From rice husk to high performance shape stabilized phase change materials for thermal energy storage

Abstract

A novel shape-stabilized phase change material (SSPCM) was fabricated by using a vacuum impregnation technique. The lightweight, ultra-high specific surface area and porous activated carbon was prepared from waste material (rice husk) through the combination of an activation temperature approach and a sodium hydroxide activation procedure. Palmitic acid as a phase change material was impregnated into the porous carbon by a vacuum impregnation technique. Graphene nanoplatelets (GNPs) were employed as an additive for thermal conductivity enhancement of the SSPCMs. The attained composites exhibited exceptional phase change behavior, having a desirable latent heat storage capacity of 175 kJ kg⁻¹. When exposed to high solar radiation intensities, the composites can absorb and store the thermal energy. An FTIR analysis of the SSPCMs indicated that there was no chemical interaction between the palmitic acid and the activated carbon with GNPs. The thermal conductivity of the prepared composites improved by more than 97% for the highest loading of GNPs (6 wt%) compared with that of pure palmitic acid. Moreover, the SSPCMs exhibit high thermal stability, with a stable melting–freezing enthalpy and excellent reversibility. The prepared SSPCMs with enhanced heat transfer and phase change properties provide a beneficial option for building energy conservation and solar energy applications owing to the low cost of raw materials and the simple synthetic technique.

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1. Introduction

Concerns regarding fossil fuels have increased interest in efficient energy utilization. Phase change materials (PCMs), which are capable of storing and releasing significant amounts of energy by melting and solidifying at certain temperatures, have been applied in many fields, including solar energy, energy efficient buildings, and waste heat recovery.^1–3 Organic and inorganic PCMs are the two most typical types of these materials. The advantages of organic over inorganic PCMs are negligible supercooling and corrosivity, melting without phase segregation, and high thermal storage density.⁴ Palmitic acid (PA) is one the organic PCMs attracting interest, mainly due to its excellent thermal properties, including high phase change enthalpies (approximately 200 kJ kg⁻¹) and negligible supercooling or phase separation. However, leakage of PA during melting and solidifying and its low thermal conductivity limit its areas of application.⁵

Several techniques for enhancing the thermal conductivity and avoiding leakage of PCMs have already suggested, and have been successfully tested previously including shape-stabilization and encapsulation of PCMs. Currently, form-stable PCM composites with good thermal conductive supporting materials that can conserve the solid form even through the phase change process are currently the objective of much research.^6–13 This technique may be the most efficient way to eliminate leaks and the low thermal conductivity problems of PCMs. Carbon materials are the most useful additives for improving the effective thermal conductivity, in part because just a small volume fraction is required, and the PCM subsequently has high thermal conductivity and low density.^14–17 Currently, numerous studies have been reported on the preparation of shape-stabilized phase change materials (SSPCMs) employing a variety of carbon materials including carbon nanotubes,^18,19 carbon fibers,^20,21 graphite,^22–28 and graphene.^29,30 All of these approaches provide a significant improvement in the effective thermal conductivity of a PCM having a carbon material as a filler.

Lately, activated carbon (AC) has attracted much as an option for providing SSPCMs with good shape stability properties. Moreover, AC has a low density and is abundant, chemically stable, simple to prepare and inexpensive. As a result, AC is highly economic for use as a supporting material for the preparation of SSPCMs. However, technical reports are limited on the utilization of AC as a supporting material in the preparation of SSPCMs.

Feng et al. prepared polyethylene glycol (PEG)/AC composite PCMs with an absorption ratio of 80 wt%.³¹ PEG/AC PCMs prepared by Wang et al.²⁶ had a maximum absorption ratio of 70 wt%. Khadiran et al.³² utilized peat soil AC to prepare n-octadecane/AC SSPCMs. The results suggest that n-octadecane is simply adsorbed into the porous networks of the AC, improving the thermal conductivity of the composites. The optimum content of n-octadecane found in the form-stable composites was 41.4%, and caused the thermal conductivity to increase from 0.106 to 0.165 W m⁻¹ K⁻¹, representing a 55% enhancement. In this case, 58.6 wt% of the prepared SSPCM was AC, and this decreases the PCM's latent heat storage capability. The thermal conductivity enhancement in these studies was low due to the low thermal conductivity of the AC.

