system-level thermal modeling

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System-Level Thermal Modeling and Co-simulation with Hybrid Power System for

Future All Electric Ship

Ruixian Fang1, Wei Jiang

1, Jamil Khan

1, Roger Dougal

2

Department of Mechanical Engineering1and Department of Electrical Engineering

2

University of South Carolina, Columbia, SC, USA

Abstract

This paper presents an approach to performing thermal-

electrical coupled co-simulation of hybrid power system and

cooling system of future all-electric Navy ships. The goal is

to study the transient interactions between the electrical and

the thermal sub-systems. The approach utilizes an existing

Solid Oxide Fuel Cell (SOFC) / Gas Turbine (GT) hybrid

electrical power model and the ship cooling system model

developed on the Virtual Test Bed (VTB) platform at

University of South Carolina. The integrated system

simulation approach merges the thermal modeling capacity

with the electrical modeling capacity in the same platform.

The paper first briefly discusses the dynamic SOFC / GT

hybrid engine system combined with propulsion plant

model. It then describes ship cooling system model and the

interactions between the electrical and the thermal sub-

systems. A simple application scenario has been

implemented and analyzed to illustrate the simulation.

Dynamic responses of coupled thermal-electrical systems

are explored under a step change of the service load to

reveal important system interactions.

Keywords: thermal-electrical co-simulation, SOFC, hybrid

power system, ship cooling system, thermal modeling

1.

INTRODUCTION

For future all electric Navy ships, which will rely on

increasing amount of power electronic components, high

power sensors and advanced weapons systems, thermal

issues become more important due to the large amount of

additional heat load generated. To evaluate the impact of the

transient load (e.g. high power, rapid transients and harsh

environment) expected to be experienced on both the ships

electrical systems and thermal systems, it will be necessary

to better co-design the electrical and thermal systems, in

particular with respect to transient responses during

dynamic events due to electrical-thermal system

interactions. Such a simulation approach will permit ship

designers to address thermal management earlier in the

design process to produce more efficient, less costly ship

power systems.

Coupled thermal-electrical transient studies have

traditionally only been carried out on the apparatus or, at

best, at the sub-system level [1]. Recently, Chiocchio and

Steurer et alperformed a real-time co-simulation of electro-

thermal coupled power systems to study the transient

interactions between the electrical and the thermal systems

[2]. The approach utilizes real-time simulation of a

synchronous gas turbine generator powered electrical

system established at Florida State University on the Real

Time Digital Simulator platform in conjunction with real-

time simulation models of a simplified thermal subsystem

from University of South Carolina implemented on the VTB

platform. The initial simplified application scenario did

reveal some insights into the co-simulation.

This paper presents an integrated approach to performing

system level thermal-electrical coupled co-simulation of a

hybrid power system and thermal system on VTB platform.

The integrated system simulation approach merges the

electrical modeling capacity with the thermal modeling

capacity on the same platform.

In order to allow such studies on the overall system, a few

challenges have been overcome based on the previous

simulation work at University of South Carolina. First, a

hybrid power system simulation model has been created by

Jianget al [3]. In that model, a dynamic SOFC / GT hybrid

engine system combined with propulsion plant is developed

and simulated. The model feature sufficient detail to study

power system transients and dynamics. Important model

components, such as compressor, gas turbine, SOFC,

propeller and ship, were modeled and verified at differentdetail levels individually. Detailed model development and

system configuration will be presented in the following

sections.

A detailed level thermal system simulation model of a ship

cooling system has been developed on VTB platform by

Fang et al [4]. Dynamic simulations for two most essential

cooling schemes used in the ship thermal management were

investigated. One configuration is freshwater cooling sub-

system using seawater as the secondary coolant. The other

configuration is chilled water cooling sub-system from the

ships air conditioning plants. The Pilot Ship, an Arleigh

Burke class destroyer (DDG-51) with an integrated electric

propulsion system, was chosen as the basis for the thermalmanagement simulation work. Recently, the system level

thermal management of the DDG-51 class chilled water

system[5], which has four 200-ton A/C plants on board and

is designed to supply 440F chilled water throughout the ship,

was modeled and simulated by combining the above two

cooling schemes.

