Solar Still Efficiency Enhancement by Using Graphene Oxide/Paraffin Nano-PCM

Abstract

Solar-driven water desalination technologies are rapidly developing with various links to other renewable sources. However, the efficiency of such systems severely depends on the design parameters. The present study focused on using graphene oxide (GO) with the Φ = 0.2, 0.4 and 0.6 wt.% dispersed in paraffin, as phase-change materials (PCMs), to improve the productivity of a solar still for desalination applications. The outcomes showed that by adding more graphene oxide to paraffin, the melting temperature got reduced. Solar still with GO/paraffin showed 25% productivity improvement in comparison with the solar still with only PCM. The obtained Nusselt number during the melting time also represented that free convection heat transfer into the melted region of the solar still has been enhanced by adding dispersed GO to the PCM, compared to the base paraffin. Also, increasing the hot wall temperature augments the Nusselt number. Finally, an empirical equation was derived to correlate the average Nusselt number as a function of Rayleigh number (Ra), the Stefan number (Ste), the subcooling factor (Sb), and the Fourier number (Fo). The obtained correlation depicted that Nusselt number enhancement has a reverse relation with Fourier number.

1. Introduction

Over the latest decades, phase-change materials (PCMs) have been employed in the heat transfer systems like solar stills [1,2,3]. As is clear from Figure 1, phase-change materials are categorized as Eutectic, inorganic, and organic [4].

Among the variety of proposed PCMs, paraffin, as an organic material, is evaluated as one of the most futuristic PCMs, due to its desired specifications, like remarkable latent heat [5,6,7]. As an example, Kumar et al. [8] enhanced the productivity of a traditional solar still by employing paraffin and attaching some fins. Their results revealed that the daily and night-time exergy efficiency of the modified solar still is augmented by 20% and 74%, respectively. In a similar work, Kabeel and Abdelgaied [9] utilized the paraffin in a solar still in Egypt. Their outcomes demonstrated that paraffin is a decisive factor in water productivity of the solar still, as it can increase it up to 67.18%. However, low thermal conductivity of paraffin causes lower heat transfer rates within melting process [10]. For improving the heat transfer rate of PCMs, different solutions have been proposed in the literature. In recent years, the concept of nanofluids was considered among researchers to improve the thermal conductivity of the phase-change materials, by dispersing the nanomaterials into the base PCMs [11,12]. Considering these facts, paraffin is a good candidate for using in solar stills. However, its low conductivity problem can be reduced by adding nanoparticles [13,14].

Table 1. Synopsis of the investigations about different nano-paraffin composites.

Author	Used Nanomaterials	Results
Rufuss et al. [15]	Copper (II) oxide	35% improvement in the productivity.
Sahan et al. [16]	Iron (II,III) oxide	Thermal conductivity of paraffin raised by 48% and 60%, by employing 10% and 20% wt.% of Fe3O4 nanoparticles.
Jiang et al. [17]	Aluminum oxide	15% latent heat decrement and 27% thermal conductivity increment.
Yang et al. [18]	Silicon Nitride	Thermal conductivity rising by 35%, utilizing 10% nanoparticles.
Park et al. [19]	Iron (II,III) oxide	45% latent heat decrement and 45% thermal conductivity enhancement by dispersing 6.6% nanoparticles in the base PCM.
Fan et al. [20]	Graphene	164% thermal conductivity increment, by dispersing 5% nanomaterials in the paraffin.
Jesumatty et al. [21]	Copper (II) oxide	6%, 6.7%, and 7.8% thermal conductivity augmentation, by dispersing 2%, 5%, and 10% nanoparticles in the base PCM.
Shi et al. [22]	Graphite nanoplatelets	Augmenting the thermal conductivity of PCM from 0.25 to 2.7, by dispersing 10% of nanomaterial in the base PCM.
Nourani et al. [23]	Aluminum oxide	Adding 10% nanoparticles to the base paraffin afford a 0.09 W/m °C thermal conductivity improvement.
Li [24]	Graphite	Adding 10% graphite nanomaterial to the base PCM, cause a 640.43% thermal conductivity enhancement.
Warzoha et al. [25]	Graphite nanofibers	180% thermal conductivity increase and 10% latent heat decrease by adding 11.4% graphite nanofibers to the base paraffin.

shows the synopsis of the investigations about different nano-paraffin composites. As can be seen from the table, graphene-based nanomaterials have a promising potential to enhance the thermal conductivity of paraffin.

Although many researches have focused on thermal enhancement of nanofluids by adding more nanomaterials to the base fluid [26,27], present study has concentrated on using paraffin with dispersed graphene oxide with the Φ =0.2, 0.4, and 0.6% to improve the productivity of a solar still. The outcome of this investigation may find applications to develop highly efficient solar stills to secure more drinkable water in warm, dry lands [28,29,30,31,32].

