Solar earth-water stills

Solar Energy, Vol. 20, pp,

387-391.

Pergamon Press 1978. Printed in Great Britain

SOLAR EARTH-WATER STILLS J. AHMADZADEH Chemical Engineering Department, Pahlavi University, Shiraz, Iran (Received 1 August 1976; in revisedform 15 May t977; receivedfor publication 1 September 1977) Abstract--Solar earth-water stills are investigated as a method of producing drinking water in an arid region. The technique involves vaporisation of the moisture in the soil and subsequent condensation. About 11. of drinking water per square meter of soil has been obtained from these stills in a 24 hr period during during the summer in the Fars province of Iran. The amount of water obtained however, drops off rapidly each day.

about 20km North of Shiraz. The location is ideal for this form of experiment due to the nature of the soil. It is necessary to sink a well 150 m before deep water can be reached. The condition of the soil is such that most of the rainfall is held within the first few metres below the surface. A site was chosen where no irrigation had been given for approximately 2 yr. The only moisture in the soil was that from previous rainfall. The moisture content at the surface was approximately 1 per cent and this increased with depth as shown by Fig. 1. Experimental work was started at the beginning of July and carried out through August and up to the middle of September, during which time there was no rainfall. The climate in Shiraz during this period is very warm, the temperature reaching around 45°C and the solar radiation reaching a maximum of about 1.4cal. min -~ cm -2 at midday. The air is also extremely dry. Initial experiments were carried out with a greenhouse-type of still as shown in Fig. 2. The still consisted I of a wooden frame 1 × ~ m, with a glass window inclined at an angle of 18° to the horizontal. An aluminum channel

1. I N T R O D U C T I O N

Large quantities of moisture are collected in the soil, even in the arid regions of the earth, due to rainfall in the winter months. This moisture is evaporated by the heat of the sun during the hot summer months and is returned to the atmosphere as part of the natural hydrologic cycle. The earth-water still is a technique for collecting the vaporised moisture and converting it back to water which can be collected for consumption. The technique is based on the greenhouse effect. The still is similar to the simple greenhouse still[I] used for water desalination, without the blackened base. The still is placed directly on the ground. Solar energy is transmitted through the sloping glass window and heats the soil beneath it. The high soil temperature results in a high rate of moisture vaporisation. The water vapour then passes up to the underside of the glass, which is at a lower temperature, and condensation takes place. Condensed water droplets run down the sloping glass window and are subsequently collected. A limited amount of work on this technique has been reported by Kobayashi[2] in Japan. Most of the work was carried out in a suburb of Tokyo during the winter months and the data obtained indicates yields of around 11. per square meter of soil per 24 hr period. Kobayashi's work is useful in the pioneering sense, however, Tokyo is not an arid region and since work was carried out during the winter months the data obtained cannot be considered representative. A small amount of work was carried out in the Quetta desert of Pakistan but no data has been given. In the Fars province of lran, of which Shiraz is the capital, there are many regions which are faced with severe water problems during the summer. The climate is such that there is no rainfall during the summer months and in some villages, it is necessary for the villager to walk long distances to the nearest water supply. With this in mind, it was decided to investigate the solar earth-water still as a technique which might assist in the drinking water requirements of such a region.

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Fig. 1. Variation in soil moisture content with depth.

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as shown in Fig. 3. Temperatures were measured up to a depth of 35 cm beneath the surface, however there was no variation in temperature with depth at depths greater than 15 cm. Further experiments consisted of removing the topsoil and then placing a still on the more moist soil. A still (1 x ½m) was placed at a level of about 8 cm below the topsoil at a point where the moisture in the soil was approximately 9 per cent. After 24 hr, 370 cm 3 of water were collected. The amount of water product was measured every 24 hr just after sunrise and the results are shown in Fig. 4. A second still (1 x ~' m) was placed at a depth of about 28 cm below the topsoil at a point where the soil humidity was I 1.2 per cent. 500 cm 3 of water was obtained in the first 24 hr and the daily results are shown in Fig. 5. Work was also carried out on earth-water stills in the form of holes in the ground with a cover to condense the water vapour. A circular hole was dug in the ground and a beaker was placed centrally in the bottom of the hole as shown in Fig. 6. A transparent plastic sheet was then placed over the hole and covered round the sides with a layer of soil to prevent any vapour leakage. The sheet was then pushed down at the centre and weighted with a stone, thus providing the necessary slope down which the condensed water droplets would run to be collected in the beaker. Three stills were made in this way each with a diameter of 50 cm. The stills were made with depths of 34cm (11.3 per cent soil moisture) 30 cm (12.0 per cent soil moisture) and 44cm (13.2 per cent soil moisture). The daily output from these stills is shown in Fig. 7. Solar radiation recordings were taken throughout the experimental period. A number of observations were made during the course of the experiments. (1) As can be seen from the experimenta] results, the general trend is for the output of each still to fall off daily. However, there are some days in which the output increased compared to the previous day or else the drop in output was very small. Solar radiation data were

