A series of electrospun fatty acid ester/polyacrylonitrile phase change composite nanofibers as novel form-stable phase change materials for storage and retrieval of thermal energy

Abstract

In this paper, a series of fatty acid esters, including ethyl laurate (EL), butyl stearate (BS), ethyl palmitate (EP), ethyl stearate (ES) and methyl palmitate (MP), were selected as the solid–liquid phase change materials (PCMs), and then embedded inside the porous network structure of polyacrylonitrile (PAN) nanofibers supporting the skeleton by electrospinning technology, respectively. Morphological structures, chemical structures and thermal energy storage properties of electrospun fatty acid ester/PAN composite nanofibers were characterized by field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy and differential scanning calorimetry (DSC), respectively. Observations by FE-SEM images showed that the PAN nanofibers acting as the supporting polymer matrices can perfectly maintain the fiber shape and effectively prevent the leakage of the molten fatty acid esters. Maximum loaded weight percentages of the EL, BS, EP, ES and MP in the composite solutions could reach up to about 70, 45, 55, 65 and 60 wt.%, respectively. DSC results indicated that the prepared EL/PAN, BS/PAN, EP/PAN, ES/PAN and MP/PAN composite nanofibers had appropriate melting peak temperatures (about 1.26℃, 21.20℃, 29.37℃, 29.66℃ and 31.93℃, respectively) based upon climatic requirement, and the corresponding melting enthalpies were about 84.11, 55.10, 95.37, 93.35 and 110.4 kJ/kg, respectively. It can be considered that electrospun EL/PAN, BS/PAN, EP/PAN, ES/PAN and MP/PAN composite nanofibers would be promising form-stable PCMs for the applications related to the storage and retrieval of thermal energy, such as solar energy storage, building energy conservation, indoor temperature controlling and smart textiles and fibers.

Keywords

form-stable phase change materials fatty acid esters polyacrylonitrile thermo-regulating nanofibers thermal energy storage properties

Phase change materials (PCMs) are latent heat storage materials, which can absorb, retain and release large amounts of latent heat energy when they change phase from one physical state to another state as the ambient temperature alters. PCMs possess the following advantages: high energy storage density, small temperature variation from storage to retrieval, stable phase change energy, etc.^1–3 Typically, PCMs are classified into inorganic materials (e.g. salt hydrate, calcium chloride hexahydrate), organic materials (e.g. paraffins, fatty acids, fatty acid esters) and their mixtures (e.g. hexadecane-tetradecane mixture, fatty acid mixtures). These PCMs have been widely applied in many fields, such as the utilization of solar energy, building energy conservation systems, space and water heating, waste heat recovery systems, air-conditioning systems, medical application, cooling of engines, heat management of electronics, agricultural greenhouse, telecommunications and microprocessor equipment, thermal regulating textile materials, etc.^1–9

