Performance improvement of ocean thermal energy conversion organic Rankine cycle under temperature glide effect

Abstract

The temperature glide effect of zeotropic mixtures on ocean thermal energy conversion (OTEC) cycle driven by a narrow temperature difference, which is significantly different from that in conventional low-grade energy technologies, is yet to be thoroughly studied. In this study, the binary zeotropic mixtures-based OTEC cycle is investigated. Comparative analysis of the classical zeotropic ORC and six types of zeotropic ORCs configured with or without series/parallel multi-pressure evaporators and single-/dual-outlet liquid-separated condensers were conducted. The results showed that zeotropic mixtures could be beneficial in ocean thermal energy conversion. Multi-pressure evaporation could significantly reduce the irreversible loss in the heat exchanger, and the series multi-pressure evaporator-based zeotropic ORC (SMZO) performed better than the parallel cycle (PMZO), with 0.09%–0.14% higher thermal efficiency, 2.27%–3.11% higher turbine power output, and 0.89%–1.46% higher exergy efficiency. Liquid-separated condensation could improve the condensation effect by increasing the heat transfer coefficient, and liquid-separated dryness dominant the performance. Dual-outlet liquid-separated condensation could also increase cycle-levelised energy cost. Comparingly, the single-outlet liquid-separated condensation based cycle could reduce the levelised energy cost by 7.93% and 4.81%, respectively.