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. þ962 6 5679773; Fax þ962 6 4613452;email: o_badran@yahoo.com

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 forabout a century in land-based plants and onships to provide water for a crew. The regularuse of distillation technologies accelerated afterWorld War II, as the demand for fresh water inarid countries. The cost for distillation has beendecreasing rapidly, especially in recent yearswith the introduction of efficient, more cost-

effective technologies. Distillations are one ofmany processes available for water purification,and sunlight is one of several forms of heatenergy that can be used to power that process.Sunlight has the advantage of zero fuel cost butit requires more space (for its collection) andgenerally more costly equipment. In principle,the water from a solar still should be quite pure.The slow distillation process allows only purewater to evaporate from the basin and collecton the cover, leaving all particulate contami-nants behind.

Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005.

European Desalination Society.

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

*Corresponding author.

Desalination 183 (2005) 137–142

doi:10.1016/j.desal.2005.02.046

The distillation using the solar still is verylimited, so that different methods have to bechosed to improve the productivity [1,2].

Many experimental and numerical investi-gations have been done on different designsof solar stills, such as [3–11]. Despite theadvantages of solar stills, they are recognizedas low productivity devices in comparisonwith the thermal desalination methods andthey depend on sunshine periods. Nowadaysmany research work are moving towardsincreasing the efficiency of the solar stills byusing enhancers such as solar collectors[2,6,11], which is examined in this work.

In areas where saline sources have beentapped by boreholes, and the water is too saltyfor humans to consume without serious conse-quences (as the case in the region of Al Alzrakin the north-east of Jordan), the introduction ofdistillation promises to enhance the quality ofwater and to improve health standard.

On June, 1998, the Al-Rai and Al-DustorNewspapers stated that published reports ofimpurities in some Amman water systems andnational concern over carcinogens in drinkingwater had created a growing market for whatare called ‘‘home water purifiers’’.

The solar desalination project will playgreat role in the Jordan badia (desert inhabi-tant) mainly to quench the thirst of smallcommunities at isolated sunny areas and canhave a limited supply to the local market withdistilled water. Also the still product is suita-ble for chemical use in laboratories and forcharging and topping up the batteries andsuitable to some extent for medical uses,also to electric irons, and any place wheredissolved solids should be avoided for notclogging up the appliance.

The present solar still is relatively simple inconstruction with low maintenance cost andcan be operated by any laborer amongst theinhabitants. The utilization of solar still sys-tems is becoming very active in Arab world

where the solar radiation intensity is veryhigh.

Our goal for the present single slope solarstill project is to design and develop plans fora still which could be replicated using ‘‘off theshelf’’ materials, also to improve the outputof the simple basin solar still through thecoupling of a flat plate collector under Jorda-nian climatic conditions.

2. Experimental setup

Lack of good drinking water kills morechildren (especially in the Third World) thanalmost anything else. Microorganisms in awater supply can cause dysentery, which canlead to diarrhea and fatal dehydration.Recently, many health workers throughoutthe world have developed inexpensive solar-powered distillation units, or stills, and pas-teurization ponds that provide people with allthe fresh water they need.

The present solar still consists of asym-metric green house type solar still coupledwith solar collector. It has a black paintedbasin of 1 m2 area filled with brackish watersupplied to it from a collector which preheatsthe water to act as an enhancer to the solarstill. The evaporating basin is covered by asheet of clear glass (to allow sunlight toreach the water) which is tilted at a slightangle (35�) to let the fresh water that con-denses on its underside trickle down to a col-lecting trough. A trough running along thebottom side of the glass cover ensures thecollection of the distilled water towards thecollecting vessel. The glass also holds theheat inside the still.

An inlet pipe is also fixed at the rear wall ofthe still for feeding brackish water. Holes weredrilled in the body of still to fix thermocouplesto measure the temperature of water in thebasin, the inner and outer glass temperature,and the vapor inside the still. A flat plate

138 O.O. Badran, H.A. Al-Tahaineh / Desalination 183 (2005) 137–142

collector (shallow box, 1.75 m long, 0.6 mwidth and 0.15 m thicknesses) has been usedto preheat the water entering the still; the col-lector is made of seven parallel steel tubes, 1/2inch diameter and 1.8 m length. The tubes arewelded to 0.7 mm thin sheet coated by a blackselective layer fixed on insulator (rock wool).

In the design of the present solar still thefollowing facts are highly considered:-

1. To be simple in construction, operationand maintenance.

2. To be rigid and firm enough to resist theworst prevailing environmental condition.

3. Localmaterials to be used as far as possible.The schematic diagram of the system is

shown in Fig. 1. The greenhouse type solarstill has glass cover (4 mm thick) at an incli-nation of 35� facing south. A rectangulartrough is fixed at the downstream end of theslope for the collection of the distilled waterwhich leads it to the collecting vessel. The stillis filled each morning or evening, and theday’s production is collected at the time.