In our earlier work, graphene nanoplatelets (GNPs)³³ and graphene oxide (GO)³⁴ were used as supporting materials for shape stabilization to enhance the thermal conductivity of composite PCMs. However, most of these supporting materials are expensive and difficult to synthesize, especially GO, and thus not economic, particularly for building and solar applications. We have found that the best way to achieve an economic SSPCM is to use a mixture of two types of carbon nano fillers; this approach permits the prepared SSPCMs to have both high latent heat storage and high thermal conductivity, while maintaining a low cost of preparation.

In the present study, we establish a simple production approach for AC with high specific surface area from rice husk, which is a waste material, by employing chemical activation. The pores of the prepared AC are chosen as a platform for the preparation of the SSPCM composite. The AC possesses numerous pores having a high surface area, which allows it to be simply loaded with the melted PA. The GNPs is used as an additional carbon filler to enhance the thermal conductivity of SSPCMs. The mixture of the AC and GNPs are prepared with specific mass ratios, and SSPCMs are prepared using a one-step impregnation method. The PA/AC/GNPs composite PCMs demonstrate high thermal conductivity as well as low loss in the phase transition enthalpy, making them promising candidates for thermal energy storage applications.

2. Experimental

2.1. Preparation of highly porous activated carbon

As shown in Fig. 1, in a regular process, the rice hull is pre-carbonized at 400 °C at a heating rate of 5 °C min⁻¹ for 2 h in a N₂ atmosphere. Subsequently, the resulting rice hull carbon (RC) is grounded into powder and mixed with NaOH pellets at a mass ratio of 3 : 1. The mixture is then heated (at a heating rate of 5 °C min⁻¹) to 800 °C in the N₂ atmosphere. The activated sample ends is washed with 10 wt% HCl to eliminate any inorganic impurities and then dried at 80 °C for 6 h.

Fig. 1 Activated carbon production process from rice hulls.

2.2. Preparation of PA/AC/GNPs composite PCMs

Palmitic acid (95%, Fisher Scientific) with a melting temperature of 60–65 °C is used as phase change material. The graphene nanoplatelets (GNPs) are purchased from XG Sciences (USA) have a specific surface area of 750 m² g⁻¹ (grade C).

The prepared activated carbon (AC) is mixed with water and then GNPs by using bath sonication for different GNP mass ratios (1, 2, 4 and 6 wt%) and then dried at 80 °C for 6 h. The form-stable nanocomposite PCMs are prepared by the vacuum impregnation method following the process presented by us earlier.³³ The samples are named S1 to S5 for PA/AC and PA/AC/GNPs composites with varying mass percentages of AC and GNPs, as shown in Table 1. The prepared SSPCMs were placed in an oven for 2 h above the melting point of PA at 80 °C to eliminate the PA that was not adsorbed by the porous structure of the AC and AC/GNPs.

Table 1 Composition of PA/AC/GNPs composite PCMs

Sample identifier	Palmitic acid (wt%)	Activated carbon (wt%)	GNPs fraction (wt%)
S1	90	10	—
S2	90	9	1
S3	90	8	2
S4	90	6	4
S5	90	4	6

