Investigate a hybrid open-Rankine cycle small-scale axial nitrogen expander by a camber line control point parameterization optimization technique

During the last few decades, low-grade heat sources such as solar energy and wind energy have enhanced the efficiency of advanced renewable technologies such as the combined Rankine cycle, with a significant reduction in CO2 emissions. To address the problem of the intermittent nature of such renewable sources, energy storage technologies have been used to balance the power demand and smooth out energy production. In this study, a detailed thermodynamic analysis of a hybrid open Rankine cycle was conducted by using engineering equation solver (EES) software in order to investigate the performance of such a cycle using a liquid nitrogen energy storage system. In this cycle configuration, the conventional closed loop Rankine cycle (topping cycle) is combined with a direct open Rankine cycle (bottoming cycle) for a more efficient system which can solve the problem of discontinuous renewable sources. In the direct open-Rankine cycle, the small expander is the main component that can improve the cycle’s performance and as a result, this small expander needs to be optimized for maximum efficiency to achieve high system performance levels. In this work a small-scale nitrogen axial expander has been optimized and modeled to be incorporated into a hybrid open-Rankine cycle, using a one-dimensional preliminary design and CFD three-dimensional ANSYS design exploration and a novel camber line control point parametrization technique, which is outlined in detail. The design optimization approach has been proven as an effective tool that could enhance turbine efficiency from 72% to 76.3% and output power from 2076 W to 2597.6 W. The optimized turbine using the control points’ approach could also improve the cycle’s thermal efficiency by 3.38% compared with the baseline design. Such results underline the potential of full simulation optimization by using a blade camber line control point’s parametrization technique for a small-scale expander with low flow rate and rotational speed.