Performance analysis of modified basin type double slope multi–wick solar still

Desalination 422 (2017) 68–82

Contents lists available at ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Performance analysis of modified basin type double slope multi–wick solar still

MARK

Piyush Pala,⁎, Pankaj Yadava, Rahul Deva, Dhananjay Singhb a b

Department of Mechanical Engineering, Motilal Nehru National Institute of Technology Allahabad, Allahabad 211004, Uttar Pradesh, India Department of Chemical Engineering, Institute of Engineering & Technology Lucknow, Lucknow 226021, Uttar Pradesh, India

A R T I C L E I N F O

A B S T R A C T

Keywords: Desalination Instantaneous efficiency Multi–wick passive solar still Wick

This paper presents an outlook to enhance the productivity of a basin type double slope multi–wick solar still by introducing the wicks. The experimental data for different months are presented, and analyzed the effect of climatic and operational parameters on the performance of modified basin type double slope multi–wick solar still (MBDSMWSS). The study has been conducted at Motilal Nehru National Institute of Technology Allahabad (MNNIT Allahabad), Uttar Pradesh (U.P.), India. A significant increase in the heat input, yield, and overall thermal efficiency have been obtained. In the instantaneous efficiency equation, the yield output and the heat input to the solar still is modified as input from both the glass covers and transparent walls are considered for the modified solar still. The result shows that, the maximum yield is obtained as 9012 ml/day (4.50 l/m2 day) for black cotton wick in comparison to 7040 ml/day (3.52 l/m2 day) for the jute wick at 2 cm water depth in MBDSMWSS. Also, for same basin condition, the overall thermal efficiency of MBDSMWSS with the jute and black cotton wicks are 20.94% and 23.03%, respectively.

1. Introduction Fresh water is the need of every human being and agricultural purposes but the sources of water around the world keep declining due to huge consumption and population growth. Most of the sources of fresh water are contaminated due to the inclusion of the chemicals (pesticides, fertilizers, etc.), high content of heavy metals, and high concentrations of salt in aquifers. The palatability of drinking water has been determined by catalog of tasters in relation to its Total Dissolved Solids (TDS) level as follows: excellent (< 300 mg/l); good (between 300 and 600 mg/l); fair (between 600 and 900 mg/l); poor (between 900 and 1200 mg/l); and unacceptable (> 1200 mg/l) [1]. The areas around the world, where conditions arises from lack of access to clean and germs free potable water, and grid–based electricity has been absent, there is an utterly need for facilitating such a cost effective technique, which is easy to handle, pollution free, and produces sufficient amount of potable water to fulfill the needs of society. Most of the developed and advanced techniques for water purification have rely on coal–based electricity, which causes damage to the environment and adversely affects the water quality. Thus, keeping the constraints of the present situation, a renewable energy based water purification technique is required. Solar desalination is one of such an efficient process that uses solar thermal energy for obtaining clean water from brackish

⁎

Corresponding author. E-mail addresses: [email protected] (P. Pal), [email protected] (R. Dev).

http://dx.doi.org/10.1016/j.desal.2017.08.009 Received 4 April 2017; Received in revised form 28 June 2017; Accepted 17 August 2017 0011-9164/ © 2017 Elsevier B.V. All rights reserved.

water. Solar distillation is primarily a minor–scale replica of the natural hydrological cycle that originates rain, which is the elementary source of fresh water worldwide. Solar still is used as a structure for solar distillation process [2,3], which utilizes solar energy to drive thermal distillation processes. Solar desalination suitable in the areas where drinking water is either limited in supply or is present in impure form and solar energy is abundant in supply. Arab Alchemists in 1551 [4] presented the work of distillation in the earlier stages of desalination methods. Malik et al. [5] studied the design and performance of different types of solar distillers. Sodha et al. [6] analyzed the performance and presented the design of a multi–wick solar still, in which blackened jute wick is used to form the wet surface, and oriented to absorbed the maximum solar radiation to achieve high temperature. Tiwari and Tiwari [7] shows the effect of climatic parameters like the radiation intensity of solar rays, temperature of an environment, and operational parameters like the brackish water depth on the performance of solar distillation system. Shukla and Sorayan [8] presented and validated a thermal model for a multi–wick solar distiller. In this work, a computer model had been evolved and based on modified heat transfer coefficients, validation of thermal model had been accomplished. Mahdi et al. [9] built a tilted wick–type solar still to characterize its performance. In this work, the charcoal cloth was used as a wick to absorb the saline water. Rajaseenivasan et al. [10] were

Desalination 422 (2017) 68–82

P. Pal et al.

Fig. 1. Schematic diagram of modified basin type double slope multi–wick solar still.

performance of the system was studied. Tiwari and Tiwari [25] investigated the effects of the inclination of a condensing cover and depth of the water on the yield and convective coefficient of heat transfer of a passive solar distiller for the climatic conditions of New Delhi, India. Deniz [26] investigated the improving of inclined solar water distiller system performance under the environmental conditions of Turkey. In this work, the system was experimented with bare, shaded bare plate, and with black cloth, shaded black cloth wick. Ayoub et al. [27] depicted a sustainable alteration in the design of a solar still in the form of a slowly swirling drum, this facilitates the creation of thin water films that evaporate hurriedly and are incessantly renewed. Kalita et al. [28] reviewed the effects of various geometric and operating parameters, and energy losses and balance were calculated using the second law of thermodynamic on the performance of a solar still. Sharshir et al. [29] reviewed the various elements affecting solar still yield like climatic conditions, design parameters, and operations. An augmentation of yield by using wicks, stepped solar still, nanoparticles, internal and external condensers, phase change materials, and internal and external reflectors have been discussed. From the above literature review, it is found that, the multi–wick solar still is associated with following drawbacks: (i) improper supply of water into the wick; (ii) improper use of incident solar radiation; and (iii) improper collection of yield. In this paper, a modified design of basin type double slope multi–wick solar still is presented for its performance evaluation in the climatic conditions of Allahabad (U.P.) (Latitude 25°27′ N & Longitude 81°44′ E), India. The experiments are performed and effect of climatic parameters like solar radiation, operational parameters like feed water depth and wick material on MBDSMWSS have been studied.

used the different wicks, energy storing materials, porous materials and variable depth of water in both single basin double slope and double basin double slope solar distiller to increase the yield of a double slope solar still by adding an extra basin. Dev et al. [11–13] developed the characteristic equations (linear and non–linear) for an active and passive solar stills. In this work, the inclination angles 15°, 30°, and 45° of glass cover have been selected in winter and summer conditions both. Pal and Dev [14] studied the performance of modified basin–type double slope, and modified basin–type double slope multi–wick solar stills. Morad et al. [15] studied the performance of solar still and flat–plate solar collector solar still, which was the function of a thickness of glass cover and the brine depth in the basin. Furthermore, based on equations of external, internal heat transfers and energy balance, a thermal analysis was accomplished. Saurabh and Sudhakar [16] presented a vast review of the various designs of solar distillers used at domestic level. Prakash and Velmurugan [17] scrutinized various parameters of the solar stills, which influenced the yield. The review showed an increase in the absorber area increases the productivity. The height of the water in the basin is the main parameter for the yield of the solar still. Pal et al. [18] studied the design of modified basin–type double slope multi–wick solar still. Manikandan et al. [19] reviewed different configurations and designs of wick type solar distillers and concluded that researchers have taken concerted efforts to make various new designs and configurations of solar distiller for higher yield. Hansen et al. [20] found that water coral fleece with heat transfer coefficient, absorbency, porosity, and capillary rise are 34.21 W/m2 °C, 2 s, 69.67%, and 10 mm/h, respectively is best working wicking material among selected material for the higher yield of the solar still. Kaviti et al. [21] reviewed different design and configurations used to improve the productivity of inclined solar stills. Rufuss et al. [22] studied a detailed review of the various active and passive, single and multi–effect passive and active solar stills, reviewed design of the solar stills, and stated various modifications for improved overall performance and productivity. Kumar et al. [23] presented a detailed observation of the single and multi–effect type with passive and active configurations. Al-Kharabsheh and Goswami [24] investigated a water distiller system used solar heat as a low–grade energy. Researchers presented the experimental result and theoretical analysis. In this work, the effect of various operating conditions such as depth of water body, heat source temperature, and temperature of the condenser, on the

2. Experimental setup and working principle 2.1. Description of experimental setup The constructed experimental modified multi–wick solar still is installed on the rooftop of ‘Heat and Mass Transfer & Solar Energy Laboratory’, Mechanical Engineering Department, MNNIT Allahabad, Allahabad, U.P., India. Fig. 1 shows the schematic diagram of the modified basin type double slope multi–wick solar still. The solar still consist of a rectangular basin of area 2 m2 (length 2 m and width 1 m), which is made up of thickness 5 mm of Fibre 69

Desalination 422 (2017) 68–82

P. Pal et al.

Reinforced Plastic (FRP). The basin is painted black from inside to absorb the maximum amount of solar radiation. The east (E) and west (W) walls (along the solar still's width dimension), which has length of 1 m and 0.12 m height from the solar still base, are made up of an acrylic sheet of thickness 3 mm, and both wall have same area of 0.12 m2. The south (S) and north (N) walls of solar still, both have 2 m in length and 0.12 m in height at the end and maximum height of 0.38 m at the centre. The S–wall is made up of an acrylic sheet of 3 mm thickness whereas N–wall is made up of a FRP of thickness 5 mm which is painted black from inside to absorb more solar radiation. Three walls i.e. E, W, S are made up of a transparent acrylic sheet of 3 mm thickness, which is equivalent thickness of acrylic sheet with respect to 5 mm thickness of FRP on the basis of the same heat transfer rate. The equivalent thickness for an acrylic sheet with respect to 5 mm FRP is calculated using Fourier's law of heat conduction (one–dimensional):

T − T1 ⎞ T − T1 ⎞ = KACRY × AACRY × ⎛ 2 QCond = KFRP × AFRP × ⎛ 2 ⎝ LFRP ⎠ ⎝ LACRY ⎠ ⎜

⎟

⎜

Table 1 Specifications of MBDSMWSS.

