). At the
5. Conclusions The hydrated salt of SAT could be applied for low temperature heat storage due to its high latent heat density. However, it always suffers from a high degree of supercooling and low thermal conductivity. The corrosion effect for long term application is seldom investigated. In this study, the compatibility of the SAT was first tested with aluminum alloy and copper. Then, several nucleating agents were used to modify the SAT. Finally, the modified SAT was composited with two kinds of copper foams. A laboratory-scale experiment was conducted to investigate the heat-charging and discharging performance of the copper foam/SAT composite-based heat storage units. Based on the analyses of the results, the conclusions can be drawn as the following. (a). The compatibility experiment was conducted by filling the aluminum alloy tubes and copper tubes with SAT and keeping the SAT in the liquid phase for 270 days. Then, the corrosion effect was tested by taking SEM images and performing EDS analysis. The results show that the oxide layer was formed on the inner surface of all tubes. Although the corrosion effect on the copper material was more obvious, the SEM image of the cross section of the copper tube shows that the corrosion did not infiltrate into the copper, which means that the corrosion effect could be neglected, and the aluminum alloy and copper could be used with the SAT for long term applications. (b). The SAT was modified using QS, BDH, SCN, SCD and DHPD as nucleating agents and CMC as the thickening agent in the high-low temperature test chamber. The results reveal that the modified SAT with the additives of 2 wt% CMC and 2 wt% DHPD has the best performance and that the degree of supercooling was approximately 4.6 °C. The phase change enthalpy of the modified SAT was 253.6 J/g. (c). The copper foam/SAT composite PCMs were prepared by applying a vacuum impregnation method. Unlike most former studies, the heat-charging process was preceded by a heat flux boundary
Fig. 17. Heat-discharging process of heat storage units with different cooling temperatures. 180
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condition. The results indicate that the natural convection has a weak effect on the heat-charging performance, and the heat-charging rate is based mainly on the power level. Under the conditions of the fixed power level and the ambient temperature, the heat storage unit with fewer thermal conductivity enhancers shows better heat storage performance than the units that could store more heat. (d). The heat-discharging performance of the copper foam/SAT composite PCM-based heat storage units was conducted in the high-low temperature test chamber. Both of the units show a larger degree of supercooling than the modified SAT, possibly indicating that the copper foam with the PPI of 20 may have no effect on further reduction of the degree of supercooling of the SAT. Meanwhile, the supercooling degree of the units increases when the ambient temperature decreases. The heat-discharging process would be reduced with a lower ambient temperature. The copper foam/SAT composite PCM may be a promising PCM for thermal energy storage, while the supercooling should be paid more attention in the actual application.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Thermal performance of sodium acetate trihydrate based composite phase change material for thermal energy storage
T
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Liang Zhao , Yuming Xing, Xin Liu, Yegang Luo School of Aeronautic Science and Engineering, Beihang University, Beijing 100083, PR China
H I GH L IG H T S
compatibility of sodium acetate trihydrate was tested. • The sodium acetate trihydrate was modified with five kinds of nucleating agents to reduce the supercooling. • The heat storage units based on the copper foam/ modified sodium acetate trihydrate were proposed. • The • The heat charging and discharging performance of heat storage unit was investigated.
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal energy storage Sodium acetate trihydrate Copper foam Compatibility Supercooling Heat storage performance
Sodium acetate trihydrate (SAT) as a phase change material (PCM) suffers from large supercooling, unclear compatibilities and low thermal conductivity. In this paper, the compatibility of SAT was tested for 270 days with aluminum alloy and copper. The corrosion phenomenon was evaluated by the scanning electron microscope (SEM) images and EDS (energy dispersive spectrum) analyses, which indicated that the corrosion effect could be neglected. Then, SAT modified with the additives of 2 wt% disodium hydrogen phosphate dodecahydrate (DHPD) and 2 wt% carboxyl methyl cellulose (CMC) showed the best performance in reducing supercooling. Finally, a laboratory-scale experiment was conducted to investigate the heat-charging and discharging performance of the heat storage units based on a copper foam/SAT composite PCM. The findings indicated that the heat-charging rate was based mainly on the heat power level, and the composite PCM with fewer thermal conductivity enhancers showed better heat storage performance. The heat discharging process revealed that the heat storage units still had more supercooling than the modified SAT. Based on the results obtained, the copper foam/SAT composite PCM appears to be a promising heat storage material, while the supercooling still needs to be considered in application.
