Transcript ARIES: Fusion Power Core and Power Cycle Engineering

An Assessment of the Brayton Cycle for High
Performance Power Plants
R. Schleicher1, A. R. Raffray2, and C. P. Wong1
1General Atomics,
P.O. Box 85608, San Diego, CA 92186, USA
2University of California, San Diego, 460 EBU-II, La Jolla, CA 92093-0417, USA
14th Topical Meeting on the Technology of Fusion Energy
Park City, Utah
October 15-19, 2000
October 15-19, 2000
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
1
Brayton Cycle Offers Best Near-Term Possibility
of Power Conversion with High Efficiency
•
Application of closed-cycle gas turbine (CCGT)
technology to fusion power plants
-
•
Maximize potential gain from high-temperature fusion in-reactor
operation
Compatible with in-reactor He coolant or other coolant through use
of IHX
High efficiency translates in lower COE and lower heat load
Identify key design parameters influencing cycle
efficiency and their likely improvement based on
near-term technology development
Estimate CCGT performance improvements for
fusion power plant
October 15-19, 2000
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
2
A Brief CCGT History Indicates a Resurgence in
Technology Development
1939
First fossil-fired CCGT plant commissioned in Switzerland. (Air as
working fluid)
1978
Last of seven CCGTs, Oberhausen II plant, commissioned in
Germany
(helium as working fluid) – 50 MWe rating
1970’s Strong development program in U.S. and Germany for coupling direct
helium CCGTs to high temperature gas-cooled reactors – work was
discontinued due to lack of incentive with tubular recuperators
(efficiency ~40%)
1980
Work on German HHV CCGT nuclear prototype is discontinued due
to
oil bearing leaks.
1987
Work at MIT demonstrates that high effectiveness plate-fin recuperators
can elevate net efficiency of nuclear gas turbines to ~50%
1990s
Strong U.S. DOE effort to design a 350MWe nuclear CCGT for new
production reactor (NPR). Work was discontinued with close of NPR
program.
2000
Republic of South Africa and U.S./Russia engaged in well-funded
design
programs to design and construct nuclear CCGT prototypes.
October 15-19, 2000
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
3
350 MWE Nuclear CCGT (GT-MHR)
Currently Being Designed by U.S./Russia
Power Conversion
Module
Reactor
Generator
Turbine
Reactor
Recuperator
Compressor
Inter-Cooler
Pre-Cooler
October 15-19, 2000
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
4
Ideal CCGT for Fusion Power Plant
• Multi-stage compression
with inter-coolers to
reduce compression
work
• Split-shaft turbine to
allow independent
optimization of
compressor and turbine
aerodynamic
performance
October 15-19, 2000
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
5
The Cycle Efficiency is Optimized by Setting
System Parameters Under the Designer’s Control
• Compressor turbine inlet temperature, Tin
• Recuperator effectiveness, erec
• System fractional pressure drop, DP/Pout
• Turbine and compressor adiabatic efficiencies, hT,ad and hC,ad
• Overall compression ratio, PC=Pout/Pin
-
PC also sets the in-reactor component or IHX return temperature, Tout
which is constrained by material limits
• The power conversion system (PCS) is likely to be a small fraction of
the
overall capital cost (~10-20%)
power cost optimization will be driven by efficiency gains over PCS
component cost
-
for fusion we can assume that PCS component designs are limited
mainly by technology.
October 15-19, 2000
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
6
Current and Near-Term Technology Values of CCGT Design
Parameters and Corresponding Gross Cycle Efficiency
Independent
Variable
Tin
erec
Current
Value
850oC
95%
(@ ~510oC)
Near-Term
Value
1,200oC *
96%
(@ ~800oC)
Pout
hT,ad
hC,ad
DP/Pout
7 MPa
93%
89%
0.07
15 MPa
94%
92%
0.04
hcycle
51%
64%
Minimum He Temp. in cycle (heat sink) = 35°C
*10-20 years in the future
October 15-19, 2000
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
7
Sensitivity of CCGT Performance to Independent
Parameters for the Case of Optimized PC
Brayton Cycle Gross Efficiency
0.62

0.6
0.58
Ti n

DP/P

erec

h

hT,ad
0.56
Changing the turbine
inlet temperature from
850°C to 1200°C has
the major effect on
increasing hcycle from
51% to ~60%.
•
Changing the other
parameters within the
stated range play a
lesser but still
significant role,
cumulatively pushing
hcycle to 64%.