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2.3. Analysis methods

High-resolution FEI Quanta 200F field emission scanning electron microscopy (FESEM) is used to investigate the morphology of the AC and SSPCMs. The Brunauer–Emmett–Teller method (BET-Autosorb-iQ2) is used to measure the specific surface area and pore distribution of the AC. Fourier Transform-Infrared (FT-IR) absorption spectra of the composites are recorded using a Bruker FT-IR (Bruker Tensor 27) spectrometer at room temperature in the range 4000–400 cm⁻¹ using the ATR mode. The X-ray diffraction (XRD) pattern of the powders and composites are obtained using an automated X-ray powder diffractometer (XRD, PANalytical's Empyrean) with a monochromated CuKα radiation (λ = 1.54056 Å). Raman spectra are obtained using a Renishaw Invia Raman Microscope using laser excitation at 514 nm. The melting and freezing temperatures and latent heats of the SSPCMs are obtained a by differential scanning calorimeter (DSC, METTLER TOLEDO 820C-Error ±0.25–1 °C) at a heating rate of 5 °C min⁻¹. The weight loss and thermal stability of the PCMs are obtained by thermogravimetric analysis (TGA, SETARAM 92 apparatus – error ±1 μg) at a heating rate of 10 °C min⁻¹ and a temperature of 50–500 °C in a purified nitrogen atmosphere. The thermal reliability of the composites is investigated after 1000 thermal cycles using an accelerated thermal cycling system.⁵ The laser flash technique (Netzsch LFA 447 NanoFlash) is used to measure the thermal diffusivity of prepared samples at room temperature. The temperature distribution photos are taken by an infrared camera (FLIR i5) with a thermal sensitivity of ±0.1 °C.

2.3.1. Photo-to-thermal energy conversion. Specimens having a diameter of 20 mm and thickness of 3 mm are prepared for the solar-thermal test. Fig. 2 shows the apparatus for the solar heating/cooling thermal examination. It can be seen there that a specimen which is a thermal meter placed on its center is fixed to a holder box, which is thermally insulated by plastic foam panels and has interior dimensions of 200 × 200 × 300 mm³. A 150 W infrared lamp functions as the thermal radiation supply through heating process. Inside the box, the specimen first undergoes a 45 min long heating process, and then is cooled to room temperature without the help of cooling instruments. The temperature of the specimen's surface as well as the temperature inside the box are recorded by the data logger and computer, as shown in Fig. 2.

Fig. 2 Schematic of the solar-thermal apparatus.

3. Results and discussion

3.1. Characterization of activated carbon

In previous studies, activated carbon samples with high specific surface area and narrow micro-pore distribution were synthesized using KOH and carbonaceous precursors such as coals, chars and others.³⁵ In contrast with the earlier reports, the chemical activation with NaOH is employed here to produce AC with an ultra-high specific surface area and large pore volumes from rice hull. The chemical equations suggested are as follows:³⁶

6NaOH + 2C → 2Na + 3H₂ + 2Na₂CO₃

Na₂CO₃ + C → Na₂O + CO₂

2Na + CO₂ → Na₂O + 2CO

2NaOH + SiO₂ → Na₂SiO₃ + H₂O

Na₂O + SiO₂ → Na₂SiO₃

The NaOH activation in particular leads to a porosity improvement and an increase in surface area, as well as additional advantage, which is the formation of –OH surface functional groups on the carbon surface.

Fig. 3(a) demonstrates the common XRD pattern of the activated carbon (AC). Two wide peaks are seen at angles of 24° and 44°, which correspond to the (002) and (100) planes, respectively. The amorphous behavior of activated carbon can be discerned from the XRD results.³⁷ In addition, no impurity diffractions are discovered, verifying the effectiveness in elimination of primarily inorganic impurities of the activation procedure and acid washing. Fig. 3(b) displays the Raman spectrum of AC which has two clear peaks at 1341 and 1588 cm⁻¹. These match the D and G bands of disordered and graphitic carbons, respectively. The D and G band ratio of AC (I₁₃₄₁/I₁₅₈₈) is 0.67, indicating the amorphous carbon structure along with a large content of lattice edges or plane defects of AC. The N₂ adsorption–desorption isotherms along with the pore size distribution of the AC material are shown in Fig. 3(c). As in previous reports, a strong N₂ adsorption is seen, which verifies the existence of various pore sizes of micro-, meso and macropores.³⁶ The high specific surface area of 3900 m² g⁻¹ is determined using the Brunauer–Emmett–Teller (BET) model. The SEM images from Fig. 4 show that the AC is an extremely porous sample with rough surfaces. The existence of these consistent mesoporous perforation like structures on the surface are mostly responsible for the PCM adsorption onto the AC surface.

