Güneş enerjili bir damıtıcıda emici yüzey alanının ekserji verimi üzerindeki etkisinin deneysel olarak incelenmesi

The demand of fresh water goes up continuously with the increasing global population and diminishing supply of fresh water. One of the methods to fulfil the need is to obtain fresh water from salty water. Solar energy can be utilized as an energy source in this process. Solar still systems are cheaper and easy to assemble. In the process of distillation with solar still; salty water in solar still get evaporated using the heat from the sun then allowed to condense (in a glass plate) that provides decontamination of salt. In the present study, the exergy efficiency of a solar still having an enlarged surface was experimentally analysed. Solar stills are in the dimension of 1000mmx1000mm having a glass plate angle of 35º. They are manufactured from galvanized plates. In order to prevent heat loss, glass wool 10mm in thickness was used. Experimental setup was oriented to the south direction. In order to enlarge absorber surface in the experiments, balls are used. Both the balls and interior surface of the solar still are dyed in black to absorb solar radiation. A nozzle is implemented that provides motion of the balls. Water spray is placed at the bottom section in order to provide rotational motion of the balls. A water pump of 25W is used in the setup. The experiments were conducted in July 2007 at the department of mechanical engineering in Firat University. The experiments were carried out between the hours 09:00 am and 05:00 pm. Wind speed, solar radiation, water production and temperatures are recorded in the experiments with a time span of 30 minutes. Radiation values are 640 W/m2, 1010 W/m2 and 346 W/m2 at 09:00 am, the time of maximum readout in a day and 17:00 pm, respectively. Instantaneous exergy efficiency in a conventional system (n=0) changes between 0.01% and 5.35%. Exergy efficiencies vary between 0.01% and 4.38% for 200 balls, 0.01% and 3.52% for 400 balls and 0.01% and 2.74% for 600 balls, respectively. Exergy efficiency decreases with increasing number of balls in a passive system. Exergy efficiency in a conventional system (n=0) changes between 0.01% and 5.32%. Exergy efficiencies vary between 0.01% and 5.99% for 200 balls, 0.01% and 6.85% for 400 balls and 0.01% and %7.83 for 600 balls, respectively. In an active system, increasing the number of ball also raises the exergy efficiency. Instantaneous water temperature increases over the time for both system. Correspondingly, evaporative exergy fraction and efficiency go up in progress of time which are in the range of 0.32 and 0.77. Convective fraction exergy is between 0.07 and 0.1 in case. Likewise, radiative fraction ratio is from 0.61 to 0.16. Exergy efficiency of the passive system was observed to be decreased with increasing ball number. In contrary, number of ball was observed to be increased exergy efficiency of the active system. In general, very low exergy efficiencies were observed in the solar stills. Maximum instantaneous exergy efficiency was identified 7.93% for the active system with a ball number of 600 (n=600). Besides, maximum average daily exergy efficiency specified to be 2.199%. Minimum instantaneous exergy efficiency is 0.758% corresponding the ball number of 600 in the passive system. A passive system can be implemented in order to reduce amount of evaporation from the water. However, in the case of rotational motion of the balls in an active system, the exergy efficiency gets increased. Similarly, it is also known to be enhanced exergy efficiency and amount of produced water. If it is introduced for the purpose of reducing evaporation, the motion of the ball should be prevented (Bilgiç, 2015)