STUDY OF SOLAR DISTILLER INCORPORATED WITH PHASE CHANGE MATERIAL

Abstract

Clean air, water, food and energy are the birthrights of human being. Potable water is one of the necessities for sustainability, food production, economic growth, and well-being. Moreover, global challenges are prevalent due to its increasing scarcity and non-sustainable supply in different regions, particularly in the developing countries. Globally, water resources are under pressure and supply of potable water is running out due to increasing population, industrialization, and draught at various locations, followed by desertification. With the increase in population, the water consumption rate is increasing twofold in every 20 years. A freshwater scarcity of ~40-80% has been reported in China, India, and the United States and emerged as one of the potential problems. Around one billion people suffer from freshwater scarcity, which will continue to increase due to population and industrial growth. The problem of non-availability of pure water is one of most serious health crisis and need more attention and resources than available today. According to a recent report by the NITI Aayog, a large number of Indians face high to extreme water stress. Over the last decade, the World Bank has supported government’s national groundwater program, the ‘Atal Bhujal Yojana’, to improve the groundwater management and to provide clean drinking water to rural communities. The world economic forum report emphasized the inter connectedness of water with economic stability, climate action, and food security, urging a re-evaluation to manage and value water resources. Diverse desalination methods include multiple-effect humidification (MEH), multi-stage flash distillation (MSF), multiple-effect distillation (MED), multiple- effect boiling (MEB), humidification–dehumidification (HDH), and solar stills. Morever, conventional methods of water purification are not capable anough in remote locations vii because of large centrelized systems, highly energy intensive, large inventory and skilled manpower requirment. The laying down of water pipelines for remote locations is also uneconomical and its supply using road transports is unpredictable and costly. In such circumstances, direct use of solar energy is pivotal to obtain fresh water by the use of solar still, eliminating the major operating cost and address energy and environmental challenges due to their self-sustainability and will assist countries in reaching carbon neutrality targets by reducing carbon emission levels to 50–60% by 2050. Solar distillation using solar stills is viewed as a means to attain self reliance and secure regular dispense of water to meet the requirement of small communities located remotely and suited where demand is less than 200 m3/day and blessed with solar radiation for the 200-350 sunny days a year. Depending on design, mode of operation, and add on, the solar still are broadly classified as (a) passive, and (b) active solar stillls and various scientists throughout the world have carried out numerous research works on design, fabrication methods, testing, and performance evaluation. Moreover, low productivity from traditional passive solar stills is the main issue. The aim of most of the researchers in the field of solar still is to enhance the productivity and can be obtained either by enhancing the basin water temperature or by increasing the difference between water and condensing cover temperature or by both. Moreover, about 28.0% of heat is lost from the basin and sidewalls. Further, availability of solar energy during night and heat losses from the basin during peak radiation to environment is challenging. These drawbacks can be compensated by storing solar energy during peak and utilizing the same during low/off sunshine period using energy storage materials. The use of latent heat energy storage using phase change materials (PCMs) is the most efficient way of storing thermal energy with enhanced cumulative productivity and increased operational time, compensates heat losses, and is preferred over sensible heat storage material due to their viii larger storage capacity/m3 and energy retaining time. The higher difference in water- condensing cover temperature can be obtained by feeding the pre-heated water in the basin from the external collectors/concentrators or by decreasing the surface temperature of the condensing cover or both to enhance the productivity. Evacuated tube collectors (ETCs) are generally considered more useful than the flat plate collectors (FPCs), and even favourable in less sunny climatic conditions. Forced circulation integration with collector is found to be better than natural circulation due to better heat extraction, prevents internal recirculation, inter-mixing of water and stagnation of water at lower end. From the literature, it has been revealed that no performance and energy-exergo- environmental-economic assessment of ETC integrated single slope solar still with PCM under forced circulation has not been investigated yet. The integration of PCM and ETC under forced circulation with solar still (ETSS-PCM) provides a novel approach for achieving higher desalination efficiency. The present research investigates the thermal and distillation performance of ETSS-PCM under forced circulation mode with a primary aim to improve the efficiency and productivity of systems by combining thermal energy storage with advanced solar collection and controlled fluid dynamics. A forced circulation loop, driven by a controlled pump, ensures continuous and efficient heat transfer from the ETC to the solar still. The work encompasses the thermal modeling for utilizing solar energy conversion including heat and mass transfer associated with the proposed system. A thermal model has been developed for the proposed system and numerically simulated. The results obtained for the hourly basin water temperatures are validated with the existing results available for the similar geometry under natural mode with paraffin wax and system without PCM. The synergy with parameterized PCM mass, optimal water depth and utilization of well regulated PCMs is crucial to address the low productivity issue of conventional solar still. The ix comparative assessment of using some PCMs is also presented. The system's performance is evaluated in terms of hourly and daily freshwater yield, basin temperature profile, thermal efficiency, and PCM behavior. The results are benchmarked against a conventional solar still and the same setup under natural circulation. The paraffin wax is considered as a most preferred PCM due to various advantages during the investigation. Exhaustive energetic and exergetic evaluation of the proposed system is further carried out to find the irreversibility and efficiency of various components. The application of 15 kg PCM is found to be optimum for the present system. The yield, energy and exergy efficiencies are estimated as 5.064 kg/m2, 39.56% and 4.05%, respectively, on typical summer day, which are higher by 29.88%, 17.03%, and 35.8%, respectively, than the system without PCM. Minimum irreversibility is found at PCM layer. The effects of operational parameters such as water depth and wind velocity on the performance are also investigated. A comprehensive environmental-energy-exergo-economic assessment is carried out for the sustainability and socio viability of the present system with variable interest rate and system life. The performance of the proposed system is found superior compared to system without PCM and reasonable range compared to other designs. The minimum cost is found to be Rs.0.31/kg at interest rate of 2.0%, 20 years system’s life, and accounting environmental cash flow earned due to carbon credit. This research highlights the synergistic benefits of combining PCM-based thermal storage with ETC and forced convection, offering a viable solution for sustainable and efficient small-scale water purification, particularly in arid and off-grid regions.