Fig. 3 (a) XRD (b) Raman spectra (c) nitrogen adsorption/desorption isotherms of AC. The inset in (c) is the BJH pore size distribution.

Fig. 4 SEM images of the prepared AC.

3.2. Morphology of SSPCMs

Fig. 5 shows the SEM images of samples S1 to S5. In the images, the white areas represent PA. As shown in Fig. 5(a), the activated carbon has multiple pores with numerous internal surfaces, which can those be saturated simply with the melted PA.

Fig. 5 SEM images of (a) S1, (b) S2, (c) S3, (d) S4, and (e) S5. The yellow arrows show the PA/AC network and the red arrows show the distribution of GNP nanosheets in the composite PCMs.

It is noticed from the fracture surface of the PA/AC composite that the AC network is covered by PA, and the porous space is almost completely filled. These micro and nano scale pores of AC collectively draw in PA (as a result of the capillary effect) preventing it from leaking out during the melting process. As observed in Fig. 5(b–e), the GNPs with various mass percentages (1, 2, 4 and 6 wt%) inside the PA/AC composites are adsorbed and distributed in the porous network of the activated carbon. It can be seen from the SEM images that GNP nanosheets are dispersed homogeneously in the PA/AC composite. Fig. 5(e) shows that the PA/AC network is entirely covered by GNPs nanosheets and provides a different morphology.

3.3. FT-IR analysis

The FT-IR spectra of PA, PA/AC and the prepared PA/AC/GNPs form-stable PCMs are presented in Fig. 6. All absorption peaks of the primary functional groups of PA can be found in the spectra of the PA/AC SSPCMs, with merely a slight shift. No major new absorption peaks occur in the spectra of PA/AC/GNPs composites compared with pure PA, simply indicating that no new chemical bonds are generated between the PA and the matrix. These results indicate that there should not be significant changes in energy storage properties of composite PCMs during the melting and solidification processes.

Fig. 6 FT-IR spectra of PA, PA/AC and PA/AC/GNPs composites.

3.4. Crystallization behavior

Fig. 7 shows the X-ray diffraction spectroscopy of the pure PA, AC and SSPCMs. The 2θ positions of the main peaks in SSPCMs (see Fig. 7) are not considerably dissimilar from that of pure PA, implying that the PA is confined in the AC pores and has the same crystal structure as the pure PA. From the peak resolution, the degree of crystallinities of pure PA and PA in the SSPCMs was calculated by considering that our prepared mesoporous AC is amorphous; hence its crystallinity can be ignored. The crystallinity of the PA is calculated to be about 0.36 while the PA/AC and PA/AC/GNPs composites have crystallinities of 0.32 and 0.31, respectively. These results suggest that the crystallinity of PA is decreased by adding AC and GNPs. We believe that the interference with PA crystal growth is caused by amorphous AC and GNPs, acting as contaminants in the PA. In addition, in the composites, most PA segments are confined within the pores of the AC or adsorbed on the surface of the GNPs, which hinders their abilities to crystallize and agglomerate.

Fig. 7 XRD patterns of AC, PA, PA/AC and PA/AC/GNPs SSPCMs.

3.5. Energy storage properties

Fig. 8 displays the DSC curves of the PA/AC PCMs for various GNP weight ratios. As seen in Table 2, the melting point of the PA/AC PCMs is decreased compared to pure PA. The phase transition enthalpy decreases with the reduction in PA weight percentage.

Fig. 8 DSC curves of PA, PA/AC and SSPCMs for various GNPs weight percentages.

