Experimental study on performance of passive and active solar stills in Indian coastal climatic condition

Abstract

This present work is aimed to examine the effect of mass flow rate on distillate output and performance of a solar still in active mode. Outdoor experiments were conducted at the coastal town, Kakinada (16°93′N/83°33′E), Andhra Pradesh, India. A solar still with a 30° of fixed cover inclination, 1m² of effective basin area, and a flat-plate collector (FPC) with an effective area of 2 m² were used. An attempt was also made earlier in passive mode to optimize the water depth for the same solar still for maximum yield and distillation efficiency. For the passive still, it is observed that the capacity of heat storage and heat drop are significant parameters that affect the still performance. For the selected still design, the study reveals that 0.04 m water depth is the optimum value for specific climatic conditions. In the active solar still, with the optimum water depth, different flow rates of 0.5, 1 and 1.5 L/min are considered through FPC. It is observed that both the mass flow rate and the variation of internal heat transfer coefficients with the mass flow rate have a significant effect on the yield and performance of the still. The experimental results show that the combination of 1.5 L/min mass flow rate and an optimum water depth of 0.04 m leads to a maximum yield for the active solar still. The enhanced yield of the active solar still is 57.55%, compared with that of the passive solar still, due to increase in area of radiation collection and more heat absorption rate.

Keywords

distillation efficiency / solar still / heat transfer coefficient / water depth / optimum and mass flow rate

Introduction

Many processes are available for obtaining purified water in which distillation is one of them. During this process water gets heated and evaporated. This evaporated vapor will be condensed and pure water is formed. The required heat for the process is powered from the Sun. In Indian coastal regions, potable water may not be readily available and ground water is mostly used as drinking water. By considering the availability of solar energy and ground water, the practical alternative, a solar distillation of ground water especially in Indian costal conditions, was studied.

This type of solar distillation never demands any hidden fuel costs and is eco-friendly. The solar radiation is abundantly available in India compared to its effective usage. So the capacity of distillation can be raised by increasing the area which receives solar radiation. There are two types of solar distillation systems, namely the passive and the active solar system.

Definition of the problem

The internal heat and mass transfer processes affect the performance of solar distillation unit. Dunkle [¹] proposed a relation between internal heat and mass transfer coefficients. A wide variety of solar stills with different geometries for different climatic conditions and different modes (passive and active) were analyzed based on Dunkle’s relation.

For the selected design, an attempt was made in this paper to find an optimum water depth required in the still for maximum daily yield. Various water depths such as 0.02 m, 0.03 m, 0.04 m, 0.05 m, and 0.06 m were considered for performance prediction of the unit.

This paper also described an attempt to compare various mass flow rates of an active solar still for maximum yield for the selected design. Three different mass flow rates of 0.5 L/min, 1 L/min, and 1.5 L/min were considered for an active solar still. The convective mass transfer relations were determined for both passive and active solar stills. The mutual effect of convective and evaporative internal heat transfer were estimated by a relation Nu = C(Gr·Pr)ⁿ.

Dunkle [¹] recommended taking the fixed values for C and n as 0.075 and 1/3 respectively whereas Kumar and Tiwari [²] used practical data to obtain the values of constants from outdoor experimentation. In this paper, the values of heat transfer coefficients for the solar still unit were evaluated and compared with both Dunkle’s and K&T models.

