TY - JOUR
AB - Abstract Phase change materials (PCMs) are widely used in energy storage and energy transportation system as chill medium because of its large apparent heat and latent heat during phase change process. The studies on stability of PCM slurry and microencapsulation phase change materials (MEPCMs) slurry with different surfactants have been carried out in the experiments. The turbidity of the slurry was tested in order to analyze the stability of the slurry. The different surfactants polyethylene glycol400 and sodium alginate were selected to mix with PCMs' slurry. The results show that the surfactants have positive improvement effect on MEPCM slurry and negative improvement effect on PCM slurry. 1 INTRODUCTION Energy is a driving force for social advance. But fuel energy consumption is at the expense of the environment. According to the current researches, the problem of environmental pollution is becoming more and more serious. People have studied a variety of ways to solve this problem of energy utilization and environmental pollution [1, 2]. The use of clean energy can effectively save energy without producing environment pollution. However, the clean energy such as solar energy have problems such as the mismatch time between energy generation and utilization. The current studies that show some materials can well alleviate this problem [3]. Especially in the field of materials, the scholars have made great progress. For example, the phase change materials (PCMs) have the ability to store the energy through the phase change process which means that such materials can improve the energy utilization efficiency without producing environmental pollution [3]. PCMs are extensively used in many applications such as energy generation, construction, textile and transportation. Especially the PCM can be used in solar energy, building materials and energy storage application. The stability characteristics of PCMs' slurry has been experimentally verified to well enhance the energy storage capacity. The study of PCMs has attracted the attention to more and more scholars. PCM can be divided into three general categories on the basis of their different melting points: organic, inorganic and co-crystal. Inorganic materials mainly include crystalline hydrate salts and metals. General inorganic materials exist in varying degrees of phase separation. Co-crystal materials are composite with organic materials and inorganic materials. PCM applied to energy storage system has potential ability to improve the energy efficiency of the system. In fact, PCM cold storage system could reduce the fluctuation of building and energy consumption. The PCM could be integrated into building materials for building envelopes such as wall panels, floors, roofs and windows. In active applications, PCM could be used in heating ventilation and air-condition systems in order to improve the system efficiency [4]. It is necessary to solve several problems such as leaking, phase separation and corrosion of PCM. A novel encapsulation method ‘microencapsulation’ of PCM (MEPCM) has been proposed [5]. MEPCM refers to package technology that encapsulate the PCM in a resilient polymer shell. The particle size of MEPCM is 1–100 μm. The MEPCM shell could insulate the core material from external materials. This technology could enhance the heat transfer performance of the fluid and control the fluctuation of room temperature. PCMs could be combined with different additives such as water, ethylene glycol and silicone oil in order to make the mixed slurry. The slurry is a kind of a latent heat functional fluid [5]. Different properties of PCMs have been studied by further researchers such like the density, thermal conductivity and stability. Kenisari and Mahkamov [6] focused on the thermal properties and energy transfer enhancement methods of PCM. The experiment on commercial latent heat storage capacity was tested at least 1000 thermal cycles in order to study the thermal performance of PCM. Tzivanidis et al. [7] simulated the phase change process according to an effective thermal capacity function. The result showed that the main parameters of the PCM cold storage system were pipe spacing, the thickness of layer pipe depth within the ceiling and thermal properties of PCM (thermal conductivity, phase change temperature range). Zhang [8] experimented the MEPCM with the surfactant sodium alginate (SA) in order to study the stability of the mixed slurry. The result showed there was no chemical reaction between these two materials. The rate of heat storage/release was increased by 23% due to 3 wt% iron powder. There is a lack of experience on studying the stability of the slurry. Schalbart et al. [9] analyzed the physical stability through low-energy emulsification methods. All the samples were stored at setting temperature without insulation. The results showed there existed stratification phenomenon. In order to prevent this phenomenon, the droplet was added to increase the homogeneity of the samples. Gunther et al. [10] used thickeners to mitigate the stabilization process, which increased the pumping power in application. The PCM slurry is usually unstable due to the influence of gravity and has stratification phenomena [9]. Because the density of water and PCMs are different, the density of mixed slurry is inconsistent. The turbidity method is a common method in order to evaluate the property of slurry stability [10]. In this paper, a novel way was used to investigate the stability of slurry. The experiment tested the turbidity of MEPCM and PCM slurry with water and two kinds of surfactants polyethylene glycol400 (PEG400) and SA in order to research whether the surfactant could improve the stability of mixed slurry. The researches of PCM and MEPCM stability revealed a more prospective application for PCMs in energy storage/transportation systems and further optimize the design of the cold storage/transportation systems. 