EXPERIMENTAL INVESTIGATION ON THE PERFORMANCE OF SOLAR STILL

AUGMENTED WITH PIN FINNED WICK

W M Alaian, E A Elnegiry and Ahmed M. Hamed*

Mechanical power Engineering Department, Faculty of Engineering, Mansoura University,

Egypt.*E-mail: amhamed@.mans.edu.eg

ABSTRACT:

This paper presents an experimental investigation on the performance of solar still augmented

with pin-finned wick evaporation surface. The experimental system involves two identical solar

stills, one of the stills is conventional and the other have an evaporation wick-finned surface.

The fins are supported vertically on the basin of the still using steel wires. Outdoor experimental

tests are conducted to investigate the effect of using pin – finned wick on the still productivity.

Tests are carried out at wide range of ambient temperatures and solar radiation. Temperatures at

different locations (glass surface, water in the still, wick surface and air-vapor mixture) as well

as ambient temperature are recorded with time. Solar radiation as well as collected distillate are

recorded during the experiments at different operating days. Experimental measurements

indicate that the increase in distillate varies with ambient conditions. Enhancing the still

productivity is proved when pin-finned wick is applied in the still. System efficiency of about

55% is recorded when pin-finned wick is applied.

Keywords: Wick ; distillate; solar ; water; still; pin fin; desalination.

INTRODUCTION

Investigation of solar still for water desalination has increased steadily during the past several

years. From the view point of the global environment and energy sources, these systems are

considered as a good alternative for obtaining potable water. A comprehensive review and

bibliography of the solar desalination can be found in literature [1-6 ]. An improvement to the

performance of solar still can be made by using porous black wicks to make what is now called

the Wick-Type solar still [7-10 ]. The productivity of a solar still increases with increases in

evaporation surface area. Enlarged areas by integrating fins with the solar still is investigated by

Velmurugan et al. [11]. They found that the average daily productivity increased by 30%. In a

fin type solar still with black rubber, sand, pebble or sponge immersed in brine; respectively, the

productivity was increased by 58% to 70% [12]. Sponge cubes are placed in a basin solar still to

increase the brine free surface and the evaporation rate [13]. It is reported that the increase in

distillate production reaches 273% compared with the still without sponge cubes under the same

condition. Floating tilted-wick type solar still is investigated by Janarthanan et al. [14]. The brine

flow over an inclined surface paved with thin wicks. Compared with a basin solar still, less time

is needed to get fresh water in a tilted wick type solar still at the beginning and the productivity

can be enhanced by 16 to50% [15].

Performance of basin type double slope solar still with different wick materials like light black

cotton cloth, light jute cloth, sponge sheet, coir mat and waste cotton pieces is investigated[16].

It is reported that light black cotton cloth was the most effective compared with other wick

materials. Aluminum rectangular fin covered with cotton cloth and arranged in lengthwise

direction was more effective and gave slightly higher production than the light black cotton

cloth. Shukla et al. [17] used jute cloth for increasing the evaporation rate. One end of the jute

cloth was dipped into the water reservoir while the surface of the jute cloth was spread over the

basin exposed to sunrays.

For increasing productivity of solar still new approaches are highly welcome. An attempt to

improve the performance of solar still should include an investigation on the performance of new

wick surface configurations. To the best of our knowledge, pin finned wick has not been

investigated in the literature. In the present study it is objected to investigate the performance of

solar still it is augmented with pin finned wick to enhance the evaporation surface. It is also

aimed to evaluate the effect of ambient conditions on the still performance.

Solar Radiation Model

In the recent subsection, the total radiation incident on the tilted surface could be evaluated in

terms of the location, day of the year and time of the day. The total perceived solar radiation can

be estimated by the following relationship[18]:

(1)

where, is beam radiation on a horizontal surface, is beam radiation tilt factor, is beam

radiation at normal incidence, W/m2, is the diffuse sky radiation, W/m2, C is diffuse radiation

factor, is the surface tilt angle, and is solar reflectance of the Earth's surface. The three

terms in the above equation represent the direct, diffuse, and reflected components, respectively.

The terrestrial beam radiation within the atmosphere and on the earth's surface on a typical clear

day is calculated using the following relation:

(2)

where, A is an empirically determined constant which represent the apparent solar radiation at air

mass zero, W/m2, B is an apparent atmospheric extinction coefficient and is the solar altitude

angle. The altitude angle can be evaluated from the following expression:

(3)

where , and are the latitude, declination and hour angles, respectively. The declination

angle can be calculated as a function of the day number, as:

(4)

The hour angle is defined by

(5)

where the value of is assumed positive in the afternoon period.

