Techno-economic evaluation of geothermal-solar power plant in Nisyros island in Greece

The environmental degradation caused by the increased use of fossil fuels in the energy sector, call for an urgent switch to alternative energy sources which can assure sustainability, energy security and reliability. So far, solutions based on Renewable Energy Sources (RES) have been proved reliable in providing energy even in autonomous networks, if they combine different sources (e.g. solar and wind) or combined with conventional energy generators. Among the promising solutions to the energy problem is the geothermal energy which consists of stored energy in the earth in the form of water or steam at high pressure and temperature. The countries which are most likely to possess a high geothermal potential are those located in seismic zones close to destructive and constructive plate margins. Greece is among the countries that, due to location, possesses a remarkable geothermal potential of medium and high enthalpy fields. The interest in exploiting geothermal energy in Greece for electricity generation is mainly focused on non-interconnected islands, where energy is produced from oil-based generators with high energy cost, mainly during the summer months which is a period of rapid increase in energy demand. In this respect, the present study investigates, through an economic evaluation, the optimum sizing of a hybrid solution, which includes a geothermal power plant and a solar field of concentrated collectors able to cover the energy needs of an isolated island. The combination of solar and geothermal energy source aims to maintain the long-term productivity of the geothermal field by reducing its degradation ratio. The thermal energy, generated by the solar installation, increases the temperature of the geothermal brine before injected it back into the well and therefore reduces its energy degradation. In this study, different cases of solar field sizes are evaluated in order to achieve the optimum combination of the geothermal power plant and the concentrated solar array. According to the results, the proposed installation can guarantee energy autonomy with increased economic efficiency and competitive energy production cost, compared to the current prices in small isolated Aegean Sea islands.