Received: 24 Oct 2019
Revised: 08 Dec 2019
Accepted: 06 Jan 2020
Published online: 09 Jan 2020
Polyethylene Glycol/Carbon Black Shape-Stable Phase Change Composites for Peak Load Regulating of Electric Power System and Corresponding Thermal Energy Storage
Xiang Lu 1,*, Huanyu Liu 2, Vignesh Murugadoss 4, Ilwoo Seok 5, Jintao Huang 3,
Jong E. Ryu 6 and Zhanhu Guo 4,*
1 Key Laboratory of Polymer Processing Engineering of the Ministry of Education, National Engineering Research Center of Novel Equipment for Polymer Processing, Guangdong Key Laboratory of Technique and Equipment for Macromolecular Advanced Manufacturing, South China University of Technology, Guangzhou, 510641, China
2 School of Mechanical Engineering, Dongguan University of Technology, Dongguan 523808, China.
3 Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China
4 Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular Engineering, University of Tennessee, Knoxville, USA
5 Mechanical Engineering, Arkansas State University, Jonesboro, Arkansas, 72401 USA
6 Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695 USA
In this work, a unique electrically conductive polyethylene glycol (PEG)/carbon black (CB) shape-stable phase change composite (SSPCC) for peak load regulating of electric power system was prepared via a vacuum impregnation approach. Scanning electron microscope (SEM), differential scanning calorimeter (DSC) and X-ray diffraction (XRD) were used to study the micro morphology, crystallization behavior, crystallization structure and thermal properties. Leakage test and DSC results showed that when the PEG content in PEG/CB SSPCC was as high as 86 wt%, the phase change enthalpy and relative enthalpy efficiency were up to 147.7 J/g and 95.1 %, respectively. Fourier transform infrared (FITR) and XRD showed no chemical reaction between CB and PEG. Thermal cycling test showed that the PEG/CB SSPCC had excellent thermal stability and thermal reliability. Moreover, the good electro-to-thermal conversion ability of PEG/CB SSPCC (0.2 S/m) provided rich possibilities for peak load regulating of electric power system and corresponding thermal energy storage.
Table of Content
A novel [email protected] SSPCC is first obtained for electro-to-thermal conversion and storage
Carbon black polyethylene glycol electro-to-thermal thermal energy storagen SSPCC
With the expansion of economic society and the increase of population, the demand for energy around the world is increasing day by day.1-3 And the limited fossil energy in nature is gradually decreasing due to the massive consumption of humans in the last hundred years.4, 5 Renewable sources on the earth, for example, solar power, water power, etc., have raised extensive attention by researchers due to their high renewal rate and sustainablity in nature. In many cases, energy supply and demand are unbalanced and have a strong time dependence. Thus, in addition to developing and utilizing new renewable sources, improving the use efficiency of existing energy (such as fossil energy and renewable energy) is also an important issue.6-9
As one of the most important energy forms in modern society, electric energy, generated by hydroelectric power, thermal power, and wind power, etc., is widely used in all aspects of national life, and it also provides great convenience for human life. For a variety of reasons, the generation of electric energy is usually huge and continuous. But the human demands for electric energy have a strong time dependence. Generally speaking, the daily life and work of human beings is mainly carried out from 8 am to 10 pm, therefore, the demand for electric energy during this period is very high. But from 10 pm to 8 am at the following day, most people and machines are at rest, and the demand for electric energy is relatively low. At night, the generated electric energy is largely idle. Thus, it is necessary to store the idle electric energy. In order to ease the situation, the electric energy storage technology based on batteries 10, supercapacitors 11, etc. has emerged.
Thermal energy, as the biggest energy source in nature, plays a very important role in human daily life. Consequently, to efficiently use the relatively surplus and cheap electric energy at night and convert it into the heat that can be used directly in the human daily life is a meaningful alternative way. In addition, compared to electrical energy and chemical energy storage technologies, thermal energy storage (TES) has a longer history, and it is simpler and easier to comply.12-15 Furthermore, the use of phase change materials (PCMs) based on latent heat storage technology has raised extensive attention recently owing to its comprehensive advantages of constant temperature, easy process control, high energy storage density (about 15 times of sensible heat storage materials), and repeated use.16-18 Up to now, thermal energy storage technology based PCMs has become the most important and common way to store heat energy in the area of waste-heat recovery in industry and solar energy, etc..