KACRY × LFRP KFRP

Specification(s)

Orientation Body material

East–West Basin and north wall made of FRP; east, west, and south walls made of Acrylic sheet; east and west glass covers of simple window glass 2 m × 1 m, black 5 mm 3 mm 0.12 m 0.38 m 1.03 m × 1.03 m × 0.004 m

Basin area and colour Thickness of FRP Thickness of Acrylic Height at ends Height at centre Glass cover dimension Quantity of glass Inclination angle of glass cover Colour of north wall inside Number of inlet to solar still Number of outlets with troughs at the ends

⎟

(1)

For the same rate of heat transfer, when the two sides of FRP is maintained at temperature T1 and T2. The thickness of acrylic is given by:

LACRY =

Component

2 15° Black 1 4

photograph of MBDSMWSS showing condensation on south wall is shown in Fig. 2. The orientation of the MBDSMWSS has been kept in the East–West direction to receive the solar radiations for maximum hours of sunshine. The details of system design parameters and its specifications are presented in Table 1. In the solar still fabrication, 19 hanging wicks with increasing pattern of heights towards the centre of MBDSMWSS are arranged, the centre wick has the maximum height and wicks on E and W side have the minimum height. The horizontal gap between the two consecutive wicks is 10 cm. The jute and black cotton wick have dimension (strap–layer of the wick from one side to another side including dip portion in water) of 2.02 × 0.80 × 0.002 m3 and 2.02 × 0.80 × 0.001 m3, respectively. The height of wicks and their position from the centre on both side (E and W) of MBDSMWSS are given in Table 2. In MBDSMWSS 19 rods which support the layers of wick and maintain the gap between two successive wicks in order to proper feeding of water to the wicks. This type of wick arrangement leads to the continuous supply of water to the top surface of wick [26]. Figs. 3 and 4 show the photograph of MBDSMWSS using jute wick and black cotton wick, respectively. The jute cloth has wicking properties (or characteristics) like: (i) heat transfer coefficient (15.4 W/m2 °C); (ii) porosity (16.7%); (iii) capillary rise (10 mm/h); and (iv) absorbency (128 s) [20]. Similarly, the black cotton cloth has wicking properties like: (i) heat transfer

(2)

where KACRY = 0.2W/m ‐ K, KFRP = 0.351W/m ‐ K. The equivalent thickness of acrylic after calculation is obtained as LACRY = 2.85 mm ≈ 3 mm, and 3 mm thickness of acrylic sheet is locally available in the market (Allahabad (U.P.) local market). So, acrylic sheet of 3 mm thickness has been selected for the fabrication of MBDSMWSS. Two simple flat glass covers are fixed to the solar still frame at an inclination angle of 15° by window putty used as sealant. The dimension of glass covers are 1.03 × 1.03 × 0.004 m3 (1.03 m both in length and width, and 0.04 m in thickness). The angle of glass cover has been chosen as 15° for easy collection of condensed water by the action of cohesion, adhesion, gravity, and to avoid bulkiness [19,30,31]. Also, 15° inclination angle allows the height of the MBDSMWSS to be optimum at the centre since more height will cause problem in capillary action and the water will not be fed properly for effective evaporation. For the collection of water after condensation and avoid mixing of condensed water into feed water, troughs are provided on E, W, S walls. The troughs are kept at an inclination of 7 cm height at one end and 5 cm height at another end from the base to trickle the water outside for collection. One inlet pipe is connected through N–wall opening to fill the feed water into the basin of solar still. Four outlet pipes are connected to the storage tank to collect distillate. Among the four outlets, two outlet pipes are provided to collect the condensate of S–wall and one pipe each at E and W side to collect the condensate of east and west glass cover, respectively. The

Fig. 2. Photograph of MBDSMWSS showing condensation on south wall.

70

Desalination 422 (2017) 68–82

P. Pal et al.

incident into the basin and water through the glass covers and transparent acrylic walls and get absorbed by the water and basin. Due to transparent E and W walls, solar radiations start falling into the basin water during sunrise and sunset hours. In the conventional solar stills, which were previously used, due to opaque walls, there were no provision for taking the advantage of solar radiations during sunrise and sunset hours, until the tilted surface of solar stills exposed to the solar radiations, so that the incident solar radiation is absorbed by the basin and water [36]. In the modified design of solar still, water in the basin gets heated as well as the thin film of water starts evaporated from the wicking layers. Due to both heating and evaporation, the temperature of water vapour increases and with the increase of temperature, kinetic energy of water vapour also increases, which tends to increase the random motion of water vapour molecules. The random movement causes collision of the water vapour molecules with each other and with the inner surface of the walls, and glass covers of the solar still. An adhesive force is acts between the water vapour, walls, and glass covers. Due to adhesive force, water vapour sticks to the inner surface of walls and glass covers. After sticking, water vapour gets condensed on the inner surface of walls and glass covers by releasing its latent heat of vaporization. As the time passes in a day, more solar radiations are absorbed by the basin and the heat stored in the basin is transferred to the feed water by convection which increases the rate of evaporation of water. After condensation of water vapour at walls and glass covers, the more condensed droplets of water (in comparison to conventional solar stills) trickles down to the trough provided at E, W, and S wall. The troughs carried the condensate to the storage tank through the outlet pipes provided at E and W walls. This process removes microbiological organisms and impurities such as salts and heavy metals leading to the production of pure water. The hourly yield which is obtained from solar the still, the same (hourly yield) is added again into the solar still to maintain the constant level of feed water in the basin. After completing the entire process the fed water is replaced by removing the glass covers and cleaning the solar still at regular interval to remove algae, fungus and sediments from the basin of solar still.

Table 2 The wick heights and positions in MBDSMWSS. Wick position (east wall to centre with horizontal gap of 10 cm)

Wick height (cm) (including dip portion of the wick)

First wick Second wick Third wick Fourth wick Fifth wick Sixth wick Seventh wick Eighth wick Ninth wick Centre wick

10 13 16 19 22 25 28 31 34 37

Note: These values of the wick heights is also taken for the wick's position from West wall to centre of the MBDSMWSS.

coefficient (36 W/m2 °C); (ii) porosity (28.5%); (iii) capillary rise (120 mm/h); and (iv) absorbency (1 s) [20]. The jute fibre has chemical properties like: (i) jute fibres have weak resistant to alkalies; (ii) jute fibres are weakened and dissipated by acids (cellulose chains disintegrate due to hydrolysis in presence of acids); and (iii) moisture regain for the jute fibre is about 12.5% and 36% at 65% and 100% relative humidity (RH), respectively [32]. The cotton fibre has chemical properties like: (i) cotton fibre disintegrates in hot diluted and cold concentrated acid solutions; (ii) cotton fibre shows very good resistance to organic solvents and alkalies; (iii) cotton fibres degrade by long exposure to sunlight; and (iv) moisture regain for the cotton fibre is about 8.5% and 24% at 65% and 100% relative humidity (RH), respectively [33,34]. An acrylic has chemical properties like: (i) acrylic fibre have good resistance to chemical and biological agents and very less affected by organic solvents, weak acids, weak alkalies and oxidizing agents; (ii) acrylic fibre degrade by strong alkalies and shows good resistance to acids; and (iv) acrylic fibres have good resistant to sunlight [35]. 2.2. Working principle of MBDSMWSS

2.3. Modifications

Tap water is fed into the solar still through an opening provided on the N–wall. The water reaches the top of the wick's surface due to capillary action and a layer of water is formed on the layer of jute/black cotton wick, which is 5 cm below the glazing cover. Solar radiation

To augment the evaporation rate of multi–wick solar still, few modification has been tried in this work. Fig. 3. Photograph of MBDSMWSS using jute wick.

71

Desalination 422 (2017) 68–82

P. Pal et al.

Fig. 4. Photograph of MBDSMWSS using black cotton wick.

