The present work analyzes the performance of unshielded receiver tube integrated solar parabolic trough collector where Al.sub.2O.sub.3/deionized (DI) water nanofluid of low concentrations was used as heat transfer fluid (HTF) element. Nanofluid is synthesized at various volume fractions starting from 0.2 to 1.0% with surfactant-free condition, by ultrasonic technique. Several researchers investigated the performance of higher nanofluid concentrations (1.0-5.0%) with and without surfactants on parabolic trough solar collector. The outdoor experiments are conducted for two HTF flow rates of 0.010 kg s.sup.-1 and 0.015 kg s.sup.-1. When the nanofluid is subjected as HTF, the DI water acted as a base fluid. While DI water is allowed to flow through the absorber, it performs both as HTF and heat storage fluid. The synthesized nanofluid at various volume fractions is allowed to flow through the receiver for the purpose of analyzing the thermal performance and compare the results with DI water. The collector efficiency increases with the mass flow rate as well as the concentration of nanofluid. For 0.015 kg s.sup.-1, the maximum efficiency was calculated as 59.13% (hourly) and 58.68% (average).