Samy M. Elsherbiny

Effect of adding a passive condenser on solar still performance

Abstract A theoretical and experimental study was conducted to investigate the effect of adding a passive condenser on the performance of the single slope, basin type solar still. A theoretical model based on the Dunkle (Int. Devs Heat Transfer 5, 895 (1969); Ref. [1]) mass transfer (evaporation) rate was developed. The model assumes that the transfer of water vapour from the still to the condenser is due to one or more of the following mass transfer modes: (i) diffusion, (ii) purging, and (iii) natural circulation. The theoretical results indicate that the diffusion contribution is relatively small. The contribution through purging represents the fraction [condenser volume/(condenser volume + still volume)] of the still yield. About 75% of the still yield is contributed through natural circulation. An experimental study that simulates the purging mass transfer mode was investigated. The experimental results show good agreement with the theoretical predictions and an increase of 50% in the still efficiency.

Free convection in inclined air layers heated from above

An experimental investigation of steady-state natural convection heat transfer was carried out in finite rectangular air layers heated from above. Two different aspect ratios, namely A = 20 and 80, and perfectly conducting boundary conditions on the end walls were used. The angle of inclination was varied from Φ = 0 (heated from below) to Φ = 180° (heated from above). A total of 226 test points were taken for heat transfer measurements in air layers heated from above at four different orientations in the range 120 ⩽ Φ ⩽ 180° for Rayleigh numbers between 102 and 2 × 106. Additional test points have been carried out to show the effect of the angle of tilt in the range O ⩽ Φ ⩽ 180° on the average Nusselt number for fixed values of the Rayleigh number. Local measurements of the Nusselt number over discrete portions of the air layer are reported to show the Nusselt number distribution over different flow regimes.

Transient analysis of a new humidification-dehumidification solar still

A numerical study was carried out to investigate the transient thermal performance of a new humidification-dehumidification solar still. The still body is a relatively thin rectangular box with a top glass cover and bottom condensing sheet. The still body is divided (by a central insulated stepped sheet carrying a group of basins) into two chambers: the upper evaporation chamber and lower condensation chamber. Air is circulated between the upper evaporation chamber (where it is heated and humidified) and the lower condensation chamber (where it is cooled and dehumidified for water production). The influence of different environmental, design, and operational parameters on the still productivity was investigated. The results indicated that increasing the solar intensity, ambient temperature, basin absorbitivity, and initial saline water temperature increases the system productivity. On the other hand, increasing wind velocity, basin insulation thickness, evaporation and condensation surface areas, condenser emissivity, and saline water mass has little effect on productivity. The interesting result is that the decreasing airflow rate has insignificant influence on system productivity. Decreasing airflow rate from 0.1 to 0.001 kg/s partially increases the productivity from 5.2 to 5.3 L/m2. The physical explanation and implications of these findings are explained.

Solar distillation under climatic conditions of Egypt

Solar distillation in a single basin was studied theoretically under the climatic conditions of Alexandria, Egypt. The unsteady energy equations for the glass cover, water and basin were simultaneously solved. The effects of saline water depth, insulation thickness and wind speed on the still productivity were evaluated. A year-round study showed that the productivity of the still strongly depends on the solar radiation and ambient temperature. The daily still productivity varies from 1.1 to 5.2 kg/m2 of basin area with an annual average of 3.16 kg/m2. The solar still efficiency variesfrom 0.34 to 0.49 with an annual average of 0.42. The daily total still production increased with decreasing the water depth and increasing the insulation thickness. Increasing the wind speed resulted in a relatively small reduction in still productivity. The maximum production rate occurred after the peak in solar radiation and the time lag increases with the increase in water depth.

PERFORMANCE OF DIFFUSION STILLS UNDER EGYPTIAN CLIMATIC CONDITIONS

The performance of diffusion stills used in solar distillation was studied theoretically under the climatic conditions of Alexandria, Egypt. The governing equations for unsteady heat and mass transfer inside vertical and inclined stills were solved numerically for the glass cover, absorber plate and the water layer. The study covered all angles of tilt between horizontal and vertical positions for the twelve months of the year. The effects of the air gap spacing, inlet water flow rate, insulation thickness and wind speed on the daily production were evaluated. The daily still productivity strongly depends on the solar radiation and ambient temperature and varies between 0.5 and 5 Kg/m2. An optimum angle of tilt is given for each month of the year. The daily yield increased with increasing the insulation thickness and decreasing the gap spacing or inlet water flow rate. The effect of wind speed was almost negligible up to 5 m/s.