On the design of small to medium scale radial inflow turbines for supercritical CO2 power cycles

Abstract

In rural Australia, concentrating solar power at sub 10 MWe scale is a candidate technology to displace current fossil fuel based technologies [1]. For concentrating solar power to be competitive for this application, coupling with an advanced power cycle is essential. A candidate is the supercritical CO 2 Brayton cycle, which is suitable for higher turbine inlet temperature operation, and has the potential to exceed the efficiency of the steam Rankine cycle. Furthermore, due to lower volumetric flow variation over the cycle, simpler turbomachinery may be used, which enables the cycle to be downscaled while maintaining turbomachinery and cycle efficiency. Of the constituent components in the cycle, turbine efficiency has the greatest impact on cycle efficiency [2]. Radial turbomachinery is a key technology for energy conversion in the supercritical CO 2 Brayton cycle at small scales (i.e. below 30 MW shaft power). While the design of efficient turbines for the supercritical CO 2 Brayton cycle is critical to realising efficient concentrating solar power plants, only a limited number of prototypes have been tested, and none at representative inlet conditions. Radial inflow supercritical CO 2 turbines are characterised by small dimensional scale and high shaft speeds. Prototype turbine designs are characterised by low expansion ratio and medium specific speed to accommodate mechanical restrictions on shaft speed. Medium specific speed designs are selected for prototypes designs due to their implied optimal efficiency. Demonstration test facilities have utilised multiple stages, or lower expansion ratio cycles in order to accommodate designs of this specific speed without exceeding mechanical limits. Increasing inlet conditions to representative cycle conditions for concentrating solar power applications will require higher shaft speeds, or additional stages if specific speed of the stage is to be maintained. Alternatively, the use of single stage low specific speed designs may pose a feasible alternative, however these designs are generally disregarded owing to the implied efficiency penalties shown on gas turbine derived architecture selection charts. Better understanding of loss characteristics of low specific speed supercritical CO 2 turbomachinery is required in order to make an assessment on the feasibility of single stage expansions. Further to sizing of the stator and rotor, stage design also includes components upstream and downstream of the rotor. The broadest challenge for supercritical CO 2 turbomachinery is designing components within a system for high pressure, high density, high temperature, and high shaft speed operation. These constraints may have significant impact on the geometry of these hot gas path features. Details of these features are not clearly detailed in published prototype designs. Considering the knowledge gap for supercritical CO 2 turbines, one key aim of this thesis is to quantify the performance of low specific speed radial inflow turbines using numerical methods. A further aim is t...

Investigation into Taylor-Couette-Poiseuille flow heat transfer for supercritical carbon dioxide turbomachinery

“…This method of providing power is prohibitively expensive and is a non-renewable source of energy. Recent research has shown that the use of sCO 2 power cycles in conjunction with a Concentrating Solar Thermal (CST) heat source, could provide a simple, efficient, renewable replacement to the diesel generators these communities currently rely on [8,23].…”