Comparative thermodynamic analysis of Kalina cycle and ORC configurations for the utilization of low to high grade geothermal sources

The Kalina cycle and Organic Rankine Cycle (ORC) are among the most promising technologies for utilizing low to medium temperature heat sources for electricity generation. In the present study, the Kalina cycle, along with subcritical and transcritical ORC configurations using novel and environmentally friendly working fluids, are investigated thermodynamically, considering geothermal sources across a temperature range of 75-300 ℃. The scenarios examined include: the Kalina cycle; subcritical ORC with R600a, R601a, R1233zd(E), and the R601a/R600a mixture; and transcritical ORC with R600a, R601a, and R1233zd(E), with corresponding models created using the Engineering Equation Solver (EES) and MATLAB coupled with the CoolProp tool. Each system is optimized under its respective decision variables to maximize electricity generation, and the optimized systems are compared across the full temperature range. The findings show that the Kalina cycle is the most effective option at low source temperatures, achieving up to 50% higher power generation than subcritical ORC with pure fluids and around 5% more than the zeotropic mixture at the lower end of the range. At medium temperatures (150–175 ℃), the subcritical ORC performs best, with R601a/R600a being the optimal choice at 150 ℃ and R600a at 175 ℃. At higher temperatures, the transcritical ORC configurations are the best performing, with R1233zd(E) proving to be the most effective working fluid. Overall, different systems are favored at different source temperature ranges, highlighting the importance of identifying the most suitable option for maximizing electricity generation.