978-1-4244-3439-8/09/$25.00 2009 IEEE 547


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Since these models are available, comprehensive thermal-

electrical coupled transient simulations of electric ship

systems will be possible. The object of this paper is to

combine the hybrid power system model with the ship

cooling system model and perform co-simulation to study

their interactions behavior during transient events. In

addition to the details on how to connect the different

system models, an illustrative example simulation is given

which demonstrates the transient behavior of the large scale

combined system.

2. LAYOUT OF THE TWO SYSTEMS AND THEIR

CONNECTION

2.1.

The SOFC/GT Hybrid Power System Layout

The conceptual system layout is shown in Figure 1. Two

sets of SOFC/GT hybrid engine systems 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 powergenerated by the SOFC stack is fed to the DC distribution

bus. The synchronous gas turbine generator provides extra

power to the same DC bus after power generation and

conversion. The power is consumed by the ship propulsion

system and other ship services, like combat systems and

electric auxiliaries, through further power conversion. The

ship is propelled by two propellers, which are driven by two

motors.

Figure 1: Conceptual layout of electric ship hybrid

power system

2.2.Ship Cooling System Description

The conventional thermal management system of a

combatant ship consists of three main types of cooling

approaches [6] : freshwater cooling, seawater cooling and

chilled water cooling. The freshwater cooling approach uses

seawater as the secondary coolant; it is mainly used for

power conversion module (PCM) cooling or electronic

device cooling. The seawater cooling approach uses direct

cooling with a centralized seawater cooling system. The

devices using this approach include the power generation

module (PGM), the power distribution module (PMM), air

conditioning plants, and steam condensers. The chilled

water cooling system is mainly used for compartment heat

and cooling. The chilled water comes from the ships air

conditioning plants, and the rejected heat from the air

conditioning plants is transferred to the centralized seawater

cooling system.

Figure 2 shows a freshwater cooling approach with

seawater as the secondary coolant. In this configuration,

cooling of PCM is achieved by passing cold freshwater

through a liquid heat sink (LHS) attached directly onto the

high heat flux electronic components (the PCMs) in a

cabinet. Heat absorbed by the freshwater is pumped out of

each PCM cabinet. The mixed hot freshwater releases the

waste heat to the centralized seawater system via the

freshwater-seawater heat exchanger. This completes the

freshwater loop.

Figure 2: Conceptual layout of freshwater cooling model

The work presented here utilizes this cooling module for

the co-simulation. There are other ways to model the PCM

cabinet, such as a chilled-air closed loop inside the cabinet,

with an air-freshwater heat exchanger bringing the heat out.

This type of cabinet model had been developed by Georgia

Institute of Technology [7], and was inserted into VTB

simulation directly by replacing the above type of LHS-

PCM cabinet. It shows the ability of implementation of

alternative cooling technologies into VTB system-level

simulation.

2.3.

Layout of Co-simulation

Figure 3 below illustrates the connection between the

hybrid power system and the simplified zonal thermalsystem for a ship. The interaction between the electrical

system and the thermal system is implemented through a

thermal port on each power consumption component. The

power consumption component includes PCM, PGM,

PMM, and high power sensors, etc. The losses resulting

from the efficiency calculation in each electrical component

serve as the forcing function for the thermal system. The

SOFC Stack

SOFC Stack

GT Set

GT Set

GEN

GEN

Motor

Ship Service

Combat S stems

Converter

Converter

Motor

Power

ConversionPower

Generation

Power

Consumption

Freshwater

Seawater

HEX Unit

PCM

LHS

Cold Freshwater

Hot Freshwater

PCM

PCM

PCM

LHS

LHS

LHS

Seawater

Central

Loop

Seawater

Central

Loop

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waste heat from the electrical components such as the

converters is transferred to the heat sink module in the

thermal system. Heatsink temperature is considered to be

the same as the electrical component being cooled. At

present, the temperature effects are not modeled in the

electrical components used in this simulation. Hereby the

heat losses from those components are temperature

independent.