2 EXPERIMENT METHODOLOGY Surfactant is a kind of substance that can change the interface properties. It could reduce the surface tension between PCM and water. Surfactants are composed of hydrophobic, lipophilic hydrocarbon chain and hydrophilic, hydrophobic polar groups. These two parts are located at both the ends of the surfactant molecule to make the surfactant be lipophilic and hydrophilic, take the structure of PEG400, for example, in Figure 1 [11, 12]. Different polar substituents have different hydrophobicities, which indicate that different surfactants have different improvement effects on stability of mixed slurry. Figure 1. View largeDownload slide The structure of PEG400. Figure 1. View largeDownload slide The structure of PEG400. 2.1 Experimental setup The turbidity could be used in order to investigate the stability of the slurry. Since the density of the material is smaller than water, the gravitation would cause the instability of the slurry. In this experiment, different slurry at volume concentrations of 15, 30 and 40%, respectively, were tested. The slurry added with surfactant was stirred for 15 min by ultrasonic oscillation instrument. The mixed slurry were stood for 48 h in order to observe the dispersion phenomenon. Then the top section of mixed slurry was diluted 1000 times. After that the turbidity of top section of mixed slurry were tested by the turbidity instrument. The US AHCH2100Q portable turbidity instrument (Figure 2) was used to test in the experiment. Lower turbidity implies higher stability of mixed slurry. Figure 2. View largeDownload slide US AHCH2100Q portable turbidity instrument. Figure 2. View largeDownload slide US AHCH2100Q portable turbidity instrument. 2.2 Principles of turbidity instrument The turbidity of slurry could be tested through turbidity instrument effectively. The data of turbidity are accurate by scientific calculation [13]. When the light passes through the homogeneous slurry, the radiation intensity will decay along the propagation direction. This phenomenon is defined as Lang–Beer’s law. The formulas are as follows [13]: P1(x)=P1(0)exp(kαx) (1) P1(0) Incident light intensity P1(x) Outgoing light intensity kα  Absorption coefficient of slurry x   Optical path length If the slurry is inhomogeneous, there will exist the scattering effect while the light passes through the slurry. The formulas are as follows: P(x)=P(0)exp(kx) (2) P(0) Incident light intensity P(x) Outgoing light intensity ks  Scattering coefficients of slurry k   Extinction coefficient k=kα+ks Take the logarithm of both ends of the equation (2) logP(0)P(x)=kxln10 (3) While the logP(0)P(x)=A, A is defined as the capacity of light absorbance. The capacity of light absorbance is proportional to the optical path length.The scattering effect is proportionate to the turbidity of the slurry. While the Extinction coefficient is defined as: k=αc (4) α Unit concentration of light absorbance c  The turbidity of slurry According to the formulas (3) and (4), the light absorbance slurry is proportional to the turbidity of slurry. Therefore, when the light passes through the slurry, the turbidity instrument could calculate the turbidity of the slurry. 2.3 Experiment procedure The turbidity of PCM slurry and MEPCM slurry at different concentrations 15, 30 and 40% added with two surfactants has been tested. The primary volume concentration of all slurry is 42.5%. First, the surfactant PEG400 was added into PCM and MEPCM slurry, respectively, in order to make the mixed slurry at different concentrations. The turbidity of mixed slutty was tested in order to research the improvement effect on stability of PCMs. The slurry added with surfactant was stirred for 15 min by ultrasonic oscillation instrument. The mixed slurry were stood for 48 h in order to observe the dispersion phenomenon. Then the top section of mixed slurry was diluted 1000 times. After that, the turbidity of top section of mixed slurry was tested by the turbidity instrument. The mixed slurry has stratification phenomenon. The surfactant will suspend on the top of the mixed slurry which will lead to the high turbidity of the mixed slurry. Therefore, the higher turbidity refers to the lower stability. The different concentration factors just aim to confirm the experiment research. Then, the turbidity of mixed slurry would be tested by the turbidity instrument. The result showed that PEG400 had positive improvement effect on stability of MEPCM rather than PCM. It indicates that the stability of MEPCM could be improved by surfactant rather than PCM slurry. Therefore, in order to further research the improvement effect on stability of MEPCM by surfactant, the other surfactant SA was added into MEPCM slurry and the turbidity of this slurry was tested. 