Thermo-economic analysis of a supercritical CO₂-based waste heat recovery system

Politecnico di Milano – Dipartimento di Energia, Via R. Lambruschini 4, 20156 Milano, Italy

^* Corresponding author: antonio.giuffrida@polimi.it

Abstract

This work investigates the performance of a supercritical CO₂ cycle as the bottoming cycle of a commercial gas turbine with 4.7 MW of electric power output. In detail, the partial heating cycle is the layout chosen for the interesting trade-off between heat recovery and cycle efficiency with a limited number of components. Single-stage radial turbomachines are selected according to the theory of similitude. In particular, the compressor is a troublesome turbomachine as it works near the critical point where significant variations of the CO₂ properties occur. Efficiency values for turbomachinery are not fixed at first glance but result from actual size and running conditions, based on flow rates, enthalpy variations as well as rotational speeds. In addition, a limit is set for the machine Mach numbers in order to avoid heavily loaded turbomachinery. The thermodynamic study of the bottoming cycle is carried out by means of the mass and energy balance equations. A parametric analysis is carried out with particular attention to a number of specific parameters. Considering the power output calculated for the supercritical CO₂ cycle, economic calculations are also carried out and the related costs compared to those specific of organic Rankine cycles with similar power output.