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10 MW Supercritical CO2 Turbine Test

29 Jan 2014-
TL;DR: The Supercritical CO2 Turbine Test (SCOT) project as discussed by the authors demonstrated the inherent efficiencies of a s-CO2 power turbine and associated turbomachinery under conditions and at a scale relevant to commercial concentrating solar power (CSP) projects, thereby accelerating the commercial deployment of this new power generation technology.
Abstract: The Supercritical CO2 Turbine Test project was to demonstrate the inherent efficiencies of a supercritical carbon dioxide (s-CO2) power turbine and associated turbomachinery under conditions and at a scale relevant to commercial concentrating solar power (CSP) projects, thereby accelerating the commercial deployment of this new power generation technology. The project involved eight partnering organizations: NREL, Sandia National Laboratories, Echogen Power Systems, Abengoa Solar, University of Wisconsin at Madison, Electric Power Research Institute, Barber-Nichols, and the CSP Program of the U.S. Department of Energy. The multi-year project planned to design, fabricate, and validate an s-CO2 power turbine of nominally 10 MWe that is capable of operation at up to 700°C and operates in a dry-cooled test loop. The project plan consisted of three phases: (1) system design and modeling, (2) fabrication, and (3) testing. The major accomplishments of Phase 1 included: Design of a multistage, axial-flow, s-CO2 power turbine; Design modifications to an existing turbocompressor to provide s-CO2 flow for the test system; Updated equipment and installation costs for the turbomachinery and associated support infrastructure; Development of simulation tools for the test loop itself and for more efficient cycle designs that are of greater commercial interest; Simulation of s-CO2 power cycle integration into molten-nitrate-salt CSP systems indicating a cost benefit of up to 8% in levelized cost of energy; Identification of recuperator cost as a key economic parameter; Corrosion data for multiple alloys at temperatures up to 650oC in high-pressure CO2 and recommendations for materials-of-construction; and Revised test plan and preliminary operating conditions based on the ongoing tests of related equipment. Phase 1 established that the cost of the facility needed to test the power turbine at its full power and temperature would exceed the planned funding for Phases 2 and 3. Late in Phase 1 an opportunity arose to collaborate with another turbine-development team to construct a shared s-CO2 test facility. The synergy of the combined effort would result in greater facility capabilities than either separate project could produce and would allow for testing of both turbine designs within the combined budgets of the two projects. The project team requested a no-cost extension to Phase 1 to modify the subsequent work based on this collaborative approach. DOE authorized a brief extension, but ultimately opted not to pursue the collaborative facility and terminated the project.

Summary (5 min read)

Introduction

  • Commercial demonstration of the s-CO2 Brayton cycle is imminent, although not for the conditions necessary for CSP.
  • Project member Echogen fielded a 250-kW prototype system in 2010 and started testing of the larger EPS100 in 2012.
  • The goal of the three-phase project was to demonstrate the efficiency of s-CO2 turbomachinery and operation of the power cycle under conditions relevant to CSP – including high turbine inlet temperatures, high compressor inlet temperatures (indicative of dry cooling), and frequent transient operation.
  • Produce a matrix of candidate materials showing their corrosion performance at time intervals of 1000 hours over the range of operating conditions from 300°C to 650°C, up to 200 bar, also known as Milestone (Task 1.1).

Go/No-Go Decision Phase 1

  • Successful completion of all Phase 1 milestones, including:.
  • An s-CO2 cycle design which modeling shows achieves 50% thermal-to-electric efficiency.
  • A turbine design with the details on the materials, geometry and operating parameters for the test loop which modeling shows achieves 80% isentropic efficiency.
  • A transient performance model for the 10MW test loop (Task 1.4).

Project Results and Discussion

  • Task 1.1 Corrosion and Materials Analysis Under Task 1.1 team member UW-Madison tested several commercial alloys for their suitability in s-CO2 at various conditions.
  • The summary of results of the weight gain measurements and some surface and cross sectional analysis is presented below.
  • Oxidation curves fitted to the measured data points describing the reaction kinetics are also shown along with error bars at each measured data point.
  • At elevated temperatures, alloy 800H offers resistance to oxidation, carburization, and sulfidation along with rupture and creep strength.
  • Failure of the oxide layer occurred in part because chromium was no longer available for protective oxide formation.

