Geothermal power is one of the renewable energies which is extensively used in Italy, especially in Tuscan region, and which is becoming very discussed all around Europe. Geothermal energy exploits the geological heat sources, which are naturally present and it is considered as a renewable energy when the short time period is taken into account. However, the continuous exploitation of the geothermal resource can affect its conditions and particularly its temperature, reducing the production rate of the power plant.
In order to reduce the impact of the geothermal power plant on the resource, a total re-injection configuration of the power plant is taken into account in this study. Particularly, a closed-loop ORC power plant layout including complete gas reinjection is studied for the geothermal location of Castelnuovo Val di Cecina, Italy.
The resource condition is saturated vapour, with a high content (8%) of non-condensable gases (NCGs); therefore, a suitable configuration of the power plant is analysed, both from design condition, as well through the development of a sensitivity analysis of the main operating conditions, such as temperatures, mass flow rates and pressures, both of the power plant and of the NCG reinjection compressor train. The re-compression train configuration is comprised of a 3-stage compressor with one precooler and two intercoolers; the NCG stream is directed downwards in the external annulus of the reinjection well, while the liquid brine flows through the inner pipe section developing progressively the piezometric head. Mixing is achieved at a certain depth (400-600 m) through the use of a reverse gas lift valve. Various operating conditions are analysed, finding the optimal design point, both from an energy and exergo-economic point of view.
Furthermore, the interaction between the binary cycle and the geothermal resource is studied through a dynamic simulation. The compressor train conditions are defined and the conditions of the fluid at well-head are assessed. The flow conditions under various operating transients are analysed, especially regarding the time required in order to achieve steady state conditions. The operational requirements are determined in terms of height of the water column and depth of the NCGs reverse gas-lift mixing valve in the reinjection well in order to match the compressor outlet pressure.