was fitted under the lower side of the window to collect the condensed water drops. The channel passed out of the side of the still and the water was collected in an external vessel. The still was well sealed to ensure no vapour leakage. The first experiment consisted of placing a still directly on the topsoil and sealing round the bottom of the frame with soil to ensure no vapour leakage. The still was left for 3 days and no water product was obtained. Thermocouples were then placed inside the still and at various depths in the soil beneath the still. In this way the temperature profile was obtained over a 24 hr period

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Fig. 4. Daily output from a wooden frame still (1 x ½m) with soil moisture content of 9 per cent.

Solar earth-water stills

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Fig. 5. Daily output from a wooden frame still (I x } m) with soil moisture content of 11.2 per cent. Stone weight

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Fig. 6. Solar earth-water still in the form of a hole in the ground.

studied and it was noted that these days of increased output corresponded to days of decreased solar radiation and ambient temperature. (2) Of the total daily output from a still, approximately 90 per cent of this output was obtained between sunset and sunrise.

so there is no doubt that the moisture in the topsoil is vaporised at a high rate. Since no water product is obtained, the rate controlling step must then be Step 3, the condensation of vapour under the glass window. Condensation of water vapour can be appreciated by examining the saturated humidity-temperature curve for the air-water system shown in Fig. 8. An air-water vapour mixture given by the point A must reach a saturated state before water will condense. This can be achieved by increasing the amount of water vapour in the mixture to point B or by decreasing the temperature to point C. If pure water is being vaporised within a still, vaporisation will take place until the saturated humidity is reached. However, for the case of water within soil the situation is more complicated. Water is held in the soil by capillary forces and thus the vapour pressure of water vapour just above the soil, p~, is less than the vapour pressure of pure water. Vaporisation will only take place as long as

3. DISCUSSION

The rate of water production from a solar earth-water still is dependent upon three steps. (1) Capillary flow of moisture below the surface of the soil up to the surface. (2) Vaporisation of surface moisture. (3) Condensation of water vapour. The first attempt at producing water by placing a still on the topsoil produced a negative result. This can be explained by considering the three steps noted and the temperature profile shown in Fig. 3. The experiment was carried out with a topsoil moisture content of 1 per cent and therefore there is moisture present at the surface of the soil at the start of the experiment. Thus, initially, Step 1 does not affect the rate of water production. It should then be noted that temperatures of up to 90°C are obtained within the still and

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where Prl2o is the partial pressure of water vapour in the bulk of the air mixture. Pv increases with the moisture content of the soil and will therefore have a very low value for a soil of low moisture content. Equilibrium between tlie earth moisture and the water vapour will be reached at a very early stage leading to a very low equilibrium humidity of the air within the still. In order to condense this vapour, a very low temperature is required. From Fig. 3 it can be seen that the minimum temperature of the glass window is about 20°C during the night. For a soil of low moisture content this temperature is evidently not low enough to condense the water vapour. This theory was tested by running ice water over the

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Fig. 7. Daily output from three hole in the ground-type stills (50 cm dia.) with soil moisture contents of 11.3, 12.0 and 13.2 per cent. A. Soil moisture content. 13.2 per cent; O. Soil moisture content, 12.0 per cent; x, Soil mixture content, 11.3 per cent• 0.16 0.14 0.14

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Fig. 8. Saturated humidity-temperature curve for air-water system. glass window of the still. Almost immediately the underside of the glass began to cloud over, indicating that condensation was taking place. It can be concluded, therefore, that for the case of the still located on the topsoil where the moisture content of the soil is very low, a limited amount of vaporisation takes place and then stops when equilibrium is reached. The air never reaches a saturated state and thus no condensation takes place.

