An experimental investigation on a solar still with an ...

Desalination 347 (2014) 131–137

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An experimental investigation on a solar still with an integrated flat plate collector T. Rajaseenivasan a, P. Nelson Raja a, K. Srithar b,⁎ a b

Department of Mechanical Engineering, Fatima Michael College of Engineering and Technology, Madurai, 625020 Tamil Nadu, India Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai, 625015 Tamil Nadu, India

H I G H L I G H T S • • • •

Flat plate collector basin and conventional stills were fabricated and tested. A horizontal flat plate collector (FPC) is integrated into the basin of the still. FPC arrangement considerably enhances the distillate of the still. The FPC basin still has about 60% higher distillate than the conventional still.

a r t i c l e

i n f o

Article history: Received 2 January 2014 Received in revised form 20 May 2014 Accepted 21 May 2014 Available online xxxx Keywords: Solar still Flat plate collector Integrated still Extended surface Solar desalination

a b s t r a c t This work promotes the performance of the single basin solar still by means of preheating the saline water using an integrated flat plate collector arrangement. A conventional single slope single basin still and a single slope flat plate collector basin still (FPCB still) are fabricated with the same basin area of 1 m2. The FPCB still is fabricated similar to a conventional still, with the integration of a horizontal flat plate collector arrangement to form six small compartments in the basin. The projected space between the consecutive basins acts as an extended surface which increases the temperature of the basin as well as the flat plate collector where the saline water is preheated before it enters the basin. Due to separate compartments (absorber plate) in the basin, the mass of water reduces and the evaporation rate increases for the same depth of water in the basin. Experiments are carried out by varying the water depth in the basin and using the wick and energy storing materials in basins of both stills. The FPCB still gives about 60% higher distillate than the conventional still for the same basin condition. Economic analysis shows that the cost of distilled water for the FPCB still is lower than that for the conventional still. © 2014 Elsevier B.V. All rights reserved.

1. Introduction A single basin solar still is a simple device used for desalination purposes. The productivity of a simple solar still is low. The performance of the solar still depends on various factors, such as: solar intensity, wind velocity, ambient temperature, water–glass temperature difference, free surface area of water, absorber plate area, temperature of inlet water, transparent cover angle and depth of water [1]. Several modifications were made in the solar still to improve its productivity. Reducing the water depth in the basin enhances the daytime productivity and reduces the nocturnal productivity [2–6]. Placing of energy storing materials [glass, black rubber, gravel, asphalt, quartzite rock, red brick pieces, cement concrete pieces, washed stones and iron scraps] in the basin improves the heat storing capacity and results in higher productivity of the still [7–11]. The latent heat thermal energy ⁎ Corresponding author. Tel.: +91 9842185302; fax: +91 4522483427. E-mail address: [email protected] (K. Srithar).

http://dx.doi.org/10.1016/j.desal.2014.05.029 0011-9164/© 2014 Elsevier B.V. All rights reserved.

storage systems have many advantages over sensible heat storage systems including a large energy storage capacity per unit volume and almost constant temperature for charging and discharging [12]. In a single slope single basin solar still, a thin layer of stearic acid was used as a latent heat energy storage material in the basin [13]. The evaporation rate of a still strongly depends on the surface area of water exposed to the sun. The surface area exposure of the water can be increased by providing sponge cubes in the basin [14,15]. In stills, evaporated water is condensed on the inside surface of the glass cover that releases the latent heat energy to the surroundings. It may be effectively used by providing an additional basin to the still, referred as a multi-effect still. Different methods to improve the performance of the multi-effect solar still were reviewed by Rajaseenivasan et al. [16]. Active methods significantly improve the temperature of water in the basin by integrating the still with external heat sources. A detailed review on the active solar distillation was made by Sampathkumar et al. [17]. Performance of the solar still integrated with a flat plate collector was studied using the tap and saline water [18].

