Magnetic Bearings for Supercritical CO2 Service

The technology development for the supercritical CO2 (sCO2) power plant is well underway, and the first pilot plants are now under construction. The new pilot plants are sized for a 10 MW net power output; a good size to gain valuable data for larger utility scale power plants. This is also a very good size for the smaller, distributed power applications. The distributed power application is envisaged as the primary application for the roll out of the technology. The reliability of the plants during the introductory phase will be very important. A panel of power plant executives at the recent DOE conference on sCO2 has made it clear that the initial cost, operating cost, and up-time reliability must be demonstrated before any significant investment at the public utility level is seen. Pursuant to this point, the reliability of the system will be the next great focus for the sCO2 systems. When a Failure Modes and Effects Analysis (FMEA) is done at a system level, machinery design with fault tolerance will be a priority. Machinery up-time and system contamination effects will be high on the list of risks. The new machines proposed for the pilot plants use oil bearings and gas seals. A turbocompressor with two gas seals and another machine with integral gear and 4 gas seals will be installed in the plants. It is well understood that reliability for turbomachinery is highly dependent upon the bearing and sealing systems. Upsets, surges, fluid-driven instability, start-up variations, and variation in thrust loading are all acceptable if the bearings and seals can tolerate them. Concepts NREC has proposed a design approach to improve reliability by addressing the bearing and seals. This approach includes: 1) Reduce the number of seals down to one seal. 2) Eliminate the oil entirely with magnetic bearings. Elimination of the oil removes a source of seal distress, and also serves to eliminate the possibility of fouling of the heat exchangers. The magnetic bearings will provide improved monitoring capability and can be used for active fluid force measurement in a machine.