THEORETICAL PERFORMANCE ASSESSMENT OF A PARABOLIC TROUGH HUMIDIFYING SOLAR COLLECTOR-BASED SOLAR STILL

Abstract

In this paper a parabolic trough humidifying solar collector-based solar still (PHSC-SS) is proposed. Its purpose is to apply some important performance improvement techniques to the flat plate humidifying solar collector-based solar still (flat plate HSC-SS) to significantly improve overall system performance. These included the use of parabolic trough solar concentrators and the design of humidifying solar collectors from evacuated tube collectors. The results reveal that unlike flat plate HSC-SS which must operate with a turbulent airflow regime to achieve optimum overall performance PHSC-SS must operate with a laminar airflow regime and high inlet and outlet temperatures of air (at least 55 degrees C and less than 100 degrees C at atmospheric pressure) in the heat collector element. For 900 W/m2 of incident solar irradiance 2 m2 of solar collector area and 000042 kg/s of air flow rate the maximum energy efficiency exergy efficiency and daily freshwater productivity of PHSCSS were found to be 6812% 1487% and 1697 kg/h respectively. Whereas for the same incident solar irradiance and solar collector area and 01 kg/s of air flow rate those of the flat plat HSC-SS were 729% 112% and between 107 - 2923 kg/h (for inlet and outlet temperatures of air less than 30 degrees C at atmospheric pressure) respectively. Although in some extreme cases freshwater productivity of flat plate HSC-SS can be higher than that of PHSC-SS it should be noted that laminar airflow regime confers great advantages to PHSC-SS. These are higher air temperatures at condenser inlet (which ease water condensation process) no need of an auxiliary cooling device (needed in the flat plate HSC-SS) less mechanical vibrations of system reduced condenser size and less energy consumed by air blowers. Furthermore the upper limit of the PHSC-SS is a PHSC-SS that operates without air flow but rather by vaporization of water droplets at boiling point from absorber followed by their suction to condenser similarly to a flash evaporation.