The aim of this book is to develop area of direct absorption solar collector (DASC) by identifying its thermal performance using nanofluids. This technology gives improved thermal performance as compared to conventional tube-in-plate type flat plate solar collectors. The DASC is based on the principle of utilizing incident direct solar radiations in the thin film of fluid over the base plate and utilizes volumetric absorption of incoming solar radiation. This technique of directly absorbing the solar energy i.e. eliminating the role of the absorber plate seems to be of great importance as the conventional collector's employs absorption of solar energy first to a black surface collector, then to the tubes and finally heat is transferred to a working fluid, this leads to high losses of heat. After a detailed study of DASC, this book presents a research on DASC working with water and nanofluids at various concentrations, after analysis it can be concluded that direct absorption is aunique and better technique to utilize the solar energy for higher thermal performance of solar collector.