Thermodynamic study of organic Rankine cycle based on extraction steam compression regeneration in the supercritical state

Abstract

The regeneration process is a crucial factor in enhancing the thermal efficiency of an organic Rankine cycle (ORC). This study introduces a novel approach called the supercritical organic Rankine cycle (S-ORC), which utilizes extraction steam compression regeneration in the supercritical state. This breakthrough addresses the existing limitation of subcritical pressure regeneration in ORC research. The study involved the construction of two configurations: the S-ORC and the supercritical regeneration ORC (SR-ORC), which incorporates turbine exhaust regeneration. The working fluid employed for both cycles was R245fa. The thermal efficiency of the S-ORC increased from 17.86 % to 18.85 %, and its exergy efficiency increased from 46.23 % to 48.79 %. Similarly, the thermal efficiency of the SR-ORC increased from 22.66 % to 23.49 %, and its exergy efficiency increased from 58.65 % to 60.8 %. Using the thermal cycle splitting method, we analyzed the S-ORC and found that it can be considered a superposition of an ORC and a single-regeneration Brayton cycle. The equivalent cooling process of the Brayton cycle did not release heat into the environment but rather transferred it to the mainstream of the ORC through regenerative processes. This resulted in an efficiency increase, as the network of the Brayton cycle is equivalent to ± 1. When the network was greater than zero, the S-ORC was superimposed on top of the network of the ORC, thereby increasing overall efficiency. This explains the mechanism behind the enhanced efficiency of the S-ORC. Furthermore, by examining the essential parameters of the cycle and considering various working fluids, we further demonstrated the efficiency advantage of the S-ORC. This study explored the regeneration potential in the supercritical region and proposed an ORC based on compression regeneration in the supercritical state. The proposed approach significantly improves the thermal efficiency of the cycle and achieves structural optimization.