Figure 3: Power system and thermal management co-

simulation layout

3.

SIMULATION SCENARIO ON VTB PLATFORM

A simple simulation scenario has been fully implemented

and partially tested using the layout described above as

shown in Figure 4Error! Reference source not found.

below. This section first briefly describes the hybrid ship

power system model and ship cooling system model

separately and then discusses how the two systems are

linked in this VTB co-simulation approach.

3.1.

SOFC/GT hybrid power model

The mathematical description and model development of

main component models for the SOFC/GT hybrid power

system such as the propeller, ship, compressor and gas

turbine are described in [3]. A one-dimensional dynamic

model of a tubular SOFC with internal reforming, capable

of system integration, is presented [8]. This model, based on

the electrical quantities, chemical reaction equilibrium and

energy balance, can predict the SOFC characteristics at the

steady states and also at transient operating states. All the

component models are independent entities. The system is

built by connecting the component needed on the VTB

platform.

The left part in Figure 4 illustrates the detailedconfigurations of the SOFC/GT hybrid power subsystem.

The methane-steam mixture is supplied to the external

reformer before being delivered to the anode of the SOFC

stack. In the SOFC stack, electrical energy is produced

along with the generation of heat during the electro-

chemical reactions at the electrodes. The produced electrical

energy is then supplied to the switchboard. Under the

assumed adiabatic condition, the heat generated from the

SOFC electrochemical reaction and SOFC internal

resistance is partly used to reform the fuel and partly used to

heat up the gases. For further energy extraction, un-reacted

hightemperature gases are channeled to the combustor for

a complete combustion. In this system, part of the methane

is directly channeled to the combustor and mixed with the

exit gas from SOFC stack to control the combustion

temperature.

The detailed configuration of the Gas Turbine Power plant

is also illustrated in Figure 4. To obtain 1-15 bar air

pressure, a two-stage compressor design is chosen to satisfy

the operating condition. To achieve higher compression

efficiency, an intercooler is applied to cool the inlet air

temperature of the second compressor. The compressed air

is then channeled to the cathode of the fuel cell. The high

temperature exhaust gas from SOFC stack expands through

the two shafts gas turbines whereby mechanical power is

generated. The power generated by the first gas turbine is

fully consumed by the compressors. Subsequent gas

expansion through the power turbine produces additionalmechanical power for electrical power generation. 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. A motor is applied

for system start-up, which will be cut off automatically after

reaching a specific rotational speed.

3.2.

Power conversion / distribution / consumption

The configurations of power conversion and the power

consumption are illustrated in the middle part of Figure 4.

In this simulation scenario, power conversion and

distribution is highly simplified. The power produced by gas

turbines and SOFC stacks, passing through the cables and

converters for voltage conversion are supplied to a DC

distribution bus. From there the power is further converted

by several DC/DC converters and distributed to power

consumption devices. Part of them is supplied to the two

motors, which are used to derive the two propellers.

Technically, the motors should be AC motors with inverters

driving them. In the present work, the motor is represented

as power load that hides some of the waveform detail. The

rest of the power will be consumed by ship service load. In

this paper the ship service load is represented by several

resistors for simplification. Component connection can be

controlled by opening or closing of switches.

3.3.The zonal thermal subsystem model

The right portion of Figure 4 illustrates the

configuration of the freshwater cooling subsystem

implemented in the co-simulation. The components

presented in the subsystem include the freshwater-seawater

heat exchanger, plate/fin heat exchangers, heat sinks,

SOFC

SOFC

GT

GT

GEN

GEN

Thermal Port

Load

DistributionShip

Service

Motor

Motor

Heat Sink

Heat

Exchanger

Heat Sink

Heat Sink

Heat Sink

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Figure 4: VTB schematic of the co-simulation

