In the present study, a two-phase closed thermosyphon flat-plate solar collector with a shell and tube
heat exchanger was investigated experimentally under the field conditions of Cairo, Egypt. The collector
was designed, constructed, and tested at transient conditions to study its performance for different cooling
water mass flow rates at different inlet cooling water temperatures. Also the effect of the number of the
thermosyphon tubes on the performance of the collector was investigated. Under different climate conditions,
the experimental results showed that the optimal mass flow rate is very close to the ASHRAE
standard mass flow rate for testing conventional flat-plate solar collectors. Also, the experimental results
indicated that the number of the thermosyphon tubes has a significant effect on the collector efficiency. The
performance of the present collector with optimum number of thermosyphon tubes was compared with the
performance of two-phase closed thermosyphon flat-plate solar collectors with tube in tube heat exchangers
of previous investigators and a better performance for the present collector was obtained at high inlet water
temperature.