Performance and operation of micro-ORC energy system using geothermal heat source

In the electricity production sector, geothermal energy is considered a reliable energy source because of its independence of seasonal, climatic and geographical conditions. Low-temperature geothermal wells present a huge potential of exploitation, as the development of binary cycles and the technological improvement in drilling make this heat source a competitive solution for electricity generated distribution and self-consumption. The Organic Rankine Cycle (ORC) is currently the best solution to convert heat into electricity using low enthalpy heat sources. The ORC technology is already mature and widespread for medium and large-scale power plants, applying for geothermal, solar, biomass or waste heat recovery exploitation. Micro-scale ORC applications are still not diffused in the market: the system layout, the working fluid selection and the expander architecture can significantly vary depending on the specific realization requirements, thus a standard configuration has not established yet. In this paper, a particular case study of a micro-ORC power system using a geothermal well is presented. The application in analysis is a plug-and-play ORC facility, currently installed and operating in a pool centre. The system layout and the main components are described. The heat source is a geothermal well, which continuously supplies (by pressure difference) liquid water at a temperature lower than 60°C to a binary Rankine cycle working with R134a. The ORC system is driven by a prototypal radial-piston expander and adopts an external-gear feed pump and a recuperative cycle. It is developed for working continuously, delivering the generated electricity directly into the grid. The facility is provided with temperature, pressure and electric power sensors for monitoring the operation and for a preliminary evaluation of the performance. The global efficiency of expander and feed pump and the ORC net efficiency have been evaluated at the regular working conditions of the geothermal well, showing values equal to, respectively, 53 %, 41 % and 4.4 %