Experimental Analysis of Modified Stepped Solar Still

Abstract

Fresh water is the basic requirement of human life. Water for drinking purpose, industrial and agricultural purposes demands ample quantity of fresh water. The solar desalination systems have the advantage of low maintenance and operational cost. In spite of high investment cost of solar stills, they are friendly to the nature. In the present article, a modified stepped solar still has been designed, fabricated and tested. The modification includes fabricating steps on which small earthen lamps are placed. The maximum temperature on the steps was found to be 67oC. In case of conventional solar still the maximum temperature recorded was 53oC. The maximum temperature of water at the steps was 26.41% higher than the basin temperature of conventional solar still. The maximum productivity of the modified solar still was 326.66% higher than conventional solar still. The optimum water input flow was found to be 250 ml/hr. I. Introduction Fresh water is primary requirement of human race. Water for drinking purpose and other industrial and agricultural purposes demands large quantity of fresh water (1). The solar desalination systems have the advantage of low maintenance and operational cost. Although solar stills have high investment cost, they are compatible with the environment (2). About 6.3 billion people are living on earth in which 400 million people are at living in water scarce areas which would rise to four billion by mid-century (3). The fabrication of solar still is simple and can be constructed by locally available materials and unskilled people. Solar stills are widely used for desalination in small scale. (4). According to an estimation, about 97% of available water sources are saline and/or consist of harmful bacteria and 2% are in the form of frozen glaciers and polar ice. Hence, only 1% of the earth's water can be used for drinking and domestic purposes (5).The arising requirement for water is primarily due to the increase of population and fast growth of industries (7). The heat energy of sun can be used in solar collectors, solar cooking, desalination and cooling systems (8). Generally, for steam generation, water heating processes and CPCs are used, as they are capable of producing higher temperatures in comparison of flat plate collectors, they can also be used for desalination purpose (9).Most of the arid, remote and semi-arid regions are lack natural fresh water in the southern part of the country and are dependent heavily on underground water for drinking purpose (10). Higher productivity is obtained in multiple-effect diffusion solar still (MEDS) (11). Solar stills are largely used for water purification in rural and remote areas with limited demand and low population when the potential of brine water sources and solar energy are available (12). II. Literature review There has been a lot of work in the field of solar stills to enhance its performance. Researchers have improved the performance by various means such as change in geometry, employment of mirrors in various orientation, use of waste heat etc. Park et al. (2016) designed a multiple-effect diffusion (MED) hybrid solar still with dual heat sources of solar thermal energy and waste heat. Performance tests with waste heat were performed with three operational parameters: (1) the seawater level in the basin (2) the seawater flow rate to the wick and (3) the amount of heat input into the hybrid still. It was observed that the productivity of the hybrid still increases with the increasing heat input, which recorded productivity of 18.02 kg/m 2 at waste heat amount of 22.37 MJ/d. The maximum productivity of distillate was obtained at the lowest seawater level with the experiment including waste heat source. The maximum performance of the MED solar still was achieved at the operation condition obtaining maximum productivity at the second effect plate (13). El-Agouz modified a stepped solar still using a storage tank for salt and sea water with continuous water circulation. Investigation was made on effect of installing a storage tank and cotton black absorber for modified stepped solar still on the distillate productivity. The indication was given by the results that, the productivity of the modified stepped still was higher than conventional solar still by 43% and 48% for sea and salt water with black absorber respectively, while 53% and 47% of sea and salt water, respectively with cotton absorber. The daily efficiency for conventional solar still was 20% lower than the modified stepped still. At a feed water flow rate of 3 LPM for salt water and 1 LPM for sea water, maximum efficiency of modified stepped still was obtained (8).