Transcritical CO2 power cycle – Effects of regenerative heating using turbine bleed gas at intermediate pressure

• Thermodynamic model for Transcritical CO2 cycle with bleed gas are developed.
• Effects of bleed ratio, pressure, and regenerator exit gas temperature are studied.
• 1st and 2nd law efficiencies are estimated.
• An optimum bleed pressure for maximum 1st and 2nd efficiencies is obtained.
• Maximum value of 1st law efficiency is limited by regenerator size.

For energy utilization from low temperature waste heat, CO2 is a potential working fluid due to its lower critical temperature. In this work, assuming finite quantity of flue gas available at low temperature (200 °C), a thermodynamic model is developed for a transcritical CO2 power cycle utilizing turbine bleed gas for regenerative heating. Analysis show that the cycle performance improves with higher value of bleed ratio. However, for a specified bleed pressure and bleed gas temperature at the regenerator exit, maximum practical value of bleed ratio may be fixed by considering the exponential growth of the regenerator size (specified by NTU (number of transfer unit)). Most significant observation is the existence of optimum bleed pressures corresponding to maximum 1st law efficiency or minimum cycle irreversibility for specified values of remaining cycle parameters.