• In the conventional solar stills, and previous authors work [37,38],

•

• • •

glass was used which was prone to damage in transportation, installation, and during operation. Hence, a plastic material of low thermal conductivity which is flexible, and retains its high transparency for the longer duration under severe weather conditions can be suggested to be used in the fabrication of modified solar still. With this modifications, a modified solar still can have a long lifetime. In this presented work, a material (Acrylic) available in the market is taken for study purpose to analyze the performance of MBDSMWSS. The previously used solar still's wall were not transparent, although the inclined glass surface and solar stills were exposed to the solar radiation [6,8,10–12,20,31]. This amount of solar radiation was neglected in the calculation of efficiency of previous solar stills also. One can utilize this amount of thermal energy (solar energy) by making the E, W, and S walls transparent using an acrylic sheet. It allows the solar radiations to enter into the basin of solar still through transparent walls for the whole day length, thus increases the heat input and water temperature and so, increases the yield rate. Due to adhesion of vapour on the S–wall, the condensation also takes place on the inner surface of S–wall, thus improving the yield. The use of acrylic, which possesses high mechanical strength, transparency, insulating property, light in weight also results ‘no corrosion’, which was a problem associated with metals, which were used in the fabrication of previous solar stills [9,10,15,20,26,36,39]. In the presented solar still, no metal is used for fabrication. An acrylic is low density material in comparison to metals, which reduces the weight of the MBDSMWSS and facilitate to easy transportation, and installation at the site. The wick arrangement is installed with 19 rods holding the wicks together and making a strap layer on the rods close to the glass surface. The capillary action enables the water to reach the top of the wick's surface and a layer of water is formed on the wicks. So, the distance for water vapour to reach glass surface is reduced. Hence, more effective evaporation and condensation is takes place with obviously reduced vapour loss. The conventional solar stills had wicks one upon the other separated by some plastic liner, the proper feeding was a problem in these setups, which leads to dry patches in the wicks. So, the new design has the arrangement of

•

hanging wicks for proper feeding of water and high evaporation rates. The experimental setup also incorporates basin which was not used in the conventional multi–wick solar stills to store the thermal energy of received solar radiations.

In the previous setups of solar stills, which was installed in India, Rajaseenivasan et al. [10] constructed double slope single basin solar distiller with mild steel plate and thermocol was used as an insulation material. The thermal conductivity of mild steel is higher than FRP, so the losses from the basin and walls of the constructed solar still with mild steel plate are higher than the proposed solar still. The losses increases as the day proceeds and temperature inside the solar still increase. Though thermocol was used as an insulation which retards the loss of energy up to some limit, but it breaks down gradually when exposed to direct sunlight and it also not suitable for prolonged period at different temperatures range. Weather and corrosive nature of mild steel are also the constraints for proper working at different climatic conditions of India. Murugavel et al. [39] constructed the double slope solar distiller with mild steel plate. For the insulation, glass wool and thermocol were used on the outer layer of the solar still, and for insulation of basin interfaces, concrete used. To keep the productivity of the solar still same as that of presented solar still, a very high thickness of concrete insulation is needed. This will make the solar still more heavy (with mild steel plate) than the presented solar still (FRP insulated). So, the presented solar still come up with suggested modifications to increase the yield as well as the heat input through transparent walls to enhance the thermal efficiency of solar still for the climatic conditions of India. 3. Instantaneous and overall thermal efficiency of solar still 3.1. Instantaneous thermal efficiency Instantaneous thermal efficiency [30,40] is defined as the ratio of total heat lost by feed water due to condensation and total heat input to the solar still through E, W, and S walls and both glass covers due to solar radiation. The total input solar radiation (instantaneous) to the solar still 72

Desalination 422 (2017) 68–82

P. Pal et al. i = 24

through glass covers (E and W side) and transparent walls (E, W, and S wall) is given by the following equation:

∑ ηO =

I (t )input = IEg (t ) × AEg + IWg (t ) × AWg + IE (t ) × AE + IW (t ) × AW + IS (t ) × AS

ṁ ew × L vap I (t )input × 3600

× 100

4. Experimental procedure and instrumentation 4.1. Experimental procedure To start the system, before sunrise, the inside space of solar still is cleaned and filled with tap water at desired level and up to 5 cm of maximum depth. As solar radiation incident on the solar still, temperatures inside the solar still are raised. The distillation process drives by the difference in temperature of the condensing glass covers and feed water. The solar radiation, ambient temperature, basin temperature and temperatures of whole desalination system are recorded every hour and taken for 24 h from sunrise to next day sunrise. Also, the total amount of yield in scaled cylindrical tank is measured at an hourly interval for 24 h. All the experiments were conducted from August 2015 to July 2016. The experiments are carried out in a typical clear day under the climatic conditions of Allahabad, U.P., India. All the observations were taken for 24 h from 07:00 a.m. to 06:00 a.m. on next day morning.

(5)

(6)

The total daily yield of the MBDSMWSS is given by:

4.2. Instrumentation and error analysis

i = 24

Ṁ ew =

∑ ṁ ew

(7)

i=1

To analyze the experimental data–climatic and operational parameters, the accurate and precise instrumentation is required for monitoring the performance of desalination system. For measuring the temperatures of different locations in the solar still, calibrated Copper–Constantan type T thermocouples, integrated with temperature indicator (auto temperature scanner) with selected channel and holding switch are used. Thermocouples are fixed at the following locations: inner and outer walls of the solar stills, basin, inner glass surface, and one thermocouple for basin water temperature. The outer glass surface temperatures are measured using FLUKE (Model 62 max) infrared thermometer. The outside wall temperatures are also measured by FLUKE infrared thermometer. To measure solar radiation, AMPROBE (Model SOLAR–100) Solar Power Meter (solarimeter) is used. The ambient and water temperatures are measured using mercury–in–glass thermometer. The yield (distilled water) obtained is measured by collecting the condensate in a four similar scaled cylindrical tank of a least count 2 ml of each cylindrical tank. The minimum error (uncertainty) occurred in a measuring instrument is equal to the ratio between instrument least count and minimum value of the output measured by that equipment [42]. Using this definition, the error analysis for various measuring instruments used in the experiments is shown in Table 3.

The fraction of total input solar radiation (instantaneous) on both the glass covers comparing to total input solar radiation (I(t)input) is given by:

F1 =

IEg (t ) × AEg + IWg (t ) × AWg I (t )input

(8)

The Fraction of total input solar radiation (instantaneous) on walls (E, W, and S wall) with respect to total input solar radiation (I(t)input) is given by:

F2 =

I (t )input − (IEg (t ) × AEg + IWg (t ) × AWg ) I (t )input

(9)

Here, the solar radiations incident on glass covers and on E, W, and S walls i.e. the sum of all the input solar radiations (I(t)input) have zero values for off sunshine hour. In conventional solar stills with opaque walls, the total yield obtained and the total heat input to the solar still are considered through the two glass covers. But, in the presented design of solar still, the total yield obtained from the two glass covers and S wall i.e. ṁ ew (Eq. (5)) and the total heat input to the solar still through the glass covers (E and W side) and the transparent E, W, and S walls (Eq. (3)) are considered. Hence, there is a modification in the formulation of instantaneous thermal efficiency of the MBDSMWSS. In modified design of solar still, the net heat input increases due to three transparent walls as well as yield produced leading to the overall increase in instantaneous thermal efficiency.

5. Economic analysis The utility of MBDSMWSS depends upon the cost of production of the distilled water and its applicability. An economic analysis provides an alternative source to improve the performance of the still from the Table 3 Accuracies and error limits for various measuring instruments.

3.2. Overall thermal efficiency The overall thermal efficiency of passive solar still is given by Tiwari et al. [41] is as follows:

ηO =

(11)

(4)

Mathematically, instantaneous thermal efficiency can be expressed as equation:

ηi =

× 100

Here, the solar radiations (I(t)) and (I(t)input) appearing in the denominator has zero value for off sunshine hour.

In the present experimental setup, the height of E and W walls have been taken 12 cm only, hence, because of space constraint, troughs on the inner surface of the E and W walls for yield collection have not provided. Although for large water capacity of solar stills, heights of all the walls can be increased, which provides space for the troughs to be placed at E and W sides also. Due to this reason, the hourly yield takes the following form:

ṁ ew = ṁ eS + ṁ eEg + ṁ eWg

∫ I (t )input × dt

(3)

The hourly yield is given by the following equation:

ṁ ew = ṁ eE + ṁ eW + ṁ eS + ṁ eEg + ṁ eWg

m˙ ew × L vap

i=1

∑ ṁ ew × L vap ∑ (I (t ) × ASS × 3600)

(10)

The overall thermal efficiency of the MBDSMWSS can be written as: 73

Sl. no.

Instrument

Accuracy

Range

% error

1 2 3 4 5

Thermocouple Solarimeter IR thermometer Mercury in glass thermometer Scaled cylindrical tank

± 1 °C ± 1 W/m2 ± 1.5 °C ± 1 °C ± 2 ml

− 40:350 °C 0–1999 W/m2 − 30:500 °C − 10:110 °C 0–250 ml

3.57 0.38 0.80 5.88 10

Desalination 422 (2017) 68–82

P. Pal et al.

cost incurred in the system, where initial capital investment, annual salvage value, and annual maintenance cost are used for estimating the total annual cost [43]. For the present economic analysis of the solar still, the useful life (n) of each component of the solar still has been considered as 15 years. The total cost of the still after fabrication is taken as capital cost (Pcost). The salvage value (SV) and interest rate (ir) have been taken 15% of the capital cost and 12%, respectively. The annual maintenance cost (AMC) has been assumed to be 15% of the annual first cost (AFC), which includes regular filling of ordinary tap water, cleaning of glass cover, collection of distilled water, salt and scales removal, wick replacement (or proper arrangement), and maintenance of sealant. The average daily yield of the MBDSMWSS with black cotton and jute wicks are determined as 4.0 kg/m2 and 3.75 kg/m2, respectively. For the calculation of annual cost of distilled water per kg or kWh, two cases have been assumed. In case (a): all days i.e. 365 days and case (b): 300 days in a year (65 days assumed as rainy or cloudy days), is considered as clear days in a year. The annual first cost (AFC) of the MBDSMWSS is calculated as:

Fig. 5. Monthly variation of global solar radiation for a typical day in different months at Allahabad, U.P., India.