In equation (1), the diffuse solar radiation is estimated from:

(6)

where, Fss = is the angle factor between the surface and the sky and is the tilt

angle of the solar collector. The beam radiation tilt factor is defined by:

(7)

where , are the beam radiation on a tilted surface and on the horizontal surface,

respectively. The incidence angle, , and zenith angle are calculated from the following

expressions,

(8)

(9)

The day length, which is the period from sunrise to sunset, can be evaluated from,

(10)

EXPERIMENTAL STUDY

The objective of the experimental work is to study the performance as well as comparison

between two solar stills; one of them is conventional still and the other is augmented with pin –

finned wick. For this purpose, two identical solar stills are designed and fabricated at the

Mechanical Power Department, Faculty of Engineering, Mansoura University. One of the two

units is augmented with a pin-finned wick surface located at the basin of the still. The still is

made of galvanized steel sheets (1.0 mm thick).. Dimensions of the basin are given as 0.8 m and

1.25 m. The basin is covered with glass sheet of 4 mm thickness, inclined with nearly 17° to

horizontal. Rubber and Silicon are used for filling to prevent leakage from any gap between the

glass covers and the still edge. All sides and the base are insulated by glass-wool of 5 cm

thickness. The insulation layer is supported by aluminum frame. Still basin as well as the wick

material are coated with black paint for good absorption.

The pin- finned wick elements of 7 cm height have 3 cm immersed in the water to have capillary

action. The condensate flows downward on the inner surface of the glass cover towards a

collecting V-trough, allocated at the lower end of the glass cover. The end of the V-trough is

connected with a tube to the graduated flask. The tube exit and flask inlet are covered by plastic

sheet to prevent leakage of vapor outside the still. Schematic of the experimental system is

presented in figure 1.

Fig. 1 Schematic of the experimental test unit

Each of the desalination units is provided with a level controller to keep the water level in the

solar still at a level of 3 cm. The air – vapor mixture temperature inside the still, wick surface

temperature, glass cover temperature, water temperature in the basin and solar radiation are

recorded versus time. Thermocouples of type K are used to measure the temperatures. Distillate

at the outlet of the collection trough is also recorded at regular intervals. Solar radiation intensity

is measured by TES-1333R solar power meter with an accuracy of ± 10 W/m2, a resolution of

0.1 W/m2 and measuring range of 0 - 2000 W/m2.

Fig. 2 View of the experimental test units

The distillate is collected in a calibrated flask located at the lower edge of the collection trough. Elements of the pin-finned wick is shown in Fig. 3.

Fig.3 Pin- finned wick elements located in the basin of the solar still

RESULTS AND DISCUSSION

Presented set of experiments were carried out in the month of April; 2015.

Experimental measurements are carried out during a short period but with a wide range of

ambient temperatures. As shown in Fig.4, it can be noticed that maximum ambient temperature

varies from about 38 C to 10 C during the test period. This period includes raining days with

fluctuation of solar radiation. Measured and calculated values of solar radiation are plotted

versus time during this period as shown in Figs.5, and 6. Clear sky day (7/4/2015) show a

reasonable agreement between measured and calculated values of solar radiation. On the other

hand, the difference between measured and calculated values of solar radiation data for other

days can be explained by the fact that the radiation model is a clear sky model. The system

accumulated productivity as well as the accumulated solar radiation for the days of the period is

plotted against time.

Enhancement of system productivity with application of the pin finned wick can be noticed.

The hourly variation of solar intensity and ambience temperature corresponding to April 2015 is presented in Figure ---

The variations in the cumulative daily amount of solar radiation incident on a 1 m2 basin area, , as well as the cumulative daily amount of distillate per unit area are shown in Figs. , , , for different days. It can be observed that the productivity is enhanced when the pin finned wick is applied.

Table 1 summarizes the daily efficiency for this set of experiments. More than 18% increase in system efficiency is attained for the day of 8/4/2015. The increase in efficiency is expected to be affected by both radiation intensity and ambient temperature which are consider the outside driving forces for the solar powered desalination unit.

Table(1) Daily Efficiency Values

Efficiency(%) DateCase(2) with wickCase(1) without wick

%47.30 %40.732015/04/07 %52.14 %33.862015/04/08 %55.05 %44.732015/04/09 %49.04 %43.972015/04/10

The maximum temperature of ambient air has been recorded as ------ °C. For the water in the basin, maximum temperatures for this set of experiments varies from --- to--- whereas the wick shows much higher temperature of -----°C.

Experimental variation of wick temperature was in the range of ----°C to ---°C.

CONCLUSION

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