Study of the performance of various types solar desalination plants in different weather conditions

Abstract

Salinity in water poses a serious problem in the coastal region of Bangladesh. The major difficulty in developing appropriate water supply system for the communities in that region is that aquifers containing sweet water are not always found at convenient locations. There are various desalination techniques practiced all over the world but those are expensive for our rural areas. Renewable energy sources specially solar energy has appeared an attractive solution. The technology involved in desalination of saline water using the solar energy is relatively simple and maintenance can be carried out by semi-skilled or unskilled operators, Therefore, in this study, an attempt has been made to study the performance of various types household and low-cost solar desalination plants. In this study, different types of single basin solar still were constructed. These designs differ in structure and material of construction, but basically, incorporate common elements for different functions. The principal criteria for selection of materials were cost involvement, efficiency, and ease of construction and maintenance. The stitls were characterized by a single basin (base and wall) to store saline water and one or more transparent covers on the top of the basin. The study was carried out in a two phases. In phase 1, although different types of materials were considered for construction of still basins, only basins constructed with ferrocement and brick were operated as these produced reasonable amount of yield. In phase 2, field investigations were commenced in the coastal belt simultaneously with laboratory investigations. A comparative study was conducted on the performance of the various types of the solar stills. Among all the materials studied, brick appeared to be cheap and durable materials for the construction of solar plants in comparison of other materials like ferrocement, steel etc. Glass and transparent plastic sheet were used as transparent cover of the plant. Glass was found superior to plastic sheets for use as transparent cover of solar stitls from view point of productivity, efficiency and ease of operation, although glass required extra care and attention. Three plants were constructed with same design and dimension except different cover slope. The cover slope 10°37' was found the optimum inclination of cover of the plant. Brick plant were constructed as single and double layers. Single layer brick plant was cost-effective in producing distill from saline water comparing to double layer brick plant. It was evident from the study that, of all the plants studied, the design of Brick Plant 1 was the best suited solar desalination plant for Bangladesh. The plant was constructed with brick having base and wall thickness 3.25 inch and 3.50 inch respectively. 5 mm glass sheet was used as transparent cover with an inclination of 10037'. The maximum, minimum and average yield of Brick Plant 1 was found 2.64 liter/m2/day, 0.28 liter/m2/day and 1.40 Iiter/m2/day respectively. The maximum, minimum and average efficiency of Brick Plant 1 was found 49,30%,8.58% and 33.82% respectively, The design life of Brick plants and Ferrocement Plant was considered 30 years. The lowest cost of water per liter was found Tk, 0.92 for Brick Plant 1. The water obtained from the plants was almost free from salinity. The salinity of water in the coastal areas of Bangladesh varies in general from 1,000 to 2,000 ppm and the acceptable limit for drinking purpose is 600 ppm. If, tubewell water was mixed with the distillate with an equal amount, then the cost of drinking iv water would be half of the production cost. Convective heat transfer (cover to air), convective heat transfer (basin to cover) thermal radiation (cover to air) and thermal radiation (basin to cover) were calculated. Convective heat transfer (cover to air) was the highest (an average 6.41 cal.cm2) among four modes and convective heat transfer (basin to cover) was the lowest (an average 0.74 cal.cm2) among the four modes. A mathematicai equation was developed for the prediction of the diurnal plant output from the climatic factors: average temperature, radiation, sunshine hour and relative humidity and plant temperatures: water temperature and glass temperature. This model could be satisfactorily used for the prediction of the diurnal yield of the plants. It was observed that the calculated average monthly diurnal yield had the maximum deviation of data within :t 14.60 for all cases and within :t 7.00 if 4 nos. data was excluded.