Binary ORC (Organic Rankine Cycles) power plants for the exploitation of medium–low temperature geothermal sources – Part B: Techno-economic optimization

This two-part paper investigates the potential of ORC (Organic Rankine Cycles) for the exploitation of low-medium enthalpy geothermal brines. Part A deals with thermodynamic analysis and optimization, while Part B focuses on economic optimization. In this part, an economic model was defined and implemented in the Matlab® code previously developed. A routine was also implemented to estimate the design of the turbine (number of stages, rotational speed, mean diameter), allowing to estimate turbine efficiency and cost. The tool developed allowed performing an extensive techno-economic analysis of many cycles exploiting geothermal brines with temperatures between 120 °C and 180 °C. By means of an optimization routine, the cycles and the fluids leading to the minimum cost of the electricity are found for each geothermal source considered. Cycle parameters found from the techno-economic optimization are compared with those assumed and found from the thermodynamic optimization. Quite relevant differences show the necessity to perform optimization on the basis of specific plant cost. As a general trend, it is however confirmed that configurations based on supercritical cycles, employing fluids with a critical temperature slightly lower than the temperature of the geothermal source, lead to the lowest electricity cost for most of the investigated cases. - Highlights: • We investigate 6 different cycle configurations adopting 54 working fluids. • We define components cost correlations and a turbine efficiency estimation method. • Techno-economic optimization is performed for various geothermal brine temperatures. • We set general rules for working fluid and cycle configuration selection