Performance analysis of solar still with separate evaporatively cooled condenser

Abstract

Keywords: Air cooler; Distillate output; Economic analysis; Separate condenser; Solar still

Introduction

Freshwater scarcity is an escalating challenge as population growth, urbanization, and industrial activity strain conventional sources. In many regions, declining rainfall, over-drawn aquifers, and rising salinity now limit supplies for both drinking and agriculture. Salinity in particular reduces soil fertility and crop yields, while pollutants from industry, sewage, and agricultural runoff further degrade surface and groundwater quality. These factors collectively heighten public-health risks and sustain the burden of waterborne disease. Numerous high and medium-tech techniques for purifying water that rely on conventional energy sources have been developed as a result of scientific and technological advancements. But “solar distillation” stands out as an easy, economical, and ecologically acceptable method for generating drinkable water that is dependent on renewable energy sources rather than traditional energy sources. A device that uses solar energy to clean water is called solar still (SS). Its principle is basically vaporization and condensation. Several designs for passive solar stills have been extensively investigated and examined by researchers in an effort to increase their efficiency. The designs include inverted absorber solar still, floating solar still, and hybrid membrane solar stills powered by fossil fuel [1-8]. Recent studies have shown growing interest in improving the efficiency of solar stills, particularly through design modifications like adding external condensers, using nanomaterials, using PCM, or coupling with other renewable systems. These advancements aim to boost water output and address limitations like low productivity [9-17] In a SS, solar energy heats impure or salty water, causing it to evaporate. The water vapour then condenses on a cool surface and is collected as pure water, leaving behind impurities and salts. The temperature difference between the water (Tw) in basin and condenser surface (Tc) is the driving factor responsible for getting pure water (condensate). Studies [18–22] have demonstrated that the difference in temperature between the water (where evaporation takes place) and condenser (Tw-Tc) is decreased when water vapour condenses inside a solar still. It has been suggested that the condenser and the SS chamber be separated in order to increase this temperature differential. In this configuration, water vapour passes from the main chamber of the solar still into a condensation chamber that is separated from the main chamber (SS chamber). As a result, the still’s glass surface experiences relatively little condensation. The (Tw-Tc) between the evaporative and condenser rises as a result of the majority of condensation occurring in the condensing chamber, which accelerates evaporation and increases distillate output [23–32]. This design of SS was called as SS with separate condenser (SSSC). The impact of integrating a connected passive condenser on a passive solar still’s efficiency was investigated by Fath and Elsherbiny [23]. They used both theoretical

analysis and experimental in their approach to this subject. Diffusion, purging, and spontaneous circulation are the three mass transfer modes in a solar still-condenser system that the researchers examined. The diffusion and purging modes were the focus of their studies, and they discovered that the outcomes closely matched the theoretical expectations. They claimed that employing the separate condenser increased production by almost 70% in purging mode. A SS with an integrated passive condenser and a natural circulation method for humidification and dehumidification was studied by Fath and colleagues [33]. The system produced 5.1L/m2 each day, according to their research. Madhlopa and Johnstone’s [34] analytical studied on the performance of a passive SS with a separate condenser. They proposed the design consists of three basins: Basin 1 is located in the evaporation chamber and is covered with a glass cover, while Basins 2 and 3 are situated in the condenser chamber. Basin 3 is covered with an opaque condensing cover. Basin 1 produces the first effect, Basin 2 the second effect, and Basin 3 the third effect. When compared to a conventional SS, they found a 62% increase in productivity. Three major components contribute to the total distillate output: the first effect, which has one evaporator basin, makes up 60% of the output; the second effect, which has an additional condenser basin, makes up 22%; and the third effect, which has two additional evaporator basins, makes up 18%. Monowe et al. [35] proposed, a new design for a portable thermal-electrical solar still with an external condenser and reflecting booster. The system is engineered to reduce the loss of latent heat from condensation by capturing and storing it in the condenser. This stored heat can subsequently be used to operate the still at night or to preheat saline water for household usage. According to preliminary results, the still can operate as efficiently as 77% when the preheated water is used for household use and as efficiently as 85% when the stored heat is used for both nocturnal operation and system recharging with fresh preheated saline water. A modified SS with an external condenser and an Al2O3water nanofluid was studied by Kabeel et al. [36]. They discovered that when the external condenser was the only one used, the productivity of the solar still improved by 53.2%. When the nanofluid was incorporated along with the external condenser, the productivity enhancement rose to approximately 116%. Rabhi et al. [37] examined the effectiveness of a modified single-basin, (SSSS) with a condenser and pin fins for heat absorption. Their findings showed that using absorber fins on the basin liner was not as effective as incorporating an external condenser. Interestingly, they discovered that, in comparison to a traditional SS, the external condenser increased water productivity by 32.18%. On the other hand, the SS with basic pin fins only had a 14.53% rise. The thermal performance of an inflatable plastic SS with a passive condenser was investigated by Bhardwaj et al. [38]. According to their findings, the device produced about

