Analysis of solar water desalination using hybrid nanofluids An experimental study

1.5. wt.% carbon black nanofluid based wicked corrugated

basin along with 3 wt.% carbon black nanoparticle embedded paraffin wax under the basin. They reported that modifying the solar still lead to a reduction of about 22.47% in the cost. Abdelgaied et al. [23] investigated the thermo-economic characteristics of a modified hemispherical solar still. The modifications were carried out by employing CuO nanofluid in the solar still basin, phase change material beneath the basin and their combination. They reported that using the CuO nanofluid and phase change material increased the productivity of the solar still by about 60.41% and 29.17% respectively. The best solar still performance with maximum productivity enhancement of about 80.20% was attained with the simultaneous use of CuO nanofluid and the phase change material in the solar still. Shoeibi et al. [24] numerically studied the influence of glass cooling using Al2O3-TiO2 hybrid nanofluid on the productivity of a double-slope solar still. Pumping the hybrid nanofluid over the glass cover increased the temperature

gradient between the glass cover and saline water, further improving the convective heat transfer within the solar still. An optimum nanofluid concentration of about 0.45% was found for the glass cooling after a CFD simulation. The fresh water yield, energy efficiency increased by about 11.09%, 28.21% respectively using the hybrid nanofluid for glass cooling. Rabbi et al. [25] studied the performance of a solar still assisted with a heat exchanger within the basin to carry out heat supply to the saline water. They prepared Al2O3, CuO mono-nanofluids and Al2O3-CuO, Al2O3SiO2 hybrid nanofluids respectively and used them in the heat exchanger. They reported that the maximum fresh water productivity increment of about 298.3% was attained by employing the Al2O3-SiO2 hybrid nanofluid within the heat exchanger in the solar still basin. Thus, it becomes obvious post-discussion that the solar stills have a huge potential to offer appreciable performance by employing justified modifications in their arrangement. Nanofluids and especially the hybrid nanofluids can be used to augment the heat transfer to the saline water while offering enhanced thermophysical properties and improving the desalination process. However, the effect of direct mixing of the nanoparticles within the saline water to produce fresh water from may lead to certain health-related complications when consumed directly. To avoid such drawbacks, the nanofluids can be used as the thermo-fluids without directly interacting with the saline water to enhance the solar still performance while overcoming the health hazard related to the nanoparticle usage. Hence in the current study contemplates to enhance the performance of a pyramid solar still by coupling it with a solar heater and using mono, hybrid nanofluids as heat transfer fluids.

1.0. wt.%

(c) (d) Figure 3. Microscopic images of nanoparticles (a) SEM image of CuO nanoparticles, (b) TEM image of CuO nanoparticles, (c) SEM image of GO nanoparticles (d) TEM image of GO nanoparticles.

produced water vapors then started to move upwards due to density difference towards the cover plate and got condensed over its inner surface due to temperature gradient between them. The distilled water in the form of condensate was then allowed to slide downwards through the inclined surface of the cover plate utilizing the gravitational force. Once the condensate reached at the bottom edge of the cover plate, it was collected and drained out of the solar still into a collection cum measuring jar through the collection arrangement. The solar heater coupled solar still was also operated by following the procedure similar to that of the conventional solar still. However, in this operational mode the solar heater was also operated. In order to operate the solar heater, the tube of the solar heater was filled with heat transfer fluid

and then allowed to gain the thermal energy of the solar irradiation crossing the cover plate. Following it, the temperature of the heat transfer fluid increased significantly and then this high temperature heat transfer fluid was made to flow through the tube towards the solar still basin such that once the heat transfer fluid came in indirect contact of the saline water, the temperature of the latter increased appreciably by absorbing the heat energy rejected by the former. Such increment in the saline water temperature, augmented its rate of evaporation, further enhancing the fresh water productivity rate. The heat transfer fluid after releasing its sensible heat was then pumped back towards the solar heater through the closed network of the tube to maintain its cyclic flow.

