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Practical Aspects of
Supercritical Carbon Dioxide
Brayton System Testing
Bechtel Marine Propulsion CorporationBettis Atomic Power LaboratoryWest Mifflin, PA
Page 1Supercritical CO2 Power Cycles Symposium September 9-10, 2014
Brayton System Testing
Eric Clementoni
Timothy Cox
Presentation Summary
• S-CO2 Brayton Cycle Integrated Systems Test (IST)
Overview and Operating Experience
• Best Practices for S-CO2 Power Cycle Operation
– Design Considerations
– Loop Cleanliness
– System Fill and Leakage
Page 2Supercritical CO2 Power Cycles Symposium September 9-10, 2014
– System Fill and Leakage
– Instrumentation
• Conclusions
• 100 kWe IST has been main S-CO2
development focus of BMPC
• Simple Brayton cycle
– Single variable speed turbine-compressor
– Single constant speed turbine-generator
IST Overview
Page 3Supercritical CO2 Power Cycles Symposium September 9-10, 2014
– Single constant speed turbine-generator
– Single recuperator
• Focus on system control
– Rapid startup
– Power changes
– Shutdown
CompressorTurbine
Heat
Source
Generator Turbine
Recuperator
Precooler
Sea WaterCooling
Water
IST Physical Layout
Turbo-Generator
Page 4Supercritical CO2 Power Cycles Symposium September 9-10, 2014
Recuperator
Precooler
Turbo-Compressor (not
visible)
Operational Experience• Shakedown started in Fall 2011
• Brayton cycle operation started in Spring 2012
• Over 165 hours of Brayton system operation
• Achieved 40% of design power
Page 5Supercritical CO2 Power Cycles Symposium September 9-10, 2014
**40 kW on
power
analyzer
System Transient Modeling
• IST design supported by transient model
predictions
– Pipe size, valve resistance, valve stroke time,
support equipment operating requirements
Page 6Supercritical CO2 Power Cycles Symposium September 9-10, 2014
support equipment operating requirements
• Transient model valuable for startup and low
power operation
– Closed system more difficult to startup and get to
full power condition than open Brayton cycle
Mechanical Connections
• IST utilizes the following fittings in CO2 systems
– Grayloc fittings for main loop piping/component
connections
– ¼” Swagelok fittings welded to pipe for instrumentation
– ¼”-2” Swagelok fittings and 2” ANSI flanges for support
system connections
Page 7Supercritical CO2 Power Cycles Symposium September 9-10, 2014
system connections
• Fittings all leak tight when installed properly
• Bolt selection considerations– Thermal fatigue from heatup
and cooldown cycles
– Brittle fracture from low
temperature of leaking CO2
Thermal Stress Consideration• Small equipment allows for compact system arrangement
• Compact arrangements can make thermal stress more difficult
to accommodate
• IST design considered thermal stress from the beginning
– 4 thermal expansion loops included in Brayton loop piping to
accommodate thermal expansion
Page 8Supercritical CO2 Power Cycles Symposium September 9-10, 2014
Material Quality / Cleanliness
• Supercritical CO2 is a strong solvent
• Materials testing and loop operating
experience has shown that some polymer
materials are incompatible with the
Page 9Supercritical CO2 Power Cycles Symposium September 9-10, 2014
materials are incompatible with the
supercritical CO2 environment
• BMPC test loops have implemented material
quality and cleanliness controls which have
shown effective at eliminating system
contamination
Turbine Nozzle Erosion
Page 10Supercritical CO2 Power Cycles Symposium September 9-10, 2014
System Fill and Leakage
• IST uses Coleman Instrument grade (99.99%)
CO2 for fill
• Three cycles of pulling vacuum and filling to
atmospheric pressure before full fill
Page 11Supercritical CO2 Power Cycles Symposium September 9-10, 2014
atmospheric pressure before full fill
• Low leakage levels from valve stem packing
and large shaft seals
Instrumentation• Temperature and pressure measurements give good
indication of loop conditions
• Coriolis flow meters used to determine both flow
rate and density of CO2
– Flow meters give good results for single phase or
supercritical flow
Page 12Supercritical CO2 Power Cycles Symposium September 9-10, 2014
supercritical flow
– Can give invalid readings during loop startup if installed
orientation allows for low point for liquid collection
Density Calculations using
REFPROP with T & P
Page 13Supercritical CO2 Power Cycles Symposium September 9-10, 2014
Summary
• No inherent issues with S-CO2 Brayton power
cycle operation identified in IST operation
• Off-design operations, thermal stress analysis,
instrumentation orientation, cleanliness, and
Page 14Supercritical CO2 Power Cycles Symposium September 9-10, 2014
instrumentation orientation, cleanliness, and
material compatability are important design
considerations
• Experience from small-scale testing will
minimize rework and operational issues for
larger systems
Acknowledgements
• This paper summarizes work that has been
performed a number of devoted engineers,
scientists, technicians, and support personnel
at the Bechtel Marine Propulsion Corporation
Page 15Supercritical CO2 Power Cycles Symposium September 9-10, 2014
at the Bechtel Marine Propulsion Corporation
and our subcontractors. This paper would not
be possible without the efforts of this team.
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