Mathematical Analysis

Thermal network of the proposed system (Fig. 2). and Figure 3 shows different internal and external heat transfer of the proposed still.

Referring Figure 3 Energy balance equation of proposed system considering unit area are as follows i. The solar distillation unit is vapour-tight. ii. The heat capacity of glass, aluminum and condenser is neglected. iii. The thickness of water film considered very small. Glass Cover (1) In Eq.1, ϕ is the fraction of water vapour incident to external condenser, the value of ϕ is considered 1 when all vapours are forced by internal fan towards external condenser. Eq. 1 can be written as, (2) Or, (3) (4) (5) Basin Water (6) Or, (7) Basin Liner

Figure 3. Internal & external heat transfer rate of the proposed still.

(11) Water Film Considering the thickness of water film very small the energy balance of strip of thickness (dx) Figure 4, will be

The values of R1, R2 and R3 are determined by the fitting (polynomial degree 2) from the saturation vapour pressure table in the temperature range of 7o C to 55o C. Integrating Eq. 18 we have

(13) From Eq. (3) and Eq. (9) the glass temperature and temperature of basin liner will be given by, (14)

In above equation Tf0 is the initial film temperature flowing over cover at x=0 The exit film temperature at x=d will be,

In Eq. 15 the expressions for constants K1, K2 and K3 are given in Appendix-I Solving Eq. (15) considering the water temperature at time t=0 as Tc0 (16) by,

By substituting the value of Tfa into Equation (17), we can determine the average glass temperature ( ), and then obtain the water temperature using Equation (16). Hourly yield is given by, (21) The value of various constants and the correlation for determining different heat transfer coefficients used in mathematical model are given in Appendix-II Expressions for Determine τ1, τ2 and τ3 [18] (22) (23) (24)

Description of Proposed Design Figure 1 shows a schematic and a picture of the proposed design. Various dimensions of the design have been

shown in Figure 5. The whole setup consists of two different parts separated by an insulated partition. The part at left side is evaporator unit and side towards right is called as condenser unit. A low-power (5W) solar exhaust fan is used to drive the vapours produced by the water in the evaporative chamber absorbing solar energy into the condenser unit. The condenser plate is made of aluminum sheet and it is continuously cooled by cooled tank water pumped from the cooler tank connected with condenser unit. With flow of cooled water (inlet temperature equal to WBT of ambient air) the condenser helps to carry the heat generated by vapours condensed in the bottom of the condenser plate. As a result, the condenser cover and basin water have a greater temperature differential. Both the condenser unit and the glass cover (evaporator unit) have a surface area of one metre square.

Figure 6. Solar radiation and ambient temperature during the experiment.

The water flowing over condenser is collected and recirculated back to cooler tank. To maintain the mass flow rate of water flowing over condenser a valve has been provided with inlet pipe (Fig.1) The hot water recirculated back to cooler tank is not going to affect the performance of air cooler [44]. Validation of Model To validate the proposed mathematical model, an experiment was carried out on May 22, 2023, at Raipur, Chhattisgarh, India (21°14΄40˝N, 81°37΄50˝E). The hourly solar radiation and ambient temperature and relative humidity during experiment is recorded and shown in Figure 6. An image of the experimental setup is shown in Figure 7, and Table 1 lists the specifics of the instruments used in the experiment. The uncertainty of various

0. C to 150 C

instruments utilized in experiment is shown in Table 2. The initial temperatures of the water, glass cover, and condensing cover were noted before the experiment started. The mass flow rate (MFR) of the cooled water passing over the condensing cover was measured at 0.065 kg/s, while the water depth in the solar still’s (SS) basin was kept at 2 cm. From 7:00 AM to 7:00 PM, hourly readings of the basin water, glass cover, and condensing cover temperatures were taken. A mathematical model was used to determine the corresponding theoretical values for the temperature of the condensing cover, glass cover, and basin water. The experimental results nearly match the theoretical values, as seen in Figure 8, 9, 10 and 11. Root mean square of percentage deviation e represents the closeness of the theoretical and experimental values. ‘e’ is given by [13],

