Systematic Comparison of ORC and s-CO2 Combined Heat and Power Plants for Energy Harvesting in Industrial Gas Turbines

Gas turbine power plants are widely employed with constrained efficiency in the industrial field, where they often work under variable load conditions caused by variations in demand, leading to fluctuating exhaust gas temperatures. Suitable energy harvesting solutions can be identified in bottoming cycles, such as the conventional Organic Rankine Cycles (ORC) or the innovative supercritical CO2 (s-CO2) systems. This paper presents a detailed comparison of the potential of ORC and s-CO2 as bottomers of industrial gas turbines in a Combined Heat and Power (CHP) configuration. Different gas turbine models, covering the typical industrial size range, are taken into account and both full- and part-load operations are considered. Performance, component dimensions, and operating costs are investigated, considering ORC and s-CO2 systems specifics in line with the current state-of-the-art products, experience, and technological limits. Results of the study show that the s-CO2 could be more appropriate for CHP applications. Both the electric and thermal efficiency of s-CO2 bottoming cycle show higher values compared with ORC, also due to the fact that in the examined s-CO2 solution, the cycle pressure ratio is not affected by the thermal user temperature. At part-load operation, the gas turbine regulation strategy affects the energy harvesting performance in a CHP arrangement. The estimated total plant investment cost results to be higher for the s-CO2, caused by the higher size of the heat recovery heat exchanger but also by the high specific investment cost still associated to this component. This point seems to make the s-CO2 not profitable as the ORC solution for industrial gas turbine heat recovery applications. Nevertheless, a crucial parameter determining the feasibility of the investment is the prospective carbon tax application.