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T. Rajaseenivasan et al. / Desalination 347 (2014) 131–137

Performance of the double slope solar still with a flat plate collector under natural circulation mode was investigated by Dwivedi and Tiwari [19]. A double effect solar still with integrated flat plate collector and water flow over the transparent cover was presented by Kumar and Tiwari [20]. Desalination system coupled with the parabolic collector with heat exchanger was theoretically and experimentally studied by Abdel Rehim and Lasheen [21]. An experimental analysis on a double slope solar still with non-tracking cylindrical parabolic concentrator with an electrical pump was presented [22]. The concentrator coupled still resulted in higher production than the passive and flat plate collector stills at various depth conditions. Kumar and Tiwari [23] conducted an experimental study on a hybrid Photovoltaic/Thermal (PV/T) active solar still. Life cycle cost analysis of a single slope hybrid (PV/T) active solar still was presented by Kumar and Tiwari [24]. A solar still integrated with a pulsating heat pipe collector (PHP) was investigated by Sharif Abad et al. [25] with various filling ratios in PHP, various water depths in the still and various inclination angles of the flat plate collector. A double basin still integrated with a vacuum tube collector was used to enhance the distillate of the still by Panchal [26]. The productivity of this system was 56% higher than that of the conventional double basin still. A study was conducted in a single slope single basin still integrated with the solar water heater by Sampathkumar and Senthilkumar [27]. The research works described above clearly show that the performance of a still depends on mass of water in the basin, water temperature and exposure area. In active methods, a separate energy collector is used to supply the preheated water to the still. It raises the overall cost of the system as well as requires isolated space. The objective of this work is to accommodate the flat plate collector system into the single basin still, to enhance the distillate of the still by supplying the preheated water. It evades the additional space of the collector and reduces the collector system cost. In this work a horizontal flat plate collector is integrated in the basin of a single slope single basin solar still. This flat plate collector basin still (FPCB still) has the following advantages compare with the conventional basin still. (i) The flat plate collector arrangement provides preheated saline water supply to the basin that increases the basin water temperature. (ii) The fin arrangement increases the heat transfer rate from the basin to water. (iii) Due to separate compartments in the basin, the mass of water reduces and the evaporation rate increases for the same depth of water in the basin. The experiments are conducted by varying the water depth in the basin, using black gravels and wick materials in the basin of conventional and FPCB stills. 2. Experimental setup and procedure This work mainly consists of two systems namely conventional single slope single basin and flat plate collector basin stills. Both stills

consist of a wooden box made by plywood having four sides with dimensions of 1.1 × 1.1 m2 and thickness of 0.025 m. The outer sides of the wooden box are covered by the sheet metal. The basins of both stills are fabricated with a mild steel plate and effective basin area of 1 m2. The basins are placed in the inner side of the plywood box. The space available between the basin and plywood box is filled by the saw dust as insulation material. Window glass with a thickness of 4 mm is used as transparent covers for both stills and fixed at an inclination of 10° to horizontal, which is latitude of Madurai. The net evaporation areas of conventional and FPCB stills are 1 m2 and 0.65 m2 respectively. The remaining area of 0.35 m2 in the FPCB still is used for flat plate collector arrangement as shown in Fig. 1. The FPCB still is fabricated similar to a conventional basin still and following modifications are carried out in the basin. In the FPCB still, the basin is divided into six small compartments with the size about 0.108 m × 1 m each. These compartments are used as basins for the FPCB still. The gaps available between the compartments are fabricated as a rectangular fin with the size of 0.07 m × 1 m each. These five rectangular fins are used as an absorber plate for the flat plate collector and horizontally placed in the basin. Five mild steel pipes are used as risers with a diameter of 12.7 mm and these are connected with the upper and lower header (25.4 mm diameter) pipes (Fig. 2). This arrangement is placed above the horizontal absorber plate (rectangular fins). The basins of both stills, absorber plate and risers of the FPCB still are coated in black to absorb more solar energy. A water storage tank with a capacity of 50 l is used to supply the saline water to conventional and FPCB stills and is controlled by three valves (V1, V2 & V3) as shown in Fig. 2. V1 is used to control the flow between the saline water storage tank and conventional still. V2 controls the flow between the storage tank and inlet riser of the flat plate collector. V3 controls the flow between the outlet of the flat plate collector risers and basin of the FPCB still. Before starting the experiment, the required water is filled in the conventional still by opening V1. In the FPCB still V2 and V3 are opened first; the water fills the risers of the collector by V2 and then fills the basins of the still up to a required depth by V3. After the required depth of water filled in the FPCB still the valves V2 and V3 are closed. Thus saline water is available in the risers and gets heated by the solar radiation. In stills, the saline water gets evaporated and condensed at the inner surface of the glass cover as fresh water. As the water evaporates, the level of the water is reduced in the stills. So, water level is maintained in the conventional still by opening valve V1. In the case of the FPCB still, V2 is opened initially, so that the preheated saline water is supplied to the basins. Then V 2 is closed and V 3 is opened for water to fill the risers again for preheat. This is repeated every 30 min. Copper-constantan thermocouples integrated with a temperature indicator and selector switch are used for temperature measurements.