3 EXPERIMENT RESULTS AND DISCUSSION 3.1 PEG400 with PCMs and MEPCMs The PCM and MEPCM slurry at different volume concentrations of 15, 30 and 40% slurry were added with surfactant PEG400 in order to make the mixed slurry. PEG400 is a water-soluble polymer carbon chain, which is widely used in biomedical materials because of the good biocompatibility. The dispersion of MEPCM in the slurry is realized by the molecular chain of the reactive [14]. The slurry added with surfactant was stirred for 15 min by ultrasonic oscillation instrument. The mixed slurry were stood for 48 h in order to observe the dispersion phenomenon. Then, the turbidity of mixed slurry would be tested by the turbidity instrument. The PCM and MEPCM slurry with PEG400 are showed in Figures 3 and 4. Figure 3. View largeDownload slide PCM slurry with PEG400. Figure 3. View largeDownload slide PCM slurry with PEG400. Figure 4. View largeDownload slide MEPCM slurry with PEG400. Figure 4. View largeDownload slide MEPCM slurry with PEG400. As shown in Figure 3, the dispersion of PCM slurry with PEG400 is obvious. This is due to the different disperse abilities of surfactant [15]. As shown in Figure 4, the dispersion of MEPCM is not obvious. The turbidity values of slurry were tested by the turbidity instrument. The values are showed in Figures 5 and 6. As shown in Figure 5, the turbidity of the PCM slurry are 505, 807 and 950 mg/l, respectively. As shown in Figure 6, the turbidity of the MEPCM slurry with PEG400 are 395, 610 and 910 mg/l, respectively. The results show the turbidity of PCM slurry are higher than the PCM slurry with surfactant PEG400. It indicates that such surfactant could not improve the stability of the PCM slurry. The results show that the turbidity of MEPCM slurry with PEG400 is less than PCM with PEG400. It indicates that the MEPCM slurry with PEG400 has better stability. Surfactants have positive improvement effect on the stability of the MEPCM slurry rather than PCM slurry. Figure 5. View largeDownload slide The turbidity values of PCM slurry with PEG400. Figure 5. View largeDownload slide The turbidity values of PCM slurry with PEG400. Figure 6. View largeDownload slide The turbidity values of MEPCM slurry with PEG400. Figure 6. View largeDownload slide The turbidity values of MEPCM slurry with PEG400. 3.2 SA with MEPCMs In order to further study the improvement effect on stability of MEPCM slurry, another normal surfactant SA was used in the experiment. SA is an anionic electrolyte with highly hydrophobic molecule, which is insoluble in ethanol and strong acid (pH < 3) liquid [16]. The MEPCM slurry at different volume concentrations of 15, 30 and 40%, respectively, were added with surfactant SA. The slurry added with surfactant SA was stirred for 15 min by ultrasonic oscillation instrument. The mixed slurry were stood for 48 h in order to observe the dispersion phenomenon. Then the turbidity of mixed slurry would be tested by the turbidity instrument. The MEPCM slurry with SA are shown in Figure 7. Figure 7. View largeDownload slide MEPCM slurry with SA. Figure 7. View largeDownload slide MEPCM slurry with SA. As shown in Figure 7, the dispersion phenomenon of MEPCM mixed slurry was obviously at volume concentration of 15%. After 48 h, the turbidity of mixed slurry was tested by turbidity instrument. The turbidity values of MEPCM slurry with SA, PEG400 and are shown in Figure 8. Figure 8. View largeDownload slide The turbidity values of MEPCM slurry with SA and PEG400. Figure 8. View largeDownload slide The turbidity values of MEPCM slurry with SA and PEG400. As shown in Figure 8, the turbidity of MEPCM with SA is lower than the MEPCM with PEG400, slurry which indicates that surfactant SA has positive improvement effect on stability of MEPCM slurry. The improvement effect of surfactant SA is better than surfactant PEG400. SA dissociates could charge acid ions (A–COO–) negatively in alkaline aqueous liquid and it is suitable to be used as an electrolyte under alkaline conditions. The pH values of the mixed slurry was 8.78, the surface of the microcapsule particles of the slurry was covered with a layer of acid molecules (A–COO–), which prevented communication between the particles and repulsion in order to ensure the slurry is stable [17]. 4 CONCLUSIONS In this experiment, the stability of different slurry was analysed by testing the turbidity of slurry. The chemical modification methods and ultrasonic oscillation methods were used to mix the slurry with different surfactants. The improvement effect on stability of PCM and MEPCM slurry were carried out. The conclusions are as follows: The surfactant has positive improvement effect on stability of MEPCM slurry rather than PCM slurry. Both PEG400 and SA slurry have positive improvement effect on stability of MEPCM slurry. The improvement effect of SA is better than PEG400 slurry. The experiment of the improvement effect on stability of PCMs slurry could be used to find more efficient and environment-friendly surfactants, which could improve the stability of MEPCM slurry. The studies of PCM and MEPCM stability revealed a more prospective application for PCMs in energy storage/transportation systems and further optimize design the cold storage/transportation systems. FUNDING This project is funded by the National Natural Science Foundation of China (No. 51508352), Science & Technology Department Foundation of Chengdu City, China (No. 2015-HM01-00244-SF) and Key Laboratory Deep Underground Science and Engineering Foundation of Sichuan University, China (No. DUSE201702). REFERENCES 1 Zhao GY , Liu ZY , He Y , Cao HJ , Guo YB . Energy consumption in machining: classification, prediction, and reduction strategy . Energy 2017 ; 133 : 142 – 57 . Google Scholar CrossRef Search ADS 2 Sansaniwal SK , Sharma V , Mathur J . 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TI - Experimental study on improving stability of PCM and MEPCM slurry with different surfactants
JF - International Journal of Low-Carbon Technologies
DO - 10.1093/ijlct/cty027
DA - 2018-09-01
UR - https://www.deepdyve.com/lp/oxford-university-press/experimental-study-on-improving-stability-of-pcm-and-mepcm-slurry-with-nB6PY00wKt
SP - 272
EP - 276
VL - 13
IS - 3
DP - DeepDyve
ER -