In solar collectors, the absorbed incident solar radiation is converted to heat.The working fluid conveys the generated heat
for different applications. Solar collectors are categorized in tow types, non-concentrating and concentrating collectors. Non-concentrating solar collectors are usually used for low and medium temperature applications such as space heating and cooling, water heating, and desalination. While concentrating solar collectors are exploited in high temperature applications such as
electricity generation. However these systems are acquiring more and more attention, prevailing to low efficiency is still a big deal. Nanofluid has shown a good ability in enhancing the efficiency of solar systems. In this part, there search over employing nanofluid in solar collectors are reviewed.