pumps, valves, pipes, valve controllers, and two seawater

models (one as source and the other one as a sink). The

mathematical descriptions, model development for the

major thermal components, and the cooling system steady

state performance validation are described in detail in [4].The pumps supply the circulation flow through the

freshwater loop and the seawater loop. For the freshwater

loop, system will distribute the flow into each of the eight

cabinets internally based on the fluid system characteristic,

which is determined by each individual fluid mechanical

model included in the fluid system. The seawater loop is

configured as an open-loop in this 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 simplified example,

only the heat losses from those power converters are

dissipated into the thermal plant. Similar to the approach

used in [2], these losses are computed from theirinstantaneous component through power values

multiplication with loss coefficients between 1% and 5%.

3.4.The control strategies for the co-simulation

In order for this simulation to perform well under transient

conditions simple control systems for the hybrid power

system, the DC/DC converter, and the thermal plant were

implemented.

The main control strategies applied in hybrid power

system are listed below:

The mass flow rates of methane, water and air arecontrolled by adjusting the openings of three associated

valves.

Specific percentage of methane, which is directly

channeled to the combustor, is controlled by the methane

valve.

In the external reformer, methane partly takes part in

the reforming reaction to produce hydrogen and carbon

dioxide.

The connection between different electrical components

is controlled by opening or closing of several switches.

For the thermal plant, a simple control approach is

implemented by adding a valve controller model as can be

seen in Figure 4. The goal of this controller is to maintaineach individual PCM temperature at a desired value when

its disturbed from the steady state. The valve controller

model checks the PCM temperature every time step. When

the temperature is greater than the desired value, the valve

controller model will change the opening of the

corresponding valve model. As the result, the mass flow rate

of freshwater through that cabinet will change accordingly.

Zonal thermal plant

Power generation conversion and distribution

Gas turbine set

SOFC stack

PCM_1

PCM_2

PCM_3

PCM_4

PCM_5

PCM_6

PCM_7

PCM_8

LOAD

Valve

Controller

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3.5.

Simulation start-up and time step

The system start-up is controlled by a voltage-driven DC

motor. The start-up motor drives the compressors and the

first gas turbine through a connector at the beginning of

system simulation. When the compressors rotational speed

reaches a specific value, the connector is switched off from

the compressor shaft and the compressors are driven by theconnected gas turbine thereafter.

An important computational aspect of any coupled

thermal-electrical transient system simulation is the vast

differences between the electrical and the thermal response

times [2]. The simulations of electrical systems typically

require from one to hundred microsecond time steps, those

of thermal system simulations range from 100 milliseconds

to several tens of seconds. Unfortunately, at this time the

simulation on VTB platform has to be run at the same time

step. Multi-time-rate step simulation capability is under

development. In this example co-simulation, a one

millisecond time step is adopted for the whole system.

4.

EXAMPLE SIMULATION AND RESULTS

4.1.Steady state operating point and parameters

This section provides illustrative examples of typical

system responses of the co-simulation. The hybrid power

system is developed for a 50-ton electric ship in this

demonstration. In this example, a rated 580 kW power is

generated by one SOFC stack while 250 kW power is

produced by one gas turbine set. The cycle steady state

operating point conditions are listed in Table 1.

Table 1: Cycle operation conditions

Parameter Value Unit

Turbine Adiabatic Efficiency 85 %CH4 Flow Rate 0.024 kg/s

H2O Flow Rate 0.054 kg/s

Inlet Temperature of Fluids 25 C

Steam Carbon Ratio (S/C) 2 NU

Pressure Drop of SOFC Stack 2 %

External Reforming Degree 1 %

By Pass Percent of CH4 0.01 %

Speed Control for Start Up 2500 rad/s

Fuel Pressure Ratio 10 bar

Ship Weight 50000 kg

Propeller Diameter 0.885 m

The thermal plant is designed to have a rated coolingcapability of 200 kW. Seawater is assumed to enter the

plate-frame heat exchanger at temperature of 35C. Also the

thermal system startup temperature is set at 35C. The

physical parameters of each LHS heat exchanger, which

locate inside of each PCM cabinet, are designed differently

according to their maximum heat load capacity. Some of the

parameters of the thermal plant are summarized in Table 2.

