Thermal System Modeling and Co-Simulation
with All-Electric Ship Hybrid Power System
Ruixian
1
Fang ,
Wei
1
Jiang ,
Jamil
1
Khan ,
Roger Dougal
2
Departments of Mechanical Engineering1 and Electrical Engineering2, University of South Carolina, Columbia, SC
The main goal of the present work is to comprehensively
model the thermal plant on board of future all-electric ship at
the system-level to resolve the dynamic interactions between
the hybrid power system and the thermal system.
Specific objectives are:
• Integrate an existing Solid Oxide Fuel Cell / Gas
Turbine hybrid electrical power model with the ship
cooling system model on the same Virtual Test Bed (VTB)
platform.
• study co-simulation issues between the coupled
electrical and thermal systems such as start-up, system
control, etc.
• Investigate dynamic responses of the coupled thermalelectrical systems under a step change of the service load
to reveal important system interactions.
• Outline a typical portion of such a configuration for the
whole ship systems on VTB platform and serves as the
baseline for future All Electric Ship simulation.
Detailed SOFC/GT hybrid
power subsystem
Detailed zonal thermal
subsystem
System Configuration
Objectives
Two sets of SOFC/GT hybrid engine
subsystems are used for power
generation. The high quality heat
source exhaust from the SOFC stack
is channeled to the gas turbine to
produce extra power. After power
conversion, the electrical power
generated by the SOFC stack is sent
to a common electrical bus for
allocation. The synchronous gas
turbine generator provides extra
power to the same DC bus after
power generation and conversion.
Through flexible distribution and
switching architecture, the common
electrical bus can supply electrical
power to both non-propulsion and
propulsion electrical loads and
instantly
redistribute
power
as
necessary.
Figure on the right illustrates the
configuration of the zonal freshwater
cooling subsystem implemented in the
co-simulation. The pumps supply the
circulation flow through the freshwater
loop
and
the
seawater
loop
respectively. For the freshwater loop,
system will distribute flow into each of
the eight PCM cabinets internally
based
on
the
fluid
system
characteristic. The seawater loop is
configured as an open-loop in this cosimulation. It will be a closed
centralized loop in the ship’s whole
cooling system.
For propulsion, electrical power from the bus is sent to motor to drive the propellers. Non-propulsion power includes ship’s service
load and electric auxiliaries. The interaction between the electrical system and the thermal system is implemented through a
thermal port on each power consumption component. The losses resulting from the efficiency calculation in each electrical
component serve as the forcing function for the thermal system. The heat load from the electrical components such as the
converters is transferred to the heat sink module in the thermal system. Heat sink temperature is considered to be the same as the
electrical component being cooled.
Simulation Results
Gas turbine validation
The responses of the PCMs heat losses to
the step change of the ship’s service load
are shown in Figure (A)-(F). All those figures
are obtained from VTB simulation plotting
directly, with the Y-axis representing the
characteristic concerned and the X-axis
representing the time in seconds.
Fuel
Combustor
Bleeding
compressor
Turbine
inlet
In the SOFC stack, electrical energy is produced along with the
heat generation during the electro-chemical reactions at the
electrodes. The produced electrical energy is then supplied to
the switchboard. For further energy extraction, un-reacted high–
temperature gases are channeled to the combustor for a
complete combustion there. The exhaust gas from the power
turbine passes through two heat exchangers to preheat fuel
mixture and compressed air for maximum utilization of the
residual heat.
• N = 11427 rpm,
•Mass
the dark lines in Figure (C), (D) and (E)
represents the power converters heat
generation, while the grey lines in those
figures represents the heat dissipation by
their corresponding heat sinks.
flow rate for
compressor = 21.018
kg/s.
Characteristic curves near
design
point
were
extracted and put into
VTB model.
To obtain 1-15 bar air pressure, a two-compressor with an
intercooler design is chosen to satisfy the operation conditions.
The compressed air is then channeled to the cathode of the fuel
cell. The high temperature gas from the combustor expands
through the two-shaft gas turbines whereby mechanical power is
generated.
Because of the symmetric arrangement of
components in both the power generation
subsystem and the propulsion, the values of
temperature and heat load of those
symmetric PCMs, such as PCM 1 and PCM
4, are always the same.
The gas turbine is validated by comparing VTB model results with GasTurb
commercial simulation software. Boundary conditions for the VTB model
match those for GasTurb: compressor inlet conditions, air bleed, design point
of shaft speed, etc.
Design point:
Design Point Validation
Compressor
•Pressures Outlet pressure error 2%,
•Outlet temperature error 9%
•Error caused by the assumption of
ideal compression.
Turbine
•Outlet pressure error 4%
•Outlet temperature error 8%
Shaft power error 4%
Thermal subsystem responses to the
step change of the service load
PCM temperature variations
Off- Design Point validation
Off-design validation
•Same engine settings
•Different operating point.
•N = 9999 rpm 10% below design point.
Compressor
•Outlet
temperature
consistent
GasTurb
with
•Outlet pressure error 42%
•Error caused by the modeling method of
the characteristic curve
Note: GasTurb uses Generic fuel, while VTB
assumes methane for these comparisons, we adjust
the methane flow rate to match the compressor
exhaust temperature.
The heat load received by each heat sink comes from each individual
electrical component. In this example co-simulation, only the heat losses
from those power converters are dissipated into the thermal subsystem. The
interaction with the thermal system is through the thermal port on each PCM.
Any loss resulting from the efficiency calculation is supposed to be the
forcing function for the thermal subsystem. These losses are computed from
their instantaneous component through power values multiplication with loss
coefficients between 2% and 5%.
Turbine
•Outlet temperature error 15%
•Outlet pressure error 9%
Shaft power error 8%
Conclusion / Future work
This work presented an integrated approach for system level thermo-electrical co-simulation. An example simulation by integrating
the SOFC/GT hybrid power generation subsystem, power distribution subsystem, propulsion subsystem with a zonal thermal
subsystem on VTB platform has been implemented. Both steady state and dynamic simulations are performed. With a step change
of the ship’s service load, the transient responses of the heat losses and temperatures of the power converters and the
performances of the thermal plant are analyzed in detail. The comprehensive analytical models and the system-level co-simulation
methodology provided in this paper lead to an improved understanding of thermal management of large scale complex systems.
As next stage for such a co-simulation, it’s necessary to enhance the level of detail represented in the thermal plant, the power
distribution system, and the power conversion system. Such as developing a heat generation model of electronic power converter
leg at the switching-averaged detail level, with temperature dependent parameters and averaged heat loss calculations; and
Incorporating more sophisticated control system onto the thermal plant.