In this study, numerical analysis has been used to investigate the thermal efficiency of the different types of flat plate solar collectors. Different numerical models were employed in order to evaluate the influence of geometrical characteristics and operating conditions on thermal performance of solar collectors. The effects of important parameters such as absorber thickness, riser position, shape of tube area section, absorber material, absorber absorptivity, glass transmissivity, and mass flow rate have been investigated. Results show that the efficiency of collector with risers on top of the absorber plate is 4.2% more than that of the collector with risers at bottom of the absorber plate. Also the efficiency of collectors with circular tubes is 38.4% more than collectors with triangular tubes. Thermal efficiency of solar collectors increases with increasing the fluid flow rate, plate absorptivity, absorber thickness and glass transmissivity. Collectors with working fluids of MWCNT-H 2 O, Al 2 O 3 -H 2 O and CuO-H 2 O nanofluid with 1, 2 and 3 wt% nanoparticle concentration at different reduced temperature difference ((Ti-Ta)/G) were studied and results compared with collectors with water as working fluids. Results show that collector with any type of nanofluid has more efficiency compared to a collector with the same pure base fluid and with increase of nanofluid concentration, the collector efficiency was increased.
