Stanciu and Stanciu [1] mentioned that when using the Hottel and Woertz model for estimating the incident solar radiation, the daily optimum tilt angle B-opt,B-d for a flat plate collector could be computed as simplest as B-opt,B-d = L - delta function on the latitude, L, and solar declination, delta, with emphasizing that this equation is applicable all over the world from 0 degrees latitude to 80 degrees and the novelty of their work consists in finding this global optimum instead of "local" ones for which monthly adjustment is required. The present comments are focused on these statements and the novelty of this work. (C) 2015 Elsevier Ltd. All rights reserved.