AFA-OC6

  • The weight gain and surface morphology of the advanced forming austenitic alloy AFAOC6 at 450oC, 550oC, and 650oC are shown below Figure 7 and in SEM micrographs .
  • AFA-OC6 exhibited more rapid weight gain and oxide growth than alloy 800H and 347SS at and above 550oC, which can be seen by comparing the previous figures.
  • The structure of the oxide layer was similar in that it consisted of an outer Fe3O4 layer and an inner (Fe, Cr) spinel oxide layer.
  • As in the case of 316SS, oxide spallation was observed.

Summary of Phase 1 Materials Tests

  • In summary, Table 4 below provides a qualitative upper temperature limit for the tested alloys with some additional comments based on SEM/EDS analysis discussed above.
  • This is a preliminary assessment and more analysis is needed to determine the metric of <30 micron/year attack.
  • Selection of the alloys for the different temperature ranges were loosely based on the anticipated lowest cost material that should be sufficient with respect to corrosion.
  • The recommendations did not include an assessment of the cost and wall thickness for pressure vessel code restrictions.
  • The data summarized here form part of two journal articles [16, 17].

F91 - 500hrs

  • The purpose of this Task was to outline the commission and test of the SunShot Heat Engine System at the Sandia test site in New Mexico and is based on the EPS100 test program at Dresser-Rand facilities in Olean, NY.
  • The project system was planned as a modification of the Echogen EPS100 system where turbine inlet temperature and compressor inlet temperature would be raised to levels that are relevant to applications in CSP.
  • The test team will consist of Echogen test personnel experienced with the operation of the EPS100 and Sandia Laboratory test personnel.
  • This review would help ensure that proper data were recorded and determine if the team can move on to the next test or must repeat the just completed test.
  • It receives high-temperature, pressure s-CO2 from the process skid through the main heat exchanger, and returns the lower temperature and pressure CO2 after expanding through the power turbine.

Power Turbine Design

  • The power turbine for the EPS100 system is a single-stage radial design.
  • The design and manufacture of the SunShot power turbine would utilize current commercial turbomachinery technology.
  • The testing of this unit would then provide a strong foundation and confidence for the progress to full scale systems.
  • TurboAero is a well-accepted, commercial turbine design package.
  • The design process was an iterative process, back and forth through the steps to reach the appropriate design.

Materials

  • Two materials were considered during the design analysis.
  • It is an oxidation and corrosion resistant material for service in environments subjected to heat and pressure.
  • These two alloys have similar properties at temperatures of 1400°F (760°C).
  • Either material is acceptable for turbine components, but because of its expected better corrosion resistance, IN740 was expected to be the preferred choice.
  • For the turbine housing ASTM A336 Grade 91 was the default.

High Temperature Recuperator

  • The planned system had a high-temperature recuperator to handle the power-turbine discharge temperature as shown in Table 7.
  • The allowable pressure drop through each side of the recuperator was 0.15 MPa (22 psi).
  • The preferred material of construction for the high-temperature recuperator was 316/316L, although it was unclear is this allow would be suitable and a higher-cost alloy would be required.
  • The manufacturer applies a nominal 0.5 multiplier on the ASME design stress numbers to provide a commercially sensible design pressure limitation.
  • Decreasing the maximum turbine inlet temperature to 600ºC or attemperating the flow with cooler CO2 could limit the recuperator inlet temperature to approximately 500ºC and allow the entire recuperator to be fabricated from 300-series stainless steel.

Heat Rejection System

  • An air-cooled heat exchanger (ACHE) system was selected as the best arrangement for the proposed tests.
  • After reviewing several quotes, Abengoa selected Hammco Air Coolers (Owasso, OK) as the preferred provider.
  • Hammco quoted a 5-bay unit, each bay containing three forced-draft, 13-ft diameter fans.
  • Page 22 of 34 A revision of the design point conditions to an ambient air temperature of 33ºC and a CO2 outlet temperature of 45ºC was proposed to reduce the ACHE to four bays with a commensurate reduction in capital and operating cost.
  • The reduction in size will preclude testing the system during the hottest summer afternoons in Albuquerque, but this is not expected to impact the test schedule.

