Thermo-economic performance improvement of butane/isopentane mixtures in organic Rankine cycles by liquid-separated condensation method

Abstract

Zeotropic mixtures can achieve a better thermodynamic performance in organic Rankine cycle (ORC) systems than frequently-used pure fluids, but their economic performance is generally worse because they need larger condenser areas. This study used the liquid-separated condensation (LSC) method to improve the thermo-economic performance of zeotropic mixtures in ORC systems. Typical butane/isopentane mixtures were selected. Influences of different heat source temperatures on the thermo-economic performance enhancement effects of LSC method were studied. Optimal selections of liquid-separated locations and system parameters were obtained. Application potential of butane/isopentane mixtures with LSC in ORC systems was studied. Results show that the LSC method can achieve larger decrements in specific investment cost (SIC) for zeotropic mixtures than for pure fluids. The SICs of butane/isopentane mixtures are reduced by 4.0%–8.8% at most by using LSC method, compared with using conventional condensation. Decrements in SIC of butane and isopentane are only 1.2%–1.3% and 1.9%–2.0%, respectively. Furthermore, the minimum SICs of butane/isopentane mixtures are always higher than butane (optimal pure fluid) for using conventional condensation. However, using LSC method helps butane/isopentane mixtures obtain lower SICs meanwhile larger net power outputs than butane, which contributes to the applications of ORC systems using zeotropic mixtures.