An investigated sea-shell solar concentrator is non-imaging, asymmetric, stationary and has long trough. This concentrator consists of a parabolic, cylindrical trough reflector, a flat double-sided flat absorber (for heat production), and a flat aperture. Three concentrators of such design were investigated by specular solar ray tracing with the software CATIA. They differ in the location of the absorber inside the concentrator. The optical efficiency of direct solar ray transfer inside these concentrators was investigated for different solar ray altitudes at the concentrator aperture for the concentrators installed in Kragujevac, Serbia. For the solar ray altitudes between 50 degrees and 69.75 degrees, the highest optical efficiency was obtained at the solar concentrator where the absorber contains the focus line of the parabola reflector. For the solar ray altitudes between 21 degrees and 50 degrees, the highest optical efficiency was obtained by the solar concentrator where the absorber contains the vertex line of the parabola reflector. (C) 2014 Elsevier B.V. All rights reserved.