Fatty acids are an important class of organic solid–liquid PCMs for the storage and retrieval of thermal energy because of their extraordinary properties of relatively high energy storage capacity, good thermal reliability and chemical stability, non-corrosivity, cost effectiveness, no supercooling and phase segregation behaviors.¹⁰ Nevertheless, according to our previous works,^11,12 it is very interesting to note that the melting peak temperatures of fatty acids such as lauric acid (LA), myristic acid (MA), palmitic acid (PA) and stearic acid (SA) were about 44℃, 56℃, 64℃ and 70℃, respectively, which are typically higher than what would be preferred for climatic requirements, and this limits the applications of fatty acids in practice. Fortunately, the phase change temperatures of fatty acids can be adjusted into lower values through preparing the fatty acid eutectics or preparing fatty acid esters by the esterification reaction of fatty acids. Fatty acid esters belong to the class of fatty acid derivatives, which can be regarded as potential energy storage materials for thermal energy storage applications in terms of the lower melting temperatures and considerable high phase change enthalpies. However, fatty acid esters will flow and leak during the melting and freezing processes due to the characteristics of solid–liquid PCMs; therefore, special packages or sealed devices are also required to encapsulate them for preventing their leakage in the melting state. This leads to the reduction of the heat transfer efficiency and increase of operational costs. This disadvantage can be overcome by preparing form-stable PCMs, which are usually formed by physical entanglement or chemical linkage between the solid–liquid PCM and polymer matrix. Form-stable PCMs can maintain a solid and stable state without any leakage, even if the environment temperature rises up to their melting temperatures. This kind of polymeric-based form-stable PCMs, such as paraffin/polyethylene,¹³ paraffin/polypropylene,¹⁴ polyethylene glycol (PEG)/polyacrylic acid or PEG/poly(ethylene-co-acrylic acid),¹⁵ PEG/epoxy resin,¹⁶ PEG/polyurethane,¹⁷ fatty acid/polyaniline,¹⁸ fatty acid/SMA (styrene maleic anhydride copolymer),¹⁹ fatty acid/polymethylmethacrylate (PMMA)²⁰ and fatty acid/PnBMA (poly(n-butyl methacrylate)),²¹ has been investigated in recent years for the applications related to storage and retrieval of thermal energy.

According to the literature, thermal energy storage and retrieval properties of fatty acid esters, such as ethyl laurate (EL), butyl stearate (BS), ethyl palmitate (EP), ethyl stearate (ES) and methyl palmitate (MP), have never been studied and reported. In addition, there are no reports about the preparation and investigation of morphological structures, chemical structures or thermal properties of form-stable phase change composite nanofibers consisting of these fatty acid esters acting as solid–liquid PCMs and polyacrylonitrile (PAN) nanofiber membranes acting as supporting materials. Therefore, the objective of this research is to develop an innovative type of form-stable PCMs with suitable phase change temperatures based on the climatic requirement for the storage and retrieval of thermal energy by encapsulating and/or/ embedding of five kinds of fatty acid esters (i.e. EL, BS, EP, ES and MP) into the three-dimensional network structure of the PAN nanofiber matrices on the nanoscale through the technique of electrospinning, respectively. These nanofiber-based form-stable PCMs possess remarkable properties of light weight, small diameter, controllable and high surface-to-volume ratio in comparison with conventional form-stable PCMs. Morphological structures, chemical structures and thermal energy storage properties of the prepared fatty acid ester/PAN phase change composite nanofibers were investigated by field-emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC), respectively.

Experimental details

Materials

Chemicals of fatty acid esters, such as EL (CH₃(CH₂)₁₀COOCH₂CH₃), BS (CH₃(CH₂)₁₆COO(CH₂)₃CH₃), EP (CH₃(CH₂)₁₄COOCH₂CH₃), ES (CH₃(CH₂)₁₆COOCH₂CH₃), MP (CH₃(CH₂)₁₄COOCH₃) and N,N-dimethyl formamide (DMF), were all obtained from the Sinopharm Group Chemical Reagent Co., Ltd (Shanghai, China). PAN powder (M_w = 50,000–70,000) was provided by Aldrich. All of the chemicals were used as received without further purifications.

Preparation of fatty acid ester/PAN phase change composite nanofibers

The PAN powder was first dissolved in the DMF solvent through magnetic stirring at ∼50℃ to achieve a 10 wt.% uniform polymer solution. Subsequently, each of the five fatty acid esters was added into the solution with maximum dissoluble mass percentages of about 70, 45, 55, 65 and 60 wt.% for EL, BS, EP, ES and MP, respectively. Thereafter, these mixture solutions were magnetically stirred to obtain the homogeneous solutions for electrospinning.

