Darryn Fleming

Performance Characteristics of an Operating Supercritical CO2 Brayton Cycle

Supercritical CO 2 (S-CO 2 ) power cycles offer the potential for better overall plant economics due to their high power conversion efficiency over a moderate range of heat source temperatures, compact size, and potential use of standard materials in construction. Sandia National Labs (Albuquerque, NM) and the U.S. Department of Energy (DOE-NE) are in the process of constructing and operating a megawatt-scale supercritical CO 2 split-flow recompression Brayton cycle with contractor Barber-Nichols Inc. (Arvada, CO). This facility can be counted among the first and only S-CO 2 power producing Brayton cycles anywhere in the world. The Sandia-DOE test-loop has recently concluded a phase of construction that has substantially upgraded the facility by installing additional heaters, a second recuperating printed circuit heat exchanger (PCHE), more waste heat removal capability, higher capacity load banks, higher temperature piping, and more capable scavenging pumps to reduce windage within the turbomachinery. With these additions, the loop has greatly increased its potential for electrical power generation, and its ability to reach higher temperatures. To date, the loop has been primarily operated as a simple recuperated Brayton cycle, meaning a single turbine, single compressor, and undivided flow paths. In this configuration, the test facility has begun to realize its upgraded capacity by achieving new records in turbine inlet temperature (650°F/615 K), shaft speed (52,000 rpm), pressure ratio (1.65), flow rate (2.7 kg/s), and electrical power generated (20 kWe). Operation at higher speeds, flow rates, pressures, and temperatures has allowed a more revealing look at the performance of essential power cycle components in a supercritical CO 2 working fluid, including recuperation and waste heat rejection heat exchangers (PCHEs), turbines and compressors, bearings and seals, as well as auxiliary equipment. In this report, performance of these components to date will be detailed, including a discussion of expected operational limits as higher speeds and temperatures are approached.

Supercritical CO2 direct cycle Gas Fast Reactor (SC-GFR) concept.

This report describes the supercritical carbon dioxide (S-CO{sub 2}) direct cycle gas fast reactor (SC-GFR) concept. The SC-GFR reactor concept was developed to determine the feasibility of a right size reactor (RSR) type concept using S-CO{sub 2} as the working fluid in a direct cycle fast reactor. Scoping analyses were performed for a 200 to 400 MWth reactor and an S-CO{sub 2} Brayton cycle. Although a significant amount of work is still required, this type of reactor concept maintains some potentially significant advantages over ideal gas-cooled systems and liquid metal-cooled systems. The analyses presented in this report show that a relatively small long-life reactor core could be developed that maintains decay heat removal by natural circulation. The concept is based largely on the Advanced Gas Reactor (AGR) commercial power plants operated in the United Kingdom and other GFR concepts.

Supercritical CO2 recompression Brayton cycle : completed assembly description.

Through multi-year funding from DOE-NE, Sandia National Labs supercritical carbon dioxide (SCO2) closed Brayton cycle (CBC) research and development team have recently overseen the completion of the SCO2 CBC recompression test assembly (TA), and delivery from the development contractor’s facility to Sandia, Albuquerque. The primary components of the completed TA include two turboalternator-compressors and associated motor/controllers, three printed circuit heat exchangers, and six shell-and-tube heaters and associated controllers. Principal supporting components include a cooling tower, electricity-dissipating load bank, and leakage flow management equipment. With this milestone completed, significant increase in CBC R&D is anticipated with the objective of advancing the technology readiness level of components seen by industry as immature. This report presents detailed descriptions of all components and operating software necessary to operate the recompression CBC.

Corrosion and Erosion behavior in Supercritical CO2 power cycles.