1. Introduction With the progressive intensification of energy shortages and environmental pollution [1–3], it becomes more urgent to improve energy efficiency, energy saving and emission reduction. Thermal energy storage is an important component of an energy application system, which could ease the mismatch in time and location between energy demands and supply [1,4–7]. In general, thermal energy storage can be divided into three groups: sensible energy storage, chemical energy storage and latent thermal energy storage. Compared with sensible energy storage and chemical energy storage, latent thermal energy storage is a more effective energy storage method because of the high storage energy density, suitable phase change temperature, stable chemical properties and low cost, and latent thermal energy storage has been widely used in
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the field of solar energy storage [8], building energy management systems [9,10], waste heat recovery systems [11,12] and thermal management systems [13,14]. The several kinds of phase change materials (PCMs) could be divided into three groups: the organic PCMs, the inorganic PCMs and the eutectic PCMs [15,16]. Many organic PCMs such as paraffin, sugar alcohols and fatty acids always have the drawbacks of relatively high cost, low thermal energy storage density and flammability, which are non-negligible and constrain the practical application of organic PCMs. Comparatively, the inorganic PCMs, especially hydrated salts, whose melting point ranges from a few degrees Celsius to over 100 °C always possess superior properties such as higher thermal energy storage density, better thermal conductivity, nonflammability and lower cost, which indicate that hydrated salts can be widely applied under the working conditions of human activities,
Corresponding author. E-mail address: [email protected] (L. Zhao).
https://doi.org/10.1016/j.applthermaleng.2018.07.094 Received 7 April 2018; Received in revised form 14 July 2018; Accepted 17 July 2018 Available online 18 July 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.
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For the hydrated salts, the compatibility of the thermal conductivity additives and the encapsulation materials with the PCM is a key issue to ensure the long-term applications of PCM-based thermal energy storage systems. The corrosion resistance of commercial metals in contact with several hydrated salts with the melting point ranging from 32 to 58 °C was tested by Cabeza et al. [34,35] with a duration up to 70 days. They found that the SAT had a slight corrosion effect on the copper and had almost no effect on the aluminum. Moreno et al. [36] experimentally evaluated the corrosion rate of two metals and two metal alloys that were in contact with different hydrated salt PCMs by weighing the mass change. Five kinds of container materials were immersed by Browne et al. in five different PCMs for a period of 722 days to investigate the compatibility of PCM with various materials [37]. The results showed that the steel was the most suitable container materials for all the PCMs. Considering the above-mentioned papers, various methods have been proposed for reducing the degree of supercooling, diminishing the phase separation, improving thermal conductivity and investigating the compatibility of SAT. The thermal conductivity of the solid SAT ranged from 0.17 W/mK to 0.7 W/mK [27,32,38] at different temperatures, and the thermal properties of the SAT composite varied significantly with different additives at different mass fraction [20,32,39]. Most related papers either reported the thermal properties of the composite SAT or focused on the thermal energy storage performance. Few research studies were systematically related to the modification of the SAT and its thermal energy storage performance. The compatibility of SAT with metals was tested with the immersion corrosion method and evaluated by weighing the mass changes. Little information about the analysis of the microphenomenon of corrosion was reported in detail. Consequently, it is valuable to systematically investigate the thermal property modification of the SAT and the thermal energy storage performance of the composite SAT-based PCM, and the compatibility of the SAT with commonly used metals. In the present study, the compatibility of SAT was investigated first. Second, the SAT was composited with different additives (disodium hydrogen phosphate dodecahydrate (DHPD), sodium carbonate decahydrate (SCD), sodium silicate nonahydrate (SCN), borax decahydrate (BDH), and quart sand (QS)) to modify the supercooling, and the carboxymethyl cellulose (CMC) was used as a thickening agent. Finally, two kinds of copper foam/hydrated salt composite PCMs were prepared by using the copper foams as supporting matrixes and the modified SAT as PCM, and the thermal energy storage performance of the composite PCMs was evaluated by funding a small-scale experimental test unit.