The as-prepared solutions were loaded in 20 ml plastic syringes, respectively, and then were fed using a syringe pump at feed rate of 1 ml/h. The inside and outside diameters of the spinneret were 0.6 and 0.9 mm, respectively. Room temperature and relative humidity (RH) were kept constant at about 25℃ and 30%, respectively. An electrode was clamped on the needle and connected to a power supply with a positive high voltage of 18 kV. Electrospun nanofibers were collected onto the aluminum foil, which was wrapped on a grounded rotating drum with the diameter of 25 cm. The collected distance between the blunt tip of the needle and the aluminum foil was set at 16 cm. The rotating speed of the grounded drum was fixed at 100 rpm. The obtained electrospun nanofibrous membranes were dried under vacuum at room temperature for 24 h to remove the residual solvents.

Characterizations

Field emission scanning electron microscopy

Field emission scanning electron microscopy (FE-SEM; Hitachi S-4800) was employed to observe the morphologies of electrospun phase change composite nanofibers. All samples were sputter coated with gold to avoid charge accumulations before FE-SEM observation. Average fiber diameters (AFDs) were analyzed from the FE-SEM images through measuring 100 randomly selected fibers using the LeicaIMGRead software (http://dell.chem.sunysb.edu). The same method of measurement has been used and reported in our previous research paper.²²

Fourier transform infrared spectroscopy

In order to investigate the compatibility and interaction between fatty acid esters and PAN supporting materials, FT-IR spectra of samples were recorded in the spectral range of 500–4000 cm⁻¹ using a Nicolet iS10 FT-IR spectrometer (Thermo Fisher Scientific).

Differential scanning calorimetry

Thermal energy storage and release properties of the five fatty acid esters and electrospun fatty acid ester/PAN phase change composite nanofibers were measured by a DSC-Q200 thermal analyzer. DSC experiments were carried out in dry nitrogen with a flow rate of 50 ml/min. The DSC curves of specimens were recorded from –40℃ to 80℃ at the linear heating and cooling rate of 8℃/min in a sealed aluminum pan. The DSC data of the peak onset temperatures (T_o), melting peak temperatures (T_m), freezing peak temperatures (T_c), peak end temperatures (T_e), melting enthalpies (ΔH_m) and freezing enthalpies (ΔH_c) were extracted from DSC curves using the DSC analysis software. A similar analytic method has been reported in the literature.^23,24

Results and discussion

Morphological structures of fatty acid ester/PAN phase change composite nanofibers

The FE-SEM images in Figure 1 show the representative morphologies of electrospun pure PAN nanofibers and the five kinds of electrospun EL/PAN, BS/PAN, EP/PAN, ES/PAN and MP/PAN phase change composite nanofibers. Figure 1(a) reveals that electrospun pure PAN nanofibers exhibited quite uniform diameter distribution, and the AFD was about 100 nm. These results indicated that the PAN solution was readily electrospun into PAN nanofibers in this 10 wt.% concentration. However, the surface morphologies of electrospun phase change composite nanofibers loading different kinds of fatty acid esters were slightly different from those of pure PAN nanofibers, as shown in Figures 1(b)–(f). Rough or coarse surface morphologies could be occasionally observed from SEM images, and some adhesions between composite nanofibers could be also found. The AFDs of electrospun EL/PAN, BS/PAN, EP/PAN, ES/PAN and MP/PAN phase change composite nanofibers were about 197, 130, 150, 183 and 189 nm, respectively, which were slightly larger than that of pure PAN nanofibers. This could be primarily attributed to the variations of electrospinning solution properties, such as an increase of viscosity and a decrease of conductivity. A similar phenomenon has been studied and reported in our previous research papers.^25–28 The maximum loaded weight percentages of the EL, BS, EP, ES and MP in the composite solutions were about 70, 45, 55, 65 and 60 wt.%, respectively. It is noteworthy that the continuous composite nanofibers cannot be obtained when the weight percentages of fatty acid esters in composite solutions increased to above the corresponding values, which could be attributed to the decline of the mixture solution spinnability as a result of the addition of the non-spinnable fatty acid esters. Interestingly, the morphological structures of electrospun fatty acid ester/PAN phase change composite nanofibers loading with different kinds of fatty acid esters were found to be very close to each other. This also indicates that these fatty acid esters could be easily combined into the supporting matrices of the PAN nanofibers through the electrospinning method. It is important to note that the obtained EL/PAN and BS/PAN phase change composite nanofibers could maintain their overall fiber shapes in the solid state, even though EL and BS showed the liquid state at room temperature. In other words, the PAN nanofiber supporting materials could efficiently prevent the fluidity at the temperature beyond the melting temperature of the loaded fatty acid esters to overcome the liquid leakage problem. Moreover, the electrospun PAN nanofibers supporting skeleton could also provide the mechanical strength to the composite phase change systems. Therefore, we concluded that the prepared electrospun fatty acid ester/PAN phase change composite nanofibers could be considered as a novel type of form-stable PCM for applications related to the storage and retrieval of thermal energy.

