Improved Still

Three improved solar stills were presented to the students in the Drafting class. The students were responsible for further research into solar still construction and were required to prepare detailed drawings for the construction of their design. A 1m2 collection area and a deeper basin containing sponge cubes were included in all the designs. The water depth could be controlled in two of these designs, while the cascading steps of the third design prevented water depth control. The cascading step design was chosen to decrease the volume of air between the seawater and glass covering. To

provide an insulation barrier to the perimeter of the water layer, each step was insulated on the lateral sides and underneath by a layer of coconut fibers covered by stucco reinforced with wire mesh. The entire interior of the still is finished with asphalt paint. The distilled water is also collected in the same manner as the prototype.

Подпись: Fig. 3. Flat-plate collector connected to improved solar still

Due to time constraints, only one still — the three step design — was selected for construction. Among design changes for the second pilot model, a flat-plate collector was added to preheat the input seawater. Sponges were added, to test research performed by Bassam Abu-Hijleh in 2002, in which the use of sponge cubes increased the amount of distilled water by 18% to 273% depending on the variables used [i. e. the water salinity: ‘fresh water, brackish water or saltwater’; water depth, and length of sponge cube].5 See figure 3 that shows the improved solar still system. Sponge cubes increase the surface area for distillation, and the porosity of the sponges’ aids in the evaporation rate. From Bassam’s conclusions these design considerations are not viable for seawater due to its high density, but could be useful in saltwater-infused groundwater [brackish water]. Current advances in solar distillation incorporate the use of membranes. These designs were also researched but not pursued because of the lack of PTFE membrane material in the market place.

Financing for the project continuation was provided by the Luxembourg Cooperation, which also supports the Technical Schools in Cape Verde. The 12th grade Civil Construction students built the improved solar still during the final trimester of the 2007/2008 school year.

The system consists of a seawater reservoir, a constant head tank, a solar flat-plate collector and the solar still. The cost for the entire system was 28,500 CVE [approximately 380USD], including the formwork and reservoir costs 6,200 CVE [~83USD].