Application of shallow geothermal energy systems to pressure reduction stations of the natural gas pipeline network

Climate change and fossil fuel depletion are urging all industrial sectors to reduce the use of non-renewable energy sources and replace them with renewables. The management of gas pipeline networks and, in particular, gas pressure reduction stations (PRS) represent a niche with good potential to increase energy efficiency. PRSs are plants connecting the international high-pressure pipeline network, operating at about 60 bar and managed by the State-owned company SNAM, to the local low-pressure network, operating at about 5 bar and managed by local companies. The expansion operated at PRS causes a temperature drop of gas, even below 0 °C, which could damage pipelines. Therefore, PRSs need to warm up natural gas upstream the expansion valve. The heat is generally provided by gas boilers, with consequent fossil fuels consumption and pollutant emissions into the atmosphere, but also the risk of fires and explosions. Alternative solutions for gas preheating at PRS have therefore been sought and, among them, heat pumps. Indeed, heat pumps combine the benefits of eliminating combustion on site and reducing overall greenhouse gases (GHG) emissions. This thesis presents the case study of a PRS NW Italy which delivers about 16.5 million Sm3 of gas per year. The line-heater consumes about 18500 Sm3 of gas per year (i.e., about 180 MWh/y), implying GHG emissions of about 40 tCO2 equivalent. Natural gas thermodynamic processes and properties were studied to determine inlet and outlet temperatures at the shell and tube heat exchanger and the overall heat transfer coefficient. Calculations on the preheating system led to an estimation of 40 kW as peak thermal power needed by the PRS. The option to replace the gas boiler with a geothermal heat pump was evaluated. Since the site lies on an alluvial plain, an open-loop system was proposed. Although the local stratigraphy and hydrogeological setting were not studied widely so far, merging different datasets allowed to derive an acceptable conceptual model of the site. A single-well pumping test performed at an existing borehole led to an estimation of transmissivity and hydraulic conductivity. Based on the results of these elaborations, the open-loop system was simulated with a numerical flow and heat transport model implemented on the finite-element code FEFLOW. Modelling results confirmed the operational sustainability of the open-loop system. As an alternative, a closed-loop system was designed adopting the Eskilson method implemented on Earth Energy Designer (EED). A payback time of 7 and 10 years was found for the open-loop and the closed-loop geothermal systems, respectively, with a reduction of GHG emissions of more than 70% in both cases. The work carried out in this thesis sheds light on the potential to implement renewable energy sources in the gas distribution network, reducing its impacts on environment and increasing its sustainability and safety.