(12)

AFC = CRF × Pcost where

CRF (Capital Recovery Factor) =

ir × (1 + ir )n (1 + ir )n − 1

hours in summer season is longer compared to winter due to the inclination of earth's axis of rotation. In the morning and evening hours, the amount of diffuse radiations in the environment is higher, as the day proceeds the amount of direct solar radiations in the environment increases. The direct solar radiations enter into the solar still only if the glass covers and walls (E, W, and S wall) are receiving the direct sun rays. The diffuse radiations enter into the solar still even when the glass covers and walls are not facing the sun. The absorbed radiation raises the temperature of glass and walls, and the glass and walls then transmit this heat to the outside environment. Fig. 6 shows the hourly ambient temperature (°C) with respect to time (h) for a typical clear day of each month. It is observed from the figure, in the month of April 2016; May 2016; and June 2016, the ambient temperature was higher and lower in the month of December 2015; and January 2016. The maximum ambient temperature was 47 °C at 13:00 h in May 2016. Fig. 7 shows the hourly variation of water temperature (°C) with respect to time (h) for a typical clear day of each month. The feed water temperature is an important variable in the desalination process. The higher thermal energy requirements of the solar system to offset the

(13)

The annual salvage value (ASV) of the still is calculated as: (14)

ASV = SFF × SV where

SFF (Sink Fund Factor) =

ir (1 + ir )n − 1

(15)

The total annual cost of the still (TAC) is determined by:

TAC = AFC + AMC − ASV

(16)

Annual yield (AY) of the still is calculated by:

AY = Average daily yield × Number of clear days in a year

(17)

Then, the annual cost of distilled water (ACDW) per kg is obtained by dividing the TAC byAY. Annual useful energy (AUE) is estimated as:

AUE = AY × L vap

(18)

where Lvap value in kWh/kg is taken as 0.627 kWh/kg. Then, the annual cost of distilled water (ACDW) per kWh is obtained by dividing the TAC byAUE. 6. Results and discussion The following is the description of result and discussion. The data observed during the experiment was calibrated and used for the analysis. Fig. 5 shows the monthly variations of the global solar radiation (W/m2) with respect to time (h) for a typical clear day of each month. It is observed that the solar radiation starts falling on solar still at the beginning of sunrise by the small quantity and increases gradually to a maximum value, and then decreases till the end of the sunshine. The maximum measured solar radiations were 1150 W/m2 in May 2016 at 11:00 h and 720 W/m2 in December 2015 at 13:00 h. Based on the latitude of Allahabad (U.P.) (25°27′ N), the intensity of solar radiation in summer season is extensively higher than winter season because the sun position in the sky during the summer season is nearly overhead, but during the winter season, the rays of the sun are far more tilted. From the figure, it is clear that the maximum solar radiation received around 11:00–12:00 h in summer while the maximum solar radiation received in winter was around 12:00–13:00 h. since, in the northern hemisphere where Allahabad (U.P.) is located, the duration of sunshine

Fig. 6. Hourly ambient temperature for a typical day in different months at Allahabad, U.P., India.

74

Desalination 422 (2017) 68–82

P. Pal et al.

Fig. 9(a). Total incident solar radiation (instantaneous) for a typical day in the month of December 2015. Fig. 7. Hourly variation of water temperature for a typical day in different months at Allahabad, U.P., India.

latent and sensible heat loads for evaporation is claimed by higher temperature of feed water. Whereas, lower temperature of feed water results in smaller solar still size, lower scaling rates and lesser start–up time. The highest water temperature was in May 2016. The maximum measured water temperature was 67 °C at 14:00 h. Fig. 8 shows the hourly variation of yield (ml) with respect to time (h) for a typical clear day of each month. With the increment in the temperature difference between condensing glass covers and feed water, the rate of an evaporation–condensation process also increases. Furthermore, temperature gradient existing between the solar still system and ambient environment also increases the heat losses from the solar still. In MBDSMWSS, feed water is heated rapidly taking very less start–up time to reaches a temperature that is adequate to evaporate because of more heat gain through transparent acrylic walls. Then, the water vapour originated during the process is converted to distillate water by continuous condensation on the glass covers as well as on the walls of the solar still. The maximum yield was 1118 ml at 13:00 h in May 2016 and highest total yield measured was 9012 ml/day (4.50 l/ m2 day) in May 2016. Fig. 9(a) shows the total incident solar radiation (W), (i.e. energy

input) with respect to time (h) for a typical day in the month of December 2015. The maximum incident solar radiation (instantaneous) was 1873 W for 2 m2 area at 12:00 h. The total input solar energy (for 2 m2 area of basin) increases as the amount of solar radiations incident on MBDSMWSS increases till 12:00 h and then decreases due to sun's position in sky and latitude of Allahabad (U.P.). Using Eq. (8), Fig. 9(b) shows the fraction (F1) of total input solar radiation (instantaneous) on both the glass covers with respect to time (h) for a typical day in the month of December 2015. Based on the latitude, location, local climate of Allahabad (U.P.) and orientation (E–W) of solar still, it is depicted in figure, most of the fraction (F1) of total input solar radiation (range 65–80%, avg. 74%) were falling on the glass covers, only a fewer percentage (range 6–8%, avg. 6%) of total input solar radiation was falling on the E and W walls. It will increase when the area or height of E and W walls increases but increment in the height of E and W walls increases the size and material requirements of solar still. After neglecting the total input solar radiation on E and W walls, most of the portion of wall's input solar radiation was falling on S wall. About 20–34% (avg. 26%) of total input solar radiations were incident on the walls of solar still, in which about 17–26% (avg. 20%) of input solar radiation was incident on the S wall alone. From the figure, it is also shown that, the amount of the fraction of total input solar radiation (instantaneous) which was incident on the glass covers and walls were higher in morning and evening hours because the amount of diffuse radiations in the environment was higher in morning and evening hours, as the day proceeds the direct solar radiations

Fig. 8. Hourly variation of yield for a typical day in different months at Allahabad, U.P., India.

Fig. 9(b). Fraction of total input solar radiation (instantaneous) on both the glass covers for a typical day in the month of December 2015.

75

Desalination 422 (2017) 68–82

P. Pal et al.

Fig. 9(c). Fraction of total input solar radiation (instantaneous) on walls (east, west, and south wall) for a typical day in the month of December 2015.

Fig. 10(b). Fraction of total input solar radiation (instantaneous) on both the glass covers for a typical day in the month of April 2016.

incident on the glass covers and walls increases (diffuse radiation decreases), from 08:00 h–14:00 h, this fraction increases and reaches to maximum value of 79.2% of total input solar radiations (instantaneous). After 14:00 h, this fraction decreases due to lesser amount of direct solar radiations incident on the glass covers and walls. After 16:00 h, again this fraction increases due to increment of diffuse radiation in the environment. Using Eq. (9), the fraction (F2) of total input solar radiation (instantaneous) on walls (E, W, and S wall) with respect to time (h) is shown in Fig. 9(c). Similarly, for the typical day in the month of April 2016, the total incident solar radiation (W), (i.e. energy input), fraction (F1) of total input solar radiation (instantaneous) on both the glass covers (Eq. (8)), and fraction (F2) of total input solar radiation (instantaneous) on walls (Eq. (9)) with respect to time (h) are shown in Figs. 10(a), 10(b), and 10(c), respectively. For April 2016, the maximum incident solar radiation (instantaneous) was 2658 W for 2 m2 area at 11:00 h. The fraction (F1) of total input solar radiation (instantaneous) on glass covers range from 80–89% (avg. 84%). This fraction increases in the summer season because of the sun's overhead position i.e. sun approaches to the zenith angle. About 2–10% (avg. 7%) of total input solar radiation was incident on the E and W walls. About 11–21% (avg. 16%) of total input solar radiations were incident on the walls of solar still, in which about 5–12% (avg. 9%) of input solar radiation was incident on the S wall alone. Fig. 11 shows the determination of range of low variation of solar

Fig. 10(c). Fraction of total input solar radiation (instantaneous) on walls (east, west, and south wall) for a typical day in the month of April 2016.

radiation (W/m2) with respect to time (h) for a typical day at Allahabad, (U.P.), India. It has been observe from the figure that, some of the values of instantaneous thermal efficiencies have also been found like either ηi > 1 or ηi < 0 (before 08:00 h and after 16:00 h) when experimental or theoretical data has been used and hence, these efficiency values have not been taken for obtaining the characteristic curves for analyzing the performance of MBDSMWSS and termed as unrealistic results [11–13,40]. These unrealistic results have been obtained because of two reasons: (i) low solar intensity in the morning and evening hours and (ii) cooling of water takes place in off–sunshine hour. Because of these reasons and from the point of view of accuracy, and to analyze the solar still, the time period between 10:00 h–14:00 h has been chosen to plot the characteristic curves as it is the high solar intensity period (the solar still absorbed the maximum amount of direct solar radiations and quantity of diffuse radiation in the environment is less) and also the variation of incident solar radiation remain closer to the average value of solar radiation of this period. In this time solar still also works under quasi–steady state condition which gives realistic results of ηi [11–13,40]. The variation of obtained values of instantaneous thermal efficiency (y = ηi) of MBDSMWSS with respect to x = (Tw − Ta)/I(t)input has been plotted (i.e. characteristic curve). With the help of characteristic curve linear and non–linear characteristic equations (with coefficient of correlation R2) has been obtained. It was noted from the figure that the ηi curve was increasing linearly with increase in x. It is also noticed that the slope of the linear characteristic curves was positive due to higher

Fig. 10(a). Total incident solar radiation (instantaneous) for a typical day in the month of April 2016.