2. Experimental

2.1. Thermophysical Properties

Paraffin wax (industrial grade) purchased from Merck company (Gernsheim, Germany), with melting temperature of 44 °C was employed as the PCM. Also, graphene oxide nanomaterial was purchased from US Research Nanomaterials, Inc. to be used as an additive to PCM to improve its thermal performance. Thermal properties of PCM and graphene oxide have been shown in

Table 2. Thermal properties of PCM and graphene oxide.

Thermophysical Property	Paraffin	Graphene Oxide
Density (kg/m3)	802	3600
Specific heat (J/kg K)	2320 (liquid)	765
Thermal conductivity (W/m K)	0.23 (liquid)	3000
Dynamic viscosity (kg/m s)	1.3 × 10−3	-
Thermal expansion coefficient (1/K)	9.1 × 10−4	1.25 × 10−5
Latent heat (kJ/kg)	226	-
Melting temperature (°C$)$	44	-

.

The thermophysical properties of the nano-PCM can be calculated using below equations [33,34]:

$${\rho }_{npcm}=\varnothing {\rho }_{np}+{\left(1-\varnothing \right)}_{\rho cm}$$

(1)

$$c_{p,npcm}=\frac{\varnothing {\left(\rho c_p\right)}_{np}+\left(1-\varnothing \right){\left(\rho c_p\right)}_{pcm}}{{\rho }_{npcm}}$$

(2)

$$h_{fgnpcm}=\frac{\left(1-\varnothing \right){\left(\rho h_{fg}\right)}_{pcm}}{{\rho }_{npcm}}$$

(3)

The dynamic viscosity of the nano-PCM can be obtained by [35,36]:

$${\mu }_{npcm}=0.983e^{12.959\varnothing }\mu {}_{pcm}$$

(4)

The thermal conductivity of the nano-PCM, subject to Brownian motion, is defined as [35]:

$$k_{npcm}=\frac{k_{np}+2k_{pcm}-2\left(k_{pcm}-k_{np}\right)\varnothing }{k_{np}+2k_{pcm}+2\left(k_{pcm}-k_{np}\right)\varnothing }{k}_{pcm}+5\times {10}^4{\beta }_k\zeta \varphi {\rho }_{pcm}{c}_{p,pcm}\sqrt{\frac{BT}{{\rho }_{np}{d}_{np}}}f\left(T,\varphi \right)$$

(5)

where $\beta$ is the Boltzmann constant, 1.381 × ${10}^{–23}$ J/K and

$$\begin{gathered} {\beta }_k=8.4407{\left(100\varphi \right)}^{-1.07304} \\ \beta =0ifT<T_{solidus} \\ \beta =1ifT<T_{liquidus} \end{gathered}$$

(6)

$\zeta$ is the correction factor and is defined as [37]:

$$\zeta =\frac{T-T_{solid}}{T_{liquid}-T_{solid}}if{T}_{solidus}<T_{liquidus}$$

(7)

f is a function, which can be defined as [36]:

$$f\left(T,\varnothing \right)=\left(2.8217\times {10}^{-2}\varnothing +3.917\times {10}^{-3}\right)\frac{T}{T_{ref}}+\left(-3.0669\times {10}^{-2}\varnothing -3.91123\times {10}^{-3}\right)$$

(8)

Paraffin and graphene oxide used in the present study can be seen in Figure 2.

2.2. Preparation of the Nanocomposite

Figure 3 demonstrates the nanocomposite preparation. The black plate receives heat by radiation during the day time. As a result, this heat will be transferred to the paraffin/graphene oxide. The present study focused on using graphene oxide (GO) with the Φ = 0.2, 0.4, and 0.6 wt.% dispersed into melted paraffin at the temperature of 45 °C, as phase-change materials (PCMs), to improve the productivity of a solar still. Mixing was applied for 2 h with 95% power intermittently to prevent overheating of the nanofluid. During the day time, especially after 2 p.m., paraffin starts to melt, and its temperature remains constant until the melting is complete. After 7 p.m., paraffin starts to cool, but the temperature remains constant until all the paraffin becomes solid; then cools back to the surrounding temperature. During the day time, when no paraffin exists, the water temperature is high, but after 7 p.m., T_w with PCM is higher. A thin layer of paraffin/graphene oxide has been integrated beneath the basin (as shown in Figure 3) to increase the efficiency of solar still during the night time.

Figure 4 also provides sedimentation level pictures of the prepared suspension. As is clear from the figure, the graphene oxide can disperse in the organic solvents easily, without any noticeable sedimentation.