The correlation between the predicted values and experimental values is given by coefficient of correlation (r). The correlation coefficient is given by the following equation

The experimentally measured water temperatures closely match the computed values. The standard error (“e”) ranges from 6.79 to 8.55, while the correlation coefficient (“r”) lies between 0.998 and 0.999, indicating an excellent fit between the theoretical and experimental results (see Figs. 8–10). This confirms that the model predicts water temperature with high accuracy. However, the deviation between theoretical and experimental distillate yield is higher, at about 14.6 %. For solar distillation systems, such a difference is generally considered acceptable while calculate the distillate output. Moreover, as shown Table 2, the combined total uncertainty of all measurement instruments is 2.661 %. This low level of uncertainty indicates that the reported trends and the solar still’s performance remain reliable, and the small variations do not significantly affect the conclusions drawn from the study.

Table 2. Uncertainty analysis Uncertainty Parameters Temperature (UT) K thermocouple [Sensor accuracy (sa), calibration (c), Resolution (R), Repeatability (Re)] Temperature (UT1) RTD [Sensor accuracy (sa), calibration (c), Resolution (R), Repeatability (Re)] Air velocity Measurement UA [Accuracy(a), Calibration (c), Resolution (R), Reeability (Re)

Solar Radiation (Us) [Calibration (c), Cosine error (co), Repeatability (Re)] Total Uncertainty Uo

Figure 10. Theoretical and experimental condensing cover temperature.

Numerical Computations

Effect of MFR and Depth of Water in Basin Numerical analysis has been done on the impacts of the water depth in the basin and the mass flow rate (MFR) of water passing over the condensing cover of a solar still (SS). The solar still’s total daily distillate output during a summer day was computed. In this analysis, the beginning glass temperature was set at 1°C below the water temperature, and the initial condensing cover temperature was set to the surrounding air’s wet bulb temperature (WBT). Furthermore, it was thought that the water’s initial temperature was taken

Figure 11. Theoretical and experimental hourly distillate output.

same as the surrounding air temperature. These computations were performed for a day (24h) in summer for climate of Raipur, Chhattisgarh, India. Hourly ambient temperature solar radiation and wet bulb temperature for a day in summer is shown in Figure 12. To analyse the effect of water depth on stills performance MFR is kept constant. The water depth in the basin was varied at 1 cm, 2 cm, 3 cm, 4 cm, 5cm and the MFR of water flowing over the cover was set at 0.065 kg/s. Raipur’s average wind speed was considered as 2 m/s (monthly average for May) As the basin’s water depth increased, the daily distillate output slightly decreased (Fig. 13). In particular, the daily

Figure 12. Hourly solar radiation, ambient temperature and WBT of summer (May).

distillate output decreased by just 4.9% when the water depth increased from 1 cm to 5 cm. On the other hand, the distillate yield of traditional solar stills (SS) usually decreases more noticeably as the water depth increases. In this design as evaporation and condensation zones are separated and condensation rate is now actively enhanced by flowing cooled water over condenser. The cooled condenser continuously maintains the temperature difference between water and condenser and hence the depth of water does not affect significantly. Additional numerical calculations were carried out to evaluate the daily distillate output of the proposed design by altering the MFR of water running over the condensing cover in order to examine the impact of the MFR of water flowing over the cover. The basin’s water depth was 2 cm, meteorological data considered are same as given

Figure 14. Effect of varying MFR of water flowing over condensing cover.

Figure 13. Effect of increasing the depth of water in basin.