76

Desalination 422 (2017) 68–82

P. Pal et al.

Fig. 11. Determination of range of low variation of solar radiation for any clear day at Allahabad, U.P., India.

for the climatic condition of Allahabad (U.P.), more solar energy input (solar radiation) was available through its walls to heat the basin and raised the temperature of water significantly. So, the wide range of x was available in time period 09:00 h–15:00 h for experimental data under quasi–steady state. In the summer season the time period 09:00 h–15:00 is more suitable for characterization of the solar still for the Allahabad (U.P.) climate. In the winter season, because of short day–length, fog present in the environment, lower temperature in the morning and evening hours, and late receiving of direct solar radiation, the time period 10:00 h–14:00 h is more suitable for analysis. Further, for the purpose of precise analysis of MBDSMWSS, two graphs have been plotted, one with time duration 10:00 h–14:00 h (Fig. 12(a)), and other in 09:00 h–15:00 h (Fig. 12(b)). In Fig. 12(a) the coefficient of correlation were 0.9517 and 0.9779 for the linear and non–linear characteristic curve, respectively and in Fig. 12(b) the coefficient of correlation were 0.8909 and 0.9747 for the corresponding linear and non–linear curves. One can observe that both the non–linear curves have similar R2 values (but the linear curves have more variation in R2 value) and one can conclude that the non–linear curves are found to be more accurate than that of linear curves because of non–linear behavior and variability in the climatic parameters such as solar radiation, wind velocity, ambient temperature, etc. Hence, non–linear characteristic curves are recommended to predict the performance of MBDSMWSS more precisely. Fig. 13 shows the hourly variation of instantaneous thermal efficiency, ηi (%) with respect to time (h) for a typical day in different

Fig. 12(a). Linear and non–linear efficiency curves for the MBDSMWSS for a typical day in the month of March 2016 (duration from 10:00 h–14:00 h) in Allahabad, U.P., India.

Fig. 12(b). Linear and non–linear efficiency curves for the MBDSMWSS for a typical day in the month of March 2016 (duration from 09:00 h–15:00 h) in Allahabad, U.P., India.

evaporative heat transfer between water (evaporating) surface and glass (condensing) cover. Similar kind of observations has been obtained in previous studies [11–13], in which time period 10:00 h–14:00 was taken for study because in this time period the solar still is work under quasi–steady state and curve has less error. But in MBDSMWSS

Fig. 13. Hourly variation of instantaneous thermal efficiency for a typical day in different months at Allahabad, U.P., India.

77

Desalination 422 (2017) 68–82

P. Pal et al.

Table 4 Hourly observations of various parameters for MBDSMWSS at 1 cm water depth on December 17, 2015. Time (h)

07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00 01:00 02:00 03:00 04:00 05:00 06:00

Temperature (°C)

Solar radiation (W/m2)

Hourly yield (ml)

Tb

Tw

Ta

ṁ eEg

ṁ eWg

ṁ eS

IG(t)

ID(t)

IEg(t)

IWg(t)

IE(t)

IW(t)

IS(t)

16 18 20 26 31 35 40 37 33 30 27 26 24 23 22 21 20 19 18 18 17 16 16 15

15 17 19 24 28 33 37 35 32 29 25 25 23 21 20 20 19 18 17 16 15 15 15 14

16 20 23 25 26 27 28 27 26 25 23 21 20 19 19 18 18 17 17 17 16 16 15 17

0 0 44 89 190 290 330 287 232 174 126 102 84 54 38 30 22 15 10 8 4 2 0 0

0 0 23 50 178 268 340 321 282 227 187 135 112 76 56 44 32 21 20 14 8 5 2 0

0 0 10 23 47 65 73 61 52 44 36 25 18 14 10 4 2 1 0 0 0 0 0 0

36 200 382 487 652 690 720 490 300 155 23 0 0 0 0 0 0 0 0 0 0 0 0 21

22 95 160 193 218 250 226 180 140 90 16 0 0 0 0 0 0 0 0 0 0 0 0 14

40 187 356 531 625 632 500 350 223 105 15 0 0 0 0 0 0 0 0 0 0 0 0 16

38 160 318 440 595 676 640 520 340 229 19 0 0 0 0 0 0 0 0 0 0 0 0 19

46 280 455 535 400 212 180 120 74 45 9 0 0 0 0 0 0 0 0 0 0 0 0 20

10 60 110 145 175 225 312 450 375 250 17 0 0 0 0 0 0 0 0 0 0 0 0 9

30 220 415 553 665 700 440 280 236 194 10 0 0 0 0 0 0 0 0 0 0 0 0 14

Total hourly yield (ml) ∑ ṁ eEg = 2131, ∑ ṁ eWg = 2401, ∑ ṁ eS = 485. Total daily yield of the solar still (ml/day) is sum of all the total hourly yield (∑ ṁ eEg + ∑ ṁ eWg + ∑ ṁ eS ) = 5017 ml/day.

increase of 11.94% in yield when the depth of feed water in basin decreases from 2 cm to 1 cm in MBDSMWSS. Showing the effects of the wicks on the performance of the MBDSMWSS, experiments were performed on the solar still with jute and black cotton cloth used as a wick on March 17, 2016 and March 25, 2016, respectively. In both the typical days the depth of feed water in solar still was 2 cm. From Fig. 11, it is observed that there was slight variation in the average solar intensity on March 17 and March 25, 2016. Hence, the comparison can be illustrated by assuming the same amount of solar radiations on modified solar still. The yield obtained with black cotton wick was greater than yield obtained with jute wick. This is because of more solar radiation absorbing power of black cotton wick than jute wick. Due to capillary action, a thin layer of water is formed at top of the wicks. The water layer in the black cotton wick gets evaporated at much faster rate than the water evaporated from the jute wick. Thus, yield obtained with black cotton wick is more than the jute wick. The total yield obtained in modified solar still with black cotton and jute wicks were 7740 ml/day (3.87 l/m2 day) and 7040 ml/day (3.52 l/m2 day), respectively. Thus, an increase of 9.04% in yield produced from MBDSMWSS, when black cotton wick was used in place of a jute wick. Also, the overall thermal efficiency of MBDSMWSS with jute and black cotton wicks were 20.94% and 23.03%, respectively. The dates of experiments for the basin conditions with both the wicks and maximum daily yield are given in Table 6. The tap water was directly filled to the solar still for experimentation and its analysis after distillation. The quality of feed water and final distillate for MBDSMWSS with jute wick and black cotton wick are noted and tabulated in Tables 7–8, and further compared with EPA standards for potable water. The distilled water after distillation process have clean taste, bacteria free, odourless, and eliminating harmful chemicals. It can be used in various chemical processed industries, scientific and research laboratories, batteries, hospitals or private clinics etc. The pH of distillate in both the cases was lower compared to feed water and under permissible limit of EPA standard. The lower value of pH signifies the removal of hydroxide ions (OH−) from the

months at Allahabad, (U.P.), India. It is seen that the maximum instantaneous thermal efficiencies were achieved in evening hours (16:00 h and after on). This is due to the fact that, at evening hours solar radiation is less but due to heat stored in the basin water, evaporation continues and yield is obtained. So, due to low heat input but good yield output, instantaneous thermal efficiency is maximum in the evening time. In the summer season, the heat input to solar still was high as solar still absorbed the maximum amount of direct solar radiation for longer duration. Along with heat input, the yield obtained from the solar still in large quantity during 12:00 h–14:00 h. So, the variation in the instantaneous thermal efficiency was less in that period. But during the winter season, the variation in instantaneous thermal efficiency after noon (12:00 h) was high, and in the evening hours, solar still have maximum efficiency. The maximum instantaneous thermal efficiency achieved was 54.80% at 16:00 h in December 2015. Tables 4–5 show the details of solar radiations, ambient, basin, feed water temperatures, and hourly yields for two typical days in the month of December 2015, i.e. December 17, 2015 and December 20, 2015. One can observe that there was little variation in the global solar radiation on December 17, 2015 and December 20, 2015. So, one can show the comparison of effect of different depths of feed water in the basin. The depth of feed water in the solar still basin was 1 cm and 2 cm on December 17, 2015 and December 20, 2015, respectively. The yield collected in the solar sill at 1 cm water depth was greater than the 2 cm water depth. This is because of the fact that, 1 cm water depth has less volume of water in comparison to 2 cm water depth. So, lower water depth implies lower heat capacity of water which results in higher water temperature in the basin. Hence, 1 cm water depth evaporated early in comparison to 2 cm water depth. Also, due to more heat capacity of 2 cm water depth, the distillate of solar still at 2 cm water depth increases in the night in comparison to distillate obtained at 1 cm water depth. The maximum yield obtained was 743 ml at 13:00 h in December 17, 2015 and 688 ml at 13:00 h in December 20, 2015 corresponding to 1 cm and 2 cm water depth, respectively. The total yield obtained at 1 cm and 2 cm water depth were 5017 ml/day (2.50 l/ m2 day) and 4418 ml/day (2.21 l/m2 day), respectively. Also, an 78

