Preliminary Design of an Axial-Flow Turbine for Small-Scale Supercritical Organic Rankine Cycle

Abstract

A small-scale organic Rankine cycle (ORC) with kW-class power output has a wide application prospect in industrial low-grade energy utilization. Increasing the expansion pressure ratio of small-scale ORC is an effective approach to improve the energy efficiency. However, there is a lack of suitable expander for small-scale ORC that can operate with a high efficiency under the condition of large expansion pressure ratio and small mass flow rate. Aiming at the design of high-efficiency axial-flow turbine in small ORC system, this paper investigates the performance of a kW-class axial-flow turbine and proposes a method for efficiency improvement. First, the preliminary design of an axial-flow turbine is conducted to optimize the geometric parameters and aerodynamic parameters. Then, the effects of tip clearance and trailing edge thickness on turbine performance are analyzed under design and off-design conditions. The results show that the efficiency of the two-stage or three-stage turbine is evidently better than that of the single-stage one. The output power and efficiency of the three-stage turbine are close to that of the two-stage turbine while the speed is lower. Meanwhile, the trailing edge loss and leakage loss can be significantly reduced via reducing the trailing edge thickness and tip clearance, and thus the turbine efficiency can be improved significantly. The estimated efficiency arrives at 0.82, which is 33% higher than that of the conventional turbine. Considering the limitation of turbine speed, three-stage axial-flow turbine is a feasible choice to improve turbine efficiency in a small-scale ORC.

Development of 180 kW Organic Rankine Cycle (ORC) with a High-Efficiency Two-Stage Axial Turbine

“…Peng et al [6] investigated the performance of a kilowatt-class axial-flow turbine and proposed a method for improving the turbine efficiency. The preliminary design of the axial-flow turbine was performed for parameter optimization, and the effect of the tip clearance and trailing edge thickness on the turbine performance was analyzed.…”

“…Table 1 summarizes the characteristics (type, number of stages, rotational speed, efficiency, and power output) of the aforementioned turbines and ORC thermal efficiency. In particular, in the case of a single-stage axial turbines of 200 kW or less, high rotational speed (>40,000 rpm) was applied to increase the efficiency [6,[14][15][16]. However, a highrotational-speed turbine requires high-performance bearings such as frictionless bearings.…”

Performance Analysis of Organic Rankine Cycle with the Turbine Embedded in a Generator (TEG)

“…It was found that turbine power at nominal total expansion ratio at 3.9 is 71.2 kW. Peng et al [13] investigated the performance of a kW-class multi-stage axial turbine, which is suitable for a small-scale ORC. In particular, the off-design for turbine performance was performed using parameters such as the number of turbine stages, trailing edge thickness, and tip clearance.…”