Thermo-economic analysis of a geothermal-driven district heating network: Comparison between a water-based and a CO2-based grid

The implementation of geothermal-based district heating networks has a substantial impact on mitigating greenhouse gas emissions. Efficient and environmentally friendly heating infrastructures can be established exploiting the heat resources available in existing geothermal reservoirs. Furthermore, the establishment of district heating networks could facilitate a more cost-effective transition towards renewable sources for the end-user, if compared to the need of installing an individual heat pump. This could lead to a reduction in social conflict when pursuing the environmental goals set by policymakers. In this framework, the present work compares two different district heating network layouts: a water-based and a CO2-based network, with the goal to identify the most promising configuration. While water-based networks are quite known and already installed in multiple locations in Europe, the CO2-based concept is innovative and could provide significant benefits when coupled with a geothermal reservoir. The network is composed of two pipes, respectively filled with saturated liquid and vapor, at relatively low temperatures (0 °C to 30 °C), and heat is provided to final users by compressing vapor to reach the required outlet temperature. This makes the system very flexible and allows it to operate even with very cold and shallow geothermal reservoirs, thus significantly reducing the installation cost. A complete thermodynamic and economic analysis has been performed for both cases based on an existing geothermal reservoir in Gavorrano (southern Tuscany, Middle Italy), finding that the CO2 network reduces the necessary drilling depth from 5 to 4 km with respect to water and the Levelized Cost of Heat (LCOH) from 0.21 to 0.16 € /kWh. Although the CO 2 -based surface plant can achieve very high COP due to the use of the geothermal well as a heat source, the economic optimum is a COP of 3, since higher COPs would require more heat extraction from the well, increasing drilling costs.