Figure 1.

Field emission scanning electron microscopy images showing the representative morphologies of electrospun polyacrylonitrile (PAN) nanofibers and phase change composite nanofibers consisting of the five fatty acid esters and PAN: (a) PAN; (b) ethyl laurate/PAN; (c) butyl stearate/PAN; (d) ethyl palmitate/PAN; (e) ethyl stearate/PAN; (f) methyl palmitate/PAN.

FT-IR analyses

Figure 2 shows the FT-IR spectra of (a) electrospun PAN nanofibers, (b) pure MP powder and (c) electrospun MP/PAN phase change composite nanofibers. As shown in Figure 2(a), there was a typical characteristic absorption peak at about 2242 cm⁻¹, which was ascribed to the stretching vibration of nitrile groups (–C≡N) in PAN molecules. The strong characteristic absorption peaks at the wave numbers of about 2928 and 1450 cm⁻¹ were assigned to the asymmetrical and symmetrical bending vibrations of methylene groups (–CH₂–), respectively.²⁹ This result was consistent with the standard infrared spectrum of PAN molecular chains. Figure 2(b) reveals that pure MP powder exhibited two characteristic absorption peaks at the wave numbers of 1740 and 1173 cm⁻¹ corresponding to the stretching vibrations of C=O and C–O groups of the acyclic saturated ester, respectively. The characteristic absorption peaks at about 2919 and 2845 cm⁻¹ were attributed to the asymmetric and symmetric stretching vibrations of the C-H bond, respectively. Moreover, it also showed characteristic absorption peaks at about 1464 and 722 cm⁻¹, which could be associated with the CH₂ or CH₃ deformation vibration and the rocking vibration in (–CH₂–) _n (n ≥ 4) groups. Compared with the FT-IR spectra of electrospun PAN nanofibers and pure MP powder, the FT-IR spectrum of electrospun MP/PAN phase change composite nanofibers in Figure 2(c) shows that no significant new characteristic absorption peak was observed, except for the original absorption peaks belonging to pure MP and PAN molecules, suggesting that there was no chemical reaction between the MP molecules and the PAN molecules during the electrospinning process. In addition, it could be clearly seen that the characteristic absorption peaks of the MP molecules well overlapped with the absorption bands of the PAN molecules in the spectrum, which also indicated that the MP molecules were successfully loaded into the supporting matrices of electrospun PAN nanofibers and they had good compatibility in the composite nanofibers.

Figure 2.

Fourier transform infrared spectra of electrospun polyacrylonitrile (PAN) nanofibers, pure methyl palmitate (MP) powder and electrospun MP/PAN phase change composite nanofibers.