Literature review

Due to the shortcomings in Dunkle’s relation, new empirical equations were proposed for determining the constants of C and n for calculating internal heat transfer coefficients by Kumar and Tiwari [²]. Rajamanickam and Ragupathy [³] reported that the double slope solar still with a north–south orientation had an edge over the single slope solar still for constant water depth. Rai and Tiwari [⁴] suggested that the production rate of the solar still with a flat-plate collector is higher than that of the simple solar still. Singh and Tiwari [⁵] conducted a quasi steady-state analysis on an hourly basis and formulated functions for various climatic parameters. They finally concluded that the annual yield varied with the water depth, the inclinations of the condensing cover and the collectors. Dwivedi and Tiwari [⁶] investigated the performance of a double slope active solar still in a natural circulation mode. The results showed that the active solar still gave a higher yield compared to the passive solar still. Feilizadeh et al. [⁷] suggested that the water surface-cover distance could affect the distillate rate of a solar still. Tghvaaei et al. [⁸] indicated that the active solar still yield increased with increased collector area. Taghvaei et al. [⁹] concluded that the increase in water depth would increase the yield and the efficiency of an active solar still. Bharadwaj et al. [¹⁰] studied the method of maximizing the water production by increasing the area of condensation surface of a solar still. Dev et al. [¹¹] proposed that an inverted absorber solar still gave a higher basin water temperature than a single slope solar still. Ahsan et al. [¹²] recommended improvements in a triangular solar still, capable of providing clean water from salt. Tripathi and Tiwari [¹³] observed that at lower altitude angles, solar fraction played a very important role. Furthermore, the heat transfer coefficient was inversely proportional to the water depth in the basin. Elango and Kalidasa Murugavel [¹⁴] compared the production from the single basin and the double basin solar still during the heating period and the cooling period. Somanchi et al. [¹⁵] reported improvements in the efficiency of solar water distillation by using phase change materials. Durkaieswaran and Kalidasa Murugavel [¹⁶] reviewed the work on different special designs of single basin passive solar stills. Tabrizi et al. [¹⁷] studied the design of a cascade solar still with the effect of water flow rate on internal heat and mass trasfer as well as distillate yield of it. The results showed that mass flow rate was inversely proportional to the internal heat, the mass transfer, and the production of the cascade solar still. Panchal et al. [¹⁸] compared the analysis of an active solar still and a passive solar still. It showed that the productivity of the active solar still was higher than that of the passive solar still while decreasing the water depth increased the productivity of the solar still. Kumar et al. [¹⁹] proved that the productivity of a solar still with an evacuated tube collector is higher in a forced mode compared with that in a natural mode. Panchal [²⁰] suggested that the productivity of a solar still increased with the sensible heat gain of potable water, which was experimentally studied by using various materials inside a solar still along with water. The enhancement of distillate of a double basin solar still was studied by Panchal and Shah [²¹] by introducing vacuum tubes and different sensible energy storage materials. Through experimentations, Singh et al. [²²] optimized the solar still system for combinations of the number of evacuated tubes and water depth. El-Agouz et al. [²³] studied the performance of an inclined solar still under the condition of continuous water flow over the absorber plate located beneath the condensing cover. A considerable increase in productivity of still was observed when basin water was circulated over the absorber plate continuously. Sathyamurthy et al. [²⁴] reviewed the influence of factors such as water depth, water flow rate, intensity of radiation, forced/natural circulation etc. for improving the yield of the solar still coupled with various solar collectors.

From the above literature, it is observed that this paper intends to enhance the productivity of an active solar still with optimum mass flow rate in the coastal regions in Indian, which has not yet been attempted before. It is a simple technique to improve the yield of the still and provides cost effective solution with better results, and hence it is a novel proposal.

Experimental setup

Distillation unit

The schematic line diagram of an active solar still coupled with a flat-plate collector is shown in Fig. 1, whereas the photographs of the experimental setup are depicted in Figs. 2 and 3. The experimental setup consists of a passive/active solar still with a condensing cover inclination of 30°. The bottom surface of the still was painted Nichrome black for a greater absorptivity. The height of the lower vertical wall of the still was kept at 0.30 m to avoid the spilling of basin water into the distillate canal and to avoid the contact of distillation canal with the condensing cover as well as with the water level. The height of the higher vertical wall was kept at 0.88 m. The effective basin area of the still was 1m × 1m and it was made of FRP of 6 mm in thickness, which provided insulation for heat flow. The condensing cover made of the plain glass of 4 mm length was fixed to the top of the vertical wall of the still, using a rubber gasket. The yield from the still was collected through a channel, fixed at the height of the smaller vertical wall of the basin. A hose pipe was connected to this canal to collect the yield to a measuring jar.

