Performance enhancement of a single basin single slope solar still ...

Desalination 399 (2016) 198–202

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Performance enhancement of a single basin single slope solar still using agitation effect and external condenser R. Arun Kumar, G. Esakkimuthu, K. Kalidasa Murugavel ⁎ Centre for Energy Studies (CES), Department of Mechanical Engineering, National Engineering College, Kovilpatti-628503, Tamilnadu, India

H I G H L I G H T S • • • • •

The conventional still is modified with basin water agitating system, fan and external condenser. A shaft coupled with a dc motor and exhaust fan used for agitatiing of water and extracting vapor to external condenser. The agitation of basin water breaks the boundary layer of water and increaes the area of contact with air. The exhaust fan and external condens enhances the circulation of air and vapor condensation. The productivity was enhanced by 39.49% as compared with conventional still.

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Article history: Received 12 April 2016 Received in revised form 2 September 2016 Accepted 8 September 2016 Available online xxxx Keywords: Solar still With external condenser Agitation of basin water Still performance

a b s t r a c t Solar still is one of the best solutions to solve water scarcity. The major drawback of the still is its low productivity. Improving the evaporation and condensation rate will increase the productivity of the still. In this work, a conventional and a modified single basin single slope solar stills were used for experimental analysis. Two single basin single slope solar still of same dimensions were fabricated. One of the still was attached with a provision to give agitation effect and external condensation. Agitation effect was given by a shaft coupled with a dc motor and an exhaust fan was used to extract the vapor from still to external condenser. Experimentation was carried out to compare the performances of conventional still and the modified still under same atmospheric conditions. The agitation effect breaks the surface boundary layer of water and increases the area of contact between the water and air. The exhaust fan and external condenser enhanced the circulation of air and condensation of vapor. The experimental result shows that the modified still enhanced the distillate productivity by 39.49% as compared to the conventional still. Also modified still is more economical than the conventional still. © 2016 Elsevier B.V. All rights reserved.

1. Introduction In arid and remote areas, water is scare, impure and saline, not suitable for drinking and requires treatment. Basin type solar still is the sustainable solution for this problem. It is very simple device, can be fabricated using locally available cheaper materials and require no skill to operate and maintain. Due to low productivity, this still has not been familiarly used. To improve the production and to make it economical, lot of researches have been progressing in improving the performance of both passive and active solar stills [1–6]. The productivity mainly depends on depth of water, evaporation area, radiation absorption capacity of the basin, transmittance of the cover, volumetric heat capacity of the basin, thermal properties of the basin and water, circulation of air inside the still and heat loss through ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (K.K. Murugavel).

http://dx.doi.org/10.1016/j.desal.2016.09.006 0011-9164/© 2016 Elsevier B.V. All rights reserved.

the bottom and side walls of the still [1]. Maintaining minimum depth increases the production rate during day time [7–9], however the still with higher depth yields more production during night time and in consecutive days [10]. Use of wick and porous materials in the basin enhances the evaporation area [8,11], use of sensible and latent heat storing materials enhances volumetric heat capacity of the basin [9,12, 13], use of dye materials improves the radiation absorption of basin [1]. Thermal properties of the water used in the still greatly affects the still performance. Thermal behavior of the still significantly depends on the thermal conductivity of the water used in the still. This property can be increased by converting the water into water nanofluid by adding suitable nano particles. Kabeel et al. [14,15] and Elango et al. [16] conducted experiment experiments with different water nanofluids in basin and compared their performances. Lovedeep and Tiwary [17] claimed, solar still with Al2O3 water nanofluid enhanced the production by 12.2% while Omara et al. [18] achieved 255% higher production using same nanofluid under vacuum condition.

R.A. Kumar et al. / Desalination 399 (2016) 198–202

199

Fig. 2.1. Schematic diagram of conventional solar still.

Another novel method of enhancing the evaporation is physically stirring, agitating or spraying of basin water using external energy. During these processes, water mass split into small particles which increases the surface area contact between air and water in turn helps in diffusing the water particles into vapor. Mohammed and Zhao [19] used a small wind turbine to stir water using impeller. Khaled [20] designed and demonstrated a new still with vibrator to vibrate the still which agitated the basin water leading to the increase of production rate from 3.4 to 5.8 l/m2 day. Significant parameters affecting the condensation are cover area, cover thickness, cover material, cover temperature and ambient conditions. When a still is activated for evaporation process, cover area available will not be sufficient to cool the air and to condense maximum mass of vapor present in the still air. Providing additional condensing area with the help of external condenser is the best solution for this problem. Recently, Kabeel et al. [21] presented a detailed review on solar still with external condensers. In this work, two new single slope single basin solar stills have been fabricated. One is conventional still. In another still, agitator is provided in the basin to agitate the basin water. An external condenser is also provided to collect part of the humid air from the still and to condense the vapor. Experiments were conducted to study the comparative performance of the modified still with conventional still under same climatic condition.

