Effects of non-condensable gas on thermodynamic performance of transcritical organic Rankine cycle

Abstract

The development and utilization of renewable energies and waste heat contribute to alleviating global issues of fossil energy shortage and environmental pollution, and energy consumption structure transition. Organic Rankine cycle (ORC), specifically transcritical ORC (tORC), garners significant attention for its potential in low-grade heat power generation. However, the existence of non-condensable gases (NCGs) caused by an incomplete vacuum process or organic fluid pyrolysis may have negative effects on system performance. Analyzing and quantifying the effects of different NCGs is of great significance for putting forward the relevant solution measures and helps improve the mechanism of pyrolysis products' influence. In this work, a novel thermodynamic model is proposed for tORC systems considering off-design performance to evaluate the effects of NCGs. Five different hydrocarbons including n-butane, n-pentane, i-pentane, cyclopentane, and n-hexane, are selected as working fluid candidates, while four NCGs are chosen as study objects including nitrogen, methane, ethane, and propylene. The established model realizes the function of analyzing and comparing the effects of different NCGs on thermodynamic performances of tORC systems. Results showed that the tORC system using cyclopentane had the best thermodynamic performances after the optimized design process. The case study indicated that an increase in the mass fraction of NCG caused the condensing pressure increase and system's thermodynamic performance degradation. Furthermore, larger variations were exhibited in the net power output when the design condensing temperature was higher. Finally, the comparative study showed that methane had the greatest negative effects on thermodynamic performances of tORC systems, especially for using n-butane.