Fig. 1. Flat plate collector basin still — Sectional view.


133

Fig. 2. Top view of conventional and FPCB stills.

The temperatures are measured at following locations: basin plate, water at basin, vapour, and inner and outer glass. Solar radiation is measured by using the calibrated solarimeter. A calibrated glass jar of 1litre capacity is used to measure the hourly yield. Vane type digital anemometer is used to measure the wind velocity. The observations are recorded from 9 AM to 6 PM. The solar intensity, wind velocity, ambient temperature, yield and temperatures at various parts of the stills are recorded in an hourly basis. The experiments are carried out in both conventional and FPCB stills to compare the performance of the proposed system. In the first set of experiments, performance of the systems is studied with various depths of water in the basin (1, 2, 3, 4 cm). In the second set of experiments, wick and energy storing materials are used in the basin to improve the heat storing and evaporation rate of the still. All the experiments are performed in the actual solar condition, during the period of March– July 2013. The experimentations are carried out at the Mechanical Engineering Department, Fatima Michael College of Engineering and Technology, Madurai, Tamil Nadu, India. 3. Efficiency of solar still The daily efficiency, η, is obtained by the summation of the hourly condensate production m, multiplied by the latent heat hfg; hence the result is divided by the daily average solar radiation Is over the whole area A of the device: X η¼

m hfg X : Is

ð1Þ

A

4. Uncertainty analysis The uncertainty analysis for the measuring instruments such as thermocouples, solarimeter, anemometer, measuring jar and daily efficiency is calculated from Eqs. (2) and (3) as given by Rahbar and Esfahani [28] and experimental errors are calculated as given by Velmurugan et al. [29]. a u ¼ pffiffiffi 3

ð2Þ

" #1 = uðmÞ2 uðIs Þ2 2 uðηÞ ¼ η þ m2 Is 2

ð3Þ

The daily uncertainty for efficiency is varied from 0.03 to 0.04% and 0.05 to 0.06% for conventional and FPCB stills respectively. Table 1 shows the range, accuracy and uncertainty of various instruments used in the experimentation work. The hourly productivity m = f(H). Total uncertainty for the hourly condensate production was given by Omara and Eltawil [30] " um ¼

∂m u ∂H H

2 #0:5 ð4Þ

where m is the hourly productivity of the still, H is the depth of water in the measuring jar and um be the results of the uncertainty.

Table 1 Accuracy and experimental uncertainty for various measuring instruments. Sl. no.

Instrument

Accuracy

Range

Standard uncertainty

% error

1 2 3 4 5

Thermometer Thermocouple Kipp–Zonen solarimeter Anemometer Measuring jar

±1 °C ±0.1 °C ±1 W/m2 ±0.1 m/s ±1 ml

0–100 °C 0–100 °C 0–2500 W/m2 0–15 m/s 0–1000 ml

0.6 °C 0.06 °C 0.6 W/m2 0.06 m/s 0.6 ml

0.25% 0.50% 2.50% 1.00% 1.00%

134


Fig. 3. Hourly variation of different parameters.