Desalination 422 (2017) 68–82

P. Pal et al.

Table 5 Hourly observations of various parameters for MBDSMWSS at 2 cm water depth on December 20, 2015. Time (h)

Temperature (°C)

07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 24:00 01:00 02:00 03:00 04:00 05:00 06:00

Solar radiation (W/m2)

Hourly yield (ml)

Tb

Tw

Ta

ṁ eEg

ṁ eWg

ṁ eS

IG(t)

ID(t)

IEg(t)

IWg(t)

IE(t)

IW(t)

IS(t)

15 17 19 24 30 33 38 36 33 28 25 24 23 21 21 20 18 17 16 17 16 15 14 13

14 16 18 23 27 31 35 34 31 27 25 24 22 20 20 18 17 17 16 15 14 13 12 12

15 17 21 23 25 26 27 27 25 23 22 19 18 17 17 16 15 14 13 12 12 11 11 12

0 0 38 80 167 265 302 254 214 156 109 88 75 43 30 26 18 12 7 4 2 0 0 0

0 0 18 38 161 244 321 300 262 206 160 124 89 57 41 35 25 16 12 8 4 2 1 0

0 0 6 18 38 54 65 54 42 36 28 22 15 11 7 4 2 1 1 0 0 0 0 0

30 184 348 435 618 656 695 471 258 141 17 0 0 0 0 0 0 0 0 0 0 0 0 18

24 112 165 178 201 240 207 169 130 118 15 0 0 0 0 0 0 0 0 0 0 0 0 20

36 187 344 514 598 618 488 324 209 98 11 0 0 0 0 0 0 0 0 0 0 0 0 14

34 147 294 425 564 658 628 500 322 220 17 0 0 0 0 0 0 0 0 0 0 0 0 15

40 268 440 512 388 197 169 105 67 38 7 0 0 0 0 0 0 0 0 0 0 0 0 17

8 52 102 130 164 218 290 442 356 238 12 0 0 0 0 0 0 0 0 0 0 0 0 5

24 205 395 539 641 673 426 270 224 187 7 0 0 0 0 0 0 0 0 0 0 0 0 11

Total hourly yield (ml) ∑ ṁ eEg = 1890, ∑ ṁ eWg = 2124, ∑ ṁ eS = 404. Total daily yield of the solar still (ml/day) is sum of all the total hourly yield (∑ ṁ eEg + ∑ ṁ eWg + ∑ ṁ eS ) = 4418 ml/day.

feed water. Turbidity values were found to be under permissible limit with both wicks. The obtained values of turbidity are indicator of water clarity and elimination of suspended matters, which are organic and inorganic substances such as sediments, algae, and other contaminants. TDS value after distillation was found to be below standard limit with both jute and black cotton wick, which represents the effective removal of organic and inorganic elements. The obtained value of electrical conductivity signifies the effective removal of dissolved inorganic solids from the feed water. The total hardness as CaCO3 obtained with both the wicks was under EPA standard. All the values obtained after solar distillation with both the wicks were under permissible limits of EPA [44] and WHO standards [45] for drinking water quality, which indicates the MBDSMWSS is effective and feasible in climatic condition of Allahabad, U.P., India. The result of cost analysis shows that, the annual cost of the distilled water (ACDW) production per kg for case (a) was found to be Indian Rs. 1.51 (0.023 US$), and Rs. 1.59 (0.024 US$); and ACDW per kWh was found to be Rs. 2.41 (0.037 US$), and Rs. 2.54 (0.039 US$); and for case (b) ACDW per kg was found to be Rs. 1.84 (0.028 US$), and Rs. 1.94 (0.030 US$); and ACDW per kWh was found to be Rs. 2.93 (0.045 US$), and Rs. 3.09 (0.047 US$) for the MBDSMWSS with black cotton and jute wicks, respectively. In present, average market cost of distilled water (MCDW) is assumed as Rs. 10/kg (0.155 US$/kg). The net profit for case (a) was found to be Rs. 12,431.89 (192.57 US$), and Rs. 11,566.81 (179.17 US$); and for case (b) the net profit was found to be Rs. 9830.85 (152.28 US$), and Rs. 9071.17 (140.51 US$) for the MBDSMWSS with black cotton and jute wicks, respectively. The payback period for case (a) was found to be 386 days and 410 days; and for case (b) payback period was found to be 401 days and 430 days with black cotton and jute wicks, respectively. The total fabrication cost of the MBDSMWSS components is given in Table 9. The economic analysis of MBDSMWSS is presented in Table 10. In the present study, jute and black cotton cloth were used as wick materials. The cost percentage was only 1.92% (≈ 2%) and 3.04% of total cost of the system for jute and black cotton cloth, respectively. In MBDSMWSS the jute and black cotton wicks have three to six months of

Table 6 Date of the experiments for basin conditions with jute and black cotton wicks and maximum daily yield. Sr. no.

Date of the experiments

Basin condition

1

December 17, 2015

2

December 20, 2015

2

March 17, 2016

3

March 25, 2016

4

May 16, 2016

1 cm water depth, wick 2 cm water depth, wick 2 cm water depth, wick 2 cm water depth, cotton wick 2 cm water depth, cotton wick

Total daily yield (ml/day) jute

5017

jute

4418

jute

7040

black

7740

black

9012

Table 7 Quality parameters analysis of distillate from MBDSMWSS with jute wick (March 17, 2016). Parameters

Feed water sample

Distillate

EPA std.

pH Turbidity (NTU) Total dissolved solids (mg/l) Electrical conductivity (μS/cm) Total hardness as CaCO3

8.1 4 352 766 224

7.3 1 46 39 48

6.5–8.5 5 500 1000 200

Table 8 Quality parameters analysis of distillate from MBDSMWSS with black cotton wick (March 25, 2016). Parameters

Feed water sample

Distillate

EPA std.

pH Turbidity (NTU) Total dissolved solids (mg/l) Electrical conductivity (μS/cm) Total hardness as CaCO3

8.4 5 398 745 238

7.2 1 40 37 44

6.5–8.5 5 500 1000 200

79

Desalination 422 (2017) 68–82

P. Pal et al.

two MBDSMWSS was unimportant (costly to stock the jute and cotton cloths). Hence, fresh wick is required during replacement. In the case of requirement of a fresh wick, if there is a delay in the replacement because of unavailability of jute or cotton cloth in the market, the MBDSMWSS can work without wick as a modified basin type double slope solar still which can provide the potable water with lesser amount. Hence, there is no urgency of replacing and keeping the stock of wicks. The performance of previous authors work is given in Table 11. The maximum amount of productivity was found 3.58 l/day for double basin double slope solar still during the months of March–May 2012 in Tamil Nadu, India [10]. The productivity for presented solar still was 7.040 l/day (jute wick), 7.939 l/day (black cotton wick), and maximum 9.012 l/day (black cotton wick) for a typical day in the month of March 2016; April 2016; and May 2016, respectively. When compared in terms of productivity only, and taking 1610 ml/day quantity of yield as reference for comparison, there were 77.13% (jute wick), 79.72% (black cotton wick), and 82.14% (black cotton wick) increase in yield for a typical days in March, April, and May 2016, respectively for the presented solar still. Hansen et al. [20] achieved the maximum distillate in the inclined type solar still was 4.28 l/day by using water coral fleece with weir mesh–stepped absorber plate. Elango et al. [37] found maximum yield of 5327 l/m2 day at 1 cm water depth by the insulated double basin double slope glass still. Dwivedi and Tiwari [46] achieved the maximum daily yield in the double slope passive solar still was 2.27 kg/m2 day (4540 kg/day) in the month of April 2006. Tiwari and Selim [47] found maximum yield was 8750 ml/day in double slope FRP multiwick solar still for a typical day in May 1984. Though, the yield was higher in that still, but the body of the solar still was made up of FRP (FRP mould was constructed with the help of a wooden die). The cost of FRP sheet per m2 in present market is much higher than old (1984) market price. So, the solar still is not economical in present scenario due to high investment in FRP sheet, wooden die and mould preparation. In these stated [10,20,37,47] research work, authors mainly focused on productivity of distillate obtained, authors not discussed about the overall thermal and instantaneous thermal efficiency of solar still. In the presented solar still, maximum instantaneous thermal efficiency and overall thermal efficiency were 54.80% and 28.27% for a typical day in December 2015 and May 2016, respectively.

