Over the last year, Billington Process Technology AS (BPT) has been involved with configuration, enhancement and technology development of a dynamic simulator for a Brayton cycle using supercritical CO2 (sCO2) as the fluid. The sCO2 is used as the energy carrier in a concentrated solar power (CSP) system.
“BPT has a strong believe in new sustainable energy systems with sCO2 as the energy carrier, as it has clear economic benefits over traditional steam-based systems, with technical benefits as higher efficiency and significantly smaller size rotating equipment”, explains a technology executive in BPT. “We have invested in closing technology gaps in current simulator technologies, enabling high-fidelity dynamic simulators to validate and de-risk new concepts at an early stage as well as providing a reliable foundation for full-fledge life cycle simulators”.
The background
The Brayton cycle can be operated at high temperatures with a variety of heat sources. Concentrating solar power (CSP) offers an interesting potential for renewable power generation, especially when in combination with a high temperature heat storage to provide continuous power generation.
Supercritical carbon dioxide is a non-toxic, stable material that is under so much pressure it acts like both a liquid and a gas. The carbon dioxide stays within the system and is not released as a greenhouse gas. Because so much energy is lost turning steam back into water in the comparable Rankine cycle, at most a third of the power in the steam can be converted into electricity. In comparison, the Brayton cycle has a theoretical conversion efficiency upwards of 50 percent.
The challenges
The cycle includes closely integrated compressors and turbo expanders, with strongly connected dynamics that may cause instabilities. It is well known that native dynamic model of expanders often causes instabilities and in a supercritical CO2 Brayton cycle, the operation of the expanders is key to the system. In addition, the native electrical motor can only be used under specific conditions, resulting in that full startup and load control scenario requires customization.
For parts of the system, the operating conditions are kept just above the critical point where thermodynamic CO2 behavior is challenging, especially if there are small amounts of impurities and during dynamic scenarios.
REFPROP is an acronym for REFerence fluid PROPerties – a program developed by the National Institute of Standards and Technology (NIST) - calculates the thermodynamic and transport properties of industrially important fluids and their mixtures. REFPROP, by many considered as the best fitted approach for supercritical CO2, has shown to cause serious challenges in simulators, with significant impact on the simulator performance and with questionable creditability in use for design and validation purposes.
Importantly, supercritical CO2 requires an accurate equation of state as the cycle operates very close to the critical point. Unfortunately, these high accuracy models (REFPROP) have not been created with dynamic modelling in mind. The simulation speed is such that it takes hours to model an event that takes minutes in reality. The presence of impurities exacerbates the speed issues and accuracy is not guaranteed. The speed drops so low that modelling becomes impractical (it takes days to model minutes).
The entire energy system requires proper holistic and integrated system engineering including the various parts as heat source, heat storage, energy carrier and energy utilization units.
System controllability is challenging, among others due to relatively small system volumes meaning fast interactions, sensitive thermodynamic behavior and operational limitations for the rotating equipment.
All the above are serious challenges when applying industry-standard steady state and dynamic simulators to validate and improve design of such energy systems. The challenges are related to accuracy of model and thermodynamics, sensitivity regarding impurities of CO2, behavior during dynamics like startup and load changes, robustness and calculation speed.
The solution
Based on BPT’s extensive and in-depth experience with modelling the energy solution in different steady state and dynamic simulation tools, we have addressed the technology gaps and developed simulator software extensions to the existing industry-standard process simulators. For pilot plants, often we also support the customer with simulator configuration and simulation services safeguarding and optimizing the simulator system.
BPT extensions include unit operations for rotating equipment and fit-for purpose thermodynamic package tailormade to enhance simulator performance. One of the examples BPT-TEX that overcomes most of the instabilities caused by the native expander operation.
In addition, BPT has worked on various control schemes and custom models, e.g. electrical starter motor.
BPT, in collaboration with thermodynamic experts, has created a thermodynamic package fitted for supercritical CO2. It is essentially the Span-Wagner formulation for pure component CO2, with mixtures containing impurities based on generalized corresponding state theory - impurities that can disproportionally affect the key properties such as density and viscosity. It has been validated for accuracy against both experimental data and REFPROP. The fit-for-purpose property package that overcomes both the speed challenge for pure CO and for CO2 containing impurities are currently used in pilot projects and under continuous performance enhancement.
For a new technology with a fast-track implementation roadmap it is crucial that the dynamics of the system are validated to avoid risks during startup and operation of the pilot or full-scale unit. Verification of system controllability, startup, shutdown and load change scenarios can only be assessed properly when the instabilities caused by certain unit operations and the thermodynamic package have been resolved.
The benefits
Tailormade modelling of electrical motor and use of BPT-TEX turbo expander unit operation extension improved simulator stability issues.
The fit-for-purpose property package has improved the simulation speed without scarifying accuracy and added a firm approach in handling uncertainty related to the thermodynamic properties involved.
Hence, dynamic scenarios like startup, shutdown, load change, etc. can be run efficiently and add reliable input to design validation and enhancements.
The overall main value is de-risking the design at an early stage.
This is also an important foundation for high-fidelity lifecycle simulators adding more benefits to detailed design, commissioning, plant startup and lifetime operation.
Billington Process Technology (BPT) is an independent digital solution, simulation and service company with Headquarter outside Oslo, Norway. BPT has unique domain knowledge within production and process facilities. We are world-class users of process simulators, and among the specialties are compressor design as well as process safety. The BPT Digital Production Twin includes an unmatched solution for holistic sensor-correction providing invaluable data fundament for a number of advanced digital applications (ala machine learning) as well as calibrated steady-state and dynamic simulators for efficient production optimization. BPT is a frontrunner in modernizing field development approach together with innovating oil companies. A BPT specialty is to apply integrated multiphase flow and dynamic process simulators throughout the field development, commissioning, and life of field to validate and improve design as well as troubleshoot and perform production optimization.
For more information about this press release, please contact Knut Erik Spilling, BPTs Vice President for Sales & Marketing (e-mail: kes@bpt.no).