above (Fig 14). The effect of changing the MFR of water flowing over the condensing cover was found to be nearly insignificant, as seen in Figure 14. While flowing evaporatively cooled water over cover due to thermal saturation the surface cover temperature of condenser remains low even at lower MFR with increase MFR further does not significantly lower condenser temperature hence distillate output does not increase noticeably. Effect of Relative Humidity and Wind Velocity To analyze the effect of relative humidity of ambient air and wind velocity we considered a typical set of parameters to compute hourly yield from the proposed device. For analyzing the effect of relative humidity, we considered ambient temperature as Ta=35oC, I=800W/m2 and wind velocity was taken as va=2m/s. Numerical computation was performed to determine hourly yield by varying relative humidity from 0.2 to 0.9. The hourly yield with varying relative humidity for typical data set was computed and shown in Figure 15. The result is as expected like other evaporative cooling devices the system works less effectively when the humidity is high. The hourly output decreases with increase in relative humidity of air. The inlet temperature of water flowing through condenser is considered equal to WBT. An increase in ambient humidity raises the wet-bulb temperature of air, thereby reducing the cooling potential and effectiveness of the condenser. For a typical set of parameters when the relative humidity increases from 0.2 to 0.9 the decrease in hourly output is 5.7%. To study the effect of wind velocity we considered Ta=35oC, I=800W/m2 and γ=0.2 Numerical computation was performed to determine hourly yield by varying wind velocity as 1m/s, 2m/s, 3m/s and 4m/s. The hourly yield has been computed at different wind velocity and shown in Figure 16. Unlike conventional solar distillation system were the distillate output increases with increase in wind velocity. Here in this design, we found the slight decrease

Figure 15. Effect of relative humidity of air on distillate output (Ta=35oC, I=800W/m2).

Figure 16. Effect of relative humidity of air on distillate output (Ta=35oC, I=800W/m2, Relative Humidity=0.2).

in hourly distillate output with the increase in wind velocity. Since the proposed design has a separate condensing unit which is evaporatively cooled by cooled water flowing over it. The condenser is already very effective so the evaporation is more dominating factor. Glass cover gets cooled due to increase in convective coefficient between glass and ambient air. The air inside gets cooled this reduces the greenhouse effect. The rate of evaporation gets reduced at high wind velocity thereby decreases the distillate output produced by system. When the wind velocity increases from 1m/s to 4m/s the decrease in the hourly distillate output is 2.7%. Performence of the Proposed Design Numerical computation has been performed to determine the performance of the proposes SS for a summer

(May) winter (Jan.) and autumn (Oct.) day in the climate of Raipur Chattisgarh. The climate data for Jan. and Oct. is shown in Figure 17. Slight improvement in output was observed with a lower basin water depth due to enhanced thermal response. Despite this, maintaining an intermediate water depth is recommended to reduce the need for frequent replenishment. Additionally, operating with a reduced MFR of cooling water is advisable, as it minimizes pumping power requirements without significantly compromising productivity. For further computations authors considered depth of water in basin as 2cm and MFR as 0.020kg/s. The performance of the propose design in May, January and October has been computed. Climate data for (May) is shown in Figure 12. Climate data for January and October

Figure 17. Climatic data for (a) Winter (Jan.), (b) Autumn (Oct.).

is shown in Figure 16. the total solar radiation received in a day for May, January, and October is 740.3 MJ/m2, 441.7 MJ/m2 and 535.7 MJ/m2. The maximum ambient temperature reached in day for May, January and October is 43.6oC, 27.1oC and 33.4 oC, respectively. Hourly water temperature, temperature of glass cover and condenser cover has been computed and shown in Figure 18,19, and 20. During the summer month of May, the highest temperature difference between the basin water and the condenser was seen as 29°C around 12:00 PM, resulting in the peak distillate yield of approximately 0.93 kg for that hour. In contrast, during January (winter), the maximum temperature difference reached 26.6°C at 1:00 PM, producing

Figure 20. Computed temperature of basin water, glass cover & condenser during autumn (October).

Figure 18. Computed temperature of basin water, glass cover & condenser during summer (May).

Figure 19. Computed temperature of basin water, glass cover & condenser during winter (January).

a corresponding maximum hourly distillate output of 0.55 kg. For the month of October, the peak temperature difference of 28.4°C occurred at 2:00 PM, with the associated maximum distillate generation being 0.67 kg per hour. The continuous cooling of condenser by cooled water helps to maintain the temperature difference between water and condensing cover during day time (6.00AM-6.00PM) as well as in night (6.00PM-6.00AM). This helps to produce distilled water during day as well as during night. The hourly distillate output produced by the proposed system during different months have been computed and shown in Figures 21, 22 and 23. The total distillate output produced in May, Jan. and Oct.is 7.83kg/day, 3.45kg/day and 5.30kg/day, respectively. The system helps to produce

Figure 21. Hourly distillate output produced in a day for May.