2. Experimental setup and procedure In this work, two basin stills were designed, fabricated and constructed to compare the performance of the solar desalination system. One of them is a conventional type and the other is the modified basin still as shown in Figs. 2.1, 2.2 and 2.3. The conventional still is made from galvanized iron sheet (1.5 mm thick) with a basin area of 0.25 m2 (500 mm × 500 mm). The low-side wall height is kept at 100 mm and the high side wall depth is 420 mm for 30° inclination. The inner walls of the basin are coated with black to improve the absorptivity. The still is insulated at the bottom and side walls with thermocole of 25 mm thickness to reduce the heat loss from the still to ambient. The basin is covered with glass cover of 4 mm thickness. The gaps between the glass cover and the still was coated with silica gel to prevent leakage to the atmosphere. The modified basin still has the same dimensions and construction of conventional still. In addition to that a shaft coupled with a dc motor of 4 W capacity for agitation effect and an exhaust fan of 0.45 W capacity is attached. The shaft was attached 20 mm above the bottom surface of basin along the centre of side wall. Five blades of length 15 mm height were used to disturb the water surface. The exhaust fan was fixed in the top corner of same side wall of the basin The output duct of exhaust fan is extended to an external condenser in cylindrical shape (400 × 300 × 300 mm) made of GI sheet and insulated using

Fig. 2.2. Schematic diagram of modified solar still.

200


improvement in performance of the still. Both the stills were maintained at 1 cm of water level depth [22]. The solar radiation, atmospheric temperature, basin temperature, basin water temperature, glass temperature and distilled water productivity were measured for every half an hour. 3. Results and discussions

thermocole. Copper tube of length 1500 mm and 16 mm diameter was used in the condenser with 5 passes of 300 mm in one pass. Both dc motor & exhaust fan were powered by using a 10 W PV panel. Air medium was used to absorb the heat from the water vapor. Table 2.1 shows the accuracy, range and error of the different instruments used. Experimentation was carried out to analyze the comparative performance of the conventional and modified stills at Energy Park, National Engineering college, Kovilpatti (9°11′N, 77°52′E). Both the stills were tested under same atmospheric condition to study the exact

Solar Intensity (W/m²)

Ambient Temperature ( C)

Wind Velocity(m/s) 40

Solar Intensity, W/m²

1000

35 800

30 25

600

20 400

15 10

200

5

Ambient Temperature, C Wind Velocity, m/s

Fig. 2.3. Experimental setup of conventional and modified still.

Experimentation was carried out during sunny days of April month, between 6:30 am to 6:30 pm. Fig. 3.1 shows the variation of input parameters such as solar intensity, ambient temperature and wind velocity with time. Solar intensity and ambient temperature reaches maximum around 12.00 pm to 2.00 pm because of abundant perpendicular solar radiation during these hours, but it is low in the morning and the late evening session. Wind velocity is minimum during start of the experimental hour and it keeps on increasing and reaches maximum during the end of the experimental hour. Figs. 3.2 and 3.3 show the graph plotted for temperatures against time in conventional and modified solar still respectively. Temperatures reaches maximum during peak hours as the solar intensity reaches maximum. It can be studied that the temperatures of both the stills were almost same. The inner glass temperature in modified still is lesser than that of the conventional still because a part of vapor is extracted to the external condenser. From the experimentation carried out, it was observed that the modified system showed a better output of 39.49% more than the conventional system. Comparison of productivity of conventional and modified stills against time duration is shown in Fig. 3.4. Improvement in the productivity of the still was due to the agitation effect and

0

0 6:30

7:30

8:30

9:30 10:30 11:30 12:30 1:30

2:30

3:30

4:30

5:30

6:30

Time, h Fig. 3.1. Average variation of solar intensity during the experimental days.

80 75 70 65 60 55 50 45 40 35 30 25 6:30

7:30

8:30

9:30 10:30 11:30 12:30 1:30

2:30

3:30

Time, h Fig. 3.2. Variation of different temperatures for conventional still

4:30

5:30

6:30


80 75 70 65 60 55 50 45 40 35 30 25

6:30

7:30

8:30

9:30 10:30 11:30 12:30 1:30

2:30

201

3:30

4:30

5:30

6:30

Time, h Fig. 3.3. Variation of different temperatures for modified still.