domestic instruments and equipment. Sodium acetate trihydrate (CH3COONa·3H2O, SAT) with the melting point of ∼58 °C is a representative of hydrate salt PCMs, which could easily be integrated with space heating and domestic hot water preparation [17], solar heating systems [18], and radiant floor heating systems [19]. However, SAT always suffers from serious supercooling problems and low thermal conductivity during the thermal energy storage process as most hydrated salts are [20]. Aiming at overcoming the drawbacks of SAT, extensive efforts have been conducted. Jin et al. [21] performed an experiment involving the cooling processes of a partially melted SAT. The results showed that the degree of supercooling of partially melted SAT increased with the elevation of the heating temperature. Zhou and Xiang [22] experimentally investigated the stable supercooling characteristics of an SAT mixture within three types of thermal storage units, indicating that the inner surface roughness, cooling rate, salt-water mass ratio and carboxyl methyl cellulose could all affect the stable supercooling of the SAT. Several kinds of nucleating agents and thickeners were selected by Mao et al. [23] to improve the phase-change energy storage properties of SAT. The experimental results indicated that adding disodium hydrogen phosphate dodecahydrate can reduce the degree of supercooling. Cabeza et al. [24] conducted an experiment to find the effective thickeners for SAT and discovered that SAT could be thickened successfully with starch and bentonite, with the mixtures showing a melting point similar to the SAT, and the enthalpy would decrease with the type and the amount of changing of the thickeners. Fashandi and Leung [25] found that adding a bio-derived chitin nanowhisker was an effective way to suppress the supercooling of SAT. Different mass fractions of silver nanoparticles were experimentally studied by Ramirez et al. [26] to reduce the supercooling of SAT. The heat transfer enhancement is another key issue for the SAT applied as a PCM for thermal energy storage. Some thermal conductivity enhancers such as nanoparticles, metal foams and graphite materials can not only reduce the degree of supercooling but could also effectively improve the thermal conductivity of SAT. Cui et al. [27] reported that adding Nano-Cu could not only reduce the degree of supercooling of SAT but also increase the thermal conductivity of the composite SAT by nearly 20% compared to the pure SAT. Johansen et al. [28] investigated the effects on the thermal conductivity and supercooling stability of SAT of adding graphite powder. The results showed that the graphite powder was a promising thermal conductivity enhancer. A series of expanded graphite-based composite PCMs using an SAT-urea non-eutectic mixture was prepared and tested by Fu et al. [19]. The test results showed that the composite PCMs with the mass fraction of 8% had the thermal conductivity of 2.076 W/mK and the degree of supercooling of 1.54 °C. Mao et al. [29] prepared a novel composite phase-change material based on SAT by adding a certain proportion of expanded graphite, which showed a relatively small degree of supercooling and larger thermal conductivity. They [30] also proposed a series of SAT-disodium hydrogen phosphate dodecahydratecarboxyl methyl cellulose/expanded graphite composite PCMs. They concluded that the composite PCM with 3 wt% expanded graphite had the most suitable thermal properties and that the degree of supercooling and the thermal conductivity were 2 °C and 1.37 W/mK, respectively. Shin et al. [31] proposed a series of SAT composites containing different mass fractions of expanded graphite and carboxymethyl cellulose. The measured results manifested that expanded graphite could effectively improve the thermal conductivity and diminish the supercooling effect of SAT. Dannemand et al. [32] prepared a series of SAT-based composite PCMs and measured their thermal conductivity. The results revealed that the composite PCM with SAT, 1% xanthan rubber and 5% graphite flakes had the highest thermal conductivity of 1.1 W/m K. Li et al. [33] studied the supercooling and thermal conductivity of the SAT composited with copper foam, which showed smaller degree of supercooling and larger thermal conductivity than the pure SAT.
2. Materials and instruments SAT, DHPD, SCD, SCN, BHD, QS and CMC (analytical reagent grade, purity > 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Copper foams with the pore size of 20 PPI (PPI means the numbers of pores in per inch) and the porosity of 0.98 and 0.88 were offered by Zhuoer Technology Co. Ltd. (Changchun, China). The apparatus used for the experiments included a differential scanning calorimeter (DSC, 214 Polyma, Netzsch, Germany), an energy dispersive spectroscope (EDS, Hitachi Inc., Japan), scanning electron microscope (SEM, Hitachi S4800, Hitachi Inc., Japan), several PT100 temperature sensors (precision ± 0.1 °C), DC power supply module by Beisina Tchnologies (SNK-22H06, 0-220 V/0-6 A), data acquisition module (ADVANTECH ADAM-4015), and a high-low temperature test chamber (BPH-120B, Shanghai Everone Precision Instruments Co., Ltd., China). 3. Experimental work 3.1. Preparation of SAT composites SAT was selected as the phase change material for thermal energy storage. However, the pure SAT has a considerably large degree of 173
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supercooling. The SAT composites containing different additives (DHPD, SCD, SCN, BHD, QS and CMC) were prepared by applying a simple blending method. First, the SAT and the additives were weighed with the selected contents of nucleating agents and CMC and were then well mixed in the porcelain crucible. Second, the mixture was poured into a sealed glass tube and placed into a thermostatic water bath and heated at the temperature of 75 °C until the mixture was completely melted. Finally, the mixture was cooled and solidified as the sample for a supercooling experiment. By repeating the above procedures, several kinds of modified SATs could be prepared. The copper foam/modified SAT composite PCM was prepared by using a vacuum impregnation method in which the modified SAT composite was utilized as the PCM, and the copper foam was applied as thermal conductivity enhancer. The preparation procedure contained three steps with vacuum assistance to increase the filling rate. First, the modified SAT composite in the stainless-steel container was heated at the temperature of 75 °C in the constant temperature water bath until the modified SAT composite was completely melted. Second, the prepared copper foam with the dimensions of 100 mm × 100 mm × 21 mm was completely immersed in the liquidmodified SAT composite. The vacuum pump connected to the container started working to maintain the pressure in the container below 50 Pa. To fill the porous space of the copper foam with the liquid-modified SAT composite completely. This filling process lasted for 2 h with the liquid-modified SAT composite maintained at the temperature of 75 °C. Finally, the sample in the stainless-steel container was cooled to the temperature of 25 °C in the thermostatic water bath until the modified SAT composite was entirely in solid phase. Then, the vacuum pump was switched off, the copper foam filled with modified SAT composite was brought out, and the surplus modified SAT composite on the sample was cleared away. The copper foam enhanced composite PCM was obtained.
