Eco-friendly coffee-based colloid for performance augmentation of solar stills
Introduction
Solar energy is one of the most promising renewable energies which contributes in cleaner production of freshwater and electricity via reducing fossil fuel consumption and greenhouse gas emissions (Modi et al., 2017). SS is a low cost solar-based desalination system in which solar energy is utilized to convert saline water into freshwater via evaporation/condensation process. In a typical SS, the saline water inside the still basin is heated up using the absorbed solar energy and begins to evaporate to fill the sealed still trough with water vapor; which consequently condensed on the glass surface and collected as freshwater. Different designs and techniques have been proposed to maximize the productivity and efficiency of SSs such as rotating-drum SS (Abdullah et al., 2019), tubular SS (Sharshir et al., 2019a), inclined SS (P.K, N. et al., 2017), modified absorber plate (Kabeel et al., 2017a), heat storage materials (Sharshir et al., 2019b), baffle plates (Sathyamurthy et al., 2016), wick-metal chips pad (Sharshir et al., 2019c), cascaded phase change materials (Zayed et al., 2019b), solar bubble column humidifier (Srithar and Rajaseenivasan, 2017), Fresnel lens (Mu et al., 2019), integration with photovoltaic panel (Elbar et al., 2019) and thermoelectric cooling (Nazari et al., 2019), water heater (Bait, 2019), bilayered structures (Elsheikh et al., 2019b), and nanofluids (Sharshir et al., 2017b). Saline water has low photothermal and heat transfer properties (Liu et al., 2015), which may impair the harvesting of solar energy and consequently the overall efficiency of the system. In recent years, nanofluids have been proposed to enhance heat transfer and photothermal properties for base fluids. Nanofluids have been applied in different engineering fields such as photovoltaic/thermal panels (Hjerrild et al., 2018), flat plate solar (Zayed et al., 2019a), heat exchangers (Karimi et al., 2019), electronic cooling application (Sardarabadi et al., 2019), metal cutting (Shuang et al., 2019), internal combustion engines (Ali et al., 2018), and solar energy applications (Chen et al., 2019). Many studies have been carried out to investigate nanofluids applications in solar collectors and solar-based desalination units to enhance solar energy harvesting.
Elango et al., 2015 experimentally compared the performance of SSs with water and water nanofluids of Al2O3, ZnO, Fe2O3 and SnO2 nanoparticles. The results revealed that the SS with Al2O3 nanofluid has 29.95 % higher freshwater productivity than the SS with water. Using water colloid with Al2O3 nanoparticles in a SS integrated with external condenser was investigated by Kabeel et al., 2014. The use of nanofluids enhanced the freshwater productivity of the SS compared with that of SS with an external condenser. Effects of different concentrations of graphite and copper oxide nanoparticles on SS yield was studied by Sharshir et al., 2017b. The optimal concentration of graphite and copper oxide nanoparticles in saline water was 1 %. The productivity of SS using the copper oxide micro-flakes and graphite micro flakes increased by about 43.10 % and 37.02 %, respectively, compared with CSS. Omara et al., 2015 studied the effect of adding nanofluids and providing vacuum on corrugated wick solar still performance. The productivity of the SS was increased by 64 % and 1.5 %, respectively, when aluminum oxides and cuprous nanoparticles were used compared with CSS. The performance of a SS equipped with a heat exchanger and applying nanofluids in it was investigated by Mahian et al., 2017. Chen et al., 2017 investigated stability, optical properties and thermal conductivity of saline water after adding SiC nanoparticles to it in solar desalination systems. Kabeel and El-Said, 2014 carried out a numerical study to calculate water productivity and cost for a flashing desalination technique coupled with nanofluid-based solar collector as a heat source. The results showed that this proposed system is able to produce water at 7.7 L/m2/day with cost of 11.68 $/m3. Applying gold nanofluids on steam production in solar heater under 280 Suns was studied by Amjad et al., 2017 and results showed that gold nanofluids with concentration of 0.04 w% increased photothermal conversion efficiency by 95 %. Han et al., 2011 studied thermal properties of carbon black nanofluids for solar absorption and reported that the nanofluids of high volume fraction have better photo-thermal properties. Ladjevardi et al., 2013 studied the effect of different diameter and volume fraction of graphite nanoparticles on nanofluids properties which are used in solar collectors. The results showed that using graphite nanofluid with volume fraction of ∼0.0000.5 % led to solar radiation of ∼50 % while pure water solar collector can absorb only ∼27 % of solar radiation. Carbon nanotubes was added to distilled water to act as a working fluid in direct absorption solar collectors and it was observed that water thermal conductivity is enhanced with 32.2 % by adding only 150 ppm of carbon nanotubes to it, as reported in Karami et al., 2014. Mercatelli et al., 2011 investigated absorption and scattering properties of nanofluids consists of aqueous and glycol suspension single-wall carbon nanohorns and results revealed that scattered light is only ∼5 % with respect to the total attenuation of light. Nanofluids obtained by adding carbon nanohorns to base fluid were also investigated in other studies (Moradi et al., 2015; Sani et al., 2010). Recent advances in nanofluids applications in solar energy were reported in a detailed review presented by Elsheikh et al., 2018b.