The flat-plate collector was a box section made up of plywood of 5mm in thickness with a size of 2 m × 1m × 0.1 m. It consisted of ten copper tubes of 10 mm in diameter and 2mm in thickness which were equally spaced along the longitudinal direction. The tubes were connected with header and footers of 30 mm in diameter to assist the flow through the tubes. The copper tubes were closely packed on both sides with Nichrome black coated copper sheets of 1mm in thickness. The top of the FPC was fitted with a glass cover of 4 mm in thickness and the top edges were sealed with rubber gasket.

Procedure

The experiments were performed in summer climatic condition in JNTUK, Kakinada, Andhra Pradesh, India. April and May are usually the hottest months of the year in this region and the typical results for 8 days during the period were reported here. In passive mode, various water depths, namely 0.02 m, 0.03 m, 0.04 m, 0.05 m, and 0.06 m, were used. In active mode, various mass flow rates, namely 0.5 L/min, 1 L/min, and 1.5 L/min, were tested. 1.5 L/min was the maximum possible flow rate for the selected still. The condensing cover of 4 mm in thickness and the inclination of 30° were fixed for both passive and active modes. In an active solar still collector, an inclination of 5° is fixed. To maintain the high temperature gradient between the glass and the water surface in the still, water was circulated through the collectors with the help of a pump. To avoid heat losses caused by reverse flow, the pump was turned off during off-sunshine hours. The experiments were conducted on alternate days. The experiment duration was 24 h, starting from 8 am for both passive and active solar stills to increase the accuracy of the experiments. The parameters, viz, glass outer and inner surface temperatures, vapor temperature, water temperature, ambient temperature, incident radiation, relative humidity (g) inside the still, and distillate output were measured on an hourly basis for both passive and active solar still experiments.

A digital temperature display was used to assess the temperatures of water, condensing cover and vapor temperatures, measured with Copper-Constantan thermocouples having a least count of 0.1°C.

A mercury thermometer was used to record the ambient temperature and outer glass temperature. The yield was measured using of a measuring flask at time intervals. The solar intensity was measured with the help of a calibrated solarimeter with a least count of 1 W/m². The hygrometer was used for measuring the relative humidity inside the basin. The ranges and least counts of the measuring instruments are listed in Table 1. The above measured parameters were used to calculate the average values of each for further calculations.

Experimental uncertainty

This indirect approach of approximation of heat transfer coefficient was based on the mass of distillate collected from the still. During experimentation, an experimental uncertainty was considered for all factors. The estimation of uncertainty [²⁶] was conducted separately for passive and active solar stills.

The uncertainty can be estimated by

(1)\mathrm{U}=\sqrt{{\displaystyle \frac{{\displaystyle \sum {\sigma }^{\mathrm{2}}}}{N^{\mathrm{2}}}}},

where s is the standard deviation of each sample and N is the total number of samples.

The cumulative uncertainty values are 24% and 21% for passive and active solar stills.

The results may vary with the thermal storage effect. The percentage error can be estimated by

\mathrm{E}\mathrm{rror}={\displaystyle \frac{\mathrm{D}\mathrm{istilate}\hspace{0.17em}\mathrm{output}\hspace{0.17em}\mathrm{during}\hspace{0.17em}\mathrm{non}-\mathrm{sunshine}\hspace{0.17em}\mathrm{hours}}{\mathrm{D}\mathrm{istilate}\hspace{0.17em}\mathrm{output}\hspace{0.17em}\mathrm{during}\hspace{0.17em}\mathrm{day}\mathrm{}\mathrm{light}\hspace{0.17em}\mathrm{hours}}}\times 100\%

The errors caused by thermal storage effect for passive and active solar stills have been estimated to be 16% and 13 % respectively.

Governing equations and thermal models

The following equations are applicable for obtaining convective heat transfer coefficient.

(2)Q={{\displaystyle h}}_{\mathrm{c}\mathrm{w}}\cdot A({{\displaystyle T}}_{\mathrm{s}}-{{\displaystyle T}}_{\mathrm{a}})={{\displaystyle h}}_{\mathrm{cw}}\cdot A\cdot \Delta T.

From the relation Nu = C (Gr·Pr)ⁿ, the convective heat transfer is written as

(3){{\displaystyle h}}_{\mathrm{cw}}=({{\displaystyle K}}_{\mathrm{v}}/{{\displaystyle L}}_{\mathrm{v}})C{(Gr\cdot Pr)}^n.