4. The cost of items is based on current market prices (as

shown in Table-3). 5. The cost of the desert cooler is excluded from the calculation. (Since desert cooler generally used in household for room cooling)

8. The system uses a 5 W PV-powered exhaust fan, and its

cost has been accounted for, while its auxiliary energy demand is considered negligible The TAC of the plant will be given by TAC = CC x CRF + AMC - SV x SFF TAC = 2269.9 CRF and SFF is given by,

Figure 22. Hourly distillate output produced in a day for January.

SFF = 0.017 Cost/L of water produced will be given by, Cost/Litre = TAC/ TAD = Rs.1.30/L(US$0.015)

Figure 23. Hourly distillate output produced in a day for October.

Sensitivity Analysis Energy Equivalence and Water Quality To assess the robustness of the proposed system, a sensitivity analysis was performed considering variations in solar availability and pump reliability. The results (Table 4) show the effect of reduced or enhanced solar radiation (±25%) and pump failure scenarios (10% and 20% downtime) on annual water output, cost of water, and payback period. We considered sensitivity scenario based on seasonal bands:

some amount of distillate during night time also. The distillate output produced during night for May, Jan. and Oct. are 0.45kg, 0.43kg and 0.61kg, respectively.

Economic Analysis An economic analysis was conducted to determine the cost per liter of water produced by the proposed solar still (SS). The following assumptions were made for the analysis:

2. The annual maintenance cost (AMC) is estimated at

(i) Winter (low) representative winter solar availability and ambient conditions (ii) Annual-average (base); and (iii) Summer(high) representative summer conditions. Outputs are computed with the same model using seasonal inputs. While higher insolation generally increases yield, productivity does not scale strictly linearly due to seasonal changes in (Tw–Tc) and convective losses. The payback period (PBP) will be given by PBP=CC/Annual Net Saving Annual Net Saving = (Annual Distillate Output) x (Selling Price) In pump failure scenario the system still run as conventional solar distillation system having separate condenser without cooling. The daily distillate output considered with pump failure as 3kg/day The analysis indicates that although the total annual cost remains constant, lower output under adverse conditions increases the unit cost of water and extends the payback period, whereas favourable conditions shorten it. For different scenario discussed in table the cost/kg of water produced is between Rs 0.94/kg to Rs 1.43/kg and the payback period between 0.236-0.535year.

`In addition, an energy equivalence analysis was carried out (Table 5) to quantify the environmental benefit of the system under the same scenarios. The annual distillate output was converted into equivalent energy savings, coal displaced, and associated CO₂ emissions avoided. This parallel evaluation highlights both the economic feasibility and sustainability advantage of the system, demonstrating its potential for potable water generation even under variable operating conditions. Energy equivalence (QE) and the mass of coal burned (Mc) will be given by,

In above Equation (CV)coal calorific value of coal is taken as 20,000KJ/kg and plant efficiency ηplant is considered 0.35. The energy equivalence analysis shows that the system can significantly offset coal consumption and associated CO₂ emissions. Depending on the operating scenario, the

Conclusion

The proposed design of SS contains two different sections separated by a common plate the section towards left is called evaporative section and two the right is called as condenser section. The water vapours produced in the evaporator section is forced towards condensing section by a low power exhaust fan in between. The condenser plate is evaporatively cooled by flowing cooled tank water from an air cooler. The cooled condenser behaves as a heat sink

Table 7. Summary of solar still modifications integrated with passive condensers Reference

Single-slope still with passive condenser; studied diffusion, purging, and circulation.

Natural convection-based humidification system with builtin condenser.

2. As the basin water depth increased, a slight reduction in

daily distillate output was observed. Specifically, when the water depth increased from 1 cm to 5 cm, the daily distillate yield decreased by only 4.9% 3. The mass flow rate of water flowing over the condenser plate has negligible affect on the distillate output produced.

4. For a typical set of parameters, it is seen that the hourly

distillate output decreases by 5.7% when the relative humidity of air increased from 0.2 to 0.9

6. Considering 2cm depth of water in basin and at mass

flow rate of 0.020kg/s the system generates 7.83kg/day, 3.44kg/day and 5.30 kg/day total distillate for a day in May, Jan. and Oct. The total radiation received for a day in May, Jan and Oct. is 740.3MJ/m2, 441.7MJ/m2 and 535.7MJ/m2, respectively.