5. Results and discussion Each experiment is conducted for three times to investigate the performance of the still at different days. The deviation in operational conditions (solar intensity and ambient temperature) within 10% is chosen for comparison and discussion purposes. Fig. 3 compares the performance of conventional and FPCB stills at 1 cm depth with various environmental and still parameters. It can be noticed that the distillate output and hourly efficiency of the FPCB still is always higher than those of the conventional still for the same environmental condition. It is due to the occupation of considerable area by the arrangement of the flat plate collector (FPC) in the basin (Fig. 1). This causes reduced mass of water in the FPCB still compared to the conventional basin still for the same depth of water in the basin. Due to the separate compartments and fin arrangement, heat transfer rate increases from the basin to water and leads to higher evaporation rate. Conventional stills need more time to get warm up than FPCB stills due to higher mass of water available in the single compartment. It reduces the speed of evaporation process in the conventional still and lowers the production rate. Further enhancement is achieved in the FPCB still by the preheated water supply. The variation of solar intensity, ambient temperature and production rate for conventional and FPCB stills with different

depths of water is shown in Fig. 4. It is noted that higher production rate is obtained for the lower depth due to reduced mass of water in both stills. Maximum distillate yields of 0.58 kg/m2 and 0.42 kg/m2 are obtained for 1 cm depth at 2.00 PM for FPCB and conventional basin stills respectively. Comparison of cumulative production rate for FPCB and conventional basin stills with different materials in the basin is shown in Fig. 5. The water depth of 1 cm is used for all modifications in both stills. It shows that provision of jute cloth has higher production rate at morning time and black gravel in afternoon hours. Black gravel is a sensible heat storing material. It absorbs the radiation and stores the heat within it at sunshine hours. At nighttime, this stored heat is released and it leads to increases in water temperature and production rate in both stills. The production rate is higher for both stills with jute cloth and black gravel in the basin. It is due to the higher heat carrying capacity of black gravel and exposure area of wick materials in the basin. Fig. 6 shows the variation of glass and water temperature of conventional and FPCB stills. Water temperature of the FPCB still is always higher than that of the conventional still due to the preheated water supply and fin arrangement. Glass temperature of the FPCB still is little higher than that of the conventional still, due to the higher condensation rate of vapour in the glass cover.

Fig. 4. Hourly variation of distillate for different depths of water in conventional and FPCB stills.


135

Fig. 5. Variation of cumulative productivity with different materials in basin and FPCB still.

Fig. 7 compares the overall performance of both stills for all basin conditions. The average solar radiation and ambient temperature are provided in Fig. 7. It shows that efficiency and productivity of both stills increase with materials in the basin. The conventional basin and FPCB stills achieve the maximum efficiency of 37% and 60% and maximum distillate of 3.62 kg/day and 5.82 kg/day. Fig. 8 compares the day–night productivity of both stills with different modifications in the basin. It shows a considerable improvement in the daytime production rate for the FPCB still. Daytime productivity of the still is higher with jute cloth and hot water supply in the basin. Black gravel significantly increases the nighttime production rate in both stills. The maximum productivity of both stills is obtained with the combination of jute cloth and black gravel in the basin. Maximum distillate yields obtained at daytime for conventional and FPCB stills with jute cloth and black gravel in the basin are 3.02 and 4.90 kg/m2 respectively. The percentage of increase in distillate for conventional and FPCB stills with different basin conditions is compared in Table 2. The distillate yield is enhanced about 60%, when the FPCB still is used instead of the conventional basin still at the same basin condition. 6. Economic analysis Economic analysis is used to estimate the unit cost of the distillated water by stills. The unit cost of the distilled water can be calculated by

Fig. 6. Hourly variation of water and glass temperature in conventional and FPCB stills.

using Eq. (2). Here the average year around productivity of the solar still is taken about 60% of its daily original productivity, due to the year around variation in climatic condition. The solving methodology and other parameters used for the economic analysis are provided in Appendix A [31,32]. Cdw ¼