Table 2:Thermal plant parameters and operating conditions


System initial temperature 35 C

Rated freshwater flow rate 11.8 kg/sec

Rated seawater flow rate 6.75 kg/sec

Specific heat for water 4186 J/kg.0K

P&F heat exchanger surface area 5.4 m2

Each Heat sink mass 50 kg

Each Heat sink specific heat 390 J/kg.0K

LHS surface area for PCM_1, 4 0.42 m2



LHS surface area for PCM_6,7 0.25 m2

PCM's temperature control point 70 C

4.2.Steady state operation results

Based on the above specific design point, the steady state

operation results are summarized in Table 3 below.Table 3:Steady operation results


Total Power Output 1.658 MW

Total Power Efficiency 67.9 %

SOFC Stacks Power Output 1155 kW

Gas Turbines Power Output 503 kW

Cell Voltage 0.54 V

Mean Current Density 0.34 A/cm2

Turbine Inlet Temperature 1268 C

System exhaust Gas Temperature 382 C

Outlet Pressure of Second Compressor 11.6 bar

Outlet Pressure of First Turbine 4.36 bar

Air Flow Rate 0.664 kg/sFuel Flow Rate 0.0243 kg/s

Main Swichboard Voltage 6600 V

Mechanical Power Deliveried to Propell1645 kW

Propeller Efficiency 18.6 %

Ship velocity 4.67 knots

The cycle simulation achieves an electrical power output

of 1.658MW, around 30% of which is produced by the

power turbines. The mass flow rate of pressurized air

supplied to the SOFC stack is 0.664 kg/s. The fuel mixture

consists of 0.024 kg/s methane and 0.054 kg/s steam. The

single SOFC stack yields a 580 kW electrical output power

at the conditions of 1106 C mean temperature, 0.541V cellvoltage and 90.7% fuel utilization. 306 kW thrust power is

produced by the propeller, which moves the ship at the

steady velocity of 4.67 knots.

The steady state heat load and temperatures of each PCM

are shown in Table 4. Because of the symmetric

arrangement of components in both power generation

system and propulsion system as shown in Figure 4above,

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the values of temperature and heat load of those symmetric

components, such as PCM 1 and PCM 4, are always the

same.

Table 4:PCMs heat load and temperature


Heat load from PCM_1, 4 28.9 kw



Heat load from PCM_6,7 6.29 kw

Temperature of PCM_1, 4 50.4 oC



Temperature of PCM_6,7 41.3 oC Seawater exits the plate-frame heat exchanger at a

temperature of 40.1C. The freshwater enters this heat

exchanger at 41.7C and exit at 38.8C. The total heat

dissipated from the thermal plant is about 144.7 kW.

4.3.

Dynamic simulation

After the system reaches its steady state, a step change

was imposed on the system by increasing the service load

up to 100% (double the load). The transient system

responses are shown in Figure 5 trough Figure 7.

Figure 5: System response to electrical transient

Figure 5 shows the responses of PCM heat losses to the

step change. While the service load increases, the power

supplied to drive the propeller decrease as a result of power

redistribution. The ship moves at a velocity 4.65 knots,

which is 0.02 knots lower than the speed at steady state. The

total electrical power feed to the DC bus slightly lowered,

since the power from SOFC stack reduced about 8 kW and

the power from the GT/Generator set increase about 1 kW.

Heat losses from PCM 6 / PCM 7 together with the heat loss

from the service load increase from 6 kW to 12 kW,

whereas the heat losses from PCM 1, PCM 4, PCM 5, and

PCM 8 decrease about 200 W respectively. Heat load from

PCM 2 and PCM 3, which are the converters for

GT/Generator sets, keeps almost unchanged. The total heat

loads for the thermal plant rise about 11kW as shown in the

bottom curve in Figure 5. The gray lines in the figure show

the heat dissipation history of each heatsink attached onto

the PCMs. At steady state, the heat dissipated from each

heatsink is equal to the instantaneous PCM heat load.