Thermal energy storage and release properties

Thermal energy storage and release properties are generally considered as important indicators to evaluating form-stable PCMs for storage and retrieval of thermal energy. Figure 3 shows the representative DSC curves of the five fatty acid esters, namely EL, BS, EP, ES, and MP, during the heating and cooling processes. The extracted thermal data, including the peak onset temperatures (T_o), melting peak temperatures (T_m), freezing peak temperatures (T_c), peak end temperatures (T_e), melting enthalpies (ΔH_m) and freezing enthalpies (ΔH_c), are summarized in Table 1. As seen in Figure 3 and Table 1, the pristine EL, BS, EP, ES and MP exhibited high melting and freezing enthalpies, whilst the melting peak temperatures taking place in the solid–liquid phase transition were about 0.11℃, 20.68℃, 25.93℃, 26.45℃ and 31.18℃ for EL, BS, EP, ES and MP, respectively, which were close to room temperature, except for that of sample EL. This result suggested that the fatty acid esters investigated in this paper could be considered as novel types of solid–liquid PCMs with excellent thermal energy properties for thermal energy storage and release applications. Figure 4 illustrates the DSC curves of electrospun EL/PAN, BS/PAN, EP/PAN, ES/PAN and MP/PAN phase change composite nanofibers during the heating and cooling processes. Table 2 summarizes their corresponding thermal energy storage data. It can be seen from Figure 4 that the thermal energy storage performances of the five electrospun fatty acid ester/PAN phase change composite nanofibers were different from each other, which could be attributed to the difference of the loaded fatty acid esters. It is notable that the electrospun EL/PAN, BS/PAN, EP/PAN, ES/PAN and MP/PAN phase change composite nanofibers possessed reversible phase transition behaviors similar to those of pristine EL, BS, EP, ES and MP, respectively. As shown in Tables 1 and 2, the phase change enthalpies of these five kinds of electrospun fatty acid ester/PAN composite nanofibers were lower than those of the corresponding pristine fatty acid esters combined in composite nanofibers, respectively, which was expected because there was no contribution of phase change enthalpies provided by the electrospun PAN nanofiber skeleton in the temperature range of the DSC measurement. In other words, the phase change enthalpies of electrospun fatty acid ester/PAN phase change composite nanofibers were affected by the maximum loaded weight percentages of the EL, BS, EP, ES and MP in the composite solutions. Interestingly, there is no appreciable difference between the phase change temperatures of electrospun fatty acid ester/PAN phase change composite nanofibers and those of the corresponding loaded fatty acid esters. DSC results indicated that the phase change temperatures of electrospun fatty acid ester/PAN phase change composite nanofibers could be adjusted to suitable regions by combining fatty acid esters with different phase change temperatures for practical applications.

Figure 3.

Differential scanning calorimetry curves of the five kinds of fatty acid esters during the heating and cooling processes. EL: ethyl laurate; BS: butyl stearate; EP: ethyl palmitate; ES: ethyl stearate; MP: methyl palmitate.

Table 1.

The peak onset temperatures (T_o), melting peak temperatures (T_m), freezing peak temperatures (T_c), peak end temperatures (T_e), melting enthalpies (ΔH_m) and freezing enthalpies (ΔH_c) of the five fatty acid esters

	Melting				Freezing
	T_o	T_m	T_e	ΔH_m	T_o	T_c	T_e	ΔH_c
Fatty acid esters	(℃)	(℃)	(℃)	(kJ/kg)	(℃)	(℃)	(℃)	(kJ/kg)
EL	−1.49	0.11	4.88	128.0	−5.18	−4.34	−10.59	123.0
BS	16.67	20.68	23.79	133.2	18.92	16.23	11.44	132.3
EP	23.33	25.93	30.56	191.8	19.18	15.18	11.73	179.3
ES	18.66	26.45	29.13	153.0	24.79	22.69	14.05	153.4
MP	27.48	31.18	34.35	197.3	26.17	24.58	19.21	194.1

EL: ethyl laurate; BS: butyl stearate; EP: ethyl palmitate; ES: ethyl stearate; MP: methyl palmitate.

Figure 4.

Differential scanning calorimetry curves of the five kinds of electrospun fatty acid ester/polyacrylonitrile (PAN) phase change composite nanofibers during the heating and cooling processes. EL: ethyl laurate; BS: butyl stearate; EP: ethyl palmitate; ES: ethyl stearate; MP: methyl palmitate.

Table 2.

