Thermoeconomic design and optimization of geothermal energy use in hydrogen production and liquefaction

Six models are developed for the use of geothermal energy in hydrogen production and liquefaction and thermoeconomic analysis and optimization of these models are performed. The aim is to use geothermal energy in hydrogen production by appropriate combination of systems at optimum operating conditions. Optimum operating conditions that minimize the unit hydrogen cost are obtained. A liquid geothermal resource at 200°C with a flow rate of 100 kg/s is considered. Among the models involving hydrogen production, the minimum exergetic cost of hydrogen production is obtained in Model 2 with a value of 1.088 US$/kg H2. In Model 2, the power generated from a combined flash-binary geothermal power plant is supplied to an electrolysis unit for hydrogen production, and geothermal water at the exit of the power plant is used to preheat electrolysis water. In the models involving hydrogen liquefaction, the minimum exergetic cost occurs in Model 4 with a value of 1.114 US$/kg H2. In this model, geothermal heat is used for absorption cooling of hydrogen gas and geothermal electricity is used to supply power to a liquefaction unit. In Model 6, in which geothermal energy is used for both hydrogen production and liquefaction, the exergetic cost is calculated as 1.993 US$/kg H2, which is 9.5% less than the sum of the values in Model 2 and Model 4. Parametric studies indicate that unit cost of hydrogen decreases by an increase in geothermal resource temperature.