Alberto Scotti Del Greco

The ORegen™ Waste Heat Recovery Cycle: Reducing the CO2 Footprint by Means of Overall Cycle Efficiency Improvement

The growing concern for the role of man-made CO2 emissions with respect to global warming combined with the large increase in energy demand spurred by developing nations and a growing global population that is foreseen over the next 15 years have recently turned attention to potential CO2 -neutral energy supply solutions. Waste heat recovery cycles applied to fossil fueled plants offer a local zero-emission solution to producing additional electric energy, thereby increasing the overall plant efficiency with a considerable reduction in the emission of CO2 per unit of energy produced. GE Oil & Gas with GE Global Research Europe has developed a new and attractive solution for recovering waste heat energy from a variety of thermal sources ranging from reciprocating combustion engines to gas turbines. This new recovery cycle is called ORegen™. The ORegen™ recovery cycle is a rankine cycle, with superheating, that recovers waste heat and converts it into electric energy by means of a double closed loop system. The ORegen™ system represents one of the very few viable solutions for recovering heat from sources (such as mechanical drive gas turbines) whose load may vary dramatically over time or where the equipment is located at a site where water is not readily available. For the temperature range of interest, a thorough comparison between many working fluids was performed, leading to the conclusion that the substance that delivers the highest efficiency is Cyclopentane. A high-efficiency Rankine cycle based on such a working fluid places a particularly high demand on the expansion ratio, which influences some of the basic architectural choices of the expander machine. This article introduces the ORegen™ recovery cycle and describes the process used in GE Oil & Gas to design the family of double supersonic stage turboexpanders, covering the power range of 2–17MW. Examples of the application of the ORegen™ cycle to gas turbine are also provided to demonstrate attractive opportunities to increase the overall plant efficiency.Copyright

Design and Optimization of Radial Flow Wheels for a Waste Heat Recovery Double Supersonic Stage Turbo-Expander

Waste heat recovery cycles equipped with radial inflow turbines (turbo-expanders) typically dictate large pressure ratios per stage in order to increase the overall cycle efficiency. Depending on the operating conditions, supersonic flow may be reached at some location within the stage. The design of turbo-machinery in such an environment poses several challenges, the most important of which are preventing performance deterioration and High Cycle Fatigue (HCF) failure of the rotating, stressed material by avoiding resonance frequencies in the operating range. Turbo-expander wheels, being uncooled components, are generally not affected by high temperature gradients; therefore LCF (Low Cycle Fatigue) doesn’t constitute their main limiting life factor. This paper describes the process used in GE Oil & Gas to design and optimize the wheels of a 17MW double supersonic stage turbo-expander. The initial design phases, preliminary design assessments, CFD analyses and structural analysis optimization are described. Special focus is given to the modal analysis and resonance identification (i.e., Modal Cyclic Analysis) used in the design phase. A critical review of the use of the SAFE interference diagram in place of the Campbell diagram is also provided.Copyright

INTRODUCTION OF CIRCUMFERENTIALLY NON-UNIFORM VARIABLE GUIDE VANES IN THE INLET PLENUM OF A CENTRIFUGAL COMPRESSOR FOR MINIMUM LOSSES AND FLOW DISTORTION

In the Oil & Gas industry, large variations in flow rates are often encountered which require compression trains with a wide operating range. If the stable operating range at constant speed is insufficient, variable speed drivers can be used to meet the requirements. Alternatively, variable guide vanes (IGVs) can be introduced into the inlet plenum to provide pre- or counter-swirl to the first stage impeller, possibly eliminating the need for variable speed. This paper presents the development and validation of circumferentially non-uniform IGVs that were specifically designed to provide maximum angle variation at minimum losses and flow distortion for the downstream impeller. This includes the comparison of three concepts: a baseline design based on circumferentially uniform and symmetric profiles and two circumferentially non-uniform concepts based on uniquely cambered airfoils at each circumferential position and a multi airfoil configuration consisting of a uniquely cambered fixed part and a movable part. The idea behind the circumferentially non-uniform designs was to take into account non-symmetric flow features inside the plenum and a bias towards large preswirl angles rather than counter-swirl during practical operation. The designs were carried out by CFD and first tested in a steady, full-annulus cascade in order to quantify pressure losses and flow quality at the inlet to the impeller at different IGV setting angles (ranging from −20° to +60°) and flow rates. Subsequently, the designs were mounted in front of a typical Oil & Gas impeller on a high speed rotating rig in order to determine the impact of flow distortion on the impeller performance. The results show that pressure losses in the inlet plenum could be reduced by up to 40% with the circumferentially non-uniform designs over the symmetric baseline configuration. Furthermore, a significant reduction in circumferential distortion could be achieved with the circumferentially non-uniform designs. The resulting improvement in impeller performance contributed approx. 40% to the overall efficiency gains for inlet plenum and impeller combined.© 2015 ASME

Special Challenges in the CFD Modeling of Transonic Turbo-Expanders

High pressure ratio turbo-expanders often put a strain on CFD modeling. First of all, the working fluid is usually characterized by significant departures from the ideal behavior, thus requiring the adoption of a reliable real gas model. Moreover, supersonic flow conditions are typically reached at the nozzle vanes discharge, thus involving the formation of a shock pattern, which is in turn responsible for a strong unsteady interaction with the wheel blades. Under such circumstances, performance predictions based on classical perfect gas, steady-state calculations can be very poor. While reasonably accurate real gas models are nowadays available in most flow solvers, unsteady real gas calculations still struggle to become an affordable tool for investigating turbo-expanders. However, it is emphasized in this work how essential the adoption of a time-accurate analysis can be for accurate performance estimations. The present paper is divided in two parts. In the first part, the computational framework is validated against on-site measured performance from an existing power plant equipped with a variable-geometry nozzled turbo-expander, for different nozzle positions, and in design and off-design conditions. The second part of the paper is devoted to the detailed discussion of the unsteady interaction between the nozzle shock waves and the wheel flow field. Furthermore, an attempt is made to identify the key factors responsible for the unsteady interaction and to outline an effective way to reduce it.Copyright