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.
Highlights
Improving energy efficiency is important to achieve the goal of carbon emission peak and carbon neutrality
A large amount of low-grade energy is wasted in industrial processes, and it has great potential to recover this part of energy via organic Rankine cycle (ORC) technology
The analysis showed that the two-stage axial-flow turbine had a higher efficiency than the single-stage turbine, indicating the potential application of two-stage axial flow turbine in low-temperature small-scale ORC
Summary
Improving energy efficiency is important to achieve the goal of carbon emission peak and carbon neutrality. Al Jubori et al [32] proposed a three-dimensional multi-objective optimization model for turbine stage to improve turbine efficiency and ORC system performance, based on a low-temperature small-scale. A low mass flow rate of the working fluid and a high pressure at the turbine inlet require a high volume expansion ratio, which puts forward a great challenge to the design of high efficiency turbine. A preliminary design model of multi-stage axial-flow turbines is established and the feasibility for the design of small ORC turbines with a high pressure ratio and a mass low flow rate is explored. The results indicate that reducing trailing edge thickness and tip clearance can significantly improve turbine efficiency for small kW-class multi-stage axial-flow turbines
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