Obviously, most of proposed nanoparticles found in literature are inorganic materials which may be harmful to environment and human health. As observed by Klaine et al., 2008, the increasing use of nanoparticles may lead to major environmental concerns because of the cytotoxicity of most of them as ensured by other studies (Jia et al., 2005; Muller et al., 2005). Moreover, inorganic nanoparticles are so expensive because of their high technology production techniques (Ahmed Ali et al., 2019; Essa et al., 2019). Depending on above mentioned reasons, organic-based colloids should be seriously investigated to replace inorganic nanoparticles with them. Alberghini et al., 2019 presented sustainable, stable and inexpensive coffee-based colloid and studied its photo-thermal performance in direct solar collector and compared it with traditional indirect selective surface collector. It has been reported that the proposed coffee-based colloids is stable for six months which is very long period compared with other inorganic based nanofluids.
In this study, totally organic coffee-based colloid is fabricated and applied on a SS basin water. The performance of the SS with coffee-based colloid basin water which is denoted as a modified solar still (MSS), and a conventional solar still (CSS) with the same dimensions and materials are experimentally compared under the same conditions. The remainder of the paper presented the follows:
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The experimental setup is illustrated.
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The theoretical background is introduced.
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Then the results are presented and discussed.
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The conclusions are presented.
Section snippets
Experimental setup
The experiment was carried out in outdoor conditions of Tanta city, Gharbia Government, Egypt (latitude 30.79 °N and longitude 31 °E) for 12 h (from 6 AM to 6 PM) on 26th June 2019. Two identical conventional solar stills were fabricated from 1 mm thick galvanized steel sheet. All inner surfaces of steel sheets were painted in black to maximize absorbed amount of solar radiation while all outer surfaces were covered by suitable insulation material (foam sheet of 3 cm-thickness) to minimize
Material characterization
Coffee-based colloid used in MSS basin was prepared by mixing 92 g of distilled water with 7.25 g of Arabica coffee powder. Then the mixture is subjected to multiple steaming-filtration process to increase suspended coffee particles in the colloid. The used procedure for coffee-based colloid preparation was inverted from (Alberghini et al., 2019). The morphology of the developed coffee-based colloid is shown in scanning electron microscope (SEM) images with different magnifications presented in
Theoretical background
There three main heat transfer modes coexist inside the SS trough: convection between basin water and air in the trough, evaporation from the water basin, and radiation between the basin water surface and the inner surface of the glass cover. The rate of heat transfer between basin water and adjacent air by convection can be calculated from the following equation (Sharshir et al., 2017a):
Convective heat transfer coefficient between basin water and adjacent air; can
Uncertainty analysis
The uncertainty analysis was conducted according to procedures developed by (Holman and Gajda, 2001). If the process response (PR) is calculated using a set of measurement for m process control variables (V), Then
the response uncertainty UPR can be computed as a function of the independent variables uncertainties U1, U2, U3,……Um as follows:
The uncertainty in measured productivity and the computed exergy and energy efficiencies
Observations and results
During experiments, variation of ambient temperature and intensity of solar radiation were measured with respect to time and plotted as shown in Fig. 3. At the beginning of experiment (at 6:00 h), solar radiation intensity was ∼195 W/m2, and then this value was increasing till recording its maximum value at 12:00 h which was ∼1076 W/m2 and started to decrease till reaching value of ∼195 W/m2 at 18:00 h. Ambient temperature variation looks like variation of solar radiation intensity (inverted
Cost analysis
Cost analysis is presented for CSS and MSS in this section. The accumulated fresh water productivity for CSS and MSS were 3600.45 and 4865.73 ml/m2, respectively. In Egypt, the sun rises approximately all the year but we assumed that stills works only 270 days per year (Kabeel and Abdelgaied, 2019). This assumption was for calculating annual fresh water productivity with reliable results. To calculate the total cost of fresh water per liter, Table 1 is established depending on the equations
Conclusion
In this paper, coffee-based colloid was introduced as organic-based colloid used instead of nanofluids, which are harmful to environment, in the basin of a SS. Two identical SSs were investigated experimentally under Tanta city (north of Egypt) weather conditions. Coffee-based colloid was added to basin water of one of stills (denoted as MSS) to improve photo-thermal properties of water and improve freshwater productivity. Depending on experimental analysis on MSS and CSS, we can conclude the
Declaration of Competing Interest
The authors of the submitted manuscript “Eco-friendly coffee-based colloid for performance augmentation of solar stills” declared that there is no conflict of interest.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (E050902, E041604).
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