Table 2. Technical details of the measurement devices. Parameter

The heat transfer fluids namely water, MNF-1, MNF-2, MNF-3, HNF-1, HNF-2, HNF-3, MNF-4 were filled one by one inside the tube of the solar heater and were investigated to identify the best out of all the investigated heat transfer fluids. In order, proper cleaning of the tube and pump was ensured. Propper cleaning of the whole experimental set-up was ensured to reduce the effect of a day of experiment over the another. It was necessitated to effectively avoid the influence of the usage of one heat transfer fluid over the other. The water temperatures, glass temperatures, distilled-water yields and the solar irradiation intensities were measured on hourly basis during the experiments. All the temperatures were measured employing the K-type thermocouples connected to a data-logger. The solar irradiation intensity was measured using a solar power meter. The technical specifications of all the measurement devices used during the experiment are specified in Table 2. Uncertainty Analysis The experiment method is an indirect way to estimate the performance of a solar still. This results in a significant uncertainty in the measurands. In order to properly assess the uncertainty in the measurands, the data of the distinct parameter was recorded was recorded during the experiment and an approximation for the uncertainty in the individual sample was calculated. The estimation of uncertainty (Ui) was carried out using the formula: The principle of the photo-thermal energy conversion is converting the energy of the incident radiation to thermal energy, solar thermal systems collectors are one of the examples. The energy equation for the solar system collector, accounting for the volumetric heat release, can be written as:

As the thermal energy storage effect was negligible since insignificant evaporation occurred in the absence of sunlight (during non-sunshine hours). Hence the error due to thermal energy storage came to be nearly equal to 0%.

58.2. °C and 44.9 °C respectively. In the initial hours of the

experiment, the hourly difference between the water and glass temperatures were not much prevalent, attributed to the lower solar irradiation intensity during the same time period. However, with further progression in the time, the increment in the solar irradiation intensity was observed. This resulted in appreciable temperature difference between the water and glass cover and eventually resulted in the improved distillate water yield.

saline water during the indirect contact with each other within the basin. It further augmented the evaporation rate of the saline water enhancing the productivity of the solar still. Using water as the heat transfer fluid (Case 2) resulted in a maximum increment of about 28.80% in the distillate water yield of solar still relative to that in Case 1.

Figure 6. Variation of hourly yield and temperatures of solar still with solar intensity using water in the heater.

Figure 5. Variation of hourly yield and temperatures of conventional solar still with solar irradiation intensity. The Figure 6 illustrates the hourly variation in the water, glass temperatures and distilled water yield (production) of solar heater coupled still with solar irradiation intensity using water as the heat transfer fluid (Case 2). The glass temperature variation in this arrangement was found to be almost similar to that in the Case 1, attributed to almost similar solar irradiation intensity. Similar to Case 1, the water temperature and distillate water yield of Case 2 were also observed to showcase direct proportionality with the solar irradiation intensity. This could be attributed to the dependence of the sensible heat aborbed by saline water and its evporation rate on the intensity of the solar irradiation. Coupling the solar heater with the solar still increased the water temperature, attributed to the heat transfer from the high temperature water (of the solar heater) to the

Figure 7 (a)-(c) illustrate the hourly variation in the water, glass temperatures and distilled water yield of solar heater coupled still with solar irradiation intensity using MNF-1 (Case 3), MNF-2 (Case 4) and MNF-3 (Case 5) respectively as the heat transfer fluids. The hourly distillate water yield in the Cases 3-5 were found to vary directly proportional with the solar irradiation intensity. The hourly distillate water yield was observed to be lower during the initial hours and attain peak at 14:00 hours in the Cases 3-5. It can be regarded to the lower solar irradiation intensity during the initial hours and attainment of peak value at 14:00 hours of a day. The water, glass temperatures were also observed to vary with the solar irradiation intensity and attained peak values at 14:00 hours. Using CuO MNFs as heat transfer fluids in the solar heater coupled still (Cases 3-5) significantly increased the saline water temperature relative to that attained in Cases 1, 2 respectively. This could be attributed to the additional supply of heat energy to the saline water due to utilization of CuO MNFs as heat transfer fluids. Another reason behind the improved performance of solar still in the Cases 3-5 could be attributed to superior thermophysical properties of the MNFs relative to that of water used as heat transfer fluid. Suspending the CuO nanoparticles in water (base fluid) to prepare MNFs appreciably augment the thermal conductivity. The increment attained in the base fluid thermal conductivity due to nanoparticle addition/suspension depends significantly on the concentration of the latter in the former i.e., higher the nanoparticle concentration (in the base fluid), higher

