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Solar collector water heating system use solar thermal energy to provide hot  water for domestic and industrial use. These systems are operated either as  open-loop or closed-loop flow circuit. The former loop systems are not  recommended for the cold climates having water freezing problem. Although  previous studies on solar collectors have used closed-loop operation with  water as the working fluid; however, it must have high boiling and low  freezing points for the colder regions and thus arises the need for  antifreeze mixtures of water. Another solution to the same problem is the  use of heat transfer oil as intermediate working fluids. In the present  study, the energy and exergy analysis of a boiler supported vacuum tube  solar collector system working with closed-loop in different working fluid  flow rates have been performed and evaluated. Heat transfer oil has been  used as an intermediate working fluid in the closed loop system at different  flow rates of 0.277 kg/s, 0.383 kg/s, 0.494 kg/s. The results show that the  collector temperature difference as well as the outlet temperature decrease;  however, the collector inlet temperature increases by increasing the flow  rate. Moreover, with the increase in flow rate, it was ascertained that the  energy and exergy efficiency of the system and the collectors increase. The  main finding of the present study is that the intermediate fluid used in the  closed-circuit operation of the solar collectors has a direct effect on the  energy and exergy efficiency of the system.

The effect of the intermediate fluid-flow rate on the system performance in the closed circuit applications of the solar collector