7. The system is capable of generating distilled water even

during nighttime due to the effective cooling provided by the separate condenser. The distillate output produced during night (6.00PM-6.00AM) is 0.45kg/m2, 0.43kg/m2 and 0.61kg/m2 for a day in months of May, Jan. and Oct. This output represents an additional benefit, as it utilizes the stored heat without requiring significant extra energy input apart from minimal auxiliary power. While this quantity is modest, it can still contribute to daily freshwater availability, particularly in water-scarce areas

8. The cost of water produced is Rs 1.30/L (US$ 0.015/L)

Applicability of the Proposed Design • Rural Households: The system’s low maintenance, passive solar powered, and simple construction make it suitable for decentralized water purification in rural areas lacking reliable potable water access. • Off-grid Communities: Since the system operates without electrical power (except minimal pumping for condenser cooling, which can be solar-powered), it is highly suitable for off-grid or remote communities where grid access is limited. • Disaster Relief: The compact design, ability to produce clean water using only solar energy and minimal infrastructure, makes it a promising option for rapid deployment during natural disasters or emergency relief operations. Additionally, we have estimated daily water output under typical rural solar insolation conditions, indicating that the unit can provide 3–8 liters/day, which is sufficient

for basic drinking water needs of a small family. A comparison table (Table-7) has been given to compare the present work with some past works on solar distillation system having separate condenser. When compared we found that the proposed system works reasonably well to fulfill the pure water requirements of small communities in rural areas. Limitation of the present work

1. Economic analysis did not include the cost of the air

cooler, as it was assumed to be a pre-existing household appliance. In communities without coolers, the overall cost and payback period would be higher. 2. Scale formation effects were not studied. In hard-water areas, scaling can reduce heat transfer efficiency. Practical adoption will require mitigation strategies such as cleaning, water softening, or anti-sealant coatings. 3. Implementation aspects such as user training, largescale deployment, and socio-economic challenges were beyond the scope of this work. These will be addressed through field trials and user studies in future work.

Nomenclatures

Area of basin, m2 Area of glass cover, m2 Area of condensing plate, m2 Specific heat of water, Jkg-1K-1 Total internal heat transfer coefficient, W / m2K Total external heat transfer coefficient, W / m2K Sum of convective & radiative heat transfer coefficient between water and water film, W / m2K Radiative heat transfer coefficient between water and glass cover, W / m2K Convective heat transfer coefficient between water and glass cover, W / m2K Evaporative heat transfer coefficient between water and glass cover, W / m2K Convective heat transfer coefficient between glass and air, W / m2K Radiative heat transfer coefficient between glass and air, W / m2K Heat transfer coefficient between basin liner and water, W / m2K Convective heat transfer coefficient between condensing plate and water film, W / m2 Annual rate of interest Latent heat of vapourization of water, J / kg Mass flow rate of water flowing over condensing plate (film flow rate), kg / s Distillate output produced per hour, kg / h Mass of water in basin of solar still, kg Saturated vapour pressure at water temperature, N / m2 Saturated vapour pressure at glass temperature, N / m2 Rate of heat added by solar still into tank, W

qrw qcw qew qca qra . qcf . qcfa . qrfa I Tg Tw Ta Tb Tw0 Twt Twln Us Ut Ub va αg σ ρw

Rate of heat taken by make-up water, W Rate of heat loss from top of storage tank, W Rate of heat loss from sides of storage tank, W Rate of heat loss from bottom of storage tank, W Rate of radiative heat transfer from water to glass, W Rate of convective heat transfer from water to glass, W Rate of evaporative heat transfer from water to glass, W Rate of convective heat transfer from glass to air, W Rate of radiative heat transfer from glass to air, W Rate of convective heat transfer from condensing plate to film, W Rate of convective heat transfer from film to air, W Rate of radiative heat transfer from film to air, W Solar radiation, W / m2 Temperature of glass cover, °C Temperature of water in basin, °C Ambient temperature, °C Temperature of basin liner, °C Initial water temperature, °C Tank (reservoir) water temperature, °C Exit water temperature from nth row of plant, °C Side loss coefficient of the storage tank, Wm-2K-1 Top loss coefficient of the storage tank, Wm-2K-1 Bottom loss coefficient of the storage tank, Wm-2K-1 Velocity of air, m /s Absorptivity of glass cover Stefans-Boltzmann constant, W / m2K4 Density of water, kg / m3

Abbreviations

SS Solar still MFR Mass flow rate TAC Total annual cost TAD Total annual distillate produced CC Capital cost CRF Capital recovery factor AMC Annual maintenance cost SV Salvage value SFF Sinking fund factor CF Cash flow U Uncertainty WBT Wet bulb temperature

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