Closed Brayton cycle.
NASA is evaluating various power-conversion technologies for future space nuclear power system applications. One potential option is the closed Brayton cycle (CBC). CBC-based power conversion uses an inert gas working fluid (instead of air) and recirculates the fluid in a Brayton conversion loop. The Brayton loop consists of a heat source heat exchanger, turboalternator-compressor, recuperator, and gas cooler as shown in the preceding figure. The turbine and compressor are mounted on a single shaft with gas foil bearings. The heat source can be solar, fission, or radioisotope; however, this activity is focused on space fission power systems. The recuperator is a gas-to-gas heat exchanger that uses turbine exhaust to preheat the working fluid before it reenters the heat source. The gas cooler is a gas-to-liquid heat exchanger that transfers the Brayton waste heat to a radiator where it is rejected to space. The alternator provides three-phase alternating-current electrical power that can be modified as necessary via a power management and distribution subsystem.
A single heat source could serve multiple Brayton loops, precluding the possibility of single-point failures in the power-conversion subsystem. In the event that one of the Brayton units is shut down, the remaining units could continue to produce power. If the Brayton units share a common fluid-containment loop, there will be various interactions among the units that affect system operation and performance. Some of these interactions had been evaluated analytically. However, there had been no previous means to experimentally evaluate the operation of a multi-Brayton system with a common gas-containment loop.
In April 2006, the NASA Glenn Research Center awarded a contract to Barber Nichols (Arvada, CO) for the design, fabrication, and delivery of a dual-Brayton-unit power system with a common gas fluid loop coupled to an electrical resistance heater as shown in the following figure. The system is designed to produce approximately 30 kW with nitrogen working fluid at a turbine inlet temperature of 1000 K, a compressor inlet temperature of 315 K, and a compressor inlet pressure of 110 kPa. Waste heat is removed with a pumped-water cooling system. The Brayton system is designed for full-power operation with both turboalternator-compressor units at design speed or for partial power operation with either unit at operating speed and the remaining unit in standby. The overall system utilizes existing commercial components where available in order to reduce cost and minimize development schedule.
Dual Brayton power system.
The system will be used as a laboratory testbed at Glenn to evaluate the performance and operational characteristics of closed Brayton power systems for future space power applications. The technical objectives include the development of operational control strategies; the investigation of design, off-design, and transient performance; and the validation of Government computer models.
As of February 2007, the system was in the final stages of assembly at Barber Nichols. All of the components had been acquired or fabricated. Initial acceptance testing is planned in late March prior to NASA delivery in April. In parallel with the contract, NASA has developed a detailed transient system model that will be used for pretest predictions. As a result of the testing, the analytical code will be experimentally validated to provide a more accurate and reliable model for future space Brayton design studies.
Find out more about the research of Glenn’s Thermal Energy Conversion Branch: http://www.grc.nasa.gov/WWW/TECB/
Glenn contacts:
Lee S. Mason, 216-977-7106, Lee.S.Mason@nasa.gov
Harold F. Weaver, 216-433-8869, Harold.F.Weaver@nasa.gov
Author:
Lee S. Mason
Headquarters program office:
Exploration Systems Mission Directorate
Programs/projects:
Prometheus, Fission Surface Power
Last updated: December 14, 2007
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