TAC M

ð2Þ

Table 3 shows the economic analysis of conventional and FPCB stills with different modifications in the basin. It shows that the cost of distilled water per kg is lower for the FPCB still. The initial cost for the FPCB still is 15% higher than that for the conventional still and the distillate output of the FPCB still is enhanced up to 60% higher than that of the conventional still. 7. Conclusion A flat plate collector basin still and conventional basin still are fabricated and tested under local climatic condition with different modifications in the basin. Jute cloth and black gravels are used in the basin to improve the evaporation rate and heat capacity of the still. Result indicates that the FPCB still has higher evaporation rate than the conventional basin still. The effect of extended surface and preheated water supply increases the distillate of the FPCB still about 60% than that of the conventional still for the same basin condition. Stills with jute cloth enhance the productivity in sunshine hours and the black gravel has a significant effect at afternoon hours. The maximum productivity values obtained for conventional and FPCB stills are 3.62 and 5.82 kg/m2·day respectively. Economic analysis shows that the costs of distilled water for the conventional and FPCB stills with wick and energy storing materials in the basin are 0.0362 and 0.0276 $/kg/m2 respectively. Nomenclature AMC annual maintenance cost, $ ASV annual salvage value, $ A area, m2 a accuracy of instrument unit cost of distilled water, $/kg/m2 Cdw CRF capital recovery factor FAC annualized capital cost, $ FPCB flat plate collector basin H depth of water in measuring jar, m enthalpy of evaporation at Tw, J/kg hfg

136


Fig. 7. Comparison of different parameters in conventional and FPCB stills for different basin conditions.

Fig. 8. Variation in distillate production rate with various modifications in basin and FPCB still.

Is i M m n P Prod SFF S TAC Tw u

solar intensity, W/m2 interest rate, % annual productivity, kg/m2 hourly productivity, kg/m2 number of life years of the system capital cost, $ daily productivity, kg/day sinking fund factor salvage value, $ total annualized cost, $ temperature of water, °C uncertainty

um η

uncertainty result daily efficiency

Appendix A FAC ¼ P ðCRFÞ The capital costs (P) of the conventional and concentrator stills are $167 and $192 respectively. CRF ¼

ið1 þ iÞn ð1 þ iÞn −1

Table 2 Comparison of distillate yield in conventional and FPCB stills. S. no.

Basin condition

Conventional basin still

FPCB still

Productivity (kg/day)

% increase

Productivity (kg/day)

% increase

1 2 3 4

Still without materials (1 cm) Jute cloth Black gravel Jute cloth + black gravel

2.69 (ref) 3.04 3.38 3.62

– 13.01 25.65 34.57

4.35 4.81 5.34 5.82

61.71 78.88 98.51 116.36


137

Table 3 Economic analysis of stills with different modifications in basin. S. no

Still type

Modification in basin

P ($)

TAC ($)

M (kg/m2)

Cdw ($/kg/m2)

1 2 3 4 5 6 7 8

Basin still

Still without materials (1 cm) Jute cloth Black gravel Jute cloth + black gravel Still without materials (1 cm) Jute cloth Black gravel Jute cloth + black gravel

166.7 166.7 166.7 166.7 191.7 191.7 191.7 191.7

30.6 30.6 30.6 30.6 35.1 35.1 35.1 35.1

633 712 750 845 950 1090 1160 1275

0.0483 0.0429 0.0407 0.0362 0.037 0.0322 0.0303 0.0276

FPCB still

The interest per year (i) and the number of life years of the system (n) are assumed as 12% and 10 years respectively. AMC ¼ 0:1FAC The maintenance cost is associated with cleaning the glass covers and water collection trough and washing the basin to avoid the scale deposition. For the above process, costly equipments are not required and only waste cloths and water are sufficient. However it may require coating of black paint in the basin to maintain the higher radiation absorption. Therefore 10% of the capital cost is taken as the maintenance cost. ASV ¼ S SFF S ¼ 0:2 P Here the salvage value of the system is taken as 20% of capital cost (P) of the system SFF ¼

i ð1 þ iÞn −1

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