Supercritical Carbon Dioxide (S-CO2) is emerging as a potential working fluid in power-production Brayton cycles. As a result, concerns have been raised regarding fluid purity within the power cycle loops. Additionally, investigations into the longevity of the S-CO2 power cycle materials are being conducted to quantify the advantages of using S-CO2 versus other fluids, since S-CO2 promises substantially higher efficiencies. One potential issue with S-CO2 systems is intergranular corrosion [1]. At this time, Sandia National Laboratories (SNL) is establishing a materials baseline through the analysis of 1) “as received” stainless steel piping, and 2) piping exposed to S-CO2 under typical operating conditions with SNL’s Brayton systems. Results from ongoing investigations are presented.A second issue that SNL has discovered involves substantial erosion in the turbine blade and inlet nozzle. It is believed that this is caused by small particulates that originate from different materials around the loop that are entrained by the S-CO2 to the nozzle, where they impact the inlet nozzle vanes, causing erosion. We believe that, in some way, this is linked to the purity of the S-CO2, the corrosion contaminants, and the metal particulates that are present in the loop and its components.Copyright

Scaling Considerations for a Multi-Megawatt Class Supercritical CO2 Brayton Cycle and Path Forward for Commercialization

Small-scale supercritical CO2 demonstration loops are successful at identifying the important technical issues that one must face in order to scale up to larger power levels. The Sandia National Laboratories (Sandia) Supercritical CO2 Brayton cycle test loops are identifying technical needs to scale the technology to commercial power levels such as 10 MWe. The small demonstration loops provide a scalable approach to identify cost, technical hurdles, and future commercialization plans of commercial applications.The small size of the Sandia 1 MWth loop has demonstration of the split flow loop efficiency and effectiveness of the Printed Circuit Heat Exchangers (PCHEs) leading to the design of a fully recuperated, split flow, supercritical CO2 Brayton cycle demonstration system. However there were many problems that were encountered such as; the high rotational speeds in these units identified the need to address bearing, seals, thermal boundaries, and motor controller problems to prove a reliable power source in the 300 kWe range. Although these issues were anticipated in smaller demonstration units, we also understood that commercially scaled hardware would eliminate these problems caused by high rotational speeds at small scale.The economic viability and development of the future scalable 10 MWe solely depends on the interest of DOE and private industry. The Intellectual Property collected by Sandia proves that the ∼10 MWe Supercritical CO2 power conversion loop to be very beneficial when coupled to a 20 MWth heat source (either solar, geothermal, fossil, or nuclear). This paper will identify a commercialization plan, as well as, a roadmap from the simple 1 MWth supercritical CO2 development loop to a power producing 10 MWe supercritical CO2 Brayton loop.Copyright

Evaluation of Recent Data From the Sandia National Laboratories Closed Brayton Cycle Testing

At Sandia National Laboratories (SNL), The Nuclear Energy Systems Laboratory / Brayton Lab has been established to research and develop subsystems and demonstrate the viability of the closed Brayton cycles (CBC), and in particular, the recompression CBC. The ultimate objective of this program is to have a commercial-ready system available for small modular reactors. For this objective, R&D efforts must demonstrate that, among other things, component and the system behavior is understood and control is manageable, and system performance is predictable. Research activities that address these needs include investigating system responses to various anticipated perturbations, and demonstrating that component and system performance is understood. To these ends, this paper presents system response to a perturbation, and turbomachinery performance results during steady state operation. A long duration test, with an extensive period at steady state, was completed in the simple CBC configuration. During this period, a cooling perturbation was initiated. Data from this test are presented and evaluated to explain the sequence of events following the perturbation. It was found that a cascading series of events ensued, starting with the fluid condensing effect of the cooling perturbation. The explanation of events emphasizes the highly interactive and nonlinear nature of CBC’s. The comparisons of measured and predicted turbomachinery performance yielded excellent results and give confidence that the predictive methods originally envisioned for this system work well.© 2016 ASME

Steady State Supercritical Carbon Dioxide recompression Closed Brayton Cycle Operating point comparison with predictions.

