Simple Cycle Test Validation of the

STEP Dynamic Simulation Model

Darryl Hino (GTI Energy)

8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 1
 System Modeling An Increasingly Valuable Tool

Steady state modeling Dynamic modeling Facility simulator

▪Development ▪Development ▪Development
▪Started Aspen Plus modeling ~12 years ▪Created Flownex model to support STEP ▪Flownex model used for hardware physics
ago for system studies ▪Calibrated against steady state model ▪Ties into a virtual controller accesses real time data
▪Continuous improvement with realistic ▪Use ▪Human machine interface mimics facility controls
pressure loss & equipment performance ▪Provide feedback on piping & equipment ▪Use
designs ▪Operator training – runs at real time
▪Cross verification w/other models
▪Test controls narrative & methodology ▪Operation planning
▪Use
▪Operations planning (startup, normal
▪Cycle performance, pipe sizing, shutdown, and trips/shutdowns)
equipment specification
▪Evaluate other sCO2 cycles &
applications
Models to be validated against STEP data for future use
The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 2
 Dynamic Modeling Configurations and Purpose
• Steady state and transient analysis performed using Flownex software
• 2 models: one for Simple Cycle and one for RCBC configurations
• sCO2 properties are taken from NIST REFPROP
• Custom component models have been created and benchmarked against vendor
predicted performance data
• Various transients have been analyzed, such as startup, shutdown, load level changes,
and emergency trips
• Validation of the model will be performed as test data becomes available

The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 3
 Flownex SE Overview
Reference: Flownex Simulation Environment

• Flownex Simulation Environment (SE) is

an object-oriented simulation environment
that was developed in the late 1980’s
– ISO 9001:2015 and ASME NQA-1
compliant
• Allows for 1D integrated steady state and
transient fluid flow and heat transfer
system modeling to calculate pressure
drop and heat transfer effects on the
system
– Typical uses include analysis, design and
optimization
• Flownex SE has been applied to various
industries:
– Power, Gas Turbines, Aerospace, HVAC,
Nuclear, etc
Flownex Model: Simple Cycle Configuration

The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 4
 Simple Cycle Overview

Simple Cycle Flownex Model

Simple Cycle Simplified Diagram

The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 5
 Component Implementation: Turbine
• STEP turbine is modeled through a
combination of Flownex elements linked to
a spreadsheet
• Spreadsheet houses calculations such as:
– Turbine flow function
– Balance piston leakage
– Inlet and exit pressure losses
• Dry gas seal flows coming from the IMS
are modeled

The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 6
 Component Implementation: Recuperators
• High temperature recuperator, low temperature recuperator, main and bypass
coolers are modeled using plate heat exchanger elements with customized heat
transfer correlations
• Error deltas were <2% in predicted outlet temperatures and <0.2 bar in calculated
pressure drops

The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 7
 Component Implementation: Heater
• Heater has been modeled as a series of tubes with composite heat transfer elements
• Modeling only the heat exchanger portion of the heater

The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 8
 Component Implementation:
PID Controllers & Compressors
• 17 total PID (proportional, integral, derivative) controllers in the model, used for
automatic or manual control
– Controlling speed, load, pressures, temperatures, mass flows
• Compressors modeled using Flownex compressor elements
– Vendor compressor maps using real gas properties incorporated

The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 9
 Test Validation Process
• Simple Cycle major component
commissioning efforts have been
completed
• Main Compressor maps have been updated
and incorporated into the steady-state and
dynamic models

The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 10
 Application of Dynamic Model: Emergency Trip L3
• One purpose of the dynamic model is
to study and evaluate control methods
and operational procedures for various
scenarios
• Scenarios simulated include:
– Nominal Startup and Shutdown
– Load Level Changes
– Emergency Trips
• One example is the Level 3
Emergency Trip simulation and results
– A Level 3 Trip is a manually activated
shutdown trip sequence via the Emergency
Shutdown (ESD) button
Level 3 Emergency Trip Valve Sequence
– It is intended to completely and rapidly de-
energize the plant

The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 11
 Results of Dynamic Model: Emergency Trip L3
• These plots illustrate the shutdown
sequence and system response
– Showing Turbine and Compressor speeds,
system pressures and temperatures
• Simulation verifies all requirements are
satisfied with current model
assumptions

The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 12
 Applications of the Dynamic Model:
Implementation of Digital Simulator
• A virtual digital Simulator has been
developed for the STEP facility
• The Simulator development was
completed in 2022, but has been
paused with plans to reinitiate the
simulator during RCBC commissioning
• The Simulator will be used during
operational planning, preparation of
operating procedures and operator
training

The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 13
 Applications of the Dynamic Model:
External Applications
• The dynamic model was successfully used to
predict transient shutdown and startup events in
a sCO2 demonstration plant
• This demonstration is to simulate a pulsed
input: 3-hour at power, 1-hour off
– Power generation pattern expected for the United
Kingdom Atomic Energy Authority (UKAEA)
• Observations:
– System shutdown is < 5min, paced by the inertia of the
rotating equipment
– Power-Off: hot condition
– Startup is < 5min, paced by the inertia of the rotating
equipment
• Please refer to paper “Evaluation on the rapidity
of sCO2 cycle power up and down events using
the STEP dynamic simulation model,” authored
by M. McDowell and J. Acres

The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 14
The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 15
 References

1. Marion, J., Kutin, M., McClung, A., Mortzheim, J., Ames, R. (2019) “The STEP 10 MWe sCO2 Pilot Plant Demonstration”, Proc. ASME
Turbo Expo GT2019-91509, Phoenix, AZ.
2. Huang, M., Tang, CJ., McClung, A. (2018) “Steady state and Transient Modeling for the 10 MWe SCO2 Test Facility Program”, Proc. 6th
International Symposium Supercritical CO2 Power Cycles, Pittsburgh, PA.
3. Flownex® version 8.2.0.1735
4. Follett, W. (2022) “DOE Research Performance Progress Report: Supercritical Carbon Dioxide Pilot Plant Test Facility”
5. McDowell, M., Acres, J. (2024) “Evaluation on the rapidity of sCO2 cycle power up and down events using the STEP dynamic
simulation model”, The 8th International Supercritical CO2 Power Cycles Symposium, San Antonio, TX.

The 8th International Supercritical CO2 Power Cycles ● February 27 – 29, 2024 ● San Antonio, TX, USA 16