Study on a family size solar desalination plant for the coastal region of Bangladesh

Abstract

In the coastal region of Bangladesh salinity in water pose a serious problem.The chief difficulty in the development of appropriate water supply system for the communities in the affected area is that - deep aquifers containing sweet water are not found at all possible locations in the coastal region. There are various desalination techniques- practised all over the world but the conventional desalination process is energy intensive and will be an expensive option particularly in the rural area of Bangladesh. Solar energy on the other hand is freely available in nature in sufficient amount . Moreover, the technology involved in distillation 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 develop a family size solar desalination plant. Presently ,solar still plants, for desalination purpose, are not in use in this country. A plant operation was performed for the months of January to April, 1994 at the Department of Civil Engineering, Bangladesh University of Engineering and Technology, to observe the variation of diurnal distillate production of such a plant . The maximum and minimum diurnal experimental output of the plant as found in the study period was 0.211m2 to 2.3llm2 respectively with an average of 1.25 11m2. The estimated variation of efficiency was 3% to 40% with an average of 19%.The salinity removal by the plant is almost 100%. Therefore, the pure water obtained from solar still canbe mixed up with feed water prior to consumption provided the feed water is free from other impurities. However, the mixture should not have salt concentration of more than 600 ppm. During this period, the variation of different climatic parameters was observed to investigate the variation of the plant yield with the change in climatic condition. Depending on the variation of the distillate amount with climatic changes- four mathematical models were developed to predict diurnal plant output TRS model considers the air temperature, solar radiation and sunshine hours; TRH model considers the air temperature, solar radiation and relative humidity; TRW model considers the air teniperature, solar radiation and wind speed. The fourth model predicts diurnal plant yield from easily available temperature data only. Application of all the models are also presented in predicting the plant yield with respective estimates of error. Since the plant output varies with the climatic condition, the analysis of seasonal variation of these climatic parameters have also been performed ,with the data collected from BMD, for past few years . This forecasts a high plant output in April and a low output in the months of December -January . The mathematical model developed (Eq. 4.2) for the prediction of the diurnal plant output from the given values of the average daily air temperature simulates the similar tretid of variation of the observed plant yield. This model can be satisfactorily used for the prediction of the diurnal yield for the whole year and therefore, such predictions can be considered for the decision making purpose regarding the installation of solar still plants in this region. Solar radiation data is of prime importance in any desalination technique using solar energy. This is , however ,not possible to measure solar radiation at all the localities of Bangladesh since its measurement requires 'Pyranometer'- a costly apparatus. A model has been developed to estimate the radiation value at this region from the easily available data of bright sunshine hours. This study describes the key points of a solar still plant construction with ferrocement and wire mesh reinforcement with detailed steps of operation and maintenance. The plant constructed, had glass as cover material . The various problems encountered at various stages of construction ,operation and maintenance have been depicted clearly.