Heat Addition System

  • The test loop system cycle described above was designed around a gas-fired Heat Exchanger (HX) inlet temperature equal to 1000°C with an exhaust gas mass flow rate equal to 35.2 kg/s.
  • As Echogen’s development of their IPSEpro model advanced, NREL dropped further development of the EES model and dedicated those resources to addressing the issue of the overall project cost exceeding the planned budget.
  • The EPS100 cycle architecture was designed to maximize the output power from a heat source that is limited in its total heat availability by the allowable temperature decrease.
  • Based on discussions with potential commercial partners, two years of pilot-scale operation was held as a reasonable threshold for obtaining bank financing for a fullscale CSP plant utilizing sCO2 technology.

S-CO2 Cycle Design-Point Modeling

  • IPSEpro was utilized to optimize the design point parameters as well as predict the offdesign performance of the s-CO2 power cycles listed in Table 10.
  • Prior to establishing the design point performance, the cycle parameters for each cycle configuration were optimized.
  • After setting bounding constraints on maximum cycle pressure, ambient conditions and minimum main compressor inlet temperature a parametric search over other possible parameter values was performed, and the combination of parameters which yielded the lowest capital cost over production ratio was selected.
  • Cycle efficiency and T across the solar receiver were noted as key metrics, and Rankine power cycle performance was used as a baseline case.
  • The partial cooling cycle was found to be slightly less efficient than the recompression cycle, but it does benefit from an increased ΔT of ~40°C.

Off-Design S-CO2 Cycle Modeling

  • The off-design cycle performance was predicted with IPSEpro with the range of operation parameters experienced when coupled to a CSP system.
  • The three operational parameters of interest were: (i) solar HTF inlet temperature to S-CO2 cycle, (ii) solar HTF mass flow rate to S-CO2 cycle, and (iii) ambient temperature.
  • The mass flow rate and efficiency of the turbine were predicted with a relative performance map given the pressure ratio and speed.
  • Both the relative performance curve and map were created from absolute performance curves/maps supplied by Echogen.
  • The control methodology can be summarized as: Inventory control utilized to maintain maximum system pressure of 250 bar by varying the inlet pressure to the main compressor.

CSP S-CO2 Annual Simulations

  • The CSP systems described Table 10 were evaluated on an annual transient performance and cost basis by Abengoa using their internal TRNSYS-based cost/performance model.
  • First, each plant was designed to meet the target power ratings at design conditions.
  • The solar field optical performance was calculated using DELSOL3 for tower fields and using internally developed performance curves for Abengoa’s trough collectors.
  • A 13.5% reduction was possible using pure NaNO3 salt and lower recuperator costs.
  • Heat exchanger costs have a strong influence on system optimization and cost, and greater understanding is needed regarding the potential for reduction and innovation in these units.

CSP Roadmap

  • Commercial deployment of s-CO2 power cycles for CSP will likely go through two phases due to current CSP deployment economics and the state of heat transfer system development.
  • Historically, due to limitations in cost or materials, new turbines are first tested at lower temperatures and/or pressures than they ultimately achieve.
  • If the authors run a performance test of the power turbine at some inlet pressure and inlet temperature other than the design inlet temperature and pressure, one can plot the data in terms of pressure ratio (Pin / Pout) vs. equivalent mass flow, ṁeq = ṁ*T 1/2 / Pin, for lines of equivalent speed, Neq = N / T 1/2.
  • Data taken at similar equivalent speeds will fall on the same line .
  • The objective of this phase of deployment is to use s-CO2 cycles to enable high-efficiency, low-cost CSP plants for long-term commercial deployment.