Fig. 2. Schematic diagram of the supercooling experimental apparatus.
bubbles, precipitates, surface changes, and a pitting process. The EDS mapping of the tube inner surface was provided to further investigate the corrosion phenomenon. 3.3. The supercooling experiment The step cooling curve method and DSC were adopted in the supercooling experiment. The cylindrical metal tubes that were made of stainless steel containing the composite PCMs (approximately 10 g) were placed in the high-low temperature test chamber for SAT melting and solidification by changing the temperature of the chamber as shown in Fig. 2. The PT100 temperature sensors (precision ± 0.1 °C) were installed at the center of the tubes to monitor the temperature variation during the experiment.
3.2. The compatibility experiment In this study, the compatibility experiment contained four steps: (1) the preparation of samples, (2) the corrosion test of copper and aluminum alloy, (3) sample clean-up, and (4) corrosion evaluation. Before the experiment, two aluminum alloy test tubes and two copper test tubes were designed for containing the SAT as shown in Fig. 1(a). Because pure SAT has considerably large supercooling that could be in the liquid phase even at room temperature, the four test tubes filled with liquid SAT (approximately 10 g) were maintained in the high-low temperature test chamber at the temperature of 70 °C for 240 h, and then maintained at room temperature for 270 days as the corrosion experiment. During the corrosion experiment, the tubes were sealed with nuts to avoid the contact with environmental agents. After the test, the SAT in the tubes was removed, and the tubes were cleaned up and cut into small pieces for corrosion evaluation as shown in Fig. 1(b). The corrosion evaluation was conducted by applying the SEM to seek for
3.4. Experimental heat storage unit Two small-scale heat storage units applying the copper foam/SAT composite PCMs were prepared as presented in Fig. 3(b). The copper foam (20 PPI, the porosity of 98% and 88%) was cut to the size of 100 mm × 100 mm × 21 mm, as shown in Fig. 3(a). A simple experiment was founded to investigate the thermal performance of copper foam/SAT composite-based heat storage unit. The heat storage was put into the high-low temperature test chamber, heated by a film heater at the bottom, and cooled by natural convection in the chamber. The schematic diagram of the experiment is shown in Fig. 4. A 100 × 100 mm2 film heater was applied as the mimicry of heat generation, which was pasted on the bottom of the heat storage unit. The power required by the heater was provided by a DC power supply module. As the key parameter being measured in this investigation,
Fig. 1. Compatibility test samples: (a) Four tubes, (b) The test samples. 174
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Fig. 3. Photographs of the copper foam (a) and heat storage unit (b).
nine PT100 temperature sensors that had a very high sensitivity of ± 0.1 °C were pasted onto the heater as shown in Fig. 5. The data acquisition module was connected to a computer and recorded the temperature at intervals of 2 s. The transient temperature variations were the average value of the nine PT100 temperature sensors.
4. Results and discussion 4.1. The compatibility of SAT As the SAT shows alkalinity, it may cause corrosion of the container material and metal foam that affects the thermal performance and the reliability of the heat storage unit for long-term application. Aluminum alloy and copper are the most common container and thermal enhancement materials. During the storage time, the SAT was stored in the aluminum alloy test tubes and copper test tubes in the liquid phase. After the experiment, the tubes were cut into the samples as shown in Fig. 1(b) to scan the inner surface of the tubes to investigate whether corrosive phenomena appeared. For comparison, the samples with the same original metal materials and same machined pattern were regarded as the samples before the compatibility experiment and analyzed, which had never been contact with SAT. The inner surface of the aluminum alloy tubes showed no color variation. The color of the inner surface of the copper tubes turned into the gray and black. Further investigating the corrosion effect, the SEM images of samples before the compatibility experiment and after the compatibility experiment were taken as shown in Figs. 6 and 7. No obvious corrosive phenomena such as bubbles, precipitates, surface changes, and pitting processes appear on the surface of the aluminum
Fig. 5. Distribution of PT100 temperature sensors.