Among thousands of PCMs that have been discovered, organic PCM, such as polyethylene glycol (PEG),19, 20 fatty acid,21 capric-stearic acid 22 and paraffin,23 etc., has become a research hotspot due to its low cost, high phase transition enthalpy, moderate phase transition temperature, little supercooling, etc. However, the biggest drawback for most organic PCMs is the typical solid-liquid transition and leakage risk during the energy storage and release. Therefore, a variety of strategies, including chemical crosslinking, polymer shaping, microcapsule coating and porous materials adsorbing etc., have been applied to fabricate shape-stable organic PCMs, which can maintain their original shape and avoid the risk of leakage during use. For example, Sundararajan et al. 24 synthesized a series of PEG-based hyperbranched polyurethanes as shape-stable PCMs (SSPCCs) using oligomeric A2 + B3 approach for thermal energy storage. Guo et al. 25 prepared diatomite stabilized paraffin/wood flour/high-density polyethylene PCM with acceptable TES performance and good mechanical strength in building energy conversion. Jiang et al. 26 prepare new paraffin wax/poly (methyl methacrylate-co-methyl acrylate)/nano Al2O3 microencapsulated PCMs through the emulsion polymerization, and discussed their potential applications in energy-saving building. In addition to this, porous materials have also widely been used to prepare SSPCCs via a vacuum impregnation approach to improve the shape stability, and this approach was easy to operate and industrialize. For example, Tang et al. 27 prepared PEG/graphene oxide aerogel (GOA) SSPCCs by introducing PEG into GAs via vacuum impregnation, and they pointed out that the prepared PCM exhibited good photo-to-thermal energy conversion property. Yuan et al. 28 prepared a erythritol/expanded graphite (EG) SSPCC via three-step technique for mid-temperature thermal energy storage.
As described above, the modified organic SSPCCs can efficiently store the thermal energy because of their high phase transition enthalpy, moderate phase transition temperature and little super-cooling. According to the Joule's Law, when the current passes through the conductor, heat is generated, and the generated heat (Q) can be calculated by Eqn (1):
Q = I2Rt (1)
where I is the current intensity in the circuit, R is the resistance value of the conductor, and t is the time. Based on this, if the prepared organic SSPCCs are electrically conductive, it may be used for electro-to-thermal energy conversion to achieve peak load regulating of electric power system at night to improve the use efficiency of idle electric energy.
Compared with carbon nanotubes and graphene, conductive carbon black (CB) is an inexpensive and relatively efficient electrically conductive filler, has the characteristics of small particle size (nanoscale), large specific surface area and rough surface, and is widely used in electrically conductive polymer composites.29-31 Theoretically, there will be strong interactions and adsorptions between CB nanoparticles and organic PCM molecular chains because of the small particle size (nanoscale), large specific surface area and rough surface of CB. The organic PCM/CB composites with high electrical conductivity can be obtained via the simple vacuum impregnation, and the obtained SSPCCs may be applied in electric-to-thermal energy conversion and storage according to the Joule's Law. PEG, as a typical solid-liquid transition organic PCM, has received extensive attention from researchers recently. However, as far as we know, the application of CB to the field of electrical conductivity PEG-based PCMs via electro-to-thermal conversion for peak load regulating of electric power system has been rarely reported.
In this study, PEG/CB composites were prepared via the simple vacuum impregnation and the effects of CB on the shape-stable performance, thermal properties, crystallization behavior, electrical conductivity, and electro-to-thermal energy conversion behavior of the obtained PEG/CB SSPCC were studied. The research results provide rich theoretical support for expanding the application of PCMs in the aspect of peak load regulating of electric power system, which has important scientific and practical significances.
Polyethylene glyol (PEG-10000) was provided by Shanghai Aladin Industrial Co., Ltd.. Conductive carbon black (ENSACO 250G: Brunauer-Emmett-Teller (Bet) surface=65 m2/g) was purchased from TIMICAL Co., Ltd., Switzerland.