Table 9 Fabrication cost of the MBDSMWSS components. Components

Quantity

Cost/unit (Rs.)

Costa

FRP sheet (0.005 m thick)

2.65 m2

3451/m2

Acrylic sheet (0.003 m thick) Glass cover (0.004 m thick)

1.0 m2 2 pieces

600/m2 500/piece

Iron stand Outlet nozzle Black paint Silicon rubber (gaskets) Glass putty Fabrication cost, labor cost

8 kg 0.1 kg 0.5 kg 9.0 m 2.5 kg Lump sum

60/kg 500/kg 180/kg 35/m 20/kg

Jute cloth (for jute wick) Black cotton cloth (for black cotton wick) Total cost of the still (with jute wick) Total cost of the still (with black cotton wick)

6.25 m2 6.25 m2

40/m2 64/m2

Rs. 9146 (141.67 US$) Rs. 600 (9.29 US$) Rs. 1000 (15.5 US $) Rs. 480 (7.43 US$) Rs. 50 (0.77 US$) Rs. 90 (1.39 US$) Rs. 315 (4.88 US$) Rs. 50 (0.77 US$) Rs. 1000 (15.5 US $) Rs. 250 (3.87 US$) Rs. 400 (6.2 US$) Rs. 12,981 (201.08 US$) Rs. 13,131 (203.4 US$)

a 1 US$ = Indian Rs. 64.52 on 27/05/2017, (costs are based on Allahabad (U.P.) market rate).

Table 10 Economic analysis of the MBDSMWSS. Cost types

MBDSMWSS Jute wick

Black cotton wick

Value Total cost of the still (Pcost) Salvage value (SV) Annual salvage value (ASV) Annual first cost (AFC) Annual maintenance cost (AMC) Total annual cost (TAC) Case (a): 365 days operation Annual yield (AY) Annual useful energy (AUE) Annual cost of distilled water (ACDW) Annual cost of distilled water (ACDW) Net profit (NP) Payback period (PP) Case (b): 300 days operation Annual yield (AY) Annual useful energy (AUE) Annual cost of distilled water (ACDW) Annual cost of distilled water (ACDW) Net profit (NP) Payback period (PP)

201.08 30.16 0.90 30.16 4.52 33.78

203.4 30.51 0.92 30.51 4.57 34.17

US$ US$ US$/year US$/year US$/year US$/year

1368.75 858.20 0.024 0.039 179.17 410

1460 915.42 0.023 0.037 192.57 386

kg/year kWh/year US$/kg US$/kWh US$/year Days

1125 705.37 0.030 0.047 140.51 430

1200 752.40 0.028 0.045 152.28 401

kg/year kWh/year US$/kg US$/kWh US$/year Days

7. Conclusions In this work, a modified basin type double slope multi–wick solar still have been designed, fabricated and analyzed its performance under the climate condition of Allahabad, (U.P.), India. The following conclusions can be drawn on the basis of this study:

• The MBDSMWSS, in which FRP and Acrylic were used for fabrica-

Note: NP = AY × (MCDW−ACDW);PP = (Pcost/NP) × solar still operation days per year.

satisfactory working life depending upon the feed water quality. The yearly recurring cost was only 3.78% (≈ 4%) and 5.91% (≈6%) of total cost of the system for jute and black cotton cloth, respectively which was very nominal cost. So, one can be neglecting the nominal recurring cost of the wicks with respect to the total cost of the system. Hence, in the present study one can consider only one or two replacement of jute or black cotton wicks in a year (depending upon the requirement if needed). Also, length of storage (stock), moisture content, amount of high–moisture foreign matter, variation in moisture content throughout the stored mass, temperature of the jute and cotton during stock, weather factors during stock like temperature; relative humidity; rainfall, and protection of the fibres from rain and wet ground all affect fibre quality and deteriorate wicks during storage [33]. So, it was very cumbersome and to maintain small stock of wick material for one or

• •

80

tion of the solar still in combination with wicks arrangement. Making the E, W, and S walls (acrylic) transparent in the solar still allows more solar radiation to impinge on the basin throughout day length. This increases the heat input and feed water temperature inside the solar still and so, increasing the temperature difference between condensing glass cover and water surface. Hence, with reduced vapour loss and increased temperature difference, more effective evaporation and condensation were obtained, thereby significantly improving the yield of the solar still. The yield was collected from the south (S) wall also due to condensation on the S wall's surface of the solar still. Therefore, the net yield of the solar still has been improved. The maximum measured solar radiation was 1150 W/m2 in May 2016. The average 84% and 74% of the fraction (F1) of total input solar radiation (instantaneous) incident on both the glass covers for a typical day in the month of April 2016 and December 2015, respectively. The average 9% and 20% of the fraction (F2) of total

Desalination 422 (2017) 68–82

P. Pal et al.

Table 11 Comparison in productivity for double slope solar still at different basin conditions by various authors. Sr. no.

Name of the paper and author

Basin condition

Total production

% increase in yield production

1

Comparative study of double basin and single basin solar stills–T. Rajaseenivasan, T. Elango, K.K. Murugavel [10]

2

Performance study on basin type double slope solar still with different wick materials and minimum mass of water–K.K. Murugavel, K. Srithar [39]

1610 ml/day 1775 ml/day 1850 ml/day 3.36 kg/day 3.49 kg/day 3.58 kg/day

Reference 9.29 12.97 52.08 53.86 55.02

3

Annual energy and exergy analysis of single and double slope passive solar stills–V.K. Dwivedi, G.N. Tiwari [46] Double slope fibre reinforced plastic (FRP) multiwick solar still–G.N. Tiwari, G.A.M. Selim [47] In presented work

2 cm water depth Jute cloth Black cotton cloth Jute cloth Black cotton cloth Aluminium rectangular fin length wise with cotton wick 1 cm water depth (April 2006)

4540 kg/day

64.54

Black jute cloth

8750 ml/day

81.60

2 cm water depth with jute wick 2 cm water depth with black cotton wick

7040 ml/day 9012 ml/day

77.13 82.14

4 5

•

•

• • •

ID(t) IE(t) IEg(t) IG(t) ir I(t) IS(t) IW(t) IWg(t) KACRY KFRP LACRY LFRP Lvap MCDW ṁ eE ṁ eEg ṁ eS ṁ ew Ṁ ew

input solar radiation (instantaneous) on walls were incident on the south wall for a typical day in April 2016 and December 2015, respectively. The yield was higher at the lower water depth, the total yield obtained in the typical days of December 2015 at 1 cm and 2 cm depth of feed water was 5017 ml/day (2.50 l/m2 day) and 4418 ml/day (2.21 l/m2 day), respectively. Thus, an increase of 11.94% in yield was obtained when the depth of feed water decreases from 2 cm to 1 cm in MBDSMWSS. The yield obtained in MBDSMWSS for a typical days in March 2016 with black cotton and jute wicks were 7740 ml/day (3.87 l/m2 day) and 7040 ml/day (3.52 l/m2 day), respectively at 2 cm water depth. Thus, an increase of 9.04% in yield obtained from presented solar still, when black cotton wick was used in place of a jute wick. So, the black cotton wick is better wick in comparison to jute wick. The maximum yield obtained was 9012 ml/day (4.50 l/m2 day) in May 2016 at 2 cm water depth with the black cotton wick. The maximum overall thermal efficiency of MBDSMWSS with jute and black cotton wicks were 20.94% and 28.27%, respectively. The maximum instantaneous thermal efficiency achieved was 54.80% in December 2015. The optimum annual cost of distilled water per kg and per kWh were Rs. 1.51 (0.023 US$) and Rs. 2.41 (0.037 US$), respectively with the black cotton wick, when the useful life and interest rate for the MBDSMWSS have been considered 15 years and 12%, respectively.