alloy tube. On the surface of the copper tube, the granulated particles were uniformly distributed, reflecting that the SAT had a corrosive effect on copper. To investigate the corrosive phenomena on the inner surface of the tubes digitally, the main compositions of the original metal materials of the tubes and the inner surface of the metal tubes after the experiment were analyzed by an EDS as presented in Fig. 8. The detailed data, listed in Table 1, showed that after the experiment,
Fig. 4. Diagrammatic sketch of the experimental system. 175
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Fig. 6. SEM photograph of the inner surface of the aluminum alloy tube: (a) before the experiment, (b) after the experiment.
and the thin corrosion film could not be observed from the cross section of the copper sample as shown in Fig. 7(c), which demonstrated that the corrosion did not infiltrate into the copper material. The copper oxide layer was also reported to increase the corrosion resistance of the copper [41,42]. Therefore, the SAT has a limited corrosive effect on aluminum alloy and copper, which could be used for container materials and thermal conductivity enhancement materials for long-term use. And due to the much larger thermal conductivity of the copper, the copper foam was utilized as the thermal conductivity enhancer, the aluminum alloy was used as container material.
the main composition of the inner surface of the aluminum alloy tube in weight ratio were magnesium 1.12%, aluminum 70.49% and oxygen 28.39%, and the main composition of the inner surface of the copper tube in weight ratio were copper 62.44%, zinc 4.7% and oxygen 32.86% after the experiment. Compared with the original metal materials, the element of the oxide increased significantly in weight. The original copper sample had a little element of oxide which was induced mainly by the unclear surface of the sample. The composition of magnesium, aluminum and copper all appeared as aluminum oxide, magnesium oxide and copper oxide, which indicated that the SAT had a corrosive effect on both aluminum alloy and copper. However, synthesizing the phenomenon in the SEM images, we can conclude that the corrosive effect on aluminum alloy induced by SAT could be neglected, and it is well known that the aluminum oxide is a stable oxidizing material that could prevent further corrosion [40]. While the corrosive phenomenon on copper induced by the SAT seems to be more serious, a thin corrosion film appears. However, the corrosion rate was very slow,
4.2. The effect of different nucleating agents on the supercooling of SAT For most of the supercooling experiments, the SAT composites were heated in the constant temperature water and cooled under the natural convection conditions [23,24,29]. In this study, “Scientific method” was applied for the formulation test, five kinds of nucleating agents
Fig. 7. SEM photographs of the inner surface of the copper tube; (a) before the experiment, (b) after the experiment, (c) SEM photograph of the cross section of the copper tube. 176
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Fig. 8. EDS photographs of the inner surface of tubes; (a) the aluminum alloy tube before the experiment, (b) the aluminum alloy tube after the experiment, (c) the copper tube before the experiment, (d) the copper tube after the experiment.
Table 1 The EDS analysis results of the surface of samples before and after the compatibility experiment. Tube
Before
After
Element
Weight %
Element
Weight %
Aluminum alloy tube
Al Mg O
98.41 1.59 –
Al Mg O
70.49 1.12 28.39
Copper tube
Cu Zn O
92.13 7.38 0.49
Cu Zn O
62.44 4.7 32.86
were selected as proposed in some former research [20,23,29,33], and the SAT composites were heated and cooled in the high-low temperature test chamber by regulating the temperature of the chamber to address the degree of supercooling. To search the best ratio of the SATbased composite and reduce the experimental workload, the different mass ratios of the nucleating agent were added into the pure SAT, and 2 wt% CMC was applied as a thickener [29] introduced to investigate the degree of supercooling. The step cooling temperature variations of pure SAT and three kinds of SAT-based composites containing 8 wt% BDH, 8 wt% QS and 8 wt% SCD were compared as shown in Fig. 9. It shows that the nucleating agent QS and BDH could almost not reduce the degree of supercooling of SAT, and the supercooling degree of SAT could be improved to a limited extent by SCD, which was approximately 13 °C. Fig. 10 shows the step cooling temperature curves of the SAT composited with different mass ratios of SCD, which shows that the degree varied with the mass ratio of SCD in the composite. The SAT composite containing 4 wt% SCD exhibited the minimum degree of supercooling of approximately 6.85 °C. The temperature variations presented in Fig. 11 indicate that when the mass ratio of SAT to SCN was 100:6, the minimum degree of supercooling of the SAT-based composites was approximately 4.83 °C. The SAT-based composites containing 2 wt% and 4 wt% showed a slightly
Fig. 9. Cooling curves of pure SAT and SAT composites.
