Optimization of Closed Brayton Cycles for Space Power Generation 929449

The Space Exploration Initiative, (SEI), has renewed interest in examining various technologies for generating electrical power in space as well as on the surface's of the Moon and Mars. One technology that is of interest is the Closed Brayton Cycle, (CBC). Recent work has further enhanced the capability to perform accurate preliminary design and optimization of CBC power conversion systems for space applications. The basic building block of the computer codes used for this work has been the Closed Cycle Engine Performance, (CCEP), code developed by John Klann at NASA Lewis Research Center. This model was originally developed for the preliminary design and performance analysis of the Space Station Freedom dynamic power conversion system.

Improvements to the model have included linking CCEP with the public domain optimization code COPES/ADS. This linkage has greatly enhanced the utility of the program and streamlined the design process. The designer now has the capability of defining design variables, design constraints and optimization functions. This facilitates studies that explore the interrelationship between system mass, cycle efficiency and radiator area. In a recent study the relationship between system mass, radiator area, cycle efficiency and recuperator effectiveness was examined.

In addition to coupling the design codes with the optimizer, additional modifications have been made to enhance the versatility of the model. A subroutine has been added to allow for an isotope heat source. The radiator code has been modified to allow for the preliminary sizing of the radiator using a closed form solution to the governing equations. To take advantage of the Brayton Cycle's ability to produce significantly more power than when operating at design speed and inlet pressure, the alternator sizing algorithms have been modified to allow for off-design sizing of the alternator. This capability allows the designer to specify the fraction of alternator design output that will be required at the turbomachinery design point. Off-design operation can then be examined at significantly higher power levels.

The final modification that will be discussed is the linking of the finite-element pumped-loop radiator design code with the optimization code COPES/ADS. The radiator code is run externally to the main design routines simply due to the execution time required to converge. Output from the optimized radiator designs are used to update the inputs for the radiator specific mass and fin effectiveness used in CCEP.