2.3. Experimental Apparatus

Figure 5 demonstrates the picture as well as schematic of the studied solar still. The experimental setup includes two parts, a solar still and an effluent settling tank. The solar still is the main part of the experiments and the effluent settling tank is used to eliminate any possible bacteria.

This settling tank is made from galvanized iron sheets with a diameter of 50 cm and a height of 60 cm. For enhancing the surface contact area, three rectangular fins (40 × 80 × 1 mm) are welded at the bottom of the tank basin. The settled effluent from the storage tank is traversed to the solar still.

The test unit comprised of a stepped-type solar absorber plate was installed on a metal base. The absorber plate was manufactured from galvanized iron sheet of 1 mm thick and 1300 mm length and 1000 mm width, which was coated with black color paint. The gap between the stepped absorber plate and the box was covered with polyurethane foam (PUF) to prevent heat loss from the sides and bottom of the unit. The cover is made of 4 mm thick glass. Raw water was used for the tests, flowing from the tank, down the steps of an absorber. This water is warmed by solar radiation and therefore, it evaporated. The vapor was condensed at the inner glass surface and the droplets were slid along the glass. Gathered water dripped to the distilled water container at the bottom, and collected in a beaker. This was done by a pipe that was installed on the side of the unit. The makeup water was refilled every 30 min, by opening a valve to keep a constant concentration for the water. Excess saline water flowed through the absorber into the tank.

It is noteworthy that one of the factors influencing the production rate is the slope of the roof, which is a function of latitude, the angle of the solar radiation, and the seasons of the year [38]. As a rule of thumb, the angle is considered equal to the latitude [39]. For the studied city, the latitude is 36.31° [40]. The most proper direction for the installation of solar panels that are deployed on a fixed basis is geographic east–west direction [41]. In the northern hemisphere, the unit must be headed toward the south; so that solar energy could be best exploited. Experiments are performed from 8 a.m. to 6 p.m., during July 2018–August 2018.

Three different thermocouples were installed to measure the temperature of the absorbent plate (T_b), the temperature of the inner surface of the glass (T_gi) and temperature of water desalination unit (T_w). For monitoring evaporator and condenser surface temperature, a set of K-type thermocouples were used, which were connected to a portable data logger. The uncertainty in the measurement of temperature was equal to ±1 K, according to the temperature monitoring system. The environment, basin plate, solution and glass temperatures as well as solar radiation and distilled water productivity were measured every 1 h.

Table 3. Design parameters for the studied solar still.

Parameters	Dimensions
Length of tank	600 mm
Width of tank	400 mm
Height of tank	2800 mm
Inclination angles	32.5°
Length of the glass covers	1300 mm
Width of the glass covers	1000 mm
Thickness of the glass covers	4 mm

presents a summary of design specifications of the solar stills.

Uncertainties

Errors of the measuring instruments are displayed in

Table 4. The accuracies of various measurement instruments [42]. Table is reprinted with permission from publisher.

No.	Instrument	Range	Accuracy	% Error
1	Solarimeter	0–900 W/m2	±4 W/m2	5
2	Thermocouple	0–300 °C	±1 °C	4
3	Thermometer	0–95 °C	±0.1 °C	0.4
4	Anemometer	0–12 m/s	±0.1 m/s	10
5	Measuring jar	0–1000 mL	±9 mL	9

. These errors have been indicated, based on the manufacturer’s instructions.

3. Results and Discussion

Experimental investigation was performed to study the thermal performance of GO/PCM in solar still. During the day time, solar energy can be divided between the water, which we wanted to distill, and the PCM/ graphene oxide. As is clear from Figure 6, water temperature without PCM is the highest between 9 a.m. and 12 p.m., as during this time, a part of solar energy is stored in PCM. After that and for no-PCM case, the temperature got reduced sharply but temperature for solar still with paraffin diminished gradually. This is because the stored energy in PCM is released, when the sun sets. Adding more nanomaterials to the base PCM caused higher temperature for the time duration from 2 p.m. to 7 p.m.

The distillation amount is the primary factor in determining the performance of a solar desalination system. The volume of the existing water varies due to different utilized nanocomposites in solar still system. Figure 7 illustrated the produced fresh water per day time. The figure shows that adding more GO to paraffin causes more fresh water production which indicates solar still efficiency enhancement. The reason of this phenomenon is related to the ability of nano-PCM to make the basin water warmer, which means more water can be distilled. Also, with nano-PCM energy released during the sunset, the distilled water production still continued. All of these reasons caused the maximum production at the end of the day to have about 100% enhancement.