larger degree of supercooling of 6.44 °C and 6.02 °C, respectively. As shown in Fig. 12, it is clear that when the mass ratio of DHPD was 2%, the degree of supercooling of the SAT-based composite was relatively small, approximately 4.6 °C. After the supercooling analysis, among these five kinds of nucleating agents, the DHPD with the mass ratio of 2% could solve the problem of supercooling more efficiently. Then, the latent heat of the SAT with 2 wt% CMC and 2 wt% DHPD was determined by applying a DSC thermal analysis at a heating rate of 3 °C/min under a N2 atmosphere as shown in Fig. 13. Compared with the latent heat of the pure SAT of 275.8 J/g, the SAT composite had a smaller value of 253.6 J/g. While the phase temperature varied a little, the value for the pure SAT was 57.85 °C, and it was 57.76 °C for the SAT composite containing 2 wt % DHPD and 2 wt% CMC.
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Fig. 13. DSC curves of pure SAT and modified SAT. Fig. 10. Cooling curves of SAT composites with different mass ratios of SCD.
foam. Recall that the heat-charging process was preceded by heat flux, and the heat discharging process was driven by natural convection in the high-low temperature test chamber.
4.3.1. The heat-charging process The PCM-based heat storage units have a thermal energy storage mechanism that includes three steps: pre-heating, phase change process and super-heating. In the first step, heat is absorbed by the specific heat of the solid PCM to raise its temperature to its melting point. In the second step, the PCM begins to melt at an approximately constant temperature until the PCM is fully in the liquid phase, during which time heat is absorbed by the latent heat of the PCM. In the third step, the PCM is completely in the liquid phase, and heat is absorbed by the specific heat of the liquid PCM. Unlike heating with temperature and convection conditions, the temperature of the PCMs would continually increase with further heat absorption. In general, most of the heat is absorbed by the latent heat of the PCM during the phase change process. In the first section, the heat-charging process was experimentally investigated under different working conditions. To identify the influence of natural convection on the charging process, the comparative experiment was conducted. The units were wrapped in glass wool with a thickness of 2 cm for the insulation conditions, and the units were put into the chamber by controlling the temperature at 20 °C to realize the natural convection condition with a temperature of 20 °C. An average temperature reading of the temperature sensors on the bottom of the unit was used to record the time-temperature response. When the units were wrapped by the glass wool, the heat generated by heater could be almost completely absorbed by the unit. The heat-charging rate could be reflected from the temperature variation due to the isothermal operating characteristics of the PCM. The time-temperature history for two heat storage units with insulation and with the ambient temperature of 20 °C is shown in Fig. 14 for the power level of 20 W. The heat-charging process could be divided into three steps, as mentioned before. The unit enhanced by the copper foam of 88% porosity and insulated with glass wool is used as an example. The temperatures rose quickly as the heat power was loaded. The discrepancy between the temperature change curves for the four cases is small in the pre-heating and phase change processes as marked in Fig. 14. This discrepancy is induced by two reasons: one reason is that the heat power is relatively small, and another reason is that the container material of aluminum alloy also has a much higher thermal conductivity, which demonstrates that they all had the ability to transfer and dissipate the heat as soon as possible at the power level of 40 W, indicating that both units had a similar heat-charging rate. As the heat-charging processes, the third step, superheating starts at different
Fig. 11. Cooling curves of SAT composites with different mass ratios of SCN.
Fig. 12. Cooling curves of SAT composites with different mass ratios of DHPD.
4.3. Heat storage performance of the copper foam/SAT composite PCM For investigating the transient behavior of the copper foam/SAT composite PCM-based heat storage unit, the modified SAT composite (SAT + 2 wt% CMC + 2 wt% DHPD) was composited with copper 178
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Fig. 14. Heat-charging process of heat storage units with different boundary conditions.