During transient process, because of the heat capacitance of

the heatsink material and the charged water inside theheatsink, its always delayed to keep up with the PCM heat

load.

Figure 6: PCM temperature variation

The temperature variations of each PCM are shown inFigure 6 above. Its obvious that PCM 6 and PCM 7 have a

larger temperature change due to the load doubled. Its

interesting that other PCMs temperature rises about 0.2 oC

respectively, though their heat losses slightly dropped or

keep unchanged. Thats because of the temperature of the

freshwater entering into each PCM cabinet is increased

through out the system. Figure 7 shows the freshwater and

Service load [W]

Losses [W]

Losses [W]

Losses [W]

Losses [W]

Losses [W]

Step change electrical load

PCM 6 / PCM 7 + Pload

PCM 1 / PCM 4

PCM 2 / PCM 3

PCM 5 / PCM 8

Total thermal loss of system

Time

T [oC]

T [oC]

T [oC]

T [oC]

Time [s]

PCM 6 / PCM 7

PCM 1 / PCM 4

PCM 2 / PCM 3

PCM 5 / PCM 8

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seawater temperature variation while they are flowing

through the plate-frame heat exchanger. They are increasing

since the total heat load for the whole thermal plant is

increased.

Figure 7: Freshwater and seawater temperature

variation

In addition, the co-simulation will respond to otherparameter changes during simulation, such as change of fuel

supply, or ships drag coefficient, etc, to investigate their

dynamic variation for both hybrid power system and

thermal system.

5.

FUTURE DEVELOPMENTS

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. In detail,

Develop a heat generation model of electronic power

converter leg at the switching-averaged detail level,with temperature dependent parameters and averaged

heat loss calculations.

Incorporate more sophisticated control system onto

simulation model for hybrid power generation including

solid oxide fuel cell power plant.

Incorporate more sophisticated control system onto the

thermal plant.

6. CONCLUSION

This paper presented an integrated approach for electro-

thermal simulation. A simple simulation scenario by

combining the hybrid power generation and ships

propulsion model with a zonal cooling model on VTBplatform has been implemented and partially tested. While

the simplified example simulation did reveal some insights

into the system interactions as a step change of the service

load. The main goal of this research is to assess dynamic

issues in a complex system such as an All Electric Ship on

VTB platform. Though it is simplified, this paper outlined a

typical portion of such a configuration for the whole ship

systems. More work need to done on power conversion,

distribution and thermal systems before we can eventually

integrate those systems to perform system-level co-

simulation and get fidelity results.

7. ACKNOWLEDGMENTS

The authors acknowledge support for this research fromthe Office of Naval Research under ESRDC consortium and

also from contract number N00014-06-1-0052, program

managed by Dr. Mark Spector.

8. REFERENCES

[1]

Storti-Gajani, G.; Brambilla, A.; Premoli, A., Electrothermal

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on Fundamental Theory and Applications Circuits and Systems I,Volume 48, Issue 8, Aug. 2001 pp. 997 1005

[2]

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Khan, J. Ordonez, M. Sloderbeck, and S. L. Woodruff. Webb,T.Kiehne, S.Haag, A Co-Simulation Approach for Real-Time

Transient Analysis of Electro-Thermal System Interactions on Board

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Prediction and Dynamic Simulation of Electric Ship Hybrid Power,Electric Ship Technologies Symposium, 2007 , pp. 490 - 497

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R. Fang, W. Jiang, A. Monti, M. Zerby, G. Anderson, P. Bernotas, J.Khan, System-Level Dynamic Thermal Modeling and Simulation for

an All-Electric Ship Cooling System in VTB, in Proc. of the IEEE

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T [oC]

Time [s

Freshwater inlet temperature

Freshwater outlet temperature

Seawater outlet temperature

Seawater inlet temperature at 35 oC

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