In this paper, we derived the collector efficiency factor, F, of two solar collectors of parallel plates for water, which is based on the efficiency factors of air heaters. This solar collectors were built in the past but without considering their efficiency factors. The efficiency factors: F and F, correspond to the basic model of Hottel Whiller Bliss (widely adopted in the literature) and they are useful for simulating the behavior of solar collectors. Finally we assess the theoretical potential of parallel plates collectors; in this case, the parallel plates collector "absorbent cover", is more efficient than the conventional one, assuming an application of a thermosyphon system with laminar velocity. (C) 2013 Elsevier Ltd. All rights reserved.