Fig. 15. Heat-charging process of heat storage units with different ambient temperatures.
times for the four cases. The same unit needs more time to complete the phase change process when it is put in the ambient conditions with the temperature of 20 °C because natural convection could release heat during the heat-charging process. For the copper foam/modified SAT composite PCM, the quantity of the modified SAT in the copper foam per unit volume is directly related to the bulk porosity density (ε ). Thus, the estimated storable latent thermal energy could be calculated as shown in Eq. (2) [43]. The effective thermal conductivity of copper foam/modified SAT composite PCM could be calculated by the Bhattacharya model [44] as shown in Eq. (3), which has been verified by Li et al. [33].
chamber with different ambient temperatures. The temperature evolution of the units is shown in Fig. 15, which could also be divided into three steps. These steps are illustrated using the unit with ambient temperature of 40 °C and enhanced by copper foam of 88% porosity as an example. When the power level was increased to 60 W, although the ambient temperature had a difference of 20 °C, the temperature curves nearly overlap for the four cases during the pre-heating and phase change process, as shown in Fig. 15, indicating that both units have the similar heat-charging rate during the first two steps. The discrepancy of the temperature appears until the superheating region is reached, indicating that the ambient temperature change rarely has an effect on the temperature of the unit for the heat-charging. For the same unit, the phase change process seems to be a little longer at an ambient temperature of 20 °C because the weak natural convection could play a small role to release heat during the heat-charging process. The temperature at the onset phase change process is much higher than the melting point of the modified SAT because the thermal resistance during the heat-charging process would be more evident as the power level increases. A review of the above analysis shows that the natural convection has an influence on the charging time but rarely has an effect on the temperature of the unit during the charging rate. Unlike the temperature and convection boundary conditions, the effective thermal conductivity of the PCM is not the main factor to decide the heat-charging rate. The heat-charging rate is almost the same when the units are loaded with the same power level. In the second section, the heat-charging performance of two heat storage units with different heat levels (20 W, 30 W and 40 W) was assessed. The measured temperature variations during the heat-charging process are plotted in Fig. 16. The turning point of the start of melting appears earlier, and the heat-charging process would be reduced as the power level increases, which means the heat-charging rate is based mostly on the power level. The turning point of the start of melting has a higher temperature with higher power level, which reflects that the overheating will be more serious with a high level of power. The temperature curves for different units with the same power level nearly overlap during most of the heat-charging process, and they will depart from each other at the end stage of the heating process due to the different amounts of heat stored in the unit. The heat-charging rate is decided mainly by the power level. The SAT-based composite PCM with more thermal conductivity enhancer seldom reflects any advantages. However, the lost amount of latent heat is evident. As a result, the SAT-based composite PCM with the lower porosity of copper foam has much better heat-charging performance when the power level
Epcm = L pcm ρpcm
(1)
Eeff = Epcm ε = L pcm ρpcm ε
(2)
k eff = A [εk pcm + (1−ε ) k skeleton] +
1−A ε kpcm
+
1−ε k skeleton
(3)
where ε is the porosity of the copper foam, ρpcm is the density of the modified SAT (kg m3 ), L pcm is the latent heat of the modified SAT (kJ kg ), Epcm is the latent heat stored by the modified SAT per unit volume (kJ m3 ), Eeff is the latent heat stored by the copper foam/ modified SAT composite PCM per unit volume (kJ m3), k eff is the thermal conductivity of the copper foam/modified SAT composite PCM (W mK ), k skeleton is the thermal conductivity of the copper foam (W mK ), k pcm is the thermal conductivity of the modified SAT (W mK ), A is the empirical value from experimental data and set as 0.35, which is proposed by Bhattacharya et al. [44] Eq. (3) indicates that due to the large difference between the thermal conductivity of SAT (k = 0.59 W/ mK as given in Ref. [33]) and the copper foam (k = 398 W mK ), the thermal conductivity of the copper foam/modified SAT composite PCM (k eff = 3.38 W mK for ε = 98%, k eff = 17.33 W mK for ε = 88% ) would increase with the decreasing porosity of the copper foam. Combining the equation of the latent heat stored by the copper foam/modified SAT composite PCM per unit volume with the experimental results, the conclusion could be reached that less time is needed to complete the phase process when PCM is enhanced by copper foam with a lower porosity under the same charging conditions because more thermal conductivity enhancer would decrease the energy stored in the unit, although the thermal conductivity is improved efficiently. To further investigate the influence of ambient temperature on the heat-charging process of the heat storage units, two units were tested with a larger power level of 60 W in the high-low temperature test 179
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beginning step of the cooling process, the temperature of the six cases dropped quickly as the sensible heat of the units was removed by natural convection. All the six cases had obvious supercooling during the heat-discharging process. The degree of supercooling was more than 6.76 °C, which is larger than the degree of supercooling of the modified SAT, mainly attributed to the different measurement modes. The degree of supercooling of the modified SAT was measured by inserting the temperature sensors into the PCM, while the degree of supercooling of the units was measured by pasting the temperature sensors on the bottom surface of the units. The copper foam with the PPI of 20 may have no effect on further reduction of the supercooling of the SAT. Moreover, the larger degree of supercooling of the heat storage unit should pay more attention to the actual application. In addition, Fig. 17 shows that a distinct temperature plateau appears, whereby the units seem to remain at the highly slow degression rate of temperature of approximately one hour, which implies that the phase change of the SAT composite was taking place from liquid phase to solid phase. The temperature during the phase change process was lower than the melting point of the modified SAT, as the lower ambient temperature and the lower temperature of the heat storage units show. This observation can be explained by the temperature being affected by the natural convection and the container material having a good ability for heat dissipation. When comparing two heat storage units at the different ambient temperatures, the heat storage unit enhanced by the copper foam with the porosity of 88% shows a higher temperature during the phase change process. Such a result indicates that the composite PCM with lower porosity copper foam has better heat transfer ability under convection conditions. However, the phase change process is a little shorter due to the loss of latent heat, as mentioned before.
