A techno-economic assessment of sCO2 Brayton cycles for low-grade geothermal reservoirs

Abstract

CO2-based power cycles have recently received much attention because they have several advantages over traditional power cycles in a wide variety of applications. For low-grade heat sources such as geothermal energy, the research has focused on the transcritical CO2 Rankine cycle (TCRC). However, the TCRC requires relatively low-temperature heat sinks to condense the CO2 and operate properly, preventing its deployment in many locations. A straightforward solution is to use a supercritical CO2 Brayton cycle (SCBC) instead. Nonetheless, in the context of binary cycle geothermal power plants, there is a lack of information about the feasibility of implementing SCBCs. Therefore, to elucidate the viability of employing SCBCs for power generation using geothermal heat, this paper presents an assessment of the thermodynamic and economic performances of four SCBC configurations. For this purpose, thermodynamic and economic models of the system, along with a thermal-hydraulic model of the heat exchangers, were developed. Net power output and net present value were used as technical and economic performance indicators, respectively. Subsequently, base case simulations were conducted, and multi-objective optimization using NSGA-II was performed. Results show that, using 20 kg/s of geothermal brine at 150 °C as the heat source and cooling water at 25 °C as the heat sink, an intercooled recuperated Brayton cycle (IRBC) is superior to the other considered configurations. It can achieve up to 932.07 kW of net power output at the optimal thermodynamic conditions and 4.45 × 103 USD of net present value at the optimal economic conditions. Besides, balancing the thermodynamic and economic performances, the IRBC attains the highest overall performance. In conclusion, using SCBCs in binary cycle geothermal power plants could be feasible, and the IRBC is a suitable layout.