Techno-economic comparison of different cycle architectures for high temperature waste heat to power conversion systems using CO2 in supercritical phase

Abstract

Bottoming thermodynamic systems based on supercritical carbon dioxide as working fluid (sCO2) are a promising technology to tackle the waste heat to power conversion at high temperature levels and that might outperform the conventional power units based on Organic Rankine Cycles. In fact, CO2 is an inexpensive, non-toxic, non-flammable, thermally stable and eco-friendly compound. Moreover, CO2 in its supercritical state shows an extreme increase in density that allows turbomachinery downsizing and a high cycle efficiency due to the reduced work required by the compression stage. In addition, supercritical CO2 permits a better temperature glide matching within the heat source which increases the overall efficiency of waste heat utilization. With the aim of identifying pro and cons of different sCO2 cycle layouts, this paper investigated four design Joule-Brayton configurations at increasing complexity: simple regenerative, with recompression, with reheating and with recompression and reheating. The research methodology is based on 1st and 2nd laws thermodynamic analyses and includes correlations to estimate the investment costs of the equipment. With reference to a high temperature industrial waste heat source, performance, costs and exergy losses in the different cycle layouts are compared. Furthermore, a parametric analysis regarding the effects of the cycle pressure ratio on net power output and back work ratio is carried out.