Table 2 Phase transition temperatures and latent heats of PA/AC composite PCMs

Sample identifier	Palmitic acid (wt%)	Melting		Solidification
		Melting peak temperature, Tpm (°C)	Melting latent heat, ΔHm (kJ kg^−1)	Freezing peak temperature, Tpf (°C)	Freezing latent heat, ΔHf (kJ kg^−1)
PA	100.00	65.34	202.29	59.16	205.48
S1	86.40	65.50	174.98	59.60	175.31
S2	85.84	64.95	173.65	60.12	174.29
S3	84.28	64.92	170.51	60.14	171.94
S4	82.76	64.60	167.43	60.15	169.32
S5	82.63	64.50	167.16	60.15	169.11

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The melting and solidifying latent heats are measured to be 202.29 kJ kg⁻¹ and 205.48 kJ kg⁻¹ for PA, and 174.98 kJ kg⁻¹ and 175.31 kJ kg⁻¹ for the PA/AC composite, respectively. Since only crystallized PA contributes to the latent heat during melting and solidifying, the mass fraction of crystallized PA within the composite can be calculated as follows:

where PCM (wt%) represents the mass percentage of crystallized PA in the SSPCMs, ΔH_SSPCM is melting latent heat of the composites, and ΔH_PA denotes the melting latent heat of the pure PA as determined by the DSC analysis. The results indicate that 86.4 wt% of crystallized PA was loaded in the PA/AC composite, and is adsorbed by the porous structure of the activated carbon. By adding 6 wt% of GNPs, this value decreases to 82.63 wt%.

The theoretical enthalpy of composite PCMs can be determined as follows:

ΔH_Theo = (1 − η) × ΔH_PA

where ΔH_Theo is the theoretical enthalpy of the composite PCMs, η is the mass fraction of the fillers, and ΔH_PA is the actual latent heat of the pure PA. The calculated theoretical value is 182.06 kJ kg⁻¹ and is constant for all prepared composites due to the constant total mass percentage of AC/GNP in each composite. For all composites, the actual values are lower than theoretical values, while the actual values are reduced by adding GNPs due to their lower specific surface areas. The latent heat of melting for S4 is almost the same as for S5, even though the mass ratio of the GNPs is higher. This suggests that PA is also adsorbed by the surface of the GNPs and that the GNPs also develop a network which can avoid PA seepage above its melting point. The melting points of the SSPCMs are decreased by adding more GNPs because of the crystallinities decrement of the PA, which is confirmed by XRD results.

In realistic applications, the supercooling of PCMs should be considered. Using the DSC measurement leads to Fig. 9, in which the level of supercooling is considered to be the difference between the melting and solidification peak temperatures. From Fig. 9, we realize that the degree of supercooling in pure PA is larger than that in the PA/AC SSPCMs. This shows that the blending of mesoporous AC with pure PA reduces the degree of supercooling in PCMs. It is also seen that by adding GNPs as a nanofiller, the level of supercooling is reduced as well. These results indicate that both porous walls of AC and GNP surface are acting as nucleation agents.

Fig. 9 Extent of supercooling for PA/AC composites.

3.6. Thermal stability

The TGA and DTG curves for PA and S1–S5 are presented in Fig. 10(a) and (b), respectively. The amount of charred residue at 450 °C and the maximum weight loss temperature are shown in Table 3. As observed in Fig. 10, there is a one-step thermal degradation processes. The weight loss of S1 is larger than those of S2 to S5 throughout the one-step thermal degradation stage. This is because of the higher mass of PA inside S1 compared to that inside S2 to S5. The initial step is seen in Fig. 10 to occur at a temperature between 200 and 310 °C. This temperature is related to the thermal degradation of PA molecular chains.

Fig. 10 (a) TGA curves of pure PA and composite PCMs and (b) the corresponding DTG thermograms.

Table 3 TGA results of PA/AC composite PCMs

Sample	Maximum weight loss temperature (°C)	Charred residue amount (%) (at 450 °C)
Pure PA	276.32	0
S1	289.12	16.32
S2	289.22	17.59
S3	289.74	18.67
S4	289.48	20.13
S5	278.48	20.85

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Consequently, the activated carbon is effective at creating carbonaceous layers that accumulate on the surface, which provides a physical safety barrier on top of the composites. This protective barrier can restrain the transfer of flammable molecules to the gas phase and the transfer of heat from the flame to the compacted phase. This result suggests that the activated carbon can enhance the thermal stability of the composites.

