Laminar falling film condensations over a vertical plate with an accelerating vapor flow is analyzed in this work in the presence of condensate suction or slip effects at the plate surface. The following assumptions are made: (i) laminar condensate flow having constant properties, (ii) pure vapor with a uniform saturation temperature in the vapor region, and (iii) the shear stress at the liquid/vapor interface is negligible. The appropriate fundamental governing partial differential equations for the condensate and vapor flows (continuity, momentum, and energy equations) for the above case are identified, nondimensionalized, and transformed using nonsimilarity transformation. The transformed equations were solved using numerical, iterative, and implicit finite-difference methods. It is shown that the freestream striking angle has insignificant influence on the condensation mass and heat transfer rates, except when slip condition is present and at relatively small Grl/Re2 values. Moreover, it is shown that increasing the values of the dimensionless suction parameter (VS) results to an increase in dimensionless mass of condensate (Γ(L)/(μl Re)) and Nusselt number (Nu(L)/Re1/2). Thus, it results in an increase in condensation mass and heat transfer rates. Finally, it is found that the condensation and heat transfer rates increase as Jakob number, slip parameter, and saturation temperature increase. Finally, the results of this work not only enrich the literature of condensation but also provide additional methods for saving thermal energy.

An experimental investigation is presented of the thermal performance of an agricultural greenhouse (GH) with a built-in solar distillation system. A set of solar basins with saline water was placed on the greenhouse roof to reduce the GH cooling load and to produce the required fresh irrigating water by solar distillation. The ventilation air enters the GH through an evaporative cooler for cooling in summer (and hot days), and is partially recirculated for heating in winter (and cold days). The experimental work was carried out and the GH was constructed at the King Abdul Aziz University, the city of Jeddah, Saudi Arabia. The system transient performance (temperatures, relative humidity, and water productivity) is presented for the summer of July 2004 in Jeddah. Measures to improve GH performance are also highlighted.

The transient performance of a stepped solar still with built-in latent heat thermal energy storage was studied. Thestill was designed for heating and humidification of agriculture greenhouses (GH) in remote areas. The solar still consists of five stepped basins with an inclined glass cover and is insulated on the bottom. The basin was placed on a slab filled with a layer of paraffin wax (phase change material, PCM) that acts as a latent heat thermal energy storage system (LHTESS). Air from GH enters the still from the bottom, flows between the basins and glass cover where it is heated and humidified, and then flows back into the GH. The still performance parameters investigated were analyzed, and the results compared with the case of a still without the LHTESS. The results showed that the still with LHTESS has an efficiency of 57%, and the total daily yield is about 4.6 L/m2. The still temperature as well as outlet air temperature and GH heat load are more uniform compared to the sinusoidal trends for the still without LHTESS. It was found that the relative humidity of circulating air increased along the still and always leaves at saturation conditions. The results indicated that decreasing the air flow rate has an insignificant influence on the still yield, while the GH heat load experiences a decrease. For a selected design and operational parameters, the still was able to provide heat for the GH for 24 h/d. This finding is important since heat could be provided to the GH at night and when it is most needed.

The potential of coupled solar stills as a mean for improving solar distillation yield is investigated. A model for a simple solar still assisted by an external solar collector is presented. The governing coupled heat and momentum balance equations are solved for a quasi-steady-state condition with temperature dependent physical properties. The results show that for coupled stills the fresh water productivity increases as the solar collector area of the assisting device increases. For an ideal system, neglecting the thermal inertia, the yield is linearly dependent upon the solar insolation. The thermal inertia causes a significant drop in the system yield and deviation from linearity. The net efficiency of the coupled system is higher than that of a similar simple still by a value that depends mainly upon the system configuration and independent of the meteorological conditions. Comparison of the analysis with experimental data is satisfactory.