A concentrating photovoltaic (CPV) plant is a complex system that integrates different technologies as single or multi-junction photovoltaic cells and optical devices. The CPV system performance analysis should take into account the malfunctions that can occur during the working, especially when the system operates with high values of sunlight concentration. A critical analysis of the solar cells is necessary to define the CPV system potential. In this paper a specific configuration of a CPV system is considered, and the experimental analysis of a system with a degraded triple-junction InGaP/GaAs/Ge solar cell is investigated. In particular, the triple-junction solar cell is stressed in an accelerated aging process of about 500 operating hours with a concentration of 310 suns without using a cooling system. After this process, for another 100 working hours the system has been monitored in order to compare, corresponding to different light concentration factors, the solar cell electric characteristics, the energy production and the power conversion efficiency in the pristine and degraded states. The results show the effect of the overheating of the triple-junction solar cell caused by the excessive increase of the light intensity. Under the same irradiance of 930 W/m(2) the short circuit current, open circuit voltage and fill factor values of the aged solar cell, compared to the pristine device, result to be strongly reduced, while the extracted value of the series resistance increases and the values of the shunt resistances decrease. The increased value of the diode ideality factor m after the thermal stress indicates a non-negligible contribution of non radiative recombination within the solar cell. Similar findings are deduced comparing the electroluminescence spectra of the pristine and degraded solar cell. The thermal stress induces a marked drop of electroluminescence signal intensity in the whole investigated wavelength range. It should be note that high electroluminescence efficiency is a good indicator in solar cells for high power conversion efficiencies. In particular, the power conversion efficiency is reduced by 50% referring to a CPV system with a degraded cell, while the electric output power is decreased by 30%. Hence, it is clear that the triple-junction cell inefficiencies, principally caused by a strong thermal stress, lead to a drastic drop of the CPV system performances. Finally, an active cooling system is absolutely necessary when high values of light concentration are reached.