Research Papers: Gas Turbines: Cycle Innovations

Control of a Supercritical CO2 Recompression Brayton Cycle Demonstration Loop

[+] Author and Article Information
T. Conboy

e-mail: tmconbo@sandia.gov

D. Fleming

Advanced Nuclear Concepts,
Sandia National Laboratories,
P.O. Box 5800,
MS 1136,
Albuquerque, NM 87185

Contributed by the Cycle Innovations Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received June 27, 2013; final manuscript received July 16, 2013; published online September 17, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 135(11), 111701 (Sep 17, 2013) (12 pages) Paper No: GTP-13-1198; doi: 10.1115/1.4025127 History: Received June 27, 2013; Revised July 16, 2013

The U.S. Department of Energy is currently focused on the development of next-generation nuclear power reactors, with an eye towards improved efficiency and reduced capital cost. To this end, reactors using a closed-Brayton power conversion cycle have been proposed as an attractive alternative to steam turbines. The supercritical-CO2 recompression cycle has been identified as a leading candidate for this application since it can achieve high efficiency at relatively low operating temperatures with extremely compact turbomachinery. Sandia National Laboratories has been a leader in hardware and component development for the supercritical-CO2 cycle. With contractor Barber-Nichols Inc., Sandia has constructed a megawatt-class S-CO2 cycle test-loop to investigate the key areas of technological uncertainty for this power cycle and to confirm model estimates of advantageous thermodynamic performance. Until recently, much of the work has centered on the simple S-CO2 cycle—a recuperated Brayton loop with a single turbine and compressor. However, work has recently progressed to a recompression cycle with split-shaft turbo-alternator-compressors, unlocking the potential for much greater efficiency power conversion, but introducing greater complexity in control operations. The following sections use testing experience to frame control actions made by test loop operators in bringing the recompression cycle from cold startup conditions through transition to power generation on both turbines, to the desired test conditions, and finally to a safe shutdown. During this process, considerations regarding the turbocompressor thrust state, CO2 thermodynamic state at the compressor inlet, compressor surge and stall, turbine u/c ratio, and numerous other factors must be taken into account. The development of these procedures on the Sandia test facility has greatly reduced the risk to industry in commercial development of the S-CO2 power cycle.

Copyright © 2013 by ASME
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Fig. 1

S-CO2 recompression Brayton cycle layout at Sandia

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Fig. 2

Schematic of the S-CO2 recompression Brayton cycle

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Fig. 3

Schematic of the turbo-alternator-compressor

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Fig. 4

Temperatures and speeds during start-up, from t = 0–4000 s

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Fig. 5

Turbine u/c ratios for units A and B, respectively

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Fig. 6

Turbo-alternator-compressor speeds and net power output to break-even condition t = 0–4000 s

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Fig. 7

Compressor A and B inlet density and speed versus time

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Fig. 8

Compressor A and B position on their shared performance map; A (light green), and B (dark green)

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Fig. 9

Transient in mass flow and speeds at t = 4820 s due to surge

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Fig. 10

Density and speed ratios during test progression

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Fig. 11

The T-S diagram for compressor A (red) and B (green) inlet conditions

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Fig. 12

Speeds and mass flow for A and B; 2-phase oscillations at A

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Fig. 13

Speed and power of both units t = 4000–8000 s

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Fig. 14

Progression of temperatures and speeds during the test operation

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Fig. 15

Progression of pressures and speeds during the test operation

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Fig. 16

The T-S diagram for the recompression cycle at t = 5800 s

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Fig. 17

Gas bearing's film temperatures during test operation

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Fig. 18

Thrust load versus load capacity: unit A

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Fig. 19

Thrust load versus load capacity: unit B

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Fig. 20

Speed and power of both units t = 6000–8000 s




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