Subsequent experiments were carried out by removing the topsoil or by making holes in the ground in order to reach a more moist soil. From the results shown in Figs. 4, 5 and 7 it can be observed that the output from a solar earth-water still drops rapidly during the first few days and then slowly levels off to a near constant value. It was also observed that about 90 per cent of the total output from a still is obtained during the night and that the output increased on days of decreased ambient temperature. The high output from a still during the first few days is due to the high moisture content of the soil, obtained by removing the topsoil. Due to the high soil temperature the rate of water formation is controlled by Step 3, the rate of condensation. After the first few days, as the surface soil begins to dry out, the rate starts to become dependent upon Step 1, the rate of capillary flow, as well as Step 3. Moisture has to travel by capillary action thi'ough the dry layer up from the moist layer beneath it. However, due to the dryness of the topsoil the water vapor pressure above the surface, Pv, is low. Vaporisation of moisture takes place during the day when the temperature in the still is high, however, very little, if any, condensation takes place due to the high temperature of the glass window. The amount of vaporisation is limited and depends on the value of pv. The still reaches an equilibrium state and no further vaporisation takes place. After sunset, the glass temperature drops, condensation commences and the water product is obtained during the night. However, although the still is now no longer at equilibrium, there is very little further vaporisation due to the low soil temperature.

Solar earth-water stills Step 3 is the rate controlling step. If condensation could be induced during the day time, the still would not reach an equilibrium state and vaporisation would be continuous throughout the day. The rate would then be controlled by Step I. The high temperature of the glass window is due to heat transfer from the soil to the window. Heat transfer from the window to the atmosphere is dependent upon ambient conditions. Due to the high ambient temperature and lack of wind during the summer months in Fars province, the rate of heat loss is very low. On days of reduced ambient temperature, the temperature of the glass window decreases, a limited amount of condensation then takes place during the day and the still output increases. During the winter months, however, the amount of sunshine is still very high while ambient temperature drops appreciably. It is, therefore, expected that under winter conditions a solar earth still will give a much higher output due to the onset of condensation during the day, however, rainfall during winter is fairly abundant and water shortage during these months is not a problem.

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At the present stage of development the solar earth still is not capable of acting as a reliable permanent drinking water source during the summer months since the output falls off too drastically after just a few days. By removing the topsoil, using the frame and glass window type of still, an output of about l 1. of water per square meter of soil can be obtained during the first 24 hr. However this levels off to a value of about 150 cm 3 per square meter per day after about 10 days. Stills made by making holes in the ground have the advantage of their simplicity. This form of still offers itself as a very attractive emergency water supply for people travelling in arid regions. By digging a few small holes in the ground and utilising a few transparent plastic sheets, this technique can easily provide several litres of drinking water per day. 5. REFERENCES

1. S. G. Talbert, J. A. Eiblingand G. O. G. L6f, Manual on Solar Distillation of Saline Water. OSW Report No. 546 (1970). 2. M. Kobayashi, A Method of obtaining water in arid lands. Solar Energy 7, 93 (1963).

Resumen--Son investigados los destiladores solares de la humedad del suelo como un m6todo de producci6n de agua potable en una regi6n firida. La t6cnica comprende la vaporizaci6n de la mezcla en el suelo y la condensaci6n subsecuente. Ha sido obtenido cerca de un litro de agua potable por m6tro cuadrado de estos destiladores del suelo en un lapso de veinticuatro horas durante el verano en las provincias alejadas de Irfin. Sin embargo, la cantidad de agua obtenida decae r:ipidamente cada dia. R6sum6--On 6tudie ici les distillateurs solaires d'eau contenue dans la terre en tant que m6thode pour la production d'eau potable en r6gions arides. La technique requiert une vaporisation de I'humidit6 du sol suivie d'une condensation. Un litre d'eau potable environ par m6tre carr6 a 6t6 obtenu h partir de ces distillateurs en 24 heures pendant 1'6t6dans les confints de l'Iran. Cependant, la quantit6 d'eau retir6e diminue rapidement chaque jour.