Abstract A novel configuration of a hybrid binary geothermal biomass power plant is proposed that generates electricity through the Organic Rankine Cycle (ORC), which receives the thermal power provided by a biomass heat source through intermediate geothermal fluid. The plants are located in regions with extreme environmental conditions where water is not available, making the use of air-cooled condensers in the ORC necessary, and the seasonal variations of the ambient air temperature are remarkable. As a further novel aspect, the modification of the biomass mass flow rate is used to overcome the simultaneous harmful effects of a considerable reduction in the geothermal fluid temperature during the operative life of the plant and the more significant seasonal change of the ambient air temperature. Our original approach involves developing a simulation model of the proposed plant using the commercial software Aspen® to determine the energy performance in off-design conditions, i.e., in the presence of the simultaneous changes of the biomass flow rate, ambient air temperature and geothermal fluid temperature. The biomass flow rate is controlled to maximize the net electric power or net thermodynamic efficiency of the plant with varying ambient air and geothermal fluid temperatures. In comparison to the first operating mode, the second enables a saving of the biomass used annually in the range of 28.3%–42.6%, corresponding to the maximum and minimum geothermal fluid temperature, respectively, with the resulting detrimental effect on the yearly produced electric energy in the range of 9%–23.6%.