Energy, Exergy and Exergo-economic analyses of supercritical CO2 cycles for the exploitation of a geothermal resource in the Mt. Amiata region

An alternative powerplant layout to the commonly applied flash technology is proposed for the geothermal location of Monte Amiata, Italy. The reservoir conditions correspond to a pressurized brine field, with relevant contents of CO2, acid gases/contaminants, and dissolved salts. The present solution avoids flashing the brine stream, proposing instead to install a borehole pump capable of maintaining pressurized conditions. By applying this solution, the amount of non-condensable gases released when reaching the ground level with reduced pressure can be conveniently limited; the gas stream is recovered, compressed and reduced in liquid conditions, suitable for complete reinjection into the reservoir. The necessity of a gas treatment section is thus completely avoided. The heat recovered from the hot brine, placed at ground level, is transferred to the supercritical CO2 cycle. Different CO2 cycle configurations were considered. Exergy and exergo-economic analyses of the whole system are carried out. The optimal calculated exergy efficiency of 45.5% was achieved for the recuperative cycle with intercooling and reheat configuration and the lowest obtained produced cost of electricity was 7.4 c€/kWh for the recuperative cycle configuration. Furthermore, the influence of the pressure losses to the heat exchanger has been assessed, allowing evaluating of the loss in efficiency for each cycle.