Fig. 16. Heat-charging process of heat storage units with different power levels.
is fixed. 4.3.2. The heat-discharging process The heat-discharging performance is another main aspect of thermal energy storage. The thermal performance of the heat storage units applying copper foam/SAT composite PCM was investigated by putting the units in the high-low temperature test chamber, setting the temperature at different values. Before the heat-discharging experiment, the heat storage units were put into the chamber with the temperature of 75 °C for 4 h to ensure that the composite PCM in the units was all in the liquid phase and at the same temperature. Then, the temperature of the chamber was set at 20 °C, 15 °C and 5 °C to provide the cold source to activate the heat-discharging process. Fig. 17 displays the temperature variations of the two heat storage units during the heat-discharging process at different ambient temperatures. Unlike the heat-charging process, the heat-discharging process seems to be much longer because the units were cooled by weaker natural convection. The unit enhanced by copper foam of 88% porosity and cooled at the temperature of 20 °C was used as an example. The whole discharging process can be divided into three steps as marked in Fig. 17: pre-cooling (sensible heat storage of the liquid phase), phase change process (latent heat storage of phase change from liquid to solid) and supercooling (sensible heat storage of the solid phase). At the
5. Conclusions The hydrated salt of SAT could be applied for low temperature heat storage due to its high latent heat density. However, it always suffers from a high degree of supercooling and low thermal conductivity. The corrosion effect for long term application is seldom investigated. In this study, the compatibility of the SAT was first tested with aluminum alloy and copper. Then, several nucleating agents were used to modify the SAT. Finally, the modified SAT was composited with two kinds of copper foams. A laboratory-scale experiment was conducted to investigate the heat-charging and discharging performance of the copper foam/SAT composite-based heat storage units. Based on the analyses of the results, the conclusions can be drawn as the following. (a). The compatibility experiment was conducted by filling the aluminum alloy tubes and copper tubes with SAT and keeping the SAT in the liquid phase for 270 days. Then, the corrosion effect was tested by taking SEM images and performing EDS analysis. The results show that the oxide layer was formed on the inner surface of all tubes. Although the corrosion effect on the copper material was more obvious, the SEM image of the cross section of the copper tube shows that the corrosion did not infiltrate into the copper, which means that the corrosion effect could be neglected, and the aluminum alloy and copper could be used with the SAT for long term applications. (b). The SAT was modified using QS, BDH, SCN, SCD and DHPD as nucleating agents and CMC as the thickening agent in the high-low temperature test chamber. The results reveal that the modified SAT with the additives of 2 wt% CMC and 2 wt% DHPD has the best performance and that the degree of supercooling was approximately 4.6 °C. The phase change enthalpy of the modified SAT was 253.6 J/g. (c). The copper foam/SAT composite PCMs were prepared by applying a vacuum impregnation method. Unlike most former studies, the heat-charging process was preceded by a heat flux boundary
Fig. 17. Heat-discharging process of heat storage units with different cooling temperatures. 180
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condition. The results indicate that the natural convection has a weak effect on the heat-charging performance, and the heat-charging rate is based mainly on the power level. Under the conditions of the fixed power level and the ambient temperature, the heat storage unit with fewer thermal conductivity enhancers shows better heat storage performance than the units that could store more heat. (d). The heat-discharging performance of the copper foam/SAT composite PCM-based heat storage units was conducted in the high-low temperature test chamber. Both of the units show a larger degree of supercooling than the modified SAT, possibly indicating that the copper foam with the PPI of 20 may have no effect on further reduction of the degree of supercooling of the SAT. Meanwhile, the supercooling degree of the units increases when the ambient temperature decreases. The heat-discharging process would be reduced with a lower ambient temperature. The copper foam/SAT composite PCM may be a promising PCM for thermal energy storage, while the supercooling should be paid more attention in the actual application.
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