October 15-19, 2000 R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton...

October 15-19, 2000 R. Schleicher, A. R. Raffray, and C. P. Wong, An Assessment of the Brayton Cycle…, TOFE 2000

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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, USA2University 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


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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


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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.


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350 MWE Nuclear CCGT (GT-MHR) Currently Being Designed by U.S./Russia

ReactorPower ConversionModule

Generator

Turbine

Recuperator

Compressor

Inter-Cooler

Pre-Cooler

Reactor


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Ideal CCGT for Fusion Power Plant

IP LPHP

Pout

Compressors

RecuperatorIntercoolers

Pre-Cooler

Generator

CompressorTurbine

To/from In-ReactorComponents or Intermediate

Heat Exchanger

1

2

3

4

5 6 7 8

9 10

1BPin

TinTout

η ,C ad η ,T ad

εrec

5'

1

22'

38

9

4

7'9'

10

6

T

S

1B'

1B

• Multi-stage compression with inter-coolers to reduce compression work

• Split-shaft turbine to allow independent optimization of compressor and turbine aerodynamic performance


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The Cycle Efficiency is Optimized by Setting System Parameters Under the Designer’s Control

• Compressor turbine inlet temperature, Tin

• Recuperator effectiveness, εrec

• System fractional pressure drop, P/Pout

• Turbine and compressor adiabatic efficiencies, ηT,ad and ηC,ad

• Overall compression ratio, C=Pout/Pin

- C 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.


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Current and Near-Term Technology Values of CCGT Design Parameters and Corresponding Gross Cycle Efficiency

Independent Current Near-Term

Variable Value Value

Tin 850oC 1,200oC *

εrec 95% 96%

(@ ~510oC) (@ ~800oC)

Pout 7 MPa 15 MPa

ηT,ad 93% 94%

ηC,ad 89% 92%

P/Pout 0.07 0.04

ηcycle 51% 64%

*10-20 years in the future0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

400

500

600

700

800

900

1000

1100

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

Overall Compression Ratio

Near-Term Technology

Current Technology

Near-TermTechnology

CurrentTechnology

Minimum He Temp. in cycle (heat sink) = 35°C


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Sensitivity of CCGT Performance to Independent Parameters for the Case of Optimized C

▲

▲

▲

▲

▼▼

▼▼

❍ ❍ ❍ ❍

❒❒

❒❒

✖ ✖ ✖ ✖

0.5

0.52

0.54

0.56

0.58

0.6

0.62▲

▼

❍

❒

✖

CurrentTechnologyValue

Near-TermTechnologyValue

Tin

/P P

εrec

η ,C adη ,T ad

• Changing the turbine inlet temperature from 850°C to 1200°C has the major effect on increasing ηcycle from 51% to ~60%.

• Changing the other parameters within the stated range play a lesser but still significant role, cumulatively pushing ηcycle to 64%.


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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


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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


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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 ε 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.


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Key Challenge for Advanced Helium CCGT Recuperator is Accommodation of Increased Temperature and Pressure

Differential

500

550

600

650

700

750

800

850

3

4

5

6

7

8

9

10

800 900 1000 1100 1200 1300

Turbine Inlet Temperature (°C)

Pout=7 MPa

Pout=15 MPa

• 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


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High Pressure Operation Would Minimize Fractional Pressure Drop and Increase Cycle Efficiency

• Brayton cycle efficiency is dependent on the fractional pressure drop (P/P)

• High pressure losses, as expected from high volume flow rates through fusion in-reactor 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

• P/P of 0.04 seems achievable based on past studies


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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...

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