This paper presents the unsteady, one-dimensional performance analysis of cylindrical parabolic concentrating collectors through the first and second laws of thermodynamics. These laws, together, help to define the optimum system that satisfies the imposed thermal and economical constraints and minimizes exergy loss. The analysis considers the thermal masses of the absorber pipe, pyrex envelope and the working fluid of the system individually, in an unsteady state. The instantaneous exergetic and energy efficiencies for the cylindrical parabolic collectors are compared for daily insolation at different flow rates. The values measured using the MKE Institute Collector Test Facility are in good agreement with the predicted simulation results. The simulation results indicate that the exergetic efficiency is highly dependent on the mass flow rates and the arrangement of the cylindrical parabolic collector array. The optimum flow rates which provide minimum exergy loss were obtained from the simulation model. (C) 1998 Elsevier Science Ltd. All rights reserved.