Thermal System Modeling and Co-Simulation

1
Thermal System Modeling and Co-Simulation with
All-Electric Ship Hybrid Power System
Ruixian Fang1, Wei Jiang1, Jamil Khan1, Roger
Dougal 2 Departments of Mechanical Engineering1
and Electrical Engineering2, University of South
Carolina, Columbia, SC
Detailed zonal thermal subsystem
System Configuration
Objectives

• 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
  thermal-electrical 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.

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 co-simulation. It will be
a closed centralized loop in the ships whole
cooling system.
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.
For propulsion, electrical power from the bus is
sent to motor to drive the propellers.
Non-propulsion power includes ships 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.
Detailed SOFC/GT hybrid power subsystem
Simulation Results
Gas turbine validation

• The responses of the PCMs heat losses to the step
  change of the ships 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.
• 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 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.

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 hightemperature 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.
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
• N 11427 rpm,
• Mass flow rate for compressor 21.018 kg/s.
• Characteristic curves near design point were
  extracted and put into VTB model.

Thermal subsystem responses to the step change of
the service load
PCM temperature variations

• 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
• Note GasTurb uses Generic fuel, while VTB
  assumes methane for these comparisons, we adjust
  the methane flow rate to match the compressor
  exhaust temperature.

• Off- Design Point validation
• Off-design validation
• Same engine settings
• Different operating point.
• N 9999 rpm 10 below design point.
• Compressor
• Outlet temperature consistent with GasTurb
• Outlet pressure error 42
• Error caused by the modeling method of the
  characteristic curve
• Turbine
• Outlet temperature error 15
• Outlet pressure error 9
• Shaft power error 8

Conclusion / Future work
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.
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 ships 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, its
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.