The paper presents the preliminary operational performance results of the of two 3.5 kWp Concentration PhotoVoltaic (CPV) devices. Each system consists of eight modules installed on a chassis for a total number of 1152 triple junction PV cells whose active area is 5.5 x 5.5 mm. The optics has a total geometrical concentration ratio of 476x. Two solutions for the primary PMMA Fresnel lens were tested, one with constant Fresnel pitch and one with variable pitch. The secondary optics is the same for both systems and consists of a truncated pyramid made of high reflective material. The two-axis tracking system is an azimuth-elevation device driven by two electrical motors and controlled by a sun sensor. Results allow to evaluate the efficiency of the plant as well as significant operational parameters under real outdoor operating conditions. The overall AC electrical efficiency is up to 31% and the power production peak is 2.54 kW. Electric power output has a linear dependency with the available Direct Normal Irradiation (DNI) while the Air Mass (AM) spectrum has a negligible effect on the performance. The system equipped with a variable pitch Fresnel lens performs slightly better (about 3.5% more power) with respect to the one with a constant pitch. The effect of lens fouling has a much higher impact: with a dirty lens the system generates over 12% less power and efficiency decreases by 3-5%, at equal solar irradiation. The performance ratio of the CPV system peaked at 82% and it has a monthly value over 70% in spring and summer months. The tracking mechanism has showed, in the worst scenario, an inaccuracy of 0.26 which is lower than the acceptance angle of the optics (0.4). (C) 2015 Elsevier Ltd. All rights reserved.