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The effect of coupling a flat-plate collector on the solar stillproductivity

O.O. Badran*, H.A. Al-Tahaineh

Faculty of Engineering Technology, Mechanical Engineering Department, Al-Balqa0 Applied University,PO Box 331006, Amman 11134, Jordan

Tel. 962 6 5679773; Fax962 6 4613452;email: [email protected]

Received 3 January 2005; accepted 21 February 2005

Abstract

Experimental Investigation to study the effect of coupling a flat plate solar collector on the productivity of

solar stills was carried out. Other different parameters (i.e. water depth, direction of still, solar radiation) to

enhance the productivity were also studied. Single slope solar still with mirrors fixed to its interior sides was

coupled with a flat plate collector. It has been found that coupling of a solar collector with a still has increased

the productivity by 36%. Also the increase of water depth has decreased the productivity, while the still

productivity is found to be proportional to the solar radiation intensity.

Keywords: Solar still; Solar collectors; Productivity enhancement

1. Introduction

Distillation technologies have been used for

about a century in land-based plants and on

ships to provide water for a crew. The regular

use of distillation technologies accelerated after

World War II, as the demand for fresh water in

arid countries. The cost for distillation has been

decreasing rapidly, especially in recent years

with the introduction of efficient, more cost-

effective technologies. Distillations are one of

many processes available for water purification,

and sunlight is one of several forms of heat

energy that can be used to power that process.

Sunlight has the advantage of zero fuel cost but

it requires more space (for its collection) and

generally more costly equipment. In principle,the water from a solar still should be quite pure.

The slow distillation process allows only pure

water to evaporate from the basin and collect

on the cover, leaving all particulate contami-

nants behind.

Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 2226 May 2005.

European Desalination Society.

0011-9164/05/$ See front matter 2005 Elsevier B.V. All rights reserved

*Corresponding author.

Desalination 183 (2005) 137142

doi:10.1016/j.desal.2005.02.046

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The distillation using the solar still is very

limited, so that different methods have to be

chosed to improve the productivity [1,2].

Many experimental and numerical investi-

gations have been done on different designsof solar stills, such as [311]. Despite the

advantages of solar stills, they are recognized

as low productivity devices in comparison

with the thermal desalination methods and

they depend on sunshine periods. Nowadays

many research work are moving towards

increasing the efficiency of the solar stills by

using enhancers such as solar collectors

[2,6,11], which is examined in this work.

In areas where saline sources have been

tapped by boreholes, and the water is too salty

for humans to consume without serious conse-

quences (as the case in the region of Al Alzrak

in the north-east of Jordan), the introduction of

distillation promises to enhance the quality of

water and to improve health standard.

On June, 1998, the Al-Rai and Al-Dustor

Newspapers stated that published reports of

impurities in some Amman water systems and

national concern over carcinogens in drinking

water had created a growing market for whatare called home water purifiers.

The solar desalination project will play

great role in the Jordan badia (desert inhabi-

tant) mainly to quench the thirst of small

communities at isolated sunny areas and can

have a limited supply to the local market with

distilled water. Also the still product is suita-

ble for chemical use in laboratories and for

charging and topping up the batteries and

suitable to some extent for medical uses,

also to electric irons, and any place where

dissolved solids should be avoided for not

clogging up the appliance.

The present solar still is relatively simple in

construction with low maintenance cost and

can be operated by any laborer amongst the

inhabitants. The utilization of solar still sys-

tems is becoming very active in Arab world

where the solar radiation intensity is very

high.

Our goal for the present single slope solar

still project is to design and develop plans for

a still which could be replicated using off theshelf materials, also to improve the output

of the simple basin solar still through the

coupling of a flat plate collector under Jorda-

nian climatic conditions.

2. Experimental setup

Lack of good drinking water kills more

children (especially in the Third World) than

almost anything else. Microorganisms in a

water supply can cause dysentery, which can

lead to diarrhea and fatal dehydration.

Recently, many health workers throughout

the world have developed inexpensive solar-

powered distillation units, or stills, and pas-

teurization ponds that provide people with all

the fresh water they need.

The present solar still consists of asym-

metric green house type solar still coupled

with solar collector. It has a black painted

basin of 1 m2

area filled with brackish watersupplied to it from a collector which preheats

the water to act as an enhancer to the solar

still. The evaporating basin is covered by a

sheet of clear glass (to allow sunlight to

reach the water) which is tilted at a slight

angle (35) to let the fresh water that con-

denses on its underside trickle down to a col-

lecting trough. A trough running along the

bottom side of the glass cover ensures the

collection of the distilled water towards the

collecting vessel. The glass also holds the

heat inside the still.

An inlet pipe is also fixed at the rear wall of

the still for feeding brackish water. Holes were

drilled in the body of still to fix thermocouples

to measure the temperature of water in the

basin, the inner and outer glass temperature,

and the vapor inside the still. A flat plate

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collector (shallow box, 1.75 m long, 0.6 m

width and 0.15 m thicknesses) has been used

to preheat the water entering the still; the col-

lector is made of seven parallel steel tubes, 1/2

inch diameter and 1.8 m length. The tubes arewelded to 0.7 mm thin sheet coated by a black

selective layer fixed on insulator (rock wool).

In the design of the present solar still the

following facts are highly considered:-

1. To be simple in construction, operation

and maintenance.

2. To be rigid and firm enough to resist the

worst prevailing environmental condition.

3. Local materials to be used as far as possible.

The schematic diagram of the system is

shown in Fig. 1. The greenhouse type solar

still has glass cover (4 mm thick) at an incli-

nation of 35 facing south. A rectangular

trough is fixed at the downstream end of the

slope for the collection of the distilled water

which leads it to the collecting vessel. The still

is filled each morning or evening, and the

days production is collected at the time.