The distillate output in kg from the unit can be obtained by the relation

(4){{\displaystyle m}}_{\mathrm{ew}}={\displaystyle \frac{{{\displaystyle q}}_{\mathrm{ew}}}{L}}{{\displaystyle A}}_{\mathrm{w}}\cdot t,

where

(5){{\displaystyle q}}_{\mathrm{ew}}={{\displaystyle h}}_{\mathrm{ew}}({{\displaystyle T}}_{\mathrm{w}}-{{\displaystyle T}}_{\mathrm{ci}}).

Evaporative heat transfer coefficient

(6){{\displaystyle h}}_{\mathrm{ew}}=\mathrm{0.016}{{\displaystyle h}}_{\mathrm{ew}}\left[{\displaystyle \frac{{{\displaystyle P}}_{\mathrm{w}}-{{\displaystyle P}}_{\mathrm{ci}}}{{{\displaystyle T}}_{\mathrm{w}}-{{\displaystyle T}}_{\mathrm{ci}}}}\right].

Distillation efficiency

The distillation efficiency ({{\displaystyle \eta }}_D) of the solar still has been defined as the ratio of distilled water produced to the total solar radiation energy received. Integration over a given period of time results in the following distillation efficiency:

For passive solar still,

(7){{\displaystyle \eta }}_{\mathrm{D}}={\displaystyle \frac{{\displaystyle \sum {{\displaystyle m}}_{\mathrm{ew}}L}}{A\mathrm{g}{\displaystyle \sum I(t)}}}\times \mathrm{100}\%.

For active solar still,

(8){{\displaystyle \eta }}_{\mathrm{D}}={\displaystyle \frac{{\displaystyle \sum {{\displaystyle m}}_{\mathrm{ew}}L}}{A\mathrm{g}{\displaystyle \sum I(t)+A\mathrm{c}{\displaystyle \sum {{\displaystyle I}}_{\mathrm{c}}(t)}}}}\times \mathrm{100}\%.

In this paper, the effect of water depth and mass flow rate on maximum yield and distillation efficiency for both passive and active solar stills were experimentally studied. Further, the constants C and n of both K&T model as well as the Dunkle’s model were used for calculating the various parameters h_cw and h_ew etc., and the comparative performance of both passive and active solar stills were also presented.

Results and discussion

In this paper, the comparative performance of solar still operating in passive and active modes was reported. Five different water depths of 0.02 m, 0.03 m, 0.04 m, 0.05 m, and 0.06 m were tested for the optimum performance of a passive still and the water depth of 0.04 m was found to be more suitable for the selected still design. Hence, the water depth of 0.04 m was adopted for the mass flow rate experimentation. Tables 2 and 4 tabulate the 24 hourly average values of h_cw, h_ew, h_cwDM and h_ewDM calculated from experimental data corresponding to the water depth of 0.04 m in passive mode and a mass flow rate of 1.5 L/min in active mode respectively. Similar values for other four water depths and two mass flow rates were also calculated. The C and n values obtained as per the K&T model [²⁵] for passive and active mode experiments are indicated in Tables 3 and 5 respectively.

Figure 4 shows the comparative values of h_cw for a passive still using the K&T model with five different water depths and the variation is obtained with respect to water depth. It is observed that the values of h_cw are almost constant throughout the 24 h duration for each water depth.

Figure 5 illustrates the values of hew for the five water depths. Their variations look similar within 24 h.

Figure 6 demonstrates the variation of the daily yield of five distinct water depths. A maximum yield of 1.694 kg is obtained at a water depth of 0.04 m whereas the yield is 1.273 kg for a water depth of 0.02 m, 1.411 kg for a water depth of 0.03 m, 1.369 kg for a water depth of 0.05 m, and 1.222 kg for a water depth of 0.06 m. It is clearly seen from the above data that the water depth of 0.04 m is the optimum water depth among the five. This is due to the following reasons: less heat storage capacity and more heat drop for the water depths of 0.02 m and 0.03 m and more time requirement for heat storage in the water depths of 0.05 m and 0.06 m compared to the water depth of 0.04 m.