3.7. Form-stability properties

The shape-stability properties of the SSPCMs are examined using a hot plate (at 91 °C). Shape changes are seen via the images in Fig. 11. The pure PA starts to flow once its temperature reaches the melting point. But there clearly is no seepage of the PA from the surfaces of the composites even when the temperature exceeds the melting point of the PA. The PA is a type of fatty acid that contain long chains, and also the AC possesses an intercross-linked network. The capillary force and surface tension caused by the AC network limit greatly the seepage of liquid PA. As a result, the SSPCMs preserve their initial shape after the phase change process.

Fig. 11 PA/AC composites with varying GNPs contents.

3.8. Thermal conductivity and thermal imaging

The thermal conductivities of the composites are calculated using values of thermal diffusivity based on our previous work.¹² Table 4 lists thermal conductivity values for PA and PA/AC composites loaded with various mass fractions of GNPs. The thermal conductivity of the PA/AC composite is almost the same as that for pure PA and improved when GNPs are loaded into the system. In particular, NaOH activation leads to an improvement in porosity. The increase in surface area is accompanied by an additional effect: the formation of –OH surface functional groups on the carbon surface. The –ONa groups form on the carbon surface after NaOH activation, which changes to –OH groups by an ion exchange reaction once the samples are rinsed with water. Therefore, voids can be created when Na is eliminated, which in turn creates a large number of polar functional groups, including –OH, which then creates the carbon surface hydrophilic. These functional groups affect the thermal conductivity of the activated carbon and for that reason a significant enhancement is not observed for the thermal conductivity of the PA/AC composite. The thermal conductivity of the SSPCMs with 1, 2, 4, and 6 wt% GNPs are 0.292, 0.334, 0.393 and 0.551 W (m K)⁻¹, respectively. These results indicate that the thermal conductivity of the PA/AC composite is enhanced by up to 95% by adding 6 wt% GNPs while the enthalpy of melting is decreased only by ∼4%.

Table 4 Thermal conductivities of the PA/AC SSPCMs

Sample	GNP fraction (wt%)	Thermal conductivity (W m^−1 K^−1)	Thermal conductivity enhancement^a (%)
			E1	E2
a E1: thermal conductivity enhancement compared with PA. E2: thermal conductivity enhancement compared with PA/AC.
PA	—	0.280	—	—
S1	—	0.283	1.07	—
S2	1	0.292	4.28	3.18
S3	2	0.334	19.28	18.02
S4	4	0.393	40.35	38.86
S5	6	0.551	96.78	94.69

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The enhancement of thermal conductivity is also examined by evaluating the thermal images of SSPCMs in the melting process (see Fig. 12). The thermal images are taken over the same time by putting the SSPCMs on a hot plate at 100 °C.

Fig. 12 IR thermal images of (a) S1, (b) S2, (c) S3, (d) S4, (e) S5.

The results demonstrate that, after a fixed time, the temperatures are 62, 62.2, 62.7, 63.4 and 74.2 °C for S1, S2, S3, S4 and S5, respectively. The temperature difference between S1 and S5 is 12.2 °C, confirming that the thermal conductivity of the PA/AC composite is enhanced significantly by adding GNPs.

3.9. Photo-to-thermal energy transfer

The photo-to-thermal energy conversion of pure PA and SSPCMs is investigated under simulated solar radiation as shown in Fig. 13. The temperature of the SSPCMs rapidly rises by absorbing the radiation and at equilibrium reaches at 70.5 °C. The rise in the melting point of composites illustrates that the samples undergo a phase transition as they are struck by solar radiation, due to the fact that AC and GNPs absorb the solar radiation and convert it to heat. The highest temperature for pure PA is 47.6 °C and no visible melting is observed because of its low solar-to-thermal conversion efficiency. It is clear that the pure PA has a white surface and reflects the radiation, which means that the temperature of the pure PA cannot reach its melting point and store the solar energy. However, in SSPCMs, the black surface of the AC permits the capture of solar energy and heats the PA molecules, allowing the PA to store the thermal energy through its phase transition.³⁸ It is clearly seen that adding GNPs to PA/AC composites significantly increases the heating rate, due to the higher thermal conductivity of PA/AC/GNP composites. The temperature of composites declines rapidly after removing the radiation source. The freezing plateau illustrates the solidification process of the PA in the composite. The cooling rate for the PA/AC/GNP composite is higher than for pure PA and the PA/AC composite because of the enhanced thermal conductivity. Based on these results, PA/AC/GNPs composite are seen to exhibit superior performance than pure PA in terms of light-to-heat transduction.