Figure 8 shows the alteration of the average Nusselt number at the hot wall, while the cold bottom surface is at 23 °C. As is evident, the Nusselt number follows the same trend of previous investigations like [43]. Adding more GO to the base paraffin enhances the average Nusselt number. This is owing to the thermal conductivity increment of PCMs, by dispersing nanomaterials in them. Also, increasing the Fourier number reduces the average Nusselt number.

The mean Nusselt number based on a recent obtained correlation [43,44] can be written as below:

$$\overline{Nu}=0.18{\left(\frac{Pr}{0.2+Pr}Ra\right)}^{0.29}$$

(9)

Also, the mean heat transfer coefficient can be given as:

$$\overline{h}=\frac{\overline{Nu}{k}_m}{H}$$

(10)

The Rayleigh and the Stefan numbers can be defined based on the temperature difference in the melted zone and the characteristic length of the solar still [45]:

$$R{a}_f=\frac{g{\rho }_f^2{c}_{\rho .f}{\beta }_f\left(T_h-T_c\right)H^3}{k_f{\mu }_f}$$

(11)

$$R{a}_m=\frac{g{\rho }_f^2{c}_{\rho .f}{\beta }_f\left(T_h-T_c\right)H^3}{k_m{\mu }_m}$$

(12)

and

$$St{e}_f=\frac{c_{\rho .f}\left(T_h-T_c\right)}{h_{ls.f}}$$

(13)

$$St{e}_m=\frac{c_{\rho .m}\left(T_h-T_c\right)}{h_{ls.m}}$$

(14)

In the final step, an empirical correlation according to the experimental outcomes has been developed to correlate the average Nusselt number in relation to the Rayleigh number (Ra), the Stefan number (Ste), the subcooling factor (Sb) and the Fourier number (Fo) by using the below formula:

$$\overline{Nu}={\mathrm{aRa}}_m^b{\left[\frac{St{e}_{m}F{o}_m}{\left(1+Sb\right)}\right]}^c$$

(15)

This formula is similar to that reported by Ho and Gao [43] for Al₂O₃/paraffin nano-PCM. To obtain the constant variables of the equation, a dataset was used to compute the variables by using Microsoft Excel. Least square regression method was employed to acquire the correlation values based on the dataset points. The values of variables in Equation (15) for studied dispersed amounts of nanoparticle into the PCM are shown in

Table 5. Coefficient, exponents and ranges of parameters for Equation (15).

Φ	a	b	c	Average Deviation (%)	Parameter Ranges
0%	0.6	0.3	−0.2	4.23	$F{o}_m=0.25–2.5;$ $R{a}_m=1.32\times {10}^6–2.38\times {10}^6$ Sb = 0.075–0.85; $St{e}_m=0.05–0.2;$
0.2%	0.65	0.35	−0.23	7	$F{o}_m=0.25–2.5;$ $R{a}_m=2.59\times {10}^6–4.85\times {10}^6$ Sb = 0.075–0.85; $St{e}_m=0.05–0.2;$
0.4%	0.7	0.4	−0.3	9	$F{o}_m=0.25–2.5;$ $R{a}_m=5.13\times {10}^6–5.49\times {10}^6$ Sb = 0.075–0.85; $St{e}_m=0.05–0.2;$
0.6%	0.8	0.5	−0.4	10.5	$F{o}_m=0.25–2.5;$ $R{a}_m=3.76\times {10}^6–7.21\times {10}^6$ Sb = 0.075–0.85; $St{e}_m=0.05–0.2$.

.

4. Conclusions

In this research, graphene oxide nanomaterials with Φ = 0.2, 0.4, and 0.6 wt.% are dispersed in paraffin (GO/PCM) and utilized in a solar still for desalination applications. Based on the outcomes, following points were summarized:

1. Solar still efficiency with GO/paraffin showed an average of 25% improvement compared to the solar still with only PCM.

2. The daily productivity of solar still with PCM was 2 kg/0.4 ${\mathrm{m}}^2$ whilst the solar still with Φ = 0.6 wt.% GO/ PCM yielded 2.5 kg/0.4 ${\mathrm{m}}^2$.

3. Adding more GO to PCM caused higher Nusselt number and consequently higher heat transfer coefficient, which means high potential to obtain remarkable reduction in heating times. However, the samples containing 0.2 wt.% nanomaterial showed least performance levels in feasible energy storage during phase change.

4. Dispersed graphene-based nanomaterials in paraffin exhibits better potential, compared to the use of raw PCMs in solar applications.

5. An empirical equation has been derived to correlate the average Nusselt number as a function of Rayleigh number, the Stefan number, the Fourier number, and the subcooling factor.