Utility Stakeholder Workshop

  • An EPRI-hosted workshop on s-CO2 technology occurred on July 31, 2013.
  • The workshop queried representatives from 18 different utilities regarding the perceived benefits and threats to the s-CO2 cycle.
  • During Phase 1 Sandia made preparations to run a natural gas extension to NESL to fuel the approx.
  • Sandia assessed three different options for power offtake from the planned test: use of a mechanical water or air brake, renting electrical load banks, or connection to the local grid.
  • Combining with the project led by SwRI in San Antonio was a viable option, but requires relocation to San Antonio, TX.

Conclusions

  • The 10 MW s-CO2 Turbine project was a major development effort with multiple partners and significant hardware requirements.
  • The project brought together a diverse and complementary set of stakeholders with the common goal of advancing the s-CO2 power cycle technology toward commercial deployment.
  • While the accomplishments described above were considerable, Phase 1 incurred many challenges.
  • NREL transient model development terminated as Echogen assumes greater role in cycle modeling.

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DE-EE0001589
Nonproprietary Final Report 10 MW Supercritical CO
2
Turbine Test
NREL
Page 1 of 34
Final Report
Project Title: 10 MW Supercritical CO
2
Turbine Test
Project Period: 10/01/12 10/31/13
Project Budget: $1,799,974
Submission Date: 01/27/2014
Recipient: National Renewable Energy Laboratory
Address: 15013 Denver West Parkway
Golden, CO 80401
Award Number: DE-EE0001589
Awarding Agency: DOE EERE SETP CSP subprogram
Project Team: Sandia National Laboratories
University of Wisconsin at Madison
Echogen Power Systems
Barber-Nichols, Inc.
Abengoa Solar
Electric Power Research Institute
Cost-Sharing Partners: Echogen Power Systems
Abengoa Solar
Electric Power Research Institute
Principal Investigator: Craig Turchi, PhD
Senior Engineer
Phone: 303-384-7565
Fax: 303-384-7495
Email: craig.turchi@nrel.gov
GO Contracting Officer: Golden, CO
Tech. Project Officer: Christine Bing
DOE Technical Mgr.: Mark Lausten

DE-EE0001589
Nonproprietary Final Report 10 MW Supercritical CO
2
Turbine Test
NREL
Page 2 of 34
Executive Summary
The goal of this project was to demonstrate the inherent efficiencies of a supercritical
carbon dioxide (s-CO
2
) power turbine and associated turbomachinery under conditions
and at a scale relevant to commercial concentrating solar power (CSP) projects, thereby
accelerating the commercial deployment of this new power generation technology. The
project involved eight partnering organizations: NREL, Sandia National Laboratories,
Echogen Power Systems, Abengoa Solar, University of Wisconsin at Madison (UW-
Madison), Electric Power Research Institute (EPRI), Barber-Nichols, and the CSP
Program of the U.S. Department of Energy (DOE).
The multi-year project planned to design, fabricate, and validate an s-CO
2
power turbine
of nominally 10 MWe that is capable of operation at up to 700°C and operates in a dry-
cooled test loop. Many stakeholders are interested in the potential of the s-CO
2
Brayton
cycle; for solar applications, advanced s-CO
2
Brayton cycles have the potential to
achieve the SunShot goal of greater than 50% thermal-to-electric conversion efficiency.
The project plan consisted of three phases. System design and modeling occurred in
Phase 1, followed by fabrication in Phase 2, and testing in Phase 3. The major
accomplishments of Phase 1 included:
Design of a multistage, axial-flow s-CO
2
power turbine,
Design modifications to an existing turbocompressor to provide s-CO
2
flow for the
test system,
Updated equipment and installation costs for the turbomachinery and associated
support infrastructure,
Development of simulation tools for the test loop itself and for more efficient cycle
designs that are of greater commercial interest,
Simulation of s-CO
2
power cycle integration into molten nitrate salt CSP systems
indicating a cost benefit of up to 8% in LCOE,
Identification of recuperator cost as a key economic parameter,
Corrosion data for multiple alloys at temperatures up to 650ºC in high-pressure
CO
2
and recommendations for materials-of-construction, and
Revised test plan and preliminary operating conditions based on the ongoing
tests of related equipment.
This report describes the progress made during Phase 1 and compares Phase 1 results
to the stated milestones. The report then outlines proposed modifications to the original
statement of project objectives (SOPO) designed to achieve the primary goal and major
objectives of the project while staying within the funding provided by the four
contributing organizations.
Phase 1 established that the cost of the facility needed to test the power turbine at its
full power and temperature would exceed the planned funding for Phases 2 and 3. The
team proposed to derate the test facility from 700°C to 600°C to save on materials cost
and presented this alternative to DOE. Toward the end of Phase 1 a unique opportunity