Fig. 13 Time–temperature curves of the PA and SSPCMs under solar irradiation.

3.10. Thermal reliability

Accelerated thermal cycling is performed for 1000 cycles for SSPCMs to evaluate their thermal reliability. The melting latent heat of the composite PCMs with varying mass percentages of GNPs are shown in Fig. 14 before and after 1000 thermal cycles. The melting latent heat value changes by −1.6%, −1.2%, −0.8%, −0.3% and −0.1% for the S1, S2, S3, S4 and S5, respectively, after the cycling. The small and irregular changes in latent heat appear at a reasonable level (less than 2%) for thermal storage applications.³⁹ Therefore, the SSPCMs with activated carbon as a supporting material are shown to possess good thermal reliability, as confirmed by the variations in their energy storage capacities.

Fig. 14 Latent heat of composite PCMs before and after thermal cycling.

Moreover, thermal properties and preparation methods for recently reported PCM composites are shown and compared in Table 5. It can be seen that using high thermal conductive fillers (GNPs) and highly porous carbon (AC) together makes the resulting composite more suitable for thermal energy storage applications. This new method significantly enhances the heat storage capacity of the PCMs by using a minimal amount of additive. The low graphene content in prepared SSPCMs and their enhanced heat transfer and phase change properties provide a beneficial option for building energy conservation and solar energy applications. This is mainly attributable to the low cost of raw materials, in contrast to other composite PCMs that typically utilize a high graphene load, and the simple synthesis technique.

Table 5 Comparison of shape-stabilized composite PCMs^a

Composite	Preparation method	ΔHm (J g^−1)	ω%	K (W m^−1 K^−1)	Ref.
a ΔHm: latent heat of melting, ω: mass fraction of crystalized PCM, K: thermal conductivity, EG: expanded graphite.
Palmitic acid/GO	Vacuum impregnation	100.21	50	1.24	29
n-Octadecane/activated carbon	Solution intercalation	95.4	41.4	0.255	32
Lauric acid/activated carbon/EG	Solution intercalation	65.14	33.3	0.308	40
n-Octadecane/activated carbon	Solution intercalation	101.8	42.5	0.255	41
Palmitic acid/GNPs	Solution intercalation	188.98	92	2.11	33
Palmitic acid/AC	Vacuum impregnation	174.98	86.4	0.283	This study
Palmitic acid/AC/GNPs	Vacuum impregnation	167.16	82.63	0.551	This study

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4. Conclusions

New SSPCMs are prepared using ultra-high specific surface area AC that is prepared from rice husk through the combination of the activation temperature approach and the sodium hydroxide activation procedure. The composite PCMs are prepared by an impregnation method that is simple and convenient.

Based on the DSC method, the mass percentage of PA in the PA/AC composite PCM is found to be about 86.4 wt%. The PA is adsorbed by the porous structure and surface tension of the AC. The addition of GNPs to the composite enhances the thermal conductivity of the SSPCMs by up to 95% and gives them high latent heat storage capacities. The results indicate that the PA/AC/GNPs composite PCM is a promising candidate for solar thermal energy storage applications due to its large latent heat, suitable phase change temperature, good thermal reliability, as well as excellent chemical compatibility and thermal stability.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

The manuscript was written with contributions of all the authors. All authors read and approved the final manuscript.

Acknowledgements

This research work has been financially supported by the High Impact Research (MOHE-HIR) grant UM.C/625/1/HIR/MOHE/ENG/21, and the University of Malaya in Malaysia.

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