Figure 5 also displays the variation of distillation efficiency for the five water depths. The variation is analogous to the daily yield, shown in Fig. 5. The efficiencies of still at the water depths of 0.02 m, 0.03 m, 0.05 m, and 0.06 m are 12.72%, 13.05%, 13.71% and 12.54% respectively. The maximum efficiency of 15.02% is obtained at the optimum water depth of 0.04 m.

Figure 7 exhibits the comparative values of h_cw for an active still using the K&T model with three different mass flow rates and the variation is obtained with respect to mass flow rate. It is observed that the values of h_cw are almost constant throughout the 24 h for each mass flow rate.

Figure 8 shows the comparative values of h_ew for an active still using the K&T model with three different mass flow rates and the variation is similar throughout the day. The maximum values of h_ew, 32.451 W/(m²·°C), 51.984 W/(m²·°C), and 36.119 W/(m²·°C), are obtained for mass flow rates of 1.5 L/min, 1 L/min, and 0.5 L/min respectively.

High yields are obtained, as presented in Fig. 9, for an active solar still coupled with various mass flow rates compared to a passive solar still with varying water depths. Figure 9 shows the daily yield for an active still with three different mass flow rates. A maximum yield of 2.669 kg is obtained at the mass flow rate of 1.5 L/min whereas the yield is 2.475 kg for the mass flow rate of 1 L/min and 2.259 kg for the mass flow rate of 0.5 L/min. It is apparently observed from the above description that the mass flow rate of 1.5 L/min gives a higher yield compared to other mass flow rates. The reason for this is that the water is able to absorb more heat while flowing through the tubes of the collector and hence more heat flows into the still continuously at the mass flow rate of 1.5 L/min.

Figure 9 also shows the variation of distillation efficiency for the three mass flow rates. This variation is similar to that of daily yield change, as shown in Fig. 9. The efficiencies of an active still for the mass flow rates of 1 L/min and 0.5 L/min are 5.73% and 4.27% respectively. The maximum efficiency of 6.82% is obtained for the mass flow rates of 1.5 L/min. The increase in area of active solar still offsets the advantage of increase in yield for the still with FPC, and the efficiency has decreased compared to a passive solar still.

Hence ∆T plays an important role in obtaining the maximum yield, distillate efficiency and ultimately the convective mass transfer, and partial pressure difference as well.

The characteristics of the initial and final samples are indicated in Table 6. The pH, the total dissolved salt, the hardness and electrical conductivity values of the distilled sample are much lower and in acceptable limits compared to the original sample.

Conclusions

The present experimental study optimized the water depth and mass flow rates for maximum productivity in the still in passive and active modes for specified conditions. The important conclusions are summarized below:

1)For the selected solar still in passive mode, the water depth of 0.04 m is found to be the optimum value.

2)More yields are obtained for an active solar still, due to the increase in both the area of radiation collection and the heat absorption rate.

3)In active mode, an optimum performance is observed at a mass flow rate of 1.5 L/min.

4)The K&T model is found to be superior due to the realistic values for the constants C and n, while the convective and evaporative heat transfer coefficients are found to be the key parameters which affect the yield and distillation efficiencies.

5)The studies will be useful for designing efficient solar distillation systems for the coastal climatic conditions.

References

[1] Dunkle R V. Solar water distillation, the roof type still and a multiple effect diffusion still, international developments in heat transfer. In: Proceedings of ASME International Heat Transfer Conference, Part 5, University of Colorado, 1961, 895–902

[2] Kumar S, Tiwari G N. Estimation of convective mass transfer in solar distillation system. Solar Energy, 1996, 57(6): 459–464

[3] Rajamanickam M R, Ragupathy A. Influence of water depth on internal heat and mass transfer in a double slope solar still. Energy Procedia, 2012, 14: 1701–1708

[4] Rai S N, Tiwari G N. Single basin solar still coupled with flat plate collector. Energy Conversion and Management, 1983, 23(3): 145–149

[5] Singh H N, Tiwari G N. Monthly performance of passive and active solar stills for different Indian climatic conditions. Desalination, 2004, 168(1–3): 145–150