will be the increment attained in the thermal conductivity. The glass temperature in the Cases 3-5 were found to be almost similar to that of Cases 1, 2 respectively. The maximum increment attained in the distillate water yield was found to be about 53.30%, 78.80% and 62.05% using

(c) Figure 7. Variation of hourly yield and temperatures of solar still with solar intensity using (a) MNF-1 (Case 3), (b) MNF-2 (Case 4), (c) MNF-3 (Case 5) in the heater.

MNF-1 (Case 3), MNF-2 (Case 4) and MNF-3 (Case 5) respectively as the heat transfer fluids. It is clear that out MNF 1-3, the maximum increment in the distillate water yield was attained using MNF-2. This can be attributed to the trade-off attained between the dynamic viscosity and thermal conductivity at 1.0 wt.% concentration in CuO MNF. Hence, the 1.0 wt.% concentration was taken as the optimum nanofluid concentration and all the HNFs used in this study were prepared to be of 1.0 wt.% concentration. Figures 8 (a)-(c) illustrate the hourly variation in the water, glass temperatures and distilled water yield of solar heater coupled still with solar irradiation intensity employing HNF-1 (Case 6), HNF-2 (Case 7) and HNF-3 (Case 8) respectively as the heat transfer fluids. Employing hybrid nanofluids within the heater significantly augments the performance of the coupled solar still. Experimenting with the hybrid nanofluids, higher saline water temperatures were realized relative to that attained in Cases (1-5). This can be attributed to the superior thermal and flow properties of HNFs relative to that of the MNF (1-3) and base fluid (water). The increment in the water temperature resulted in augmenting the distillate water yield due to realization of higher evaporation rates. The water temperature and the distillate water yield were found to showcase sharp slopes during the initial hours of the experiment attributed to the increasing solar irradiation intensity and the support of using hybrid nanofluids in the heater. It was found that the solar still performance improved with the increase in the proportion of GO nanoparticles in the HNFs. Since, a maximum increment attained in the distillate water yield were found to be about 89.30%, 101.33% and 127.46% in the Cases 6,7 and 8 respectively relative to that attained in the Case 1 (conventional solar still) regarded to the high thermal conductivity of the GO nanoparticles. This resulted in higher thermal conductivity of the HNFs having larger proportion of GO nanoparticles. Realization of the increasing thermal conductivity of HNFs with the increase in the GO nanoparticle concentration resulted in efficient transfer of the heat energy absorbed by the hybrid nanofluids (in the solar heater) to the saline water. Improved supply of heat energy to the saline water increased its temperature which improved its evaporation rate further resulting in the improved hourly productivity or the distillate water yield of the solar still. The glass temperatures were observed to remain almost similar in the Cases 6-8. However, slight increment in the same was observed relative to that attained in the Case 1 attributed to augmented realization of water vapour condensation rates within the solar still on the inner glass surface and the increment in the solar irradiation intensity during the final days of the experiment due to progression of summer season with the passing days. It was observed during the experiments that the concentration, proportions of the nanoparticles added in the base fluid influence the performance of the solar still if utilized as the heat transfer fluids. This can be stated by the fact that out of all the prepared CuO MNFs, the 1.0 wt.%