900

Conventional still

Modified still

Productivity, ml

800 700 600 500 400 300 200 100 0 6:30

7:30

8:30

9:30 10:30 11:30 12:30 1:30

2:30

3:30

4:30

5:30

6:30

Time, h Fig. 3.4. Comparison of productivity of conventional and modified still.

external condenser. Agitation effect breaks the surface boundary layer of the water. This leads to water disturbance and increases the surface area of contact between water and air leading to increase in rate of evaporation. External condenser carries out a part of condensation in it. As a part of vapor is extracted, the temperature of the inner glass cover decreases which in turn increases temperature difference between the basin water and inner glass cover leading to more amount of condensation. Also the evaporation rate gets increased as a part of vapor is extracted. The incident radiation received by the absorber plate increases as a part of vapor is extracted to the condenser leading to more amount of evaporation rate and condensation rate. Condensation took place in both the still and external condenser. 4. Efficiency comparison Efficiency of the still can be calculated by using following procedure [23]: η¼

m hfg A Is

Production rate of water in kg/h latent heat of water in kJ/kg = Effective solar energy receiving area hourly solar radiation in W/m2 The efficiency of the conventional still was calculated based on the basin area of 0.25 m2. For the modified still, the solar receiving area was calculated based on basin area of 0.25 m2 and PV panel area of 0.06 m2. Efficiency of conventional still is 27.17% for the yield of 0.595 kg/day and for the modified still is 30.57% for the yield of 0.830 kg/day. The productivity of the conventional and the modified stills were calculated per m2 area available for solar radiation. The condensate yield from the conventional still is 2.38 kg/m2/day and that of the modified still is 2.667 kg/m2/day. 5. Cost analysis Cost comparison can be made by using:

Σm hfg 1000 =½ðA ΣIs Þ 3600

ð1Þ

Table 2.1 Accuracy and error limits for various measuring instruments.

Profit ¼ Product Value–Annual Running Cost of Still where,

SI·NO

Instrument

Accuracy

Range

%error

1 2 3 4

Thermometer Thermocouple PV type sun meter Measuring jar

+1 °C +0.1 °C +1 W/m2 +10 ml

0–100 °C 0–100 °C 0–2500 W/m2 0–1000 ml

0.25% 0.5% 2.5% 10%

Annual running cost ¼ First annual cost þ Annual maintenance cost–Annual salvage value The annual maintenance cost is considered as 5% of the first annual cost. So the maintenance cost of conventional and modified still is Rs.

202


Table 5.1 Comparison between conventional and modified still.

Type

First Annual Life annual Annual decline time cost maintenance value (years) (Rs.) cost (Rs.) (Rs.)

Conventional 10 still Modified still 10

Annual salvage value Yield Profit (Rs.) (ml) (Rs.)

4000

200

210

3790

595

1375

6850

342.50

365

6485

830

1782.50

200 and Rs. 342.50 respectively. Life time of the still is considered as 10 years and the salvage value as 50% of first annual cost. Salvage value of dc motor and glass is considered as zero at the end of 10 years. Thus the annual decline value for conventional and modified still is Rs.210 and Rs.365 respectively. Input energy used to drive the DC motor for agitation and exhaust fan for condensation is obtained from solar radiation using a PV panel. As the cost of the panel is included in first annual cost and the energy obtained is a free energy, the input energy is neglected. Table 5.1 shows the comparison between conventional and modified still for various parameters. The market price of the distilled water is Rs.10 per litre. By using the above formula, profit of the yield from conventional still is Rs.1375 per year and that of the modified still is Rs.1782.50 per year. Thus by the above cost analysis, modified still is more effective than the conventional still. 6. Conclusion Two single basin single slope solar still of same dimensions were fabricated. One of the still was attached with a provision to give agitation effect and external condensation. Both conventional and modified still were investigated experimentally at Energy Park, National Engineering college, Kovilpatti (9°11′N, 77°52′E). Agitation effect increases the area of contact between the water surface and air by breaking the surface boundary layer of water, leading to enhanced evaporation rate. External condenser carries out a part of condensation in it. As a part of vapor is extracted, the temperature of the inner glass cover decreases which in turn increases temperature difference between the basin water and inner glass cover leading to more amount of condensation. The incident radiation received by the absorber plate increases as a part of vapor is extracted to the condenser leading to more amount of evaporation rate. From the experimentation it was found that the yield of condensate from the modified still was 39.49% more than the conventional still. Efficiency of conventional still was found to be 27.17% and that of modified still was found to be 30.57%. Cost comparison was made between both the stills. Modified still was economically efficient when compared with conventional still.

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