A comprehensive exergy evaluation of a deep borehole heat exchanger coupled with a ORC plant. The case study of Campi Flegrei

The paper presents a comprehensive energy and exergy analysis of a possible geothermal power plant located in the geothermal district of Campi Flegrei (Italy), made of a coaxial WellBore Heat eXchanger coupled to an Organic Rankine Cycle. We have accounted for all system components: the ground source, the WellBore Heat eXchanger, the Organic Rankine Cycle cycle, and cooling system. The energy and exergy performance indexes of each subsystems and overall system have been evaluated, thus calculating the net power, the First-Law efficiency, the Second-Law efficiency, the irreversibilities. The results indicate a good potential of the WellBore Heat eXchanger – Organic Rankine Cycle technology in the area, as the estimated performances have similar values to those of classical binary geothermal power plants: a First-Law efficiency of 11.67% and a Second-Law efficiency of about 43.80%. The overall system performances decrease respectively to 10.62% due to the fans energy requirements in the cooling tower and to 23.15% due to the large exergy destruction occurring in the WellBore Heat eXchanger. A deep exergy analysis of the WellBore Heat eXchanger has highlighted that the overall irreversibility is strongly affected by the insulation performance between the two coaxial pipes and by the temperature deviation between the ground and the fluid. The latter one is mainly due to the continuous heat extraction from the geothermal source, therefore proposed improvement strategies consist of both the increasing of thermal resistance of the material insulating the upward pipe and the reduction of the equivalent thermal radius of the well optimizing the heat extraction profiles over the plant lifetime.