higher thermal conductivity of the HNFs with higher concentration of GO nanoparticles. But it was realized during the experiments that if the key to attain superior solar still performance is the utilization of higher proportions of high thermal conductivity GO nanoparticles, then the 1.0 wt.% GO (MNF-4) could result in further augmentation in the productivity of the solar still. Therefore, later in the study, 1.0 wt.% GO nanofluids were also tested for their employability as the heat transfer fluid in the solar heater coupled still. Figure 9 illustrates the hourly variation in the water, glass temperatures and distilled water yield of solar heater coupled still with solar irradiation intensity using MNF-4 (Case 9). It was observed during the experiment that employing the MNF-4 as the heat transfer fluid in the experimental set-up improved the performance of the solar still relative to that attained in the Case 1 with the attainment of a maximum increment of about 54.93% in the distillate water yield. However, upon further comparison with the Cases 2-8, it was observed that the performance augmentation attained in the Case 9 could only surpass that attained in the Cases 2, 3 respectively i.e., using water and MNF-1 as the heat transfer fluids with an increment of about 20.30%, 1.07% respectively in the distillate water yield. It was clear post that HNFs pose higher potential for performance augmentation of the solar still relative to the MNFs and water.

(c) Figure 8. Variation of hourly yield and temperatures of solar still with solar intensity using (a) HNF-1 (Case 6), (b) HNF-2 (Case 7), (c) HNF-3 (Case 8) in the heater. concentration was found to be optimum to attain best solar still performance out of the Cases 1-5. Another supporting fact for the same is that the best solar still performance was realized using HNF-3 as the heat transfer fluid i.e., Case 8 out of the Cases 1-8 respectively. This was attributed to the

Figure 9. Variation of hourly yield and temperatures of solar still with solar intensity using MNF-4 in the heater. It gets clear from the discussion that solar heater coupled still offer better productivity relative to that attained in the conventional solar still attributed to superior thermal and flow properties of heat transfer fluids and enriched availability of thermal energy for the saline water. The pumping power required to circulate the different heat transfer fluids through a close loop between the solar still and solar heater is an essential parameter to be considered during the selection of the best performing heat transfer fluid. An ideal heat

transfer fluid for such application would be the one resulting high distillate water yield of solar still with minimum pumping power required to maintain its circulation. It is crucial to minimize the required pumping power of heat transfer fluids to make the solar heater coupled still set-up operable from the economic perspectives. During the experiments it was observed that the pumping power of the nanofluids (used as the heat transfer fluids in the current study) varied with the variation in the nanofluid physical properties. Since, the pumping power required in Case 2 was quite lower relative to that in Cases 3-9 attributed to the lower dynamic viscosity of water. Suspension of nanoparticles in the base fluid increases its dynamic viscosity. The high dynamic viscosity of nanosuspensions results in higher flow resistance. This eventually poses requirement of high pumping power to maintain the fluid flow. Such increment in the pumping power due to increased dynamic viscosity can be regarded as the function of the nanoparticle type, its shape, size and its concentration in the base fluid. The flow characteristics of the base fluid also influence the same of their nanofluids. It was observed post experimentation that the pumping power required for different heat transfer fluids and the advantage attained using them in terms of augmented distilled water yield contradict each other during the selection of best performing heat transfer fluid. Hence, the authors decided to assess the performance index for different heat transfer fluids. The performance index of a heat transfer fluid contrasted in this study was defined as the “distillate water yield of solar heater coupled still (in cc) per unit pumping power consumption of the heat transfer fluid (in Watts)”. Figure 10 illustrates the comparison of the performance index and distillate water yield of different heat transfer fluids tested in the current study. Out of all the tested heat transfer fluids, the maximum fresh water yield was attained using HNF-3 (2907 cc) followed by HNF-2 (2573 cc), HNF-1 (2419 cc), MNF-2 (2285 cc), MNF-3 (2071 cc), MNF-1 (1959 cc), water (1646 cc). However, it gets clear after assessing the performance indices of all the heat transfer fluids that HNF-2 was the most suitable out of all the tested heat transfer fluids in the experimental set-up. Since the maximum performance index of 134.64 cc/W was attained using HNF-2 followed by HNF-3 (127.22 cc/W), HNF-1 (126.71 cc/W), MNF-2 (120.26 cc/W), MNF-1 (104.75 cc/W), MNF-3 (98.61 cc/W) and water (91.44 cc/W) respectively in the set-up. HNF-3 offered the maximum distillate water yield but considering the pumping power, HNF-2 can be regarded as the most suitable heat transfer fluid since it realized the maximum performance index i.e., maximum distillate water yield for unit pumping power consumption out of all the tested heat transfer fluids. The nanofluids offer high thermophysical properties when utilized as heat transfer fluids. It is the reason that when the nanofluids are filled in the tubes of the solar heater, then they readily gain the thermal energy of the

