Performance analysis of an organic Rankine cycle with an internal heat exchanger considering turbine pressure ratio and efficiency

Abstract

This study investigated an organic Rankine cycle (ORC) that utilizes waste heat from ships. Steam at 170 °C generated by an economizer and either surface or deep seawater were used as the heat source and heat sink, respectively. Environment-friendly working fluids (hydrofluorocarbons, HFO) and conventional R-245fa were used for comparison. Energy and exergy analyses of the basic ORC and ORC with an internal heat exchanger (IHX) were conducted. In the basic ORC, R-1233zd(E) exhibited the highest efficiency. Comparatively, R-1336mzz-Z showed significantly improved efficiency upon IHX application. The IHX application allowed the working fluids to achieve efficiency improvement and decreased irreversibility of components, thereby increasing exergy efficiency. Although the higher pressure ratio at turbine produced a better output, a clear limitation in increasing pressure ratio more than 3.0 was observed due to the choking phenomenon. A distinguishing characteristic of the IHX cycle is its ability to compensate for efficiency decrease that occurs when lowering the pressure ratio to avoid choking by increasing the superheating degree. At a constant turbine inlet temperature (the sum of evaporation temperature and superheating degree the system efficiency at a pressure ratio of 2.99 was relatively 1.39 % higher than that at 3.58 owing to IHX application.