Experimental Study of Thermal Performance of One-Ended Evacuated Tubes for Producing Hot Air

The thermal performance of an evacuated tube solar air collector is experimentally investigated at different air flow rates. Air is used as a working fluid in experimental setup and tested in Indian climatic conditions. The evacuated tube solar air collector consists of fifteen evacuated tubes and manifold channel. The manifold channel consists of a hollow pipe (square pipe) in centre through which air flows. The temperature difference and efficiency are studied with different air flow rates. The reflectors are used to enhance the performance of evacuated tubes solar air collector. It is observed that in case of reflector evacuated tube solar air collector gives higher outlet temperature and temperature difference and has better thermal performance as compared to the case without reflector. The maximum outlet temperature and temperature difference of air are found to be 97.4°C and 74.4°C at a flow rate of 6.70？kg/hr. 1. Introduction Nowadays the demand of conventional resources of energy is very high, so these resources are continuously depleting day by day. The development of renewable energy technology is important for the future to balance global energy resources. For many years the evacuated tube has been used to heat the water but it has never been used for the generation of hot air. One-ended glass evacuated tubes are widely used and are widely popular in the world because evacuated tubes have better performance. The performance of flat plate collectors is lower than evacuated tube solar collectors for high temperature operation. The evacuated tube consisting of two concentric tubes is used to heat the fluid flowing inside the inner tube. The outer portion of the inner glass tube is coated with an absorbing material to absorb heat. A review of the literature suggests that considerable efforts have been made in the research and development of the evacuated tube solar collectors. Garg and Chakravertty [1] developed an empirical relation for finding overall heat loss coefficient in case of evacuated tubular collector for all possible ranges of variables and values compared with the analytically calculated values and found that efficiency decreased in series combination and remained constant for parallel combination. Kumar et al. [2] studied a thermal performance of a community type solar pressure cooker based on evacuated tube solar collector. Morrison et al. [3] developed a number of heat extraction methods from all glass evacuated tubes and found the water in glass concept to be the most successful due its simplicity and low manufacturing cost.