ṁ eW ṁ eWg n NP PP Pcost QCond SFF SV t TAC Tw

Nomenclature AACRY ACDW AE AEg AFC AFRP AMC AS ASS ASV AUE AW AWg AY CRF F1 F2

area of cross section of acrylic, m2 annual cost of distilled water, US$/kg or US$/kWh area of east wall, m2 area of east glass cover, m2 annual first cost, US$/year area of cross section of FRP, m2 annual maintenance cost, US$/year area of south wall, m2 area of solar still, m2 annual salvage value, US$/year annual useful energy, kWh/year area of west wall, m2 area of west glass cover, m2 annual yield, kg/year capital recovery factor, dimensionless fraction of total input solar radiation (instantaneous) on both the glass covers fraction of total input solar radiation (instantaneous) on walls (east, west, and south)

diffused solar radiation, W/m2 solar intensity incident on the east side of solar still, W/m2 solar intensity on the east glass cover, W/m2 global solar radiation, W/m2 interest rate, % solar radiation on solar still, W/m2 solar intensity incident on the south wall of solar still, W/m2 solar intensity incident on the west side of solar still, W/m2 solar intensity on the west glass cover, W/m2 thermal conductivity of acrylic, W/m-K thermal conductivity of FRP, W/m-K thickness of acrylic, m thickness of FRP, m latent heat of vaporization, J/kg market cost of distilled water, US$/kg hourly yield from the east wall of solar still, ml hourly yield from the east glass cover of solar still, ml hourly yield from the south wall of solar still, ml hourly yield of the solar still, ml total yield of the solar still, ml/day hourly yield from the west wall of solar still, ml hourly yield from the west glass cover of solar still, ml still useful life in years, y net profit, US$/year payback period, day capital cost of the still, US$ conduction heat transfer, W sink fund factor, dimensionless salvage value, US$ time, h total annual cost, US$/year water temperature, °C

Subscripts a b E g N S SS w W

81

ambient basin east glass cover North south solar still water west

Desalination 422 (2017) 68–82

P. Pal et al.

[21] A.K. Kaviti, A. Yadav, A. Shukla, Inclined solar still designs: a review, Renew. Sust. Energ. Rev. 54 (2016) 429–451. [22] D.D.W. Rufuss, S. Iniyan, L. Suganthi, P.A. Davies, Solar stills: a comprehensive review of designs, performance and material advances, Renew. Sust. Energ. Rev. 63 (2016) 464–496. [23] P.V. Kumar, A. Kumar, O. Prakash, A.K. Kaviti, Solar stills system design: a review, Renew. Sust. Energ. Rev. 51 (2015) 153–181. [24] S. Al-Kharabsheh, D.Y. Goswami, Theoretical analysis of a water desalination system using low grade solar heat, J. Solar Energy Eng. 126 (2004) 774–780. [25] A.K. Tiwari, G.N. Tiwari, Effect of cover inclination and water depth on performance of a solar still for Indian climatic conditions, J. Solar Energy Eng. 130 (2008) 1–4. [26] E. Deniz, An investigation of some of the parameters involved in inclined solar distillation systems, Environ. Prog. Sustain. Energy 32 (2) (2013) 350–354. [27] G.M. Ayoub, M. Al-Hindi, L. Malaeb, A solar still desalination system with enhanced productivity, Desalination Water Treat. 53 (2015) 3179–3186. [28] P. Kalita, A. Dewan, S. Borah, A review on recent developments in solar distillation units, Sadhana 41 (2016) 203–223. [29] S.W. Sharshir, N. Yang, G. Peng, A.E. Kabeel, Factors affecting solar stills productivity and improvement techniques: a detailed review, Appl. Therm. Eng. 100 (2016) 267–284. [30] R.V. Singh, R. Dev, M.M. Hasan, G.N. Tiwari, Comparative energy and exergy analysis of various passive solar distillation systems, Proceeding of World Renewable Energy Congress–2011, 14 Linkoping University, Sweden, 2011, pp. 3929–3936. [31] V.K. Dwivedi, G.N. Tiwari, Comparison of internal heat transfer coefficients in passive solar stills by different thermal models: an experimental validation, Desalination 246 (2009) 304–318. [32] http://nptel.ac.in/courses/116102026/9 (Last accessed on 25/05/2017). [33] S. Gordon, Y.L. Hsieh, Cotton: Science and Technology, Woodhead Publishing Ltd. and CRC press, 2007. [34] http://www.swicofil.com/products/001cotton.html (Last accessed on 25/05/ 2017). [35] http://nptel.ac.in/courses/116102026/40 (Last accessed on 25/05/2017). [36] B.A. Akash, M.S. Mohsen, W. Nayfeh, Experimental study of the basin type solar still under local climate conditions, Energy Convers. Manag. 41 (2000) 883–890. [37] T. Elango, K.K. Murugavel, The effect of the water depth on the productivity for single and double basin double slope glass solar stills, Desalination 359 (2015) 82–91. [38] T. Rajaseenivasan, A.P. Tinnokesh, G.R. Kumar, K. Srithar, Glass basin solar still with integrated preheated water supply–theoretical and experimental investigation, Desalination 398 (2016) 214–221. [39] K.K. Murugavel, K. Srithar, Performance study on basin type double slope solar still with different wick materials and minimum mass of water, Renew. Energy 36 (2011) 612–620. [40] R. Dev, Thermal modeling and characteristic equations for passive and active solar stills (Ph.D. Thesis), Centre for Energy Studies, IIT Delhi, 2012, pp. 119–123. [41] G.N. Tiwari, V. Dimri, A. Chel, Parametric study of an active and passive solar distillation system: energy and exergy analysis, Desalination 242 (2009) 1–18. [42] A. Kianifar, S.Z. Heris, O. Mahian, Exergy and economic analysis of a pyramid–shaped solar water purification system: active and passive cases, Energy 38 (2012) 31–36. [43] A.E. Kabeel, A.M. Hamed, S.A. El-Agouz, Cost analysis of different solar still configurations, Energy 35 (2010) 2901–2908. [44] Parameters of Water Quality–Interpretation and Standards, (2001) (http://www. epa.ie/pubs/advice/water/quality/parametersofwaterquality.html ). [45] Guidelines for Drinking-water Quality: Incorporating the First Addendum, fourth ed., World Health Organization (WHO), 2017. [46] V.K. Dwivedi, G.N. Tiwari, Annual energy and exergy analysis of single and double slope passive solar stills, Trends Appl. Sci. Res. 3 (3) (2008) 225–241. [47] G.N. Tiwari, G.A.M. Selim, Double slope fibre reinforced plastic (FRP) multiwick solar still, Solar Wind Technol. 1 (4) (1984) 229–235.

Greek dt ηi ηO

small time interval, h instantaneous thermal efficiency (hourly basis) overall thermal efficiency of the solar still

Acknowledgement The present work is carried out under the Institute Project Fund sanctioned under letter Ref. no. 134/R & C/2013–14, dated: 23/08/ 2013 of Motilal Nehru National Institute of Technology Allahabad, Allahabad, Uttar Pradesh–211004, India. References [1] Guidelines for Drinking-water Quality, 2nd ed., Vol. 2 World Health Organization, Geneva, 1996 Health criteria and other supporting information. [2] G.N. Tiwari, Solar Energy – Fundamentals, Design, Modeling and Applications, Narosa Publishing House, New Delhi (India), 2005. [3] A.E. Kabeel, S.A. El-Agouz, Review of researches and developments on solar stills, Desalination 276 (2011) 1–12. [4] H.P. Garg, Solar desalination techniques, First Exposition and Symposium for New and Renewable Energy Equipment, 1991, pp. 1–38. Tripoli, Libya. [5] M.A.S. Malik, G.N. Tiwari, A. Kumar, M.S. Sodha, Solar distillation, Pergamon Press, Oxford, UK, 1982. [6] M.S. Sodha, A. Kumar, G.N. Tiwari, R.C. Tyagi, Simple multiple wick solar still analysis and performance, Sol. Energy 26 (1981) 127–131. [7] G.N. Tiwari, A. Tiwari, Solar Distillation Practice for Water Desalination Systems, Anamaya, New Delhi, India (2007). [8] S.K. Shukla, V.P.S. Sorayan, Thermal modelling of solar stills: an experimental validation, Renew. Energy 30 (2005) 683–699. [9] J.T. Mahdi, B.E. Smith, A.O. Sharif, An experimental wick–type solar still system: design and construction, Desalination 267 (2011) 233–238. [10] T. Rajaseenivasan, T. Elango, K.K. Murugavel, Comparative study of double basin and single basin solar stills, Desalination 309 (2013) 27–31. [11] R. Dev, G.N. Tiwari, Characteristic equation of a passive solar still, Desalination 245 (2009) 246–265. [12] R. Dev, H.N. Singh, G.N. Tiwari, Characteristic equation of double slope passive solar still, Desalination 267 (2011) 261–266. [13] R. Dev, G.N. Tiwari, Characteristic equation of the inverted absorber solar still, Desalination 269 (2011) 67–77. [14] P. Pal, R. Dev, Experimental study on modified double slope solar still and modified basin type double slope multi–wick solar still, Int. J. Civil Environ. Struct. Construct. Architect. Eng. WASET 10 (2016) 70–75. [15] M.M. Morad, H.A.M. El-Maghawry, K.I. Wasfy, Improving the double slope solar still performance by using flat–plate solar collector and cooling glass cover, Desalination 373 (2015) 1–9. [16] S. Yadav, K. Sudhakar, Different domestic designs of solar stills: a review, Renew. Sust. Energ. Rev. 47 (2015) 718–731. [17] P. Prakash, V. Velmurugan, Parameters influencing the productivity of solar stills: a review, Renew. Sust. Energ. Rev. 49 (2015) 585–609. [18] P. Pal, P. Yadav, R. Dev, Design of modified basin type double slope multi-wick solar still, BLB–Int. J. Sci. Technol. (2015) 304–309. [19] V. Manikandan, K. Shanmugasundaram, S. Shanmugan, B. Janarthanan, J. Chandrasekaran, Wick type solar stills: a review, Renew. Sust. Energ. Rev. 20 (2013) 322–335. [20] R.S. Hansen, C.S. Narayanan, K.K. Murugavel, Performance analysis on inclined solar still with different new wick materials and wire mesh, Desalination 358 (2015) 1–8.

82