	Melting				Freezing
	T_o	T_m	T_e	ΔH_m	T_o	T_c	T_e	ΔH_c
Samples	(℃)	(℃)	(℃)	(kJ/kg)	(℃)	(℃)	(℃)	(kJ/kg)
EL/PAN	−3.05	1.26	8.27	84.11	−9.13	−12.46	−19.37	85.07
BS/PAN	15.56	21.20	25.51	55.10	18.15	12.61	7.21	55.13
EP/PAN	21.22	29.37	34.17	95.37	18.99	12.00	4.56	95.85
ES/PAN	23.38	29.66	34.87	93.35	26.90	19.83	12.68	95.59
MP/PAN	25.10	31.93	37.58	110.4	24.33	18.68	11.52	110.8

EL: ethyl laurate; BS: butyl stearate; EP: ethyl palmitate; ES: ethyl stearate; MP: methyl palmitate.

According to the literature, Jeon and coworkers³⁰ reported that a kind of composite PCM with melting peak temperature of about 18℃ can be considered as energy saving building materials for a residential building using radiant floor heating system. Chalco-Sandoval et al.³¹ developed three kinds of microcapsule PCMs with the phase change temperature set at –1.5℃ for thermal energy storage systems of food superchilling applications. Moreover, Mondal⁸ reported that a PCM that has a melting point from 15℃ to 35℃ is one of the most effective ideas for effective utilization in the textiles field. Table 3 summarizes the thermal energy storage properties of electrospun fatty acid ester/PAN phase change composite nanofibers and their comparison with some electrospun ultrafine phase change composite fibers reported in the literature.^{27,28,32–39} It was found that the melting peak temperatures of most electrospun ultrafine phase change composite fibers reported in the literature were higher than the actual climatic comfortable temperatures (i.e. 15–30℃), except for the electrospun phase change composite fibers with the binary fatty acid eutectics loading reported in our previous literature.¹² Interestingly, the melting peak temperatures of electrospun BS/PAN, EP/PAN, ES/PAN and MP/PAN phase change composite nanofibers were in these climatic comfortable temperature range, as shown in Table 2. Moreover, Tables 2 and 3 also illustrate that the melting/freezing enthalpies of electrospun EL/PAN, BS/PAN, EP/PAN, ES/PAN and MP/PAN phase change composite nanofibers were about 84.11/85.07, 55.10/55.13, 95.37/95.85, 93.35/95.59 and 110.4/110.8 kJ/kg, respectively, which suggested that these electrospun nanofiber-based phase change composite materials, except for the EL/PAN composite nanofibers, also exhibited the desired phase change enthalpies compared to those of the electrospun ultrafine phase change composite fibers summarized in Table 3. Therefore, we concluded that these innovative form-stable PCMs of electrospun fatty acid ester/PAN phase change composite nanofibers have a high potential for thermal energy storage applications, such as building energy saving applications, by incorporating these materials into building materials, temperature regulating fibers and smart textiles.

Table 3.

Comparison between the melting and freezing peak temperatures (T_m and T_c), melting enthalpies (ΔH_m) and freezing enthalpies (ΔH_c) of electrospun fatty acid ester/polyacrylonitrile (PAN) phase change composite nanofibers and those of other electrospun phase change composite nanofibers reported from the literature

