Collectors for building-scale solar thermal systems are generally fixed, so that energy capture is optimized over the course of a year, but is generally not optimal on any given day. In particular, abundant sunlight available during summer mornings and late afternoons is not captured. Moreover, ancillary systems (pumps, piping, etc.) are designed to operate at peak, meaning that they are underutilized for the majority of the time. In this study, reflective surfaces are used to reflect morning sunlight onto fixed collectors. The orientation of the reflectors is optimized using simulation. The simulation results are corroborated by experimental data from a 150 kW, system located on the roof of the University of New Mexico's Mechanical Engineering system in Albuquerque, New Mexico. The study also considers economic and performance benefits deriving from using reflectors in combination with fixed collectors. It is found that the simulation correctly predicts system performance, and that a reduction in the cost or solar cooling of approximately 20% can be obtained by installation of the mirrors, without adding any extra components to the system. Perhaps the most important outcome of this study is that performance of booster mirrors is extremely sensitive to local climatic conditions and to the constraints placed on the positioning and orientation of the booster reflectors, making it impossible to generalize findings for one particular location. Thus, computer simulation is essential for designing and optimizing thermal systems with booster reflectors. (C) 2011 Elsevier Ltd. All rights reserved.