2.2 Fabricating method of [email protected] SSPCC
The PEG/CB SSPCC was fabricated by a direct vacuum impregnation method. Briefly, 3 g CB powder and 22 g solid PEG were put into a glass container first, and then moved in a vacuum oven to achieve the adsorption saturation at 80 oC for 12 h under high vacuum. After the vacuum adsorption process, the specimen was placed onto a sheet of filter paper, and then put into a vacuum oven to remove PEG residue at the specimen surface at 80 oC for 2 h. The filter paper changed periodically and ended when no PEG leaked from the sample. Finally, PEG/CB SSPCC was obtained and identified as [email protected] The PEG loading (W) in the [email protected] was calculated by Eqn. (2):
where m1 is the quantity of pristine CB and m2 is the quantity of the prepared [email protected] composite.
2.3 Characterization Methods
Fourier transform infrared (FT-IR) test was carried out by an equipment from PerkinElmer labeled by Spectrum 2000 over the scanning range 4000-400 cm−1. The BET method was introduced to evaluate the pore volume and specific surface area, and the samples were tested on BELSORP-max (Microtrac BEL, Japan) at 77 K. Morphology observations of CB and [email protected] were performed on a field emission scanning electron microscopy (Quanta FEG 250). XRD tests (diffraction angle 5–60°) were conducted on a Bruker D8 Advance powder diffractormeter. The heat storage behavior of samples was characterized by a DSC equipment (Netzsch 204c). The samples were heated and cooled under a nitrogen atmosphere in test at a rate of 10 °C/min. TGA was performed to evaluate the thermal stability by Netzsch TG209. The samples were tested from 30 to 600 °C and heated under nitrogen flow at a rate of 10 °C/min. The accelerated thermal cycling test was used to evaluate the thermal reliability and reusability of [email protected] via high-low temperature chamber. The volume electrical resistivity was measured at 25 °C using an electrical conductivity analyzer with four probes (RTS-4). The thermal conductivity of pure PEG and [email protected] were measured at 25 °C by the transient plane source method using a laser thermal conductivity analyzer (LFA-427). The electro-thermal conversion and storage behaviors of the obtained [email protected] were studied via a self-built circuit. Infrared thermography camera was used for the collection of temperature change during test.
3. Results and Discussion
3.1 Leakage test
Fig. 1. The quantity of PEG leakage at different time periods.
As described in Section 2.2, 3 g CB powder and 22 g solid PEG were put into a glass container first, and then moved in a vacuum oven to achieve the adsorption saturation at 80 oC for 12 h under high vacuum. For calculating the loading of PEG in PEG@CB without any leakage, leakage tests were carried out on the obtained PEG/CB composite. The specimen was placed onto a sheet of filter paper. The quantity of leaked PEG at different time ranging from 0 to 20 hours is shown in Fig. 1. Also, the corresponding digital photos of specimen during leakage are presented. At the beginning, the specimen temperature is near the room temperature, and the sample is in a solid state. After being heated, the un-adsorbed PEG in PEG/CB composite was quickly melted to a liquid state, and was adsorbed by the filter paper on the bottom. As shown in Fig. 1, the quantity of the leaked PEG increased with increasing the test time. At about 8 hours, the leaked PEG quantity maintained at around 3.2 g and the leaking quantity reached its equilibrium state. This indicates that no more PEG leaks out from the PEG/CB composite further. The phenomenon shows that un-leaked PEG was well adsorbed by CB. According to Eqn. (2), the loading of PEG in the PEG@CB is up to 86 wt%, and it is higher than most of the values in the previously reported work, refer to Table 1.
Table 1. Comparison of Maximum PEG ratio in PCM in literatures
|PEG/Diatomite||Vacuum impregnation||50 %||32|
|PEG/Diatomite pretreatment||Direct impregnation||55%||33|
|PEG/Diatomite pretreatment||Vacuum impregnation||70%||34|
PEG/ Biological porous carbon
|PEG/Expanded perlite||Vacuum impregnation||73.93%||35|
|PEG@CB||Vacuum impregnation||86%||Present Work|
3.2 Morphology observations