In this paper, a phosphoric acid fuel cell (PAFC) - thermally regenerative electro-chemical cycle (TREC) hybrid system is investigated in terms of the different parameters including ecological function for different temperatures. Low temperature heat obtained by the PAFC is used by the TREC to generate electricity as a secondary power generator. Ecological function is applied to design an environmentally friendly hybrid system. Results are obtained numerically and an ecologically friendly operation range is attempted to determine for the considered system. According to results, the maximum power of the hybrid system is 885.60 W at 150 degrees C, 935.07 W at 180 degrees C, 949.07 W at 200 degrees C and the maximum ecological function is equal to 373.85 W at 150 degrees C, 431.63 W at 180 degrees C and 439.02 W at 200 degrees C and finally, the maximum efficiency is 0.754 at 150 degrees C, 0.780 at 1.80 degrees C and 0.784 at 200 degrees C. Finally, it is recommended that current density should be chosen between the current density at the maximum ecological function and current density at the maximum efficiency for a more efficient system. (C) 2018 Elsevier Ltd. All rights reserved.