CO2 hydrogenation for geothermal energy storage through synthetic natural gas production and byproduct of refrigeration and freshwater using solid oxide electrolyzer cell (SOEC) and methanation reactor; Techno-economic evaluation and multi-objective optimization

A multi-generation geothermal-based energy system is proposed to convert geothermal renewable power into fuel gas and other valuable products such as electricity, cooling, and freshwater. The system is analyzed and optimized from a thermo-economic perspective. This system is constituted of a flash-binary geothermal power plant integrated into an Organic Rankine Cycle, a double-effect Li/Br absorption refrigeration cycle, a multi-stage flash desalination plant (MSF), and a solid oxide electrolyzer cell (SOEC) combined with a Sabatier methanation reactor. The steam leaving the steam turbine are utilized by the SOEC to generate hydrogen, which is used in the Sabatier reactor to hydrogenate CO2 to produce methane. Cooling and Freshwater are respectively produced by the refrigeration cycle and MSF plant that are driven by the steam extracted from the intermediate stages of the steam turbine. The system can produce 22.9 g/s gas fuel with a lower heating value (LHV) of 28453 kJ/kg, of which 53% is methane, 554.4 kW net power, 3715 kW cooling, and 41.35 kg/s freshwater. The SOEC produces hydrogen with a rate of 6.52 g/s; the power demanded to generate this amount of hydrogen is 884.47 kW. The system's payback period is 7.3 years (regarding the tax). According to the optimization results, compared to baseline design conditions, the payback period and LHV of the generated fuel respectively decline by 18.7% and 35%, and the net output power enhances by 88% at optimum mode when power and payback period are objective functions.