Silicon rubber sealant is used to prevent

leakage from any gap between the glass covers

and the still box. The side walls and the base of

the unit are insulated with rock wool (thermal

conductivity =0.035 W/m2 K) of 6 cm thick.

A constant head tank was used to control the

brine level inside the still by a float type regulat-

ing valve for one level of water depth for 2 cmduring the period of the experimental work.

The basin of the solar still is made water

tight to avoid water leakage and the inside sur-

face is blackened to absorb maximum solar

radiation. It should probably be baked in the

sun for a while before it is used in order to free

the paint of any volatile toxicants which might

otherwise evaporate and condense along with

the drinking water. The bottom and sides of

the basin are insulated to reduce the heat losses

to the surrounding. The solar still has been

designed installed and operated at Faculty of

Engineering Technology at Al-Balqa0 Applied

University in Amman.

3. Results and discussion

In this paper we report on daily experimen-

tation of a single slope solar still and the same

still coupled to a flat plate collector. The system

was operated continuously for several months

1-Tgoot 2-Tgin 3-Tv 4-Tw 5-TaFig. 1. A schematic diagram showing the arrangement of the still-collector systems and the location of the

thermocouples (1-Tgoot; 2-Tgin; 3-Tv; 4-Tw; 5-Ta).

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(October to December) under different climatic

conditions, covering months with moderate

and low sunshine. The work targeted to

enhance the still output through improving the

still operations condition by using a flat platecollector. The temperatures of brackish water,

glass covers, vapors and ambient temperature

are recorded continuously.

The still unit is mounted on an angled iron

stand; it is movable to make any adjustment to

the angle of the axis of the still. The standard

orientation of the solar still is assumed to be toward

south in order to receive maximum solar radiation.

The influence of climatic conditions and

mainly solar radiation, on the system produc-

tion is investigated without coupling the collec-

tor (still alone). The variations of the daily solar

still output and the average solar radiation for

different days in October are shown in Fig. 2.

The figure shows that the still productivity is

proportional to the solar radiation intensity,

which depends on climatic condition of each

day. The effect of the ambient temperature is

shown in Fig. 3. It can be seen from Fig. 3 that

gradual increases in the ambient temperature

tend to increase the yield of the solar still.The effect of coupling the solar still with a

solar collector is shown in Fig. 4. From Fig. 4, it

can be concluded that there is proportionality

in water production with respect to the basin

water temperature. The higher the temperature

the higher the output will be from the distilla-

tion system. This high productivity is expected

as a result of coupling the collector with solar

still. This can be explained by the fact that solar

collector will preheat the feed water into the

solar stills. Solar collectors have a higher effi-

ciency than solar stills. Increased temperatureof the water in the basin increases the rate, as

well as the total output of distillate. The percen-

tage of enhancement in daily productivity due

Fig. 2. The relation of solar intensity and still output

during October.

Fig. 3. Effect of ambient temperature on passive

solar still productivity.

Fig. 4. Comparative variation of still productivity.


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to coupling of solar collector (3510 mL) is cal-

culated, and found to be 36% more than that

when the still was operated alone (2240 mL).

Fig. 5 shows the results of the experiments

performed during the month of October todetermine the optimum direction angle for the

still by changing the still direction few degrees

toward the east and west from the geographic

south, to detect the optimal angle that will give

the higher yield. Such deviation is required as

the movement of the sun varies in direction

between summer and winter. From the produc-

tivity of the still, it can be seen that the optimal

angle is found to be 10 to the west of the

geographic south during the winter season in

Jordan. These results show that tracking the

sun is one of the preferred methods to increase

the still yield. It is clear from the figure that the

effect is not significant.

Fig. 6 shows the productivity of the still as a

function of the basin water depth, it is evident

that the productivity decreases with the increase

of water depth. This increase in still productivity

as the depth decreases could be attributed to the

lower heat capacity of the basin water that results

in a higher temperature in the basin and increasethe evaporation rate. It can be concluded that the

output of the still is maximum for the least water

depth in the basin (20 mm). The 20 mm depth

was used for all experiments inorder to determine

different effects on the solar yield.

The average daily output of the solar still

for three months is shown in Fig. 7. The

maximum solar still yield occurred in October

at which the solar irradiation was the highest

during the period of the experimental tests.

Fig. 8 presents the variation of hourly

temperatures for a test carried out on 21thof Nov using a still coupled with a collector.

All the temperatures showed similar trends of

increasing with the increases of solar radia-

tion during the day. It was found that the

Fig. 5. The effect of solar still direction on the still

output.

Fig. 6. The effect of water depth on solar still

production.

Fig. 7. The average daily production for different

months of the year.

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water temperature was the highest then fol-lowed by the vapor temperature. The highest

temperatures occurred between the hours of

1416 p.m. The ambient temperatures ranges

were between 20 and 30C.

4. Conclusion

These solar energy distilling plants are rela-

tively inexpensive, low-technology systems,

especially useful where the need for smallplants exists. However, there is still much

room for innovation and improvement. It is

well, known, that solar distillation exhibits a

considerable economic advantage over other

salt water distillation processes, because of

cost-free energy and reduce operating costs.

The operation of a solar distillation system

coupled with a solar collector has been inves-

tigated experimentally. Comparison of the

output between coupled and stand alone still

was studied. It was found that the productiv-ity of the coupled still is found to be 36%

higher than the still alone. It can be con-

cluded that, the present still design leads to

higher distilled water output due to higher

basin water temperature.

Producing fresh water by a solar still with

its simplicity would be one of the best

solutions to supply fresh water to small iso-

lated communities (Jordanian badia) with no

technical facilities.

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Fig. 8. Hourly variation of temperatures.


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