[6] Dwivedi V K, Tiwari G N. Experimental validation of thermal model of a double slope active solar still under natural circulation mode. Desalination, 2010, 250(1): 49–55

[7] Feilizadeh M, Estahbanati M R K, Ahsan A, Jafarpur K, Mersaghian A. Effects of water and basin depths in single basin solar stills: an experimental and theoretical study. Energy Conversion and Management, 2016, 122: 174–181

[8] Taghvaei H, Taghvaei H, Jafarpur K, Feilizadeh M, Karimi Estahbanati M R. Experimental investigation of the effect of solar collecting area on the performance of active solar stills with different brine depths. Desalination, 2015, 358: 76–83

[9] Taghvaei H, Taghvaei H, Jafarpur K, Estahbanati M R K, Feilizadeh M. A thorough investigation of the effects of water depth on the performance of active solar stills. Desalination, 2014, 347: 77–85

[10] Bhardwaj R, ten Kortenaar M V, Mudde R F. Maximized production of water by increasing area of condensation surface for solar distillation. Applied Energy, 2015, 154: 480–490

[11] Dev R, Abdul-wahab S A, Tiwari G N. Performance study of the inverted absorber solar still with water depth and total dissolved solid. Applied Energy, 2011, 88(1): 252–264

[12] Ahsan A, Imteaz M, Thomas U A, Azmi M, Rahman A, Nik Daud N N. Parameters affecting the performance of a low cost solar still. Applied Energy, 2014, 114(2): 924–930

[13] Tripathi R, Tiwari G N. Thermal modeling of passive and active solar stills for different depths of water by using the concept of solar fraction. Solar Energy, 2006, 80(8): 956–967

[14] Elango T, Kalidasa Murugavel K. The effect of the water depth on the productivity for single and double basin double slope glass solar stills. Desalination, 2015, 359: 82–91

[15] Somanchi N S, Sagi S L S, Kumar T A, Kakarlamudi S P D, Parik A. Modelling and analysis of single slope solar still at different water depth. Aquatic Procedia, 2015, 4: 1477–1482

[16] Durkaieswaran P, Kailas Murugavel K. Various special designs of single basin passive solar still—a review. Renewable & Sustainable Energy Reviews, 2015, 49: 1048–1060

[17] Tabrizi F F, Dashtban M, Moghaddam H, Razzaghi K. Effect of water flow rate on internal heat and mass transfer and daily productivity of a weir-type cascade solar still. Desalination, 2010, 260(1–3): 239–247

[18] Panchal H N, Patel M I, Patel B, Goswami R, Doshi M. A comparatıve analysıs of sıngle slope solar stıll coupled wıth flat plate collector and passıve solar. IJRRAS, 2011, 7: 111–116

[19] Kumar S, Dubey A, Tiwari G N. A solar still augmented with an evacuated tube collector in forced mode. Desalination, 2014, 347: 15–24

[20] Panchal H N. Performance analysis of solar still with cow dung cakes and blue metal stones. Frontiers in Energy, 2015, 9(2): 180–186

[21] Panchal H N, Shah P K. Enhancement of distillate output of double basin solar still with vacuum tubes. Frontiers in Energy, 2014, 8(1): 101–109

[22] Singh R V, Kumar S, Hasan M M, Khan M E, Tiwari G N. Performance of a solar still integrated with evacuated tube collector in natural mode. Desalination, 2013, 318: 25–33

[23] El-Agouz S A, El-Samadony Y A F, Kabeel A E. Performance evaluation of a continuous flow inclined solar still desalination system. Energy Conversion and Management, 2015, 101: 606–615

[24] Sathyamurthy R, El-Agouz S A, Nagarajan P K, Subramani J, Arunkumar T, Mageshbabu D, Madhu B, Bharathwaaj R, Prakash N. A review of integrating solar collectors to solar still. Renewable & Sustainable Energy Reviews, 2017, 77: 1069–1097

[25] Panchal H N, Shah P K. Effect of varying glass cover thickness on performance of solar still: in a winter climate conditions. International Journal of Renewable Energy Research, 2011, 1(4): 212–223

[26] Nakra B C. Instrumentation Measurement and Analysis. New Delhi: Tata Mc Graw-Hill, 1985

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