DE-EE0001589
Nonproprietary Final Report 10 MW Supercritical CO
2
Turbine Test
NREL
Page 3 of 34
arose to collaborate with another turbine development team to construct a single,
shared s-CO
2
test facility. The synergy of the combined effort would result in greater
facility capabilities than either separate project could produce and would allow for
testing of both turbine designs within the combined budgets of the two projects. All
industry partners in both projects supported the collaborative effort. Subsequently, the
project team requested a no-cost extension to Phase 1 to develop a Phase 2 proposal
based on this collaborative approach. DOE allowed a brief extension for reasons
unrelated to the proposed collaboration, but ultimately opted not to pursue the
collaborative facility and terminated the project.
Table of Contents
Executive Summary ........................................................................................................ 2
Background ..................................................................................................................... 4
Introduction ..................................................................................................................... 5
Phase 1 Milestones ..................................................................................................... 6
Project Results and Discussion ....................................................................................... 9
Task 1.1 Corrosion and Materials Analysis .................................................................. 9
Alloy IN800H .....................................................................................................................10
347SS ...............................................................................................................................11
AFA-OC6 ..........................................................................................................................12
Haynes 230 .......................................................................................................................14
316SS & 310SS ................................................................................................................14
Ferritic-martensitic Steels ..................................................................................................15
Summary of Phase 1 Materials Tests ........................................................................ 15
Task 1.2 Test Plan Development ............................................................................... 16
Task 1.3 Test Loop Design ........................................................................................ 18
Power Turbine Design .......................................................................................................19
Materials ...........................................................................................................................20
High Temperature Recuperator .........................................................................................21
Heat Rejection System ......................................................................................................21
Heat Addition System ........................................................................................................22
Task 1.4 Modeling and Simulation of Cycles ............................................................. 23
Task 1.5 Commercial Power Cycle ............................................................................ 23
Task 1.6 CSP Commercial Deployment Path ............................................................ 25
S-CO
2
Cycle Design-Point Modeling .................................................................................25
Off-Design S-CO
2
Cycle Modeling .....................................................................................25
CSP S-CO
2
Annual Simulations ........................................................................................26
CSP Roadmap ..................................................................................................................27
Utility Stakeholder Workshop ............................................................................................28
Task 1.7 Site Preparation .......................................................................................... 28
Conclusions ................................................................................................................... 30
Path Forward: ................................................................................................................ 31
References: ................................................................................................................... 33