	Melting		Freezing
Phase change composite nanofibers	T_m (℃)	ΔH_m(kJ/kg)	T_c (℃)	ΔH_c (kJ/kg)	Refs.
CA (71 wt.%)/PET	37.78	107.3	23.78	104.2	[27]
LA (50 wt.%)/PET	45.14	70.76	38.57	62.14	[32]
GMS (60 wt.%)/PET	57.89	66.99	46.65	66.02	[33]
SS (33 wt.%)/PET	50.00	53.77	–	–	[34]
DADOEs (43 wt.%)/PET	48.20	66.50	45.00	65.40	[35]
CA-LA (67 wt.%)/PET	22.70	127.2	16.43	125.2	[12]
CA-MA (67 wt.%)/PET	26.02	155.2	15.08	145.9
CA-PA (67 wt.%)/PET	28.71	141.4	19.26	136.1
CA-SA (67 wt.%)/PET	31.17	156.8	22.94	147.1
LA-MA (55 wt.%)/PET	37.12	70.84	32.48	69.51	[28]
LA-PA (55 wt.%)/PET	36.37	65.53	30.43	67.30
MA-PA (55 wt.%)/PET	46.54	92.96	42.43	85.32
MA-SA (55 wt.%)/PET	45.94	94.29	39.59	83.73
PA-SA (55 wt.%)/PET	56.13	112.7	51.76	101.6
LA (60 wt.%)/PA6	44.71	74.12	40.09	62.48	[36]
PEG4000 (60 wt.%)/PA6	59.97	96.96	41.47	85.13	[37]
PEG10000 (60 wt.%)/PA6	62.01	104.2	47.34	99.70
PEG10000 (70 wt.%)/CA	63.00	120.18	40.00	104.40	[38]
PEG 1000 (56 wt.%)/PVDF	38.50	72.20	24.80	68.80	[39]
EL (70 wt.%)/PAN	1.26	84.11	−12.46	85.07	This study
BS (45 wt.%)/PAN	21.20	55.10	12.61	55.13
EP (55 wt.%)/PAN	29.37	95.37	12.00	95.85
ES (65 wt.%)/PAN	29.66	93.35	19.83	95.59
MP (60 wt.%)/PAN	31.93	110.4	18.68	110.8

EL: ethyl laurate; BS: butyl stearate; EP: ethyl palmitate; ES: ethyl stearate; MP: methyl palmitate; PET: polyethylene terephthalate; PEG: polyethylene glycol; LA: lauric acid; MA: myristic acid; PA: palmitic acid; SA: stearic acid; GMS: glycerol monostearate; SS: stearyl stearate; DADOEs: diacid dioctadecyl esters.

Conclusions

In this paper, a series of electrospun fatty acid ester/PAN phase change composite nanofibers, namely EL/PAN, BS/PAN, EP/PAN, ES/PAN and MP/PAN, were successfully developed as novel form-stable PCMs for thermal energy storage applications. The maximum loaded weight percentages of the EL, BS, EP, ES and MP in the composite solutions were determined to be about 70, 45, 55, 65 and 60 wt.%, respectively. FE-SEM images showed that electrospun EL/PAN, BS/PAN, EP/PAN, ES/PAN and MP/PAN phase change composite nanofibers exhibited rough or coarse surface morphologies, and some adhesions among the intersections of some composite nanofibers could also be observed. The AFDs of electrospun fatty acid ester/PAN phase change composite nanofibers were determined to be about 130–200 nm, which were slightly larger than that of pure PAN nanofibers (about 100 nm). DSC results demonstrated that these five kinds of fatty acid esters had been successfully combined into the three-dimensional network structure of electrospun PAN nanofiber supporting matrices by electrospinning technology. Thermal energy storage performances of the prepared phase change composite nanofibers mainly depended on the type of loaded fatty acid esters. Their melting peak temperatures and melting enthalpies were about 1.26℃ and 84.11 kJ/kg, 21.20℃ and 55.10 kJ/kg, 29.37℃ and 95.37 kJ/kg, 29.66℃ and 93.35 kJ/kg, and 31.93℃ and 110.4 kJ/kg for electrospun EL/PAN, BS/PAN, EP/PAN, ES/PAN and MP/PAN phase change composite nanofibers, respectively. We concluded that these developed phase change composite nanofibers could be considered as a promising type of form-stable PCM for applications related to the storage and retrieval of thermal energy.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Six Talent Peaks Project in Jiangsu Province (grant number 2014-XCL001) and the Science and Technology Support Program of Fujian Province (grant numbers JK2014042 and 2015H0030).

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