Silicon rubber sealant is used to preventleakage from any gap between the glass coversand the still box. The side walls and the base of

the unit are insulated with rock wool (thermalconductivity = 0.035 W/m2 K) of 6 cm thick.A constant head tank was used to control thebrine 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 watertight to avoid water leakage and the inside sur-face is blackened to absorb maximum solarradiation. It should probably be baked in thesun for a while before it is used in order to freethe paint of any volatile toxicants which mightotherwise evaporate and condense along withthe drinking water. The bottom and sides ofthe basin are insulated to reduce the heat lossesto the surrounding. The solar still has beendesigned installed and operated at Faculty ofEngineering Technology at Al-Balqa0 AppliedUniversity in Amman.

3. Results and discussion

In this paper we report on daily experimen-tation of a single slope solar still and the samestill coupled to a flat plate collector. The systemwas 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).

O.O. Badran, H.A. Al-Tahaineh / Desalination 183 (2005) 137–142 139

(October to December) under different climaticconditions, covering months with moderateand low sunshine. The work targeted toenhance the still output through improving thestill operations condition by using a flat platecollector. The temperatures of brackish water,glass covers, vapors and ambient temperatureare recorded continuously.

The still unit is mounted on an angled ironstand; it is movable to make any adjustment tothe angle of the axis of the still. The standardorientation of the solar still is assumed to be towardsouth in order to receive maximum solar radiation.

The influence of climatic conditions andmainly solar radiation, on the system produc-tion is investigated without coupling the collec-tor (still alone). The variations of the daily solarstill output and the average solar radiation fordifferent days in October are shown in Fig. 2.The figure shows that the still productivity isproportional to the solar radiation intensity,which depends on climatic condition of eachday. The effect of the ambient temperature isshown in Fig. 3. It can be seen from Fig. 3 thatgradual increases in the ambient temperaturetend to increase the yield of the solar still.

The effect of coupling the solar still with asolar collector is shown in Fig. 4. FromFig. 4, it

can be concluded that there is proportionalityin water production with respect to the basinwater temperature. The higher the temperaturethe higher the output will be from the distilla-tion system. This high productivity is expectedas a result of coupling the collector with solarstill. This can be explained by the fact that solarcollector will preheat the feed water into thesolar stills. Solar collectors have a higher effi-ciency than solar stills. Increased temperatureof the water in the basin increases the rate, aswell 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.

140 O.O. Badran, H.A. Al-Tahaineh / Desalination 183 (2005) 137–142

to coupling of solar collector (3510 mL) is cal-culated, and found to be 36% more than thatwhen the still was operated alone (2240 mL).

Fig. 5 shows the results of the experimentsperformed during the month of October todetermine the optimum direction angle for thestill by changing the still direction few degreestoward the east and west from the geographicsouth, to detect the optimal angle that will givethe higher yield. Such deviation is required asthe movement of the sun varies in directionbetween summer and winter. From the produc-tivity of the still, it can be seen that the optimalangle is found to be 10� to the west of thegeographic south during the winter season inJordan. These results show that tracking thesun is one of the preferred methods to increasethe still yield. It is clear from the figure that theeffect is not significant.

Fig. 6 shows the productivity of the still as afunction of the basin water depth, it is evidentthat the productivity decreases with the increaseof water depth. This increase in still productivityas the depth decreases could be attributed to thelower heat capacity of the basinwater that resultsin a higher temperature in the basin and increasethe evaporation rate. It can be concluded that theoutput of the still is maximum for the least water

depth in the basin (20 mm). The 20 mm depthwasused for all experiments inorder todeterminedifferent effects on the solar yield.

The average daily output of the solar stillfor three months is shown in Fig. 7. Themaximum solar still yield occurred in Octoberat which the solar irradiation was the highestduring the period of the experimental tests.

Fig. 8 presents the variation of hourlytemperatures for a test carried out on 21thof Nov using a still coupled with a collector.All the temperatures showed similar trends ofincreasing 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.

O.O. Badran, H.A. Al-Tahaineh / Desalination 183 (2005) 137–142 141

water temperature was the highest then fol-lowed by the vapor temperature. The highesttemperatures occurred between the hours of14–16 p.m. The ambient temperatures rangeswere between 20 and 30�C.

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 muchroom for innovation and improvement. It iswell, known, that solar distillation exhibits aconsiderable economic advantage over othersalt water distillation processes, because ofcost-free energy and reduce operating costs.

The operation of a solar distillation systemcoupled with a solar collector has been inves-tigated experimentally. Comparison of theoutput between coupled and stand alone stillwas 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 tohigher distilled water output due to higherbasin water temperature.

Producing fresh water by a solar still withits simplicity would be one of the best

solutions to supply fresh water to small iso-lated communities (Jordanian badia) with notechnical facilities.

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