DE-EE0001589
Nonproprietary Final Report 10 MW Supercritical CO
2
Turbine Test
NREL
Page 4 of 34
Background
Power cycle efficiency has a dramatic impact on CSP levelized cost. Higher efficiency in
the power cycle reduces the size and cost of the solar field and thermal storage system
required to achieve the desired system capacity and reduces the size of the power
block cooling loads. Higher efficiency in the power cycle also reduces plant size and
associated environmental footprint.
The current state of the art in CSP technology is the molten salt power tower. Although
power towers are capable of achieving temperatures up to 900°C, the molten nitrate salt
used as the heat transfer and thermal storage fluid is limited to temperatures less than
about 600°C. An operating limit of approximately 565°C, combined with a dry-cooled
steam Rankine power cycle, limits thermal-to-electric conversion efficiency to
approximately 41%.
This project planned to showcase the turbomachinery for a new cycle, the s-CO
2
Brayton cycle, capable of achieving DOE SunShot objectives of greater than 50% dry-
cooled efficiency with a power block cost less than $1200/kW. Originally proposed in the
late 1960s [1], this cycle has been under renewed investigation for the past decade [2-
6]. Researchers have modeled the basic thermodynamics of the cycle and used small
test rigs to explore the behavior of s-CO
2
turbomachinery and operational
characteristics of a closed Brayton cycle [7]. However, validation via operation of a
larger-scale prototype at temperatures relevant to CSP is needed to establish the true
potential of the power cycle.
While s-CO
2
power cycles hold much promise for CSP systems, there are numerous
hurdles to overcome before commercial s-CO
2
Brayton cycles achieve the efficiency
and reliability necessary for the solar application. No systems have been designed and
tested at turbine inlet temperatures greater than about 500°C. Better understanding of
material selection and corrosion mechanisms at higher temperatures, thermal stress
management, and real-gas aerodynamic performance modeling are all critical design
issues that will benefit from the execution of this program. Similarly, compressor
designs matched to dry cooling conditions are required.
Fortunately, we are able to draw on a substantial body of existing work. Over the past
several years, research teams from around the world have proposed and modeled
thermodynamic cycles using s-CO
2
. Laboratory and small-scale test systems have been
assembled to explore the behavior of s-CO
2
when compressed near and through the
critical point, and the operation of small-scale s-CO
2
turbomachinery and heat
exchangers in a closed loop cycle. Members of this proposal team have been heavily
involved in this preliminary development work, as indicated by their organization and
participation in two symposia devoted specifically to the s-CO
2
power cycles in 2009 and
2011.
Figure 1 shows how applicability of major system components varies with overall scale.
Considering bearings, seals, rotational speed, and ancillary equipment, a nominal 10-
MWe capacity is estimated to be the minimum size that allows for a viable commercial
design of the power turbine [8]. The commercial potential of the s-CO
2
turbine cannot be
evaluated unless high-efficiency, commercial-scale design elements can be

DE-EE0001589
Nonproprietary Final Report 10 MW Supercritical CO
2
Turbine Test
NREL
Page 5 of 34
incorporated in the unit and this project will validate the performance of a commercial-
scale, high-temperature s-CO
2
turbine.
Figure 1. Range of applicability of turbomachinery features. Use of commercial design elements is
essential for efficiency and reliability while reducing cost. Adapted from [8].
Introduction
Commercial demonstration of the s-CO
2
Brayton cycle is imminent, although not for the
conditions necessary for CSP. Project member Echogen fielded a 250-kW prototype
system in 2010 and started testing of the larger EPS100 in 2012. The EPS100 is
designed to run at temperatures of approximately 500°C and with wet cooling, and the
knowledge and support infrastructure developed for the EPS100 was leveraged for this
project. The ability to tap into Echogen’s existing knowledge base for instruments and
controls, ancillary equipment, and skid layout significantly reduced the cost for design
and testing of an s-CO
2
turbine of the necessary scale.
The goal of the three-phase project was to demonstrate the efficiency of s-CO
2
turbomachinery and operation of the power cycle under conditions relevant to CSP
including high turbine inlet temperatures, high compressor inlet temperatures (indicative
of dry cooling), and frequent transient operation. Testing the turbomachinery requires
assembling a full power cycle with the associated ancillary facilities for heat supply and
rejection, CO
2
supply, controls, and safety. The overall project tasks and team roles are
outlined in Table 1.
The project requires the team to specify and construct the power turbine and requisite
compressors and ancillary equipment necessary for a complete power conversion
system. The prototype would validate turbomachinery efficiency and cycle response to
transient operation and dry cooling conditions. Concurrently, the team would refine
performance models that predict steady-state and transient system response.
Experimental data would be used to validate the models, and simulations would be
made of one or more advanced cycle configurations that achieve a power block

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Frequently Asked Questions (1)
Q1. What are the contributions in this paper?

Turchi et al. this paper presented a 10 MW Supercritical CO2 Turbine Test Project ( SCTP ) to demonstrate the efficiency of s-CO2 turbomachinery and operation of the power cycle under conditions relevant to CSP.