A Comparison Between Organic Rankine Cycle and Supercritical CO₂ Bottoming Cycles for Energy Recovery From Industrial Gas Turbines Exhaust Gas

Gas turbines are often employed in the industrial field, especially for remote generation, typically required by oil and gas production and transport facilities. The huge amount of discharged heat could be profitably recovered in bottoming cycles, producing electric power to help satisfying the onerous on-site energy demand. The present work aims at systematically evaluating thermodynamic performance of organic Rankine cycle (ORC) and supercritical CO₂ energy systems as bottomer cycles of different small/medium size industrial gas turbine models, with different power rating. The thermoflex software, providing the GT PRO gas turbine library, has been used to model the machines performance. ORC and CO₂ systems specifics have been chosen in line with industrial products, experience and technological limits. In the case of pure electric production, the results highlight that the ORC configuration shows the highest plant net electric efficiency. The average increment in the overall net electric efficiency is promising for both the configurations (7 and 11 percentage points, respectively, if considering supercritical CO₂ or ORC as bottoming solution). Concerning the cogenerative performance, the CO₂ system exhibits at the same time higher electric efficiency and thermal efficiency, if compared to ORC system, being equal the installed topper gas turbine model. The ORC scarce performance is due to the high condensing pressure, imposed by the temperature required by the thermal user. CO₂ configuration presents instead very good cogenerative performance with thermal efficiency comprehended between 35% and 46% and the primary energy saving (PES) value range between 10% and 22%. Finally, analyzing the relationship between capital cost and components size, it is estimated that the ORC configuration could introduce an economical saving with respect to the CO₂ configuration.