Figure 10. Variation of productivity index and total yield attained using different heat transfer fluids.

incident solar radiation and store it in the form of sensible heat. The high temperature nanofluid when pumped through the tubes from the solar heater to the saline water then the former rejects its sensible heat to the surrounding saline water. This increases the saline water temperature further augmenting its vaporization rate while the cooled nanofluid is pumped back to the solar heater to carry out its cyclic flow. The nanofluid after coming in indirect contact with the saline water, reject significant amount of the absorbed thermal energy by virtue of its advance thermophysical properties (high thermal conductivity, heat transfer coefficient) and the nanoparticle Brownian motion. The Brownian motion i.e., the random motion of nanoparticles across the base fluid result in improved heat transfer towards the enclosing channel or tube, further transferred to the surrounding saline water. The density of the nanofluid employed in the heater also influences the performance of the coupled solar still. There exists an optimum nanofluid concentration which optimizes the distillate water yield of the solar still. Such concentration of nanofluid acts to be optimum when a favorable trade-off is attained between its thermal properties and viscosity. With the increase in the nanofluid concentration, the thermal characteristics of the nanofluid improves correspondingly. However, the same also results in increased dynamic viscosity of the nanofluid. The increment in the viscosity reduces its mass flow rate through the tube (measured in the form of pressure drop across the orifice plate). The reduction in the mass flow rate results in reduced heat rejection from the nanofluid to the saline water which degrades its evaporation rate and finally the distillate water yield in the solar still. Hence a trade-off should be attained between the improved thermal characteristics and degraded flow characteristics of the nanofluid such that maximum heat transfer occurs from the nanofluid to the saline water and

Table 3. Total fixed cost for different experimental cases Case

improves the distillate water yield in the solar still by realizing high evaporation rates. Such optimization between the thermal and flow characteristics in the current study was attained using HNF-2 at 1.0 wt.% concentration and 50:50 proportion of CuO and GO nanoparticles in water. Economic Analysis Herein the authors have tried to compare all the Cases (1-9) from economy point of view. The total fixed costs (F) of the experimental set-up in the considered cases are tabulated in Table 3. Assuming the system lifetime (n) to be 10 years and interest rate (i) of 15%, the equations involved in the economic analysis were [26]: The capital recovery factor (CRF), fixed annual cost (FAC) is evaluated as, (3) (4) The Sinking fund factor (SFF) is evaluated as, (5) The salvage value (S) and annual salvage value (ASV) is evaluated as, (6)

(7) The annual maintenance cost (AMC) is evaluated as, (8) The total annual cost (TAC) is evaluated as, (9) Finally, the distilled water cost per litre (CPL) is evaluated as, (10) Following the equations (3-8), the CPL for Cases (1-9) were found to be about 0.028, 0.026, 0.022, 0.018, 0.023, 0.016, 0.013, 0.015, 0.022 $/L respectively. Water Quality Assessment The quality of the saline water and that of the distillate water produced from the experimental set-up when operated using HNF-2 (i.e., Case 7) was compared with the Bureau of Indian Standards (BIS). The measurement of the total dissolved solids (TDS) content, conductivity, pH and fluoride composition of the saline water sample and distillate water sample (of Case 7) was carried out. The results for the specified measurements are tabulated in the Table 4. It is clear from the results presented in the Table 4 that the produced distilled water was suitable for the human consumption. However, with the addition of essential minerals in the produced distilled water, it will completely fulfill the criteria as directed by the BIS.

Table 4. Water quality comparison with BIS data Quality parameter