C ,ad
0.54
0.52


















0.5
Current
Technology
Value
October 15-19, 2000
•
Near-Term
Technology
Value
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
8
Ceramic Turbine Would Allow for High Temperature
Operation


The major issues associated with large helium turbocompressors are:
-
Material limitations of high temperature blades and disks
Dynamic stability of large flexible rotor assemblies
-
Dynamic loading capability of magnetic bearings
High temperature turbine blades/disks limited by creep and fatigue crack growth
-
Uncooled turbine components made of cast mono-crystal nickel (e.g. IN-100) and
special wrought materials
- Projected useable lifetimes of 50-60,000 hrs at 850°C
-
Cooling of blades/disks with cold bypass helium
- Higher inlet temperatures, but with diminishing improvements
-
Refractories and ceramics offers high potential gains in performance
- Arc-cast molybdenum based refractories (e.g. TZM) could allow inlet temperatures as high as 1000 °C
- SiCf/SiC composites could allow inlet temperatures of up to 1150-1200°C
- Advanced carbon-carbon materials exhibit acceptable strength at temperatures up to 1500°C in low
oxidizing helium environments
October 15-19, 2000
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
9
High Adiabatic Efficiencies Can be Achieved with
Split-Shaft and Larger-Size Compressor and Turbine
•
Helium turbine and compressor adiabatic efficiencies dependent on volumetric
flow and rotational speeds.
- Common practice is to connect turbine and compressor through a common shaft to limit runaway
speeds in the event of a loss-of-loads
•
.
-
This limits optimization of the compressor efficiency, which performs better at high rotational
speeds to compensate for lower volumetric flows
-
Splitting the turbine and compressor into two shafts gives better compressor performances but
requires development of fast-acting control techniques
It is easier to achieve higher adiabatic efficiencies with larger turbine and
compressor sizes (94 and 92%, respectively achievable for ~400 MWe)
-
Large helium turbocompressors tend to be long and flexible and operating speed will be well into
the critical speed range
-
Operation of large turbines above critical speeds is only possible with magnetic bearings, which
can actively control stiffness and damping characteristics to adjust the critical speed relative to the
operating speed
-
Fusion rotors are estimated to weight ~100 tonnes and would require 4-6 radial bearings.
-
The largest rotor suspended to-date on magnetic bearings is 23 tonnes using 5 active magnetic
radial bearings
October 15-19, 2000
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
10
High Recuperator Effectiveness Achievable Based on
Near-Term Technology
•
Development of plate-fin recuperators in last two decades was the most
important advance for improving CCGT performance.
-
Until the early 1980’s, tubular designs limited large recuperator effectiveness to ~
81-82%.
-
Development of manufacturing techniques for large plate-fin recuperators in the
early 1980s by Allied Signal and others made possible designs of helium
recuperators with e of up to 95% .
-
Presently, high temperature, high effectiveness recuperators are available from
Allied Signal, Heatrix (U.K.) and IHI (Japan).
-
OKBM of Nizhny-Novgorad (Russia) recently constructed and tested a modular
helium recuperator for nuclear CCGT service and demonstrated 95%
effectiveness at a heat duty of 628 MWt, peak temperature of 508oC and pressure
differential of 45 atm.
October 15-19, 2000
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
11
Key Challenge for Advanced Helium CCGT Recuperator is
Accommodation of Increased Temperature and Pressure
Differential
• Maximum recuperator temperature
- ~590-600°C for medium priced heat exchanger
materials (e.g. SS316)
- ~750-800°C for ~10x more expensive nickel based
alloys (e.g. Alloy 800H)
• Use of ceramic materials for high effectiveness,
high temperature, high pressure, fixed surface
recuperator still needs to be demonstrated
• High heat flux porous media as heat exchanger
configuration might also improve future
recuperator performance and needs to be
further studied
• Even based on metallic materials and
conventional configurations, recuperator
effectiveness of 96% at a temperature of 800oC
are projected for fusion reactors
October 15-19, 2000
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
12
High Pressure Operation Would Minimize Fractional
Pressure Drop and Increase Cycle Efficiency
•
Brayton cycle efficiency is dependent on the fractional pressure drop (DP/P)
•
High pressure losses, as expected from high volume flow rates through fusion inreactor elements, can be compensated by increase in the system pressure.
•
The maximum pressure considered in nuclear design studies to-date is ~7 MPa,
limited to the capability of large, uninsulated, high temperature pressure vessels.
•
Internal insulation would increase the pressure capability
•
Power conversion system should be able to achieve pressure capabilities of
current nuclear pressure vessels and piping components (~15 MPa.)
•
It is likely that He-cooled in-reactor components will establish the pressure limit
•
DP/P of 0.04 seems achievable based on past studies
October 15-19, 2000
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
13
Conclusions
•
Helium closed-cycle gas turbines are a promising technology for future fusion plants.
They can be coupled with fusion in-reactor components using the same He coolant
but also using different coolants via an intermediate heat exchanger
•
The overall compression ratio for given cycle parameters can be optimized for
maximum cycle efficiency and acceptable in-reactor inlet temperature as required by
material consideration
•
Based on current technology, He CCGT can achieve a gross cycle efficiency of ~51%
•
With technology developments related to turbine and recuperator materials and
increases in turbine size, it seems reasonable to expect an increased gross thermal
efficiency of up to ~64% on a time scale of ~10-20 years
•
As the technology is applicable to both fusion and fission reactors, fusion will benefit
from current developments and demonstrations in this area under fission programs
October 15-19, 2000
R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000
14