Energy Exploitation of High-Temperature Geothermal Sources in Volcanic Areas-a Possible ORC Application in Phlegraean Fields (Southern Italy)

This work aims to investigate the energy performances of small-scale Organic Rankine Cycles (ORCs) for the exploitation of high temperature geothermal sources in volcanic areas. For this purpose, a thermodynamic model has been developed, and a parametric analysis has been performed that considers subcritical and transcritical configurations, and different organic fluids (isobutane, isopentane, and R245ca). The investigation illustrates the significant effect of the temperature at the entrance of the expander on the ORC behaviour and the rise in system effectiveness when the internal heat exchange (IHE) is adopted. As a possible application, the analysis has focused on the active volcanic area of Phlegraean Fields (Southern Italy) where high temperature geothermal reservoirs are available at shallow depths. The work demonstrates that ORC systems represent a very interesting option for exploiting geothermal sources and increasing the share of energy production from renewables. In particular, the investigation has been performed considering a 1 kg/s geothermal mass flow rate at 230 degrees C. The comparative analysis highlights that transcritical configurations with IHE guarantee the highest performance. Isopentane is suggested to maximise the ORC electric efficiency (17.7%), while R245ca offers the highest electric power (91.3 kW(el)). The selected systems are able to fulfil a significant quota of the annual electric load of domestic users in the area.