The U.S. Department of Energy Office of Nuclear Energy (DOE-NE) supercritical carbon dioxide recompression closed Brayton cycle (RCBC) test assembly (TA) construction has been completed to its original design and resides at Sandia National Laboratories, New Mexico. Commissioning tests were completed in July 2012, followed by a number of tests in both the recompression CBC configuration, and in a bottoming cycle configuration that is proprietary to a current customer. While the test assembly has been developed and installed to support testing, a computer model of the loop, written in Fortran programming language, has also been developed. The purpose of this iterative model is to facilitate data interpretation, guide test assembly design modifications, develop control schemes, and serve as a foundation from which to develop a transient model. Of central utility is its modular nature, which has already been leveraged to develop a customer’s bottoming cycle configuration. Verification that the model uses appropriate physical representations of components and processes, is performing as intended, and validation that the model accurately reproduces test data, are necessary activities. Completion of the model’s verification and validation (V&V) supports the long-term goal of commercializing the RCBC for a sodium fast reactor. This paper presents verification results of certain subprocesses of the iterative computer model. Verification of these subprocesses was completed with positive results. While an adequate range of data for complete and thorough validation do not yet exist, comparison of subprocess predictions with data from a single, representative operating point are presented as are explanations for differences. Recommendations for activities necessary to complete subprocess and model validation are given. The RCBC iterative computer model V&V process should be revisited following completion of these recommended actions and the generation of steady state data while operating near the test assembly design point.Copyright

Testing Platform and Commercialization Plan for Heat Exchanging Systems for SCO2 Power Cycles.

Supercritical Carbon Dioxide Closed Brayton Cycle (S-CO2 CBC) systems have the potential to convert thermal energy to electricity at efficiency significantly higher than traditional steam Rankine cycles. The primary difference in the Brayton cycle that enables higher efficiency is the availability of a useful temperature difference between the high temperature, low pressure flow exiting the turbine, and the low temperature, high pressure flow exiting the compressor. In the S-CO2 CBC cycle, this temperature difference drives heat transfer through recuperation in heat exchangers. Overall cycle energy conversion efficiency increases as the extent of recuperation increases. The platform, once commissioned, can test many types of heat exchangers to investigate performance characteristics and to select which application they will be best suited for. Characterizing these heat exchangers will facilitate understanding how they scale. Plant economics will be a major factor in the selection of these heat exchangers. It has been identified that at this time, up to 90% of the cost of the S-CO2 Brayton Cycle will be in the heat exchangers. This percentage assumes the use of printed circuit heat exchangers. Although these heat exchanger are approximately 98% efficient and a relatively high cost, the use of a lower efficiency and less costly heat exchanger may make this S-CO2 technology more attractive for a path forward commercialization.

Scaling Considerations for SCO2 Cycle Heat Exchangers

Many SCO2 Brayton cycle demonstration loops have been constructed or are being planned at various thermal power levels between several kilowatts up to 50MW. However experiences at Sandia National Labs and other organizations have demonstrated that few options exist to procure off-the-shelf heat exchangers, especially for higher temperature ranges, due to high operating and differential pressures and the cost of materials with sufficient strength above 600 °C. This paper reviews fundamental mechanical scaling considerations governing SCO2 cycle heat exchangers that should be considered when designing and constructing SCO2 cycles. Current manufacturing limitations will also be reviewed.© 2014 ASME

Materials Corrosion Concerns for Supercritical Carbon Dioxide Heat Exchangers

Supercritical Carbon Dioxide (S-CO2) is an efficient and flexible working fluid for power production. Research to interface S-CO2 systems with nuclear, thermal solar, and fossil energy sources are currently underway. To proceed, we must address concerns regarding high temperature compatibility of materials and compatibility between significantly different heat transfer fluids.Dry, pure S-CO2 is thought to be relatively inert [1], while ppm levels of water and oxygen result in formation of a protective chromia layer and iron oxide [2]. Thin oxides are favorable as diffusion barriers, and for their minimal impact on heat transfer. Chromia, however, is soluble in molten salt systems (nitrate, chloride, and fluoride based salts) [3–8]. Fluoride anion based systems required the development of the alloy INOR-8 (Hastelloy N, base nickel, 17%Mo) [9] to ensure that chromium diffusion is minimized, thereby maximizing the life of containment vessels.This paper reviews the thermodynamic and kinetic considerations for promising, industrially available materials for both salt and S-CO2 systems.Copyright