Harmattan performance of a solar still in the Guinea Savannah

0960-1481/91 $3.1)0+.00 Pergamon Press plc

RenewableEnergyVol. 1, No. 5/6, pp. 799~01, 1991 Printed in Great Britain.

TECHNICAL NOTE Harmattan performance of a solar still in the Guinea Savannah S. U . EGARIEVWE Department of Physics, Bendel State University, Ekpoma, Nigeria and A . O. E. ANIMALU a n d C. E. OKEKE Department of Physics and Astronomy, University of Nigeria, Nsukka, Nigeria

(Received 10 June 1990; accepted 10 November 1990) Abstract--This paper presents the design, construction and Harmattan performance analysis of a glasscovered concrete-basin solar still located at Nsukka (6.80°N, 7.35°E) in the African Guinea Savannah. The analysis includes the effects of still materials and the weather conditions in this region, on its operational characteristics. The average daily distillate yield and efficiency during the Harmattan period are 1.0691/m2 and 17.6%, respectively. 1. INTRODUCTION The problem of inadequate good water supply in the African Savannah region has remained a major reason for the consumption of unclean water from rivers, lakes and ponds, by its people. This has led to the contraction and spread of deadly diseases, in which guinea-worm is a prominent case. The use of solar distillation in the purification of water is appropriate for small-scale water supply. It will reduce or eliminate the burning of fire-wood for heating in local purification methods, thus reducing deforestation. This, in turn, will reduce desert encroachment and the erosion of farm lands in the region. Presently in this region, solar stills are used for actual water supply in Niger, while prototypes have been constructed and used principally for research purposes in Nigeria. Dioha and Guima [1] have worked on solar distillation in food technology in Birnin-Kebbi. This paper reports the design, construction and the performance of a glass-covered concrete-basin solar still during the Harmattan season, at Nsukka (6.80°N, 7.35°E) in the Guinea Savannah. The observed operational characteristics as affected by the materials used and the weather conditions in the region, are also reported. 2. ENERGY AND ENVIRONMENTAL CONSIDERATIONS The performance of a solar still depends on a number of environmental parameters, design specifications and construction materials. The environmental parameters include insolation, geographical location, ambient temperature, humidity, windspeed and weather conditions (sunshine, clouds and rainfall). Thus, the characteristics of stills vary from place to place and from one type to another, due to variations in the environment, design and materials. The energy transfers responsible for water distillation in a still are the supply of heat for the evaporation of liquid The proofs of this article were corrected by the Editor in place of the author.

and the removal of heat from condensing vapour. The heat required for the evaporation depends on the amount of solar energy reaching the still, the fraction transmitted through the roof, and the fraction absorbed in water and basin. These, in turn, depend on weather conditions, roof materials, and basin materials, respectively. The removal of heat from condensing vapour depends mainly on the conductivity and emissivity of the roof materials, wind speed, and weather condition. As observed in this work, distillation rate increases during rainfall in the afternoon after sunshine in the morning hours. Cooper [2] has reported that for average wind velocities from 0 to 2.15 m/s the output gets increased by 11.5%, while from 2.15 to 8.81 m/s there was only an increase of 1.5%. This shows that the increasing influence of wind on still output decreases at higher wind velocities. Soliman [3], who has considered all heat and mass transfer modes in his study of the effect of wind velocity on still output concluded that at high water temperature, the increase in the difference between water and cover (roof) temperatures by increasing wind speed causes an increase evaporation rate. The Harmattan season in the Guinea Savannah in the Northern hemisphere is usually dusty, caused by dust partitles carried from the Sahara desert by the North-east trade wind. This dry, dust-laden wind, called locally "the Harmattan", also stirs up a thick dusty haze [4]. This reduces the amount of solar radiation reaching the still, due to seattering and absorption by the dust particles. As observed during the experiment, dust particles deposit on the transparent roof, and can significantly reduce its transmitivity if left without cleaning for several days.

3. EXPERIMENTAL CONSIDERATION The design of the still is shown in Fig. 1. It is constructed with a glass roof and a concrete basin, lined with black polyvinyl chloride (PVC) sheets. It has an east-west orientation with the slanty sides of the roof parallel to the equator. The parameters monitored during the experiment are insolation, still output, and temperatures of impure water, roof 799

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Fig. 2. Diurnal variation of insolation H~, output V, glass temperature Tg, water temperature Tw, and ambient temperature To.

and the ambient. Readings were taken at one hour interval from 06.00 to 18.00 h local time, and overnight. The investigations carried out included the behaviour of the still, the effects of weather conditions (sunshine, clouds and rain), and the effects of basin lining materials (PVC, charcoal and coal). The experiment was carried out in the month of December, during the Harmattan season.

curves for Tw and Tu are similar to the Hs curves. They show how the temperatures of the impure water and roof depends on insolation. The instantaneous productivity is highest in the morning and evening hours, with minimum values around 12.00 noon. The instantaneous productivity decreases with decrease in the temperature gradient between the impure water and roof (7",.- Te). Daily output increases with increase in daily insolation. See Fig. 3, showing the variation of insolation, output and efficiency with weather conditions, for various days. The highest daily efficiency was obtained on the rainy day due

4. RESULTS AND DISCUSSION The variations of water temperature Tw, glass-roof temperature Tg, ambient temperature Ta, insolation /-/5, and hourly output V, during day-time are shown in Fig. 2. the

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801

Technical Note to the cooling effect of rain-water on the glass-roof, which increased condensation rate. The daily efficiency of the still was computed using the relation :

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where ma is the daily mass productivity of the still, and Hs is daily insolation. The addition of charcoal and coal to the impure water and the basin lining material, PVC, showed improvements on the performance of the still. Mixing the impure water with fine particles of charcoal gave an improvement of 12.2% on the still efficiency. The percentage improvements by the addition of charcoal pieces, coal pieces and a mixture of charcoal plus coal pieces are 27.7, 10.8 and 8.1%, respectively. The average daily productivity of the still during the Harmattan season is 1.069 1m: of impure water surface area per day. The average daily efficiency during the same period is 17.6%.

5. CONCLUSIONS Our investigation of the effects of weather conditions and basin lining materials on the operational characteristics of a glass-covered concrete-basin solar still in the African Guinea Savannah, shows that output increases with an increase in sunshine except during rainfall, and the additions of coal (mined from underground) and charcoal (from burnt wood) improve the performance of the still. The cooling effect of rain on the roof increases condensation rate, which eventually lead to increase in evaporation of water at high water temperatures. Coal and charcoal increase the absorption of solar radiation by the impure water and basin, and hence increase evaporation rate. The emergence of charcoal pieces a little above the water surface increase the evaporation area, due to its porosity to water molecules.

REFERENCES

1. I. J. Dioha and M. A. Gulma, Ni#erian Journal of Solar Ener#), 5, 89 (1986). 2. P. I. Cooper, Solar Energy 12, 313 (1969). 3. H. S. Soliman, Solar Energy 13, 403 (1972). 4. G. C. Leong, Certificate Physical and Human Geography, p. 119. Oxford University Press, Hong Kong (1971).