PERFORMANCE ANALYSIS OF EVACUATED TUBE COLLECTOR AUGMENTED DOUBLE SLOPE SOLAR DISTILLER UNDER FORCED MODE

Abstract

It is a well-known fact that, at present, one of the foremost challenges in front of the entire world is to obtain fresh water from the natural resources of water to meet the increasing demand for freshwater for domestic as well as industrial use. The scarcity score of water is ranked high (R > 0.422) among all the Middle Eastern countries, China, Pakistan, South Africa, etc. including India. To explore the potable water, the population in these areas have to travel long, especially during summer. It is expected that by 2050, worldwide potable water demand may increase more than twice including higher energy use (20-50%) than at present ~ 350 EJ/year. India is a tropical country with about 16% of the combined population of the world, having only 4% of pure water availability with scarcity already apparent in many regions, leading to challenges for the survival of Biota. This may exaggerate further with expected population growth to 1.6 billion at the end of 2050. The water available from rivers, lakes, and underground reservoirs contain a large number of micro-organisms which may cause health hazard to human beings. However, the available water, after distillation, may be used for domestic and industrial use. The conventional methods for distillation exist, they are energy-intensive techniques and require fast depleting sources of energy. In such circumstances, solar energy, which is the oldest form of energy available to mankind and is abundant in nature, provides the best alternative to obtain fresh water by the use of solar still. The rate of distillate obtained from solar stills mainly depends on the operating temperature and shape and material of condensing cover. It is to be noted that the primary aim of most of the research work done in the field of solar distillation is to increase the yield of the distiller unit, which can be attained by maximizing the temperature difference between water vi and the condensing cover. Hence the design parameters should be employed efficiently to attain the above-mentioned aim. Intensity and temperature are interrelated to each other. Higher intensity leads to high temperature inside the solar still and hence results in the higher temperature of water in it. It is essential that the solar radiations falling on solar still should contribute towards the enhancement of temperature of the water, especially during winter months. Temperature is the most critical climatic factor during winter. The aim of most of the research work carried out in the field of solar distillation is to increase the yield. This can be achieved either by increasing the water temperature or by increasing the difference between water and glass cover temperature or by both. The higher water temperature can be obtained by feeding the additional thermal energy to the basin water after pre-heating it externally in the collector. Therefore, a new approach has been employed by designing a modified geometry, coupling double slope solar still with ETC in force mode. The force mode operation has various merits associated with it compared to the natural mode of operation. As per the literature survey, the performance investigation of ETC integrated double slope solar still has not been carried out. The objective of the present work is to develop a thermal model for the proposed geometry of solar still and carry out the numerical simulation to optimize the flow rate and water depth for the number of tubes connected in parallel. The effect of the diffused reflector has also been considered. Further, the performance of the system has been investigated for the water temperature attainable below the boiling point at an optimum flow rate. The effect of water depth in the basin on the output such as yield, energy efficiency, and exergy efficiency has also been investigated. Finally, the annual performance of the present design of solar still has been estimated and compared with the other designs reported in the open literature. The performance of the proposed model of solar still is found superior compared to other designs in terms of output per m2 of surface area. At optimal flow rate, system yields 6.644 kg using 10 tubes at 0.005 m water depth.